UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Growth and characterization of thin oxide films on SiGe Zheng, Lan 1997

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

Item Metadata

Download

Media
831-ubc_1997-0611.pdf [ 3.08MB ]
Metadata
JSON: 831-1.0061674.json
JSON-LD: 831-1.0061674-ld.json
RDF/XML (Pretty): 831-1.0061674-rdf.xml
RDF/JSON: 831-1.0061674-rdf.json
Turtle: 831-1.0061674-turtle.txt
N-Triples: 831-1.0061674-rdf-ntriples.txt
Original Record: 831-1.0061674-source.json
Full Text
831-1.0061674-fulltext.txt
Citation
831-1.0061674.ris

Full Text

GROWTH A N D C H A R A C T E R I Z A T I O N OF THIN OXIDE FILMS O N SiGe by LAN ZHENG B . S c , Fudan University, 1989  A THESIS S U B M I T T E D IN P A R T I A L F U L F I L L M E N T OF T H E R E Q U I R E M E N T S F O R T H E D E G R E E OF M A S T E R OF SCIENCE in T H E F A C U L T Y OF G R A D U A T E STUDIES Department o f Chemistry  We accept this thesis as conforming to the required standard  T H E U N I V E R S I T Y OF BRITISH C O L U M B I A September 1997 © L a n Zheng, 1997  In presenting  this  degree at the  thesis  in  partial fulfilment  of  University of  British Columbia,  I agree  freely available for reference copying  of  department publication  this or of  and study.  thesis for scholarly by  this  his  or  her  Department The University of British Columbia Vancouver, Canada  requirements that the  I further agree  purposes  representatives.  may be It  thesis for financial gain shall not  permission.  DE-6 (2/88)  the  that  advanced  Library shall make it  by the  understood be  an  permission for extensive  granted  is  for  that  allowed without  head  of  my  copying  or  my written  Abstract Atomic oxygen from a remote plasma oxidation was used to grow a high quality gate oxide on SiGe at l o w temperatures. Atomic oxygen from a remote O2 plasma was also used to form thin oxide films on SiGeo.025 that is capped with 12.5 A o f S i at ~200°C. The characteristics and thicknesses o f the oxide were determined by X-ray Photoelectron Spectroscopy ( X P S ) . It was observed that the oxidation that resulted from exposure to Oatoms, produced a thin oxide film with both S i and Ge oxidized. The interfacial trap densities were continuously monitored with an RF-probe. The changes in trap density were quite rapid during the process o f oxidation and subsequent exposure to hydrogen atoms. However, the disappearance o f the carrier traps, when the H-atoms were shut off, was found to be slower and could be followed. This is true at all temperatures ranging from 20°C to 200°C.  The kinetic analysis o f this trap removal  process reveals that, this is either a second order reaction with an activation energy o f 0.35 e V , or there are two concurrent first order processes with activation energies o f 0.21 eV and 0.16 e V . B y comparing the behavior o f S i capped SiGeo.025 with SiGeo.025, SiGeo.3 and intrinsic S i at ~200°C, it was found that all interfaces except SiGe .3 benefit from 0  exposure to atomic hydrogen after oxidation, and that after such a treatment the interfacial trap density is not significantly different on S i capped SiGeo.025, SiGeo.025 and intrinsic S i . The passivation level achieved by low temperature H-atoms treatment was also compared to a high temperature (~450°C) H2 annealing process which was conducted according to the standard industrial practice.  Similar passivation levels were achieved  with both techniques.  ii  Table of Contents  Abstract  ii  Table o f Contents  iii  List o f Tables List o f Figures  v  vi  Acknowledgments  Chapter 1 Introduction .1.1 Semiconductor Materials and Physics 1.1.1 Silicon and Germanium 1.1.2 SiGe 1.2 Semiconductor Devices  1 2 2 11 12  1.2.1 Bipolar  12  1.2.2 M O S F E T  14  1.3 Process Technology  16  1.3.1 Fabrication o f a Bipolar Transistor  16  1.3.2 Fabrication o f a M O S F E T  18  1.3.3 Oxidation, Plasma vs. Thermal Oxidation  20  1.4 Oxide/Semiconductor Interface Properties  23  1.5 Objectives of Present Research  26  Chapter 2 Experimental 2.1 Materials  29 29  2.1.1 Silicon and Silicon Germanium Samples  29  2.1.2 Chemicals  31  2.2 Apparatus  31  2.2.1 Reaction Chamber  33  2.2.2 RF-Probe  34  iii  2.2.3 Mass Spectrometer  37  2.3 Experiment Procedure  39  2.3.1 Sample Cleaning  39  2.3.2 Generation o f O-atoms and Plasma Oxidation  39  2.3.3 H-atoms Generation and Its Exposure to Samples  41  2.4 X-ray Photoelectron Spectroscopy ( X P S )  42  2.4.1 Fundamental Principles  42  2.4.2 Instrumentation  45  2.4.3 Spectral