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Characterization of epitaxial semiconductor films by scanning tunneling microscopy at ambient pressure Pinnington, Thomas H. 1992

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CHARACTERIZATION OF EPITAXIAL SEMICONDUCTOR FILMS BYSCANNING TUNNELING MICROSCOPY AT AMBIENT PRESSUREbyTHOMAS HENRY PINNINGTONB.Sc. University of Alberta, 1990A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF APPLIED SCIENCEinTHE FACULTY OF GRADUATE STUDIESDEPARTMENT OF ENGINEERING PHYSICSWe accept this thesis as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIADecember 1992© Thomas Henry Pinnington, 1992In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.c ne.r.--,-1^gi c--CDepartment ofThe University of British ColumbiaVancouver, CanadaDate^1->_.c.--^I / 19 2--DE-6 (2/88)ABSTRACTEpitaxial layers grown by molecular beam epitaxy on both silicon and galliumarsenide substrates are exposed by cleaving and studied by scanning tunneling microscopy(STM) at ambient pressure. The cleaved surfaces are prepared for imaging by wet chemicaltreatments. GaAs/AlGaAs and Si/SiGe multilayer structures are imaged with —1 nmresolution. The contrast in the STM images is believed to be topographic in origin,resulting either from selective etching during surface preparation or strain-relaxation at thecleaved surface. Layers of alternating dopant-type are also resolved. In particular,alternately doped layers on GaAs are imaged by STM for the first time in air, and the pnjunctions located to within 20 nm. The apparent topographic contrast in these images isexplained in terms of an electronic contrast mechanism associated with the carrier type.Current-voltage (IV) characteristics of the STM tunnel junction are obtained atselected locations in the epitaxial layers. IV curves acquired over n- and p-type regionsresemble the IV characteristics of Schottky barrier diodes. A new imaging method isproposed which exploits the contrast between the n- and p-type IV curves. Imaging in thismode is achieved by repetitively interrupting the constant-current feedback loop during thescan and recording the tunnel current at a new tip-sample voltage setting, preselected toyield high conductivity-type contrast. This method, which decouples the electronic contrastassociated with the carrier type from the topography, is demonstrated for an npn structureon GaAs.Modification of the treated GaAs and Si surfaces is observed during imaging, and isattributed to a chemical change in the surface enhanced by the tunneling process.Photoemission spectroscopy of the treated GaAs surface indicates that the treatment inhibitsoxidation, possibly by formation of a thin sulfur passivation layer. The photoemissionresults also suggest that selective removal of the arsenic atoms occurs during the treatment,which may also help to produce a more stable surface.TABLE OF CONTENTSAbstract ^ .Table of Contents ^List of FiguresAcknowledgements ix1. Introduction ^  12. Scanning Tunneling Microscope ^  52.1. Description ^ 52.2. Fabrication of Tunneling Probes ^ 62.3. Calibration of STM 82.3a. Lateral Calibration ^ 92.3b. Vertical Calibration  112.3c. Comparison with Theoretical Model ^ 143. Surface Preparation ^  153.1 Imaging on Chemically Treated Surfaces  153.2. Silicon  163.2a Surface Preparation Procedure ^ 163.2b. Tunneling-Induced Surface Modification 203.3. Gallium Arsenide ^ 233.3a. Imaging on Unpassivated GaAs Surfaces - Oxide DesorptionStudy ^ 233.3b. Surface Passivation Procedure ^ 273.3c. Tunneling-Induced Surface Modification ^ 293.3d. Photoemission Spectroscopy of Chemically Treated (110)GaAs ^ 314. Tip Effects 374.1. Tip Instabilities ^ 374.2. Tip Images 394.3. Multiple Tips 405. Crossectional Imaging of Epitaxial Structures ^ 445.1. Positioning of the Tunneling Probe 445.2. Imaging of Si/Ge Multilayers 455.4. Imaging of Epitaxial GaAs/AlGaAs Multilayers ^ 505.5. Carrier-type Contrast in Images of GaAs np Structures ^ 555.6. SEM Studies of GaAs Epilayer Structures ^ 586. Model for Current-Voltage Characteristics ^ 617. Two-Dimensional Characterization of Electronic Structure 677.1 Electronic Characterization Methods 677.2. Dependence on Tip-Sample Separation ^ 687.3. Dependence on Carrier Type ^ 717.4. Spatial Resolution ^  738. Conclusions and Recommendations  81References ^ 82Appendix. Current-Voltage Measurement Procedure ^ 85ivLIST OF FIGURESFig. 2.1. Free section of scanning tube, showing electrode configuration. 5Fig. 2.2. SEM micrographs of probe tips produced by mechanical^7shearing and by electrochemical etching.Fig. 2.3. Atomic resolution STM image of graphite.^ 10Fig. 2.4. STM image of 2iim x 2gm via contact hole on TiN film.^10Fig. 2.5. Hysteresis loop of vertical piezo tube deflection. ^11Fig. 2.6. Topographic scan line over a via contact hole. 12Fig. 2.7. Stepped terraces on graphite.^ 13Fig. 2.8. Topographic scan lines obtained at atomic steps on graphite. ^13Fig. 3.1. STM image of freshly cleaved, untreated (110) Si.^17Fig. 3.2. Image of Si surface etched in HF.^ 17Fig. 3.3. Image acquired on a cleaved Si surface after etching in NH4F. ^19Fig. 3.4. Image on Si following a 'refresh' treatment, 24 hours after the ^19inital cleave.Fig. 3.5. Squares on treated p-type Si produced by scans at successively ^21larger ranges.Fig. 3.6. Sideways 'H' pattern written on treated p-type Si.^21Fig. 3.7. Surface roughening on n-type Si, induced during three^22successive scans.Fig. 3.8. STM images of a polished (100) GaAs substrate and a substrate 25following oxide desorption.Fig. 3.9. Three-dimensional renderings of images of polished and ^26desorbed surfaces.Fig. 3.10. STM image of a (110) n-type GaAs surface passivated in P2S5 28solution.Fig. 3.11. Tunneling-induced roughening on treated n-type GaAs. ^28Fig. 3.12. Preferential roughening of n-type GaAs on an npn structure.^30Fig. 3.13. XPS at the binding energy of the As2p core level for cleaved ^32(110) GaAs surfaces.Fig. 3.14. XPS results for the Ga2p core level for (110) GaAs.^32Fig. 3.15. Raw data from PES measurements of the As3d core level on^35(110) GaAs.Fig. 3.16. PES of the Ga3d core level.^ 35Fig. 4.1 Effect of mechanical tip stability on image quality. ^38Fig. 4.2. Double-tip image of a GaAs/AlGaAs multilayer structure.^41Fig. 4.3. Change in tip geometry midway through a scan on a ^41GaAs/AlGaAs multilayer structure.Fig. 4.4. Change in tip structure between two successive scans of the ^42same portion of a GaAs/AlGaAs superlattice.Fig. 5.1. Orientation of scanning probe and cleaved surface for^44crossectional measurements.Fig. 5.2. Field emission SEM micrograph of a group of 20 Si/Ge ^46superlattices.Fig. 5.3. STM image of the epilayer containing the Si/Ge multilayers. ^47Fig. 5.4. Topographic scan line across the Si/Ge multiLayers.^47Fig. 5.5. Higher magnification STM image of a portion of the Si/Ge^48multilayer structure.Fig. 5.6. STM image of Si/SiGe multilayers and topographic scan line^49across the layers.Fig. 5.7. Topographic contrast at the GaAs/AlGaAs interface.^51Fig. 5.8. Cleavage steps on the (110) GaAs surface, oriented in the (111) 52direction.Fig. 5.9. STM image of a group of n and p layers grown on a ^52semi-insulating GaAs substrate.Fig.5.10. High resolution image of a 16 period GaAs/AlGaAs multilayer 54structure, and topographic scan line across the multilayers.viFig. 5.11. Three-dimensional rendering of a pnp structure on GaAs,^55showing apparent topographic contrast of about 5A.Fig. 5.12. Energy band diagram showing conduction mechanisms over n- 57type material at the scanning set point voltage.Fig. 5.13. Comparison of high resolution SEM and STM images of the^59same GaAs/AlGaAs superlattice.Fig. 5.14. SEM micrograph of epilayer containing a 16-period multilayer 60structure.Fig. 6.1 Energy band diagrams for a metal surface and an n-type ^62semiconductor surface.Fig. 6.2. Band-bending in the semiconductor induced by the tip. Zero bias 62case.Fig. 6.3. Voltage drops in the vacuum gap and semiconductor with the tip 63positively biased.Fig.7.1. Current-voltage characteristics on n-type Si and n-type GaAs^69obtained in air at various tip-sample separations.Fig. 7.2. Reverse break down in tunnel junction IV characteristic for the ^70case of small tip-sample separation.Fig. 7.3. Reverse break down for the case of a large tip-sample^70separation.Fig. 7.4. IV characteristics obtained over n- and p-type regions on Si and 72GaAs.Fig. 7.5. Series of IV curves obtained across a Si/Ge superlattice^73multilayer structure.Fig. 7.6. Series of IV characteristics and IVI scan taken across an n-p^75junction on Si.Fig. 7.7. Lateral band-bending over a p-layer in close proximity with an ^76n-region.Fig. 7.8. IVI scan and topographic scan line across an npn structure on ^78GaAs.Fig. 7.9 Constant current images obtained before and after the acquisition 79of IV characteristics.Fig. A.1. Voltage waveforms in the IV and IVI measurements.^86viiFig. A.2. Detailed schematic of the sample voltage waveform during an ^87IV measurement, showing the various time delays.ACKNOWLEDGEMENTSI would first like to express my thanks and gratitude to Dr. Tom Tiedje, whoseinsightful comments, enthusiastic assistance, and infectious optimism made this workpossible and enjoyable. I gratefully acknowledge the contributions of my collaborators, inparticular Steve Patitsas for stimulating theoretical discussions, Aaron Sanderson for hisinvaluable assistance in developing the current-voltage measurement technique, andDr. T. P. Pearsall, who suggested the Si/Ge STM study, for his encouragement andsupport. The cooperation and assistance of Christian Lavoie and Shane Johnson, whogrew the GaAs samples for these studies, is greatly appreciated. I also thank Dr.P. C. Wong and Tony Van Buuren for performing XPS and synchrotron PESmeasurements respectively, and also for helping me to interpret the data. I am indebted toDr. J. A. Dagata, whose surface preparation technique has made the GaAs measurementspossible, for his helpful advice and encouragement. The Si samples used in theseexperiments were supplied by Dr. D. C. Houghton, Dr. J. P. Noel, and Greg Mattiussi,whose interest and cooperation are gratefully acknowledged.Finally I thank Jim Mackenzie and Shane Johnson fo'r help with the figures, andYuan Gao for typing in references.ix1. IntroductionThe improved performance associated with the miniaturization of electron deviceshas motivated the evolution of characteristic semiconductor device geometries to submicronscales. In the case of integrated circuit technology, the reduction in electron transit times asdevice density is increased results in faster operation. Such devices are produced bythermal diffusion or ion-implantation/anneal, of dopant atoms, producing three-dimensionalstructures of varying dopant type and concentration.1,2 In the case of quantum welldevices, such as quantum well lasers and photodetectors, efficiency is enhanced aselectrons are confined to smaller regions.3,4 Confinement in one dimension can beachieved by growing alternating layers of lattice-matched materials of different bandgaps,such as GaAs and AlGaAs, by molecular beam epitaxy.5 Device structures are fabricatedon semiconductor substrates and the relevant geometries extend perpendicular to the planeof the surface. High resolution characterization methods are needed to monitor productionof devices and to permit meaningful comparisons between actual device behaviour andcomputer simulations based on device geometry.6 In particular, dopant concentration andchemical composition both need to be measured as a function of position in the fabricatedstuctures. Also, interdiffusion of chemical species, and defects in the crystal structure suchas dislocations arising from lattice mismatch strain relaxation must be monitored, as thesedegrade the quality of the interfaces between regions of differing composition or dopingand can dramatically affect performance.Scanning tunneling microscopy7 (STM) which is capable of atomic resolutionimaging on semiconductor surfaces8,9 and is sensitive to spatial variations in electronicproperties 10,11, is a promising candidate for semiconductor device characterization. InSTM, a sharp metal probe is scanned across the sample surface. The probe is held at afixed potential with respect to the sample, and the distance between the tip of the probe andthe sample surface is kept small enough (typically less than 20 A) to permit quantum1mechanical tunneling of electrons to occur between the tip and sample. This is achievedusing feedback control techniques12, typically by adjusting the tip-sample separation tomaintain a constant current of tunneling electrons. An image is acquired by monitoring therelative change in tip-sample separation as a function of location of the scanning probe.This image corresponds to the surface topography if the surface is electronicallyhomogeneous, although more generally both electronic and topographic information isacquired simultaneously, making data interpretation more complicated. Purely electronicinformation can be extracted through analysis of current-voltage characteristics of the tip-sample tunnel junction.9,13 These are acquired by interrupting the scan and disabling theconstant current feedback loop, and then monitoring the tunnel current as the tip-samplebias is varied. Alternatively a small, high frequency voltage modulation signal, outside thebandwidth of the feedback loop, can be added to the tip-sample bias while scanning and thecorresponding small-signal current modulation recorded.Methods involving STM as described above have been successfully applied to thecharacterization of semiconductor devices. 14 In ultrahigh vacuum (UHV), STM has beenused to examine GaAs/AlGaAs interfaces with atomic resolution15. Recently, GaAs layersof alternating doping type were resolved in UHV.10 In these experiments, a piece of thewafer containing the device structures was cleaved in situ, permitting measurements to bemade on the structures, which were exposed in crossection. Measurements on Si have alsobeen made, although the sample is usually cleaved outside of the UHV chamber andchemically treated before imaging.16 This is because it is difficult to produce a smoothcleave on Si, and anyway the electrical properties of the clean Si surface do not make itamenable to STM measurements of carrier-type.10,16 Similar techniques have been used inambient pressure STM, where chemical treatments are used to inhibit oxidation of thecleaved surfaces. Measurements on both Si 16 and more recently GaAs 17 structures havebeen made, although not with atomic resolution. Sensitivity to carrier type and carrierconcentration has been demonstrated both in air18 and in UHV16 STM.2Traditional device characterization methods complement the information providedby STM, but typically can only achieve high spatial resolution in one dimension.Photoluminescence, for example, has been used to determine the overall thickness andinterface quality of electron confinement layers.19 Capacitance-voltage (C-V)measurements19,20 yield carrier concentration as a function of depth below the substratesurface, based on the change in capacitance between a surface contact and the sample withchanging voltage. Secondary ion mass spectroscopy (SIMS), in which material is removedfrom the sample by an ion beam, measures the concentration of dopant atoms directly (asopposed to carrier concentration).20 By taking measurements at several locations andcomparing the data with computer simulations, SIMS and C-V data have been used toconstruct two-dimensional dopant and carrier-concentration profiles indirectly, with100 nm resolution.6,14The combined sensitivity to sub-nanometer topographic contrast and capability ofextracting quantitative electronic information, is an advantage of STM methods over otherelectron microscopy techniques. Scanning electron microscopy (SEM) for example, hasbeen used to locate pn junctions using the electron beam induced current (EBIC) method. 