Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Studies of oxide desoption from GaAs by diffuse electron scattering and optical reflectivity Van Buuren, Anthony W.H. 1991

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata


831-UBC_1991_A6_7 V36.pdf [ 4.25MB ]
JSON: 831-1.0084986.json
JSON-LD: 831-1.0084986-ld.json
RDF/XML (Pretty): 831-1.0084986-rdf.xml
RDF/JSON: 831-1.0084986-rdf.json
Turtle: 831-1.0084986-turtle.txt
N-Triples: 831-1.0084986-rdf-ntriples.txt
Original Record: 831-1.0084986-source.json
Full Text

Full Text

STUDIES OF OXIDE DESORPTION FROM GaAs BY DIFFUSE ELECTRON SCATTERING AND OPTICAL REFLECTIVITY  By Anthony William Henry Van Buuren B.Sc.  Simon Fraser University, 1987  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE  IN THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF PHYSICS  We accept this thesis as conforming to the required  standard  THE UNIVERSITY OF BRITISH COLUMBIA APRIL 1991 © Anthony William Henry Van Buuren  In  presenting this  degree at the  thesis  in  University of  partial  fulfilment  of  of  department  this thesis for or  by  his  or  requirements  British Columbia, I agree that the  freely available for reference and study. I further copying  the  representatives.  an advanced  Library shall make it  agree that permission for extensive  scholarly purposes may be her  for  It  is  granted  by the  understood  that  head of copying  my or  publication of this thesis for financial gain shall not be allowed without my written permission.  Department The University of British Columbia Vancouver, Canada  DE-6 (2/88)  ii ABSTRACT  We  have  determined  that  the  temperature  for  desorption  of  gallium oxide from GaAs increases linearly with oxide thickness, for oxide layers between about 6A and 26A the  oxide  desorption  temperature  was  from the diffuse the  front  determined reflectivity  surface  thicknesses  of  ranged  and  oxide  from from  thick. The temperature for  580°C the  to  630°C.  optical  The  band-gap  wafer  measured  of the sample, which was polished on  textured  layers  were  on  the  created  back  surface.  by varying the  Different exposure  time of the GaAs wafers to a low pressure oxygen plasma. The oxide thicknesses  were  determined  by  XPS  analysis.  Desorption  experiments were carried out in a V G V80H M B E system under a As4 beam equivalent pressure of 1.5 x 10"  ton.  5  Measurement  of  diffuse  light  scattering  shows an abrupt and non-reversible increase intensity  during the  oxide  desorption. This  using  a HeNe  laser  in the  scattered  light  suggests the  macroscopically roughened due to inhomogeneous  surface  is  desorption of the  oxide. The  oxide  desorption  was  also  studied  by  monitoring  the  secondary electrons produced by the high energy electrons from the R H E E D gun. After the gallium oxide desorption there is a reversible, order of magnitude,  increase  in the  number of  scattered  electrons  produced by the incoming primary beam. We interpret this result as evidence for some form of microscopic roughening.  iii T A B L E OF CONTENTS  ABSTRACT  ii  T A B L E OF CONTENTS  iii  T A B L E OF FIGURES  v  ACKNOWLEDGEMENTS  vii 1  I INTRODUCTION 1.1 1.2  Background Theory  of  3 Secondary  Electrons  and  Gallium  Droplet  Formation  5  H EXPERIMENTAL PROCEDURE 11.1  9  Wafer Preparation  9  11.2 M B E Idle Conditions  1 1  11.3 Oxide Desorption Conditions  14  m IN-SITU M E A S U R E M E N T TECHNIQUES  15  IV E X P E R I M E N T A L RESULTS  25  IV. 1 RHEED Pattern Spectrometer  25  IV.2  Mass  Measurements  26  IV.3  Ions  40  IV.4  Diffuse Electron Scattering  46  IV.5  Effect of Oxide Thickness on Secondary Current Onset  56  IV.5  Diffuse Reflectivity  64  V DISCUSSION  69  VI CONCLUSION  85  VTJ REFERENCES  87  iv  VIII.l APPENDIX 1 Optical  Temperature  91 Measurement  Technique  VHI.2 APPENDIX II Oxide Thickness Determination from XPS Results  91 93 93  V  T A B L E OF FIGURES  Figure 1. Schematic diagram of the plasma reactor  12  Figure 2. Schematic diagram of the V-80H M B E system  12  Figure 3 Schematic diagram of detector bias circuit  23  Figure  4a.  RHEED  diffraction  pattern  of  an  oxidized  substrate at 250°C  27  Figure 4b. R H E E D diffraction of wafer at 6 2 0 ° C  27  Figure 5a. Low temperature AsO peak  29  Figure 5b. High temperature AsO peak  30  Figure 6. Desorption of the high temperature A s O versus ozone oxidation time Figure  7.  Ga20  31  desorption  temperature  versus  ozone  oxidation time Figure  8.  33  Ga20  desorption  temperature  versus  plasma  oxidation time  36  Figure.9. Ga20 signal from the mass spectrometer during oxide desorption for three different oxidation thickness Figure  10.  Maximum  height  of  the  mass  38  spectrometer  Ga20 signal versus plasma oxidation time Figure  11.  Oxide  thickness  inferred  39 from  XPS  measurement of plasma oxidized GaAs Figure  12.  Gallium  oxide  desorption  41 temperature  as  a  function of the thickness of the surface oxide  42  Figure 13. Ion current as a function of negative bias on the plate and wire detectors  45  Figure 14. A rough measure of the energy distribution of the thermal ions Figure detector Figure  15.  47  Secondary electron  current as  a function  of  bias 16.  49 Energy  distribution  of  secondary  electrons  from an oxidized substrate a room temperature Figure  17. Secondary electron current recorded during a  50  vi temperature  ramp for  an  oxidized  and desorbed  GaAs  substrate Figure detector  53  18.  Secondary electron current as  a function of  bias  54  Figure 19. Energy distribution of secondary electron from substrate at 620°C Figure 20.  55  Secondary electron current as  a function of  substrate temperature for thin surface oxides Figure 21.  Secondary electron current as  58  a function of  substrate temperature for thick surface oxides Figure  22.  Mass  spectrometer  data  and  59 secondary  electron data measured during desorption of a 7A oxide Figure 23.  60  Secondary electron current measured during a  slow stepped ramp  62  Figure 24. Semilog plot of secondary electron current as a function of 1000/T °K Figure  25.  63  Diffuse  reflectivity  and  Ga20  mass  spectrometer signal measured during oxide desorption Figure 26.  Surface texture  of desorbed GaAs  65  substrates  observed with a Nomarski interference microscope Figure  27.  Temperature dependence  of  the  the  68 rising  edge of the Ga20 mass peak  71  Figure 28. Schematic of our model of the  inhomogeneous  oxide desorption process  77  Figure 29. Plot of equilibrium pressure of As2,As4, and Ga over GaAs FiguTe 30  83  Calibration curve for the  optical temperature  measurement technique Figure  31  X-ray  photoelectron  92 spectra  and Ga 3d core levels of oxidized GaAs  showing  the  As 95  vii ACKN0WLEIX3EMENTS  I would like to express sincere gratitude to my supervisor, Dr. Tom  Tiedje.  It was  his  advise  and insight  that  made this work  possible. I also wish to thank Kevin Colbow and Jim Mackenzie for their assistance in data collection and processing. A special thanks also goes to Dr. K.A.R. Mitchell and Dr. P.C. Wong for their  assistance  spectroscopy  in  obtaining  measurements.  x-ray  photoelectron-emission  1 I INTRODUCTION  The growth of high quality thin films of GaAs, by molecular beam  epitaxy  (MBE),  has  made  possible  new  optoelectronic  and  electronic devices. M B E is an epitaxial growth process involving the ultrahigh-vacuum ( U H V ) conditions. The flexibility of changing the surface  composition  temperatures  has  semiconductor Prior substrates,  varying  provided  the  new  beam  control  fluxes over  and  substrate  H I - V compound  heterostructures.  to the  by  growing wafer  epitaxial  layers  on  must be heat treated  gallium  to remove  arsenide  the  surface  oxide. A n oxide layer is formed on the wafer surface due to wet chemical etching [1,2], exposure to air, and ozone cleaning to remove surface contaminants [3,4]. Since the oxide is more volatile than the GaAs itself, it can be removed by heating with minimal damage to the wafer. Therefore a sacrificial oxide layer is often used to protect and to passivate the GaAs surface prior to epitaxial growth. In M B E systems the desorption of the oxide is conducted in an over pressure of arsenic to limit the arsenic loss from GaAs wafer after the oxide has been removed. Measurement molecular  beam  thermocouple temperature optical  to  of  the  epitaxy the  must  pyrometry.  systems  wafer  be  substrate  is  The  since  not  determined  temperature mechanical  feasible. using  temperature  at  is  difficult contact  Therefore  other which  methods a  the  of  a  wafer  such  surface  in  as  oxide  2 prepared  under  convenient since  standard conditions  desorbs  thermal calibration point. This  the desorption temperature depends  has  been  used  as  a  calibration has problems on the technique  used  to  create the oxide layer.[4,5] In the experiments monitoring  device  described here a new  was  employed.  The  optical temperature  wafer  temperature  was  inferred from the optical band gap which was determined from the diffuse reflectivity of the sample. [6] The samples were polished on the  front  surface  measurement for  samples  and  technique with  textured the  varying  on  the  desorption thicknesses  back  surface.  temperature of  was  oxide.  Using  this  determined  The  different  thicknesses of oxide were created on the GaAs surface by varying the exposure time to a low pressure oxygen plasma. In  a conventional  M B E system  any  in-situ  surface  analysis  technique must be positioned so as not to interfere with growth. For this reason reflection high energy electron diffraction (RHEED) which does not block the front surface of the sample, is ideally suited for use  in  the  inelastically  M B E system. scattered  In  this  electrons  experiment produced  by  the the  secondary high  and  energy  primary beam from the R H E E D gun are measured to give insight into microscopic roughening on the wafer surface before, during and after oxide  desorption. We also observed a new  surface roughening effect  associated  with the oxide desorption. This effect is clearly visible under intense illumination but is only marginally detectable  by electron scattering.  3 Although R H E E D  can effectively  measure the dynamics of the film  growth, [7] surface reconstructions [8] and microscopic roughness it is not  necessarily  morphology.  sensitive  changes  in  the  Macroscopic roughening was  visual inspection measuring  to  with an intense white  scattering  of  light  from  macroscopic  observed  surface  qualitatively by  light and quantitatively by the  wafer  surface  with  a  photodiode. The  data accumulated from these experiments made it possible  to formulate a model of the oxide desorption process  and provided  evidence for the formation of microscopic Ga droplets on the GaAs surface.  1.1 Background  In  M B E systems the  heating  the  sample  in  an  surface  arsenic  diffraction pattern on the R H E E D removed screen pattern  a sharp GaAs  which of  desorption  is  the  easily  is  flux,  normally removed by while  monitoring  the  screen. When the oxide has been  crystal diffraction pattern appears distinguished  amorphous  temperature  oxide  oxide  layer.  measurements  found to range between 5 6 0 - 6 8 0 ° C .  from  the  diffuse  Previous  using  this  on  the  diffraction  gallium  oxide  method  were  [9-11] Yet it has been reported  that oxides layers grown in the same manner will repeatedly desorb at  a  set  temperature  temperature.[9] was  determined  The by  first  reliable  Auger analysis  oxide of  desorption  oxidized  GaAs  4  surfaces,  during  heat  treatment in an evacuated  quartz ampoule.  SpringThorpe et al [ 5 ] determined from these measurements that the desorption  temperature for a thermal oxide  was  582+1°C.  The  thermal oxide was created by heating the GaAs wafer in air at 250°C for  2 0 minutes. The desorption temperature of an oxide created by  exposing the GaAs wafer to an ultra-violet ozone reactor for 2 0 minutes was found to be 638±1°C. These experiments are not exactly comparable to the MBE results since they are not conducted  with an  arsenic over-pressure. However, it does prove that the desorption temperature depends on the method used to create the oxide. X-ray photoelectron spectroscopy (XPS) of the surface oxide also  suggests that the composition and structure depends  technique  used  to produce the  oxide  layer.[l,2,4]  on the  According  to  conventional interpretation of XPS chemical shifts, the major species in the oxide film produced by the plasma and ozone technique are A S 2 O 5 ,  A S 2 O 3  and  G a 2 0  3.[4]  This is also the order of their  decomposition when the wafer is heated to >600°C.  This sequential  removal of the surface oxide was recently observed using a mass spectrometer  to monitor the  species that desorb from  the  GaAs  substrate.