Analysis  47  2.4.4 Application  52  Chapter 3 Results and Discussions  53  3.1 Oxidation o f Si Capped SiGe with O-atoms at Different Temperatures  53  3.2 X P S Analysis o f Si Capped SiGe  55  3.2.1 X P S o f Clean and Oxidized Samples  55  3.2.2 Angle Dependent X P S  58  3.2.3 Determination o f the Atomic Sensitivity for Ge 2p and Ge 3d  62  3.2.4 Quantitative Analysis  64  3.3 Effect o f Exposure to H-atoms on the Charge Carriers  70  3.3.1 Unoxidized Samples  70  3.3.2 Oxidized Samples  72  3.3.3 Kinetic Analysis and Activation Energy Calculation  75  3.3.4 Mechanism o f the Trap Decay  80  3.4 Comparison o f Si Capped SiGe Substrate with Other Substrates 3.4.1 SiGeo.3  85 85  3.4.2 Intrinsic S i and Uncapped SiGe .o25  88  3.4.3 Effect of H annealing  93  0  2  Chapter 4 Conclusion  95  References  97  iv  List of Tables  Table 1.1  Selected important properties o f Si and Ge at 300°K  Table 3.1  Peak intensities for the Si 2p and Ge 2p peaks as a function o f plasma oxidation time (take-off angle = 90°)  Table 3.2  57  Intensity ratios as a function of plasma oxidation time (take-offangle = 90°)  Table 3.3  10  57  Peak intensities from peak fitting of Si 2p and Ge 2p as a function of take-off angle (oxidation time = 50 min)  61  Table 3.4  Intensity ratio as a function o f take-off angle (oxidation time = 50 min)  61  Table 3.5  Integrated Ge 2p and Ge 3d peak intensities for pure Ge and mean path paths  62  Table 3.6  Calculated atomic intensity ratio and sensitivity ratio o f Ge 2p/ Ge 3d  64  Table 3.7  Peak intensities and mean free paths o f Ge 2p and Ge 3d for an H F washed sample  66  Table 3.8  Mean free paths (X) used in the calculation  69  Table 3.9  Oxide thickness for both Si and Ge as a function o f oxidation time. The last column lists the calculated values for a 45° take-off angle with a 50 m i n oxidation time  69  Table 3.10 Tabulated data o f second order rate constants  78  Table 3.11 Trap decay constants k i and k2 at different temperatures when H-atoms were turned off  83  List of Figures  Figure 1.1  Silicon crystal structure  3  Figure 1.2  M i l l e r index  5  Figure 1.3  Band structure diagram of Si and Ge  7  Figure 1.4  (a) A bipolar transistor fabricated by the methods of planar technology (b) Schematic of active elements of an npn bipolar transistor  12  Figure 1.5  The physical structure of an n M O S F E T  14  Figure 1.6  A n npn transistor fabrication  17  Figure 1.7  A Simplified n M O S F E T fabrication  Figure 2.1  Schematic of reactor and associated equipment  32  Figure 2.2  A n RF-probe and RF-circuit  35  Figure 2.3  Mass Spectrometer  38  Figure 2.4  Schematic of the X P S process  44  Figure 2.5  X-ray photoelectron spectroscopy  46  Figure 2.6  X P S spectra of (a) Si 2p and (b) Ge 2p  48  Figure 2.7  Surface sensitivity enhancement by variation of the electron  2  19  'take-off angle  51  Figure 3.1  Sketch of Si capped SiGeo.025 sample  53  Figure 3.2  Change of carrier concentration for the Si capped SiGeo.025 sample during oxidation at 25°C, 205°C and 440°C 54  Figure 3.3  S i 2p and Ge 2p spectra from Si capped SiGeo.025 sample after an H F wash and after a 50 min oxidation  56  vi  Figure 3.4  Plot o f relative intensity ratios o f Si / Si and G e I G e with 4  u  oxidation time Figure 3.5  59  Comparison o f Si 2p and Ge 2p spectra obtained at two take-off angles (90° and 45°) for a S i capped SiGe sample after 50 m i n oxidation, showing the effect of increasing the take-off angle on the relative oxide/element intensities  Figure 3.6  60  Ge 2p and Ge 3d spectra from a pure Ge sample showing the relative peak intensities  63  Figure 3.7  Structure o f the H F washed Si capped SiGeo.025 sample  65  Figure 3.8  X P S spectra of Ge 2p and Ge 3d from an H F washed sample (take off angle = 45°)  Figure 3.9  66  M o d e l used to estimate oxide thicknesses from X P S peak intensity ratios...67  Figure 3.10 Effect o f H-atoms on an H F washed Si capped SiGeo.025 sample at 198°C  71  Figure 3.11 Change o f carrier concentration for an oxidized and annealed S i capped SiGeo.025 sample during long exposure to H-atoms at 20°C, 83°C, and 173°C  73  Figure 3.12 Change o f trap density for an oxidized and annealed Si capped SiGeo.025 sample during long exposure to H-atoms at 20°C, 83°C, and 173°C  74  Figure 3.13 Plot o f the logarithm o f trap density versus time for 20°C, 83°C, 148°C, 173°C, and 192°C  76  Figure 3.14 Plot o f the inverse o f trap density versus time and the "best" straight line through the points for 20°C, 