14The high energy electron beam creates electron-hole pairs as it scans the surface, and theexcess minority carriers drift under the action of the local field in the pn junction, producingthe EBIC signal which is detected in an external circuit. The resolution of the pn junctionlocation by this method is limited to the width of the space-charge region.6 Field emissionSEM micrographs, as presented in this work, are sensitive to conductivity type, but it is notpossible to differentiate between electronic and topographic contrast in the micrographs.Also, SEM is not sensitive to topography of less than a few nanometers. Transmissionelectron microscopy (TEM) in which an electron beam is transmitted through a thinnedsample, provides two-dimensional information with high spatial resolution, but themeasurement represents an average over several hundred atomic planes, due to the finite3thickness of the sample. Furthermore, dopant profiling requires the use of selectiveetchants or stains,21 and TEM sample preparation times are much longer than for STM.In this work, ambient pressure STM as a tool for semiconductor devicecharacterization is investigated. The test structures consist of epitaxial layers of alternatingchemical composition and dopant type, grown by solid source MBE on Si and GaAssubstrates. These are imaged in cleaved-crossection following wet chemical treatments ofthe freshly cleaved surfaces to permit stable imaging at ambient pressure. The STM used inthese experiments is described in Chapter 2. Results obtained on untreated surfaces arepresented in Chapter 3, accompanied by a description and evaluation of the proceduresdeveloped to prepare the samples for imaging. Measurement artifacts associated with non-idealities of the scanning probe are discussed in Chapter 4 to permit more meaningfulinterpretation of the crossectional STM measurements. The results of these measurementsare presented in Chapter 5, along with a discussion of the electronic and topographiccontrast mechanisms present in the images. A theoretical description of conduction acrossthe STM tunnel junction is given in Chapter 6. In Chapter 7, current-voltage characteristicsof the tunnel junction obtained at selected locations in the epitaxial layers are presented, andused to explain the electronic contrast observed in the constant current images. Thecurrent-voltage measurements are accompanied by results acquired using a new imagingtechnique, developed to decouple the electronic contrast from the topographic contrast.4TO PREAMPLIFIERINPUTPIEZO TUBEPROBE TIP-Y2. Scanning Tunneling Microscope2.1. DescriptionThe scanning tunneling microscope used in these experiments uses a piezoelectricscanning device under integral feedback control of the tunneling current.The scanning device consists of a platinum-30%iridium tunneling probeconcentrically mounted to the end of a 10 mm diameter PZT-5H (lead zirconate titanate)piezoelectric tube, the other end of which is rigidly fixed. The free section of the tube,illustrated in Fig. 2.1, is 20.4 mm long and the tube wall thickness is 0.84 mm. A5Fig. 2.1. Free section of scanning tube, showing electrode configuration.ceramic (Macor) disc at the free end and another in the middle secure the probe andelectrically isolate it from the tube wall. Four high voltage electrodes on the outer wall ofthe tube are appropriately biased, up to a maximum of 206V with respect to the groundedinner wall electrode, to deflect the tube. Positive and negative bias on an electroderespectively expands and contracts that portion of the tube. Common mode voltagelengthens or shortens the tube. Scanning is accomplished by biasing opposing pairs ofelectrodes with voltage ramps of opposite phase.The sample to be scanned is mounted on a horizontal cantilevered hinge plate. Aspring-screw mechanism at the free end of the plate controls the deflection angle, allowingfine vertical positioning of the sample (-1/160 inch per turn midway along the hingeplate). This entire mechanism is itself mounted on a micrometer stage, which enables thesample to be brought to within a micron of the probe tip, at which point the piezoelectrictube can bring the tip into tunneling range.The preamplifier for the tunneling current signal is an op amp (OPA111 low noiseamplifier) current-to-voltage conversion circuit. The feedback loop consists of a 1 MQresistor in parallel with a 1 pF capacitor. This configuration provides 1 mV/nA gain overa 5 kHz bandwidth. The probe is connected directly to the inverting input of the op ampwith a short wire (about 2 cm) to minimize input capacitance. The probe tip is biased withrespect to the chassis ground by applying the tip voltage to the non-inverting input.2.2. Fabrication of Tunneling ProbesThe tunneling probes are made either by cutting 0.25 mm Ptar wire with scissorsor by electrochemically etching the wire in a saturated NaC1 solution using the 'drop-offmethod22. In the electrochemical etching an ac voltage is applied between the Pt/Ir probewire and a tungsten electrode. One end (-2 cm long) of the probe wire is inserted into anylon sheath to protect it from etching. The wire is suspended vertically near the surface of6(a)7(b)Fig. 2.2. SEM micrographsof probe tips produced bymechanical shearing (a) and (b),and by electrochemical etching (c).The length of the dotted line inlower right gives the scale.The dark grey film on the endof the etched tip is carbondeposited by the electron beam.(c)the salt solution so that the shielded end is fully immersed and only a small portion(-1 mm) of the exposed wire is below the meniscus. The etching voltage is high enough(-50 V) so that occassional sparking occurs at the Pt/Ir wire electrode, but is reduced toabout 10 V before drop-off occurs to prevent annealing of the tip by the electrochemicalcurrent. The shielded end of the wire, which drops off, is removed from the nylon sheath,rinsed in an ultrasound methanol bath, and installed in the STM so that the etched end is theprobe tip. The other end of the wire, which is connected to the voltage supply, continuesto etch after drop-off resulting in a blunt tip which is not useful as a tunneling probe.Fig. 2.2 shows SEM micrographs of both etched and cut probe tips. Mechanicalshearing produces irregular probe tips (a) and (b), with many sharp points, whereaselectrochemical etching results in more regular tips (c). Higher resolution micrographs ofthe tips indicated that a typical tip radius is about 20 nm for probes fabricated by bothmethods, although much larger radii (>100 nm) for etched tips result when the ac etchingvoltage is not reduced sufficiently before drop-off, as discussed above. Atomic resolutionimages were acquired on (100)-oriented graphite surfaces (in air) using both etched and cutprobes.2.3. Calibration of STMThe image acquisition software assumes a linear relationship between the the tubedeflection and the voltage applied to the electrodes. Measurements were performed toestablish appropriate deflection/voltage conversion factors, however nonlinearities in theactual piezo tube response result in increasingly large deflections per volt with increasingtube deflection. Systematic errors associated with this nonlinearity, which is about 20%for lateral and vertical deflections of 1 jim, result in distortion of images with largetopographic or lateral range. This distortion is minimized by ensuring that the dc8component of the electrode voltages is kept small (by careful mechanical positioning of theprobe) during image acquisition.2.3a. Lateral CalibrationThe scan range of the STM on small scales was calibrated using atomic resolutionimages of (100) graphite (ZYB monochromator calibration samples). Resolution of thehexagonal lattice (Fig. 2.3) was achieved at small values of tip bias (+0.01 V typically)and large tunneling current set points (about 5 nA). The centres of the hexagonal ringshave a much lower electron density than the atomic cores, and appear black in the image.Three locations in the hexagonal lattice appear bright. These locations may correspond tolocations of high electron density at the Fermi level. In many images, only these threepoints were resolved in each lattice. The spacing between the hexagon centres is knownfrom x-ray diffraction to be 2.46 A 23. From these measurements, the lateral deflection ofthe scanner is found to be 12 nm/volt.A sample consisting of arrays of regularly spaced holes in a film of TiN, evaporatedon a silicon substrate, was used for large scale calibration. The sample was provided byNorthern Telecom to study the quality of electrical contact holes made by photolithography.Fig. 2.4 is of a 2.0 pm x 2.0 pm square hole. These large scale scans are prone toconsiderable distortion due to nonlinearities in the piezoelectric deflection of the scanner. Itappears that the image is compressed in the y-direction. Based on this image, the deflectionis found to be roughly 14 nm/volt in the x-direction and 19 nm/volt in the y-direction.In the images presented in this work, the lateral scale given is based on aconversion factor of 12 nm/volt. As discussed above, this linear conversion is only validfor small scan ranges (less than 1 p.m).9Fig. Atomic resolution STM image of graphite. The spacing between hexagon centres is 2.46A.Fig. 2.4. STM image of 2 pm x 2 pm via contact hole on TIN film. The horizontal streaking is due tomanual adjustments in the dc level of the common mode electrode voltage, to compensate for creep in thepiezo tube (the signal was direct-coupled to the image acquisition system).102.3b. Vertical CalibrationThe vertical deflection of the scanning tube was calibrated using an optical positionsensing device, which had 0.15 pm precision.24 To reflect the sensing laser beam, acorner reflecter was mounted on the end of the tube, which was oriented vertically for thesemeasurements. The vertical deflection as a function of common-mode electrode voltage isplotted in Fig. 2.5 for one series of measurements in which the maximum tube extensionwas 1 pm. The uncertainty in the calibration factor associated with the hysteresis of thepiezoelectric is about 20% for these large-scale deflections. Based on an average of least-squares fits to data obtained for positive and negative deflections up to 1 p.m, thecalibration factor was determined to be 50±5 kvolt (random error only).The topographic scan line in Fig. 2.6 is taken across an image of a 2 pm x 2 pmTiN contact hole, obtained using an electrochemically etched probe which was sharp1110.^40^80^120^160^200common mode voltage (V)Fig. 2.5. Hysteresis loop of vertical deflection of the piezo tube, with applied common mode electrodevoltage. Each deflection measurement was made 5 seconds after resetting the voltage.1000800600nm400200enough to image the bottom of the hole. The depth of the hole, based on the calibrationfactor above, is approximately 850 nm (±10%), about 15% less than the nominal value of1 Jim .An attempt was made to obtain a small-scale calibration based on scan lines acrossatomic steps on the graphite samples. This technique is used in UHV STM, for exampleusing atomic steps on silicon surfaces.25 Fig. 2.7 shows an air STM image of steps onthe (100) graphite surface. A scan line across these steps, as well as two others acrossdifferent steps on the same surface, appear in Fig. 2.8. The spacing of the atomic planesis known to be 3.5 A from X-ray measurements.23 Although the smallest steps observedwere roughly 4 A in height, step height was found to be an essentially continuous quantity(ie. not an integer multiple of 3.5 A). Before imaging, the top few layers of graphite arepeeled away to expose a fresh surface. It is likely that this procedure results in terraceswhich have been partially lifted off, so that the height of the terraces is ill-defined at theterrace edges (steps). This interpretation is consistent with the result that the height (basedon 50 A/volt) of the smallest steps agree with the known atomic spacing.120^200 400 600 800 1000nmFig. 2.6. Topographic scan line over a via contact hole. The nominal depth was 1 pm and the dimensionshown (850nm) is based on the calibration factor determined from Fig. 2.5.Fig. 2.7. Stepped terraces on graphite. The terraces are atomically flat.50^100^150nm13200^2506040A20 -200^400^600^800^1000nmFig. 2.8. Topographic scan lines obtained at atomic steps on graphite (top two scan lines), and across theterrace steps in Fig.2.7, above (bottom).2.3c. Comparison with Theoretical ModelThe longitudinal (Az) and lateral (Ax) deflections of a piezoelectric tube (with thiselectrode geometry), with electrode voltage (V), can be described by a simple mode112,LAz = —t d31VAx =1-=2 d31 V2rt^'where L, t, and r are the tube length, wall thickness, and radius respectively. Thepiezoelectric constant, d31=2.74*10-10 m/volt for PZT-5H. For this tube geometry (see§2.1), these equations predict (Az/V) = 6 nm/volt and (Ax/V)=14 nm/volt. The actuallateral scan range also depends on the length of the probe, X, which extends beyond theend of the tube. If the curvature of the deflected tube is approximated to be zero, the scanrange is increased by a factor of ((L+X)/L) where X is 5 mm typically, resulting in a lateralscan range of 17 nm/volt. These values are in rough agreement with the measuredcalibration factors reported above. This model is expected to over estimate the deflectionsby about 10% 12, and the tube dimensions are uncertain by roughly 5%.143. Surface PreparationSTM measurements at ambient pressure were attempted on the untreated cleavedsurfaces of GaAs and Si substrates. However, it was found that stable and reproducibleimages could only be consistently obtained after the surfaces had been passivated againstoxidation by chemical treatments.The goal of the preparation procedures described here is to produce a smoothsurface, ideally atomically flat, which is stable enough to permit imaging at ambientpressure. The general approach for both the silicon and gallium arsenide substrates is tocleave the wafer along an atomic plane, followed by an immediate wet chemical treatment inwhich the surface is stabilized against oxidation. The stability of the treated surfacedepends critically on producing a smooth cleave, with no steps visible under an opticalmicroscope. The cleave is produced by first scratching a small tick at the edge of the waferwith a diamond scribe, and then forcing the scribe 'open'. This is achieved by laying thewafer, scribed side down, on a rubber mat and applying pressure at the point on the edgeof the wafer directly above the scribed tick with a tapered metal rod until the cleave isinitiated. The cleave propagates away from the scribe, becoming smoother as it gets furtheralong. For samples with epitaxially-grown layers, the scribe is made on the epitaxial side.3.1 Imaging on Chemically Treated SurfacesImage quality for both GaAs and Si surfaces was found to be dependent upon thetunneling current and tip