[12] It was found that when the arsenic oxides desorb a peak is produced in the mass spectrometer at mass 91 corresponding to AsO. This suggests that the arsenic oxides decompose to AsO and O 2 in the mass spectrometer or that the desorbed species is AsO. It is known that the A S 2 O 3  A S 2 O 5  species is unstable and will decompose to  under an electron flood gun  [2]  or on heating to  200°C  [4].  The  5 gallium  oxide  is  the  species  that  is  desorbed  at  the  highest  temperature and it is detected in the mass spectrum as G a 2 0 (masses 154, 156 and 158).  1.2 Theory of Secondary Electrons and Gallium Droplet Formation  R H E E D is a excellent probe of the translational symmetry of the surface atoms for well ordered systems. In addition the intensities of the  individual  diffraction spots are found to oscillate  under some  growth conditions of GaAs.[13,14] These oscillations were found to be synchronous with the thought  to  be  monolayer growth rate. The oscillations  caused  by  periodic  variations  monolayer terrace edges during epitaxial Growth  oscillations  have  also  in  the  density  are of  growth.[15]  been  reported in  the current  absorbed by the substrate.[16] This absorbed current is thought to be associated  with the secondary electrons  generated by the impact of  the primary electrons from the R H E E D gun, on the surface. Growth oscillations  have  also  been  reported  in  the  secondary  electron  emission from the R H E E D beam [17] and in photoemission produced by  light from an ultra-violet lamp.[18] Because of the high surface  sensitivity  of the R H E E D  measurement and the observed ability to  detect growth in the secondary electron current, one would expect a change in the secondary electron emission when the surface oxide is removed. reported.  In this  thesis,  the  first measurements  of this  effect  are  6 Electron emission from the substrate can result from secondary electron  production, or  by  elastic  or  inelastic  incoming primary electron. The main sources  scattering  of inelastic  of  the  scattering  are (1) the creation of plasmons in the solid; (2) ionization and core excitation  of  atoms  Secondary  electrons  in  the  solid;  (3)  are produced when  thermal the  diffuse  fast  scattering.  primary  electron  loses energy to the electrons in the solid. If the electron is travelling fast, the probability of an interaction is relatively low. Therefore the electron will travel a certain distance into the bulk which  increases  with the electron energy, before ionization will occur. The energy loss due to ionization for a nonrelativistic electron traveling through a solid is given by the Bethe expression shown in Equation  l.[19]  dE  47te r>  mv  4  •di n^2 =  _  2  uoffjr-iraiotfHW)  (i)  v- is the velocity of the primary electron is the density of electrons in the solid (electrons cm"3) I-is the ionization potential in the solid m-is the mass of the electron The  incident electron  inelastic  scattering  undergoes  events  in  a successive  the  series of elastic  material. As  a result  of  and these  scattering events within the material, the original trajectories of the electron  are  randomized.  dissipation extends is  The  effective  depth  then much smaller then  to  which  the Bethe  energy  range. A  better estimate of the actual electron penetration depth is given in  7 Equation 2.[20]  R  0.0276A , . 8 8 9 Ebl-67(nm)  e=  (2)  p z 0  Re-range of the electron A-is  the atomic weight in g/mol  p-density of the solid Z-atomic Using  number  this  equation  the penetration  depth of  a 12keV electron  in  GaAs is approximately 1p.m. At  a temperature  surface  forms  substrate phase  a  ~550°C  surface or by  occurs  reconstruction.[21] intensity  2x4  temperature  transition  of  under  an  As4  reconstruction.  decreasing  between  a  flux,  By  the  As4  2x4  and  the  increasing  flux a  oscillations  a first-order lxl  surface  in the  specular R H E E D diffraction spot under  These oscillations  are thought  to be due to  layer by layer evaporation of GaAs. The activation energy was  determined  eV.[21,22] This  the  On crossing this phase boundary one can observe  non-growth conditions.  process  GaAs  value  by  two  agrees with  different  authors  to  for this  be  4.6±.2  a thermodynamic calculation of  gallium loss from GaAs under an arsenic beam equivalent pressure of 1.1x10-6 Torr.[23] Gallium high fluxes  desorption from  temperatures by  GaAs  wafers  has  been  measured  at  under an As4 flux and under both As4 and Ga  modulated  beam  mass  spectrometry.[24]  The  activation  energy for Ga desorption under growth conditions was determined to  8  be 2.9  eV, which is  very close to  the  evaporation from Ga metal, which is 2.7  activation  energy  for Ga  eV. Therefore one could  infer that free Ga not incorporated into the lattice may be present as microscopic  droplets  on  the  wafer  surface  during  growth.  These  droplets have been observed under Ga rich conditions in migrationenhanced  epitaxy  using  scanning  reflection  electron  microscopy  (SREM).[25] The activation energy for Ga desorption under an As4 flux  only,  was  found to  be 3.6  eV. One would expect that  this  activation energy would be the same as that observed with the layer by  layer evaporation, namely 4.6  energy  eV. This  intermediate  activation  could be explained by assuming that there are Ga droplets  present on the surface even under arsenic rich conditions at wafer temperatures gallium  above  desorption  ~540°C. over  a  Therefore limited  intermediate between Ga droplets (2.7 eV).  The idea  of  Ga droplets  the  activation  temperature  of  would  be  eV) and Ga from GaAs  under As rich  suggested previously but not observed  range  energy  condition  directly.[24]  has  (4.6 been  9  II E X P E R I M E N T A L PROCEDURE  II.l  Wafer Preparation  The substrates used in the desorption experiments were inch,  semi-insulating  gallium  arsenide wafers.  This  material  two was  made by Johnson Matthey Electronics (JM) in Trail B . C . , using the high pressure liquid  encapsulated Czochralski ( L E C ) process. The  Czochralski ingot was cut into wafers at an angle 2° off the (100) plane, toward the (110) plane. These wafers were then etch polished to a mirror finish on both sides. To improve the radiative coupling between the heater foils and the GaAs wafer, the back surface was etched in a concentrated nitric  acid solution, producing a uniform  matte finish.[26] A l l chemical processing was conducted in a laminar flow hood to minimize the accumulation of particles on the GaAs surface. The  JM  wafers  were  given  an  ozone  treatment  by  manufacturer to remove carbon contaminations from the surface and to produce an oxide passivation layer on the GaAs  the [3]  surface.[4,5]  The ozone was created by placing the GaAs sample close to an ultraviolet radiation source while in an oxygen environment. [4] After the J M oxide was desorbed, in the M B E growth chamber the GaAs samples were reoxidized in a low pressure oxygen plasma discharge. The plasma reactor consists of a glass vessel 60 cm long  10  and  10 cm diameter, capable  of being pumped with a mechanical  pump to < 0.1 mTorr. (Fig.l) A leak valve at one end of the tube was used  to  introduce  oxygen  while  the  tube  was  being  pumped. A  coaxial resonator with a helical inner conductor was mounted on the narrow end of the reactor, downstream from the oxygen inlet. With a 60 M H z input, the coaxial resonator produced an electric field strong enough to ionize the oxygen molecules and create a plasma discharge. The 60 M H z signal was produced with a V H F oscillator in conjunction with  a 25W  R F amplifier. Ozone  and atomic  oxygen  are the  end  the  by  products of the plasma via the following reaction.  e- + 02 -» O + O +eO + 02 -> O3  The  wafer  outside  was  (3)  (4)  positioned  the plasma glow  in  the  downstream  of  to prevent damage from oxygen  reactor  ions. The  plasma was confined to the resonator with two grounded conducting rings. The ozone and atomic oxygen will reacted with the GaAs wafer as they  flow  to the reactor exhaust,  positioned  directly behind the  wafer holder. It was found that the plasma intensity  changes slowly  during the first minute after igniting the plasma. The fluctuation  is  thought to be caused by impurities in the reactor or instability of the oxygen flow. This problem disappeared if the plasma was allowed to run  for  approximately  15  minutes  prior  to  wafer  entry  into  the  reactor. After the wafer was inserted into the system, it was pumped  11  down to < 0.5 mTorr for 5 minutes. Then the chamber was flushed at a high oxygen flow for another 5 minutes. The oxygen flow was cut back to achieve a stable pressure of 200 mTorr before the coaxial resonator was switched on and oxide growth started.  II.2 MBE Idle Conditions  All desorption experiments were conducted in an ion pumped Vacuum Generators model V80H M B E system with the cryoshroud filled with liquid nitrogen. Experiments were only conducted when the background pressure in the growth chamber with the arsenic cell cold was < 1 x 10 9 _  Torr. The M B E system is divided into two  chambers one for sample preparation (Prep. Chamber) and the other for film growth (Growth Chamber) as shown in Figure 2. The GaAs wafer is transported through the M B E system in a molybdenum ring sample holder. The sample holder containing the wafer enters the preparation chamber from the exterior through an entry loadlock. This entry lock can be independently pumped, therefore the rest of the  M B E system stays at U H V during sample  preparation chamber the wafer  is  heated  loading. In the  to between  100°C and  200°C in a high temperature stage to remove any physisorbed water vapor or oxygen. The sample then enters the growth chamber from the preparation zone through a gate valve.  Once in the growth  chamber the molybdenum ring holding the sample is placed in the substrate manipulator which is heated by resistive heater foils. The  12  Figure 1. Schematic diagram of the plasma reactor  Cryoshroud K-Cells^  / j£  J  Substrate ManipulateManipulator  D  / High-Temp Stage J  •  Entry Lock  Preparation Chamber Growth Chamber  Figure 2. Schematic diagram of the V-80H MBE system  13 GaAs wafer was  then heated to the idle temperature of  1 0 0 ° C and  allowed to stabilize. This mounting technique is called indium free mounting to distinguish it from an alternative mounting technique in which  the  solder.  wafer  is  attached  to a molybdenum block with indium  [26] The indium solder technique is less convenient from the  point of view of sample mounting, and the mounting procedure is a potential  source of substrate  In  M B E growth  evaporating furnace  a molecular  ultrahigh-purity  with  contamination. beam  source  an small diameter  is  material  created in  a  aperture directed  by  thermally  self-contained  at the  This furnace is called a Knudsen cell or effusion  substrate.  cell. Inside  the  effusion cell the solid or liquid source material is held in a pyrolitic boron nitride crucible which is heated  by radiation from  resistance  heater foils. A thermocouple pressed against the base of the crucible is used to provide the temperature feedback. The prevent Before  GaAs  decomposition  is  exposed  due  starting to heat the  increased  in  temperature molecules. by  wafer  temperature the  arsenic  an arsenic  arsenic  substrate  loss  the  at  over pressure high  temperatures.  arsenic effusion  to  approximately  source  material  to  340°C.  sublimes  cell  was  At  this  into  As4  The As4 molecule can be further broken down into As2  subsequent  experiments  heating  discussed  to here  arsenic cracker cell was clogging.  to  to  800°C we  heated  in  used  the an  cracker As4 flux,  cell.  In  the  although  the  to approximately 4 0 0 ° C  The heaters were allowed  to prevent  to equilibrate for one  hour to  14 produce a uniform pressure of arsenic in the shuttered cell. During this equilibration time the R H E E D gun and mass spectrometer were switched on and allowed to stabilize.  II.3 Oxide Desorption Conditions  Before shutter  the  on the  wafer  arsenic  is  heated  effusion  above  cell  is  the  idle  opened  temperature  the  producing an As4  molecular beam. The As4 beam equivalent pressure measured with a ion  gauge is found to be approximately 1.5x10"^ Torr. Opening the  arsenic shutter also increases  the background pressure in the growth  chamber  The manipulator is then positioned  to  1.0x10"7  Torr.  to  expose the front surface of the GaAs wafer to the arsenic flux and the back to direct radiative heating from the tantalum foils. The R H E E D gun is then readjusted to get a good diffraction pattern on the screen. For  all experiments  the temperature of the wafer was ramped  automatically at a rate of 10°C/min. This ramp rate was controlled by a feedback thermocouple positioned between the heater foils and the substrate.  