83°C, 148°C, 173°C, and 192°C  77  Figure 3.15 Arrhenius plot o f the rate constants for the disappearance o f traps when H-atoms are shut off. This process was analyzed by second order kinetics  79  Figure 3.16 Trap density decay when H-atoms are turned off at different temperatures. The dashed lines are fitting lines from a double exponential decay function y = y + y i exp(-kit) + y exp(-k t) 0  2  2  82  Figure 3.17 Arrhenius plot o f trap decay rate constants (ki for a fast process and k for a slow process) when H-atoms are turned off. These 2  processes were analyzed by a two-concurrent first order kinetics  84  Figure 3.18 Change o f carrier concentration for SiGeo.3 during O-atom and H-atom treatments at 209°C  86  Figure 3.19 Change o f trap density for SiGeo.3 during O-atom and H-atom treatments at 209°C  87  Figure 3.20 Change o f trap density for an intrinsic S i surface during O-atom and H-atom treatments at 209°C  89  Figure 3.21 Change o f trap density for a SiGeo.025 surface during O-atom and H-atom treatments at 209°C  90  Figure 3.22 Change o f trap density for a S i capped SiGeo.025 surface during O-atom and H-atom treatments at 209°C  91  Figure 3.23 Comparison o f H-atom treatment at 202°C on intrinsic S i samples that (a) have been annealed in H for 20 minutes at 450°C, 2  and (b) that have not been annealed in H  2  94  Acknowledgments I would like to express my sincere gratitude to my supervisor, Professor Elmer Ogryzlo for his guidance and sound advice throughout this work. He has always been a source o f inspiration and knowledge. I am especially grateful to Dr. Hongjun L i for sharing me with his experimental experience and ideas. I would also like to express my thanks to other members in our group: Dr. L i g i a Gheorghita, Professor Jun-gill Kang and Dr. Bagher Bahavar for their assistance and thoughts. M y thanks also go to Professor B . Heinrich in the Department o f Physics at Simon Fraser University, for X P S measurements and for helping me with the interpretation o f the X P S results. It is a pleasure to acknowledge many people in the Department o f Chemistry and at the Advanced Materials and Process Engineering Laboratory ( A M P E L ) who provided their support and kindness that have made my academic studies a valuable learning experience. Finally, I would like to express my deepest appreciation to my mother, my father and my sister for their love and encouragement throughout these years of my life.  ix  Chapter 1 Semiconductors  have  made  Introduction  possible  technological breakthroughs o f this century.  one  o f the  greatest  scientific  and  The road to semiconductor devices began  with the invention o f the first germanium transistor in 1947.  Germanium is directly  below silicon in the periodic table and has the same number o f bonding electrons and the same crystal structure as silicon. The attractive features o f germanium from the point o f view o f device theory are its carrier mobilities (3900 c m / V s for electrons and 1500 2  2  2  cm / V s for holes). These are more than twice the values found for silicon (1500 cm / V s for electrons and 450 c m / V s for holes). However, the oxide o f germanium does not have 2  the low interfacial state density, good dielectric properties, and the great chemical stability o f Si02. Because o f its smaller temperature range and inferior oxide, germanium was soon replaced by less costly silicon in the 1960's. Silicon is the second most abundant element in the Earth's crust, where it is present principally as the oxide. In elemental form, it is nontoxic and is an excellent conductor o f heat.  It can be grown into ultra-pure, very large diameter crystals and  readily forms a stable insulating oxide o f high quality. A large measure o f the success of silicon as a material for electronics is due to its oxide, Si02, which is easily grown on any Si surface.  This oxide is an excellent electrical insulator and is chemically inert,  protecting the silicon from attack by other substances.  Especially important is the low  density o f states at the interface between the silicon and the Si02. Such interfacial states trap electrons, bending the valence and conduction bands so that the Fermi level at the interface is pinned at a position between the bands that is determined by the partially  1  filled traps rather than being controllable by doping or by the application o f a "gate voltage" in Metal-Oxide-Silicon-Field-Effect-Transistor devices. Even in the case o f the very good Si-Si02 interface extreme cleanliness and care are needed to keep the surface state density low enough to allow electronic devices to work. Such a favorable condition has not been consistently produced in any other system and the field effect transistor has, consequently, not been successful with any other semiconductor. The  properties  o f silicon  make  it a natural  for  integrated  circuit  (IC)  manufacturing. Yet, from a transistor designer's perspective, silicon is hardly the perfect semiconductor.  