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The preparation and operation of lithium drift germanium detectors Thompson, Albert Charles 1966

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THE PREPARATION AND OPERATION OF LITHIUM DRIFT GERMANIUM DETECTORS by ALBERT CHARLES THOMPSON B.Sc, University of B r i t i s h Columbia, 1964 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of PHYSICS We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA Ju l y , 1966 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x -t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n -c i a l g a i n s h a l l no t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t The U n i v e r s i t y o f B r i t i s h C o l u m b i a V a n c o u v e r 8, Canada ABSTRACT Lithium d r i f t e d germanium detectors have been prepared f o r use as high r e s o l u t i o n gamma ray spectrometers. The f a b r i c a t i o n procedure and the problems which can a r i s e during preparation are discussed i n d e t a i l . Using the techniques described, germanium detectors having the following c h a r a c t e r i s t i c s were prepared. Active Volume Total 1.0 cm 3 0.5 cm 2.0 cm 3 1.7 cm3 Resolution at 661 keV 5.0 KeV 4.0 KeV 4.0 KeV 2.9 KeV - i i i -TABLE OF CONTENTS Chapter 1 - INTRODUCTION 1 Chapter 2 - THEORY OF DETECTOR OPERATION 3 A. Interaction of Gamma Rays with a Semiconductor C r y s t a l i ) Interaction with c r y s t a l electrons 3 i i ) Conversion of electron energy to ionized.• charge • V B. Use of Semiconductor C r y s t a l as a Detector 8 C. Necessary Properties of Germanium f o r Detectors 10 D. Lithium Ion D r i f t i n g 12 E. , Surface. Problems 15 F. Resolution of Germanium Detectors 18 Chapter 3 - EXPERIMENTAL EQUIPMENT 22 A. Lithium Evaporation Equipment 22 B. D r i f t i n g Units 22 C. Power Supply 23 D. D r i f t Current C o n t r o l l e r 24 E. Detector Holders 23" F. Detector Preamplifier 26 Chapter 4 - DETECTOR FABRICATION PROCEDURE 27 Chapter 5 - PRESENTATION OF RESULTS 32 - i v -A. S i l i c o n Detector Fabrication 32 B. Germanium Detector Production 33 C. Detector Operation 36 Chapter 6 - CONCLUSIONS 40 Appendix 42 Bibliography 43 - V -LIST OF TABLES Table 2-1 Table 3-1 Table 5-1 Table 5-2 Energy Resolutions f o r Fano Factors of 0.075 and 0.16 Values of Resistance Used for Various D r i f t Currents Gamma Ray E f f i c i e n c y as Function of Energy Least Squares Straight Line F i t of Gamma Ray Spectra Following page 20 24 38 38 - v i -LIST OF FIGURES following Figure 2-1: V a r i a t i o n of Theoretical Absorption C o e f f i c i e n t s with —2Si£ Energy 6 Figure 2-2: The Energy Loss Process 8 Figure 2-3: Dependence of I n t r i n s i c C a r r i e r Density on Temperature 11 Figure 2-4: Dependence of Electron and Hole M o b i l i t i e s on Temperature 11 Figure 2-5: Impurity Concentration versus Distance from Surface 13 Figure 2-6: Growth of the Compensated Region 13 Figure 2-7: Normal E l e c t r i c F i e l d D i s t r i b u t i o n 14 Figure 2-8: Drifted Region Thickness versus Time f o r Various Temperatures 15 Figure 2-9: Resolution versus Energy f o r Various Fano Factors 20 Figure 3-1: a) Evaporation Assembly 20 b) Evaporation Boat 22 Figure 3-2: a) Germanium D r i f t Unit 23 b) Three Complete D r i f t Units Figure 3-3: Power Supply C i r c u i t 23 Figure 3-4: UBC D r i f t Current C o n t r o l l e r 24 Figure 3-5: a) V e r t i c a l Detector Holder 25 b) Horizontal Detector Holder Figure 3-6: Detector Holder 25 Figure 3-7: Low Noise F i e l d - E f f e c t T r a nsistor Preamplifier 26 Figure 4-1: Lithium D i f f u s i o n Heating Cycle 28 Figure 4-2: Arrangement f o r Measuring Detector Leakage Current 31 137 Figure 5-1: Cs Gamma Ray Spectrum with S i l i c o n Detector 33 - v i i -Following page 137 Figure 5-2: Cs Spectrum with Detector #2 35 137 Figure 5-3: Cs Spectrum with Detector #4 35 Figure 5-4: Typical Leakage Current versus Bias Voltage , 36 Figure 5-5: Peak Shape for Various Bias Voltages 36 57 Figure 5-6: Co Spectrum 37 134 Figure 5-7: Cs Spectrum 37 134 Figure 5-8: Expanded Cs Spectrum 37 154 Figure 5-9: Eu Spectrum 37 Figure 5-10: RdTh Spectrum 37 Figure 5-11: Resolution Squared versus Gamma Ray Energy 38 - v i i i -ACKNOWLEDGEMENTS I would l i k e to thank Dr. G. Jones, my research supervisor, f o r the h e l p f u l advice and assistance which he generously gave me in my research and the preparation of t h i s t h e s i s . The kind help of F.S. Goulding of the Lawrence Radiation Laboratory i s si n c e r e l y appreciated. The month spent i n h i s laboratory working with him, B. J a r r e t t , and W. Hansen was of great help i n learning the techniques of detector preparation. I am indebted to my fellow graduate students; D. Dalby, P. Tamminga, and, R.Bradbeer f o r t h e i r assistance i n preparing detectors. I would also l i k e to thank I. Fowler of Chalk River Laboratories f o r providing an excellent ingot of germanium. -1-CHAPTER 1 INTRODUCTION It i s frequently important to measure gamma ray energies with high accuracy and with good e f f i c i e n c y i n many f i e l d s of nuclear physics. Lithium d r i f t semiconductor detectors have recently made i t possible to measure gamma ray energies with high r e s o l u t i o n simultaneously over a wide range of energy. Previously, i t was necessary to use a very low e f f i c i e n c y magnetic spectrometer i f high energy res o l u t i o n was desired or to use a u moderate resolution (>8%) Nal s c i n t i l l a t i o n detector i f good e f f i c i e n c y was necessary. Lithium d r i f t germanium detectors combine a re s o l u t i o n close to that of magnetic spectrometers witH"the high e f f i c i e n c y and wide energy range c h a r a c t e r i s t i c of Nal s c i n t i l l a t i o n detectors. Currently the detection e f f i c i e n c y of germanium detectors i s about ten times less than s c i n t i l l a t i o n detectors but t h i s defect w i l l be overcome as the active volume of germanium detectors is increased. With germanium detectors i t i s possible to do a great range of experiments which were not f e a s i b l e before because of l i m i t a t i o n s i n the resolu t i o n or e f f i c i e n c y of previous detectors. In addition, many previous experiments can be redone with an order of magnitude increase i n accuracy. Two f i e l d s which have benefitted p a r t i c u l a r l y by the use of germanium detectors are l i f e t i m e measurements of excited nuclear states (Alexander and A l l e n 1965; Alexander, Litherland, and Broude 1965) and the precise analysis of X-Rays from mu-mesic atoms (Bardin et a l . 1966a, Bardin et a l . -1-CHAPTER 1 INTRODUCTION It i s frequently important to measure gamma ray energies with high accuracy and with good e f f i c i e n c y i n many f i e l d s of nuclear physics. Lithium d r i f t semiconductor detectors have recently made i t possible to measure gamma ray energies with high r e s o l u t i o n simultaneously over a wide range of energy. Previously, i t was necessary to use a very low e f f i c i e n c y magnetic spectrometer i f high energy res o l u t i o n was desired or to use a moderate r e s o l t i o n (>8%) Nal s c i n t i l l a t i o n detector i f good e f f i c i e n c y was necessary. Lithium d r i f t germanium detectors combine a re s o l u t i o n close to that of magnetic spectrometers with" the high e f f i c i e n c y and wide energy range c h a r a c t e r i s t i c of Nal s c i n t i l l a t i o n detectors. Currently the detection e f f i c i e n c y of germanium detectors i s about ten times less than s c i n t i l l a t i o n detectors but t h i s defect w i l l be overcome as the active volume of germanium detectors i s increased. With germanium detectors i t i s possible to do a great range of experiments which were not f e a s i b l e before because of l i m i t a t i o n s i n the resolu t i o n or e f f i c i e n c y of previous detectors. In addition, many previous experiments can be redone with an order of magnitude increase i n accuracy. Two f i e l d s which have benefitted p a r t i c u l a r l y by the use of germanium detectors are l i f e t i m e measurements of excited nuclear states (Alexander and A l l e n 1965; Alexander, Litherland, and Broude 1965) and the precise analysis of X-Rays from mu-mesic atoms (Bardin et a l . 