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Dislocations in gallium arsenide deformed at high temperatures 1987

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( DISLOCATIONS IN GALLIUM ARSENIDE DEFORMED AT HIGH TEMPERATURES By PATRICK JOHN GALLAGHER B.A.Sc. The University of B r i t i s h Columbia, 1983 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Metals and Materials Engineering) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l 1987 © Patrick John Gallagher, 1987 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of MgTTrlS Adt> M f r T g e M L S SrOanJEergJNKi The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date A P * I « - l*&7 nF.Afi/ft-n Abstract Test pieces of GaAs were cut from Czochralski grown <100> wafers. P r i o r to deformation the d i s l o c a t i o n configuration was established by cathodoluminescence (CL). Etch p i t s produced by molten KOH on examined c r y s t a l surfaces coincided with the CL images. The t e s t pieces were capped with Si 3N4, heated to between 950 and 1050"C, and p l a s t i c a l l y deformed by bending. The d i s l o c a t i o n configuration a f t e r bending was then compared to that of the undeformed c r y s t a l . I t was observed that heating to 1050°C d i d not s i g n i f i c a n t l y change the as grown c e l l u l a r d i s l o c a t i o n arrays i n the c r y s t a l . With s t r a i n the d i s l o c a t i o n configuration changed appreciably. New bands of di s l o c a t i o n s were formed, p a r a l l e l to the bend axis with d i s l o c a t i o n free regions between them. Increasing the s t r a i n increased the number of bands. Observations were made on undoped c r y s t a l s with high and low d i s l o c a t i o n densities, and S i doped c r y s t a l s . The luminescent properties of the di s l o c a t i o n s were observed to change with heating and s t r a i n . As grown, a d i s l o c a t i o n imaged as a dark spot surrounded by a bright halo, giv i n g bright d i s l o c a t i o n networks. After heating to 950°C samples showed only the dark spots without halos. A f t e r deformation, a l l the new di s l o c a t i o n s appeared as dark spots or l i n e s without halos. At very low s t r a i n s , the o r i g i n a l d i s l o c a t i o n s were s t i l l evident but were d i s t i n c t from the new arrays. In an attempt to correlate the d i s l o c a t i o n images with impurity segregation some observations of the samples were made using secondary ion mass spectroscopy (SIMS). The r e s u l t s suggest the p o s s i b i l i t y of the dark areas i n the CL images being associated with the presence of carbon. i v Table of Contents Abstract i i L i s t of Tables v i L i s t of Figures v i i Acknowledgment x i 1 Introduction 1 1.1 Li t e r a t u r e Review 2 1.2 Gallium Arsenide 6 1.3 Cathodoluminescence 7 1.4 Observing Dislocation Arrangements 9 2 Procedures 10 2.1 Deformation of Gallium Arsenide Plates 10 2.2 Bending J i g s 13 2.21 S t a t i c Bending J i g 14 2.22 Dynamic Bending J i g 15 2.23 Materials Selection 16 2.24 S t r a i n C a l i b r a t i o n of Dynamic J i g 17 2.3 Environmental Conditions 18 2.4 Temperature C a l i b r a t i o n . . 20 2.5 Specimen Preparation and Handling 22 2.51 Arsenic Containment 22 2.511 Preliminary Testing Under Boron Oxide 23 2.512 S i l i c o n N i t r i d e Capped Specimens . . . 25 2.52 Post Heating Specimen Treatment 27 2.6 Cathodoluminescence Imaging 28 2.61 Scanning Electron Microscope 28 2.62 Cathodoluminescence Detection Equipment and I n s t a l l a t i o n 33 2.7 Secondary Ion Mass Spectrometry Analysis 36 V 3 Observations 38 3.1 The Cathodoluminescence Process 38 3.2 Cathodoluminescence Imaging of GaAs 39 3.21 Cathodoluminescence Image compared to Etch P i t s 40 3.22 CL Contrast - Dots and Halos 42 3.23 Surface condition 44 3.24 GaAs Capped with S i l i c o n N i t r i d e 44 3.25 Secondary Ion Mass Spectrometry Analysis . . 45 3.3 Heated Specimens 46 3.4 S t a t i c Bend Specimens 48 3.5 C y c l i c Bend Specimens 53 4 Discussion 60 4.1 Cathodoluminescence Imaging 60 4.2 Deformation 61 4.21 Formation of Linear Arrays 61 4.22 New Array Density ; 62 4.23 Y i e l d strength 65 4.24 Calculation of Parameters from C y c l i c Tests 66 4.25 Estimation of Parameters To Move Dislocations 68 4.3 Conclusions 70 5 Future Recommendations 72 L i s t of References 99 v i L i s t of Tables TABLE I. GaAs specimens and t e s t conditions 13 TABLE I I . Density of l i n e a r arrays along specimen. . . . 64 v i i L i s t of Figures FIGURE l . Stress versus s t r a i n for S i doped GaAs at various temperatures, tested i n compression along <100> a x i s 2 . . 73 FIGURE 2. Stress versus s t r a i n for S i doped GaAs at various temperatures, tested i n compression along <111> a x i s 2 . . 73 FIGURE 3. Y i e l d stress versus temperature for various samples of doped GaAs tested i n compression along <100> a x i s 2 . . 74 FIGURE 4. Dislocation p o s i t i o n versus time from CL image of GaAs under an applied stress of 47MPa at 195°C 3. . . . . 74 FIGURE 5. GaAs c r y s t a l structure (cubic zincblende) showing the two inter-penetrating FCC l a t t i c e s of Ga and As . . .75 FIGURE 6. Schematic comparison of band structure of d i r e c t gap and i n d i r e c t gap i n momentum (k) space 75 FIGURE 7. Schematic of simple, d i r e c t gap recombination. . . 76 FIGURE 8. Schematic of recombination with an interband energy l e v e l 76 FIGURE 9. S t a t i c bending j i g (a) i n p r o f i l e and (b) showing arrangement 77 FIGURE 10. Dynamic bending j i g shown i n section 77 FIGURE 11. C a l i b r a t i o n curve for c r y s t a l growers control thermocouple using the 15cm heater 7 8 FIGURE 12. C a l i b r a t i o n curve for c r y s t a l growers control thermocouple using the 20cm heater 78 FIGURE 13. Experimental arrangement f o r t e s t i n g B 20 3 as an encapsulant f o r high temperature deformation of GaAs shown i n section 79 FIGURE 14. GaAs wafer orientation showing specimen alignment with wafer 79 FIGURE 15. Inverted CL image of polished GaAs showing the networks of as-grown d i s l o c a t i o n arrays 80 v i i i FIGURE 16. Inverted CL image of polished low grade GaAs showing d i s l o c a t i o n arrays and spots 81 FIGURE 17. Optical image of same GaAs surface as i n Figure 16 a f t e r a chemical etch i n molten K 0 H 81 FIGURE 18. CL image of K 0 H etched, low d i s l o c a t i o n density GaAs 81 FIGURE 19. Optical image of same area of GaAs as i n Figure 18 showing etch p i t s 81 FIGURE 20. CL image of sample of S i doped GaAs, KOH etched. . 82 FIGURE 21. SE image of same area of S i doped GaAs as i n Figure 20, showing KOH etch p i t s 82 FIGURE 22. CL image of a d i s l o c a t i o n p a r a l l e l to the surface of S i doped GaAs 82 FIGURE 23. SE image of same area of etched GaAs as i n Figure 22 showing absence of etch p i t s 82 FIGURE 24. CL image of elect r o n i c s grade GaAs showing "dots" and "halos" e f f e c t s around d i s l o c a t i o n s 83 FIGURE 25. CL image of GaAs a f t e r heating to 1000°C 83 FIGURE 26. Enlarged CL image of GaAs i n Figure 25 83 FIGURE 27. SIMS map of GaAs from the same region as Figure 26 showing d i s t r i b u t i o n of carbon 83 FIGURE 28. SE image of 750A t h i c k S i 3 N 4 cap on GaAs 84 FIGURE 29. SE image of p i t s i n S i 3 N 4 cap on GaAs that formed a f t e r heating to 950°C 84 FIGURE 30. CL image of capped GaAs a f t e r heating to 950°C, from the same area as Figure 29 84 FIGURE 31. Inverted CL image of capped GaAs specimen T4 . . . 85 FIGURE 32. CL image of GaAs specimen T4 a f t e r 3 60s at 955°C, cap removed and surface polished. Area of Figure 31. . . 85 FIGURE 33. Inverted CL image of capped GaAs T4, adjacent region to Figure 31 85 FIGURE 34. CL image of annealed GaAs T4 from area adjacent to Figure 32, same area as Figure 33 85 ix FIGURE 35. CL image of GaAs T2 a f t e r bending at 955°C. Area 12.5mm from bend axis 86 FIGURE 36. CL image of GaAs T2 a f t e r bending at 955°C. Area 7.5mm from bend axis 86 FIGURE 37. CL image of GaAs T2 A f t e r bending at 955"C. Area 3mm from bend axis 86 FIGURE 38. CL image of GaAs T2 a f t e r bending at 955°C. Area 1.5mm from bend axis 86 FIGURE 39. CL image of GaAs T5 a f t e r bending at 955°C. Area near bend axis 87 FIGURE 40. CL image of GaAs T5 showing new d i s l o c a t i o n arrays i n area of Figure 39 87 FIGURE 41. CL image of GaAs T5 a f t e r bending at 955°C. Area away from bend axis 87 FIGURE 42. CL image of GaAs T5 showing arrays i n Figure 41 . 87 FIGURE 43. CL image of S i doped GaAs a f t e r bending at 955°C showing old and new di s l o c a t i o n s i n low s t r a i n region . . 88 FIGURE 44. CL image of S i doped GaAs S i l a f t e r heating and bending showing new arrays i n the high s t r a i n region. . . 88 FIGURE 45. CL image of GaAs SI a f t e r c y c l i c loading at 1000 °C. Area near bend axis 89 FIGURE 46. CL image of d i s l o c a t i o n arrays at A i n Figure 45 . 89 FIGURE 47. CL image of SI a f t e r heating & c y c l i n g from the area away from the bend axis 89 FIGURE 48. CL image of d i s l o c a t i o n arrays i n Figure 47. . . .89 FIGURE 49. CL image of GaAs S2 showing d i s l o c a t i o n arrays a f t e r c y c l i n g at 1050°C. Near the bend axis 90 FIGURE 50. Thermal hi s t o r y of GaAs specimen S3 during high temperature bend 90 FIGURE 51. CL image of GaAs sample S3 capped with 8 0 0 A of S i 3 N 4 6mm from the centre of the specimen 91 FIGURE 52. CL image of area of S3 i n Figure 51 a f t e r 1 cycle at 1000°C. Low s t r a i n region 6mm from bend axis . . . . 91 X FIGURE 53. Inverted CL image map of middle 12mm of GaAs specimen S3 capped with 800A of S i 3 N 4 92,93 FIGURE 54. CL image map of GaAs S3 a f t e r 1 bend cycle at 1000°C. Same region as Figure 53 92,93 FIGURE 55. CL image of GaAs S4 a f t e r c y c l i n g at 1000"C. Highest s t r a i n region, at the bend axis 94 FIGURE 56. CL image of GaAs S4 adjacent to region i n Figure 55, near the bend axis 94 FIGURE 57. CL image of GaAs S4 region away from the bend axis, i n the bright zone 94 FIGURE 58. CL image of GaAs S4 region further away from the bend axis, i n the bright zone 94 FIGURE 59. CL image of GaAs S4 a f t e r c y c l i n g at 1000°C. Low s t r a i n area i n the bright zone near t r a n s i t i o n . . . . 95 FIGURE 60. CL image of GaAs S4 at the t r a n s i t i o n from the bright zone to the dark zone 95 FIGURE 61. CL image of GaAs S4 showing d i s l o c a t i o n d i s t r i b u t i o n i n the dark zone 95 FIGURE 62. S l i p planes and s l i p d i r e c t i o n s i n bend specimens. 96 FIGURE 63. Construction showing surface s t r a i n (e s) due to a varying y i e l d depth (YD) for a calculated YD p r o f i l e i n a 3-point bend specimen 96 FIGURE 64. Dislocation l i n e density versus distance from bend axis f o r GaAs specimen T2 plus parametric curve of "Ee11 . 97 FIGURE 65. Y i e l d stress versus temperature for doped/undoped GaAs tested i n compression along <100> a x i s 2 and i n bending about <011> axis 97 FIGURE 66. Construction showing stress d i s t r i b u t i o n and y i e l d depth (YD) i n a 3-point bend specimen undergoing contained p l a s t i c y i e l d i n g 98 FIGURE 67. Schematic showing e f f e c t i v e CL generation depth about the t r a n s i t i o n region i n GaAs specimen S4 98 x i Acknowledgement The work described i n t h i s thesis and the manuscript were prepared under the auspices of Dr. F. Weinberg. Financial support was provided, i n part, by a fellowship from Alcan. Thanks are also extended to the f a c u l t y and s t a f f at U.B.C. metallurgy f o r t h e i r assistance. Special thanks are extended to f a c u l t y and s t a f f at B.C.I.T. Physics f o r t h e i r support and access to equipment for preparing t h i s manuscript. F i n a l l y , to my wife Diana, without whose support t h i s work would not have been completed, a very sp e c i a l thanks. 1 1 Introduction Commercially produced gallium arsenide (GaAs) i s finding a p p l i c a t i o n i n integrated opto-electronic systems due to i t s semiconducting and o p t i c a l properties. The c r y s t a l s , as grown by the l i q u i d encapsulated Czochraloski (LEC) process, contain a high concentration of d i s l o c a t i o n s r e l a t i v e to s i l i c o n , usually formed into c e l l u l a r arrays. These d i s l o c a t i o n s are suspect i n the low y i e l d of integrated c i r c u i t s from GaAs wafers. In p a r t i c u l a r , the d i s l o c a t i o n s are known to be the cause of f a i l u r e of o p t i c a l devices (lasers) b u i l t onto the wafer. Attempts to reduce the number of d i s l o c a t i o n s i n the as-grown c r y s t a l s are ongoing. Attempts at removing or reorganizing the d i s l o c a t i o n networks by annealing the wafers (or boules) have been unsuccessful. The purpose of t h i s work was to determine whether the arrays of grown-in d i s l o c a t i o n s i n GaAs could be moved with application of temperature and s t r a i n . Further, i f they could be moved, then determine i f they could be moved into regular arrays, and the c r i t i c a l parameters to accomplish t h i s . To make these observations required a technique for delineating the d i s l o c a t i o n s i n GaAs. This could be attempted using several techniques. 2 1.1 L i t e r a t u r e Review Previous deformation studies on GaAs have involved the determination of macroscopic properties such as y i e l d strength as a function of temperature 1' 2. In reference (1) specimens of GaAs were deformed by bending up to 700 "C i n a i r and i n tension from 700 to 1000°C i n an arsenic (As) atmosphere. A f t e r deformation the sample surface was etched i n molten potassium hydroxide (KOH) to reveal the d i s l o c a t i o n arrangement. Some samples were also prepared for examination i n a transmission electron microscope (TEM). In a t y p i c a l t e s t , deformation reached 10 to 2 0 per cent, and the r e s u l t i n g d i s l o c a t i o n densities were on the order of 10 8/cm 2. The di s l o c a t i o n s were found to be aligned on the in t e r s e c t i o n of {111} planes with the surface. The s l i p system was determined as {111}<110> and the majority of d i s l o c a t i o n s had Burgers vectors of |<110>. In reference (2) the mechanical properties of GaAs were investigated with respect to the use of GaAs as an i n f r a red (ir) window. The study considered y i e l d strength and fracture behaviour i n compressively loaded specimens oriented with either <111> or <100> along the compressed axis over the temperature range of 250 to 550°C. Samples of undoped and S i , Cr, or Zn doped GaAs were tested. The samples were loaded at constant loading rates of 13.7, 34.3, and 68.7 MPa/s. Specimens were placed between A12C>3 buttons inside a three zone furnace. 