Compared with some o f the other semiconductors, it is quite poor in  terms o f how fast charge carriers can travel through the crystal lattice. This sluggishness limits the speed at which silicon devices can operate.  1.1 Semiconductor Materials and Physics 1.1.1 Silicon and Germanium Crystal Structure Silicon and Germanium are group I V elements, which therefore have 4 valence electrons. Those are the electrons which participate in chemical bonding when the atoms form compounds. U p o n solidifying from the liquid phase both S i and Ge crystallize with covalent bonding in the diamond structure, as illustrated in Figure 1.1.  2  Figure 1.1 Silicon crystal structure . A diamond lattice unit cell, in which each atom is surrounded by four nearest neighbors as shown by the black atoms.  3  There are 3 principal crystal planes or directions o f interest. These planes exhibit different properties and are defined in terms o f the unit cell, which in this case is a cube with side dimension a. The planes (100), (110), and (111) are shown shaded in Figure 1.2.  The three ordered digits, "1,0,0" for example, are called Miller indices and are  commonly used to identify the planes and directions in cubic crystal structures.  The  numbers are the reciprocals, normalized with respect to a, o f the intersection o f the shaded plane and the x-axis, v-axis and z-axis, respectively.  The three indices are  normally placed in round parentheses (e.g. (100)) to designate the plane. The direction normal to the plane is indicated by the same M i l l e r indices, but they are enclosed in square brackets, viz.,[100] as shown in Figure 1.2. The family o f equivalent directions is indicated by <100>.  Doping The number o f free electrons, n, in a pure semiconductor is exactly equal to the number o f holes, p, since the production o f both is due to S i -> S i + e" where S i is +  +  designated by "p" (a hole) and e" is designated by " n " (an electron).  Such pure  semiconductors are called intrinsic and an index ' i ' is appended to n and p in this case: ni = pi  (1.1)  If a silicon atom in a Si crystal lattice is replaced by an impurity atom from Group V o f the periodic table (phosphorus or arsenic, for example), four o f the valence electrons of the arsenic w i l l take part i n the covalent bonding with the neighboring Si atoms while the fifth electron w i l l be only weakly attached to the arsenic atom. A t room temperature,  4  a  5  all such dopant centered electrons are promoted into orbitals i n the conduction band which encompass the entire crystal. Such a semiconductor is called "n-type", to describe the presence o f an excess o f negative charge carriers. The corresponding doping impurity is called a donor, having 'donated' an electron to the conduction band. If a Si crystal is doped with an atom from Group III, having only 3 valence electrons, at the location o f that impurity one of the covalent bonding electrons is missing. W i t h the expenditure o f a relatively small amount o f energy, one o f the valence electrons i n S i can contribute an electron to the Group III atom. A hole is created at the position vacated by that valence electron.  The impurity atom, having accepted an  additional electron, is called an acceptor. A semiconductor doped with acceptor is rich in holes, i.e. positive charge carriers. Such a semiconductor is called p-type. Electrons in n-type material and holes in p-type are called majority carriers, while holes in n-type and electrons in p-type are called minority carriers. The concentrations of n and p i n Si at 25°C are governed by the relationship, np = 2 . 1 0 x l 0 cm" 20  6  (1.2)  Band Gap Engineering A n energy band structure, shown in Figure 1.3 for Si and Ge, is a plot of the allowed electron energies E as a function o f the momentum vector k.  