1966a, Bardin et a l . -2-1966b). Direct gamma ray spectra measurements (Freedman, Wagner, Porter, and Bolotin 1966), neutron a c t i v a t i o n analysis (Hughes, Kennett, Prestwich, and Wall 1966), and gamma ray coincidence studies w i l l a l l be made more accurate using germanium detectors. This thesis presents the work done at the University of B r i t i s h Columbia on the preparation of lithium d r i f t germanium detectors. In Chapter 2, the theory of the operation of lithium d r i f t detectors i s presented and i n Chapter 3 the apparatus which was b u i l t to fa b r i c a t e detectors i s given. In Chapter 4 the f a b r i c a t i o n procedure i s given and i n Chapter 5 the experimental r e s u l t s are presented. F i n a l l y , i n Chapter 6 some conclusions are drawn on the preparation and operation of lit h i u m d r i f t detectors. - 3 -CHAPTER 2 THEORY OiF DETECTOR OPERATION A. Interaction of Gamma Rays with a_ Semiconductor Cr y s t a l The detection of gamma rays with semiconductor detectors i s based on the i n t e r a c t i o n of gamma rays with the electrons i n a semiconductor c r y s t a l . Detection of-a gamma ray can be considered as proceeding i n two steps. F i r s t , the gamma ray inte r a c t s d i r e c t l y with one of the electrons. The re s u l t i n g energetic electrons then i n t e r a c t with the other electrons i n the c r y s t a l producing many free electrons i n the conduction band and holes i n valence band. The electrons and holes are c o l l e c t e d and t h e i r t o t a l charge measured to give the energy of the incoming gamma ray. i ) Interaction with C r y s t a l Electrons There are three ways i n which an incoming gamma ray can lose energy to the electrons i n a c r y s t a l . The f i r s t i s photoelectric absorption i n which a l l the gamma ray energy i s used to excite an electron from an atom i n the c r y s t a l . The k i n e t i c energy o f the electron, E , i s then: E e = E Y " E b ' C2-1) where: E^ = energy of the gamma ray E^ = binding energy of the electron Momentum i s conserved by the r e c o i l of the residual ion. The p r o b a b i l i t y for p h o t o e l e c t r i c absorption i s therefore largest f o r K s h e l l electrons which are most strongly coupled to the nucleus. The photoelectric absorption cross section f o r K s h e l l electrons was given by Ha l l (1936) as: = 4 /2 Z ! _ I t 137 m0c hv (2-2) assuming: hv < <m Qc ( n o n - r e l a t i v i s t i c ) where: fo r germanium: hv >>binding energy of K s h e l l electron 0^ = K s h e l l p h o t o e l e c t r i c absorption cross section Z = atomic number of atom mass of electron m o hv energy of incoming gamma ray -25 2 constant = 6.6 x 10 cm = 3.37 x 10 -27 (hv) 7/2 cm (2-3) where hv: i s given i n MeV. This process i s dominant for low energy gamma rays (less than 200 KeV) i n germanium detectors. The second way an electron may receive energy from a gamma ray i s by Compton sc a t t e r i n g . The process can be considered as an e l a s t i c c o l l i s i o n between the gamma ray and an electron i n which the energy i s shared and the the energy of the emergent photon i s less than that of the incident photon. The energy of the incoming gamma ray i s so large compared to the binding energy of the c r y s t a l electrons that the electrons behave as i f they were unbound. The energy of the scattered gamma~ray i s given by: hu' = hu The cross section f o r Compton sca t t e r i n g was given by Klei n and Nishina (1929) as: a = 2TTT 2 c 1+a J2a(l+a) \ l+2a log(l+ 2 a ) (2-5) where: 1 i r-, o •> l+3a + _ l o g ( l + 2 a ) - T I T 2 S . ) 2 , -13 = c l a s s i c a l electron radius (2 .818 x 10 cm) h v m zl hv = energy of incoming gamma ray m = rest mass of electron o Compton sca t t e r i n g i s the dominant process i n germanium detectors f o r gamma rays i n the energy range 200 KeV to 6 MeV. The t h i r d e x c i t a t i o n process i s p a i r production. In t h i s process a gamma ray inte r a c t s with the Coulomb f i e l d of a nucleus and creates a positron-electron p a i r . In the center of mass coordinate frame the positron and electron conserve momentum by going o f f i n opposite d i r e c t i o n s . Conservation of energy then requires that each has k i n e t i c energy: K.E. +'-.= e 2 m c o (2-6) The. t o t a l p a i r production cross section, a , was given by Bethe and Bacher (1936) as: % Z 28 In 2h m c o 218 27 (2-7) where: c = 5.80 x 10" 2 8 cm2 o charge of nucleus assuming: m = mass of electron o 2 2 -1/3 m c « h v « 137 m c Z ' o o The positron and electron are quickly slowed down by c o l l i s i o n s with other electrons and the positron eventually annihilates with an electron i n the 2 c r y s t a l to produce two photons of energy moc (0.510 MeV). I f one of the an n i h i l a t i o n photons escapes from the c r y s t a l the t o t a l energy released 2 1 i n the c r y s t a l w i l l be hv - mQc . This process thus y i e l d s a peak (c a l l e d the s i n g l e escape peak) i n the detected energy spectrum 0.51 MeV below the f u l l energy peak. S i m i l a r i t y , i f both a n n i h i l a t i o n photons escape, the 2 energy given to the c r y s t a l i s h v ~ ^ m 0 c producing a double escape peak i n the energy spectrum 1.02 MeV below the f u l l energy peak. The p a i r production process i s possible only f or gamma rays above 1.02 MeV and i s the dominant process above s i x MeV i n germanium. The v a r i a t i o n of the t h e o r e t i c a l absorption c o e f f i c i e n t s with gamma ray energy i n germanium for ph o t o e l e c t r i c absorption, Compton sc a t t e r i n g , and p a i r production i s shown i n Fiqure 2-1. The three processes described f or the i n t e r a c t i o n of a gamma ray 1. but the other photon i s completely absorbed (say by a pho t o e l e c t r i c process) 10 Gamma-ray energy (MeV) Figure 2-1 Variation of Theoretical Absorption Coefficients with Energy. -7-with the c r y s t a l electrons are not exclusive since the degraded photon from a Compton process or an a n n i h i l a t i o n photon can further i n t e r a c t by another Compton or photoelectric process. More of the gamma ray energy i s given to the c r y s t a l electrons by these multiple processes and' therefore the number of counts i n the f u l l energy peak i s increased r e l a t i v e to the Compton background. The p r o b a b i l i t y f or occurrence of these multiple processes increases with c r y s t a l volume and therefore large detector volumes are very desirable. i i ) Conversion of Electron Energy to Ionized Charge The energetic electrons produced by a gamma ray lose energy by i n e l a s t i c c o l l i s i o n s with the bound electrons. The p r i n c i p a l way free charges are produced i s by the energetic electrons e x c i t i n g electrons from the valence band to the conduction band. For every electron excited a hole i s produced i n the valence band and the energy l o s t i s at least the band gap energy of 0.67 eV for germanium. Another way the energetic electrons can lose energy i s by in t e r a c t i n g with the c r y s t a l l a t t i c e . The int e r a c t i o n excites the l a t t i c e into an o p t i c a l mode o f ' v i b r a t i o n with the energy exchange being quantized and -3 characterized by the Raman frequency of the l a t t i c e (5 x 10 eV for germanium)« T h i r d l y , large numbers of very low energy electrons and holes remaining a f t e r the f i r s t two processes and not having s u f f i c i e n t energy to produce secondary i o n i z a t i o n , lose t h e i r energy by thermal c o l l i s i o n s with the l a t t i c e . Goulding (1965b) gave a diagramatic representation of the energy 2.. F. 5. Goulding, ul'RL-16231, 85 (1965a) -8-loss process which i s given i n Figure 2 - 2 . B. Use of a Semiconductor C r y s t a l as a Detector A large number' of electron-hole pairs are produced when a gamma ray int e r a c t s with a semiconductor c r y s t a l . I f an e l e c t r i c f i e l d i s applied to the c r y s t a l , the electrons and holes are separated and move to opposite electrodes. The charge which i s c o l l e c t e d i n the external c i r c u i t w i l l thus be proportional to the energy of the in t e r a c t i n g gamma ray. H h - ® n Ge with Electrodes Applied Unfortunately, i n normal germanium or s i l i c o n single c r y s t a l s i t i s very d i f f i c u l t to measure the charge flucuations due to gamma rays because' the leakage current i s very" high. It would, therefore, be necessary to cool the c r y s t a l to very low temperatures where the impurity c a r r i e r s become ina c t i v e and the leakage current low i n order to use normal germanium as a detector. ^ Another way :to reduce the leakage current i s to use a reverse biased n-p junction. The c a r r i e r depleted region at the junction could then be used as a detector. This type of detector (usually s i l i c o n ) i s widely used 3 . Such operation has, i n f a c t , been employed. The resol u t i o n obtained, however, was not as good as that possible by the following technique. < ELECTRON OR I [OLE ENERGY E Probability ( 1 - Probability r ELECTRON OR HOLE (l-p)(E-e) ELECTRON P(E- e < ))/2 3 ^ 0 OPTICAL PHONON LOSS These now become parents for future generation i f th e i r energy i s adequate for production of secondaries e K= Raman phonon energy for l a t t i c e e 3 = Band gap of material p = Assumes a random value i n the range 0 to 1 Figure 2 - 2 The Energy Loss Process -9-for the detection of charged p a r t i c l e s but, because of i t s small active volume, i t . i s not normally used f o r gamma rays. Before the development of lithium d r i f t detectors, however, Donovan, M i l l e r , and Foreman (1960) used a high r e s i s t i v i t y d i f f u s e d junction counter f o r the detection of 120 KeV gamma rays. Lithium d r i f t e d detectors also use a reverse biased junction but in between the p and n regions i s an impurity compensated region. MW/VVW—1 V The impurity compensated region can be very large. Early lithium d r i f t detectors were small s i l i c o n devices which gave a f u l l energy peak i n t e n s i t y which was only one per cent of the Compton edge i n t e