This  thermocouple  was  found  to  read higher  than  the  actual wafer temperature by as much as 1 5 0 ° C in a non-reproducible way. This was  a concern both with regard to controlling the ramp  rate at 1 0 ° C / m i n particularly Using  the  if  the  optical  and for determining the true sample temperature, temperature method  to  error  changed  measure  the  during  wafer  the  heating.  temperature  found that the ramp rate of the wafer was equal to 10°C/min  we to a  15 high degree of accuracy even though the thermocouple and optical temperatures did not agree. The cooling rate of the sample was not the  same as  the  thermocouple  presumable  due  to  changes  in  the  thermal gradients in the manipulator that holds the sample.  Samples  are  rate  typically  cooled  back  approximately 2 0 ° C / m i n  to  the  idle  temperature  at  a  of  after the oxide has been desorbed.  Ill IN-SITU M E A S U R E M E N T TECHNIQUES  Accurate in-situ wafer temperature determination is a difficult problem because it is not possible to have a probe in direct contact with the wafer. Accordingly we inferred the wafer temperature from an optical measurement of the band gap of the GaAs substrate. [6] As  discussed  in  the  experimental  procedure  the  wafer  was  polished on the front surface then textured on the back surface  to  increase radiation coupling between the wafer and the heater foils. If visible light is shone on a polished GaAs wafer approximately 40% of the light is specularly reflected from the polished front surface. Light that is not reflected is absorbed by the wafer if the energy of the light is great enough to excite transitions across the optical band gap. If the energy of the light is less than the band gap, the light will be transmitted through the wafer and if the back surface is rough, it will be non-specularly reflected from the back surface. By measuring the onset of the diffuse scattering the optical absorption edge of GaAs can  be  determined.  One  can  infer  the  wafer  temperature  from  16 published  data on  the  temperature  dependence  of  the  bandgap of  GaAs. [27] See Appendix 1 for the calibration curve used to determine temperature from the wavelength  of the absorption threshold.  A U H V window mounted on an effusion light from a 150 watt tungsten-halogen  cell port allowed the  lamp to illuminate the GaAs  wafer. The white light produced by the lamp was chopped at 420 Hz. A  detector  effusion  was  cell  mounted  port.  in  a  The detector  non-specular consisted  position  of  on  a collector  another lens,  a  monochromator used to measure the spectrum of the scattered light, and a liquid nitrogen cooled Ge photodiode to measure the intensity at each  wavelength.  The resulting diode current was  synchronously  measured using a lock-in amplifier. A strip chart recorder was  used  to record the output from the lock-in amplifier. The monochromator was scanned in 100 nm intervals over the wavelength minutes  of the absorption edge. Each scan took approximately to  complete.  Therefore  the  wafer  temperature  1.5 was  measured in 1 5 ° C intervals as the wafer was heated. This data was marked  on the  chart recorder outputs  from the  and from the secondary electron detector described A  V G QX-300 quadrupole  detect the mass of the desorbed  mass  mass  spectrometer  below.  spectrometer  was  used  to  species during the oxide removal.  The mass spectrometer was mounted in one of the effusion cell ports to give the analyzer a direct line of sight to the GaAs wafer. Data from the mass spectrometer  was displayed on a storage  and recorded on a strip chart recorder.  oscilloscope  17 The spectrum measure  arsenic at  oxide  mass 91  desorption produced a peak  due  only mass 91  to  in the  A s O . The spectrometer  while  the  wafer  minute from idle temperature to 5 0 0 ° C .  was  was  ramped at  mass set  to  1 0 ° C per  Above this temperature the  gallium oxide desorption was monitored, the major species occurred in the mass spectrum at 154, These three because  peaks  there  are  in the two  156 and a smaller peak at 158 a.m.u.  mass spectrum all correspond to  isotopes  of  gallium present  in  Ga20  significant  quantities. The isotopes are Ga (69 a.m.u.) 60.4% natural abundance and Ga (71 a.m.u.) with a 39.6% natural abundance. The spectrometer was set to scan between mass 150 and 160 every 30 seconds, so all peaks  could be measured at the same  time. The spectrometer  was  easy to align quickly for there is a very large peak at mass 150 due to As2The  surface  the experiment  structure of  using reflection  the  substrate  high energy  was  monitored during  electron diffraction or  R H E E D . This technique is based on the fact that high energy electrons incident at an angle monolayers  into  the  of - 3 ° will bulk  of  the  not penetrate GaAs  before  more  than  being  a  few  elastically  scattered. If the surface structure of the wafer is periodic it will act like a two dimensional diffraction grating and diffract the incoming electrons.  These  elastically  scattered  electrons  are  diffracted  as  discrete beams forming a pattern on the fluorescent R H E E D screen. A collimated beam of high energy electrons was produced by a V G Leg 110 electron gun. The gun was 20 cm from the GaAs wafer  18  and the electron beam was incident at an angle of 3° to the surface. A phosphor screen positioned symmetrically on the opposite side of the M B E growth chamber was used to detect the diffracted electrons. Electrons were produced inside the Leg 110 electron gun by a tungsten  filament  heated  with  2A  a  current.  The  filament  is  positioned within a dome shaped wehnelt shield with an aperture on the top. The shield is at a negative large  negative  electrons  are  difference  potential forced  controls  at  the  through the  the  electron  potential slightly less than the  filament, shield  therefore aperture.  emission  current.  the  emitted  The  potential  An  emission  current of 0.2 mA was used in all experiments, because it was found to give the most stable electron beam. The electrons were accelerated through  12  a  electrostatic Finally  keV  potential  difference  then  passed  through  a  Einsel lens which was used to focus the electron beam.  the electron beam passed  through a set  of X - Y deflection  plates. These plates enable the electron beam to be moved around on the wafer surface. In this arrangement the electron gun produced a <lmm spot size on the phosphor screen. The electron  gun was  run for two  hours prior to operation,  allowing it to stabilize and prevent drift in the beam intensity. This drift  in  tungsten  beam  intensity  filament.  interference  from  It the  was was  attributed  to  discovered  cracker  cell  themal that  heaters  heating  of  the  electromagnetic running  at  high  temperatures, caused an increase in the electron beam spot size. This problem could be minimized by decreasing the cracker temperature,  19  however was  there was  always  an effect present  when the cracker cell  operating. The  emission  of  secondary  electrons  from  the  substrate  was  measured to give insight into surface morphology since a change in the microscopic roughness of the surface should affect the number of emitted  secondary  electrons.  Also during  the  oxide  desorption  the  surface composition changes, therefore one might expect a change in the work function which would also  effect  the  secondary  electron  emission. From  the  discussion  in  the  introduction we  know  that  the  ionization probability for a hot electron in a solid is a maximum near the end of the range of the electron. If the electrons from the R H E E D gun  are  incident  penetrate  a  few  on  the  sample  monolayers  at  before  a  shallow  an  inelastic  occurs. Most of the secondary electrons and  some escape into the  dimensional  features  dimensional surface features the  incident  captured  primary  by  the  probability as for a flat features  are  microscopic  will event  are captured in the  wafer  roughening occurs  substrate  surface.  If the  three three  are larger than the penetration depth of  electrons,  GaAs  they  scattering  vacuum. When  appear on the  angle,  the  wafer  secondary  with  electrons  approximately  will  the  be  same  surface. But, when the three dimensional in  size  more  secondaries  will  escape  compared to the smooth surface case. In this situation one might also expect  the  forward scattered  probability of escape.  secondary  electrons  to  have  a high  20 This effect could be detected with a direct measurement of the absorbed current in the substrate. To do this would require that the substrate  be  electrically  isolated  from  ground  and  the  absorbed  current measured with an electrical contact attached to the substrate. This  is  not  practical for  us  due  to  the  design  of  our  sample  manipulator. On the other hand it is easy to measure the emission of secondary  electrons  with  a  separate  electrically  isolated  detector  since this measurement requires no modifications to the M B E sample mounting A  technique. three pin U H V electrical feedthrough  was  mounted on an  effusion cell shutter port. The feedthrough has B N C connectors on the atmospheric side. The two detectors were attached to the pins of the feedthrough with stainless steel barrel connectors on the U H V side. A stainless  steel  prevented detectors wafer  shield  between  the  shorting to the flange extended  from  barrel  connector  due to arsenic  and  the  pin  accumulation. The  to within 15 cm from the sample, and faced the  above  at  a 4 5 ° angle  to  the  surface.  One  detector  consisted of a tantalum oval which was 5cm by 3cm mounted on a 3 mm. diameter stainless steel rod (the "plate detector") the other was a 1/16  inch diameter tungsten  wire approximately 20 cm. long  "wire detector").  The current from the  was  with  measured  electrometer.  a  Keithley  secondary electron Model  A strip chart recorder was  617  (the  collector  programmable  used to record the  output  signal of the electrometer. The  input  of  the  electrometer  consisted  of  a triaxial cable  21 terminated with three alligator clips. When the electrometer is in the current measurement inverting  mode the input triaxial  pre-amplifier.  electrometer  (red  wire)  electron detector.  The was  To bias the  cable  high  impedance  connected  directly  is  fed  into an  input to  the  of  the  secondary  secondary electron detector  the pre-  amp of the electrometer was floated with an external power supply. The electrometer input could be floated to a maximum of 500 volts, so a power supply adjustable to 450 volts was constructed for this purpose.  One  noninverting was  lead  of  input of  attached  to  the  the the  power  supply  electrometer  ground of  the  was  connected  (black wire)  to  and the  the other  M B E chamber. The electrical  schematic is shown in Figure 3. Due to the feedback circuit of the electrometer  op-amp  the  voltage  difference  between  the inverting  and noninverting input is zero. Therefore the potential between the secondary electron  detector  and the chamber will  be the  same  as  voltage produced by the power supply. The polarity of the bias can then  be  changed  by  simply  changing  the  polarity of  the  power  supply. To decrease the noise in the current measurement the alligator clips of the input cable are shielded in a Aluminum box with two B N C connectors.  One B N C was  connected  to the power  supply with a  coaxial cable and the other B N C was attached directly to the detectors B N C . The ground clip from the electrometer was attached to the case of the steel box. Leakage at high voltage bias was minimized using a high voltage B N C connection on the detector.  The wire detector  was  22 designed to simulate a detector of zero surface area. At zero bias this detector should not capture any secondary electrons, and at positive bias  the  detector  will  capture to first approximation all  electrons  with kinetic energy less than the value of the applied potential. A plot of detector current versus bias voltage will be the integral of the electron  energy  distribution up to the  applied potential. Therefore  the derivative of this curve versus bias is a measure of the energy  23  Red Win  Electrometer Preamp  0  Detector  MBE Chamber (Ground Potential) Black Wire  Power Supply  ure 3 Schematic diagram of detector bias circuit  24 distribution of the secondary Measurement scattering  is  morphology, wavelength oxidized  a  of  very  if the of  GaAs  the  electrons.  