3 A thermocouple was placed against the specimen to record temperature. Load and crosshead displacement were recorded and the r e s u l t i n g s t r e s s - s t r a i n curves are shown i n Figure 1 for the <100> and Figure 2 for the <111> oriented S i doped specimens. The r e s u l t s confirmed a s l i p system of {111}<110> since the r a t i o of y i e l d strengths of <111> and <100> oriented samples, over the temperature range, was constant. M u l t i p l y i n g each set of data by t h e i r appropriate Schmid factors collapsed the data onto one curve of c r i t i c a l resolved shear stress (CRSS) versus temperature f o r the S i doped samples. The study also showed a monotonic decrease i n y i e l d strength with temperature for a l l doping species and undoped GaAs as shown i n Figure 3 of y i e l d stress versus temperature f o r <100> oriented specimens. I t was found that S i doping increased the y i e l d strength, Zn doping lead to a drop i n y i e l d strength, and Cr doping did not seem to a f f e c t the y i e l d strength. A l l t e s t s were conducted i n a i r . Samples were strained to l e v e l s of 10 to 20 per cent. The maximum t e s t temperature was l i m i t e d to 550°C to avoid problems associated with As loss from the surface of the GaAs specimens. Microscopic studies of d i s l o c a t i o n movement associated with small stresses have also been undertaken 3' 4. In these works the 4 e f f e c t s on d i s l o c a t i o n v e l o c i t y , of stress, temperature, and i r r a d i a t i o n (electron beam) l e v e l were studied. In reference (3) a serie s of heating and stressing experiments were c a r r i e d out inside a scanning electron microscope (SEM) that had been equipped with cathodoluminescence (CL) detection apparatus. A damaged area was introduced into the GaAs surface by a diamond scrib e . This generated the di s l o c a t i o n s which were subsequently studied. By c a r e f u l manipulation of the GaAs specimen and the electron beam scan area, a condition of stress and l o c a l i z e d i r r a d i a t i o n (electron beam) was used to p u l l a singl e d i s l o c a t i o n out from the damaged region. Once t h i s had been accomplished, states of uniform heating, i r r a d i a t i o n , and stress were applied and the d i s l o c a t i o n was tracked i n the CL image. The bending apparatus was a four point system (generates constant bending moment i n the centre span, i e . constant surface stress) driven by a solenoid. The load was cont r o l l e d by the amount of current passing through the solenoid windings and had the f a c i l i t y to be eithe r held at a constant load or cycled at various frequencies. The solenoid and magnetic core were enclosed i n an aluminum case to s h i e l d the SEM from the magnetic f i e l d generated. The bending j i g i t s e l f was surrounded by shielded heating c o i l s which were used to control the temperature of the specimen. Temperatures were recorded from thermocouples placed i n contact with the GaAs specimen. 5 The positions of the d i s l o c a t i o n s were recorded from the CL image as a function of time at temperature, i r r a d i a t i o n , and stress l e v e l . A t y p i c a l set of data taken at 195°C and a stress l e v e l of 47MPa i s shown i n Figure 4. The d i s l o c a t i o n v e l o c i t y i s the slope of t h i s p l o t and t h i s was the parameter measured against temperature, i r r a d i a t i o n , and stress. Two of these were held constant while the t h i r d was varied. Accumulated r e s u l t s showed a range of d i s l o c a t i o n v e l o c i t i e s from 10~ 9 to 10~6m/s, between 27 and 427°C, with stress l e v e l s from 10 to 50MPa and i r r a d i a t i o n from 30keV electrons at current de n s i t i e s from 4xl0~ 3 to 5xl0 - 1A/m 2. At temperatures above 227°C the background r a d i a t i o n was s u f f i c i e n t to flood the CL detector, so continuous imaging was not possible. Instead images were taken before and a f t e r s p e c i f i c times at stress and temperature. The highest temperature that could be studied was r e s t r i c t e d to that below which degradation of the GaAs surface, due to loss of As, was a problem. Therefore the highest temperature studied was l i m i t e d to 427°C. This study was not d i r e c t l y concerned with the grown-in arrays of d i s l o c a t i o n s . 6 1.2 Gallium Arsenide In i t s c r y s t a l l i n e form GaAs has a cubic zinc blende structure with a l a t t i c e constant (a) of 5.65A. The l a t t i c e can be interpreted as two inter-penetrating face-centred-cubic (FCC) l a t t i c e s , one of Ga, the other of As, o f f s e t from each other by a quarter of a l a t t i c e spacing (^<111>). Figure 5 shows the unit cube of the c r y s t a l . The larger c i r c l e s (As) are shown on the FCC s i t e s of the unit cube. The smaller f i l l e d c i r c l e s (Ga) are those atoms inside the unit cube while the smaller open c i r c l e s (Ga) show the remainder of the second FCC l a t t i c e outside the unit cube. The c r y s t a l i s a semiconductor and i n the undoped state i t i s semi-insulating. I t i s a d i r e c t band gap semiconductor with a room temperature (300K) band gap of 1.423eV. Figure 6 compares the d i r e c t gap (conduction band minimum d i r e c t l y above the valance band maximum i n k space) band structure of GaAs with the i n d i r e c t gap structure of S i or Ge. A photon emitted from GaAs due to a simple electron-hole recombination has a wavelength of 871nm. Recombination can take place anywhere across the gap however, with the proper phonons available, and so a more t y p i c a l wavelength of 838nm i s observed for GaAs 5. Mechanically the c r y s t a l i s quite b r i t t l e at room temperature but w i l l deform by s l i p on {111}<110> at elevated 7 temperatures. I t s density i s 5.317xl0 3kg/m 3. The melting point i s 1238°C and the s p e c i f i c heat at 300K i s 327J/kg*K. The l i n e a r c o e f f i c i e n t of thermal expansion i s 5.73xl0~ 6 at 300K. GaAs dis s o c i a t e s at temperatures over 450°C due to the r i s e of the p a r t i a l pressure of the As. This complicates both the growth of the c r y s t a l s and also high temperature experiments on GaAs. 1.3 Cathodoluminescence One of the signals that i s generated from the GaAs specimen as a r e s u l t of bombardment by the electron beam of the SEM i s cathodoluminescence (CL). The CL sig n a l r e s u l t s from the recombination of the excess hole-electron p a i r s generated by the impinging electron beam. As each p a i r recombines they lose t h e i r energy of formation, equal to the difference i n the excited band energies and the re s t energy. This difference i s usually equal to the band gap. Figure 7 shows a schematic of the energy band structure of GaAs and the simplest recombination scenario. The l o s t energy can be released as a photon, and t h i s has a high p r o b a b i l i t y f o r a d i r e c t band gap c r y s t a l such as GaAs. In regions of the c r y s t a l where there are i n t e r band energy l e v e l s (traps) ava i l a b l e due to the presence of foreign atoms or d i s l o c a t i o n s , the recombination can s t i l l take place but with a d i f f e r e n t wavelength photon or without any photon being released. A schematic of the process i n which an electron drops from the 8 conduction band to an i n t e r band l e v e l i s shown i n Figure 8. The photon released i s lower i n energy (has a longer wavelength) than the one released when an electron f a l l s across the whole gap. This e f f e c t produces the contrast i n the CL images. The CL detector can be chosen to be most s e n s i t i v e to the wavelength emitted by the c r y s t a l as a whole ( i e . Ep n = Eg ap) . Areas that emit photons of other wavelengths or emit no photons w i l l therefore appear darker with t h i s detector. As a r e s u l t , regions of the c r y s t a l that do not contain foreign atoms (dopants or contaminants) or d i s l o c a t i o n s w i l l have a brighter l o c a l CL image, and regions that do contain impurities and/or d i s l o c a t i o n s w i l l have a darker l o c a l CL image. Dislocations, which i n t e r f e r e with the normal luminescence of the c r y s t a l , appear as dark spots or l i n e s i n CL images. These spots (lines) are c a l l e d "dark spot(line) defects" (DS(L)Ds) and have been observed i n degraded opto-electronic d e v i c e s 3 . Further observations showed these DSDs coincided with the etch p i t configuration on an etched GaAs c r y s t a l and therefore the d i s l o c a t i o n d i s t r i b u t i o n . Correlations between CL images and d i s l o c a t i o n revealing etch p i t s have previously been shown6 and are demonstrated i n t h i s work. Evidence that some of the contrast i n CL images that depicts the d i s l o c a t i o n s i t e s i s related to the l e v e l of trace impurities that have accumulated around the grown-in d i s l o c a t i o n s has been 9 demonstrated?. This was accomplished by assessing the l o c a l impurity l e v e l using secondary ion mass spectrometry (SIMS) and comparing the d i s t r i b u t i o n of contaminants with the CL images of the d i s l o c a t i o n networks. Residuals of S i , 0, Cr, and C were a l l found to be concentrated at the d i s l o c a t i o n c e l l walls i n undoped, semi-insulating GaAs. The resolution a v a i l a b l e could not r e g i s t e r the concentration difference between the d i s l o c a t i o n cores and the region immediately surrounding. 1.4 Observing Dislo c a t i o n Arrangements To examine the d i s l o c a t i o n configuration before and a f t e r heating and deformation requires two sets of observations of the same area. One way of doing t h i s i s to etch the c r y s t a l i n an etchant which reveals the d i s l o c a t i o n s at the surface as p i t s . The specimen i s then polished to remove the p i t s , heated, deformed, and then re-etched to delineate the new d i s l o c a t i o n arrangement. The process of etching and re-etching i s time consuming and i s destructive to the c r y s t a l . In the case of GaAs, and other d i r e c t band gap semiconductors, an al t e r n a t a t i v e procedure i s possible. Dislocations at or near the surface can be observed by imaging the CL sig n a l emitted from the c r y s t a l . 10 2 Procedures Test plates of GaAs were deformed at high temperatures. Two j i g s were designed and fabricated i n order to deform the GaAs specimens at the high temperatures inside a c r y s t a l grower. The GaAs specimens required preparation involving cleaning, po l i s h i n g , and capping to protect t h e i r surfaces during heating. The specimens were characterized before heating and deformation t e s t s , and re-examined afterwards. An Etec SEM was modified to be used as a CL imaging device i n order to characterize GaAs specimens with respect to d i s l o c a t i o n arrangement and density. 2.1 Deformation of GaAs Plates Samples of GaAs were prepared and loaded into s p e c i a l l y designed three point bending j i g s . The j i g and specimen were then placed inside a GaAs c r y s t a l grower. The grower chamber was then sealed and flushed with argon. A f t e r two evacuations and back f i l l s with argon the chamber was pressurized to approximately 2kPa. Concurrently with t h i s the cooling water was turned on and allowed to c i r c u l a t e . A f t e r the flow had s t a b i l i z e d the cooling l i n e s were bled to release any trapped a i r . The set point of the growers heater c o n t r o l l e r was set to approximately 220C below the desired t e s t temperature (900°C to 1050°C) and power was applied to the heaters using the automatic control mode. When the thermocouple indicated the maximum 11 overshoot (approximately 200C) the set point was reset to the overshoot temperature and the grower was l e f t to s t a b i l i z e for 3 00 seconds. The set point was then increased i n steps to the desired t e s t temperature using a rate of 0.03 3°C/s. At the desired temperature the grower was l e f t to s t a b i l i z e for 600 seconds. At t h i s point the deformation was c a r r i e d out which involved centre deflections of between 0.25mm and 1.0mm. The samples were loaded i n eith e r a s t a t i c manner, which l e f t them i n the bent state, or i n a c y c l i c manner through 1 to 5 cycles. At the conclusion of the deformation treatment the samples were allowed to anneal for 90 to 600 seconds at the t e s t temperature. The cooling sequence was accomplished by reducing the c o n t r o l l e r set point by 2 00C° at a time and allowing the temperature o f f s e t to go to zero before the next reduction. The temperature c o n t r o l l e r maintained about a 10 per cent of maximum input power during t h i s phase of cooling. A f t e r cooling to a temperature of 3 00°C the power to the heaters was turned o f f and the grower was allowed to cool to room temperature with the cooling water. Throughout the heating, bending, and cooling the pressure i n the chamber was maintained at the 2kPa by c o n t r o l l i n g both the argon i n l e t flow rate and the chamber outlet flow rate. The specimen and j i g were observed through the viewing port of the grower chamber. A f t e r the chamber had cooled completely, the cooling water was turned o f f , residual pressure i n the chamber was bled o f f , 1 2 and the chamber was opened. The j i g was removed from the grower and the GaAs specimen was recovered from the j i g . A l l parts were handled with gloved hands due to the presence of As dust and masks were worn during recovery (and loading) for the same reason. The specimens were handled by the edges with tweezers and place i n covered p e t r i dishes containing ethyl alcohol. The t e s t pieces of GaAs were then prepared for CL examination. This was accomplished by a ser i e s of polishes and rinses, to be described l a t e r , and f i n a l l y mounting on the specimen holder for the examination. The mounted samples were placed i n an Etec SEM that had been modified to image CL signals. The d i s l o c a t i o n arrangement i n the sample was imaged by CL and these image, were compared with the CL images taken from the same areas on the specimen before the heating and deformation. Table I shows a compilation of a l l the specimens tested and the t e s t conditions. 13 sample # from wafer # capping S i 3 N 4 (A) Tmax (°C) d e f l e c t i o n (mm) cycles (#,rate) anneal (s) T l 44 750 955 0.2 s t a t i c 120 T2 44 750 955 1.0 s t a t i c 120 T3 44 750 955 0.5 s t a t i c 60 T4 44 750 955 0 s t a t i c 600 T5 44 750 955 0.4 s t a t i c 600 S i l 273#45 750 955 0.25 s t a t i c 60 LI 293#9 750 955 0.25 s t a t i c 60 SI 1214S61 800 1000 0.5 5,high 30 S2 1214S61 800 1050 0.5 4,high 120 S3 1214S61 800 1000 0.25 1, low 90 S4 1214S61 800 1000 0.25 3, low 120 d e f l e c t i o n rates thicknesses : Com HP : high = 0. . J#44 = 0. 273#45 = 0. 21mm/s 40mm 48mm low = 0.014mm/s Com. EG#1214S61 = HP293#9 = 0.434mm 0.49mm TABLE I. GaAs specimens and te s t conditions. 2.2 Bending J i g s Two j i g s for bending the GaAs were designed and employed. The f i r s t was a s t a t i c bending j i g used i n early experiments to f i n d the range of temperatures where GaAs deformed p l a s t i c a l l y . The second, dynamic j i g deformed the GaAs i n c y c l i c bending under c o n t r o l l e d s t r a i n . In order to minimize the e f f e c t of the thermal gradient across the chamber the apparatus was made s u f f i c i e n t l y small to occupy only the centre region of the cr u c i b l e . The use of small bending j i g s also minimized the cost of both j i g materials and GaAs specimens, that i s , more could be made from the same quantity of stock. 14 2.21 S t a t i c Bending J i g The s t a t i c bending j i g i s shown schematically i n Figure 9a. I t consists of a large support block "A" of boron n i t r i d e (BN) with a recess cut for one end of the 25mm long specimens "B" and provis i o n for the s t a i n l e s s s t e e l (type 316) mass and s t r a i n control block "C". A ha l f round of BN "D" was cut to the same height as the recess i n the support block and provided the central bearing load f o r the specimen. A small c y l i n d e r of BN "E" contained a s l o t f or the ca n t i l e v e r end of the GaAs specimen and provided the load on the end of the specimen. The load block "E" was kept i n alignment by a v e r t i c a l s t a i n l e s s s t e e l guide rod "F". The guide rod runs i n the s t a i n l e s s s t e e l guides "G" attached to 11C" preventing the specimen from twisting. I t also serves as an indicator of specimen d e f l e c t i o n , since bending of the specimen i n c l i n e s the rod, causing the top of the rod to move ho r i z o n t a l l y , magnifying the sample d e f l e c t i o n by the bar length. The load applied to the specimen could be increased r e a d i l y by adding washers onto block "E", held i n place by the v e r t i c a l rod. The guides themselves are attached to 11C" by a locking screw and can be extended or retracted to give various amounts of s t r a i n to the specimens. 