n s i t y f o r 662 Kev gamma rays (Mayer, Bailey, and Dunlap 1960). Germanium i s preferable to s i l i c o n because of i t s greater atomic number and therefore larger cross section f o r gamma rays. Freck and Wakefield (1962) reported operation of a germanium detector 1.5 mm deep at a bias of 12 v o l t s . Since then many groups have progressively enlarged the active volume of detectors. Malm and Fowler (1966) have recently reported operation of a germanium lithium d r i f t coaxial 3 detector with an active volume of 54 cm at 600 vol t s bias. -10-C. Necessary Properties' of Germanium f o r Detectors To make good'detectors the germanium c r y s t a l used must have very high p u r i t y and few c r y s t a l ' f a u l t s because a long c a r r i e r l i f e t i m e i s desired and good compensation of impurities by lithium i s necessary. The c a r r i e r l i f e t i m e should be long because i t i s desired to have most of the'charge produced by the gamma ray c o l l e c t e d by the applied e l e c t r i c f i e l d and supplied to the a m p l i f i e r rather than l o s t by recombination processes within the c r y s t a l . Traps provide intermediate l e v e l s i n the energy band gap through which recombination and generation processes can take place. Traps are due to impurity centers or c r y s t a l imperfections (vacancies, d i s l o c a t i o n s , etc.) and may exhibit p r e f e r e n t i a l trapping properties f o r either electrons or holes thus i n h i b i t i n g c o l l e c t i o n of c a r r i e r s i n the detector. Goulding (1965c) gives a discussion of the e f f e c t s and causes of traps. He points out that heat treatments increase the number of traps and therefore should be kept to a minimum. Very high p u r i t y germanium i s necessary because, impurities l i k e oxygen can form complex' ions with lithium ( l i k e LiO +) which are less mobile + 9 than the L i ion. Oxygen concentrations greater than one part i n 10 greatly reduce the d r i f t rate of lithium (Goulding 1965d). Lithium can also p r e c i p i t a t e and lose i t s e l e c t r i c a l a c t i v i t y i f vacancies are present i n the c r y s t a l . The q u a l i t y of the i n i t i a l germanium c r y s t a l , therefore, determines to a great extent the q u a l i t y of the r e s u l t i n g detector. In order for s u f f i c i e n t l y low leakage currents to be r e a l i z e d , the conductivity of the material must be governed by the density of thermally excited c a r r i e r s rather than by impurity atoms. In other words, the conductivity must be i n t r i n s i c and not e x t r i n s i c at the operating temperature. In i n t r i n s i c material the thermally generated current density, J ^ , i s given by: J i = ^ V ^ h + V (2_8) -19 where: q = charge per electron (1.6 x 10 coul) n^ = i n t r i n s i c c a r r i e r concentration E = e l e c t r i c f i e l d y^ = hole mobility y g = electron mobility the dependence of n . , y^, and y g on temperature, T,was given by Conwell (1958) as: n.(T) - 1.76 x 1 0 1 6 • T 3 / 2 - e - 4 5 5 0 / T cm"3 (2-9) y h(T) - 1.05 x 10 9 • I " 2 , 3 3 cm 2/volt-sec (2-10) y (T) = 4.9 x 10 7 • T " 1 " 6 6 cm 2/volt-sec (2-11) where: T = absolute temperature (°K) The dependence of n^ on temperature i s shown i n Fiqure 2-3 and the dependence of y^ and y on temperature i s shown i n Fiqure 2-4. It i s necessary to operate germanium detectors at l i q u i d nitrogen o temperature (77 K) to keep the thermal generation current very low and thus contribute very l i t t l e to the t o t a l noise. At 77°K the i n t r i n s i c c a r r i e r density given by equation (2-9) i s : n.(77) = 2.5 x 1 0 - 7 cm"3 However, the highest p u r i t y germanium obtainable commercially has about TEMPERATURE (°K) Figure 2-3 Dependence of I n t r i n s i c C a r r i e r Density on Temperature I 1 1 J 1 1 i _ O .50 100 1510 ZOO 2.50 300 3SO TEMPERATURE (°K) Fiqure 2-4 Dependence of Electron and-Hole M o b i l i t i e s on Temperature 1 0 1 3 impurities per cm3 of a l l one type (acceptor or donor). At 77°K most of the impurities w i l l be ionized and therefore the c a r r i e r density due to impurities, nj.-mp» w i l l be: n. (77) = 1 0 1 3 cm - 3 imp v There i s a very great difference between n i(77) and n i mp(77). To reduce ,nj_mp'(77) compensation of the impurities i n the c r y s t a l i s done by introducing impurities of the opposite type into the c r y s t a l . I f there i s an exact balance of acceptor and donor impurities then the c a r r i e r concentration w i l l be the i n t r i n s i c c a r r i e r concentration and the conductivity w i l l be i n t r i n s i c . The electron and hole m o b i l i t i e s , however, w i l l not be the same as the i n t r i n s i c material since they are decreased by scattering of c a r r i e r s from the charged"impurities (Adler, Smith, and Longini 1964). Since the m o b i l i t i e s are decreased the charge c o l l e c t i o n time w i l l be increased by impurity s c a t t e r i n g . • I f the acceptor and donor impurities are paired impurity s c a t t e r i n g i s reduced. This ion p a i r i n g can be done with lithium ions by doing f i n a l compensation at a lower temperature where the L i + ion i s just mobile enough to be attracted by the acceptor impurity but not mobile enough to move -very f a r by d r i f t i n g . D. Lithium Ion D r i f t i n g To reduce the number of ionized' impurities i n germanium the technique of l i t h i u m ion d r i f t has been developed. A germanium c r y s t a l i s refined so that the major impurity i s " the'acceptor gallium. Lithium, which i s a donor impurity, i s used to compensate the gallium impurity and thus reduce the net impurity ion concentration. Lithium i s used because i t i s a very mobile -13-donor impurity with a low i o n i z a t i o n energy (0.0093 eV) i n germanium. The lithium i s u s u a l l y vacuum evaporated onto one face of the c r y s t a l which i s then heated to 450°C f o r f i v e minutes to allow the lithium to d i f f u s e into the c r y s t a l . The d i f f u s i o n ' o f impurities into" semiconductors i s discussed very completely by Warner and Fordemwalt (1965). A f t e r d i f f u s i o n the lithium concentration has the d i s t r i b u t i o n shown i n Fiqure 2-5 and the concentration i s given by: N. N «erfc o 2 / D T o (2-12) where: t D donor concentration at depth x from surface lithium surface concentration duration of d i f f u s i o n d i f f u s i o n constant Since the germanium c r y s t a l i s s l i g h t l y p-type and a n-type lithium layer has been formed on one surface, there i s a p-n junction at X q where: d a N = acceptor concentration i n c r y s t a l and x i s therefore obtained from: o N «erfc o o o (2-13) When t h i s p-n junction i s reverse biased the p o s i t i v e l y charged lithium ions move into the p side of the junction where they compensate the negatively charged acceptor ions by charge n e u t r a l i z a t i o n . Fiqure 2-6 shows the growth of the compensated region a f t e r a short period of d r i f t i n g . N After Diffusion Lithium donors Bulk acceptors i d N After D r i f t x = t d Figure 2-6 Growth of the Compensated Region -14-The compensation i s very exact since, f o r example, i f a p i l e up of donors occurs i n the compensated region t h e ' e l e c t r i c f i e l d gradient i s modified so as to dissipate'the excess concentration. Fiqure 2-7 shows the normal e l e c t r i c f i e l d d i s t r i b u t i o n and that r e s u l t i n g from an excess of ions. Following the arguments of Goulding (1965e) the rate of growth of the compensated ( i n t r i n s i c ) region can be estimated assuming that the current of l i t h i u m ions i s due e n t i r e l y to the e l e c t r i c f i e l d , that the d i f f u s i o n current i s n e g l i g i b l e , and that the lithium ions have already d r i f t e d a distance W. In the i n t r i n s i c region N = N, and the e l e c t r i c f i e l d i s V/W a d where V i s the applied voltage. The current of li t h i u m ions per square cm, J L , i s then given by: J L = y L-N a.V ( 2 . 