bias. In particular, ambient pressure imaging was not possiblewith the tip bias at negative polarity, as this always resulted in a very noisy current signal.This effect is not observed in experiments performed in UHV, in which stable imaging ispossible at both polarities9,13. It is believed that the electric field associated with a negative15tip promotes the chemisorption of an atmospheric species at the surface (perhapsoxidation). This interpretation is consistent with an early study26 of the electric fielddependence of Si oxidation. Large positive tip bias also tended to produce a noisy imageand resulted in damage to the sample surface. The best images were obtained with the tip at+3 to +4 volts for GaAs and +1 to +2 volts for silicon.It was necessary to keep the tunneling current low (about 0.2nA) and to scan atslow speeds (approximately 2seconds for a lp.m scan line or 8 minutes for a full 256-lineimage), with high gain in the feedback loop (RC=300p). These settings represent acompromise between minimizing damage to the surface and achieving reasonable dataacquisition rates. The concern is that scanning too close to the surface (by increasing thecurrent set point or reducing the tip bias), or scanning at a high rate, increases thefrequency and severity of tip crashes, in which the feedback control servo is unable torespond to sudden changes in surface topography. The result is either a sudden increase inthe electric field as the tunneling gap narrows or actual tip-sample contact.3.2. Silicon3.2a Surface Preparation ProcedureBefore resorting to chemical passivation treatments, an attempt was made to imagethe untreated, freshly cleaved (110) silicon surface. (100)-oriented silicon substrates werecleaved along the (110) direction. In order to produce surfaces with step-free regions, itwas necessary to either cleave a large sample (greater than 1 cm in length) or to thin thesamples to less than 100 iim before cleaving. Fig. 3.1 is a scan taken in air immediatelyafter cleaving a sample which had been thinned to 100 .t.m. Images acquired on theuntreated surface were noisy and deteriorated with time. Moreover, scanning became veryunstable or impossible within an hour of cleaving, presumably due to the build up of oxide.16Fig. 3.1. STM image of freshly cleaved, untreated (110) silicon surface. The tunneling set point was +1 Von the tip and 0.1 nA tunnel current. Horizontal scan range (x) =1 lam. Vertical greyscale (z) =180 A(white to black).17Fig. 3.2. Image of same surface as in Fig. 3.1., after etching in HF. The small, identically shaped featuresare tip images. Set point = (+2 V, 0.1 nA). x-range=2.0 pm, z-range greyscale=90 A.Fig. 3.2 shows an image taken on the same surface, with the same probe, asFig. 3.1, following a dip in concentrated (49%) hydrofluoric acid (HF) for severalseconds to remove the oxide. The cleaved surface had been exposed to room air for about20 minutes prior to this treatment, which is known13 to terminate the silicon surface withhydrogen. Following the HF dip, imaging was reproducible and stable for several hours.The prominent feature in the image may be surface damage resulting from a tip crash whilepositioning the probe. The image also contains many smaller identically shaped features,which were not observed prior to the chemical treatment. These features indicate thepresence of sharp points on the surface which image the tip, as described below (4.1). Itis likely that the HF had etched the surface nonuniformly, producing these points.Silicon surfaces treated with ammonium fluoride (NH4F) solution have beenimaged with atomic resolution in UHV STM.27 Accordingly, this procedure was appliedto imaging in air. The freshly-cleaved surface was dipped into concentrated (40%) NH4Ffor about 30 seconds, followed immediately by a 5 minute soak in de-ionized (DI) water.The NH4F solution is also believed to hydrogen-passivate the silicon surface.27 Thethorough DI water soak is necessary to prevent further etching of the surface. Exposure tothe NH4F for more than a few minutes resulted in non-uniform etching and a roughsurface. Following the DI water soak, the sample was removed from the water and blowndry with nitrogen. In some experiments this last step was done in a nitrogen-filled glovebag which contained the STM, in order to minimize exposure to room air.Fig. 3.3. shows an image acquired on an unthinned silicon substrate, which hadbeen dipped in NH4F for about one minute immediately after cleaving. The surfaceroughness is less than 15 A, suggesting that the etch is much more uniform than in the HFtreatments. Such surfaces imaged reproducibly after several hours in air, and up to24 hours in nitrogen ambient. It was also found that it was possible to 'refresh' thesurface once it had oxidized, by repeating the etching procedure. The image in Fig. 3.4was taken on such a refreshed surface. Atomic resolution of the treated surfaces was not18Fla. 3.3. Image acquired on a cleaved (110) n-type Si surface after etching in NH4FSet point = (+1 V, 0.3 nA). x-range=1 gm, z-range=100 A.19Fig. 3.4. Image of the same surface as in Fig. 3.3. following a 'refresh' treatment, 24 hours after the initalcleave. The surface roughness is about 15 A. Set point=(+1V, 0.1 nA). x-range=1.5 pt_m, z-range=60 A.achieved in air or in the nitrogen-filled glove-bag. Etching times in the range of 10 secondsto 5 minutes were tried and all produced similar results.3.2b. Tunneling-Induced Surface ModificationAs discussed previously (§3.1) scanning conditions were chosen to reduce theprobability of mechanical contact of the tunneling probe with the surface. Even with theseprecautions, modification of the surface due to scanning was observed on the NI4F-treatedsamples. This damage occured to varying extents on different samples. A possibleexplanation is that the stability of the passivation treatment is dependent on the quality ofthe cleave, which varies among samples. Surface modification was enhanced as exposureof the surface to tunnel current increased, so that scans over smaller areas resulted in moredamage than larger scale scans taken at the same rate per scan. One would expect that theopposite would be true if the damage were due to mechanical interaction of the tip andsample. The effect appears to be stronger at larger tunneling voltages, which is alsoinconsistent with a mechanical interaction mechanism. These observations suggest that thesurface modification is caused by a chemical reaction, likely oxidation, assisted by thetunneling process. This effect has been studied on both cleaved and polished siliconsurfaces by Dagata et.a1.28, who obtain similar results.Fig. 3.5 shows an image of a cleaved p-type (dopant concentration=lx 1017 cm-3)substrate. The image was acquired at a set point of (+3 V, 0.1 nA) following repeatedscans over 3 successively increasing areas, with each scan taken in a time of 5 minutes, atthat set point. The smaller area scan regions appear as progressively darker squares withinthe larger area scan regions. If this tunneling-induced surface modification werecontrolled, it could be used to fabricate device structures, enabling submicron control oflateral geometries.28 Fig. 3.6 shows the results of an attempt to 'write a sideways 'H' on20Fig. 3.5. Squares on the treated p-type silicon surface produced by successively larger range scans at(+3 V. 0.1 nA). x-range=3.0 j.tm, z-range=200 A.Fig. 3.6. Sideways 'H' pattern on treated p-type silicon. The dark pattern was produced by scanning theselected areas at (+4 V. 0.1 nA). The scanning set point was switched to (+2 V, 0.1 nA) in the regionssurrounding the 'H'. x-range=2.5 gm, z-range=50 A.21a different area of the same p-type surface. The central square was written by scanning thatarea three times at a set point of (+4 V, 0.1 nA). The sides of the 'H' were written byscanning a larger square area three times at (+2V,0.1nA), switching to (+4V,0.1nA) at thetop and bottom of regions of the scan. Due to hysteresis in the piezo tube response (see§2.3), the area to the right of the resulting dark pattern appears saturated white in theimage, and the left half of the pattern is darker than the right half.Fit-I. 3.7 is a scan taken on an n-type region, the substrate of a sample on which aSi/Ge multilayer structure (§5.2) was grown. Successive scans on the substrate sometimesresulted in the appearance of tip images, as in this example, which had been scanned 3times at (+3 V, 0.1 nA). Further scanning resulted in more tip images until the wholesurface was covered. This effect, which suggests that the tunneling process is producingH2. 3.7. Surface roughening on n-type silicon, induced during three successive scans at (+3 V. 0.1 nA).The roughening has produced four sharp features which image the tip. The horizontal lines are due to creepin the piezo tube (see Fig. 2.4). x-ran2e=2.0 p.m, z-range=80 A.22sharp points on the surface, was not observed on the p-type capping layer grown above theSi/Ge multilayers. Also, the effect observed on the p-type substrates described above wasnot observed on these n-type substrates.3.3. Gallium ArsenideThe GaAs-AlGaAs epilayers imaged in these experiments are grown on semi-insulating and conductive n-type (100)-oriented GaAs substrates. The dopantconcentration of the conductive substrates is 2x1018 cm-3.Attempts to image the freshly cleaved GaAs samples without chemical passivationwere unsuccessful, due to oxidation of the cleaved surface. As with the untreated siliconsurfaces, imaging often became impossible within a few hours of imaging, perhaps due tothe build up of oxide picked up by the tip.3.3a. Imaging on Unpassivated GaAs Surfaces - Oxide Desorption StudySome success was achieved in imaging the polished (100) substrate surfacesfollowing a 5 minute etch in 3M HC1 to remove the native oxide. This was followed by arinse in DI H20 and nitrogen blow off. No further chemical treatments were performed topassivate the surface against oxidation. This procedure did not work consistently,however, and always produced a rough surface.The HC1 treatment was used to prepare samples for imaging in an STM study ofroughness induced by oxide desorption on GaAs substrates. The thermal desorption of aprotective oxide is the final cleaning step for the GaAs substrates, prior to epitaxy.29 Thisoxide is grown by exposing the wafer to ozone in an RF plasma and is desorbed in the(UHV) MBE growth chamber by radiative heating of the substrate. The desorption resultsin roughening of the surface, which can be detected by in-situ optical monitoring of the23substrate." In order to investigate the nature of this roughenning, STM images wereacquired on the (100) surface after desorption. These were compared to images obtainedon polished substrates which had not undergone the MBE cleaning procedure.Figs. 3.8 shows images acquired on (a) a polished GaAs substrate and (b) asubstrate following desorption of an oxide grown by a 20 minute exposure to ozone. Itwas necessary to scan at slow speeds (8 minutes/image), positive tip bias (+1 Vtypically), and small tunneling currents (0.2 nA). Both of these images were acquiredreproducibly for three successive scans of the surface. The criss-crossing lines on thepolished substrate (a) may be polishing scratches. The surface roughness in this image isabout 50 A. By contrast, the large features on the desorbed surface (b) have an associatedtopography of more than 500 A. In particular, three large holes appear in this image. Thesurface density of these holes is in agreement with that of features observed on the samesample using a Nomarski phase-contrast optical microscope (roughly 0.34m2). Three-dimensional renderings of regions of these images appear in Fig. 3.9. The lateral andvertical scales in these renderings are in angstroms. Higher magnification images acquiredon the polished and desorbed surfaces showed the roughness to be greater than 100 A andcomparable for both surfaces, for lateral ranges of about 200A. Possibly this roughness onsmall scales results from the HC1 etching.Although the results above were acquired during stable imaging, many of theimages were unstable and noisy. In general it was found that reproducible images couldnot be obtained for more than an hour following the HCI etch, due to oxidation of thesurface. However, a passivation step involving the solution used on the freshly cleavedsurfaces as discussed below has been shown to permit stable imaging of the etchedsurfaces. 1724(a)(b)Fig. 3.8. STM images of (a) a polished (100) GaAs substrate and (b) a (100) substrate following oxidedesorption as described in the text. The desorption produces large holes spaced apart by about 1 lam.x-range=2.7 im for both images.25(a)26(b)Fig. 3.9. Three-dimensional renderings of (a) polished surface and (b) desorbed surface, from portions of theimages in Fig. 3.8. The hole in (b) corresponds to the large feature near the bottom left corner of3.8b. Both the lateral and vertical scales are in angstroms.3.3b. Surface Passivation ProcedureDue to its compound nature, the GaAs surface is much more difficult to passivatethan silicon. Wet-chemical passivation treatments involving sulfide-based solutions havebeen investigated for purposes of device processing.31 These treatments are believed toform a sulfur-terminated surface on polished (100) GaAs surfaces.32,33 Accordingly,attempts were made to prepare samples for imaging by dipping the freshly cleaved (110)surfaces in Na2S solution, followed by an H20 rinse and nitrogen blow dry. However,this treatment did not permit stable imaging. It is possible that the tunneling processdamages the insulating passivation layer, either because the layer is unstable or because it isthick enough that it is in mechanical contact with the the probe during scanning.It has been shown17 that stable STM imaging of GaAs is possible using a treatmentin a 1g/20mL/60mL P2S51(NH4)2S/H20 solution, heated to over 50°C. This procedure isbelieved to remove the surface arsenic atoms and leave a stable Ga0 termination.17Following this method, the freshly cleaved samples were quickly dipped into the heatedsolution (60°C) for 30 to 60 seconds, rinsed in DI water (5 min.) and blown dry withnitrogen. Stable imaging was achieved for up to 3 days on freshly-cleaved samples,following the P2S5 treatment.Fig. 3.10 is an image taken in air, at a tunneling set point of (+4 V, 0.1 nA),15 hours after this chemical passivation treatment. The scan is on the n-type substrate of asample on which epitaxial layers had been grown. The surface roughness is less than30 A. The image quality did not deteriorate noticeably with repeated scanning over thesame area, and stable images were obtained on this sample even after 30 hours exposure toroom air. The stability and longevity of samples prepared in this way varied amongsamples, and appear to be strongly dependent on the quality of the cleave. It was foundthat the passivating solution, which is typically prepared in 32 mL quantities, generally2728Fig. 3.10. STM image of a (110) n-type GaAs surface which had been passivated in P2S5 solution. Thesurface imaged stably after more than 30 hours exposure to air. x-range=500 urn, z-rang,e=50 A.Fig. 3.11. Image acquired on a treated n-type surface, following a previous smaller scale scan which resultedin the formation of sharp points. The dark area on the right is due to hysteresis in the piezo tube.x-range=1.5 tim, z-range=25A.becomes ineffective after storing for more than two days in a stoppered volumetric pyrexflask. The solution turns from a straw-yellow to a dark, orange-yellow color after thistime, possibly due to the formation of polysulfides.3.3c. Tunneling-Induced Surface ModificationAs in the case of silicon, damage due to mechanical tip-surface interaction wasminimized by appropriate choice of scanning voltage and current, as discussed in §3.1.Nevertheless, surface modification in the form of sharp points was sometimes observedwhile scanning on the n-type substrates, or in n-type regions of the grown layers. As withsilicon, this damage was found to increase with exposure to tunnel current and varied inextent among samples. By contrast, no noticeable surface modification occurred duringrepeated scans on p-type material.Fig. 3.11 shows an image taken on an n-type substrate, following a smaller scalescan at the same set point. The surface had been prepared in the same way as that inFig. 