The Y axis  represents energy and the x axis shows the magnitude of the momentum vector along some important crystal directions. The curves above the band gap (EG) are conduction bands (C.B.), while the curves below EG describe the valence bands ( V . B . ) . The  6  difference between the minimum in the conduction bands and maximum i n the valencebands is called the energy band gap and noted as EQ. Electrons i n a crystal are not completely free, but instead interact with the periodic potential o f the lattice. A s a result, their 'wave-particle' motion cannot be expected to be the same as for electrons in free space. The technique commonly used for correcting for this when equations o f electrodynamics are applied to charge carriers in a solid, is to alter the electron mass, which is then called the effective mass. The effective charge carrier mass, m * is then related to the rate o f curvature o f the band i n which the electron (or hole) finds itself.  h  2  m* = —  d EI 3k 2  (1.3) 2  here % = h / 2n, where h is Planck's constant. Table 1.1 lists the effective masses and other important properties o f Si and Ge at 300°K. From Figure 1.3 and Table 1.1, one can see that Ge has a smaller band gap and lower m * for both electrons and holes (mo is the free electron rest mass). In thermal equilibrium, the electrons in the conduction band and holes in the valence band move in random fashion with a velocity that depends on the temperature. The disturbance o f thermal equilibrium by the application o f an electric field can cause carriers to acquire a drift velocity in a direction parallel to the electric field. velocity (v<j) is proportional to the magnitude of the electric field (s).  The drift  The constant is  called the mobility (u). During the time between collisions (T), the impulse due to the force o f the electric field causes the electron (with charge q) to gain momentum. If all the momentum gained by an electron is lost to the lattice in collisions, it follows that  8  qxc = m v e  d  (1.4)  Rearranging (1.4) gives, for the electron drift velocity,  v  d  =  (1.5)  qx  —  m  e  We define qx He =  (1-6)  V  so that V =LL S d  e  (1.7)  i.e. U.e (called the mobility of electrons) is the way in which the drift velocity increases with applied voltage. Mobility is an important parameter for devices in which the current is due to drift. The higher the mobility, the faster the carriers w i l l move for a given applied field.  9  Table 1.1 Selected important properties o f Si and Ge at 3 0 0 ° K ' ' ' 2  Properties  Si  Ge  Atomic N o .  14  32  Atomic weight  28.08  72.60  Crystal structure  Diamond  Diamond  Lattice constant (nm)  0.543  0.566  Density (kg m" )  2.33* 10  Melting point (°C)  1420  937  Intrinsic carrier concentration nj  1.45x10'°  2.4xlO  1.12  0.67  1.1 mo  0.55 mo  J  5.33*10  n  (cm" ) 3  Energy gap (eV) Electron effective mass m Hole effective mass m*  n  0.56 m  p  Mobility o f electrons (cm /Vs) 2  0  0.37 m  1500  3900  450  1900  in the bottom o f the C . B . Mobility o f holes (cm /Vs) 2  near the top o f the V . B .  0  3  4  5  6  1.1.2  SiGe The objective o f SiGe technology is the utilization o f the doping process to add  the benefit o f germanium's high carrier mobility to a silicon wafer.  Preliminary work  indicated that an increase in hole mobility could be achieved with the addition o f only a few % Ge to S i . This high mobility benefits both bipolar and M O S F E T devices. In the 1960s, researchers developed epitaxy, a technique that seemed well suited to delicate fabrication tasks.  In this process, layers o f atoms are deposited onto an  existing crystalline material. The underlying crystal, or substrate, serves as a template so that the newly accumulated layers follow the same atomic arrangement as the crystal itself. Because silicon and germanium have almost the same crystal structure, a layer of one material can be deposited on the other, maintaining perfect crystallinity across the growth interface. To accomplish this, the composite film undergoes strain and this layer is called a strained SiGe layer. A frequently used technique for depositing epitaxial layers is chemical vapor deposition ( C V D ) . It utilizes gaseous molecules such as S i H and G e H that incorporate 4  4  the desired atoms i n the S i or Ge crystal when they decompose on the hot surface.  By  1986 a key step had been taken toward commercializing SiGe technology, namely the development o f ultrahigh vacuum ( U H V ) chemical vapor deposition by Bernard S. Meyerson and his co-workers at I B M ' s research facility.  