1 4 ) where: y^ = mobility of lithium i o n s . i n the semiconductor at the d r i f t temperature The number of acceptors per unit area which can be compensated i n time dt i s therefore J T «dt and, since the acceptor concentration i n the compensated Li material i s N , the increase, AW, i n the thickness W of the i n t r i n s i c a ' ' layer i n time dt i s given by: Na-dW = V . y L- N a-dt (2-15) The d r i f t rate, dW , i s : dt dW V dt = W ' y L (2-16) Figure 2-7 Normal E l e c t r i c F i e l d D i s t r i b u t i o n -15-Therefore by i n t e g r a t i o n : W2 = 2 V* y • t Lt W = /2n • V- t (2-17) Li Note that the d r i f t rate i s independent of the r e s i s t i v i t y of the s t a r t i n g material but that i t i s increased by r a i s i n g the temperature (since the L i ion m o b i l i t y increases with temperature) or by increasing the applied voltage. The applied voltage i s l i m i t e d by surface breakdown and i s t y p i c a l l y 100 to 600 v o l t s . The d r i f t temperature has an upper l i m i t above which the material becomes i n t r i n s i c and the c r y s t a l ceases to behave as a diode. D r i f t temperatures well below the i n t r i n s i c temperature are preferred to avoid compensation by the lithium of the thermally generated c a r r i e r s . This overcompensation can be reduced by d r i f t i n g at lower temperatures at the end of the primary d r i f t . The thickness of the d r i f t e d region versus d r i f t time for various d r i f t temperatures i s shown i n Fiqure 2-8. E. Surface Problems The surfaces of the c r y s t a l containing the exposed junction are c r i t i c a l regions during d r i f t . Problems can develop at t h i s stage which can s e r i o u s l y a f f e c t the c h a r a c t e r i s t i c s of the f i n a l detector. Before d r i f t i n g the edges are etched to remove the surface and expose a clean surface. P r i o r to etching the c r y s t a l faces are taped with an etch 4 r e s i s t a n t tape to prevent etching of the lithium layer. During d r i f t i n g the c r y s t a l i s kept i n a dry i n e r t atmosphere to reduce the c o l l e c t i o n 4. Scotch brand #471 Fiqure 2-8 D r i f t e d Region Thickness versus Time f o r Various Temperatures -16-of impurities on the junction. I f impurities, such as water, do c o l l e c t they can a l t e r the e l e c t r i c a l properties s u f f i c i e n t l y such that the junction can be shorted and stop the lithium d r i f t i n g . Under such conditions the impurities can be removed by re-etching the edges a f t e r which d r i f t i n g w i l l normally resume. Sometimes the c r y s t a l stops d r i f t i n g and cannot be re-started by edge etching. Staining the edges by reverse biasing i n a copper sulphate sol u t i o n to reveal the junction w i l l then often show that somewhere the junction curves abruptly (usually at a corner). L i Surface ain This indicates that the junction has h i t a bad spot i n the c r y s t a l . A f t e r the bad spot has been sawn o f f and the edges re-etched d r i f t i n g can often be resumed. When d r i f t i n g i s completed and the c r y s t a l i s ready f o r mounting, the junction surface condition i s s t i l l very important because a stable, low conductance surface i s needed to keep the t o t a l leakage current low. The surface states which are formed can have a profound influence on the f i n a l c h a r a c t e r i s t i c s of the device. For a p-n junction the surface states can r e s u l t i n an inversion layer extending across the junction. The r e s u l t of t h i s i s to greatly increase the junction area and therefore the capacitance and reverse leakage current increase. Armantrout (1966) has studied the e f f e c t of surface states on lithium d r i f t germanium'detectors. The surface-states s e n s i t i v e to ambient conditions are present i n or on the outside of the oxide layer which forms on the germanium when i t . i s exposed to .the atmosphere. The e f f e c t of these states i s to a l t e r the energy band structure of the c r y s t a l by introducing energy le v e l s i n the surface layer. For example, oxygen may gain a negative charge and behave as an acceptor s i t e . To s a t i s f y charge n e u t r a l i t y a hole i s formed i n the germanium near the surface thus creating a p-type inversion layer. S i m i l a r l y , other absorbed atoms can r e s u l t i n a n-type or p-type inversion layer. Llacer (1964) has proposed a model which provides a good explanation of the r o l e of surface states i n determining the leakage and voltage breakdown c h a r a c t e r i s t i c s of a lithium d r i f t detector. He suggests that the surface states may cause an inversion layer which extends across the i n t r i n s i c region and overlaps the opposite junction. Undrifted Material High f i e l d s are present where the inversion layer overlaps the opposite junction. Zener breakdown can occur at low bias and then the surface becomes a conducting channel r e s u l t i n g i n high leakage current. The surface breakdown may occur i n i t i a l l y as large pulses which are s i m i l a r -18-to pulses from r a d i a t i o n . It i s , therefore, desirable to have as l i g h t an inversion layer on the surface as possible since the heavier the inversion layer the longer the surface channels. Llacer (1966) has pointed-out that i f ' d e t e c t o r s could be constructed with high e l e c t r i c f i e l d s normal to the junction at' the i-p junction then the resistance of the surface channel would be increased. At present, howeveri'the rbest surface treatment found i s a methyl alcohol r i n s e a f t e r a short etch. The detector i s then immediately put i n a good vacuum to maintain the condition of the surface. F. Resolution of Germanium Detectors The r e s o l u t i o n of lithium d r i f t germanium detectors i s an order of magnitude better than that of Nal s c i n t i l l a t i o n detectors. In f a c t , the ov e r a l l r e s o l u t i o n i s li m i t e d not only by the q u a l i t y of the detector but also by the noise and s t a b i l i t y c h a r a c t e r i s t i c s of the el e c t r o n i c equipment. The ultimate r e s o l u t i o n of germanium detectors can be estimated by considering the charge production processes which occur. I f a l l the energy l o s t by a gamma ray i n a detector was converted into i o n i z a t i o n the signals produced by monochromatic r a d i a t i o n would show n e g l i b l e f l u c u a t i o n . However, i f the energy was dissipated by thermal processes then normal s t a t i s i c a l f l u ctuations i n the number of electron-hole.pairs produced would be expected. In t h i s case, the RMS f l u c t u a t i o n , <n>, of the number of pairs would be given by: E < n ' _ 7 (2-18) where: E = energy l o s t i n the detector by the gamma ray -19-E = average energy required to produce an electron-hole p a i r = 2.94 eV per p a i r f o r germanium The RMS f l u c t u a t i o n i n energy. AE . would be: 6 7 ' rms' AE = <n>-e = /elf f2-191 rms •> The actual s t a t i s i c a l f l u c t u a t i o n i n germanium detectors i s between those characterizing pure i o n i z a t i o n and normal s t a t i s i c a l f l u c t u a t i o n . Fano (194T1) introduced the Fano factor as a convenient way of expressing t h i s s i t u a t i o n . It i s defined as: n 2 • f - (2-20) where: n = observed RMS f l u c t u a t i o n o n = normal s t a t i s i c a l f l u c t u a t i o n The f u l l width at h a l f maximum, AEp^^, of an energy peak i s the convenient measure of re s o l u t i o n f o r gamma ray spectra. The AEp^^ for a Gaussian d i s t r i b u t i o n i s rela t e d to the RMS f l u c t u a t i o n by: 1/2 AEFWHM = / 8 ^ r T " eC r = /8 In 2 • e( F E ) e 1/2 = 2.355 • (e E F ) 1 / 2 (2-21) Recently, Mann (1966) has measured the Fano factor f o r lithium d r i f t detectors as a function of the applied e l e c t r i c f i e l d . A dependence of the -20-Fano factor on the f i e l d was observed. For one detector he measured F = 0.16 - 0.01 and by extrapolating to i n f i n i t e e l e c t r i c f i e l d he i n f e r r e d that 0.05 < F < 0.10. Figure 2-9 shows the expected detector contribution to the t o t a l r e s o l u t i o n f o r the values F = 0.075 and F = 0.16. Table 2-1 gives the possible r e s o l u t i o n and the percentage r e s o l u t i o n f o r both F values at various energies. Use of very high e l e c t r i c f i e l d s i s l i m i t e d by de t e r i o r a t i o n of r e s o l u t i o n due to the increase i n leakage current with higher e l e c t r i c f i e l d s . Leakage currents above one nanoampere a f f e c t the re s o l u t i o n noticably. Besides the detector, the ele c t r o n i c s used for amplifying the c o l l e c t e d charge represents a s i g n i f i c a n t l i m i t a t i o n i n obtaining good re s o l u t i o n . At present the best low noise amplifiers have a noise fiqure of O-^S'KeV plus "06 KeV per pF where the capacitance i s the t o t a l input capacitance. The detector acts as a capacitor with capacity given by: 4TT W (2-22) where: for germanium: K = d i e l e c t r i c constant of material A = area of the detector W = depletion layer thickness Det 1.37 f pF (2-23) Goulding (1965f) has presented a very good discussion of noise i n a recent paper. The s t a b i l i t y of the e l e c t r o n i c system also becomes important since 0 1 2 3 4 5 6 7 8 9 1 0 ENERGY (MeV) Fiqure 2-9 Resolution versus Energy f o r Various Fano Factors Energy of Gamma Ray • Fano Factor = 0 .075 Fano Factor = 0.16 , Resolution (KeV) FWHM . % Resolution' . Resolution (KeV) Resolution. 