3.10 and was imaged at the same tunneling set point. The image acquisition time was8 minutes for both the small scale scan and the larger scale scan, which means that thesmaller area has substantially higher exposure per unit area to the tunneling current. Incontrast with the smooth surface in Fig. 3.10, the previously scanned area appears to becovered with sharp points, resulting in identical tip images.Fig. 3.12 shows two successive scans taken on a 16-period GaAs/AlGaAsmultilayer structure. The first 8 periods of the superlattice are n-type and the remaining8 periods are p-type. To the left of the structure is the semi-insulating substrate on whichthe epilayer was grown. The material to the right of the multilayers is n-type GaAs. Notethat the n-type regions, both in the superlattice and in adjacent n-layer, are covered withsharp points. The density of the points increases during the next scan. By contrast, the p-type material in the superlattice is essentially undamaged by scanning. This preferential2930(a)(b)Fit?. 3.12. Preferential roughening of n-type material during two successive scans on a structure consistingof a p-type layer (middle dark band) between two n-type regions. No significant surface modification of p-type material was observed during constant current scans. x-range=1.51.mi. z-range= 30 A.roughening of n-type material was observed on many occasions, but the p-type materialseemed to remain smooth. One explanation of this effect is that the roughening is causedby oxidation of the surface, which is presumably enhanced by the tunneling process. Thesurface Fermi level in the n-type material will be higher than the surface Fermi level in thep-type material by almost the full band gap (1.4 eV) in the absence of surface states. Inthis case the p-type material has a correspondingly higher work function than the n-typematerial, and hence a higher affinity for electrons. Because the oxygen must removeelectrons to bond with the gallium or arsenic atoms, this electron affinity represents abarrier to oxidation, which is significantly higher for oxidation of p-type material, andhence the p-type regions are likely to oxidize at a lower rate.3.3d. Photoemission Spectroscopy of Chemically Treated (110) GaAsIn order to investigate the chemical passivation mechanism which permits the stableimaging of GaAs in air, photoemission spectroscopy5 (PES) of the Ga and As core levelchemical shifts was performed on the cleaved (110) GaAs surfaces. Semi-insulatingsubstrates, which are slightly thicker (0.8mm) than the conductive substrates, were used toimprove the measurement sensitivity. Treatments with two chemical solutions wereinvestigated: P2S5/(NH4)2S, and (NH4)2S. Both treatments were identical to thatdescribed previously (§3.2b), with the single exception that no P2S5 was added to thepassivating solution for the latter treatment. An untreated sample cleaved from the samesubstrate was used for comparison.Figs. 3.13 and 3.14 show X-ray photoemission (XPS) measurements34 of thecleaved surfaces obtained with a Leybold MAX200 spectrometer, using an Al Ka x-raysource (hv=1486.6 eV). All three samples had been exposed to room air for about 1 hourfollowing surface preparation before being loaded into the spectrometer vacuum chamber(6x10-9 TOIT operating pressure). The measurements shown correspond to the As2p and31i\-‘1/^.1/untreated^Ale.NA,..,1/4, e j!V't^• /^VA; 1,1 \1 1\4s."tv\t'l'IV ri \kti\AA,A1^ 1, \A'(NH4)2SYNAVVVV,V-11/4^ii \ \ , ‘\1Ij '^\ v\/\.\:\41\ :„NiV\WIA/j\Adl,tsItti-A:\:^, iii vAN V•-• N.,-,`V ,,v4A-00,^CI^\ ^ \I—N v,,,,,,W \VI's/\Y\P2S5/(N114)2S\livsNIA...44 V" VA" V \livtA1335^1330^1325^1320^1315bind. energy^ieV] linFig. 3.13. XPS (arbitrary units) at the binding energy of the As2p core level for cleaved (110) GaAssurfaces. Results for an untreated (oxidized) cleave are shown for comparison with the chemically treatedsurfaces. The magnitudes of the chemically shifted components (higher binding energy), relative to themagnitudes of the bulk peaks indicate that relatively little arsenic oxide is present on the treated surfaces.32 1.051 . 00r' A0.95-0.90-0.850.80P2S5/(N H4)2S0.950.900.85\0.80-"^0.75-^ %^. ‘^\//^\0.70'^ )./(0.65' untreated(NH4)2S0.60-Nrne1128^112,4 1120^:116^:112bind. enerw;^eV:Fig. 3.14. XPS results for the Ga2p core level for the same three samples in Fig. 3.13 (arbitrary units).The untreated and P2S5-treated spectra are asymmetric due to oxidation of the Ga whereas the treatmentwithout P2S5 has resulted in a symmetric peak.Ga2p core level binding energies. The higher binding energy of the 2p levels makes thesespectra more sensitive to surface composition than those of the 3d levels: photoelectronsejected from the 2p levels have a lower kinetic energy and hence a smaller escape depththan those ejected from the 3d levels. For the As2p core level, chemical shifts of +3 eVand +2 eV respectively are associated with bonding to oxygen and sulfur.35 The relativeintensities of the spectral peaks corresponding to the chemically shifted (higher bindingenergy) and the unshifted As2p binding energies show that the P2S5-treated surface hassignificantly less arsenic oxide than the untreated cleave. The chemical shifts associatedwith the formation of gallium oxide and sulfide are small (+1.3 eV and +0.6 eVrespectively35), and result in an asymmetric peak, rather than distinct peaks, at the Ga2pbinding energy. The Ga2p peak corresponding to the P2S5-treated surface appears moresymmetric than the untreated peak, indicating that the gallium oxide/sulfide content is lessthan the gallium oxide content of the untreated surface. Better resolution is needed todetermine if the asymmetry in the P2S5-treated peak is due to sulfide or oxide, however.Analysis of the spectra obtained on the surface treated in (NH4)2S solution (with noP2S5 added) reveals that no detectable oxidation of either the arsenic or gallium hasoccurred: the Ga2p peak is symmetric, and no oxide peak is detected in the As2p spectrum.A slight shoulder on the As2p peak of the (NH4)2S-treated surface, corresponding to thesulfur chemical shift, is barely detectable above the background. One interpretation is that athin sulfur passivation layer has inhibited the oxidation of this surface. Auger emissionspectroscopy was also performed34 on surfaces treated in (NH4)2S solution, with andwithout P2S5 added, and confirmed the presence of sulfur in both cases. In thesemeasurements, which were made with the same spectrometer as the XPS, sensitivity wasimproved by stacking several cleaved samples together after the treatment, to increase theedge area.To further clarify the XPS results, PES using synchrotron radiation wasperformed36 on the P2S5/(NH4)2S-treated surface. The low energy (hv=100 eV) radiation33used makes these measurements sensitive to chemical composition extending only 6 Abelow the surface as opposed to about 20 A below the surface for the XPS data(hv=1.4 keV) above, based on the universal curve37 of electron mean free path. Thesamples had been exposed to air for three days before the measurements, which werecarried out at the University of Wisconsin. The chemical shifts associated with bonding tooxygen and to sulfur respectively are +3 eV and +1.5-2 eV for the As3d core level and+1.3 eV and +0.6 eV for the Ga3d core leve1.38 Overview spectra which included theGa3d and As3d peaks in the same scan were taken on the treated and untreated surfaces.Based on these scans, the ratios of the intensities of the bulk Ga3d and As3d peaks are thesame within 10% for both surfaces. Higher resolution scans at the binding energies ofeach core level were taken individually (Figs 3.15 and 3.16). The fit to each spectrumrepresents a sum of two Gaussians, separated by the energy shown in the figure. TheAs3d peak (Fig. 3.15) reveals that for the untreated surface, the arsenic has oxidizedsignificantly more than the arsenic on the P2S5-treated sample: for the treated sample, theintensity of the bulk As3d peak is about the same as the chemically shifted component; forthe untreated sample the chemically shifted component is approximately twice as intense asthe bulk peak. By contrast, the relative amplitudes of the chemically shifted and bulk Ga3dpeaks (Fig. 3.16) are similar for both the treated and untreated samples. These results areconsistent with the XPS results above, and with previous XPS results17 which indicate thatthe chemical treatment selectively removes arsenic atoms from the surface. No peaksassociated with the sulfur chemical shift are observed for Ga3d or As3d, although thepresence of the oxide obscures the smaller sulfur chemical shift. Possibly the passivationlayer had deteriorated, permitting oxidation to occur, or the passivation treatment producesprimarily a surface oxide rather than a sulfide. The deterioration interpretation is consistentwith the observation that STM imaging of the treated surfaces became unstable within a fewdays of treatment.342Solid - DataDash - Gaussian Fit35BINDING  ENERGY ( e V )Fig. 3.15. Raw data from PES measurements of the As3d core level for an untreated cleave (a) and a cleavedsurface passivated in P2S5 soloution (b). The peak for the treated cleave is smaller and has a considerablyreduced chemically shifted (higher binding energy) component than the untreated cleave. Both samples hadbeen exposed to air for 3 days after preparation.BINDING  ENERGY (e U)Fig. 3.16. PES of the Ga3d core level. The untreated (a) and treated (b) peaks in this raw data are of nearlyequal intensity and both have large chemically shifted components (higher binding energy), corresponding tooxidation of the Ga.Note that oxygen is present in both passivating solutions, owing to the dilution withDI H20. The (NH4)2S treatment seems to inhibit oxidation of both arsenic and gallium,apparently by formation of a thin sulfide layer. By contrast, The XPS and synchrotronPES results above suggest that the P2S5 treatment used in preparing the cleaved surfacesfor imaging selectively removes the surface arsenic. Selective removal of arsenic atomsmay be a key factor in producing a surface stable against imaging in air: Arsenic oxides aremore unstable than gallium oxide, in the sense that they evaporate at a lower temperaturethan gallium oxide29 and assume various chemical stoichiometries such as As0 and As203.Possibly the arsenic oxides are removed or chemically altered by the tunneling process,resulting in surface damage and a noisy current signal.364. Tip EffectsEven when the chemical treatments had resulted in a stable surface, resistant tomodification due to tunneling, imaging was sometimes inconsistent, due to irregular tipgeometry or mechanical instability of the tunneling probe. In order to interpret the STMimages, it is necessary to evaluate how these non-ideal tip structures affect the data.Various kinds of tip artifacts were observed and are discussed below.4.1. Tip InstabilitiesImages acquired on the semiconductor surfaces treated as previously describedwere sometimes noisy, with large apparent z-range, as if the surfaces were very rough.After a new tip was installed however, the images were often immediately improved,revealing a stable and smooth sample surface. It is most likely that the effect is due to somemechanical instability of the tip. This instability might arise from insulating adsorbates, lefton the end of the tip as it was prepared, or picked up from the sample while scanning.Mechanical contact of this insulating material with the sample surface would result in anoisy current signal. As the scanning continued, adsorbates swept off the tip would bereplenished by insulating debris from the damaged passivation layer. The same effect wasnot observed while scanning on graphite or gold samples, which consistently imagedstably. Presumably any tip adsorbates are removed during scanning of these more robust,oxide-free surfaces.Fig. 4.1a shows an image of a cleaved silicon (110) surface, which had beenpassivated by dipping in NH4F. The image is noisy and suggests that the surface is rough.During acquisition of the next image, the image quality suddenly changed, and a largerscale scan was taken immediately, shown in Fig. 4.1b. Here, the white blob in the center37(a) x-range=250 rim, z-range=50 A(b) x-range=500mn, z-ran2e=100 AFig. 4.1 Effect of mechanical tip stability on image quality. It is believed that the noisy imaging in (a)was caused by a mechanical instability in the tip, which was no longer present in (b). The irregular whitefeature in the centre of the second scan is possibly a metal fragment deposited by the tip during the change.3 8of the image is believed to be a tip fragment deposited on the surface. The image is nolonger noisy and the surface appears smooth, probably because the remaining tip ismechanically stable. The dark circular area surrounding the tip fragment may be electroniccontrast, since the deposition of metal could result in local pinning of the Fermi level8 andthe tunnel current is sensitive to the surface Fermi level position (see Chapter 6).4.2. Tip ImagesIf features on the sample surface have a higher aspect ratio than that of the tip, thosesurface features will image the structure of the tip as it scans the surface. This effect is wellknown and has been observed on many different surfaces.25,39 Careful consideration ofthe orientation of the tip images can sometimes provide information about thecrystallographic orientation of the surface structures responsible.39 Tip images areobserved while imaging on the chemically treated semiconductor surfaces discussed in thiswork, as shown above (3.2,0.3). The tip images scale with scan range as would truesurface features.It is possible that the white blob in Fig. 4.1b is also a tip image, if the depositedfragment is sharper than the remaining tip. The spatial extent of the electronic contrastdiscussed in connection with this image should be comparable to the depletion width (seeChapter 6), which is roughly 80 nm for the dopant concentration of this sample(1x1017 cm-3), assuming the Fermi level is pinned midgap by the metal deposition. Theradius of the dark region in the image is about 50 nm, although only about 10 nm isvisible. An interpretation is that the small metal fragment responsible for the electroniccontrast produces an image of the irregularly shaped tip, which partially obscures thecircular depletion region.394.3. Multiple TipsOften the tunneling probe will consist of many sharp points, particularly in the caseof probes prepared by mechanical shearing as shown in §2.2. If, while scanning, morethan one of these tips is within tunneling range , the resulting image will be a sum of thecontributions from each tip. The tunnel current is expected to be reduced by an order ofmagnitude for each angstrom increase in tip height (see Chapter 6). This result, and thelow probability of two tips being at almost precisely the same separation (within 1 A)means that multiple tips are rarely a concern for atomically flat surfaces. High relief surfacefeatures, however, will be imaged multiple times.The steps of the atomically flat terraces on the graphite surface shown previously inFig. 