15 Three bending systems were b u i l t into the j i g (shown in Figure 9b) which then allowed three samples to be tested simultaneously under the same thermal and environmental conditions. In a given t e s t , variables between specimens considered were specimen material, applied load, and maximum s t r a i n . 2.22 Dynamic Bending J i g The dynamic bending j i g was designed for c y c l i c s t r a i n t e s t i n g and i s shown i n section i n Figure 10. The j i g has a three point bending system with devices for s t r a i n l i m i t a t i o n The j i g consists of a main body "A" that holds two lower support pins "B" and a movable top piece 11C" that contains the central loading pin "D", the return block "E" and the s t r a i n control "F" and "G". A mass r i n g " I " , made of s t a i n l e s s s t e e l , provides the downward load during a return cycle. The j i g was designed to bend the specimen a controlled amount on loading and then to p u l l the specimen back to i t s i n i t i a l p o s i t i o n without an excess of compressive loading. This can be done once, or several times, i n a given t e s t at various temperatures, strain(loading) rates and maximum s t r a i n values. For t h i s j i g loading was c a r r i e d out through "G" by an external source, the seed l i f t mechanism. 16 2.23 Materials Selection Materials for the bending apparatus were selected based on a number of factors. (1) The device had to perform mechanically at high temperature, without s e i z i n g or g a l l i n g . (2) A low c o e f f i c i e n t of thermal expansion was required so that cold set clearances and l i m i t s would not vary over the temperature range of i n t e r e s t . (3) The device had to be unaffected by GaAs or As at high temperatures. (4) There could be no reaction between the material and GaAs at high temperatures. The material chosen was HP grade boron n i t r i d e (BN) . P y r o l i t i c BN i s the material employed as the c r u c i b l e for containing the molten GaAs during growth of c r y s t a l s i n commercial growing operations. The compound, BN, i s therefore, s u i t a b l y i n e r t to and from GaAs. BN i s a non-seizing material when used against i t s e l f , and remains s e l f l u b r i c a t i n g to over 1600°C. I t retains i t s mechanical strength to over 2700°C, and i s e a s i l y machinable to a 4-5 micron f i n i s h , but i s b r i t t l e . HP grade BN has a s p e c i f i e d c o e f f i c i e n t of thermal expansion of O.OxlO"6 from 75°C to over 1000°C. BN i s susceptible to damage by absorption of moisture which r e s u l t s i n a drop i n strength. HP grade BN has a small amount of Ca added to i t to minimize the e f f e c t s of moisture. 17 2.24 S t r a i n C a l i b r a t i o n of Dynamic J i g The maximum s t r a i n imposed on the specimen i s l i m i t e d by the s t r a i n l i m i t i n g block 1 1F". When t h i s block reaches the top face of the lower assembly block "A" there i s no further downward movement of the loading pin. To set the maximum s t r a i n , a shim of thickness equal to the desired maximum s t r a i n i s placed between the l i m i t e r block 1 1F" and the top face of the lower assembly "A". With a specimen i n place the s t r a i n adjustment screw can be adjusted so that the centre pin j u s t contacts the specimen surface at the end of the downward t r a v e l . For a bend t e s t the shim i s removed and the additional downward t r a v e l now avai l a b l e i s the shim thickness. In actual p r a c t i c e the condition of the centre pin "just contacting" the specimen i s d i f f i c u l t to assess. There i s a further complication i n that some measure of downward load must be applied during the s t r a i n l i m i t adjustment to take up the necessary clearances and tolerances that allow the j i g to move. This needed downward load would be hazardous to the GaAs specimens which are very b r i t t l e at room temperature; a small d e f l e c t i o n applied during the adjustment would break the GaAs specimen. In order to circumvent these d i f f i c u l t i e s a long, f l e x i b l e pseudo specimen was substituted for the GaAs specimen during the s t r a i n l i m i t adjustment. This pseudo specimen was chosen long enough so that roughly h a l f i t s length extended outside the j i g . 18 The long extension provided a mechanical amp l i f i c a t i o n of the centre displacement which was needed to determine zero centre d e f l e c t i o n . That i s , the extended t i p i s displaced proportionately to the centre d e f l e c t i o n but at a higher rate. The condition of "just contacting" the specimen i s now translated into a zero d e f l e c t i o n at the end of the extended portion of the pseudo specimen. A 50mm long, 0.5mm diameter graphite rod was chosen as the pseudo specimen. Afte r adjustment, the locking nut was tightened onto the adjusting screw. A confirmation that the desired centre d e f l e c t i o n would a c t u a l l y be r e a l i z e d i s accomplished by removing the shim and applying the d e f l e c t i o n to the graphite rod. The slope of the extended portion of the rod can then be used to cal c u l a t e the actual centre d e f l e c t i o n . This was i n agreement with the shim thickness. The actual s t r a i n seen by the GaAs specimen was d i f f e r e n t than that for the graphite rod by an amount equal to the difference i n the thickness of the two, t h i s was taken into account when se l e c t i n g the shim. 2.3 Environmental Conditions At higher temperatures the vapour pressure of As i n the GaAs i s increased s u f f i c i e n t l y to cause d i s s o c i a t i o n of the GaAs. This leads to the release of free As into the high temperature environment. Thus, containment of As was a prime concern. 19 Heating of specimens, even i n an i n e r t atmosphere, leads to loss of As from the surface, and a subsequent excess of elemental Ga l e f t behind on the GaAs. Ga i s l i q u i d above 32 °C. The process leads to the formation of thermal p i t s into the surface of the GaAs specimen. A major drawback of a small specimen i s i t s r e l a t i v e l y large r a t i o of surface area to volume leading to aggravated d e t e r i o r a t i o n of the specimen by loss of surface As. Protecting the specimen surface from t h i s type of damage was also a major concern. Therefore, the whole of the experiment had to be c a r r i e d out i n a sealed environment with a mechanical feed-through, a viewing system and capacity to heat the t e s t assembly to better than 1000°C. Based on these requirements, the GaAs c r y s t a l grower was chosen as the containment vessel. The grower i s suitably designed to deal with the As released from the GaAs as t h i s i s a much more severe problem when melting a charge of several kilograms of GaAs as opposed to the small specimens used for deformation experiments. Also, the grower i s designed to run at moderate pressures over atmospheric and t h i s serves to reduce the rate of As loss from the surface of the heated GaAs specimen. The heaters were l e f t i n place to provide the high temperatures required. The seed l i f t mechanism was used to provide the necessary mechanical feed-through, and the viewing window allowed observation of the experiment. 2.4 Temperature C a l i b r a t i o n In order to carry out high temperature deformation t e s t s on GaAs specimens i t was necessary to measure the temperature of the sample during the t e s t . I t i s d i f f i c u l t to introduce thermocouples into the high pressure growth chamber and pos i t i o n them close to the t e s t sample. As an al t e r n a t i v e , the control thermocouple of the c r y s t a l grower was c a l i b r a t e d and used to determine the sample temperature thermocouple. The large thermal mass of the system and the necessary s t a b i l i t y and r e p e a t a b i l i t y of the control thermocouple made t h i s procedure reasonable. The thermocouple was ca l i b r a t e d by melting samples of known melting temperatures at the p o s i t i o n i n the chamber where the deformation t e s t s were to be c a r r i e d out. Three elements were used as c a l i b r a t i n g materials: germanium, s i l v e r , and gold. The c a l i b r a t i o n was run i n a quartz c r u c i b l e at a fix e d c r u c i b l e height of 14cm against the reference bar, using the 15cm heater. Cal i b r a t i o n s were conducted at approximately 2kPa, under the same conditions the heating and deformation experiments were carr i e d out. 21 Three quartz ampules were made from 10mm (OD) tube stock. They were made approximately 5cm long, sealed at one end and necked down to 3mm (ID) i n the middle. A small amount of c a l i b r a t i n g material was placed i n the top h a l f of each ampule. The ampules were flushed with argon then t h e i r tops were sealed. The ampules were then placed upright i n a c y l i n d r i c a l graphite holder which was then placed i n the bottom of the grower c r u c i b l e . The graphite holder supported the ampules, set them at the correct height above the bottom of the c r u c i b l e , positioned each ampule the same distance from the centre of the chamber, thus giv i n g the a b i l i t y to rotate the c r u c i b l e to bring each ampule into best viewing po s i t i o n , and provided a thermal mass to minimize small temperature transients. A f t e r the grower was sealed and pressurized the temperature was slowly raised (0.033°C/s for Ge, Ag and 0.0083°C/s for Au) through the melting points of the three standards. Melting was observed, through the grower chamber window, when the material flowed from the top section of the ampules to the bottom. The melt temperatures could then be related to the output of the control thermocouple (output equals the set point when the o f f s e t i s zero) . This gives a c a l i b r a t i o n curve of control thermocouple related to the temperature at the centre of the grower chamber. Melting temperatures were believed to be within 1.5°C f o r a l l three materials used. The p l o t of temperature versus c o n t r o l l e r thermocouple, shown i n Figure 11, has excellent l i n e a r i t y (R 2 = 0.9994) with predictor errors of l e s s than 2°C on a l l points. A s i m i l a r temperature c a l i b r a t i o n was c a r r i e d out for the 20cm heater with c r u c i b l e height of 14cm. The c a l i b r a t i o n curve obtained i s shown i n Figure 12. A l l of the heating and deformation t e s t s were c a r r i e d out between 900 and 1050°C, t h i s i s i n s i d e the range of c a l i b r a t i o n given i n Figures 11 and 12. The temperature of the GaAs during a heating and deformation t e s t i s believed to be correct to within 10°C of the stated values. 2.5 Specimen Preparation and Handling I n i t i a l t e s t s were c a r r i e d out on sections cut from low grade wafers of GaAs. These wafers were received i n an as cut condition and required a chemical p o l i s h i n a 15 per cent s o l u t i o n of bromine i n methanol for 3 00 seconds. 2.51 Arsenic Containment Samples which were to be heated had to be protected from loss of surface arsenic which r e s u l t s i n thermal p i t t i n g and surface degradation. I n i t i a l attempts were made to prevent arsenic loss by heating the specimens under a layer of boron oxide (B 20 3) . This i s the material used as the l i q u i d encapsulant during the growth of GaAs c r y s t a l s . This method was 23 rejected due to problems explained below. Later, capping the samples with s i l i c o n n i t r i d e (Si 3N 4) proved to be a workable method. 2.511 Preliminary Testing Under Boron Oxide To check the effectivness and f e a s i b i l i t y of conducting deformation t e s t s of GaAs under B2O3 i n order to protect the GaAs specimen at high temperatures, a piece of a GaAs wafer and a small block of BN were placed i n the bottom of a quartz c r u c i b l e . The c r u c i b l e was then f i l l e d with granular B 20 3 and placed i n the grower chamber. Af t e r sealing and pressurizing the chamber to 2kPa, the temperature was raised to 1000°C and held for 12 00 seconds. Power was then removed from the heaters and the grower was allowed to cool. Upon removal from the grower i t was found that the quartz c r u c i b l e had cracked. The BN piece did not appear to have been f u l l y wetted by the B 20 3. Recovery of the GaAs specimen was accomplished by d i s s o l v i n g the B 20 3 i n water. I t was found that the B 20 3 had adhered to both the BN and the GaAs. The small sample of GaAs was completely crumbled, presumably due to stra i n s during s o l i d i f i c a t i o n of the B 20 3. In a subsequent t e s t a higher maximum temperature was reached and a more controlled, slower cooling rate was used. The t e s t was also arranged to apply a deforming load to the GaAs sample, Figure 13 shows the arrangement. Two rods of BN were machined to f i t i n the bottom of a small quartz boat. The t e s t specimen of GaAs spanned these two support rods and a t h i r d BN rod was placed i n the middle of the GaAs specimen. A st a i n l e s s s t e e l assembly of a b o l t and two nuts was adjusted so that the two nuts f i t over the edge of one end of the quartz boat and allowed the b o l t head to swing down and rest on the top BN rod. This arrangement provided the deforming load to the GaAs. The boat was then packed with c r y s t a l l i n e boron oxide and the whole assembly was placed i n the bottom of the grower c r u c i b l e . A f t e r sealing and pressurizing the chamber the temperature was raised to 600°C and held there for 300 seconds to allow the B 20 3 to melt and cover the GaAs. The temperature was then raised to 1100"C and held f o r 1800 seconds. The chamber was then slowly cooled by leaving the heater power on automatic and lowering the set point 200C° at a time. This maintained an heater input of approximately 10 per cent of maximum throughout the cooling. Again the quartz boat was found to have cracked. Recovery of the GaAs specimen was accomplished by d i s s o l v i n g the B2O3 i n water. I t was found that the B 20 3 had adhered to both the BN and the GaAs. The BN rods were also found to be cracked. The small sample of GaAs had been broken into three larger pieces along with several f i n e r pieces. The surface of the larger pieces of GaAs had been eroded quite severely. This procedure for encapsulating the small samples of GaAs at high temperatures was considered unsatisfactory and thus rejected. 2.512 S i l i c o n N i t r i d e Capped Specimens Another way of protecting the GaAs surface was to cap the surface with a protective layer which was i n e r t to the GaAs and stable at high temperatures. Carbon was considered and rejected as i t could contaminate the GaAs. S i l i c o n n i t r i d e (Si 3N 4) was then considered as i t i s used as a cap for GaAs during e l e c t r o n i c p r o c e s s i n g 8 ' 9 ' 1 0 . GaAs, capped with S i 3 N 4 , can withstand short anneals to 1080°C under s t a t i c conditions. S i l i c o n n i t r i d e does not contaminate GaAs. The S i 3 N 4 cap i s normally deposited by a vapour process to thicknesses from 500 to a few thousand angstroms. Damage to the GaAs surface due to the capping by t h i s method i s confined to the upper two hundred angstroms. In choosing the thickness of the cap, i n t e g r i t y of too t h i n a layer must be traded against cracking of too thick a layer. T y p i c a l l y , the thickness of S i 3 N 4 used i s 750 angstroms. For t h i s work 750 to 800 angstrom t h i c k caps were successful i n protecting the GaAs surface. This thickness was also s u f f i c i e n t l y t h i n to obtain CL images through the deposited layer since the e x c i t a t i o n volume extends approximately one micron into the c r y s t a l , well beyond the S i 3 N 4 layer. 