250 KeV 0.54 0.22 0.79 0.32 500 KeV 0.76 0.15 1.11 0.22 750 KeV 0.93 0.12 1.36 0.18 1.00 MeV 1.08 0.11 1.58 0.16 1.50 MeV 1.32 • 0.088 1.93 0.13 2.00 MeV 1.53 0.076 2.23 0.11 2.50 MeV 1.71 0.068 2.49 0.10 3.00 MeV 1.87 0.062 2.73 0.091 4.00 MeV 2.16 0.054 3.15 0.073 5.00 MeV 2.41 0.048 3.52 0,070 • 6.00 MeV 2.64 0.044 3.86 0,064 7.00 MeV 2^86 0.041 4.17 0,060 8.00 MeV 3.05 0.038 4.46 0.056 9.00 3.24 0.036 4.73 0.053 10.00 3.41 0.034 4.98 0,050 Table 2-1 Resolution f o r Fano Factors of 0.075 and 0.16 - 2 1 -gain s h i f t s of 0 .1 % s e r i o u s l y a f f e c t the re s o l u t i o n . The use of gain s t a b i l i z a t i o n over the whole system can help reduce the gain s h i f t s of the e l e c t r o n i c equipment. -22-CHAPTER 3 EXPERIMENTAL EQUIPMENT The experimental equipment developed f o r producing detectors was ; a l i t h i u m evaporation system, d r i f t units and power supplies f o r d r i f t i n g c r y s t a l s , a d r i f t current c o n t r o l l e r , and two s t y l e s of detector holder. A. Lithium Evaporation Equipment The evaporation and d i f f u s i o n of lithium was done i n a CVE-15 vacuum system. The evaporation assembly i s shown i n Fiqure 3-la. Lithium was heated i n a tantalum evaporation boat of the shape shown i n Fiqure 3-lb. Vhe d e f l e c t o r above the boat directed most of the lithium downwards onto the c r y s t a l . Around the boat was a s t a i n l e s s s t e e l s h i e l d to keep most of the vacuum system clean during lithium evaporation. A hole i n the bottom of the s h i e l d allowed the lithium to be transmitted to the c r y s t a l . A movable s t a i n l e s s s t e e l f l a g between the boat,and the c r y s t a l prevented contaminants released during the i n i t i a l heating of the boat from depositing on the c r y s t a l . The c r y s t a l was mounted on a four inch diameter by one quarter inch thick graphite block which was used as the d i f f u s i o n heater. A f t e r evaporation, the lithium was allowed to d i f f u s e into the germanium by heating the block to.450°C using a current of 150 amperes at 4 v o l t s . B. D r i f t i n g Units The d r i f t i n g of lithium through the germanium c r y s t a l s was performed Figure 3-1 (a) Evaporation Assembly Figure 3-1 (b) Evaporation Boat -23-with the c r y s t a l mounted on a temperature co n t r o l l e d p l a t e . The d r i f t unit design i s based on a system described by Goulding and Hansen (1964). A picture of one of the d r i f t units i s shown i n Fiqure 3-2a. The c r y s t a l was placed on a chrome plated copper block which was mounted, with f i v e other u n i t s , on a r e f r i g e r a t e d (-10°C) copper plate to provide a heat leak of twenty watts from the plate on which the c r y s t a l was placed. The d r i f t unit was heated with a 120 ohm 11 watt power r e s i s t o r mounted underneath the upper plate. The high voltage d r i f t bias was applied to the c r y s t a l by a spring contact made of phosphor bronze and insulated from" the rest of the assembly. The d r i f t unit was e l e c t r i c a l l y insulated from the r e f r i g e r a t e d plate with a t h i n sheet of mylar ( s i l i c o n e thermal grease on both sides) so that the d r i f t - current passed through' the- d r i f t ' current"control to ground. The -temperature of the upper' plate-was' monitored with a resistance thermometer. Each' d r i f t unit was covered with an inverted 1000 ml glass beaker which' was f i l l e d with nitrogen during -'drift" to i n h i b i t contamination of the c r y s t a l . In Fiqure 3-2b three of the complete d r i f t units are shown. This type of d r i f t " unit has :proven"very'convenient and r e l i a b l e f o r the d r i f t i n g of c r y s t a l s . C. Power Supply The power supply.used to provide'the -high: voltage bias f o r d r i f t i n g i s s i m i l a r to that used by Hansen and J a r r e t t (1964). The c i r c u i t f o r the power supply i s shown i n Fiqure 3-3. It can supply up to 100 ma at 1000 v o l t s . A large variable r e s i s t o r i s placed i n series with the output to l i m i t the power output. The l i m i t i s set with the c r y s t a l d r i f t i n g by adjusting the var i a b l e r e s i s t o r u n t i l the output voltage i s h a l f of the r e c t i f i e d voltage. F i g u r e 3-2 (b) 1000 Volt - 100 ma Power Supply Figure 3-3 Power Supply C i r c u i t (from Hansen and J a r r e t t (1964)). -24-D. D r i f t Current C o n t r o l l e r A current c o n t r o l l e r was designed to maintain'the d r i f t current through a c r y s t a l to a preset value. The d r i f t current was co n t r o l l e d by suit a b l y heating or cooling the plate on-which the c r y s t a l was. mounted. The c i r c u i t i s based oh'that rof Hansen and J a r r e t t (1964) and i s shown i n Fiqure 3-4. The c o n t r o l l e r measured the voltage across a r e s i s t o r i n series with the d r i f t current and varied the power sup-lied to a heater r e s i s t o r underneath the d r i f t plate so as to maintain t h i s voltage constant. The d r i f t current l e v e l was set by changing the r e s i s t o r ' i n s e r i e s with the current. The lower the resistance the higher the the d r i f t current since the c o n t r o l l e r maintains the voltage across the r e s i s t o r at the value -7.5 v o l t s . Table 3^1 gives the value of t h e - r e s i s t o r for the various d r i f t currents. A d i f f e r e n t i a l a m p l i f i e r compared the voltage across the r e s i s t o r to a set l e v e l . Following the amp l i f i e r , the c i r c u i t r y d i f f e r s s i g n i f i c a n t l y from that described by Hansen and J a r r e t t (1964). In the UBC system, the output of the ampli f i e r c o n t r o l l e d the frequency of a unijunction t r a n s i s t o r o s c i l l a t o r . The o s c i l l a t o r i n turn triggered a s i l i c o n c o n t r o l l e d r e c t i f i e r (SCR) which was i n series with the r e s i s t o r under the d r i f t p l a t e . The frequency of the o s c i l l a t o r when the d r i f t current was balanced was less than s i x t y cycles per second so that the SCR was only on during part of the r e c t i f y i n g cycle. The o s c i l l a t o r c o n t r o l l e d the power to the heater since the higher the o s c i l l a t o r frequency the more often the r e c t i f i e r was on and supplying power to the heater. The maximum temperature of the d r i f t plate could be set by adjusting the maximum o s c i l l a t o r frequency with 110 V A.C. +10 V D.C. DRIFT CURRENT INPUT • W \ / V •AAAA^-^*--\A/\/\r^ -AAAA/ -^ -* -AAAA/— 5 < < >^3.9K ? > 1.5K 2N2925 A A A A f -^7—vAAA>-^Sv\AAA-| •&9—'XAAA -S^A/\/\/v4 CURRENT DEMAND SWITCH . -\AAyV 200K | V N I2N2925 15K £5.6K < • f J4.7K 1.5K GROUINID Figure 3-4 U.B.C. Current C o n t r o l l e r Drift Current Resistances Used (ohms) Total Resistance (ohms) (mA) 5 .100- + 150 + 1100 • • 1350 10 100- + 390 + 220 710 15 100* + 390 490 20 100* + 150 +47+82 379 25 100* + 150 + 47 297 30 100* + 150 250 40 ' 100* + 27 + 27 + 39' 193 50 100* +27+27 152 60 • 100* + 27 127 70 100* +12 112 80 100* 100 * - 5 watt resistor Table 3-1 Values of resistance used for various drift currents. -25-the v a r i a b l e r e s i s t o r (see Fiqure 3-4). If the d r i f t current was too low the d i f f e r e n t i a l a m p l i f i e r output would t r i g g e r the o s c i l l a t o r so that the r e c t i f i e r was on most of the time and therefore the d r i f t plate would be heating. On the other hand, i f the d r i f t current was too high the o s c i l l a t o r would be o f f and the d r i f t p l ate would therefore cool. I f the d r i f t current was at the selected value the o s c i l l a t o r would t r i g g e r j u s t enough to maintain the temperature. The advantages of t h i s c o n t r o l l e r were that the heating element was a passive element, a power r e s i s t o r , the maximum temperature was adjustable, and the control was continuous from f u l l on to o f f . E. Detector Holders The detector holders used for mounting the f i n i s h e d detectors were of two types s i m i l a r to those described by Miner (1965). The f i r s t type, Figure 3-5a, was used f o r detector t e s t i n g and for counting with radioactive sources. It was convenient f o r t e s t i n g d r i f t e d c r y s t a l s as i t was small thus making i t easy to pump, cool, and keep clean. It was also convenient when used with radioactive sources which could be mounted e a s i l y under the holder. The second holder, Fiqure 3-5b and Fiqure 3-6,was more suited to studies with the UBC Van de Graaff accelerator as the detector was mounted h o r i z o n t a l l y . Both holders use a Linde CR-10 l i q u i d nitrogen dewar which normally retains coolant f o r about a week. A one l i t e r per second Vaclon pump, Varian Model No. 913-0008, was used to maintain a pressure of around 2 x 10 ^ t o r r i n the holder. E l e c t r i c a l connections to the preamplifier were made by i n s e r t i n g Figure 3*5 (a) V e r t i c a l Detector Holder Figure 3-5 ( b ) H o r i z o n t a l Detector Holder Fiqure 3-6 Detector Holder -26-a lead through an i n s u l a t i n g Kovar connector on the detector holder. When the f i e l d - e f f e c t t r a n s i s t o r a m p l i f i e r was used a four p i n Kovar connector was used to make e l e c t r i c a l connections. F. Detector Preamplifier The preamplifier used i n i t i a l l y was-the Ortec Model 203-101XL, which used vacuum tubes as the active elements; This was later.replaced by a preamplifier u t i l i z i n g a cooled f i e l d - e f f e c t - t r a n s i s t o r (Goulding 1966). The c i r c u i t , i l l u s t r a t e d i n Fiqure-3-7,- was constructed by D. Dalby and was used because of i t s improved noise c h a r a c t e r i s t i c s . With the FET preamp the signal from-the detector went to a 2N3823 FET which was mounted inside , r the holder on a 'stainless s t e e l screw two cm fromthe cold f i n g e r . The detector was ;insulated e l e c t r i c a l l y from the cold finger by using a t h i n sheet of nylon between: two sheets of indium with films of high vacuum grease between sheets. 