2.7 (§2.3) are each imaged twice, presumably due to a double tip. Note that thevertical separation of these tips must be less than 10 A, the height of the smallest steps inthe image. In Fig. 4.2, an image of a GaAs/AlGaAs superlattice is presented. The samplehad been prepared as described above. The superlattice structure appears as alternatinglight and dark vertical bands in the image, and is imaged twice due to a double tip. Fromthe image the lateral separation between the tips is estimated to be about 200 nm.An image of another GaAs/AlGaAs superlattice appears in Fig. 4.3. At the top ofthe scan, the individual layers of GaAs (light bands) and AlGaAs (dark bands) are wellresolved. Midway through the scan, however, the resolution is suddenly lost, clearly dueto a change in the tunneling probe. Several tip images are present after the change, andfrom these we see that the probe is in fact a double tip. From these tip images, the lateraltip separation is seen to be almost exactly the same as the layer thickness (20nm). The thindark lines in the remainder of the scan (after the change in the probe), correspond to thebrief instances when both tips are simultaneously over the AlGaAs regions.Fig.4.4 shows two successive scans over the same area of a portion of aGaAs/AlGaAs superlattice. Again, the AlGaAs layers appear narrower than the GaAs4041Fig. 4.2. Double-tip image of a GaAs/AlGaAs multilayer structure consisting of four GaAs layers (brightbands) and three AlGaAs layers (dark bands). The entire structure is imaged by two tips on the same probe,separated laterally by 200nm. x-range=2 p.m, z-range=55 A.Fig. 4.3. Change in tip geometry midway through a scan on a GaAs/AlGaAs multilayer structure. The tipimages (bright dots) reveal that the change has produced a double tip, in which the lateral separation of thetips is nearly identical to the layer thickness (20nm). x-range=1.5 p.m. z-range=28 A.(a)(b)Fig. 4.4. Change in tip structure between two successive scans of the same portion of a GaAs/AlGaAssuperlattice. The contrast in the image is enhanced (b) after the change. x-ran2e=270 nm, z-range=140 Ain both images.42layers in both images, probably due to multiple tips. The apparent topography of each ofthe layers changed identically in the second scan. This suggests that the higher GaAslayers are imaging the structure of the tip, which changed between the scans.Clearly these multiple tip effects complicate the interpretation of the images, and ofthe spatial dependence of the IV characteristics obtained at selected locations in the epitaxiallayers (§7.4). It is therefore necessary to ensure that multiple tips are not present, forexample by comparing the layer thicknesses in the STM multilayer images with thethicknesses predicted by the MBE growth rate calibration or determined from SEMmeasurements. Alternatively, several measurements using different tips can be made, sincetip artifacts will materialize as inconsistencies in the data.435. Crossectional Imaging of Epitaxial Structures5.1. Positioning of the Tunneling ProbeIn the case of the samples imaged in cleaved crossection, the scanning direction isperpendicular to the polished surface, as shown in Fig. 5.1. The tip is positioned over theMBE grown layers by moving the sample using the mechanical translation device describedpreviously (§2.1), until the layers are observed in the image. Often this procedure involvesfirst locating the edge of the sample, evidenced by huge topographical contrast as the tipfalls off the edge, and then moving the tip away from the edge by applying an appropriatevoltage offset to the scanner electrodes. During sample translation it is necessary to eitherretract the tip using the piezolelectric tube or to back the sample off with the micrometerstage adjustment, to prevent tip crashes.(100)AFig. 5.1. Orientation of scanning probe and cleaved surface for crossectional measurements. The scanningdirection is across the layers, perpendicular to the plane of the polished surface.4 45.2. Imaging of Si/Ge MultilayersFig. 5.2 is a field-emission scanning electron microscope (SEM) micrograph of theepilayer shown schematically above it.40 The multilayer structure consists of twenty, 5-period Si/Ge superlattices (5 nm wide), separated by 20 nm Si spacer layers, and appearsas alternating light and dark bands in the SEM image. It is not clear from the micrographwhich bands correspond to the Si layers and Ge-containing layers. The cleaved sample hadnot been chemically treated, so the contrast is not due to selective etching. Possibly thecontrast is electronic in origin, due to the smaller band gap of the Ge. The individual Siand Ge layers in the superlattices are 0.5 nm thick, and are not resolved.An STM image41 of the same sample is shown in Fig.5.3. The surface had beenpassivated by an HF treatment and was imaged in air at a tunneling set point of(+1 V, 0.2 nA). The vertical contrast to the left of the multilayer was not present in allthe samples, and might be associated with a topographic step at the interface between thesubstrate and the buffer layer, caused by the cleaving process. The p-type capping layerappears saturated black in the image, presumably because it had cleaved at a lower height.The 20-period multilayer is resolved in the centre of the image as a series of alternating lightand dark bands. The bright bands are interpreted as the Ge-containing layers and the darkbands as the Si spacer layers, as discussed below. Contrary to this identification, thebright bands are wider than the dark bands, which is attributed to the finite aspect ratio ofthe tip: the side of the tip images the adjacent Ge-containing layer when the probe ispositioned over a Si spacer layer. In higher magnification images, which failed to resolvethe 0.5 nm layers, the bright bands appeared to be lined with white balls which limited theresolution. These might be tip images resulting either from tunneling-induced oxidation(§3.2b) or non-uniform etching by the HF (§3.2.a).A scan line taken across the same multilayer structure on the same surface at adifferent location in the epilayer, is shown in Fig. 5.4. The cleavage plane of the epilayer45020Asubstratep-type capn-typebuffer layer Si(Fig. 5.2. Field emission SEM micrograph of a group of 20 Si/Ge superlattices.46Si/Ge47500400A^300200100850 900^950^1000^1050^1100^1150^1200^1250Fig. 5.3. sTm image of the epilayer containing the Si/Ge multilayers, showing a topographic step at thesubstrate/epilayer interface. The bright bands are interpreted as the Ge-containing layers. x-range=1.4 !_tm,z-range=350 A.n mFig. 5.4. Topographic scan line across the Si/Ge multilayers. The modulation associated with themultilayer periodicity is attributed to strain relaxation of the Ge-containing layers.is not quite perpendicular to the probe tip, resulting in the 400 A bulge in the scan line.The higher frequency modulation in the scan line is due to the multilayer topography. Themagnitude of the topographic modulation is roughly 40 A and is attributed to bulging_ ofthe compressively stressed Ge-containing layers at the cleavage surface. The equilibriumlattice constants of Ge and Si are 5.66 A and 5.43 A respectively. This lattice mismatchresults in large compressive strrains in the Ge layers grown on the Si substrate.Fig,. 5.5 is a higher magnification image taken of the same multilayer structure.This sample was etched in NR4F and imaged in nitrogen ambient. No tip images arepresent, consistent with smoother etching of this treatment over HF. However, theindividual Si and Ge layers are still not resolved.Another multilayer structure, consisting of four alternating periods of 40 nm siliconand Si0.8Ge0.2 alloy layers,42 is shown in Fig. 5.6. This image was acquired in nitrogenambient following a treatment in NH4F. The multilayer structure is imaged more than48Fig. 5.5. Higher resolution STM image of a portion of the Si/Ge multilayer structure. x-range=500 nm,z-ranc.,.e=26 A.once, due to multiple tips on the scanning probe. Four of the layers are brighter than the Sisubstrate on the left and the Si cap immediately to the right of the multilayers. Based onthis observation, the bright layers are deduced to be the Ge-containing layers, which isconsistent with the designation in the previous images. A scan line across this image isshown in Fi2.5.6b. The topographic contrast associated with the strain relaxation of theSiGe is about 30 A.49(a) 30201 00600^700^800^900^1000^1100(b) n mFig. 5.6. (a) STM image of Si/SiGe multilayers and (b) topographic scan line (different lateral scale) acrossthe layers. The Si capping layer is at the top edge of the sample, just before the black region on the right.The layer thickness is 40nm. Set point=(+1.5 V, 0.1 nA), x-range=1.0 tun, z-range,=116 A.5.4. Imaging of Epitaxial GaAs/AlGaAs MultilayersFig. 5.7 shows an air STM image of a group of n and p GaAs layers43 which hadbeen grown on an AlGaAs buffer layer, which is on the far left. The contrast at theAlGaAs/GaAs interface is likely due to a cleavage step along the interface, with the AlGaAscleaving higher than the adjacent GaAs layer. In the higher magnification image of thisinterface (Fig. 5.7b), taken at a different location on the same cleave, the AlGaAs hascleaved at a lower height. The horizontal drift in this image is due to creep in thepiezoelectric tube scanner. The roughness observed in the AlGaAs region is likely due toselective removal of the aluminum during the chemical treatment. Similar roughening wasobserved in SEM micrographs obtained on samples in which the aluminum had beenselectively etched in K3Fe(CN)6 solution to enhance the contrast between the AlGaAs andGaAs layers." Presumably the selective etching is nonuniform and roughens thealuminum-containing regions.In Fig. 5.8, an image of four periods of a GaAs/AlGaAs multilayer structure,grown on an n-type substrate, is shown. The individual GaAs and AlGaAs layers are50 nm thick. The steps visible on the substrate are oriented in the (111) direction, and arecaused by imperfect cleaving. Such steps would degrade the performance ofGaAs/AlGaAs quantum well lasers, in which the mirrors of the laser cavity are the cleaved(110) facets. The substrate/epilayer interface apparently acts as a barrier to steppropagation, since only one of the cleavage steps has propagated into the grown layers.A large scale scan of the sample containing the 16 period multilayer structure (farleft) described in §3.3 is shown in Fig. 5.9. Immediately to the right of the multilayerstructure is a 1 p.m wide layer of n-type GaAs followed by 1 p.m of p-type GaAs.Another smaller (n-type) multilayer structure consisting of three AlGaAs layers and twoGaAs layers is visible on the far right. The material to the right of these multilayers is n-type GaAs. The dc component of the data was removed by high pass filtering during image50(a) x-range=3 pm, z-range=200 A(b) x-range=1.3 pm, z-range=110 A.Fig. 5.7.(a) and (b) Topographic contrast at the GaAs/AlGaAs interface, attributed to preferential cleavingalong the interface.51•Fig. 5.8. Cleavage steps on the (110) GaAs surface, oriented in the (111) direction. A GaAs/AlGaAsmultilayer structure is on the right. x-range=1.8 pun, z-range=80 A.Fig. 5.9. STM image of a group of n and p layers grown on a semi-insulating GaAs substrate. A pnjunction is sandwiched between the two multilayer structures. x-ran2e=3.3 jnn, z-range=10 A.52acquisition, so that the image appears as if it was illuminated from the left. Nonlinearity inthe piezo tube response (see§2.3) has distorted the lateral scale in this image. Thetopographic contrast associated with compositional modulation in both multilayer structuresis probably due to selective etching of the Al-containing layers. In addition, there iscontrast associated with the dopant type. The p-type regions of the epilayer all appearlower than the n-type regions. This effect is attributed to an electronic contrast mechanismdiscussed in the next section. 45A higher magnification image of the 16 period multilayer structure, acquired at atunneling set point of (+4 V. 0.1 nA), is shown in Fig. 5.10. These layers were growndirectly on the semi-insulating substrate. The MBE cleaning procedure, which involvesthermal desorption of a thick protective oxide as discussed previously (§3.3a), roughensthe substrate surface. The roughness is manifested in this image by the defect at thesubstrate/epilayer interface, encountered midway through the scan. This image indicatesthat the roughness is substantially reduced within one or two of the layers, which are20 nm thick. A topographic scan line across the layers is also shown. The finite tip radiusof the tunneling probe (§2.2) limits the resolution of the AlGaAs layers, which are lowerthan the GaAs layers. This is because the sides of the tip image the adjacent GaAs layerswhen the probe is positioned over an AlGaAs layer. The resolution of the the individualGaAs layers is about 1 nm in the p-region, based on the edge definition in the image. Therelative thicknesses of the GaAs and AlGaAs layers can be deduced from the width of theflat portion of each peak in the p-type region of the scan line, which is interpreted as theGaAs layer thickness. Note that this may not be true for the n-type multilayers, where tipimages due to roughenning of the n-type GaAs (§3.3c) obscure the resolution. Based onthis assumption the GaAs and AlGaAs layers are found to be 23 ±2 nm thick. This is inrough agreement with the nominal values of 20 nm and 19 nm for the GaAs and AlGaAsrespectively46, deduced from the growth rate calibration of the molecular beam fluxes.532010200 1200800It^I1000400^600Fr(-1154(a)nm(b)Fig.5.10. (a)Higher resolution image of the 16 period GaAs/AIGaAs multilayer structure and (b)topographic scan line (same lateral scale) across the multilayers. The first 8 periods are n-type and theremaining 8 periods are p-type. The p-type region is lower than surrounding n-type material by about 5A.From the p-type region of the scan line, the GaAs and AlGaAs layers are determined to be --.23 nin thick.5.5. Carrier-type Contrast in images of GaAs up StructuresThe image in Fig. 5.10 had not been high-pass filtered, and so the lower p regionof the superlattice appears darker than the adjacent n-regions. The scan line shows this n-pcontrast to be about 5 A. The transition from n to p-type occurs within one layerthickness, so the junction position is located with at least 20 nm resolution. This is lessthan the depletion width, which is about 30 nm for this doping (2x1018 cm-3). Possiblythe resolution is enhanced by the wider bandgap of the AlGaAs material in the depletionlayer. The three-dimensional rendering (Fig. 5.11) of a pnp structure is taken from animage obtained on a different sample, at the same tunneling set point. The dopingconcentration for these layers was - 5x1017 cm-3. Again the p regions appear about 5 Alower than the n-region. The contrast is attributed to the different electrical properties of theFig. 