26 Wafers of <100> grown GaAs were capped on both sides with a layer of S i 3 N 4 750 to 800A t h i c k using a chemical vapour deposition process i n E l e c t r i c a l Engineering, U.B.C.. The capped wafers were then cut into the specimens to be heated and deformed. A l l specimens were cut from the wafer with the long axis p a r a l l e l to the major f l a t of the wafer, that i s i n the [Oil] d i r e c t i o n . Figure 14 shows the ori e n t a t i o n of the specimen with respect to the wafer. The width of each specimen was 6.35mm. Specimens for the s t a t i c bending j i g were cut to a length of 25.4mm. These were samples Tl-5 from Cominco wafer J#44 and samples S i l and LI from Hewlett Packard wafers 273#45 and 293#9 respectively. Specimens for the dynamic bending j i g were cut to a length of 32mm. These were specimens Sl-4 from Cominco wafer 1214S61. The thicknesses of the wafers were measured as 0.40mm for J#44, 0.49mm for 1214S61, 0.48mm for 273#45, and 0.43mm for 293#9. Wafer J#44 was a low grade wafer and was received i n the as cut condition. I t was f i r s t polished i n a 15 per cent solution of bromine (Br) i n methanol for 3 00 seconds to give i t a mirror f i n i s h before capping. The two Hewlett Packard wafers had been previously etched l i g h t l y i n molten KOH, and were cleaned i n a 1:1:240 sol u t i o n of NH40H:H 20 2:deionized water (DI) before capping. Individual specimens were i d e n t i f i e d with a specimen number printed onto the surface of one end with a diamond tipped scribe. With the l a b e l l e d face up and the specimen number at the l e f t end the middle h a l f of each specimen was indicated by scribe marks place at the top l e f t and bottom r i g h t corners of the centre region. These marks were used as references which would remain stable through heating and deformation. This middle region was then mapped by CL imaging through the S I 3 N 4 cap before any treatment. A control piece from each wafer was also examined without a cap to compare with the CL images through the caps i n order to assess the e f f e c t of capping on the CL emission. 2.52 Post Heating Specimen Treatment A f t e r heating the specimens required p o l i s h i n g before CL imaging could be c a r r i e d out. The process consisted of removing the S i 3 N 4 cap i n hydroflouric acid (HF), r i n s i n g i n deionized water (DI), and pol i s h i n g i n a 1:1:1 soluti o n of NH4OH:H202:DI. This i s done to remove remnants l e f t by the HF, of which there was an appreciable amount both i n a v i s i b l e form and i n a surface f i l m which prevents the normal po l i s h i n g action of Br i n methanol. Following t h i s i n i t i a l cleaning are rinses i n deionized water and then methanol. Then, following the cleaning p o l i s h , a f i n a l chemical p o l i s h i n a 5 per cent solution of Br i n methanol was c a r r i e d out followed by a methanol r i n s e . 28 Times for each step i n the procedure depended strongly on the condition of the specimen. Each step of po l i s h i n g was c a r r i e d out while v i s u a l l y assessing the progress. When either a s a t i s f a c t o r y surface was generated or no further improvement was obvious or i f thinning of the specimen became a problem the p o l i s h was terminated. T y p i c a l l y , removal of the cap i n HF was accomplished i n 120 to 180 seconds with a water r i n s e f o r 180 to 300 seconds. The cleaning p o l i s h i n the NH4OH:H202:DI solution was normally conducted i n two steps of 300 to 600 seconds each i n fresh s o l u t i o n . The f i n a l chemical p o l i s h i n g using 5 per cent Br i n methanol was also done i n two batches of fresh s o l u t i o n with 600 seconds per bath. A l l cleaning and po l i s h i n g was conducted at room temperature (20°C) with room temperature solutions. The cleaning and po l i s h i n g solutions had to be fresh. A s a t i s f a c t o r y surface was one with a mirror f i n i s h with as l i t t l e remaining p i t t i n g as possible. At the very l e a s t the sharpness of the edges of any p i t t i n g had to be polished o f f . 2.6 Cathodoluminescence Imaging 2.61 Scanning Electron Microscope The SEM has seven basic systems, a high voltage system to generate the beam of electrons, a vacuum system to provide the necessary vacuum to allow passage of a beam of electrons, a focussing and demagnification system to guide and focus the 29 electron beam onto the specimen surface, a scan system to raster the electron beam over a s p e c i f i c area of the specimen, a detection system to pick up the secondary electron (SE) signal generated by the i n t e r a c t i o n of the electron beam with the sample surface, a video system to display the detected s i g n a l coincident with the beam scan, and f i n a l l y , a mechanical system for manipulating the specimen p o s i t i o n and ori e n t a t i o n within the vacuum chamber. The beam i s generated by applying an accelerating voltage (nominally 10, 20 or 30kV) to electrons that have been thermally emitted from a heated filament. This coarse beam i s compressed by the e l e c t r i c f i e l d of the Wenhelt cap and i s made to pass through a physical aperture (spray aperture) consisting of a pr e c i s i o n hole i n a m e t a l l i c d i s c . The beam i s further demagnified by a system of objective lenses before passing through the f i n a l aperture. Next the condenser lens i s used to further demagnify the beam, while the focussing lens i s used to set the point of minimum beam diameter at the surface of the specimen. Two sets of scan c o i l s are used to ras t e r the beam over the specimen surface. The signal generated by the beam's in t e r a c t i o n with the specimen i s picked up by the detection system and relayed to the video section which performs the raster on the cathode ray tube monitors synchronously with the beam. Magnification i s the r a t i o of the video sweep to the beam sweep, and as the beam i s made to raster over smaller and smaller 30 regions the magnification increases. Images from the SEM can be displayed on eit h e r (or both) of the two cathode ray tube monitors on the front panel. The r i g h t most screen i s screen 1 and the image displayed on t h i s screen i s the same as the one that can be sent to the camera. The other screen, to the l e f t i s screen 2 and can be set up to display either the si g n a l displayed on screen 1, some a t t r i b u t e of the image on screen 1 (brightness f o r example), or another signal e n t i r e l y from the same specimen area as that displayed on screen 1. The most useful combinations are as follows: 1) SE image on screen 1 and brightness waveform on screen 2 2) SE image on screen 1 and CL image on screen 2 3) CL image on screen 1 and brightness waveform on screen 2 Combinations (1) and (3) are useful for adjusting the contrast and brightness for picture taking of eit h e r SE or CL images and also during the procedure of centring and saturating the filament. Configuration (2) i s useful for simultaneously imaging the surface (SE image) of the specimen and the CL image for c o r r e l a t i n g p a r t i c u l a r features present i n both images. The whole of the electron system i s maintained under vacuum, the filament which i s heated to 'b o i l o f f the electrons f o r the beam i s s i m i l a r to the filament i n a l i g h t bulb. I f there are any o x i d i z i n g elements present as gasses i n the chamber t h i s filament w i l l burn out very quickly. The filaments do burn out over time and t h e i r l i f e t i m e i s greatly affected by how hot they are a c t u a l l y run. Beginning at low temperatures there are very few thermally excited electrons emitted, to increase the number emitted, that i s to make the beam brighter, the temperature of the filament i s increased. There i s though a l i m i t to t h i s i n that at a p a r t i c u l a r temperature there i s no further increase i n emission with an increase i n temperature. At t h i s point increasing the temperature only shortens the l i f e of the filament. However, t h i s i s also the best point at which to operate since i t gives the brightest beam. The procedure to accomplish t h i s i s referred to as saturating the filament and i t i s very important i n terms of maximizing beam brightness without unnecessarily s a c r i f i c i n g the filament. In order to achieve a s u f f i c i e n t electron beam current for CL imaging the filament must be centred and saturated under the conditions used for CL imaging. The procedure to accomplish t h i s i s as follows: With a secondary electron image displayed on screen 1 sel e c t the si n g l e l i n e scan mode and set the auto brightness to o f f . On the second screen choose the waveform mode and adjust the contrast and brightness controls to bring the scan i n t e n s i t y into the range of the screen. With that waveform displayed increase the filament current and monitor the height of the waveform on screen 2. Re-adjust the contrast and brightness as necessary to keep the waveform on the screen. At saturation there w i l l be no further increase i n the height of the waveform (brightness) with an increase i n the filament current. I f the filament i s not centred there w i l l , i n fact, be a drop i n brightness with an increase of filament current past saturation. To correct t h i s condition the filament current i s set i n the over saturated range and the X, Y filament p o s i t i o n adjustments are varied to maximize the brightness. Once t h i s has been accomplished the filament current i s again v a r i e d u n t i l the peak brightness condition i s regained. The process of adjusting the filament current and centring the filament may require several i t e r a t i o n s before the optimum conditions are found. F a i l u r e to obtain a s a t i s f a c t o r y brightness ( i n s u f f i c i e n t f or CL imaging) a f t e r a few i t e r a t i o n s usually indicate that the filament i s too f a r o f f centre and i t may have to be p h y s i c a l l y bent or even replaced. The high beam currents necessary for CL imaging were found to be detrimental to the filaments r e s u l t i n g i n excessive warpage or i n them becoming completely burnt through a f t e r about 2 0 hours. The specimen to be examined i n the SEM must be capable of d i s s i p a t i n g any acquired charge by conduction and must be grounded to the SEM ground. I f t h i s i s not the case then, over time, the specimen w i l l pick up and store charge from the a r r i v i n g beam electrons u n t i l t h i s accumulated charge i s s u f f i c i e n t l y large so as to a f f e c t e i t h e r the incoming electron beam or the outgoing electron s i g n a l . This e f f e c t i s known as "charging" and i s detrimental to imaging of the desired s i g n a l . The standard SEM detects the secondary electron signal generated from the specimen by the bombardment of the electron beam. This i s only one of several signals generated. The incident high energy electron beam also generates an X-ray s i g n a l , a back scattered electron signal and the CL s i g n a l . 2.62 Cathodoluminescence Detection Equipment and I n s t a l l a t i o n A s o l i d state CL detector was added to an ETEC SEM to obtain CL images. The detector, consisting of a four segment annular s i l i c o n diode with a high p u r i t y quartz window was attached with a brass c l i p bracket to the bottom of the SEM pole piece. The e l e c t r i c a l connections to the detector passed through an optional port on the SEM used to house the mounting bracket and the e l e c t r i c a l feed-through. The electron beam from the SEM passes through the central hole of the diode array i n the detector and s t r i k e s the sample d i r e c t l y . A quartz window i n the detector passes the CL s i g n a l but blocks the back scattered electron s i g n a l . The SE s i g n a l i s s t i l l swept out by the Faraday cage of the SE detector and i s a v a i l a b l e f o r normal SE imaging. The CL detector signal i s fed into a preamplifier which was i n s t a l l e d immediately outside the port on the SEM used for the detectors e l e c t r i c a l feed-through. The preamplifier receives the s i g n a l from the detector and passes i t to the main amplifier. The preamplifier also provides each diode i n the detector with the proper bia s i n g and allows for zeroing of the detector. At the main ampli f i e r each quadrant of the detector can be selected to be i n e i t h e r the on, o f f , or inverted mode. Brightness and contrast controls are also on the main amplifier. The amplifier output adds the signals selected and displays them on the video screen of the SEM. 2.63 Operating Procedure In order to obtain CL images on the SEM, the instrument had to be set at the highest voltage available and the largest spot s i z e had to be selected. Extra large apertures were required (890 or 1270 microns for the spray and 400 microns for the f i n a l ) . An indicated working distance of 2 5mm gave the best contrast. Grounding of the specimen was c r i t i c a l . The filament had to be centred and saturated before each session. The demands on the filament were high r e s u l t i n g i n considerable shortening i n the service l i f e of the filament to only about 20 hours. The detector bias and zero had to be checked and reset following every ten sessions of CL imaging. Symptoms of an unbalanced condition included poor contrast or flooding i n one quadrant of 35 the screen that could be improved by s e l e c t i v e l y turning o f f quadrants of the detector. Pictures of the CL images were most successful when taken i n the 540 scan l i n e s mode. To begin a session the SEM must be on and i n the auto mode (key above camera) . The beam and filament must be turned o f f . The SEM chamber i s vented (VENT button). The specimen, which i s mounted on a stub with carbon dag i s then put into the SEM chamber. The chamber door i s then closed and held shut while simultaneously pressing the EVAC button. This w i l l evacuate the chamber. The beam cannot be turned on u n t i l a vacuum l e v e l of 2xl0~ 4 Torr i s reached, however, delaying the turn on u n t i l the vacuum reaches 10~ 4 Torr helps to extend the filament l i f e . Once the beam i s turned on a SE image i s f i r s t used to locate the specimen and/or the area of i n t e r e s t . The i n i t i a l imaging of the specimen and focussing are done with a small electron spot s i z e (2.5-3.0 amps condenser current). The Z axis control and the coarse focus are used to bring the surface of the specimen into focus at an indicated working distance of 2 5mm (thi s corresponds with a magnification correction factor of 1.0). The spot s i z e i s then increased while simultaneously adjusting the focus controls to maintain the image. Once the largest spot s i z e has been reached (1.6 amps condenser current), the Z axis control and the coarse focus are used to bring the specimen surface into focus at an indicated working distance of 25mm. At t h i s point the centring and saturation of the