non-specular reflectivity  sensitive  technique  surface features  light  used.  wafer  is  are on the  The polished  smooth  for  and light  monitoring same  surface is  or diffuse  of  light  surface  scale as the  reflected  the  original  specularly  provided the surface is free of particulate contamination. Formation of any three dimensional defects will cause scattering and hence a rise in the diffuse light intensity. A Uniphase (model 1125P) lOmW helium-neon laser is used as the light source for the diffuse mounted  external  to  scattering experiments. This laser is  an effusion  cell  port equiped  with  a UHV  window. From this position the beam is incident on the GaAs wafer at a 2 3 ° angle relative to the surface normal when the substrate holder is in the growth position. Non-specular light is collected through the same  port  used  in  the  optical  temperature  monochromator is tuned to accept the wavelength (633  nm) and this  light is detected  measurements.  The  of the HeNe laser  by a silicon photodiode. The  voltage output of the photodiode is displayed on a chart recorder. The XPS studies were performed by P . C . Wong of the U B C chemistry department with a Leybold M A X 200 XPS surface analysis system using A l K a x-rays. (hv=1486.6eV)  25 IV EXPERIMENTAL RESULTS  IV. 1 R H E E D Pattern  In  early  M B E work the  oxide  desorption  was  monitored by  observing the changing R H E E D pattern during the desorption process. Since  the  oxide  layer on  GaAs  is  amorphous  only  weak  diffuse  scattering is observed in the presence of the oxide. This appears as a uniform haze on the phosphor R H E E D screen. At the instant the oxide is removed sharp diffraction spots due to the clean GaAs surface can be  seen.  The  pattern  consists  of  intense  corresponding to the bulk GaAs. Between  diffraction  these sharp spots are less  intense diffraction spots corresponding to the surface of  spots  the GaAs. The surface reconstruction depends  reconstruction  on the temperature  of the wafer and the intensity of the AS4 flux. Desorption temperatures  were measured  using  this  method on  the thicker oxide samples only, because the thinner samples (3-10 had  visible  bulk spot patterns  before  the oxide  was  A)  removed. This  type of R H E E D pattern is consistent with what others have observed for  thin oxides on GaAs.[12] As the oxide layer becomes thicker the  bulk  pattern  becomes  weaker  with  only  the  specular  spot  visible.  The thickest oxide films (>18A) had no visible diffraction spots. A thick oxide layer (~22A) wafer  to a low  temperatures  pressure  below  the  oxygen gallium  was created by exposing plasma for oxide  120  desorption  the GaAs  minutes.  For all  temperature  the  26 diffraction  pattern  appeared  as  a  uniform  background  on  the  phosphor screen as shown in Figure 4a. As the gallium oxide begins to desorb the specular spot begins to show in the diffraction pattern. Within  15-25°C  containing  of  other  the  specular  diffraction  surface reconstruction is diffraction  pattern  is  spot  spots  observed,  observed  appearance  due  to  shown all  the  an  a  sharp pattern  arsenic  stable  2x4  in Figure 4b. When this surface  oxide  has  been  evaporated.  IV.2  Mass Spectrometer  A  quadrupole  desorbed  Measurements  mass  spectrometer  was  used  measure  species during the oxide removal. The arsenic  measured by monitoring the change in intensity spectrum. Mass 91 500°C.  to  oxide  the was  of A s O in the mass  was monitored from the idle temperature up to  Above this temperature the  mass spectrometer  was  scanned  between mass 150 and 160 a.m.u., the mass range where the gallium oxide  species  function  of  appear.  substrate  The  desorbed  temperature  species  for  a  are  series  measured of  samples  as  a  with  different thicknesses of oxide. Experiments  were  conducted  on  a series  of  ozone prepared  oxides prepared by Johnson Matthey Electronics with exposure times in  a ozone  ramped at  reactor ranging from 0  to  5  minutes.  1 0 ° C per minute from a idle temperature  Samples of 5 0 ° C  were and  27  Figure 4a. R H E E D diffraction pattern of an oxidized substrate at 2 5 0 ° C  f  •  Figure 4b. R H E E D diffraction of wafer at 6 2 0 ° C . Diffraction pattern for 2x4 reconstructed GaAs visible.  28 continued until the gallium oxide was completely removed. Over the course  of  an  detected two  oxide  desorption  A s O peaks  experiment  the  mass  spectrometer  occurring at different temperatures  and one  peak due to Ga20 desorption. During the temperature ramp the mass 91  peak  (AsO) was  detected  at  the  beginning  of  the  ramp and  reached a maximum for the first time, usually between  1 0 0 ° C and  2 0 0 ° C . This peak shown in Figure 5a, was broad and asymmetric with a slow rise on the low temperature side followed by a sharp drop down to the background levels. Continuing the ramp a second smaller AsO  peak occurred with a maximum occurring between 3 5 0 ° C and  450°C.  This peak shown in Figure 5b was symmetric, about  100°C  wide and was similar to the AsO peak reported by SpringThorpe et. al. [12]. Figure 6 is a plot of the desorption temperature of the AsO peak  versus  ozone exposure  temperature A s O evaporates exposed  time.  The data suggest that the  at lower  temperatures,  if the  high  GaAs  is  to the ozone for longer periods of time. The thickness  of  these oxides, was not measured. The lower temperature AsO peak was first thought to be due to some hydrated form of arsenic oxide due to exposure  to room air  during the sample transfer to the M B E system. However we think a more likely explanation is that this low temperature A s O is due to A S 2 O 5 . X P S studies of ozone oxides formed on GaAs (100) have been  29  100  120  140  160  Temperature (C)  Figure  5a.  Low  temperature  A s O peak  observed  in  the  spectrometer during ramped heating of ozone oxide on GaAs.  mass  30  16  1  114  —  £12  —  1  11 1  1  1 1 1  1  1  1  1  1 1 1  1  1— I  -  T—  • •  — •  —  £10 ca  £ d  •  8 •  Q.  (I) in  D  <o 4 co A  •  • •  — •  • •  »  1  1  340  1  1  360  11 1  1  380  1 1 1  1  1  1  400  1  1  . . .  420  1 .  J  440  460  Temperature ( ° C )  Figure  5b.  High  temperature  A s O peak  observed  in  the  spectrometer during ramped heating of an ozone oxide on GaAs.  )  mass  31  440  i  — 420  h  i i i ^ i i i i I i i i i I i i i i I i i i i I i i i  i I i i i i I i i i i  O  .*—* ca  cB 400  h  Q.  E  Q)  c o  380  8  360  CD  Q  I • . . . I  • '  340 0.5  1  1.5  2  2.5  I I  3  3.5  i i i i i  4  4.5  Exposure time (min.)  Figure  6.  oxidation  Desorption time.  Note  of  the  high  desorption  temperature  temperature  A s O versus  was  determined  the mass spectrometer signal reached a maximum value.  ozone when  32  interpreted by assuming the existence of AS2O5 and AS2O3 in the oxide. It is known that  A S 2 O 5  decomposes to  A S 2 O 3  at temperatures  above 150°C [ 4 ] and is very sensitive to electron irradiation [2].  As2C»5->As203+02  (5)  Annealing for two hours at  100°C  was enough to removed this low  temperature oxide leaving only the high temperature AsO signal. Increasing  the  annealing temperature to  200°C  affected  the high  temperature AsO as well. Ramping the sample after the anneal gave results ranging from no AsO for long anneals ( > 2 hrs.) to a small AsO peak for shorter anneals. XPS studies have suggested that the  A S 2 O 3  state is the last form of arsenic oxide to desorb from the wafer surface during heating  [1,2,4].  Therefore the high temperature AsO is  interpreted as the desorption or decomposition product of  A S 2 O 3 .  The RHEED pattern did not change from its amorphous background during the arsenic oxide desorption At  higher  temperatures  the  Ga2 0  appeared  in the  mass  spectrum. Figure 7 is a graph of the temperature of maximum signal in the Ga20 verses ozone exposure. The trend of these few data points is to higher desorption temperature for increased time to the ozone. Low temperature annealing  (~200°C)  exposure had no  observable effect on the gallium oxide desorption temperature. After the gallium oxide is removed the diffraction pattern for bulk GaAs appears on the RHEED  screen. The GaAs samples  that had been  33  620 O  610  1  •  •  • 1•  T  1  1  1 — 'i  i  i  -  1  i  •  '  1  A '  1  — i — i —  A —  o  0  k=3  600  •4—'  CO  l_  <D  CL  E H c o o  CO CD Q  590  -  —  A 580  A  —  —  570  —  i  560  1  0  J  1  .  I  .  1  •  •  2  •  .  I  .  .  •  •  3  Exposure time (min.)  Figure 7. GaiO desorption temperature versus ozone oxidation time.  34  desorbed during the study of the ozone oxides are removed from the MBE system and reoxidized using the low pressure oxygen plasma reactor. The  samples with the plasma grown oxide are ramp heated in  the M B E from an idle temperature of  100°C  through the Ga20  desorption at 10°C per minute. From the previous ozone data one expects only the high temperature AsO peak to be detected in the mass spectrometer. No peak was found in the AsO mass spectrum but there  was  a  broad  approximately 350°C.  band  that  rose  above  the  background at  This band of AsO continued until the mass  spectrometer was switched to scan between mass 150 and 160 at 500°C. The mass 91 intensity was not consistent and changed with each  desorption  experiment.  These  intensity  fluctuation  were  assumed to be caused by the length of time the sample sat under idle conditions  at 100°C.  When the  samples  were heated  from room  temperature a similar AsO broad band desorption was observed, only this time the intensity of the AsO signal was larger. The only time a peak was seen in the AsO data was after the sample was ramped up to 275°C annealed for a short time then cooled to room temperature. When this sample was reramped a peak was observed. This peak occurred at approximately the same temperature and had the same shape  as the high temperature AsO peak detected in the ozone  oxides. One oxidation  explanation  for this effect  forms multiple arsenic  would be that the plasma  oxides,  which transform during  35  annealing into a single phase. This arsenic oxide seems to desorb at lower  temperatures  with  thicker  films  but  with  only  three  measurements the data is not conclusive. These results suggest that arsenic oxide layer in the plasma oxide is different than in the ozone oxide. The difference may be due to the fact that the samples with the ozone oxide sat at room temperature in air for a few weeks. During this time it is conceivable that room temperature annealing or stabilization would occur in the arsenic oxide of the ozone oxide samples. substrates  After  exposure  are immediately  to  the  plasma  put into the  oxidation  process  the  M B E system. Therefore,  stabilization of the arsenic oxide can not occur before the oxide is desorbed. Furthermore the plasma oxides are grown on previously desorbed substrates. This could conceivably affect the formation of the arsenic oxide. The gallium oxide desorption temperature was measured as a function of the exposure time to the low pressure oxygen plasma. The desorption temperature for a sample that had been exposed to the oxidation plasma for 0.5 minutes was found to be ~ 5 0 ° C than  a  sample  that  had  been  exposed  for  120  minutes.  lower The  temperature of the maximum Ga20 signal seen in the mass spectrum is plotted versus substrate exposure time in Figure 8. This plot shows that the  desorption  temperature  increased  quickly  with  exposure  time for the thinner oxides (<5 minutes exposure) and then increased progressively slower for the thicker oxides. As the oxide layer becomes thicker, one would expect the Ga20  36  630  1  620  —  610  —  600  —  ri—|—r—r i  |  i i  i  I i i  •  1  1  • 1 ~i—i—  1  i *  i •i  -  •  •  ~J—  —  590 1  -  580 570  1  .  1 .. .  20  1 . , . 1 . . i  40  60  1i  80  i  i  1  100  i  1  120  i  140  Exposure Time (min.)  Figure 8. Ga20 desorption temperature versus plasma oxidation time.  37  peak area to increase. Also the desorption process might be expected to occur over a longer time interval since there is more gallium oxide available  to  corresponding  desorb. to  In Figure  9 the  mass  Ga20 is plotted against  spectrometer  signal  wafer temperature for  different oxide thicknesses. We observe that the height of the Ga20 peak and hence the area increases with plasma exposure times. But unexpectedly the temperature range for the Ga20 desorption did not increase  with exposure  time but remained between 30° and 40°C  wide for all samples. In fact Figure 9 shows that the thinner oxide layers are completely desorbed before the thicker oxide layers even begin to desorb. This result is not consistent with a simple layer by layer evaporation process. As  mentioned in section  1.1,  SpringThorpe et al. [12]  have  measured the Ga20 desorption of a thermally created oxide and an ozone oxide, with a mass spectrometer. It is interesting to note that the Ga20 peak shape for the thermal oxide reported in Reference [12] is the same as that for the thin plasma oxide in Figure 10 and the ozone oxide is similar to the thick plasma oxide. Also the Ga20 desorption  temperature  for  the  thermally  created  oxide  ( T G a 2 0 = 5 8 2 ± l ° C ) was ~50°C lower than that of the ozone oxide (TGa20=638±l°C). In Figure 10 the maximum height of the mass 154 Ga20 signal is plotted against plasma exposure time. The relationship between peak height and exposure time in Figure 10 closely  parallels the  relationship between the gallium oxide desorption temperature and  38  Figure.9.  