5.11. Three-dimensional rendering of a pnp structure on GaAs, showing apparent topographic contrastof about 5A. Lateral and vertical scales in angstroms.55p and n layers. This interpretation is consistent with the fact that the observed heightchange is about the same as that observed in recent UHV measurements10 of an untreatedGaAs pn junction cleaved in situ and imaged at a tip bias of +3 V. Also, atomic forcemicroscopy measurements, which are insensitive to electronic structure, find47 thattopographic images of the chemically treated surface exhibit no contrast which correlateswith layers of alternating dopant type.To better understand the apparent topographic contrast in the STM images betweenthe p and n type material, the current-voltage characteristics for the STM tunnel junction arecalculated, using a model similar to that proposed by Feenstra and Stroscio9. The fittingparameters of the model are the sample surface affinity, the tip work function, the tunnelingdistance and the tunneling area. The surface band bending is determined from a onedimensional solution to Poisson's equation using degenerate statistics to accommodateinversion and accumulation at the surface.48 Surface states in the bandgap are neglectedconsistent with the chemical passivation of the samples. Conduction is by direct tunnelingfrom filled states on one side of the barrier to empty states on the opposite side, or bythermionic emission over the surface Schottky barrier followed by vacuum tunneling. Thetunneling current through the vacuum barrier is calculated from the Bardeen expression49using the WKB approximation, including image force lowering. Tunneling through thespace-charge region is neglected. For applied voltages large compared to the bandgap, theknown bulk band structure of GaAs50 along lines of symmetry is used in order to evaluatethe phase space integrals which define the tunneling current. The energies of carriers noton a symmetry line are interpolated. The phase space integrals are computed by performinga direct Monte-Carlo summation over the Brillouin zone.51Using this model, the tunneling distances at the imaging set point (+4 V. 0.1 nA)were found to be 15 A and 12 A over the n-type and p-type regions respectively. Aneffective tunneling area of 100 A2 was used along with an affinity of 4.1 eV for thesemiconductor and a work function of 5.3 eV for the metal tip. The difference between the56two tunneling distances is in rough agreement with the 5 A contrast we observe at the(+4 V, 0.1 nA) set point, and is consistent with the qualitative explanation10 of thiscontrast presented elsewhere. For the p-type case, conduction is due to tunneling fromoccupied states in the valence band to empty ones in the tip. For the n-type case thisconduction mechanism is also present, but the major current contribution comes from thatpart of the conduction band which bends beneath the Fermi level at the surface, as shownin the band diagram in Fig. 5.12. This is because the vacuum barrier height for conductionband tunneling is lower by the bandgap energy (1.4 eV) than the barrier for valence bandtunneling. This explains why the vacuum gap must be reduced over the p-type regions inorder to maintain the same current.electronaffinity57metalworkfunctionmetalFermilevelFig. 5.12. Energy band diagram showing conduction mechanisms on n-type material at the scanning setpoint voltage (+4 volts on the tip). The barrier for tunneling through the vacuum is less for electrons inthe conduction band (CB) than for electrons in the valence band (VB) by the bandgap energy.5.6. Scanning Electron Microscopy of GaAs Epilayer StructuresField emission SEM micrographs of the epitaxial structures were obtained' 4 on thecleaved (110) surface and compared with the crossectional STM measurents of the samestructures. It was found that the GaAs/AlGaAs multilayers were well resolved by theSEM, following the same chemical treatment used in preparing the samples for STMimaging. Fig. 5.13 shows a high resolution SEM micrograph (a) of a multilayer structure,accompanied by an air STM image (b) of the same structure. Since the SEM is notsensitive to topographical modulation of a few nanometers the contrast in the multilayers isbelieved to be electronic in origin. A lower magnification micrograph of the epilayercontaining the 16-period multilayer discussed previously (§3.3.c.), is presented inFig. 5.14. A cleavage step is present in the multilayer structure, which is on the far leftside. Note that the SEM is also able to resolve the n and p layers. In contrast with theSTM measurements, the n regions appear darker than the p regions, indicating that fewersecondary electrons are emitted from the n-regions. This contrast was present even forsamples which had not been subjected to any preparatory etch, indicating that the contrastmechanism is electronic. Better contrast was achieved when the sample was scanned with5 keV electrons as compared to 20 keV.An explanation for the dopant-type contrast in the SEM micrographs is as follows:The Fermi level is pinned near the middle of the band gap at the surface. This pinningarises from surface states in the band gap, which might be induced by damage from thescanning electron beam. Surface pinning results in a depletion layer at the surface for bothdopant types. The band-bending associated with the depletion layer in the n-type materialwould act as a barrier for the emission of the secondary electrons, whose energies are ofthe order of an electron volt. In the p-type material, the band-bending is such that a hotelectron in the conduction band will encounter a reduced barrier for escape from thesurface, compared to an electron in the same state in the n-type material.5859(a)(b)Fig. 5.13. Comparison of high resolution (a) SEM and (b) STM images of the same GaAs/AlGaAssuperlattice. The layer thickness is 20nni and the z-range in the STM image is 30A.000711 5.0 k V >2.0K 1.50pmFig. 5.14. SEM micrograph of epilayer containing the 16-period multilayer structure. The contrast betweenthe n and p-type material is believed to be electronic in origin (see text).606. Model for Current-Voltage CharacteristicsThe essential physics of the tunneling process is described by a model whichassumes that the only conduction mechanism for electrons from the semiconductor to themetal tip is by thermionic emission over the Schottky barrier, followed by elastic tunnelingthrough the vacuum barrier. This model is only applicable for small values of applied bias,when the Fermi level of the metal is within the bandgap of the semiconductor. At largerapplied voltages, direct tunneling occurs, either from filled states in the metal to empty onesin the conduction band of the semiconductor, or from filled states in the valence band toempty ones in the metal. A more general model which includes these conductionmechanisms is then needed, such as the one discussed on p. 55. The description givenhere is similar to the metal/insulator/semiconductor (MIS) theory of Card and Rhoderick.52Energy band diagrams at the surfaces of the metal probe tip and an n-typesemiconductor surface are shown schematically in Fig. 6.1. The semiconductor surface isassumed to be free of states in the band gap, so the Fermi level is not pinned. In thesediagrams, the horizontal axis represents distance in the direction perpendicular to the planeof the surfaces. The vertical dimension is potential energy in volts, with the conventionthat negative voltage (corresponding to higher electron potential energy) is up. Om is themetal work function, and xs and Eg are the semiconductor electron affinity and band gap,respectively. 4 is the difference between the bulk conduction band minimum, CB, and thesemiconductor Fermi level, efs, which decreases as the doping concentration is increased.wm, iva3, and xvvB represent the probability amplitudes which describe the wavefunctionsin the vacuum for electrons originating from the metal Fermi level, semiconductorconduction band minimum, and valence band maximum respectively. The decay of thesewavefunctions with distance, s from the surface, due to the vacuum energy barrier, Ovac isgiven by lif—e-ks where k=A1 2m(I)vac and m is the free electron mass. From the diagram,11261Ovac is given by Om, xs, and xs+Eg respectively for electrons originating from the metalFermi level, conduction band minimum and valence band maximum.62EFmn - type semiconductorFig. 6.1 Energy band diagrams for a metal surface and n-type semiconductor surface.VsEFm CBEFsVBFig. 6.2. Band-bending in the semiconductor induced by the tip. Zero bias case.Fig. 6.2 illustrates the situation when the metal and semiconductor wavefunctionsoverlap (ie. when the tip is within tunneling range of the sample surface). No externalvoltage is applied between the probe and sample so the Fermi levels must align. In thiscase, (0111>Xs+4 so electrons in the semiconductor transfer into the metal, establishing aspace-charge region of uncovered donor ions at the semiconductor surface. This results ina voltage drop, Vd in the semiconductor due the band-bending in the space-charge region.Continuity of the normal component of the displacement field at the surface means that anadditional voltage drop, Vs exists in the air gap, s, between the tip and the sample. If themetal and semiconductor surfaces are assumed to be planar,63V 2e ssc Vd Egapw'2e,cs Vdwhere W =^`qND • (1)Esc is the semiconductor dielectric constant (e5c-13 for GaAs and 11 for Si) and q is themagnitude of the electronic charge. eoap is the dielectric constant in the air gap, and isassumed to be unity. W is the width of the space-charge region calculated using thedepletion approximation, which assumes that the space-charge density is just the donordensity, ND.Fig. 6.3 shows the voltage drops in the gap and semiconductor when tunneling onn-type material at positive tip bias, V. The difference in the metal and semiconductor Fermilevels is seen to be:V =^Xs Vs Vd^ (2)EF. /Fig. 6.3. Voltage drops in the vacuum gap and semiconductor with the tip positively biased.This result holds only if the Fermi level remains constant in the space-charge region. Thisis a reasonable approximation to make provided the transmission coefficient through thevacuum gap is much less than unity. The net elastic tunneling current density, Jms from themetal to the semiconductor is found by summing over the contributions at each energy, e:Jms = -q Ye T(e).pm(e).Psc(e)-{fm(e).{ 1-fsc(e)} — Pc(e).{ 1-fm(E)}{, (3)where p(e) is the density of states and f(e) and { 1-f(e)} are the probabilty a state isoccupied or unoccupied, respectively. The superscripts m and sc designate metal orsemiconductor states. The summation over momentum states at each energy is notexplicitly shown. For elastic tunneling, the transverse component of momentum must beconserved. Eq. 3 simply states that each component of the current density is the productof occupied initial states on one side of the junction and unoccupied final states on the otherside, weighted by the transmission coefficient, T(e) for tunneling through the vacuumbarrier.The calculation of the current density is greatly simplified for small appliedvoltages, when the metal Fermi level is within the semiconductor bandgap, if oneapproximates the expressions for occupied states in the conduction band and unoccupiedstates in the valence band by delta functions located at the conduction band minimum andvalence band maximum respectively. This approximation is reasonable because the decayof the Fermi-Dirac distribution, f(e) with energy above the Fermi level (exponential in theBoltzmann limit), is much more rapid than the increase in the density of states p(c), withenergy above the conduction band minimum or below the valence band maximum (asquare-root dependence in the effective mass approximation). With this approximation, thesum over energies in Eq. 3 is reduced to two terms. The first term, due to the contribution6 4from the conduction band, becomes:f(Vd) 1-f(Vd-V)} — f(Vd-V) 1-(V&}= f(Vd) — f(Vd-V)qVdexpl-^[1— exP{kT}],kTwhere the transmission coefficent and density of states prefactors have been omitted andf(e) has been approximated by the Boltzmann distribution in the last step. Including the s-dependence of the transmission coefficient and converting to the standard convention (Vpositive for positive tip bias and JCB positive for electrons flowing into the tip):qV CB 1], where k–A/ 2m XsJ^c'c e- 21cs • expl-^d I Fe nI 1\-7-1kT^x. kT^ 112 (4)In this expression, the trapezoidal vacuum barrier for conduction band tunneling has beenapproximated as a rectangular barrier of height thvacXs, to estimate the s-dependence of they =transmission coefficient. Using xs=4 eV, the affinity of (110) GaAs, this exponentialprefactor can be written as exp {- —s }, where 50-0.5 A. This results in an order ofsomagnitude attenuation in tunnel current per angstrom increase in tip-sample separation.Eq. 4 is the same as that for a planar metal-insulator-semiconductor (MIS)Schottky diode. A large tip-sample separation corresponding to a thick insulating layer inthe MIS diode results in a 'leaky' diode characteristic: as s increases, Vd0 by Eq. 1, sothat in reverse bias the metal Fermi level is raised above the conduction band minimum ofthe semiconductor, resulting in a large reverse current. For very small tip-sampleseparations (s0), V—>Vd+constant (according to Eqs. 1 and 2), and the expressionqV approaches that of an ideal Schottky diode: J(V)=J0{ exp(kT) - 1}. In this case, all of theapplied voltage drop is in the semiconductor.An expression similar to Eq. 4 can be obtained for valence band tunneling. For ann-type semiconductor, this term can become important when the tunnel junction is reversebiased (negative tip voltage), so that the valence band maximum approaches the Fermi65JCBlevel. In this case electrons in the metal can tunnel directly into the empty states in thevalence band, but must be thermally excited over an energy barrier in order to tunnel intothe conduction band, if the metal Fermi level is below the conduction band minimum. Notethat the barrier height for valence band tunneling (Ovac'Eg+Xs) is larger than the barrier forconduction band tunneling, however, resulting in a smaller transmission coefficient, soeither process may dominate, depending on the details of the band-bending. For farforward bias, as discussed in §5.5, conduction band tunneling dominates when the surfaceis accumulated so that the conduction band minimum dips beneath the Fermi level. In thiscase, direct tunneling to empty states in the metal occurs from filled states in both thevalence and the conduction bands, but the transmission coefficient is higher for theconduction band electrons. Note that Eq. 1 does not apply in this case, because degeneratestatistics must be used to determine the band-bending when the Fermi level approacheseither band.667. Two-Dimensional Characterization of Electronic StructureIn order to learn more about the electronic contrast observed in the constant currentimages, current-voltage (IV) measurements were performed at selected regions in thecleaved epilayers. IV curves acquired at n and p-type regions resemble those of MISSchottky barrier diodes, discussed in Chapter 6. The contrast between the n and p typecharacteristics is enhanced