SEM filament should be checked and adjusted. I f there i s i n s u f f i c i e n t brightness to produce the i n i t i a l SE image the centring and saturation procedure may be necessary for the small spot s i z e (this i s e s p e c i a l l y true immediately a f t e r replacing a filament). However, even i f t h i s i s done, the centring and saturation must be checked and adjusted for the large spot s i z e . Next the CL mode i s selected and displayed on screen two concurrently with the secondary image on screen one. The brightness and contrast controls on the CL main amplifier are adjusted to display the best CL image. A f i n a l focus and Z axis adjustment are required to bring the CL image into sharpest focus at an indicated working distance of 2 5mm. The CL s i g n a l can be displayed i n two modes; normal and inverted. In the normal mode areas that emit l i g h t i n the detector's band width w i l l appear bright on the SEM viewing screens. In the inverse mode the l i g h t emitting areas w i l l appear dark. 2.7 Secondary Ion Mass Spectrometry Analysis Some SIMS work was c a r r i e d out i n an attempt to explain some of the CL contrast e f f e c t s i n terms of impurities. This was accomplished by mapping areas of a c r y s t a l by CL imaging and then comparing the concentrations of impurity elements between the CL bright and dark regions. Specimens of GaAs were mounted on s p e c i a l l y adapted stubs that could be accommodated i n both the SEM and the SIMS chambers. CL images were taken from several recognizable areas (adjacent edges and corners) f o r l a t e r reference f o r the SIMS imaging. The magnification used was 52x which gave the largest f i e l d i n the SIMS. The mounted specimens were then transferred to the SIMS machine. Approximately 5000A of surface material was eroded from the sample surface to remove contaminants. Then a series of spectrum sweeps were run to i d e n t i f y impurities present. Those found were mapped and t h e i r d i s t r i b u t i o n was compared to the CL images. A d i r e c t comparison i s made d i f f i c u l t since the SIMS map contains a degree of foreshortening as the specimen i s i n c l i n e d to the incident ion beam of the SIMS. A s i m i l a r study was conducted 7 and showed an increased concentration of S i , 0, Cr, and C at the d i s l o c a t i o n c e l l walls. The GaAs studied was not doped, the impurities were present only as residuals. 38 3 Observations 3.1 The Cathodoluminescence Process The electron beam from the SEM generates an e x c i t a t i o n volume i n the GaAs which extends into the material i n a tear drop shape to a depth of 2 to 3 microns. The incident beam spot, which i s large to begin with, also spreads, reducing the r e s o l u t i o n on the images. On the basis of measuring the minimum detectable separation of two d i s t i n c t points i n the image, the re s o l u t i o n i s estimated to be 6 microns, which gives a useful magnification l i m i t of 400 times. Most CL images were taken at a magnification of 32 times. For more d e t a i l e d pictures magnifications of 64, 128, and 320 times were used. At the highest magnification loss of d e t a i l i n the image i s c l e a r l y apparent. Specimen cleanliness i s also c r i t i c a l as d i r t , carbon dag (used f o r attachment and grounding of the specimen) , and scratches appear c l e a r l y i n the CL image, as dark regions. Images from d i r t and scratches f l a r e d badly when using inverted imaging, reducing the o v e r a l l q u a l i t y of the image. When a GaAs c r y s t a l i s examined for CL two additional e f f e c t s can occur. F i r s t , the high energy incident electron beam cracks the o i l present i n the vacuum atmosphere and leaves a 39 contaminating f i l m of carbon on the specimen surface. This i s not v i s i b l e i n secondary imaging but i s s u f f i c i e n t to degrade the CL image over time. Secondly, l i g h t from the filament shines down through the large apertures i n the SEM and illuminates the sample surface which a f f e c t s the CL image. This i s a persistent phenomenon which i s only detectable on the CL image. The e f f e c t i s most pronounced on clean, polished specimen surfaces and to a l e s s e r extent on capped Si3N 4 samples. I t was found that the observation time was l i m i t e d to about two hours fo r each examination. During t h i s period i n t e r n a l heating i n the SEM was s u f f i c i e n t to increase resistance losses to the point that the high voltage producing the electron beam dropped too low for CL imaging. This was aggravated further by the low voltage of 27kV which was the actual voltage when the SEM was set at 3 0kV. In some sequences of CL micrographs a gradual loss of contrast w i l l be apparent which i s due to t h i s progressive decrease of the high voltage with time. 3.2 Cathodoluminescence Imaging of GaAs An inverted mode CL image of a section of a polished wafer of as-grown GaAs i s shown i n Figure 15. The image reveals a network of dark c e l l walls surrounding regions of l i t t l e or no contrast. Since the image i s inverted the dark features are a c t u a l l y the more luminous. The contrast i s due to l o c a l differences i n CL emission around d i s l o c a t i o n s i t e s . The c e l l s (A and C) are predominantly featureless regions and vary i n s i z e from 300 to 1000 microns across. The c e l l walls (B) are darker o v e r a l l than the c e l l i n t e r i o r s and contain groups, c l u s t e r s and arrays of dark spots. In places the walls are only the width of a s i n g l e spot, elsewhere the wall thicknesses increase to 100 microns and contain several spots across the width. These spots show the l o c a t i o n and arrangement of grown i n d i s l o c a t i o n s . These d i s l o c a t i o n s form into c e l l walls i n the c r y s t a l during the growth process. In the i n t e r i o r of these c e l l s are regions of r e l a t i v e l y d i s l o c a t i o n free material. 3.21 Cathodoluminescence Image compared to Etch P i t s A f t e r etching a piece of GaAs that had been mapped by CL i t was found that the etch p i t s produced by molten potassium hydroxide (KOH), which are normally associated with d i s l o c a t i o n s at the surface, had an excellent correspondence to the spots and arrays of the CL images. This i s evident by comparing Figure 16, an inverted CL image with Figure 17, an o p t i c a l image of the same GaAs surface a f t e r etching i n KOH. C e l l s marked A, B, and C as references are c l e a r l y evident i n both figures as i s the rest of the correspondence. The CL image, i n Figure 16, displays a c l e a r e r demarkation of the c e l l walls than i n the case of the etch p i t s since dark regions surrounding the spots i n the CL image serve to "connect the dots". A s i m i l a r comparison f o r a low d i s l o c a t i o n density sample (4xl0 3/cm 2) i s shown i n Figures 18 and 19. There i s excellent r e g i s t r a t i o n between the CL spots and the etch p i t s (A). Further, the CL image shows not only the location of the d i s l o c a t i o n s that r e s u l t i n etch p i t s but also regions around the p i t s i t e s that are lower i n CL brightness. For a given group of p i t s i n an area the etch p i t pattern does not r e f l e c t the s i z e or shape of the associated dark f i e l d around them i n the CL image. This can be seen by comparing the two regions marked B and C i n Figures 18 and 19. The dark regions around the d i s l o c a t i o n s i t e s i n the CL image also make i t easier to v i s u a l i z e the network structure of the d i s l o c a t i o n arrays. The CL image and the SE image (which indicates topographical features) are compared for a S i doped sample of GaAs i n Figures 20 and 21. The etch p i t s , A, B, and C, produced by molten KOH, are seen as regular, hexagonal features i n the SE image. The CL image shows the dark spots associated with the d i s l o c a t i o n s that correspond with these p i t s . Figure 22 shows the CL image of a S i doped GaAs c r y s t a l with a long d i s l o c a t i o n evident as a dark l i n e , running along the surface of the c r y s t a l . The KOH etched surface of the same area i n Figure 23 does not show any i n d i c a t i o n of t h i s d i s l o c a t i o n . In other areas of the same sample there were instances of dark spots appearing i n the CL image but there was no corresponding etch p i t v i s i b l e i n the SE image. 3.22 CL Contrast - Dots and Halos The difference i n CL i n t e n s i t y from various areas of the specimen can be attributed to several e f f e c t s . When imaging a c r y s t a l of GaAs by CL, contrasting networks are e a s i l y resolved on the c r y s t a l . There was no corresponding feature v i s i b l e i n secondary electron image of the same area. This demonstrates that the networks imaged i n CL are not a r e s u l t of the topography of the specimen surface. In the f i r s t s e r i e s of specimens the networks appeared bright on the CL image i n the normal mode. This at f i r s t seemed to c o n f l i c t with the notion that i n the regions at and around d i s l o c a t i o n s CL emission should at le a s t be s h i f t e d i n wavelength out of the range of the detector i f i t existed at a l l , and thus they should appear dark. Upon closer examination i t was found that the bright spots that made up the arrays i n the CL images were themselves made up of a dark central spot surrounded by a bright region. Figure 24 shows t h i s i n ele c t r o n i c s grade GaAs (xl28) . At lower magnifications (x32) i n the CL image of the same area, the dark core of the spots could no longer be resolved. A d i s l o c a t i o n i s 43 therefore imaged as a dark spot surrounded by a bright halo. This dot and halo e f f e c t has been reported by other w o r k e r s 7 ' 1 0 ' 1 1 ' 1 2 . I t has been proposed that the e f f e c t i s caused by the d i s t r i b u t i o n of impurities around the d i s l o c a t i o n . The d i s l o c a t i o n i t s e l f i s a s i t e of non-radiative recombination, hence a dark spot. However, the d i s l o c a t i o n may also act as an impurity sink. Impurities would d i f f u s e from the region around the d i s l o c a t i o n to the d i s l o c a t i o n core, thus generating a region around the d i s l o c a t i o n lower i n impurities than the bulk c r y s t a l . The impurities give r i s e to recombination at wavelengths d i f f e r e n t to those of the pure c r y s t a l . These new wavelengths may be outside the detection l i m i t s of the CL detector i n the SEM. This would make regions of higher impurity concentrations appear darker, giving a dark spot, and regions swept c l e a r of impurities brighter, accounting for the halo. From the proposed mechanism i t i s not c l e a r which plays the greater r o l e i n generating the dark spot, the d i s l o c a t i o n i t s e l f or the cloud of impurities i t has gathered. Regardless of the mechanism r e s u l t i n g i n the contrast i n the CL images, the apparent d i s l o c a t i o n networks imaged by CL corresponded well with KOH etch p i t s produced on the same surface. The CL images of the di s l o c a t i o n s appear as either l i g h t or dark, depending upon the c r y s t a l and i t s h i s t o r y . In most cases the best images were obtained when the detector mode was chosen to display the networks as dark regions. As a r e s u l t , the imaging mode which shows the d i s l o c a t i o n arrays as dark i s generally used. A conscious e f f o r t i s thus required to mentally in v e r t the contrast i n the inverted CL images before drawing any conclusions on the nature of the impurity d i s t r i b u t i o n . 3.23 Surface condition In the early part of t h i s i n v e s t i g a t i o n low grade GaAs wafers with as-cut surfaces were examined. The cut surfaces were smooth but not with a mirror f i n i s h , the surface appearing matted. These surfaces could not be imaged by CL; only large topographical features could be detected. A f t e r p o l i s h i n g these samples i n a 15 per cent s o l u t i o n of Br i n methanol for 600 seconds the surface had a mirror f i n i s h which was suitable fo r CL imaging. 3.24 Gallium Arsenide Capped with S i l i c o n N i t r i d e To determine whether the S i 3 N 4 capping of the GaAs influenced the CL image, and whether an image could be obtained through the layer, two pieces of the same c r y s t a l were examined by CL imaging, one without capping and the other with a S i 3 N 4 cap 750A th i c k . The r e s u l t s showed that the d i s l o c a t i o n arrays were c l e a r l y v i s i b l e which indicated that any surface damage due to capping did not a f f e c t the d i s l o c a t i o n image. There was a small amount of added contrast i n the CL images of the capped samples, 45 appearing as a s l i g h t mottling. This may have been due to both the damage i n the GaAs as well as the actual layer of Si3N 4. 3.25 Secondary Ion Mass Spectrometry Analysis The bright and dark regions i n the CL images are possibly associated with differences i n impurity l e v e l s , the bright regions being associated with depleted regions. This has previously been i n v e s t i g a t e d 5 using SIMS. The concentration of impurities would be very low, on the order of 1 0 1 5 / C I t i 3 f that i s at the r e s i d u a l l e v e l . Attempts were made to determine whether the bright regions were impurity depleted using secondary ion mass spectrometry (SIMS). A GaAs sample was examined a f t e r heating f o r 12 00 seconds at 1000°C. CL images at two magnifications are shown i n Figures 25 and 26. The bright r i n g marked A i n the figures i s due to a surface defect. A SIMS map of the same surface area a f t e r 5000A had been removed i s shown i n Figure 27. This map shows the d i s t r i b u t i o n s p e c i f i c a l l y of carbon. The large bright c i r c l e corresponds to the surface defect at A. Comparing Figure 26 with 27 (B, C marked for reference) indicates that some c o r r e l a t i o n e x i s t s between the dark regions i n the CL image, associated with d i s l o c a t i o n s , and the bright regions i n the SIMS image associated with higher carbon l e v e l s . A d i r e c t comparison i s made d i f f i c u l t since i n the SIMS the specimen i s i n c l i n e d to the incident ion beam whereas i n the SEM the CL image i s taken with the specimen surface normal to the incident beam. This r e s u l t s i n the SIMS image containing a degree of foreshortening r e l a t i v e to the CL image. 