Ga20 signal from the mass spectrometer  desorption for three different oxidation thickness during identical 10°C/min temperature ramps.  during oxide  (oxidation times)  39  60 i  T  I  1—|  I  I  I  ~l  I  1  1  1  I  ^ r  T—i—T"—i—i—r  50  CO  40 CO CD Q.  o  CD QCO  w w  30 20  CO  1 0  J—i  0  i I i i i i • • • i • • • i  20  40  J  i i_  60 80 100 Exposure Time (Min.)  t I i  120  140  Figure 10. Maximum height of the mass spectrometer Ga20 signal versus plasma oxidation time. Note the signal height is proportional to the amount of desorbed oxide.  40  exposure time found in Figure 8. This suggests that a plot of the Ga20 peak height (proportional to the oxide thickness) versus desorption temperature should be linear. The relationship between the oxide thickness and the exposure time of the GaAs substrate to the plasma oxidation was determined by XPS analysis of similar samples oxidized in the same manner as those used in Figure 8. From this data a good estimate of the oxide thicknesses  for different exposure times to plasma oxidation was  obtained, (see Appendix 2 for the method used to determine oxide thicknesses) The thickness as a function of exposure time is plotted in  Figure  11.  The  oxide  thickness  inferred  from  the XPS  measurements shows the same dependance on the plasma exposure time as does the Ga20 peak height in the mass spectrometer. We can now assume that the mass 154 peak height is proportional to the thickness of oxide on the GaAs surface. Figure 11 is then used as a calibration curve to obtain the oxide thicknesses for the exposure time data used in Figure 8. The Ga20 desorption temperature data of Figure 8 is replotted as a function of oxide thickness in Figure 12. We find that the desorption temperature is a linear function of the oxide thickness, for oxide layers between ~5A and 26A thick.  IV.3 Ions We also studied the effect of the oxide desorption process on the  secondary  electron  current induced  by  the  RHEED  gun.  Preliminary experiments conducted with the plate detector found  41  30  "1  1  |  1  1  I  I  1  1  1  I  I  I  r  25 • Arsenic Oxide w 20 0}  CD C  Gallium oxide  •§ 15 CD  1  10  b 5  P  .  .  .  .  i  50  .  .  .  •  i  100  •  X  150  200  250  Exposure Time (min.)  Figure 11. Oxide thickness inferred from XPS measurement of plasma oxidized GaAs  42  i  Figure 12. Gallium oxide desorption temperature as a function of the thickness of the surface oxide.  43  that the background current in the M B E growth chamber fluctuated between 0 and +30 picoamps (pA). The background current did not change when the ion pump and ion gauges were turned on and off, but  increased in magnitude to approximately -3nA with the mass  spectrometer slightly  on. Increased pressure in the M B E growth chamber  increases  spectrometer.  the  negative  current associated  A desorption experiment  conducted  with the mass with the mass  spectrometer and the secondary electron detector both in operation proved  that,  associated  the  current  measured  by  the  electron  detector  with the mass spectrometer did not change when the  oxide was desorbed. With the RHEED gun on but not aligned to hit the wafer, the negative current increased to approximately -4nA. When the electron beam was positioned to produce a RHEED diffraction pattern the secondary current increased to ~-27nA. current  produced when  the  electron  beam  hit  The electron  the molybdenum  sample holder was significantly lower and no diffraction pattern was observed on the RHEED screen. Therefore by positioning the electron beam to maximize the secondary electrons  current produced, one  could align the electron gun on the substrate  without using the  RHEED screen. The  first oxide desorption experiments  were conducted with  zero bias on the secondary electron detector. A small increase of approximately  1.5nA  in  the  electron  current was  observed  at  400±40°C, during the high temperature AsO desorption. The intensity of  this effect changed for each desorption in a non-reproducible  44  manner much like what was observed in the mass 91 signal from the mass spectrometer data. This was attributed to the annealing of the GaAs wafers prior to the oxide desorption. As the wafer temperature was  increased  further  the  current remained constant  until  the  gallium oxide desorption. As the wafer temperature was ramped through the gallium oxide desorption the negative current increased , peaked and then decreased sharply, eventually becoming positive at higher temperatures. After  the  oxide  was  removed the  positive  current continued to increase with wafer temperature. We interpret the positive current as an ion current. The positive ion current was studied in greater detail with a negative bias voltage on the detector to suppress the low energy electrons. With a negative bias voltage of 100 volts on the detector, a positive  current begins  to appear about 50°C  below  the  initial  appearance of Ga20 in the mass spectrometer. It was discovered that the  positive  Rotating  the  current remained, even sample  with the electron gun  manipulator showed  that there  off.  are certain  positions that have intense ion currents. At high temperatures the heater foils  in the substrate manipulator ionize residual gas and  material desorbed from the back surface of the wafer. These ions escape through holes in the radiation shield of the manipulator. When the manipulator is rotated to give one of the holes a direct line of sight from the heater to the detector an intense ion current is measured. Figure 13 is a plot of ion current against negative bias. One can  45  200  I  1  1  1  1  I  -i—i—I—i—i—i—i—i—i—i—r  150 • Plate Detector  ^100 c  I  Wire Detector  50  ZJ  O  0 h4 -50 -500  -400  _L _L -300 -200 Voltage (V)  -i  i  i  i  i_  1 00  Figure 13. Ion current as a function of negative bias on the plate and wire detectors. Oxide has been desorbed and substrate is at an equilibrium temperature of 620°C.  46  see that at high bias voltage the ion current begins to saturate. This graph shows two curves one with a plate detector and one with a wire detector. As discussed in Section III the derivative of this plot is a rough measure of the energy distribution of the thermal ions. The derivative versus negative bias plot in Figure 14 shows that there is a peak in the ion signal at 40 volts which is approximately equal to the voltage on the heater foils. It is expected that ions will be created by the primary electron beam during desorption from the front surface of the GaAs wafer. However the thermal ions from the heater foils seem to mask this effect and evidence for the existence of the desorption ions is not conclusive from this data. These desorption ions should oscillate during epitaxial growth at the monolayer growth frequency.  IV.4 Diffuse Electron Scattering  The desorption experiment was repeated with a positive bias voltage of 100V on the plate detector. Under these conditions the ions from the manipulator were completely suppressed. In fact, +40 V was enough to remove any effect from the ion current. With a +100V bias on the plate detector the background current increased to -25pA. The electron current increased to approximately -30nA when the electron gun is on and aligned to produce a RHEED pattern. The secondary electron current increased with the acceleration voltage of the primary electrons up to 12 keV. Above this voltage the  47  Figure 14. A rough measure of the energy distribution of the thermal ions. Note peak in derivative curve at 40 volts, the voltage of the heater foils.  48  secondary current was constant until ~13 keV then decreased slowly to ~-16nA as the beam voltage increased up to the maximum of 15 keV. This peak in the secondary electron current could be due to the alignment of the gun, or due to the fact that at higher energies the primary electrons  penetrate deeper into the  sample and produce  fewer secondaries near the surface. The  secondary  electron  current was  measured  at  several  different bias voltages on an oxidized sample at room temperature. The data is shown for both detectors in Figure 15. The wire detector starts from a low current at zero bias because of its small surface area. The derivative of the current as a function of bias voltage gives a  crude measure  of  the  energy  distribution of  the  secondary  electrons. Given this interpretation one can see from Figure 16 that the secondary electron distribution peaks at zero kinetic energy. This is a typical distribution for secondary electrons. [28] The  secondary  electron  current was  measured  during  the  desorption of the plasma oxides. The current stayed constant at negative  30nA except  for some  slight drift  until  the AsO was  desorbed. The drift of ±0.5nA was thought to be due to charging effects in the inside of the MBE chamber due to the scattered high energy electrons. During  the desorption of the high temperature  arsenic oxide, a step of ~l-2nA  was  observed in the secondary  electron current. These results were inconsistent however. At times there was a step in the current during the AsO desorption peak and at other times there was no effect at all. Similar results were  49  Voltage (V)  Figure 15. Secondary electron current as a function of detector bias. Substrate is at room temperature and has a thin oxide approximately (9 A).  50  0.3  • i •—i—|—i—i—i—i—| i i—i i |  *  •  i  i  i | i l • • | l i i i | i i • i  Plate Detector  0.25  >  0.2  CD  0.15  >  • •  •• • • •••  ca  CD Q  0.1 0.05  Wire Detector  0  i i i—i  50  Figure oxidized  16.  Energy  1  0  n  •• •  •  1  i i i i  50  distribution  100 150 Voltage (V)  of  secondary  substrate a room temperature.  at zero volts bias.  • •1 1 1 — 1 1 1  i i  200  electrons  250  300  from an  Note distribution is peaked  51  obtained with no bias, on the ozone oxides. A step in the secondary current from -30nA to -32nA suggest that the wafer  surface  is  roughening during the AsO desorption. After the AsO desorption the detected current remained constant until the Ga20 desorption begins to occur. During increases  and after  the gallium oxide desorption, the current  exponentially  temperature  is  in the  negative  direction  as  the  sample  ramped up. Unlike the ion current this negative  current is produced by the incoming primary electron beam. If the beam voltage  or filament  current dropped to completely  removed  the  current was  turned off  background level.  (~605°C)  temperature was held constant. At  the  After  ramp was  the the  secondary Ga20  stopped  was  and the  this point the negative current  continued to increase until the equilibrium wafer temperature was attained. The current then remains constant (~200nA) for as long as the sample was  held at this temperature. When the sample  was  cooled the current decreased back to its original value of -31nA. This occurred at approximately the same temperature as the initiation of the secondary electron current during the rising temperature ramp. The now oxide-free GaAs wafer was allowed to equilibrate at 300°C, then was heated back up to the temperature where the Ga20 desorbed during the initial heating. As expected there was no change in the secondary current as the substrate was heated through the AsO desorption temperature. As the wafer temperature approached the point where the Ga20 began to desorb in the previous heating  52  the negative current began to increase again. This time the onset of the current increase occurred at a slightly lower temperature and the shape of the current versus time was different from the oxidized case, as shown in Figure 17. The temperature ramp was stopped at 610°C and the temperature of the sample was allowed to stabilize. The plate detector at 100 volts positive bias gave a reading of -225 nA. With the wafer still at 610°C and a negative 100 volts bias on the plate detector the measured current was +50nA. Normally we expect any ion current to be predominantly due to positive ions. Since the magnitude of the negative  current was 5x the magnitude of the  positive current for the same sample conditions we conclude that the negative current must be due to electrons and not negative ions. When the sample was cooled, the secondary electron current again decreased back to the initial value of ~-30nA.  Subsequent  current measurement on other oxidized GaAs samples during initial desorption and reheating showed that the same reversible secondary electron current. The effect of bias on the high temperature electron current was studied and compared to the results obtained at room temperature. From the plot of current versus bias voltage shown in Figure 18, one can  see  that the plate detector and wire detector have different  responses.  