2N3823 s. 4 Bottom ^- 24 * 12V are 1 % p r e c i s i o n r e s i s t o r s - 24V Fiqure 3 - 7 Low Noise F i e l d - E f f e c t T r a n s i s t o r Preamplifier CHAPTER 4 DETECTOR FABRICATION PROCEDURE The following procedure was used to prepare the lithium d r i f t detectors. The starying material was.a zone-levelled; gallium doped germanium ingot of 5.5 ohm-cm r e s i s t i v i t y and s i x t y microsecond c a r r i e r l i f e t i m e . ^ ' ^ The ingot was about 15 cm i n length with a cross section as shown: actual s i z e For s l i c i n g , the ingot was mounted on a graphite block with Apiezon black wax and cut on a,Micrq-Mech"precision diamond saw to the desired thickness. The graphite block provided a shock mount f o r the c r y s t a l during cutting. The maximum cut depthper saw blade pass was prevented from exceeding s i x mm to reduce c r y s t a l damage-" during c u t t i n g . The c r y s t a l was cut to a thickness of from s i x to ten mm and then lapped on both faces with 800 g r i t alumina grinding powder to remove c r y s t a l damage from sawing. The c r y s t a l was then cleaned with TCE (trichloroethylene) and methyl alcohol using Johnson cotton Q-Dabs f o r wiping the c r y s t a l . 5. from Sylvania E l e c t r i c Products, Towanda, Perin. 6. excellent material (20 ohm-cm and 250 micrsec l i f e t i m e ) was obtained from Socie'te' Ge'ne'rale Me"tallurique de Hoboken, Brussels, Belgium -28-The c r y s t a l was then painted on the edges and one face with Aquadag ( c o l l o i d a l graphite i n water). This treatment i n h i b i t e d lithium d i f f u s i n g into the edges and also helped to produce good thermal contact with the heater on the bottom. When the Aquadag was dry the c r y s t a l was ready for the evaporation of lithium onto the unpainted face. A piece of lithium metal about 1.5 cm x 0.5 cm x 0.3 cm was cut, washed with TCE, blown dry with nitrogen, and" placed'in the tantalum evaporation boat'in the vacuum'system. The-stainless "steel s h i e l d was put around the boat and the c r y s t a l was placed on the graphite heater under the hole i n the s h i e l d . The system was then evacuated to a pressure of'around 2 x 10 ^  t o r r . The current supply to the evaporation boat was-turned"on' and rai s e d slowly until'140 amperes (7 amperes onr the panel meter) was flowing through the boat. When the lithium deposited on ;the f l a g turned from black to grey the f l a g was moved aside and lithium was deposited onto the c r y s t a l f o r one minute. The evaporation b e l l j a r was closed off"from the'pumping system and the heater power of 600 watts turned'on. When'the thermocouple attached to the heater read 200°C, nitrogen was l e t into'the b e l l j a r . The heater power, was adjusted to give a temperature of 450^C for'seven minutes and then turned o f f . Figure 4-1 shows a t y p i c a l temperature versus time curve. This procedure gave a di f f u s e d lithium layer about.0.5 mm deep. When the crystal'had cooled, the excess lithium was washed o f f with d i s t i l l e d water and the Aquadag was removed. The c r y s t a l was given a f i f t e e n second etch*which was quenched w i t h - d i s t i l l e d water. The r e s i s t i v i t y of the lithium surface was measured-with a-fouT point probe'. I f the r e s i s t i v i t y of the.surface was above 0.2 ohm-cm"the lithium-had not dif f u s e d c o r r e c t l y 50 0-i 1 1 1 | I i 1 1 1 T-I 1 1 1 1 !-] " 1 r b 5 10 15 20 TIME (minutes) Fiqure 4 -1 Lithium d i f f u s i o n Heating Cycle -29-and the lithium evaporation procedure was repeated. I f the surface r e s i s t i v i t y was acceptable the c r y s t a l was etched for a minute i n an etch s o l u t i o n of 3:1 HNO^:-HF/'-with,'a/vsm'al':l.--amoimt.of red fuming HNO^ added to speed the-start • of'the' etch. The^-eteh-was quenched with deionized d i s t i l l e d water and then the crystal-was rinsed with methyl alcohol and b1own • dry wi th n i t rogen. Indium gallium eutectic was-applied-to the -crystal faces and the c r y s t a l was-placed l i t h i u m side-down-on-a d r i f t " unit.' A f t e r the cover was put on, the d r i f t unit was f i l l e d with nitrogen; heated to 30°C, and 100 v o l t s reverse bias applied. A f t e r one hour.the'drift voltage was increased to around 500 v o l t s and the" current"demand switch adjusted (usually 15 ma) to give" a d r i f t " temperature.around 35°C. During d r i f t the current demand" was -gradually increased to maintain a d r i f t temperature above' 30°C. I f the diode junction broke down (characterized "by an adrupt" f a l l in" temperature^) - i t was removed and given a surface treatment as' follows. A f t e r removing •  the indium" gallium eutectic with" a Q-Bud wetted with TGE, the faces were taped with #471 Scotch tape and the c r y s t a l given a one minute etch a f t e r which i t was quenched with methyl alcohol. A f t e r blowing the c r y s t a l dry with nitrogen and removing the tape the c r y s t a l was replaced on the d r i f t unit and the same procedure followed as for a new c r y s t a l . I f the c r y s t a l s t i l l would not d r i f t i t was removed from the d r i f t unit" and the" d r i f t e d region was made v i s i b l e - b y reverse biasing the c r y s t a l i n a weak copper" sulphate: s o l u t i o n (20 grams "per l i t e r of water). The d r i f t e d region" does not stain' where" asT* the -undrifted' region i s stained a brown colour". I f the junction was d i s t o r t e d at some point, the d i s t o r t e d area was sawn o f f with the diamond-saw. The; sawn surface was lapped, the -30-faces taped, the c r y s t a l etched for one minute, quenched i n methyl alcohol, and then replaced on the d r i f t u n i t . 7 I f the c r y s t a l s t i l l would not d r i f t i t was given a " f r o s t t e s t " . This consisted of placing the c r y s t a l i n a clamp-(insulated so reverse bias could be applied), immersing i t completely i n l i q u i d nitrogen f o r seven seconds, blowing on i t to form' a" layer of frost", then reverse biasing to a current of' s i x t y ma;" If" the frost~:me'lted"preferentially i n one area i t indicated that the'current was flowing across t h e " c r y s t a l in that area predominantly." This area was then'removed by" sawing and the same procedure followed as i n the previous paragraph -to put the c r y s t a l back on the d r i f t u n i t . 'When the width of the d r i f t e d "region"was estimated using Fiqure 2-8 to be about 1 mm from the undrifted face the c r y s t a l was removed and stained. I f the c r y s t a l was not the desired depth' the etch treatment descibed above was performed and the c r y s t a l replaced on the d r i f t unit for continued d r i f t i n g . I f , however, the c r y s t a l had d r i f t e d the desired depth i t was given a surface treatment' and replaced on the d r i f t e r f o r twelve hours at low current demand (5 ma) and 500 v o l t s . The temperature characterizing these-conditions was about -5°C. This p o s t d r i f f w a s done' to l e t the lithium concentration' adjust to the reduced generation current and to l e t the lithium ions p a i r with''the' acceptor impurities. To test the d r i f t e d c r y s t a l " t h e gate-and drain leads of the FET in the holder were shorted : so that the-leakage current versus voltage character-i s t i c of the c r y s t a l could be measured. The c r y s t a l was given a one minute etch : (faces taped) a f t e r which the c r y s t a l was quickly moved to a beaker of 7. E. Kashy and M. Rickey,Rev. S c i . Instr. 35, 1364 (1964). -31-methyl alcohol f o r f i f t e e n seconds and 'then-blown.dry. It was immediately mounted i n a holder with the" lithium side upwards and a small amount of eutectic placed"on~ the lithium" side- where--the; a m p l i f i e r contact pressed on the crystal". The holder" was; evacuated" and,- vwhen: the pressure was 2.x 10 ^  t o r r , l i q u i d nitrogen"was ; put i n -the~;co!d" finger- o f the holder. The- lekage'current versus "^voltage" c h a r a c t e r i s t i c " was measured using the' arrangement-shown" i n Fiqure"'4-1.' Usually several, treatments of the sort described above were required before"the ^leakage current"was below ten nanoamps' at the-operating-voltage"(about" 75 v o l t s per mm). When t h i s was achieved "the" gate-drain shorting'wire was removed and the holder re^evacuated, cooled and the Vaclon pump- valve opened-'and the~roughing valve closed. The a m p l i f i e r was turned on, detector" bias voltage' applied, and the detector 57 137 was tested f o r r e s o l u t i o n using Co and Cs radioactive sources. The f a b r i c a t i o n procedure was now compTeted"-and' the' detect or" ready. for operation. P r e c i s i o n High Voltage Supply 100 Meg VTVM (100 Meg Input Resistance) 1 v o l t = 20 nA L i face of Detector Figure 4-2 Arrangement f o r measuring detector leakage current. - 32 -CHAPTER 5 - PRESENTATION OF RESULTS A. S i l i c o n Detector Fabrication P r i o r to making germanium detectors several s i l i c o n l i t h i u m d r i f t detectors were produced. It was found r e l a t i v e l y easy to f a b r i c a t e s i