3.3 Heated Specimens Samples of GaAs were capped with 750 Angstroms of Si3N 4. To the naked eye the capped samples appeared as a blue, mirrored surface (the colour i s used as an i n d i c a t i o n of the thickness of the S i 3 N 4 layer : 700-800A appears blue). The SE image i n Figure 28 shows the topography of the capped surface at a magnification of 200 times. The surface i s quite bumpy r e l a t i v e to the clean GaAs surface. The capped samples were placed i n a c r y s t a l grower and heated to approximately 950°C, held at temperature for 120 seconds and slowly cooled. Figure 29 i s a SE image of the surface a f t e r heating. The cap i s s t i l l i n place, but there are numerous pinholes i n i t . The CL image from the area i n Figure 29 i s shown i n Figure 30. The damage features i n the cap are observed to be most prominent i n the image, overshadowing the underlying d i s l o c a t i o n structure. The remainder of the cap was removed i n HF and upon further examination i t was found that p i t s had developed i n the GaAs surface under the cap pinholes. These were estimated to be h a l f a micron deep from o p t i c a l and SE images. Attempts to image the CL s i g n a l from the GaAs a f t e r removal of the cap were unsuccessful as the p i t s themselves were the prominent feature. I t was not u n t i l much of the p i t t i n g damage was polished o f f that the CL sig n a l of the underlying d i s l o c a t i o n structure could be imaged. The e f f e c t s of heating GaAs on the CL image were determined by examining sample T4 which was heated to 955°C for 360 seconds, then cleaned and polished. Figures 31 through 34 are CL images (x32) of areas of sample T4. The CL images i n Figures 32 and 34 are from adjacent areas of the specimen a f t e r heating, taken i n the normal imaging mode. The d i s l o c a t i o n arrays appear dark and the images are quite sharp. This i s compared against the CL images i n Figures 31 and 3 3 taken from the same areas of the specimen before heating. These were taken i n the inverted imaging mode. Hence the arrays a c t u a l l y appeared bright i n the CL image before heating. The arrangement of the d i s l o c a t i o n networks remain e s s e n t i a l l y the same, with minor deviations a f t e r the heating. There i s a contrast inversion a f t e r heating and there i s an increase i n c l a r i t y or sharpness of the CL image a f t e r heating. 48 A sample of the same specimen which had been capped, but not heated, was put through the same cap removal, cleaning and p o l i s h i n g procedures as the heated portion to determine i f any of the observed e f f e c t s could be at t r i b u t a b l e to specimen treatment a f t e r heating. Subsequent CL imaging of t h i s sample gave the same fuzzy inverted image obtained from the capped sample before heating. This indicates that the contrast inversion and sharpening of the CL image are a r e s u l t of the heating and are not due to the po l i s h i n g procedure. A comparison was made of CL t o t a l brightness before and a f t e r heating. This was done by examining two specimens mounted on the same specimen stub. One specimen was as supplied and the second had been heated to 1000°C. The CL images of both samples had roughly the same o v e r a l l brightness. Moving from one specimen to the other did not require any brightness adjustment to keep the signal l e v e l constant. 3.4 S t a t i c Bend Specimens Specimens T l , T2, T3, T5, S i l , and LI were a l l bent i n the s t a t i c bend j i g shown i n Figure 9. Af t e r placing each t e s t assembly i n the grower chamber, the temperature was slowly increased. Specimens T l and T2 were tested simultaneously, as were T3, S i l , and, LI. Specimen T4 was tested with T5 but was only heated, not stressed. The deforming load i n each case was 12 grams (0.118N), applied at the free end of the specimen 14mm from the bearing support. Specimens T l and T2 deformed at approximately 950°C. After bending, the temperature was held constant for 120 seconds before cooling. The centre d e f l e c t i o n , l i m i t e d by the j i g s guides, was 0.2mm fo r each specimen . Figures 35 through 38 are a serie s of CL images taken of specimen T2 a f t e r heating, bending, subsequent removal of the cap, cleaning, and pol i s h i n g the sample surface. The sequence covers the specimen from near the free end (Figure 35) where the applied bending moment i s minimal, to near the bend axis (Figure 38), where the applied bending moment i s a maximum. The CL images between are of increasing proximity to the bend axis (increasing applied moment). The images were taken i n the normal mode at a magnification of 128 times. The dark c i r c u l a r regions that appear i n some of the images are those persistent apparitions described as side e f f e c t s e a r l i e r i n the CL imaging section. These dark c i r c l e were not immediately apparent whenever a new area of the specimen was brought into view during CL imaging. A f t e r each region was brought into focus and contrast and brightness l e v e l s were set for photographing, these c i r c u l a r features began to develop. As the time spent imaging a p a r t i c u l a r area increased, the c i r c l e i n the f i e l d grew darker. Usually the time needed f o r the photographic scan was s u f f i c i e n t for these c i r c l e s to develop from unnoticeable to the l e v e l i n the CL images shown. The d i s l o c a t i o n arrangement i n t h i s s e r i e s of CL images i s d i f f e r e n t from the as-grown c e l l u l a r structure as seen i n Figures 31-34 taken from the same wafer. In the sequence of CL images of specimen T2, the d i s l o c a t i o n s image as dark spots or l i n e s with no halos. The d i s l o c a t i o n s are no longer found i n t h e i r o r i g i n a l c e l l u l a r structure but have formed into arrays p a r a l l e l to the bend axis. Between these arrays are areas s u b s t a n t i a l l y c l e a r of d i s l o c a t i o n s . The spacing between the arrays decreases nearer the bend axis. Figure 35 was taken near the free end of the specimen 12.5mm from the bend axis. The alignment here i s minimal, and parts of the o l d c e l l u l a r structure are s t i l l v i s i b l e . Figure 3 6 was taken at 7.5mm from the bend axis. The alignment of d i s l o c a t i o n s here i s quite c l e a r with the appearance of short l i n e segments along with the aligned single spots. The CL image i n Figure 37 was taken at 3mm from the bend axis. The density of di s l o c a t i o n s i s increased from the previous figure and there are more dark l i n e segments present. F i n a l l y , Figure 38 was taken at 1.5mm from the bend axis. Again the d i s l o c a t i o n density has increased over the previous figures. There are more, longer, dark l i n e s present i n t h i s region. The d e n s i t i e s of the new arrays i n t h i s specimen measured 2, 22, 40, and 53 l i n e s per mm at 12.5, 7.5, 3, and 1.5 mm from the bend axis. Sample T5 was bent at 955°C and was l e f t at temperature for 3 60 seconds following the deformation. The centre d e f l e c t i o n achieved by sample T5 was measured as 1.0mm. The same uncapping and p o l i s h i n g procedures were applied before taking the CL images shown i n Figures 39 to 42. The d i s l o c a t i o n s again appear as dark l i n e s or spots with no halos. Figures 39 and 40 are CL images (x32 and xl28 respectively) taken from the high s t r a i n region of specimen T5 near the bend axis. The applied bending moment i s highest i n t h i s region. The image i n Figure 39 shows a very f i n e matte pattern of d i s l o c a t i o n s , with no remnant of the as-grown d i s l o c a t i o n arrays present. At higher magnification of the same area, Figure 4 0 resolves the d i s l o c a t i o n s into arrays. These arrays have l i n e a r features p a r a l l e l to the bend axis. These new arrays have a density of 70 l i n e s per mm i n t h i s region. Figures 41 and 42 are CL images (x32 and xl28 respectively) taken from the low s t r a i n region of specimen T5 nearer the free end. The applied bending moment i s lower i n t h i s region. In Figure 41 the old network patterns of the as-grown dis l o c a t i o n s are s t i l l v i s i b l e . Superimposed on t h i s however, are many fine 52 l i n e s p a r a l l e l to the bend axis. The higher magnification image i n Figure 42 c l e a r l y resolves the new l i n e a r d i s l o c a t i o n arrays with wider spacing and lower density than those i n Figure 41. Specimens LI, S i l , and T3 were tested simultaneously i n a slowly increasing temperature environment. They started deforming at close to 950°C, with bending s t a r t i n g at d i f f e r e n t times f o r the three specimens. The low d i s l o c a t i o n density specimen LI was observed to deform f i r s t , at the lowest ambient temperature, followed by T3 and f i n a l l y the S i doped specimen S i l . The temperature differences between the onset of deformation of the three specimens were not established but small. The CL images (xl28) shown i n Figures 43 and 44 were taken from regions of specimen S i l a f t e r bending. Figure 43 was taken nearer the free end i n the low s t r a i n region. V i s i b l e are three (A, B, and C i n the figure) of the as-grown d i s l o c a t i o n s (compare with Figure 20) . Also present are l i n e a r arrays of d i s l o c a t i o n s made up of e i t h e r long single l i n e s or a series of short segments each p a r a l l e l to the bend axis. The new arrays l i n e s are very much sharper than the o r i g i n a l d i s l o c a t i o n images. Figure 44 was taken from the region of S i l nearer the bend axis i n the high s t r a i n region of the specimen. New arrays of d i s l o c a t i o n s are seen as f i n e dark l i n e s and spots arranged p a r a l l e l to the bend axis. There i s a higher density of these new arrays than i n the low s t r a i n region of Figure 43, and no evidence of as-grown d i s l o c a t i o n s i s present. The top "and bottom surfaces of deformed specimen T3 were examined to compare the d i s l o c a t i o n configuration on the tension and compression sides of the specimen. CL images showed the d i s l o c a t i o n pattern was the same on both sides, and was si m i l a r to that shown i n Figures 35 to 38, with arrays of new dis l o c a t i o n s p a r a l l e l to the bend axis. 3.5 C y c l i c Bend Specimens The bending experiments described above established the temperatures at which p l a s t i c deformation occurred i n GaAs and the extent of generation of new dis l o c a t i o n s with s t r a i n . On the basis of t h i s information a more systematic s e r i e s of bending te s t s were c a r r i e d out i n which small c y c l i c s t r a i n s were applied to the specimen using the apparatus shown i n Figure 10. For t h i s s e r i e s of te s t s , specimens 32mm long and 6.35mm wide were used, cut p a r a l l e l to the major f l a t from an el e c t r o n i c s grade GaAs wafer. This i s the same orientation as used i n the s t a t i c tests and shown i n Figure 14. Pri o r to cutting, the wafer was capped with 800A of Si3N 4 which was removed a f t e r deformation. For specimen SI the maximum centre d e f l e c t i o n was set at 0.5mm, with a s t r a i n rate of 0.21mm/s. The t e s t temperature used was 1000°C. The specimen was cycled f i v e times with a 30 second pause at each end of t r a v e l . Figures 45 and 46 are CL images (x32 and xl28 respectively) taken from the high s t r a i n region of specimen SI near the bend axis. The applied bending moment i s highest i n t h i s region. A mixture of l i n e a r arrays p a r a l l e l to the bend axis and f i n e c e l l u l a r networks are j u s t v i s i b l e i n Figure 45. These become c l e a r l y resolved i n the higher magnification image i n Figure 46 from the same area. The c e l l s i z e of the new d i s l o c a t i o n networks i s approximately 3 0 microns, much smaller than the as- grown c e l l s i z e of 500 microns. Long arrays of d i s l o c a t i o n s p a r a l l e l to the bend axis are also evident. A CL image (x32) of the low s t r a i n region of specimen SI i s shown i n Figure 47. Here the boundaries of the as-grown d i s l o c a t i o n arrays are s t i l l v i s i b l e , but lack c l a r i t y . Figure 48 i s a CL image (xl28) showing region A from Figure 47 at higher magnification. The new d i s l o c a t i o n density i s higher than before the bending, but the d i s t r i b u t i o n i s more homogeneous with no strong c e l l u l a r or l i n e a r alignment. There appears to be no r e l a t i o n between new d i s l o c a t i o n s and the old network boundaries (the darker areas). 55 In the second c y c l i c t e s t with specimen S2, the maximum d e f l e c t i o n was also 0.5mm, with a s t r a i n rate of 0.21mm/s. The t e s t temperature was increased to 1050°C, and S2 was cycled four times with a 120 second pause at each end of t r a v e l . The higher temperature caused a decomposition and melting of the specimen ends. The centre region of the specimen remained i n t a c t , but the t e s t c l e a r l y established the upper temperature l i m i t f or the present series of experiments. A CL image (xl28) of specimen S2 from the high s t r a i n region near the bend axis i s shown i n Figure 49. As before, a high density of di s l o c a t i o n s has been generated, s i m i l a r to SI, with l i n e a r arrays p a r a l l e l to the bend axis and a f i n e c e l l u l a r array structure. For sample S3 the maximum centre d e f l e c t i o n was reduced to 0.25mm, and the s t r a i n rate to 0.014mm/s. The t e s t temperature was returned to 1000°C. S3 was cycled only once and allowed to anneal f o r 90 seconds at the end of the cycle. The thermal h i s t o r y f o r S3 i s shown i n Figure 50. The heating and cooling rates shown are representative of a l l t e s t s . The e f f e c t of high temperature deformation on specimen S3 i n a low s t r a i n region 6mm from the bend axis i s shown i n Figures 51 and 53. These are CL images (xl28) of the same area of the 56 specimen before and a f t e r deforming respectively. The tangle of di s l o c a t i o n s at "A" shows some realignment, but i s recognizable. The r e s t of the new d i s l o c a t i o n structure i n Figure 52 cannot be d i r e c t l y r e l a t e d to the di s l o c a t i o n s i n Figure 51. CL (x32) maps of the middle 12mm of specimen S3 before and a f t e r deforming are shown i n Figures 53 and 54 respectively (covers two pages) . The image i n Figure 53 was taken i n the inverted mode. The maximum s t r a i n took place at the centre, (A - A) . The as-grown c e l l u l a r boundary structure (examples B and C) at the low s t r a i n ends of the specimen are c l e a r l y evident a f t e r high temperature deformation. However, there has been a considerable change i n the CL image of these c e l l s . C e l l C i n Figure 53 i s shown with a bright i n t e r i o r and the boundary consists of grayed region containing dark spots. The image was taken i n the inverted mode though and therefore, the c e l l i n t e r i o r i s darker than the boundary and the di s l o c a t i o n s appear bright. In Figure 54 the i n t e r i o r of c e l l C i s bright, the boundary i s dark and the di s l o c a t i o n s appear as bright spots i n the boundary. There i s not an exact correspondence between the d i s l o c a t i o n spots i n the c e l l boundaries before and a f t e r the deformation. Upon approaching the centre, higher s t r a i n region i n Figure 54, the occurrence of the bright d i s l o c a t i o n spots i n the as-grown c e l l boundaries diminishes and the boundaries themselves lose c l a r i t y . A f i n e r network of d i s l o c a t i o n l i n e s becomes apparent. These new arrays are arranged p a r a l l e l to the bend axis . The t e s t conditions for specimen S4 were kept as those for S3 except that S4 was cycled three times with a 120 second pause at each end of t r a v e l . The specimen was cleaned and polished as before and examined. In the centre region arrays of new d i s l o c a t i o n s had formed p a r a l l e l to the bend axis. These appeared as f i n e dark l i n e s and spots and were most dense i n the highest s t r a i n region. The as-grown c e l l boundaries were s t i l l v i s i b l e but only as s l i g h t l y greyer regions with no c l a r i t y or d e t a i l . In the low s t r a i n region the CL image was o v e r a l l much darker than i n the high s t r a i n region. Here the as-grown boundaries appeared as dark regions containing bright spots, as i n the case of the low s t r a i n region of S3. Separating these two regions was an exceedingly sharp t r a n s i t i o n . The brightness of the CL image changed markedly over the t r a n s i t i o n and the bright spots i n the as-grown c e l l boundaries, v i s i b l e up to the t r a n s i t i o n i n the low s t r a i n region, were not present upon crossing into the high s t r a i n region. 