The  responses  are  also  different  from  the  room  temperature results. This difference is best seen in Figure 19 where the derivative of the current as a function of bias voltage is graphed. One can see that the electron distribution on the plate detector is  53  520  540  560 580 600 Temperature (°C)  620  640  Figure 17. Secondary electron current recorded during a temperature ramp for an oxidized and desorbed GaAs substrate.  54  ~i—i—i—i—|—i—i—i—i—|—i—i—i—r  'bo, -100  -  _-200  -  <  _ Wire detector  c  £-300  -  O -400  -  -500  -  CD  O  o J—I—I—I  -600  Plate detector  100  I  I  I  I  I  I I  200 300 Voltage (V)  _ i _ L _i  400  i_  500  Figure 18. Secondary electron current as a function of detector bias. GaAs is at a temperature of 620°C and oxide has been completely desorbed.  55  1  1 1  1  11  1 1 1  1  1i  i i |  i r TT—|—i—i—i  i  | — r - Ti i  —  o °o  —  —  o o  —  Plate Detector  o o o  °  — 1  100  1  1  1  1  1  oo o  ••••• 1  1  1  •••  o •  •  o  o  o  Wire Detector 1  100  1  i  i  i  li 300  1 i i i i  200 Voltage (V)  —  ° i  i  t  I i i  400  t  i  500  Figure 19. Energy distribution of secondary electron from substrate at 620°C.  56  peaked at 100 volts whereas, the wire detector shows a continuous increase in the number of electrons captured up to 500 volts bias. It was expected that the electron energy distribution would peak at zero bias like the room temperature results. One explanation for this observation is that the produced  in  this  high  temperature  regime  are  secondaries  preferentially  scattered in the forward direction. Since the detectors are parallel to the incoming electron beam, few of these forward scattered electrons would be captured if the bias was zero. At an equivalent bias voltage the electric field from the plate detector extends over a greater range because the plate detector has a larger solid angle compared to the wire detector. Therefore the plate detector should be able to capture these forward scattered electrons at a lower bias voltage compared to the wire detector. This is consistent with the fact that the plate detector is able to capture the scattered electrons at a 100 volt bias. On the other hand a bias voltage greater than 500 volts is required for  the  wire detector  to capture the  same  number of  scattered  electrons. For this reason no peak is observed in the derivative of the current as a function of bias for the wire detector before 500 volts.  IV.5 Effect of Oxide Thickness on Secondary Current Onset  The onset of the increase in the secondary electron current was examined closely  for a series  of plasma oxides  prepared with  different exposure times. For oxides with exposure times of less than  57  20 minutes the secondary electron current did not increase until the Ga20 peak was noticeable in the mass spectrometer as shown in Figure 20 For substrates that were exposed to the plasma oxidation for longer than 20 minutes the electron current started increasing at the  same  time  spectrometer.  as  the  initial  (Figure 21)  detection  of  Ga2 0  The data suggest  that  in the mass the  secondary  electron current is dependent on the removal of the oxide layer since there is no great increase in current until the Ga20 begins to desorb. In the thinner oxides  with the lower desorption temperatures the  current does not increase until well after the gallium oxide has been removed. This suggests that the current onset also depends on the wafer  temperature. One can attribute the increase in the negative current to two  different  effects.  There is  a large effect  which appears  as an  exponential increase in the number of secondary electrons  as the  wafer temperature is increased. The second  effect is seen as an  inflection in the exponential increase in the secondary electrons. In all samples the kink in the secondary current appears in the data at the same  temperature as the maximum Ga20 signal in the mass  spectrometer, as shown in Figure 22. When samples with the oxide layer  already  desorbed  were  reheated,  the  secondary  electron  current increased smoothly with temperature, with no feature near the Ga20 desorption temperature. The kink in the secondary electron current must then be caused by the desorption of the Ga20 layer. A inflection of this form can be created by superimposing a small peak  Figure 20. Secondary electron current as a function of temperature for thin surface oxides.  substrate  59  Figure 2 1 . Secondary electron current as a function of temperature for thick surface oxides.  substrate  60  100  r-  -i—|—i—i—i—|—i—i—>—r  < C C D i_ k. 3  o '  •  540 560 Temperature (°C)  Figure  22.  Mass spectrometer data  '  I  1  1  1—u  580  and secondary  measured during desorption of a 7A oxide.  600  electron  data  61  on a larger exponential background. There is evidence that suggests that at temperatures above 640°C  the secondary current increase  begins to slow and reach a constant value. Not enough data has been taken in this region to prove this conclusively. The temperature dependence of the secondary electron current for an oxidized sample and an oxide free sample were compared. In most  cases  the  onset  of  the  electrons  was  detected  at  lower  temperatures for the desorbed sample. In some random cases for both an oxidized and a clean GaAs wafer the exponential increase of the secondary electrons  occurred ~30°C  higher than the expected  onset temperature. It is thought that this effect is caused by changes in the As4 flux. In general one can say that the presence of the oxide layer retards the the exponential secondary electron increase and that the  desorption of the  oxide  layer causes the kink  in the  measured current. A desorbed sample was slowly heated up to 620°C and slowly cooled to see if the secondary current effect was reversible. The sample temperature was  stepped  and allowed to equilibrate each  time before the current was measured. The results shown in Figure 23 prove that for the samples with no oxide layer the current follows the same curve during heating and cooling. The natural log of the current was plotted versus 1/T to determine the activation energy of the secondary electron current. From the slope of Figure 24 the activation energy was found to be 3.6±0.4 eV.  62  140  T—i—i—|—i—i—i—r  120 _  <  T—i—i—i—i—i—r  •  decreasing temp.  •  increasing temp.  T—i—r  100  c  ~ 2 O  80 60 40  -  20  J  I  550  I  I  L  560  I  570  I  I  J  I  I  580  I  I l_ 590  I  600  Temperature (°C)  Figure  23.  Secondary  electron  current measured  during  a slow  stepped ramp. GaAs substrate was allowed to equilibrate before the current was measured.  63  5  -r—r—i—  1—r—r—i—|—r  r  4.5 ^  -•—<  4  c  CD  "  O  3.5 3 2.5 J  2 1.12  •  decreasing temp.  •  increasing temp. i  L  1.15  j  i  1.17  _L  i  j  1.19  1000/Temperature  i  1.21  •  • •  1.23  (1/°K)  Figure 24. Semilog plot of secondary electron current as a function of 1000/T °K. Activation energy for the secondary electron current was determined to be 3.6±0.4 eV.  6 4  IV.5 Diffuse Reflectivity  The bright lamp of the optical temperature monitor made it possible to observe the formation of a faint haze on the polished surface of the GaAs wafer during the Ga20 desorption. When the GaAs sample was removed from the system the haze made the wafer appear cloudy to the eye. The haze was thought to be caused by surface roughening during the oxide desorption process. Formation of the haze was monitored by measuring the diffuse  light scattering  from the surface with a HeNe laser. The laser beam had an incident angle of 28° to the wafer and the intensity of the scattered light was measured  with  a 65  mm diameter  collecting  lens  and silicon  photodiode. A  polished  wafer  would  give  a  background reading  of  approximately 0.5 microvolts, due to scattering of the specular beam inside the vacuum chamber. No scattering could be detected from the wafer initially. The background value remained constant until the beginning of the Ga20 desorption where the signal began to increase slightly. When the peak Ga20 signal is seen in the mass spectrometer the intensity of the scattered light increases abruptly, as shown in Figure 25. This effect shows up in the output of the photodiode as a 7 u.V step that occurs over a temperature range of less than 10°C. After the oxide is removed the intensity of the scattered light continues to increase with temperature, but at a much reduced rate. Both of these processes are not reversible. The voltage produced by the diode  65  remains constant even if the wafer is cooled back down to room temperature. If the oxide free wafer is now reheated there is no abrupt increase in the scattered light, but at 500°C a slow rise in scattered light intensity is observed. This rise was similar to what was seen with the oxidized wafer after the oxide was removed, only the effect occurs at a lower temperature. At 640°C the temperature was held constant and the intensity of the scattered light continued to increase until an equilibrium value of approximately  11 |iV was  obtained.  Again this process was not reversible on cooling, the diode voltage remaining constant  even after the sample had cooled. This result  suggests that the abrupt step in the scattered light is caused by surface roughening due to the gallium oxide desorption. The wafer was reoxidized to see if the plasma oxidation process would smooth the GaAs surface. Measurement of the scattered light from the reoxidized sample determined that this was not the case, for at room temperature the diode voltage was approximately 10 p V , consistent with the roughness induced by the oxide desorption. As the temperature of the wafer  was  ramped up the diode  voltage  remained constant until an additional abrupt increase at the Ga20 desorption. suppressed removed desorption  This  suggests  until the this  effect  that  oxide occurs  the  slower  roughening  layer is removed. Once the approximately  80°C  lower  effect  is  oxide  is  than  the  temperature.  Examination of the desorbed wafers with an ordinary optical  66  540  560  580  600  620  640  660  680  Temperature °C  Figure 25. Diffuse reflectivity and Ga20 mass spectrometer measured during oxide desorption.  signal  67  microscope  showed  desorbed wafers illumination.  no evidence  of  surface  roughening, yet  appeared cloudy to the naked eye  The surface  texture  is  visible  the  under intense  in a phase  contrast  microscope as shown in Figure 26. The series of progressively thicker ozone oxide layers were desorbed, and then were examined with a Nomarski  interference  contrast  microscope  at  Johnson Matthey  Electronics (Trail B.C.) by Luigi Isernia. The results obtained showed that the oxide desorption process indeed does cause the GaAs surface to roughen. Also this data showed that desorption of the thicker oxide layers caused the surface to roughen more than the desorption of the thinner oxide layers. A practical consequence of these results is that one can use the step in the diffuse light scattering, as a method for detecting the oxide desorption. The step in the diffuse reflectivity is much sharper than other methods used to detect the oxide desorption such as RHEED or mass spectrometry because these methods  are  affected  by  inhomogeneous  wafer  temperatures.  68  Figure 26.  Surface texture of desorbed GaAs substrates  with a Nomarski interference microscope.  observed  69  V DISCUSSION  Thermal desorption occurs when the bond of the absorbed species to the surface breaks due to random thermal motion of the atoms. The rate of desorption from a unit surface  area may be  written as  - ~  N s(T)co e(-E/kT) x  x  ( 5  )  N-is the number of adsorbed atoms or molecules on the surface per unit area x-is the order of the desorption s(T)-is the sticking coefficient  for a molecule or atom striking the  surface.This term is temperature dependent but expected to be weak compared to the exponential term. co-is an attempt frequency, related to the vibration frequency of the species to be desorbed E-is the activation energy of the desorption, related to the bond energy T-temperature in degrees Kelvin In the simplest case, if more than one monolayer is present on the substrate, the absorbed material will desorb layer by layer. This is a zeroth order desorption process for it is independent of the number of adsorbed species on the surface. With one monolayer of oxide remaining on the surface the desorption kinetics will change to first order, because the rate of desorption now depends  on the  oxide  70  coverage.  