l i c o n detectors so they served the purpose of t e s t i n g the d i f f u s i o n and d r i f t i n g apparatus f o r f a u l t s before t r y i n g to prepare germanium detectors. The procedure followed f o r the f a b r i c a t i o n of s i l i c o n detectors was s i m i l a r to that of Lothrop and Smith (1965) . The procedure d i f f e r e d i n several respects from that used f o r the preparation"of: germanium detectors. The d i f f u s i o n temperature was 350°C and the d i f f u s i o n time was two minutes. The l i t h i u m d r i f t i n g was done on a plate as described by Lothrop and Smith (1965) except that the heating was performed using the d r i f t c o n t r o l l e r described in Chapter 4. T y p i c a l d r i f t temperatures were 110°C and the d r i f t current was four milliamperes at 500 v o l t s . The surface treatment was the same as used by Lothrop and Smith. A t y p i c a l cross section of the compensated region i s shown i n the following diagram. / — L i + diffused layer Depletion region - 33 -The silicon detectors produced by this means had an active volume of about 0.7 cm . (12 mm. in diameter by 2 mm. deep). The drifting rate of lithium in silicon is approximately half Of; that in germanium so silicon detectors are rarely made with a compensated depth greater than 5 mm. 137 A Cs gamma ray spectrum taken with a silicon detector at 77°K with a FET preamplifier is shown in Figure 5-1. The counting rate in the f u l l energy (661 keV) channel relative to that of a channel in the Compton background is 4%. The resolution of the peak is 4.8 KeV FWHM ( f u l l width at half maximum). Because of their low gamma ray detection efficiency, silicon devices are rarely used as gamma spectrometers. For detection of charged particles, on the other hand, they are widely used because of their easy fabrication, stable characteristics, and, most important, their high resolution performance, even when operated at room temperature. B. Germanium Detector Production The f i n a l quality of a detector is very dependent on the characteristics of the germanium ingot from which i t is fabricated. Except for the last one described, a l l detectors mentioned in this thesis were produced from the same ingot which was obtained from Sylvania Electric Products. I n i t i a l measurements on a detector made from one end of this ingot gave moderately good results. Two more detectors were fabricated, with d i f f i c u l t y , from the same end of the ingot but, despite very careful techniques, the remaining two thirds of the ingot produced no useable detect o CO CC I j j o O CD -Jr. •r > '-!-!• ~!;, -j. T. „L-t 'L " J . -in!- -ir ••rf-ir^-i-T 20.0QQ OQO 30..oc;n 55.000 yo..ooo , CHRNNEL NUMBER ( X 1 0 1 IJ5. oon fjO.OGQ 5 5 . O V Q F i g u r e 5-1 Cs Gamma Ray S p e c t r u m w i t h S i l i c o n D e t e c t o r - 34 -During the l i t h i u m d r i f t stage many etching and sawing operations were required and, although a d r i f t depth of 5 mm. was obtained, the c r y s t a l s s t i l l did not make usable detectors because of high leakage current. Repeating surface treatments, sawing o f f various areas, and r e d i f f u s i n g l i t h i u m did not reduce the leakage currentto a useable l e v e l . A f t e r several months of negative r e s u l t s a new ingot was obtained from Hoboken of Belgium which produced a very good detector with the f i r s t s l i c e . One c h a r a c t e r i s t i c of the ingot which would seem to be important i s the i n i t i a l r e s i s t i v i t y . The Sylvania ingot had a r e s i s t i v i t y of 5 ohm-cm while the Hoboken ingot had a r e s i s t i v i t y of 20 ohm-cm. The f i r s t germanium detector produced from the Sylvania ingot had an i n s t r i n s i c r e s o l u t i o n of 4.0 KeV f o r 661 KeV gamma rays at an operating voltage of 175 v o l t s . The depletion depth was about 4 mm. 3 giving an active volume of approximately 1 cm . Unfortunately t h i s detector was damaged when the vacuum pump f o r the detector holder f a i l e d and became contaminated. At the time i t was not f e a s i b l e to store the detector at l i q u i d nitrogen temperatures while the pump was repaired so the detector was destroyed. The second detector had a depletion depth of 5 mm. and 3 an active volume of about 0.5 cm . The energy r e s o l u t i o n at 661 KeV was 4.0 KeV ( i n t r i n i s i c r e s o l u t i o n 1.9 KeV) at a bias voltage of 200 v o l t s . The t h i r d detector also had a depletion depth of 5 mm. 3 but the area was l a r g e r giving an active volume of 2.0 cm . At a bias voltage of 225 v o l t s i t gave an energy res o l u t i o n of 5.1 KeV (4.5 KeV i n t r i n s i c resolution) f o r 661 KeV gamma rays. - 35 -The fourth and best detector, which was prepared from the 3 Hoboken ingot, had an active volume of 1.7 cm and a depletion depth of 5 mm. The energy resolution obtained with this detector is summarized in the following table. Gamma Ray Energy Total Resolution (FET preamp) Intrinsic Resolution 122 KeV 2.5 KeV 1.3 Kev 136 KeV 2. 6 KeV 1.5 KeV 661 KeV 2.9 KeV 2.0 KeV 1332 KeV 4.1 KeV 3.5 KeV 2614 KeV 5.2 KeV 4.9 KeV 137 Figures 5-2 and 5-3 show similar Cs spectra taken with detectors #2 and #4. The higher resolution of detector #4 compared to the other detectors is clearly seen by comparing the two figures. The improved resolution of the fourth detector probably resulted from the use of a better germanium ingot. o o o o T 0 L J O 0c-> —Ico" CO . - J o CD o o o it-., CD V W H --J- -J. Hi^-r-r-ii-i - i ' 1 ' \ — , _ " 500.000 575;D00 650.0OD 725.000 C H R N N E E N U M B E R I r j — i ^ - r r - H - i i - H | — — 800.000 875.000 950.000 10£5.00 13? Ficrure 5-2 Cs Spectrum w i t h D e t e c t o r II- 2 o i n " en o O 7 0 (JO _JP7" CO • .. v > — > o D o u_ 01 51 .. no-2: O C J > CD H vO VD •:-J-3 2 U . 0 0 0 " i r: 1 1 M p y . 0 0 0 5 2 1 J . 0 0 0 6 2 A 1 . 0 0 0 7 2 4 . 0 0 0 C H R N N E L N U M B E R 8 2 U . 0 O O 9 2 ' J . O O O 1 0 2 1 . 0 0 0 F i g u r e - 36 -C. Detector Operation When using germanium detectors i t i s important to optimize the detector r e s o l u t i o n . The detector r e s o l u t i o n i s dependent on the detector bias voltage. Figure 5-4 shows a t y p i c a l leakage current versus bias voltage graph.. The best operating voltage i s a l i t t l e below the voltage where the leakage current increases r a p i d l y . Figure 5-5 0_'.v-137 gives the shape of a 661 KeV (Cs ))gamma ray peak f o r various bias voltages obtained with detector #2. Below 200 v o l t s the resol u t i o n increased as the voltage was increased because the charge c o l l e c t i o n e f f i c i e n c y increases with e l e c t r i c f i e l d . The peaks are very asymmetric with a t a i l on the low energy side due to incomplete charge c o l l e c t i o n f o r some of the gamma rays. Above 200 v o l t s , on the other hand, the peak broadens r a p i d l y due to a rapid increase i n the leakage current. The peak i s symmetric but much wider. The optimum bias voltage i s , therefore, r e l a t i v e l y e a s i l y found by increasing the bias voltage u n t i l the asymmetry i n the spectrum peaks disappears and the peak becomes symmetric but does not widen. With good .detectors the leakage current remains low at high e l e c t r i c f i e l d s and therefore the operating voltage i s not as c r i t i c a l ( e l e c t r i c f i e l d s are t y p i c a l l y 100 v o l t s per mm.). For high energy gamma rays (greater than 3 MeV), however, c a r e f u l optimization of the bias voltage can increase the resol u t i o n considerably. M -6 10 to ft e ^ l C f ' EH W K O 10 10 -9 10 -10 100 200 300 400 • BIAS VOLTAGE (volts) 500 600 700 Figure 5-4 Typical Leakage Current versus Bias Voltage CHANNEL NUMBER F i g u r e 5 - 5 P e a k S h a p e f o r V a r i o u s B i a s V o l t a g e s - 37 -The replacement of the Ortec 101XL preamplifier with the cooled FET preamplifier reduced the electronic noise contribution from 3.2 KeV to 2.1 KeV. A single differentiation time constant of l.O t^sec was found to yield the lowest noise. The gamma ray spectra shown in Figures 5-6 to 5-10 were obtained using detector #4 with an FET preamp and illustrate 57 the high resolution achievable with germanium detectors. The Co spectra of Figure 5-6 shows the resolution obtained at low energy. The separation of the two peaks is 14 KeV and the resolution is 2.5 KeV. At low energy most of the peak width is due to electronic noise. 134 154 The Cs spectra of figures 5-7 and 5-8 and the Eu spectrum of Figure 5-9 vividly illustrate the value of high resolution in the separation and measurement of complex gamma ray spectra. Figure 5-8 illustrates the region from channel 318 to 332 of Figure 5-7 with the background subtracted. The two gamma rays, which are 6.7 KeV apart, are clearly resolved. In Figure 5-9 the many gamma rays of the Eu^*^ are clearly shown. The f u l l energy peak (2614 KeV), the f i r s t escape (2103 KeV), and the double escape peak (1592 KeV) of the RdTh spectrum are shown in Figure 5-10. The number of counts in the double escape peak (with background subtracted) is 30 times the number in the f u l l energy peak. For gamma ray energies above 2 MeV the double escape peak is the dominant peak and the f u l l energy and single escape peaks give a o o o o • —. , , 1 1 1 ° 0 . 