as the tip-sample separation is reduced.The n/p contrast in the IV curves is exploited in a new current-voltage imaging (WI)technique45, which is designed to separate the electronic contrast in the STM images fromthe topography. In this technique, the current is measured at a preselected sampling voltageafter setting the tip height independently. Various combinations of tip-sample separationand bias voltage were tried. Contrast between p and n regions was improved when the setpoint voltage which determines the tip-sample separation was kept small, consistent withthe reduction in lateral band-bending described at the end of this chapter.7.1 Electronic Characterization MethodsAs discussed above, two methods — acquisition of current-voltage (IV)characteristics, and current-voltage imaging (WI) — were used to identify and investigateelectronic contrast in the epilayer structures. For both methods the current readings aremade with the tip positioned at a constant height over the sample, using the methoddescribed in Appendix A. The tip bias is left at the imaging set point throughout theprocedure, and changes in the tip-sample voltage are made by adjusting the sample bias.The tip-sample separation is set by adjusting the sample voltage with the current heldconstant by the feedback loop. IV characteristics are acquired by ramping the samplevoltage and measuring the tunneling current with the feedback loop disabled. Current67readings in the IVI scans are made at the preselected sampling voltage only. In the IVcurves the current is plotted as a function of the difference between the tip and samplevoltages; typically the data of twenty or more successive voltage ramps are averaged toreduce the noise bandwidth. The tip-sample separation is reset between each voltage ramp.In the IVI scans, the tip translation is done at the imaging set point to minimize damage tothe surface.Because the data acquisition time is long (typically 200 ms per pixel), larger pixelsand smaller scan ranges were used in the IVI scans, as compared with the constant currentscans. In this sense, the IVI technique is a practical compromise between the higherresolution of the constant current mode, and the electronically richer information in the IVcurves. Once the scan is finished, the IVI results are displayed as a greyscale image similarto the constant current images, except that the greyscale represents the range of currentreadings. Leakage currents between the probe tip and the grounded inner-wall electrode onthe piezo tube (see §2.1) are of the order of 30 pA at +3 V on the tip. Because the tip biasremains fixed, the tip leakage current contributes a dc shift to the IV and IVI currentreadings, which can be subtracted after data acquisition.7.2. Dependence on Tip-Sample SeparationFig. 7.1 shows two sets of IV curves, obtained while tunneling on the cleaved(110) surfaces of (a) n-type (1x1017 cm-3) Si and (b) n-type (2x1018 cm-3) GaAssubstrates. The surfaces had been prepared for imaging using the procedures described in§3.2a and §3.3b for Si and GaAs respectively. The curves in both (a) and (b) wereobtained at successively decreasing tip-sample separation, s, by reducing the tip-samplevoltage of the set point with the feedback enabled. The feedback was disabled while themeasurements were made. As with other work53 on passivated Si, the curves in (a)become more asymmetric as s is reduced, consistent with enhanced rectification as the68+0.8V+0.6V+0.6V+0.8V+0.5V0.30.600+0.4V-0.3junction characteristic approaches that of an ideal Schottky diode (see discussion afterEq. 4 in Chapter 6). The GaAs curves in (b) are all asymmetric, possibly because theband-bending is still significant at the largest tip-sample separations represented in the69figure.-0.6^-0.2^0.2^0.6Tip-Sample Voltage (V)-1.5^-1^-0.5^0^0.5^1^15Tip-Sample Voltage (V)Fig.7.1. Current-voltage characteristics on (a) n-type Si and (b) n-type GaAs obtained in air at various tip-sample separations determined by the operating set point voltage. The current at the set point voltage is1 nA in (a) and 0.1 nA in (b).The data also show that breakdown in reverse bias (tip-sample voltage < 0) occurssooner as s is reduced. The mechanism responsible for this behaviour54 is illustrated by70the band diagrams in Figs. 7.2 and 7.3 for tunnel junctions with small and large vacuumgaps respectively. At zero bias the band bending associated with the the difference in themetal and semiconductor work functions results in a Schottky barrier which is greater forthe junction with smaller s, as expected in the MIS model. As this junction is reversebiased, the band-bending increases until the valence band maximum coincides with thesemiconductor Fermi level (Fig. 7.2b). Degenerate statistics must be used to describe thecarrier populations in this case, so the model in Chapter 6 does not apply. The result of theinversion is that the band-bending increases at a substantially reduced rate so that theenergy difference between the metal Fermi level and the semiconductor conduction band isreduced with increasing reverse bias (Fig. 7.2c), resulting in an increase in tunnel current.Initiation of this effect requires larger reverse voltages for junctions with greater vacuumgaps, because the zero-bias band-bending is smaller (Fig. 7.3a.).V . 0^ 0 < IV « Eg^ IVI Eg -EtEgFig. 7.2. Reverse break down in tunnel junction IV characteristic for the case of small tip-sampleseparation. The zero-bias band-bending (left) is quite large so that degenerate conditions occur at a smallreverse bias voltage (middle). Larger reverse biasing (right) results in reduction of the Schottky barrier andan increase in reverse current.VAFig. 7.3. Reverse break down for the case of a large tip-sample separation. The band-bending at zero bias(left) is small due to the drop in the vacuum gap, so degenerate conditions do not occur until a relativelylarge reverse bias is applied (far right).7.3. Dependence on Carrier TypeThe results of current-voltage measurements performed over n-type and p-typeregions of (a) Si and (b) GaAs surfaces are presented in Fig. 7.4. In each case theoperating point which sets the tip height is the same for the n and p-type IV curves. Thecurves intersect at the common set point in the positive quadrant of each graph. The setpoint voltage for these measurements is smaller than the imaging voltage, in order toenhance the asymmetry in the IV curves as discussed in the previous section. Fig. 7.4ashows IV characteristics obtained at a set point of (+0.5 V, 1 nA), over the n-typesubstrate and p-type cap (both 1x1017 cm-3) of the sample on which the Si/Ge superlattices(§5.2) were grown. Fig.7.4b shows IV curves obtained over the n-type substrate(2x1018 cm-3) and p-type capping layer (3x1017 cm-3) of a GaAs sample, at a set point of(+1 V, 0.1 nA). The acceptor doping concentration of the p-type epilayer in this samplewas determined from Hall effect measurements.55 These IV characteristics are typical ofmany n-type and p-type curves obtained on Si and GaAs, and can be qualitativelyunderstood in terms of ideal Schottky diode behaviour. When the tip is negative withrespect to the sample, the p-type material is forward biased and the n-type is reverse biased.The larger reverse current of the p-type curves in Figs. 7.4(a) and (b) indicates that the thetip is closer to the p-type surface than the n-type surface at the operating set point.Presumably the reverse would hold at a negative value for the set point voltage, althoughinstabilities at negative tip bias (see §2.1) prevented the acquisition of meaningful data totest this.The curves in Fig.6.4b are accompanied by theoretical fits. The theoretical curvesare calculated with the model used in §5.5, using the same values for the semiconductoraffinity and metal tip work function. The tunneling area was set equal to 100 A2 and thevacuum tunneling gaps adjusted to fit the data. The fits shown were obtained for tunnelinggaps of 6.5 A and 3 A for the n and p-type curves respectively.5171-0.6^ 0^ 0.6Voltage (V)(a)(b)72Fig. 7.4. IV characteristics obtained over it- and p-type regions on (a) Si and (b) GaAs. The n and p curvesintersect at the operating set point which is common for both.7.4. Spatial ResolutionFig.7.5 is a series of IV curves obtained across the Si/Ge superlattice, followingacquisition of the image in Fig.3.16. The image is repeated in the inset for reference. Itwas hoped that the measurements would reveal electronic contrast in the superlattice,associated with the different band gaps of Si (Eg=1.1 eV) and Ge (Eg=0.67 eV). Datawas acquired at equally spaced intervals along a line indicated by the white bar shown inthe inset, at a set point of (+1 V, 0.5 nA). These curves are typical of more extensivedata taken across these multilayer structures. Although there are differences in the IVcurves as a function of position in the multilayer, there are no systematic differences which-0.2^-0.1^0^0.1^0.2^0.3^0.4^0.5^0.6Voltage (V)Fig. 7.5. Series of IV curves obtained across the Si/Ge superlattice multilayer structure shown in the inset.The curves, which were obtained at equally spaced points along the white bar in the inset, are spaced apartvertically for clarity. The operating set point was (+1 V, 0.5 nA).73correlate in a simple way with the layer periodicity. The depletion width (Eq.2, Chapter 6)for 0.1 V band-bending at the surface is about 300 nm for these intrinsic layers(-1015 cm-3). Most likely the band-bending, which extends laterally across the surface aswell as into the bulk, obscures the delineation of the 20 nm thick layers.A series of IV characteristics obtained at a set point of (+0.5 V. 0.5 nA) across annp junction grown on an n-type Si substrate56, is shown in Fig. 7.6. The curves, whichwere acquired sequentially at —115 nm intervals along a line perpendicular to the npjunction, are spaced apart vertically for clarity, so that the first curve in the sequence is atthe origin. The composition of the junction, in the direction of the measurement sequenceis: n+ substrate (5x1018 cm-3); 900 nm lightly doped (1017 cm-3) n-type; and p+(2x1019 cm-3) capping layer (100 nm). The first three curves, which cover a range ofabout 350 nm, resemble the n-type curve in Fig. 7.4a, which was acquired at the same setpoint on a lightly doped (1017 cm-3) n-type substrate. These curves are therefore thoughtto correspond either to locations in the 900 nm n-type layer or in the n+ substrate. Bycontrast, the remaining curves in the series are essentially symmetric. Presumably thesesymmetric curves correspond to the p+ cap. IV curves (not shown here) on graphite (asemimetal) and gold surfaces were also found to be symmetric. The p+ material, which isdegenerately doped so that the Fermi level in the bulk is below the valence band maximum,might be expected to have a similar IV characteristic.An IVI-STM image acquired over the same area and at the same operating point asthe IV data is shown in Fig. 7.6b for comparison. The image is oriented vertically for easeof comparison with the IV data and has a lateral scale which corresponds to the verticalspacing of the IV curves. The current range greyscale in the image, acquired at -0.5 V, is+1.7 nA (white) to -1.7 nA (black). The bottom region is grey (no current) for about400 nm, consistent with the reverse bias region of the corresponding n-type curves. There7 4ce)0.4-5-0.8^-0.4^0Voltage (V)302520500.8(b)(a)is a transition to saturated black as the capping layer is approached, again consistent withthe IV measurements. Note that more noise is present in the IVI scan than might beexpected from the relatively noiseless IV curves. The data in this IVI scan were obtainedfrom a single measurement, whereas the data points in the IV curves each represent theaverage of 20 current readings acquired over successive voltage ramps.75Fig. 7.6. Series of IV characteristics (a) and IVI scan (b) taken across the n-p junction on Si described in thetext. The operating point was (+0.5 V, 0.5 nA) for both measurements, and the sampling point for the IVIscan was -0.5V. The current range .-,),reyscale in the IVI image is -1.7 nA (black) to +1.7 nA.(white).The apparent width of the p+ region indicated by the IV sequence and IVI scan ismore than 700 nm. However, only 100 nm of p+ material was grown in the cappinglayer. Tip induced band-bending, which would result in a 100 nm depletion region in then-layer, does not by itself account for the poor spatial resolution. Another mechanism, inwhich the equilibrium barrier height of the np+ junction is reduced by lateral band-bendingin the plane of the surface, would further affect the spatial dependence of the IV curves. Aqualitative description of this effect is given for the case of a measurement made on a p-region in close proximity to an n-region: When the tip is positioned over a p-type region,one would ordinarily expect only a small current to be drawn at positive tip bias (seeFig. 7.4). The situation changes in close proximity with an n-type region, if the lateralband-bending induced by the tip extends into the depletion region of the pn junction(Fig. 7.7). Whent the tip-induced band bending is small, the pn junction is effectively76z(a) P^ n/TIP /--------------(b)CBVBn^ CB^ VBTIPFig. 7.7. Tip-induced band-bending over a p-layer in close proximity with an n-region. z is perpendicular tothe sample surface and x represents a lateral dimension along surface. As the tip voltage is made morepositive (reverse bias for p-type), the np junction becomes forward biased (b) due to lateral band-bendingeffects.unbiased and no net current flows across the depletion region (a). As the tip becomes morepositively biased however, the barrier for transport of electrons from the n-type side of thejunction to the p-type side is reduced and a net current flows into the p-region beneath thetip (b). In effect, the pn junction becomes forward biased at positive tip voltages, when thetip is over the p-type side but close to the junction. The contrast between the n and pmaterial is thus limited to the difference between a forward biased MI-n diode (over the n-regions) and a forward biased MI-pn diode (over the p-regions), both of which haveexponential IV characteristics. The common set point ensures that the pre-exponentialmultiplicative factor is the same for both cases, essentially eliminating any contrast. Thespatial extent over which this mechanism operates depends on the details of the band-bending, but computer simulations16 show that it can be many times the depletion width.To improve the resolution, it is therefore necessary to reduce the band-bending at theoperating set point.Fig. 7.8 shows an IVI-STM measurement taken across the GaAs npn structure inFig. 