In  this  model,  the  rate  of  desorption  increases  exponentially with temperature until most of the absorbed material has been removed then rate of desorption peaks and drops quickly to zero. Therefore the temperature of maximum desorption should increase with thickness, because the thicker layers can continue the exponential increase in desorption rate longer, since there is more material to desorb. This theoretically predicted relationship between the desorption temperature and the thickness  is logarithmic rather  than linear as is observed in our experimental data shown in Figure 12. Assuming that the gallium oxide initially desorbs via zeroth order kinetics one would expect that the temperature for the initiation of the desorption process would be constant, independent of the oxide thickness.  We observed experimentally  that the thin oxide  layers  were completely desorbed before the thicker oxides began to desorb. It was noticed that the width at half maximum of the mass spectrometer  Ga20 peaks was approximately 15°C±5°C. This width  must be regarded as an upper limit, since the experimental peak will be broadened by temperature  inhomogeneity  in the  zeroth  desorption  five  order  model  for  the  of  substrate.  A  monolayers  of  absorbate using a realistic value for the activation energy (2.55 eV) shows that the width at half maximum should be approximately 50°C.[29] An analysis of the temperature dependence  of the rising  edge of the Ga20 mass peak gives an activation energy of 10 eV as shown in Figure 27. This is an unrealistically large activation energy considering that the measured activation energy is a lower limit to  71  Figure 27. Temperature dependence of the the rising edge of the Ga20 mass peak.  72  the true value due to the experimental peak broadening mentioned above. This proves that the Ga20 desorption can not be explained by normal layer by layer evaporation. One simple explanation for the narrow peak width is that the gallium oxide undergoes a phase transition prior to desorption. This would explain the data if the phase transition produces a gallium oxide which is weakly bonded to the GaAs substrate or has a higher vapor  pressure  than  the  low  temperature  phase.  Under  these  conditions, the gallium oxide will completely desorb a few degrees after  reaching  interpretation  the could  temperature explain  why  of the  the  phase  thinner  transition. oxides  would  This be  completely desorbed before the thicker oxides start to desorb, if the composition  of the  oxides changes with thickness.  However this  model does not readily explain the roughening effect discussed in Section IV.5. An  alternate  model  that does explain  all the  experimental  observations is that the oxide desorption is associated  with a thin  film instability. In this model, we assume that at some temperature small holes or cracks appear in the oxide layer. Energy is needed to nucleate the formation of holes or cracks in the oxide layer because it involves energy  the creation of additional surface area. It requires more to form holes in the thicker oxides,  therefore  the thicker  oxides are more stable and desorb at a higher temperatures. The edges of the oxide around the hole have a high curvature and one would expect the oxide vapor pressure to be higher than for the flat  73  oxide surface. Once holes are formed the oxide evaporates rapidly around the perimeter of the holes. Eventually the holes  coalesce  exposing the clean GaAs surface. This explains qualitatively why the oxide desorption takes place over such a narrow temperature range. In the calculation below we show how the energy needed to form holes or cracks in the oxide layer increases linearly with the oxide desorption temperature. In Equation 6 we give an approximate rate equation for the formation of cracks in the surface oxide.  = co e-(Wa7kT) 0  (  6  )  M-number of holes or cracks per unit area co -an attempt frequency 0  Wd-is the energy needed to form a hole or crack in an oxide layer of thickness d. If the cross-section shape of the minimum energy hole in the oxide is independent of the oxide thickness, then the surface energy of the hole will scale linearly with oxide thickness. Therefore we can expect that 'Wd' will also increase linearly with the oxide thickness. Each crack formed will grow, and expose a small area of the substrate during the desorption process. This small area ' A ' can be defined by the density of cracks when the oxide desorbs. Therefore oxide desorption occurs when one crack appears in this area ' A ' .  74  tl  ^T =  A  dt  1  (7)  0 The  desorption  Equation  temperature  6 into Equation  can  7.  be  determined  The wafer  heating ramp is given by T=T +Pt where T 0  by  substituting  temperature during 0  the  is the idle temperature  and p is the ramp rate. Since temperature is a linear function of time we change the upper bound of the integral from ti to T  m a  x and the Wd  lower from 0 to T . By substitution of a dimensionless variable =Jcj" x  0  into Equation 7 one can write; Tmax  AWdcOo  Pk  f  e-  J  x  x  2~dx =1  (8).  Tmin  By evaluating the integral, Equation 8 can be written in the following form  -(AWdcOo/pk) {^+Ei(-x)J =1  (9).  The exponential integral can be approximated by a asymptotic series, 1 1 2 6 Ei(-x)=e-* {-- + ^ -^3 +^4  )  (10)  Since x is » 0 we only need to keep the first term of the expansion to get a good approximation of Equation 9.  75  Now we assume that - (  W d  e  -  k T m  p Wd a  A C 0 0  Rearrangement  /  k T m a x  W d  e  (e-Wd/kTmax  of  ) » -(  } s  equation  1  ( 1 2  12  /  k T m i n  ) then we can write  )  shows  that  the  desorption  temperature should be linearly proportional to the surface  energy,  neglecting the logarithmic term. Wd T ax^ m  k  ln{  kT  m a x  p  w  2Ao) d  0  }  (13)  We have assumed that the surface energy of crack formation will scale linearly with the oxide thickness, therefore one would expect the  desorption  temperature  to  also  scale  linearly  with  oxide  thickness. This model can be used to explain the narrow desorption peak of the gallium oxide and why the desorption temperature is a linear function of the oxide thickness. As described in Section IV.5 we observed surface roughening as  the gallium  oxide  was desorbed,  by measuring  the  diffuse  reflectivity. From this data we determined that there is an abrupt increase in the scattered light during the desorption followed by a slow increase with temperature after the oxide was removed. Since the wafer is polished to a mirror finish before the oxidation process one might expect that an atomically roughened GaAs-oxide interface would be created during the growth of the oxide.[30] This roughened surface would become exposed  after the desorption of the gallium  oxide. One would expect that the spacing between features  on the  76  roughened surface would be at most on the same order of magnitude as the oxide thickness (~20A).  Photographs through the Nomarski  microscope of a wafer after the oxide has been removed show a much larger scale roughness. The photographs show a textured GaAs surface with a mean separation between surface features  estimated  to be approximately 2p.m. The measured separation between features is consistent with the fact that the intensity of the light scattering is high when the scale of the surface texture is of the same order as the laser  wavelength. This  type  inhomogeneous the  thickness  of  surface  roughening  is  consistent  with  the  oxide desorption model proposed above to explain dependence  of  the  gallium  temperature. In this case some regions  oxide  of the  desorption  substrate  surface  would be free of the surface oxide while other regions would still have an oxide cap. It is reasonable to expect that the exposed areas would lose gallium by evaporation at the high wafer temperatures and therefore process  cause pits to form in the uncapped regions.  is shown schematically in Figure 28.  This  Surface roughening  would increase with thicker oxide layers since the thicker oxides desorb at higher temperatures where the evaporation rate of gallium is higher. One would expect the surface to continue to roughen to a lesser degree after the oxide desorption due to conversion of the pits to lower energy crystal facets. For example the (110) lower energy  surface than the (100)  surface is a  surface and might form in  combination with the (100) facets to lower the surface energy on a  77  Figure 28.  Schematic of our model of the inhomogeneous  oxide  desorption process. This model explains the origin of the surface texture which produces the observed increase in diffuse reflectivity.  78  textured On  surface. the  other hand,  we  believe  that  the  increase  in the  secondary electron current is caused by microscopic roughness on the GaAs surface, that is, roughness conclude that the roughness reversible  effect  on the  10-lOOA scale. We  is microscopic in size because  which caused  the  electron  scattering  was  the not  detected in the light scattering experiment. Since the secondary electron effect is reversible, one might expect the microscopic roughness  to be caused by a roughening  transition. This type of phase transition should have been observed in the many RHEED studies of the surface reconstructions of desorbed GaAs  substrates.  There has been  no mention  of  such a phase  transition of GaAs in the literature so another explanation for the microscopic roughness effect is needed. The secondary electron effect should be sensitive to changes in work function as the substrate surface changes chemical composition during heating. But the order of magnitude change in the secondary electrons can not be explained by the work function change alone. For example the work function for GaAs is ~4.5eV and for Ga metal it is 3.95 eV, for a 12keV primary electron this difference is unlikely to be large enough to produce the observed change in the secondary electrons. A proposed explanation for the large increase in the secondary electron current at temperatures T>TGa20 is given below. It has been determined that when the wafer is above the temperature of the  79  gallium oxide desorption the surface can lose gallium and arsenic even when exposed to an AS4 flux. Excess gallium on the surface can either desorb, combine with As to form GaAs or diffuse along the surface and form Ga droplets. Gallium droplets have been observed on the surface of GaAs under gallium rich conditions during pulsed beam epitaxy  [25]  and have been proposed to exist even under  arsenic stable  conditions.[24]  There are three effects which will cause a RHEED electron incident on a Ga droplet to produce more secondary electrons than a RHEED electron of the same kinetic energy incident on a flat GaAs surface.  First,  a 10-lOOA Ga droplet is  small compared to the  penetration depth of the primary electrons and is comparable in size to the escape depth of the  secondary electrons.  Therefore most  secondary electrons produced in the droplet can escape but only the secondary electrons  produced in the top layers of the GaAs can  escape, the rest being absorbed by the wafer. This effect is observed in  electron  microscopy  where  surface  roughness  affects  the  brightness of the secondary electron images. In general, for electrons of  kilovolt energy, the penetration depth into the target is of the  order of microns, while the secondary electron range is only tens of angstroms. We expect the high energy electrons incident at an angle of ~ 3 ° on  these  secondary  microscopic electrons.  droplets  to  produce  The forward scattered  forward  scattered  secondary  electrons  produced in the Ga droplet will escape into the vacuum, but forward  80  scattered electrons in the GaAs wafer are more likely to be absorbed rather than escape. The experimental secondary electron data is consistent with this interpretation, as discussed in Section IV.4. It has been reported that the primary electron interactions associated with secondary electron production can be divided into two classes, plasmon excitation, and direct valence and core electron excitation [31,32], It was further suggested that the contribution to the secondary electron yield is approximately equal for the two processes.  Therefore one  can not ignore the  effect  of plasmon  creation from the RHEED electrons when considering the secondary electron emission. The energy of bulk plasmons in GaAs has been reported  to  be  approximately 5 eV.[33] The energy of the bulk  plasmon in Ga metal is approximately 10 eV. One would expect that secondary electrons produced from the decay of plasmons in GaAs would have a relatively low probability of escaping into the vacuum because  their energy is close to the threshold energy needed to  escape. The secondary electrons produced from the 10 eV plasmons in Ga metal should have a higher probability of escape into the vacuum. The increase in the secondary electron current should then be proportional to the fraction of the surface covered by the gallium droplets. The rate of production of free Ga on the surface is equal to the rate of As loss minus the rate of Ga evaporation. A rate equation for the increase of Ga atoms on the wafer surface is given by equation 14.  