58 A sequence of CL images (xl28) was recorded beginning at the centre, highest s t r a i n region, of the sample and proceeding toward the low s t r a i n region. Figures 55 through 59 show the new d i s l o c a t i o n d i s t r i b u t i o n i n the high s t r a i n region. Figure 55 was taken at the bend axis. The new d i s l o c a t i o n arrays, made up of long l i n e a r structures and spots, are aligned p a r a l l e l to the bend axis and are c l o s e l y spaced. In Figure 56, taken adjacent the area of Figure 55, the same type of arrays appear with a s l i g h t l y greater spacing and with more instances of shorter l i n e segments making up the arrays. Further away from the bend axis i n Figure 57 the alignment of the new arrays i s s t i l l apparent, but the arrays consist more of short l i n e a r segments and dots. The spacing between the arrays i s also increased. Further away s t i l l from the bend axis the d i s l o c a t i o n arrangement i s shown i n Figure 58. Here the arrays contain only short segments and spots. The long l i n e a r features v i s i b l e i n the higher s t r a i n regions no longer appear. There i s s t i l l alignment of the new arrays p a r a l l e l to the bend axis. F i n a l l y , Figure 59 shows the d i s l o c a t i o n arrangement near the t r a n s i t i o n to the low s t r a i n region. The appearance of even short l i n e a r segments has diminished here with the majority of the d i s l o c a t i o n s appearing as spots. The d i s l o c a t i o n density i s at a minimum for the high s t r a i n region and the di s l o c a t i o n s do not show a strong alignment. The s l i g h t l y greyer regions i n the image are remnants of the as-grown c e l l boundaries. 59 The t r a n s i t i o n from the high s t r a i n (bright) region to the low s t r a i n (dark) region i s shown i n Figure 60. The width of the t r a n s i t i o n i s approximately 20 microns. The darker outer regions (one to e i t h e r side of the centre bright region) are symmetrical about the centre region and the centre loading pin. Each dark region i n t e r s e c t s the centre region with a parabolic p r o f i l e extending furthest along the centerline of the specimen. Separation of the two dark zones was 9mm along the central axis of the specimen. Figure 61 shows the CL image (xl28) of the d i s l o c a t i o n arrays i n the low s t r a i n region j u s t beyond the t r a n s i t i o n . The d i s l o c a t i o n s image as bright spots and are predominantly arranged i n the as-grown c e l l boundaries. This i s i n contrast to Figure 59 which was taken approximately 2mm closer to the centre. 60 4 Discussion 4.1 Cathodoluminescence Imaging Dislocations i n GaAs are c l e a r l y delineated i n the CL image provided the surface i s well polished. The CL image of a d i s l o c a t i o n consisted of a dark core sometimes surrounded by a bright halo. Where the halo was present i t was sometimes s u f f i c i e n t to obscure the dark core, p a r t i c u l a r l y at lower magnifications. In these cases the d i s l o c a t i o n s appeared as bright spots. There i s excellent c o r r e l a t i o n between the d i s l o c a t i o n spots i n the CL image and d i s l o c a t i o n revealing etch p i t s introduced by etching the GaAs i n molten KOH. The CL image also contained additional grey l e v e l contrast that outlined the as-grown c e l l boundaries, which made the c e l l u l a r structure more apparent than the etch p i t pattern. Heating the GaAs to 950"C served to enhance the CL image. The halos around the d i s l o c a t i o n s i t e s disappeared revealing the dark cores. The image was much sharper a f t e r heating. The d i s l o c a t i o n arrangement was not s i g n i f i c a n t l y affected by heating alone and CL images taken a f t e r heating showed general correspondence with the features present before heating. 61 4.2 Deformation The grown-in d i s l o c a t i o n arrangement of networks could be affected by s t r a i n i n g the c r y s t a l at high temperatures. The amount of change i n the d i s l o c a t i o n d i s t r i b u t i o n depended upon the amount of s t r a i n and on the amount of energy (cycles) put into the c r y s t a l . At low s t r a i n s , only minor re-arrangement was accomplished. At high s t r a i n s , complete o b l i t e r a t i o n of the o r i g i n a l arrays was possible at the expense of a f a r more dense d i s t r i b u t i o n of new arrays. Between these two extremes was found the realm of j u s t being able to remove the old arrays and replace them with a ser i e s of new arrays formed p a r a l l e l to the bend axis with d i s l o c a t i o n free regions between them. The spacing between these new arrays varied with the amount of s t r a i n introduced. At the l i m i t s approaching that of reducing the s t r a i n to l e v e l s that did not a f f e c t the o r i g i n a l arrays the new d i s l o c a t i o n s were at low d e n s i t i e s and did not exhibit strong alignment. The t r a n s i t i o n (yielded to non-yielded) occurred over a very narrow region, 20 microns wide, and i t was immediately inside t h i s y i e l d ed zone that the old arrays disappeared and the new d i s l o c a t i o n s were present at t h e i r lowest density. 4.21 Formation of Linear Arrays GaAs has i t s s l i p systems on {111}<110>. In the present specimen orientation, s l i p can only take place on two of these 62 {111} planes, both are shown i n r e l a t i o n to the specimen i n Figure 62. The bend axis i s along [ O i l ] . Both these s l i p planes, (111) and (111), in t e r s e c t the specimen surface at ri g h t angles to the specimen length, or p a r a l l e l to the bend axis. Dislocations i n the c r y s t a l can g l i d e to the surface on these two s l i p planes. Not every s l i p plane i s expected to operate, instead, s l i p i s more l i k e l y to occur on several p a r a l l e l s l i p planes separated by regions which do not s l i p . As more deformation i s required, more s l i p planes w i l l come into operation. The r e s u l t i n g arrays seen on the surface are then also p a r a l l e l to the bend axis with a spacing representative of the inverse of the amount of d e f l e c t i o n (curvature) achieved. 4.22 New Array Density The density (inverse spacing) of the new arrays formed p a r a l l e l to the bend axis i n the s t a t i c a l l y bent specimens (T1,T2,T5) was found to vary nonlinearly with the distance from the bend axis. The bending moment applied to the specimen varies l i n e a r l y from a maximum at the bend axis to a minimum of zero at the applied load. The v a r i a t i o n of s t r a i n at the specimen surface i s not expected to vary l i n e a r l y though since the e l a s t i c core diminishes while approaching the bend axis beyond the onset of surface y i e l d i n g . The s t r a i n at the interface between yielded and e l a s t i c region w i l l be roughly constant and i s given by the y i e l d stress divided by the e l a s t i c modulus for that temperature. 63 Following the assumption that planes i n i t i a l l y perpendicular to the neutral axis remain planar a f t e r the deformation, the s t r a i n i n the sample varies l i n e a r l y with the distance from the neutral axis. Therefore, the deeper the yielded depth, the deeper w i l l be the constant value of s t r a i n at the interface and the greater the surface s t r a i n w i l l be. This can be seen by the construction i n Figure 63. There w i l l also be a degree of uncontrolled s t r a i n at the bend axis due to the formation of a p l a s t i c hinge (increasing s t r a i n does not increase s t r e s s ) . This i s a l i m i t i n g approximation but stress s t r a i n curves f o r GaAs 2 show a f l a t t e n i n g stress s t r a i n curve above y i e l d i n g at increasing temperatures, Figure 2. Table II shows the new d i s l o c a t i o n l i n e d e n s i t i e s and associated moment arm and applied load for specimen T2. In Figure 64 the data of table II has been plo t t e d along with a parametric curve of surface s t r a i n versus distance from the bend axis developed from the arguments above. There i s a good f i t between the d i s l o c a t i o n l i n e density data and the form of the parametric curve, which i s scaled appropriately. I f the Burgers vector of the di s l o c a t i o n s i s taken as b = ^ [ l l O ] 1 ' 3 , then the displacement along the surface of the specimen, p a r a l l e l to [Oil] i s — per d i s l o c a t i o n . With the sc a l i n g factor (roughly 40 lines/mm corresponds to an Ee of 8MPa) from the above f i t , and a room temperature value f o r the modulus each l i n e seen i n the CL image would correspond to some 20 d i s l o c a t i o n s . I t i s expected however, that the modulus fo r GaAs at 1000°C i s considerably reduced from the room temperature value. I t i s also reasonable to assume that the dark l i n e s i n the CL images do contain more than a si n g l e d i s l o c a t i o n . The r e s o l u t i o n a v a i l a b l e i s well below that needed to disprove t h i s and TEM work 1 showed that a single l i n e of etch p i t s found i n deformed material corresponded to arrangements of several c l o s e l y spaced d i s l o c a t i o n loops. d i s l o c a t i o n l i n e frequency (lines/mm) moment arm (mm) -2 1.5 22 7.5 40 11 53 12.5 applied load = 12g centre d e f l e c t i o n = 0.2mm TABLE I I . Density of l i n e a r arrays along specimen. The degree of "uncontrolled s t r a i n " i s only uncontrolled with respect to the specimen i t s e l f . The bending apparatus used here provide an external l i m i t to the absolute amount of bending. In specimen T5 where the centre d e f l e c t i o n was increased to 1.0mm the new array density near the bend axis was increased to 70 lines/mm. 65 4.23 Y i e l d strength A simple p l a s t i c hinge t h e o r y 1 3 gives a y i e l d point for the specimens i n the s t a t i c 3-point bending apparatus. Since the specimens e x h i b i t p l a s t i c bending at a s p e c i f i e d temperature under a known moment, the r e s i s t i n g moment ( M r e s i s t ) i n the specimen at that temperature could no longer increase and therefore, the specimen can be assumed to be behaving p l a s t i c a l l y at a roughly constant y i e l d stress, YS # The r e s u l t i n g stress d i s t r i b u t i o n generates a couple about the bend axis, equal to M r e s i s t * With a specimen of width w and thickness t, Mres^s^- i s given by YS•w* t 2 ^ r e s i s t = ? ( D This value i s then equated to the bending moment applied to the specimen (load x moment arm) and a y i e l d stress at the bending temperature determined. For T l , T2, and T5 t h i s was found to be 6.5 MPa at 955°C. This compares very well with the monotonic decrease of y i e l d strength versus temperature 2 f o r 250- 550°C. Figure 65 shows the data plotted along with an expected value of zero y i e l d strength at the melting temperature. The comparison between y i e l d strength values i n compression tests along <100>2 and bending about the [Oil] axis i s v a l i d since the Schmid factors required to resolve the stress onto the s l i p planes are the same, 0.408, f o r each system. Based on the above value f o r y i e l d stress the c r i t i c a l resolved shear stress (CRSS) for GaAs at 955°C i s calculated as 2.65MPa. The assumption that the whole of the specimen y i e l d s i s reasonable, since even i f an e l a s t i c core remained i n the specimen, i t s contribution to the r e s i s t i n g moment would be small owing to the short moment arm such a core would have about the neutral axis. In e f f e c t , i f a 50% e l a s t i c core remained 92% of the r e s i s t i n g moment would be generated from the yielded region. 4.24 Calculation of Parameters from C y c l i c Tests In the case of sample S4 there seemed to be a well defined yielded zone, that i s the brighter centre region. The whole specimen was i n the same thermal environment and there were no features on the bending j i g that could be associated with the change. The only variable changing along the axis of the specimen was the magnitude of the applied bending moment. The p r o f i l e of the zone was c h a r a c t e r i s t i c of y i e l d i n g taking place under plane strain/plane stress conditions from the middle of the sample to the edges 1 4. An estimate of the actual load applied to the specimen was calculated based on the width of t h i s zone, the geometry of the bending j i g , and the estimate of y i e l d strength ar r i v e d at from the preliminary t e s t s . The construction i n Figure 66 shows a c r y s t a l of thickness t and width w, consisting of an e l a s t i c core surrounded by a yielded layer of depth YD, 67 with y i e l d strength YS. The r e s i s t i n g moment i s given by M YS'W ( -YD2 + YD-t + | ) ( 2) r e s i s t ~~ 3 The applied moment i s given by M a p p l i e d ~ (3) Where the c r y s t a l changes from the yielded bright region to the e l a s t i c dark region the y i e l d depth, YD, goes to zero. Equating the applied and r e s i s t i n g moments at t h i s p o s i t i o n gives an expression f o r the applied load P. The r e s u l t f o r P was 0.338N (34.5 grams). A value of 5.45MPa was used f o r YS. This was an extrapolated value between 6.5MPa found at 955°C and zero expected at 1238°C, the melting point. With t h i s calculated value for the applied load i t was possible to fi n d i f the centre of the specimen did go f u l l y p l a s t i c and how wide a zone about the centre loading pin was f u l l y yielded. This was accomplished by se t t i n g YD equal to h a l f the specimen thickness i n equation (2) and solving for x. The r e s u l t was that f u l l y i e l d i n g took place out to an x value of 0.4mm, or that P YS ' W-t 2 (4) 3(5 " x) 68 there was a f u l l y yielded zone 0. 