5.9, in which the p-type region is 11.1,113 wide, and the doping level is 2x1018 cm-3for all three regions. The reference tip-sample separation was set by a (+0.5 V, 30 pA)operating point, and the current image was acquired at -0.9 V. The image clearly showsthe p-type layer between the two n-type layers. The data indicates that the current over thep-region is approximately -50 pA, while no current is drawn over the n-regions at thesampling voltage. This is consistent with the interpretation that the p-region is forwardbiased at this voltage. Possibly the voltage at the operating set point is small enough thatlateral band bending has not significantly affected the contrast in this image. A scan linetaken from a constant current image of the same structure is shown below the IVI image forcomparison. The image was obtained immediately before the IVI scan, at a set point of(+4 V, 0.1 nA). The apparent topographical contrast due to the conductivity type isnearly 10 A in this image.774191161111111111111111111111111111111n-type^p-type^n-type(a)785AO-50.2^0.6^1 .0^1.4(b)Fig. 7.8. IVI scan (a) and constant-current topographic scan line (b) across an npn structure on GaAs. Theoperating point for the IVI scan was (+0.5 V, 0.03 nA) and the data acquistion voltage was -0.9 V. Thecurrent range greyscale is -0.25 nA - 0.05 nA. The constant current set point in (h) = (+4 V, 0.1 nA).The IVI data acquired over the forward biased p-regions in these scans is noisy.An explanation is that the tip is not set at the same height above the surface at each point.Tip noise, possibly due to mobile adsorbates on the sample or tip, causes the tip-sampleseparation to fluctuate in the constant current mode with amplitudes of one or twoangstroms (since the control loop servo attempts to maintain a constant current by adjustingthe tip height). This fluctuation is a source of random error when the tip feedback controlis put on hold. Because changes in tip-sample separation of 1 A cause the tunnelingcurrent to change by approximately an order of magnitude, small fluctuations in the tipelevation will cause large fluctuations in the current-voltage characteristic.79(a)(b)Fig. 7.9 Constant-current images obtained before (a) and after (b) the acquisition of IV characteristics acrossthe doped layers. The damaged areas in (b) correspond to the locations where the IV measurements weremade. x-range=2 j.im, z-range= 68 A.Fig. 7.9 shows images of a GaAs np structure before and after the acquistion of IVcharacteristics across it. The constant current image was acquired at (+4 V, 0.1 nA) andthe IV operating point was (+1 V, 0.1 nA). The tunneling set point while translating theprobe was the same as the imaging set point. The voltage was ramped between -1 V and+1 V during the IV data acquisition. The images reveal that the IV scan has resulted insurface damage at the data acquisition points. Damage of this sort was observed after mostsuch scans, as well as in the IVI scans. As discussed in §3.1, oxidation is believed to beenhanced when the tip is biased negatively with respect to the sample. It is therefore likelythat the damage is induced during data acquisition, when the tip-sample voltage is rampedto negative values.808. Conclusions and RecommendationsThe STM is shown to be capable of imaging epitaxial layers on Si and GaAs withresolution of about 1 nm at ambient pressure. In particular, multilayer structures ofSi/SiGe and GaAs/AlGaAs with typical layer thicknesses of 20 nm are well resolved. Wetchemical treatments are needed to stabilize the freshly cleaved surfaces for imaging in air.Tunneling-induced surface modification occurs to varying degrees on the treated n-typeGaAs surfaces depending on the scanning conditions. The p-type GaAs is found to beresistant to tunneling-induced modification. The ST'M measurements are sensitive to carriertype, and layers of alternating n and p-type material type are resolved on GaAs for the firsttime in air. The pn junctions are located in constant current mode with better than 20 nmresolution.Current-voltage characteristics of the STM tunnel junction obtained on n and p-typeregions in the epitaxial layers exhibit rectifying behaviour. The asymmetry in the IV curvesis exploited to obtain carrier-type contrast in a new current-voltage imaging technique. Thistechnique makes it possible to separate the electronic contrast in the STM images associatedwith the doping levels from the contrast associated with surface topography.More work needs to be done to determine appropriate values for the modelparameters, in order to obtain more quantitative information from the IV curves. Forexample, the metal work function and semiconductor electron affinity need to be measuredfor these surfaces, since it is unlikely that they are the same as the values for the 'clean'surfaces in UHV. Also, it is conceivable that the dielectric constant in the air gap is greaterthan unity, since water will be present on the surfaces at ambient humidity. This wouldeffect the dependence of the IV characteristics on tip-sample separation, since a higherdielectric constant would reduce the voltage drop in the gap.81References1. C. Hill, J. Vac. Sci. Technol. B 10, 289 (1992).2. P. Roitman, J. Albers, and D. R. Myers, J. Appl. Phys. 55, 4439 (1984).3. C. Weisbuch and B. Vinter, Quantum Semiconductor Structures, Academic Press(1991).4. C. Weisbuch and J. Nagle, Physica Scripta, T19, 209 (1987).5. A. Zangwill, Physics at Surfaces, Cambridge University Press (1988).6. S. H. Goodwin-Johansson, M. Ray, Y. Kim, and H. Z. Massoud, J. Vac. Sci.Technol. B 10, 369 (1992).7. R. J. Behm, N. Garcia, and H. Rohrer, Scanning Tunneling Microscopy and RelatedMethods, Kluwer Academic Publishers, 1990.8. D. G. Cahill and R. J. Hamers, Phys. Rev. B 44, 1387 (1991).9. R. M. Feenstra and J. A. Stroscio, J. Vac. Sci. Technol. B 5, 923 (1987).10. S. Gwo, A. R. Smith, and C. K. Shih, K. Sadra, and B. G. Streetman, Appl. Phys.Lett. 61, 1104 (1992).11. R. Chapman, M. Kellam, S. Goodwin-Johansson, J. Russ, G. E. McGuire, K.Kjoller, J. Vac. Sci. Technol. B 10, 502 (1992).12. T. Tiedje and A. Brown, J. Appl. Phys. 68, 649 (1990).13. M. B. Johnson and J. M. Halbout, J. Vac. Sci. Technol. B 10, 509 (1992).14. R. Subrahmanyan, J. Vac. Sci. Technol. B 10, 358 (1992).15. H. Salemink and 0. Albrektsen, J. Vac. Sci. Technol. B 9, 779 (1991).16. S. Kordic, E. J. van Loenen, and A. J. Walker, J. Vac. Sci. Technol. B10, 496(1992).17. J. A. Dagata, W. Tseng, J. Bennett, J. Schneir, and H. H. Harary, Appl. Phys. Lett.59, 3288 (1991).18. J. A. Dagata and W. Tseng, App. Phys. Lett., submitted (1992).19. B. G. Streetman and Y. C. Shih, J. Vac. Sci. Technbol. B 10, 296 (1992).20. W. Vandervorst and T. Clarysse, J. Vac. Sci. Technol. B 10, 302 (1992).21. Hans Cerva, J. Vac. Sci. Technol. B 10, 494 (1992).22. A. J. Melmed, J. Vac. Sci, Technol. B 9, 601 (1991).8223. W. A. Harrison, Electronic Structure and the Properties of Solids, Dover Publications(1989).24. This calibration technique was suggested to me by R. Andrews and R. Coope.25. D. Rogers and T. Tiedje, Surface Science Letters, 274, L599 (1992).26. J. Jurgensen, J. Chem. Phys. 37, 874 (1962).27. G. S. Higashi, R. S. Becher, Y. J. Chabal, and A. J. Becker, Appl. Phys. Lett. 58,1656 (1991).28. J. A. Dagata, J. Schneir, H. H. Harary, C. J. Evans, M. T. Postek, and J. Bennett,Appl. Phys. Lett. 56, 2001 (1990).29. T. Van Buuren, M. K. Weilmeier, I. Athwal, K. M. Colbow, J. A. Mackenzie, T.Tiedje, P. C. Wong, and K. A. R. Mitchell, Appl. Phys. Lett. 59, 464 (1991).30. C. Lavoie, S. R. Johnson, J. A. Mackenzie, T. Tiedje, and T. Van Buuren, J. Vac.Sci. Technol. A 10, 930 (1992).31. Takahisa Ohno, Phys. Rev. B 44, 6306 (1991).32. C. J. Sandroff, M. S. Hegde, L. A. Farrow, R. Bhat, J. P. Harbison, and C. C.Chang, J. Appl. Phys. 67, 586 (1989).33. J. Shin, K. M. Geib, and C. W. Wilmsen and Z. Lilliental-Weber, J. Vac. Sci.Technol. A 8, 1894 (1990).34. XPS and AES measurements performed by P. C. Wong.35. J. Massies and J. P. Contour, J. Appl. Phys. 58, 806 (1985).36. PES performed by T. Van Buuren and T. Tiedje37. Penn, D. R., Phys. Rev. B 13, 5248 (1976),38. T. Tiedje, K. M. Colbow, and D. Rogers., J. Vac. Sci. Technol. B 7, 837 (1989).39. M. Simard-Normandin, L. Weaver, and D. Vacca, and D. Rogers, A. Vitkin, and T.Tiedje, Can. J. Phys. 69, 290 (1991).40. SEM performed by M. Weiss. The Si/Ge sample was provided by J. Bean and theSTM study was done in collaboration with A. Sanderson and T. P. Pearsall.41. T. Pinnington, A. Sanderson, T. Tiedje, T. P. Pearsall, E. Kasper, and H. Presting,Thin Solid Films, accepted (1992).42. This sample was supplied by D. C. Houghton.43. All the GaAs samples studied in this work were grown by C. Lavoie and S. R.Johnson.44. SEM performed by C. Lavoie.8345. T. Pinnington, S. N. Patitsas, C. Lavoie, A. Sanderson, and T. Tiedje, J. Vac. Sci.Technol, accepted (1992).46. C. Lavoie, private communication.47. J. A. Dagata, private communication.48. A. Many, Y. Goldstein, N. B. Grover, Semiconductor Surfaces, North-HollandPublishing (1965).49. W. A. Harrison, Solid State Theory, Mc Graw-Hill (1970).50. J. S. Blakemore, J. Appl. Phys. 53, 123 (1982).51. The theoretical modelling and calculations were performed by S. N. Patitsas.52. H. C. Card and E. H. Rhoderick, J. App!. Phys. 4, 1589 (1971).53. W. J. Kaiser, L. D. Bell, M. H. Hecht, and F. J. Grunthaner, J. Vac. Sci. Technol. A6, 519 (1988).54. S. N. Patitsas, private communication.55. C. Lavoie, private communication.56. This sample was provided by J. P. Noel and G. Mattiussi.84Appendix. Current-Voltage Measurement ProcedureThis section gives the details of the method used to obtain the current-voltage (IV)characteristics and current-voltage imaging (IVI) data presented in §7.1 and §7.2. Theprocedure was developed in collaboration with A. Sanderson, who wrote the software andassisted in testing.For this procedure to work, the sample in the STM must be isolated from thechassis ground. The sample holder voltage input is connected to the BNC cable labelled'sample' and the tip current output from the controller is connected to the BNC labelled'current'. The dual-pin connectors 'tip bias' and 'relay' are connected to the 8-pin D-connector (STM preamp cable) and the 16-pin D-connector (computer control cable)respectively, at the back of the STM controller module. Both the IV characteristics and theIVI images are acquired after the constant-current image is obtained by exiting the image-acquisition software and running the program 'IVI'. The feedback loop is put undercomputer control by switching the integrator switch on the controller from 'ON' to'HOLD', following the program prompt. This enables the software to interrupt thefeedback control loop by means of the relay in the controller. The location at which data isto be acquired is selected by moving the cross-hairs to the corresponding location on theimage. For an IVI-STM scan, the upper left and lower right corners of the desired scanarea are selected, as well as the effective pixel size (ie. the spacing between measurementlocations).The essential features of the measurement procedure are illustrated schematically inFig. Al. After the user has entered the pertinent data acquisition parameters as discussedbelow, the scanning probe is moved to the selected location (ta). As the schematicindicates, the feedback control loop is enabled (relay 'ON') while the probe is in motion,and the tunnel current and tip bias remain at the values set by the controller. These are the85taON ^FeedbackOFFScanning\flip^VoltageVsample0 Vtb^tc^td te    ItScanningHeighto A            same conditions as for constant-current imaging. Once the probe has been positionedlaterally, a voltage is applied to the sample, which changes the net tip-sample voltage to thedata-acquisition set point (tb). The tip-sample separation, s is adjusted by the feedbackcontrol to keep the tunnel current constant. Data are acquired by ramping the samplevoltage and sampling the tunnel current, It (tc). The feedback control is disabled (relay'OFF') during data acquisition, so s is not changed. Usually more than one voltage rampis performed, and the results averaged, to improve the signal-to-noise ratio. s is resetbetween voltage ramps by momentarily enabling the feedback control loop (W. In IVImode, the current is measured at one selected voltage only. Once the run has beencompleted, the tunneling configuration is returned to the normal constant current imagingsettings (te). In the case of IVI, the probe is moved to the next location. Note that the tipbias, which is set by the control module, remains fixed throughout the entire procedure.Fig. Al. Voltage waveforms in the IV and IVI measurements. The tip voltage is unchanged throughoutthe procedure. Typically 20 voltage ramps are averaged each run, and the tip-sample separation is resetbetween eacn ramp by briefly enabling the feedback as shown.86A typical sample voltage waveform for IV data acquisition is shown in detail inFig. A.2. Note that the voltage is ramped down and up while data are acquired (boldportion of waveform). The current readings from the two ramps are averaged. The voltageis actually incremented in discrete steps, where the magnitude of the increment, or stepsize, is set by the user. During data acquisition, the current is sampled after each voltagestep. The ramp rate for voltage changes made while data is not being acquired is setseparately. If the step size is greater than the actual ramp range, the voltage ramp is madein a single step. Several time delays are used and are also set by the user. A description ofeach is given below, along with the default setting.T1:time delay after resetting relay. The mechanical relay specifications are 6ms to activate(ON) and 3 ms to disengage (OFF). Default setting: 10 msT2:time delay after setting or resetting tip-sample separation for data acquisition. This is toallow the feedback loop establish the set point current after the sample voltage has beenchanged. Default setting: 100 msT3:time delay after setting the tip-sample separation to the scanning set point in IVI mode,before moving the probe to the next imaging location. Default setting: 100 msT4: time delay after each motion of the probe. This sets the scan rate for positioning theON ^FeedbackOFF_Fig. A.2. Detailed schematic of the sample voltage waveform during an IV measurement, showing thevarious time delays (see text).87probe, in both IV and IVI modes. The probe motion is in discrete steps, with each stepcorresponding to 2 pixels on the screen or 1/256 of the image width. Default setting:10 ms. This corresponds to 2 seconds to scan the full width of the screen.In order to establish an acceptable voltage range for the IV data acquisition, the'quick run' feature may be executed. Here, allowed current limits are entered as inputs.The sample voltage is ramped up and down while monitoring the current until the currentlimits are reached or the maximum voltage range (±10 V on the sample) is exceeded. Thecorresponding voltage range is given as output.As discussed on p. 78, tip noise might cause the tip-sample separation, s to be setinconsistently, perhaps by one or two angstroms. Accordingly, s is checked indirectly bymonitoring the tunneling current immediately after the feedback is disabled. A specifiednumber of current readings are taken (default setting: 50) and averaged. If these differ bymore than the specified tolerance (default setting: 50%) from the set point current, then s isreset by enabling the feedback for T2 seconds. This process is repeated until the toleranceis met or the number of permitted attempts (default setting: 10) is exceeded, in which casethe measurement is aborted. In IVI mode, the computer beeps and a default valuecorresponding to saturated white (+10 nA) is recorded for that data point.The data are displayed immediately after acquisition and may be saved on disk asASCII files. The current range greyscale for the IVI-STM image is adjusted by the userafter data acquisition to obtain the desired visual contrast. The actual data file is unaffectedby the contrast settings, which are not stored.88


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