81  dN /dt=(l-e )RAs -(e )S(T)F G  D  D  AS  -(26 )RGa-Ga "(l-Bo^Ga (14) D  The symbols used in this expression are defined as follows; N G is the number density of Ga atoms on the surface in Ga droplets. RAS is the evaporation rate of As from GaAs under a As4flux. F A S is flux of AS4 onto the GaAs surface (beam equivalent pressure of 2xl0-  5  torr)  RGa-Ga is the evaporation rate of Ga from the Ga droplets. This term depends  on the  vapor pressure of Ga over Ga metal.(activation  energy~2.7eV) S(T) is the sticking coefficient of AS4 on GaAs at temperature T 0D is the fraction of the total surface area covered by the Ga droplets. RGa  is the rate of Ga evaporation directly from GaAs.(activation  energy~ 4.6eV) In the RGa-Ga> term the factor of 2 in 26D is a result of the surface area of a hemispherical drop (2TCR ) being twice the area of the GaAs 2  surface that it covers (TIR ). In steady state at temperature T the rate 2  is dNG/dt=0 therefore Equation 14 can be written in the following form.  (l-e )(RAs-RGa)= (eD)(S(T)F s+2R a-G ) D  A  G  a  (15)  One can solve this equation for 6D, the fraction of the surface covered by gallium droplets.  82  When  the  wafer  reaches  the  temperature  where  gallium  oxide  desorption occurs we expect that evaporation rate of Ga and As from GaAs to be similar in value because the sample is near the congruent sublimation temperature (~640°C). For this reason it is assumed that S(T)FAS  is greater than RAs-^Ga  near the congruent sublimation  temperature The fraction of the surface covered by Ga droplets is approximated by equation 17. A RAs-KGa ° -S(T)F s+2R a-Ga D  A  This  n  7  U  G  expression  . °  is reasonable  because the production of gallium  droplets only occurs when the evaporation rate of As from GaAs is greater than the evaporation rate of Ga from GaAs. This occurs at substrate (~640°C).  temperatures  near the congruent  Also, we find that the coverage  sublimation temperature of gallium droplets is  inversely proportional to the As4 flux and the rate of Ga loss from the droplets. Figure 29 is a plot of equilibrium pressure of A s 2 , As4, and Ga over GaAs.[34] In the temperature range from 600°C to 700°C we see that the vapor pressure of A s 2  over GaAs becomes much  higher than Ga over GaAs. The rate of As loss from GaAs will become greater than the rate of Ga loss from GaAs in this temperature region. Therefore one would expect the coverage of Ga droplets to increase dramatically. This model does not take into account the fact that the GaAs can  be  As  terminated  at  temperatures  below  the  congruent  sublimation point, or Ga terminated at higher temperatures. In our  83  Figure 29. Plot of equilibrium pressure of As2,As4, and Ga over GaAs. Ref. [34]  84  case this is not a problem because we are operating close to the congruent sublimation temperature where a transition from the As stable surface to the Ga stable surface should occur. Therefore the surface should be approximately 50% As and 50% Ga. An Arrhenius plot of the secondary electron current is shown in  Figure 24  and the  activation  energy  was  determined  to  be  3.6±0.4eV. Using this data in Figure 29 we calculate the activation energy for the loss of As2 from GaAs to be 4.0±0.2eV. This value is remarkably close to the activation energy for the formation of Ga droplets.  One  would  expect  the  activation  energy  for  droplet  formation to be different from that of the activation energy for As2 loss from GaAs, because droplet formation depends on the rate of Ga loss also. The reversible nature of the secondary electron effect can be explained by the Ga droplet model. Once the wafer temperature is decreased below the congruent sublimation temperature, the rate of As loss from the GaAs is less than the rate of Ga loss, hence the supply of gallium to the remaining  Ga droplets  gallium droplets is  present  on  the  surface  also stopped. The will  be quickly  consumed by evaporation or by reaction with the incoming As4 flux. It is reasonable to assume that if the droplets are small, the GaAs that is formed will be a smooth layer due to Ga diffusion on the surface. This is in fact observed in migration enhanced epitaxy where gallium pulses.  atoms  and arsenic  molecules  are supplied separately  in  85  VI CONCLUSION  In this thesis we report new effects that occur on the surface of the GaAs substrate during the oxide desorption that provide new insight  into  the  oxide  desorption process,  and new information  relating to the nature of the GaAs surface in the vicinity of the congruent  sublimation temperature.  After the gallium oxide was removed, a reversible order of magnitude increase in the secondary electron current associated with the RHEED beam was observed. We interpret the secondary electron measurements  as evidence  for microscopic roughening due to the  formation of gallium droplets on the substrate surface. A model of the droplet formation process shows that the fraction of the surface covered by Ga droplets is proportional to the difference between the rate of evaporation of As from GaAs and the rate of evaporation of Ga from GaAs. A small effect in the secondary electron current was observed due to the gallium oxide desorption. This may be due to surface  roughening during  the  desorption  or a change  in work  function of the surface. This effect may be used to determine the oxide  desorption temperature in materials that do not have  the  associated droplet formation. The  variation in oxide desorption temperatures reported in the  literature has been explained in terms of variations in the oxide thickness. We observed a linear increase in the oxide desorption  86  temperature with oxide layer thickness, for oxide layers between 6A and 24A.  In addition, we show by diffuse light scattering that the  smooth GaAs substrates roughens during the oxide desorption. One can use the step in the diffuse scattering as a accurate method for detecting the oxide desorption. We assume the intensity of the step should be a measure of the oxide thickness, since surface roughness was found to increase with the thickness of the oxide. These results are interpreted in terms of a model where the oxide layer desorbs inhomogeneously.  87  VII REFERENCES  [1] J.P. Contour, J. Massies and A. Saletes; Appl. Phys. A . , 3 8 , 45 (1985) .  [2] R.P. Vasquez, B.F. Lewis and F.J. Grunthaner; Appl. Phys. Lett. 42, 293 (1983).  [3] J.A. McClintock, R.A. Wilson and N.E. Byer; J. Vac. Sci. Tech. 20, 241 (1982).  [4] S J . Ingrey, W.M. Lau and N.S. Mclntyre; J. Vac. Sci. Tech. A4, 984 (1986) .  [5] A.J. SpringThorpe, S.J. Ingrey, B. Emmerstorfer, P. Mandeville and W.T. Moore; Appl. Phys. Lett. 50, 77 (1987).  [6] M.K. Weilmeier, J.M. Colbow, T. Tiedje, T. Van Buuren and L i Xu; Can. J. Phys. (to be published 1991).  [7] J.H. Neave, B.A. Joyce, P.J. Dobson and N. Norton; Appl. Phys. A 3 1 , 1 (1983).  [8] B.A. Joyce, J.H. Neave, P.J. Dobson and P.K. Larsen; Phys. Rev. B.  88  29, 814 (1984). [9] J.R. Arthur; J. Appl. Phys. 38, 4023 (1967)  [10] C.E.C. Wood and B.A. Joyce; J. Appl. Phys. 49, 4854 (1978).  [11] K.J. Kuhn and G.A. Pubanz; J. Vac. Sci. Tech. A7, 3265 (1989).  [12] A.J. SpringThorpe and P. Mandeville; J. Vac. Sci. Tech. B6, 754 (1988).  [13] J.M. Van Hove and P.I. Cohen; J. Crystal Growth 81, 13 (1987).  [14] P.J. Dobson, B.A. Joyce, J.H. Neave and J. Zhang; J. Crystal Growth 81, 1 (1987).  [15] P.I. Cohen, P.R. Pukite, J.M. Van Hove and C.S. Lent; J. Vac. Sci. Tech. A4, 1251 (1986).  [16] J.H. Neave, B.A. Joyce, P.J. Dobson and N Norton; Appl. Phys. A31, 1 (1983).  [17} L.P. Erickson, M.D. Longerbone, R.C. Youngman and B.E. Dies; J. Crystal Growth 81, 55 (1987).  89  [18] J.N. Eckstein, C. Webb, S.L. Weng and K.A. Bertness; Appl. Phys. Lett. 51, 1833 (1987).  [19] E . Segre, (ed.) Experimental  Nuclear  Physics (vol. 1); John  Wiley & Sons, New York, 1953, pp. 252-304 .  [20] B.G. Yacobi and D.B. Holt; Cathodoluminescence of  Inorganic  Microscopy  Solids, Plenum Press, New York and London, 1990,  pp. 56-65.  [21] J.M. Van Hove and P.I. Cohen; Appl. Phys. Lett. 47(7), 1 (1985).  [22] Takeshi Kojima et. al.; Appl. Phys. Lett. 47(3), 1 (1985).  [23] R. Heckingbottom; J. Vac. Sci. Tech. B3, 572 (1985).  [24] E.M. Gibson, C T . Foxon, J. Zhang and B.A. Joyce; Appl. Phys. Lett. 57,  1203 (1990).  [25] K. Yamada, N. Inoue, J. Osaka and K. Wada; Appl. Phys. Lett. 55, 622 (1989).  [26] A.J. SpringThorpe and P. Mandeville; J. Vac. Sci. Tech. B4, 853 (1986).  90  [27] C D . Thurmond; J. Electrochem. Soc. 122, 1133 (1975)  [28] Wells; Scanning  Electron  Microscopy,  McGraw-Hill, New  York, 1974, pp.63  [29] M . K. Weilmeier; (private communication).  [30] D.E. Aspnes and A.A. Studna; Appl. Phys. Lett. 46,1071 (1985).  [31] S.D. Berger, D. Imeson, R.H. Milne and D. McMullan; Proceedings of  the Institute  of Physics  Electron Microscopy  and Analysis  Group 1985, G.J. Tatlock (ed.) pp.99 (1985).  [32]  A . L . Bleloch;  Electron  Proceedings  Microscopy  of  and Analysis  the  Institute  Group  of  Physics  1989, (Vol. 1) P.J.  Goodhew and H.Y. Elder (ed.), pp469 (1989).  [33] J.S. Blakemore; J. Appl. Phys. 53, 123 (1982).  [34] C. Pupp, J.J. Murray and R.F. Pottie; J. Chem. Thermodyn. 6, 123 (1974)  [35] J. Massies and J.P. Contour; J. Appl. Phys. 58, 806 (1985)  [36] M.P. Seah and W.A. Dench; Surf, and Interface Anal. 1, 2 (1979)  91  VIII.l APPENDIX 1  Optical Temperature Measurement Technique  An  optical temperature measurement  technique  was  used to  determine the temperature of the GaAs substrates.[6] The optical measurement  had  an  absolute  accuracy  of  about  ±10°C  for  temperatures between 20°C and 700°C. The accuracy is limited by the accuracy with which the bandgap of GaAs is known as a function of temperature. The technique has a relative sensitivity of about ± 1 ° C from  room temperature to  450°C.  A calibration of  the optical  technique, with a thermocouple in contact with the sample, is shown in Figure 30.  92  Figure 30 Calibration curve for the optical temperature measurement technique.  93  VIII.2 APPENDIX II  Oxide Thickness Determination from XPS Results  X-ray  photo-emission  spectroscopy  (XPS)  was  used  to  determine the oxide thickness on a series of GaAs substrates which had been exposed, for various lengths of time,.in the plasma reactor. XPS measurements were conducted with a A l K a radiation source (hu=1486.6 eV). Figure 31 shows the photoemission spectra for the As and Ga 3d core levels along with the components associated with As-0 and Ga-0 bonding.These components due to oxygen bonding are chemically shifted to higher binding energies with respect to the core levels.  The thickness of the oxide layer was estimated using the  following expressions.[35]  Iox=(D ^ox)(l-eox  dox  /^ ) x  Isub=(D ub>.sub)(e- ^ ) dox  S  ox  (18)  (19)  lox , Isub-are the intensities of the photoelectrons coming from the same atomic shell in the oxide and the substrate Dox, D b-are the atomic densities in the oxide and substrate su  X . X o x  SUD  -are the inelastic mean free paths of electrons in the oxide  and the substrate d - i s the thickness of the oxide layer ox  In the calculation of thickness we assume that the atomic density of  94  the oxide is equal to the atomic density of the substrate and X . equal to A .  SUD  ox  is  which is taken as 25 A for the As 3d core level. [36]  This value was determined from a universal curve drawn through a compilation of published measurements  of electron inelastic mean  free paths in solids for energies in the range 0-10,000 eV above the Fermi level. We find the inelastic mean free path for the Ga 3d core level  on the  universal curve is  also  approximately 25A. Using  Equations 17 and 18 and the above approximations we can derive Equation 19.  (Iox+Isub)  =  e  d  Q  x  A  (  1  9  )  Isub  We know the area of an XPS peak is equal to the intensity of the photoelectrons multiplied the capture cross section. A =a xIox ox  An  0  (20)  estimate of the oxide  thickness  can be determined from the  following equation. ,  «,  fAox+Asub.  d =Mn {—j—-—} ox  The  . . .  (21)  thickness calculated in this manner is only an estimate because  the values of the capture cross section and density are close, for the substrate and the oxide, but not the same.  95  50 45 40 35 30 25 20 Binding energy (eV) Figure 31 X-ray photoelectron spectra showing the As and Ga 3d core levels of oxidized GaAs.The area of the chemically shifted peak at 45 eV due to As-0 bonding is determined with a three Gaussian fit (dotted line) The chemically shifted peak due to Ga-0 bonding is fit with one Gaussian, (a) 1 min. plasma oxidation (8A) (b) 20 min. plasma oxidation (15A) (c) 60 min. plasma oxidation (19A).  


Citation Scheme:


Citations by CSL (citeproc-js)

Usage Statistics



Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            async >
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:


Related Items