8mm. wide about the centre loading pin. This i s consistent with the assumption made i n a r r i v i n g at the o r i g i n a l value of the y i e l d s t r ess. A f i n a l c a l c u l a t i o n was made to provide i n t e r n a l consistency with the above ca l c u l a t i o n s and assumptions, and t h i s was for the generation volume depth of the CL si g n a l . Following the construction i n Figure 67, the t r a n s i t i o n width between the yielded and non-yielded zones i n the CL images i s re l a t e d to the depth of the signal generation volume and the rate of change of depth of the yielded region, dYD/dx, at x equal to 4.5mm. Di f f e r e n t i a t i o n of equation (2) and sub s t i t u t i n g 4.5mm for x gave the slope of the interface (0.03mm/mm). The t r a n s i t i o n width was measured from a 128 times CL photograph and was found to be 20 microns. This gave a CL generation volume depth of 0.6 microns. I t has been previously r e p o r t e d 1 5 that the CL signal depth i s one t h i r d of the electron beam penetration depth. Based on the above, the electron depth i n t h i s work i s estimated to be- about 2 microns. This i s a reasonable value for the penetration of 27keV electrons into GaAs. 4.2 5 Estimation of Parameters To Move Dislocations A load of 0.118N was applied to the free end of specimen T2, 14mm from the bend axis. In specimen T2 s i g n i f i c a n t rearrangement of d i s l o c a t i o n s into l i n e a r arrays p a r a l l e l to the bend axis was accomplished 7.5mm from the bend axis. The bending moment at t h i s point i s calculated from equation (3) r e a l i z i n g that 0.118N P i s —. Assuming that t h i s region i s outside the zone where surface y i e l d i n g takes place the surface stress can be calculated from the flexure formula below. At the surface c i s h a l f the thickness and I i s the moment of area about the neutral axis. The calculated value f o r surface stress i s 4.5MPa, which i s below the calculated y i e l d stress, v a l i d a t i n g the assumption. The resolved shear stress to achieve t h i s d i s l o c a t i o n arrangement i s 1.8MPa at 955°C. S i m i l a r l y , f or specimen S3 at 6mm from the bend axis a tangle of di s l o c a t i o n s was b a s i c a l l y preserved through the deformation as were the outlines of the as-grown c e l l boundaries at t h i s p o s i t i o n . Most of the dis l o c a t i o n s though had been moved within the boundaries. The conditions of loading applied to the t h i s specimen were the same as for S4 (cycles and times were di f f e r e n t ) so the value of 0.338N calculated as the centre deforming load for S4 can be assumed to have been applied to S3. The r e s u l t i n g surface stress from equations (3) and (5) i s 4.5MPa, and the resolved shear stress i s 1.8MPa. 70 In specimen S4 there was a c l e a r t r a n s i t i o n between low s t r a i n regions where the as-grown networks survived to the high s t r a i n region where only the new arrays of d i s l o c a t i o n s were present. The boundary was assumed be where the surface j u s t reached the y i e l d stress, which was estimated to be 5.5MPa at 1000°C. The resolved shear stress value i s then 2.2MPa. However, S4 was not subjected to a sing l e cycle of loading and the r e s u l t i n g development of the sharp t r a n s i t i o n may be more con t r o l l e d by the amount of thermal and mechanical energy introduced over the cycles. To estimate s t r a i n s that follow from the above stress l e v e l s requires a value for the modulus near 1000°C. I t i s c l e a r from common sense and the r e s u l t s i n section 4.22 above that the modulus at t h i s temperature i s between the room temperature value and some lower l i m i t . A good value was not established, but using a value h a l f that of the room temperature value translates the above stress l e v e l s to s t r a i n s of approximately 0.3 microstrain. The radius of curvature to produce t h i s s t r a i n l e v e l f o r the given specimens i s approximately 800mm. 4.3 Conclusions 1) The as-grown dis l o c a t i o n s i n GaAs are moved with the a p p l i c a t i o n of s t r a i n at high temperatures. Resolved shear 71 stress l e v e l s needed are between 1.8 and 2.2 MPa i n the temperature range of 950 to 1000°C. 2) Annealing alone to 955°C does not s i g n i f i c a n t l y a f f e c t the arrangement of the as-grown di s l o c a t i o n s , but does a f f e c t the CL image, inv e r t i n g the contrast and improving the c l a r i t y . 3) Bending of GaAs single c r y s t a l s leads to the formation of l i n e a r d i s l o c a t i o n arrays along the i n t e r s e c t i o n of {111} s l i p planes with the specimen surface. The density of these new arrays varies with the amount of s t r a i n . 4) At 955+10°C GaAs has a y i e l d strength of 6.5MPa along <011> and a corresponding CRSS value of 2.65MPa. 5) CL imaging can be used to characterize GaAs with respect to the arrangement of d i s l o c a t i o n s . 6) Si 3N4 i s useful as a protective cap f o r GaAs for temperatures under 1050°C. 72 5 Future Recommendations Future work i n t h i s area can extend on three fronts : 1: Improvements i n CL imaging, including better r e s o l u t i o n by increasing the beam current by means of a more sophisticated filament (pointed or LaB 6), and increasing the accelerating voltage by modifying the SEM, both of which would allow the use of a smaller electron spot s i z e . 2: More controlled deformation of GaAs samples using constant curvature j igs with parameters gained from t h i s work to generate larger regions of c r y s t a l s with uniform d i s t r i b u t i o n of d i s l o c a t i o n s for studies on e l e c t r i c a l properties. 3: I n t e r d i s c i p l i n a r y studies on GaAs between E l e c t r i c a l and M e t a l l u r g i c a l engineering including CL studies of f a i l e d devices, e l e c t r i c a l properties of deformed GaAs, and use of CL to c o r r e l a t e device performance with d i s l o c a t i o n d i s t r i b u t i o n . This l a s t point was discussed and would involve CL mapping of a region of a GaAs wafer p r i o r to f a b r i c a t i n g an array of i d e n t i c a l devices on the wafer. Subsequent performance of the i n d i v i d u a l devices could then be compared with the underlying d i s l o c a t i o n structure. 73 350°C 450°C 500°C 550°C s t r a i n (per cent) FIGURE 1. Stress versus s t r a i n f o r S i doped GaAs at various temperatures, tested i n compression along <100> a x i s 2 . 350°C 450°C 500°C 550°C s t r a i n (per cent) FIGURE 2. Stress versus s t r a i n f o r S i doped GaAs at various temperatures, tested i n compression along <111> a x i s 2 . 74 300 400 500 600 Temperature (°C) FIGURE 3. Y i e l d stress versus temperature f o r various samples of doped GaAs tested i n compression along <100> ax i s ^ . 25 50 75 100 125 150 time (seconds) FIGURE 4. D i s l o c a t i o n p o s i t i o n versus time from CL image of GaAs under an applied stress of 47MPa at 195°C 3. 75 Q - As a t o m s i n u n i t c u b e o n f e e s i t e s F I G U R E 5. G a A s c r y s t a l s t r u c t u r e ( c u b i c z i n c b l e n d e ) s h o w i n g t h e t w o i n t e r p e n e t r a t i n g F C C l a t t i c e s o f G a a n d A s . d i r e c t g a p i n d i r e c t g a p F I G U R E 6. S c h e m a t i c c o m p a r i s o n o f b a n d s t r u c t u r e o f d i r e c t g a p ( l e f t ) a n d i n d i r e c t g a p ( r i g h t ) i n m o m e n t u m ( k ) s p a c e . valence band k=0 FIGURE 8. Schematic of recombination with an interband energy level. 77 A B C D E F G support block specimen mass^/strain. control block bearing support loading block guide rod guides M M JUT FIGURE 9. Static bending jig in profile (a) and showing arrangement (b). scale 1:1 G A - main body B - lower support pins C - top assembly D - central loading pin E - return block F - strain limit block G - strain limit adjust screw / seed l i f t connecting rod H - specimen I - mass ring J - locking nut FIGURE 10. Dynamic bending jig shown in section. scale 1:1 78 1100- 15 cm heater / 1000- Temp. (°C) 900- 800- 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 8 9 10 1 11 12 1 1 13 c r y s t a l grower c o n t r o l l e r set point (mV) FIGURE 11. C a l i b r a t i o n curve f o r c r y s t a l growers con t r o l thermocouple using the 15cm heater. 1100- 20 cm heater 1000- Temp. C O 900- 800- 1 1 1 1 1 1 1 1 1 1 1 1 1 1 i 1 1 8 9 10 c r y s t a l grower c o n t r o l l e r l l I 11 12 set point (mV) 1 ! 13 FIGURE 12. C a l i b r a t i o n curve f o r c r y s t a l growers con t r o l thermocouple using the 20cm heater. 79 stainless steel quartz boat GaAs specimen BN rods FIGURE 13. Experimental arrangement for testing B 20 3 as an encapsulant for high temperature deformation of GaAs shown in section, scale 1:1. [010] [Oil] t FIGURE 14. GaAs wafer orientation showing specimen alignment with wafer. 80 FIGURE 15. Inverted CL image of polished GaAs showing the networks of as-grown dislocation arrays. 11 FIGURE 16. Inverted CL image of polished low grade GaAs showing d i s l o c a t i o n arrays and spots. 32x FIGURE 18. CL image of etched (KOH) low d i s l o c a t i o n density GaAs. 32x B : A FIGURE 19. O p t i c a l image of same area of GaAs as i n Figure 18, showing etch p i t s . 32x 82 FIGURE 20. CL image of sample of S i FIGURE 21. SE image of same area of doped GaAs, KOH etched. 128x S i doped GaAs as i n Figure 20, showing KOH etch p i t s . 128x FIGURE 22. CL image of a d i s l o c a t i o n FIGURE 23. SE image of same area of p a r a l l e l to the surface of S i doped etched GaAs as i n Figure 22 showing GaAs. 128x absence of etch p i t s . 128x FIGURE 24. CL image of e l e c t r o n i c s grade GaAs showing "dots" and "halos" e f f e c t s around d i s l o c a t i o n s . 128x FIGURE 26. Enlarged CL image of GaAs from region of Figure 25. 128x 83 FIGURE 25. CL image of GaAs a f t e r heating to 1000°C. 52x FIGURE 27. SIMS map of GaAs from the same region as Figure 26, showing d i s t r i b u t i o n of carbon. 140x 34 FIGURE 29. SE image of p i t s i n S i 3 N 4 cap on GaAs that formed a f t e r heating to 950°C. 32x FIGURE 30. CL image of capped GaAs a f t e r heating to 950°C, from the same area as Figure 29. 32x 3 5 FIGURE 33. Inverted CL image of capped GaAs T4, adjacent region to Figure 31. 32x FIGURE 32. CL image of GaAs specimen T4 a f t e r 360s anneal @ 955°C. S i 3 N 4 cap removed and surface polished. Same area as Figure 31. 32x FIGURE 34. CL image of annealed GaAs T4 from area adjacent to Figure 32, same area as Figure 33. 32x 86 FIGURE 37. CL image of GaAs T2 A f t e r heating to 955°C and bending. Area 3mm from bend axis. 128x FIGURE 38. CL image of GaAs T2 a f t e r heating to 955°C and bending. Area 1.5mm from bend axis. 128x FIGURE 39. CL image of GaAs T5 a f t e r heating to 955°C and bending. Area near bend axis. 32x FIGURE 40. CL image of GaAs T5 showing new d i s l o c a t i o n arrays i n area of Figure 39. 128x FIGURE 41. CL image of GaAs T5 a f t e r heating and bending. Area away from bend axis. 32x FIGURE 42. CL image of GaAs T5 showing new arrays from area i n Figure 41. 128x FIGURE 43. CL image of S i doped GaAs S i l a f t e r heating to 955°C and bending, showing old and new d i s l o c a t i o n s i n low s t r a i n region. 128x FIGURE 44. CL image of S i doped GaAs S i l a f t e r heating and bending showing new arrays i n the high s t r a i n region. 128x 89 FIGURE 47. CL image of SI a f t e r heating & c y c l i n g from the area away from the bend axis. 3 2 x FIGURE 48. CL Image of d i s l o c a t i o n arrays i n area of Figure 47. 128x FIGURE 49. CL image of GaAs S2 showing dislocation arrays after cycling at 1050°C. Near the bend axis. 128x 800 1600 2400 3200 4000 4800 t i m e ( s e c o n d s ) FIGURE 50. Thermal h i s t o r y o f GaAs specimen S3 d u r i n g h i g h t e m p e r a t u r e bend. FIGURE 51. CL image of GaAs sample S3 capped with 800A of Si3N4 taken 6mm from the centre of the specimen. 128x FIGURE 52. CL image of same area of S3 as Figure 51 a f t e r bending @ 1000°C. Low s t r a i n region 6mm from bend axis. 128x 92 FIGURE 53. Inverted CL image map of FIGURE 54. CL image map of GaAs S3 middle 12mm of GaAs specimen S3 after 1 bending cycle @ 1000°C. Same capped with 800A of Si 3N 4. 32x region as Figure 53. 32x 93 94 FIGURE 55. CL image of GaAs S4 a f t e r c y c l i n g at 1000°C. Highest s t r a i n region, at the bend axis. 128x FIGURE 57. CL image of GaAs S4 region away from the bend axis, i n the bright zone. 128x FIGURE 56. CL image of GaAs S4 adjacent to region i n Figure 55, near the bend axis. 128x FIGURE 58. CL image of GaAs S4 region further away from the bend axis, i n the bright zone. 128x FIGURE 60. CL image of GaAs S4 at the transition from the bright zone to the dark zone. 32x FIGURE 61. CL image of GaAs S4 showing dislocation distribution in the dark zone. 128x 9 6 FIGURE 63. Construction showing surface s t r a i n ( e s ) due to a varying y i e l d depth (YD) f o r a ca l c u l a t e d YD p r o f i l e i n a 3-point bend specimen. 97 d i s l o c S array density 80- 70- (#/inm) 60- e l a s t i c -p l a s t i c e l a s t i c -20 "Ee" (MPa) 50- -10 40- 30- 20- 10- 1 i i i 1 1 1 1 1 1 1 I ^ T ~ — 1 1 2 3 4 5 6 7 8 9 distance from bend axis 1 1 1 1 10 11 12 13 (mm) * FIGURE 64. Dislocation line density versus distance from bend axis for specimen T2 plus parametric curve of "Ee" scaled to data. ' v s (MPa) compress <100>z- © S i doped GaAs • Cr doped GaAs A Zn doped GaAs O undoped GaAs bend about <011> + undoped GaAs 200 400 600 800 Temperature (°C) 1000 1200. FIGURE 65. Yield stress versus temperature for doped/undoped GaAs tested in compression along <100> axis 2 and In bending about <011> axis. 98 FIGURE 66. Construction showing stress d i s t r i b u t i o n and y i e l d depth (YD) i n a 3-point bend specimen undergoing contained p l a s t i c y i e l d i n g , a distance (x) from the c e n t r a l l y applied load (P). ^ transition length FIGURE 67. Schematic showing e f f e c t i v e CL generation depth about the t r a n s i t i o n region i n GaAs specimen S4. 99 L i s t of References 1) D. L a i s t e r / G.M. Jenkins "Deformation of a Single Crystal of GaAs" Journal of Material Science volume 8 number [9] ppl218-32 (1973) 2) V. Swaminathan / S.M. Copley "Temperature and Orientation Dependence of P l a s t i c Deformation i n GaAs Single Crystals Doped With S i . Cr, or Zn" Journal of the American Ceramic Society volume 58 No. 11 - 12 pp482-485 (1975) 3) K. Maeda / M. Sato / A. Kubo / S. Takeuchi "Quantitative Measurements of Recombination Enhanced Disloca t i o n Glide i n Gallium Arsenide" Journal of Applied Physics volume 54 number [1] ppl61-168 (1983) 4) K. Maeda / S. Takeuchi Japanese Journal of Applied Physics volume 20 l e t t e r L165 (1981) 5) C. K i t t e l "Introduction to S o l i d State Physics" 5th e d i t i o n copyright 1976 John Wiley and Sons p 530 (1976) 6) G.T. Brown / CA. Warwick / I.M. Young / G.R. Booker "An Examination of Dislocations i n Si-doped LEC GaAs by Double Crys t a l X-ray Topography. SEM Cathodoluminescence and Chemical Etching" International Physics Conference Series Number 67 Microscopy of Semiconducting Materials (1983) The I n s t i t u t e of Physics pp371-377 7) T. Kamejima / F. Shimura / Y. Matsumoto / H. Watanabe / J . Matsui "Role of Dislocations i n Semi-Insulation Mechanism i n Undoped LEC GaAs Crys t a l s " Japanese Journal of Applied Physics volume 21 number 11 pp L721-L723 (1982) 8) N.J. Shah / H.Ahmed / L.A. Freeman / D.J. Smith "Electron microscopy of Se-implanted and electron-beam annealed GaAs" International Physics Conference Series Number 67 Microscopy of Semiconducting Materials (1983) The I n s t i t u t e of Physics ppl25-130 100 M.A. Shadid / S. Moffatt / N.J. Barrett / B.J. Sealy "Investigation of ion-implanted GaAs following electron-beam annealing" I n s t i t u t e of Physics Conference Series Number 67 Microscopy of Semiconducting Materials 1983 The I n s t i t u t e of Physics ppl31-136 10) C.A. Warwick / S.S. G i l l / P.J. Wright / A.G.Cullis "Spatial v a r i a t i o n of dopant concentration i n ^ • S i — implanted Czochralski and metal-organic vapour phase e p i t a x i a l GaAs" I n s t i t u t e of Physics Conference Series Number 76 Microscopy of Semiconducting Materials 1985 The I n s t i t u t e of Physics pp365-372 11) D.B. Holt / S. Datta "The Cathodoluminescent Mode as an A n a l y t i c a l Technigue; It s Development and Prospects" Scanning Electron Microscopy 1980 volume 1 pp259-278 (1980) 12) S.M. Davidson "Semiconductor material assessment by scanning electron microscopy" Journal of Microscopy volume 110 part 3 August 1977 ppl77-204 (1977) 13) E. Popov "Introduction to Mechanics of So l i d s " Copyright 1968 (Prentice-Hall Inc. Englewood C l i f f s , New Jersey) sections 6.8, 11.13, 12.8 14) D. Broek "Elementary Engineering Fracture Mechanics" Third E d i t i o n Copyright 1983 Martinus Ni j h o f f Publishers, The Hague pp 101-102 15) S.M. Davidson / C.A. Dimitriadis "Advances i n the e l e c t r i c a l assessment of semiconductors using the scanning electron microscope" Journal of Microscopy volume 118 part 3 March 1980 pp275-290 (1980)

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