0 0 3 0 0 ' 6 . D O 9 . 0 0 1 2 . 0 0 1 5 . 0 0 C H A N N E L N U M B E R [ X I O A ] F i g u r e 5-6 Co Spectrum o o IT.' ' O O CD CD -Jo?" cc LLJ CO O CO o o CD > > > 0) <D 0) WWW o I N .4-e e • f A O M A ^ D V O O .1-> 0> W o o > w co J. J-l- J-V - . " -!-i i r : 1 n 184.000 304.000 M24.000 544.000 664.000 C H A N N E L N U M B E R 1.367 KeV + 122 KeV > o W CO VO j -.1-A-r5ii. • * -l--H-iJr-H- -J- - i --:-:-:--;-!-f?-?-w-j- -:-.>--i-:-r -ir-;;- i-<;-H-;-;-T •-:-*r'K-;-:-!ffi-;-784.000 904.000 10211,000 1 7>4 Figure 5-7 Cs J Spectrum f NUMBER OF COUNTS (x 10 ) i -.000 NUMu i . ooc OF COUNTS (LOG) 2.000 • 3.000 y.oco 5.000 H* " ro VJl i < c h-' VJl -p-CD o 3 a fog-' i i -3^5 KeV 782 KeV •967 KeV •1092 KeV •1118 KeV o O O 1416 KeV V M CJ> 00 CD < 3 H ru -p-CD < **1 £ o 1 I—1 o & P" CO CD O c+ 3 - . G O O •J ZD CT: ' ' CD O o CD N U M B E R O F C O U N T S ( L O G ) JUL) ,40-"3 0 fi 0 0 0 J.OOi "1592 KeV double escape 2 1 0 3 KeV single escape - 2 6 1 4 KeV • f u l l energy - 38 -convenient check of the energy c a l i b r a t i o n of the analyzer since the energy difference between the peaks i s 511 KeV. In Figure 5-11 the square of the re s o l u t i o n i s plotted as a function of gamma ray energy f o r detector #4. A crude estimate of the Fano f a c t o r obtained from t h i s data i s 0.5 which i s considerably higher than that measured by Mann (1966). The large Fano f a c t o r obtained here i s at l e a s t p a r t i a l l y a t t r i b u t a b l e to a r t i f i c i a l broadening of the peaks due to e l e c t r o n i c i n s t a b i l i t y and d r i f t i n g since the counting rates were f a i r l y high and gain s t a b i l i z a t i o n was not used. Preliminary e f f i c i e n c y measurements were made with detector #4 at d i f f e r e n t energies. The r e s u l t s are tabulated i n Table 5-1. Fow low energy gamma rays the e f f i c i e n c y i s high but i t f a l l s o f f r a p i d l y so that above 2.5 MeV the f u l l energy peak e f f i c i e n c y i s l e s s than 0.1%. The double escape peak e f f i c i e n c y , however, f o r gamma ray energies above 2 MeV remains almost a constant at a value of 0.3% f o r t h i s detector. The l i n e a r i t y of the ampli f i e r (Ortec 201 Multi-Mode) and the pulse height analyser (Nuclear Data ND-160) were measured 134 154 using Cs and Eu spectra (figures 5-7 and 5-9). The peak positions were calculated and, using the tabulated energies f o r the gamma rays, a l e a s t squares s t r a i g h t l i n e f i t of the gamma ray energy as a function of channel p o s i t i o n was performed. The r e s u l t s are presented i n 134 154 Table 5-2. In Both the Cs and Eu spectra the deviation from a s t r a i g h t l i n e was le s s than the quoted accuracy of the gamma ray Table 5-1 E f E (MeV) Strength Distance #of nr's through ttCounted E f f . Source ~ Detector 1) C o 6 0 1.172 4913/iCi 14.5 cm 1.2 x 10 7 4.0 x 10 4 .32% 60 ' 7 4 Co 1.332 4913^01 14.5 cm 1.2 x 10 3.6 x 10 .29% 2) RdTh 2.614 6.6^Ci 7 cm 7.2 x 10 6 6.2 x 10 2 .08% D.E. 1.592 6.65^Ci 7cm 7.2 x l O 6 2.2 x l O 3 .31% Table 5-2a L i n e a r i t y f o r Eu Channel P o s i t i o n Energy Energy Calculatedfrom Energy Difference Straight Line F i t •  86.9 245.0 KeV 243.6 KeV - 1.4 KeV 161.4 345 0 KeV 345 5 KeV + 0.5 KeV 210.0 412 0 KeV 412 0 KeV 0.0 KeV 234.0 445 0 KeV 445 2 KeV + 0.2 KeV 481.1 7132 0 KeV 783 1 KeV + 1.1 KeV 617.2 969 0 KeV 969 3 KeV + 0.3 KeV 706. 8 10'32 0 KeV 1032 0 KeV 0.0 KeV 726.0 1118 0 KeV 1118 2 KeV + 0.2 KeV 942.9 1416. 0 KeV 1415 1 KeV - 0.9 KeV Energy = 124 KeV + 1.368 KeV/channel Table 5-2b L i n e a r i t y of Cs Spectrum Channel P o s i t i o n Energy. . Energy .Calculated Energy. Di f f e r e n c e 322.8 563.0 KeV 563.3 KeV + 0.3 327.3 569.7 KeV 569.4 KeV - 0.3 353.1 605.4 KeV 604.7 KeV - 0.7 671.5 1039.0 KeV 1039.9 KeV . + 0.9 766.2 1168.0 KeV 1169.3 KeV + 1.3 910.5 .: 1368.0 KeV 1366.5 KeV - 1.5 Energy = 122 KeV + 1.367 KeV/channel. - 39 -energies and the accuracy of the peak determination. From t h i s data the i n t e g r a l l i n e a r i t y of the system was estimated to be better than 0.2% over the channel range 100 to 1000 channels. - 40 -CHAPTER SIX - CONCLUSIONS The f a b r i c a t i o n of germanium l i t h i u m d r i f t detectors, although b a s i c a l l y a simple process, requires considerable care and cl e a n l i n e s s to produce high q u a l i t y gamma ray spectrometers. The major problems i n f a b r i c a t i o n are the i n i t i a l q u a l i t y of the material and the c o n t r o l of the surface states. I f the o r i g i n a l germanium ingot is^damaged or of poor q u a l i t y considerable e f f o r t can be wasted t r y i n g to make detectors from.it. C o n t r o l l i n g the surface state of the exposed compensated region i s a poorly understood technique. The leakage current of the detector i s determined p r i m a r i l y by the surface leakage. I f a better wiy was found to c o n t r o l and maintain neutral surface states, such as a protective Sip l a y e r , the applied e l e c t r i c f i e l d could be increased. The lise of higher e l e c t r i c f i e l d s could increase the charge c o l l e c t i o n e f f i c i e n c y , and thus improve the energy r e s o l u t i o n of the detector. Development of lower noise and higher s t a b i l i t y e l e c t r o n i c s w i l l also improve the t o t a l r e s o l u t i o n . Optimization of FET preamplifiers and developments i n parametric amplifiers should reduce the present e l e c t r o n i c noise. S t a t i s t i c a l processes, however, w i l l l i m i t the ultimate system r e s o l u t i o n to around 1 KeV at 1 MeV f o r germanium detectors (see Figure 2-9). Other semiconductor materials characterized by smaller energy gaps, such as GaAs may give better r e s o l u t i o n but, at present, such materials of s u f f i c i e n t p u r i t y are not a v a i l a b l e . The e f f i c i e n c y of germanium detectors i s l i m i t e d by the -.41 -active volume of the detector. Detector e f f i c i e n c y should gradually improve as higher q u a l i t y germanium and better f a b r i c a t i o n techniques (such as co a x i a l and two way d r i f t i n g ) make l a r g e r active volumes easier to produce and techniques f o r stacking of several detectors are improved. Because of the superior r e s o l u t i o n and high e f f i c i e n c y of germanium l i t h i u m d r i f t detectors they are r a p i d l y replacing other detectors as the major experimental instrument f o r gamma ray spectrometry. - 42 -APPENDIX A. Recipe f o r etch: Mix the following ingredients i n the following order: 7 l b s . of concentrated n i t r i c acid (70%). 2 l b s . of 48% hydrof l u o r i c acid. \ l b . of red fuming n i t r i c acid. B. Ga-In E u t e c t i c : Prepare with 12% by weight of gallium. C. Etch r e s i s t a n t tape: Scotch #471, 3 M Company, ( l o c a l supplier, Black Bros.). - 43 -BIBLIOGRAPHY Adler, R.B.,,. Smith.,..A.,C.._..and..l.onpjin.iR.L., .Introduction to Semiconductor  Physics, (John, Wiley 8 Sons-', Inc., New York, 1964), .Vol.1. Sec. 1, p.50. Alexande , T.K. and Allen, K.W. , Can.. J. Phys. 43_, 1563 (1965). Aleaander, T.K., :Ldtherland, A.E.- and Broude, C. ,. Can:. J.. Phys. 43, 2310 (1965). Armantrout:,. G. IEEE Trans. Nucl. Sci., Vol. NS-I3, No.l 85 (1966). Bardin, T.T.,.. Barrett, R., Cohen, R.C. , Devons, S., H i t l i n , D., Macagno, E.R., Sabat, C.N.,. Rainwater, .J. , Runge, K. and-Wu, CS. , Columbia , University Progress Report (1966), Columbia University Preprint (1966). Bethe, H.A. and Bacher, ,R.F;, Rev.Mod.Phys., 8_, 82 (1936). Conwell, P r o c IRE 46, 1281 (1958). Donovan, P.F.•., Miller., G.L..• and Foreman, B.M., Bull. Amer. Phys. Soc., Ser. I l l , .5, -355 .(I960). . Fano, U., Phys. Rev. 72, 26 (1947). Freck, D.V. and .Wakefield, J., Nature, 193, 669 (1962). Freedman, M.S;Wa\gner, F. j r . , Porter, F.T. and Boloton, H.H., Phys. Rev. 146, 791 (1966). • Goulding, F.S. a) Nucl. Instr. and Meth. 43_, 26 (1966)'.' b) 'Nucl.-Instr.. and Meth. 43, 27 (1966). c) Nucl. Instr'.' and Meth. 43, 8 (1966). d) . Nucl. Instr. and Meth. 43_, 20 (1966). e) Nucl. Instr.'and Meth. 43", 19 (1966). f) Nucl. Instr. and Meth. 43, 28 (1966).. Goulding, F.S., private communication (1966). Goulding, F.S. arid .Hansen, W.L.,. UCRL-11261, 2 (1964). ' Hall, H. , Rev. Mod. Phys. 8_, 35.8:('l936). Hansen, W.L. 'and Jarrett,, B.V. , UCRL-11589, Fig. 3 (1964). - 44 -Hughes, L.B. , .Kennett-', T.J. and Prestwich, W.V., and Wall, B.J. Can. J. Phys., 44, 919 (1966). Kashy, E. and Rickey, .M., Rev. Sci. Instr. 35, 1364 (1964). Klein, 0 and Nishina,- Y., Z. Physik. 52_, 853 (1929). Llacer, J.,. IEEE Trans. Nucl. Sci., Vol. NS-I1, No.3,. 221 (1964). Llacer, J. IEEE Trans. Nucl. Sci., Vol. NS-13, No.l, 93 (1966). Lothrop, R.P. and Smith,.H.E., •UCRL-16190, (1965). Malm, H.L. and Fowler, I.L, IEEE Trans. Nucl. Sci. Vol... NS-13, No.l, 62 (1966). Mann, H.M., Bull. Amer. Phys. Soc, 11, 127 (1966). Mayer, J.W., Bailey, N.A. arid Dunlap, H.L.y Eroc. of. Conf. on Nuclear • Electronics, Ser II, 5_, 355 (I960). Miner, C.E., UCRLr11946, 21. (1965). Warner, R.M. and Fordenwalt, Integrated Circuits (McGraw-Hill Book Co., • New York, 1965), Vol.1, Chap. ,3, p.72. 

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