They combined the ultrahigh  vacuum o f molecular beam epitaxy tools and a conventional chemical vapor deposition process in a technique that exploits the unique role o f hydrogen in regulating the chemistry at the silicon and SiGe growth interface. Thin strained layers o f the SiGe alloy  11  are deposited on silicon wafers by a U H V / C V D batch tool with the doping and thickness control needed for device fabrication.  1.2 Semiconductor Devices 1.2.1 Bipolar Bipolar transistors electronics.  were the first transistors  to be produced and used in  The bipolar transistor derives its name from the fact that both doping  polarities, positive and negative, are used in its structure. A perspective view o f a silicon npn bipolar transistor is shown in Figure 1.4a.  Emitter  A  Base  /  \  Collector  /  \  f Oxide  7  (a)  p-substrate  (b) n  +  Emitter  P Base  n Collector  Figure 1.4 (a) A bipolar transistor fabricated by the methods o f planar technology . (b) Schematic o f active elements of an npn bipolar transistor.  12  A n idealized, one-dimensional structure of the bipolar transistor, shown in Fig. 1.4b, can be considered as a section o f the transistor along the dashed lines in Fig. 1.4a.  The heavily doped n -region is called the emitter (E), the narrow central p+  region is called the base (B), and the lightly doped n-region is called the collector (C). If the E - B junction is forward-biased by applying a positive voltage to the base electrons flow into the base.  The base is very thin in a bipolar transistor.  Electrons  diffuse through the base away from the emitter until they encounter the high electric fields produced by the reverse bias on the collector and are rapidly swept into the collector region. The base-emitter bias essentially serves to control the current from the emitter to the collector. The bipolar transistor is mostly used as a current-controlled device, and it functions as a current amplifier. When germanium is used to dope the base region o f a silicon bipolar transistor, which i n other respects  is quite conventional, the result is a silicon-germanium  heterojunction bipolar transistor (HJBT).  This technology has the advantage o f being  compatible with normal silicon bipolar processing but offers an improvement in speed over the "silicon-only" circuits.  Steady progress has been made in the development o f  SiGe heteroj unction technology and H J B T is set to enter the marketplace. ' ' ' ' . 8  9  10  11  12  Most o f the applications o f the bipolar transistor have been in situations where extremely fast responses are needed, such as in microwave circuits. Fast logic chips also employ the bipolar transistor where speed is also a great advantage . 13  13  1.2.2 MOSFET Metal-oxide-semiconductor field-effect  transistors  (MOSFET)  are the  most  important devices for very-large-scale integration (VLSI) circuits such as microprocessors and computer memories.  They are unipolar devices, in which only one type o f carrier  predominantly participates i n the conduction process . 14  A s show n-type M O S F E T in  Figure 1.5, a M O S F E T consists o f three parts: a body o f semiconductor material (n-type in this case), an insulating film (i.e. oxide), and a conductive layer (labeled "Gate"). The transistor is a three-terminal device; the two contacts at the ends are called the source and the drain. The contact i n the middle, called the "Gate", is insulated from the other two by the oxide.  Source n-type  Drain p-substrate  n-type  Figure 1.5 The physical structure o f an n M O S F E T .  14  The M O S F E T is basically a voltage-controlled resistor.  In the n M O S F E T (n-  channel on p-type substrate) depicted i n Figure 1.5, the basic idea involved is that the application o f a positive voltage to the gate attracts electrons to the underside of the oxide, thereby creating a conducting channel from the source to the drain (see Figure 1.5). Current is driven through the channel by the application o f a positive voltage to the drain. In the absence o f a positive voltage on the gate, almost no current flows.  A t a certain  positive voltage on the gate, called the threshold voltage, the electron channel forms and current flows from source to drain, increasing with increasing gate voltage. M a n y effects make the analysis and the operation o f the M O S F E T more complex than suggested.  Electrical charges can be trapped in the oxide and at the interface  between the oxide and the silicon. Trapped charges also attract or repel electrons in the channel and change the threshold voltage. Indeed, it is only for the Si/Si02 structure that such trap densities are low enough to permit effective gate control.  The practical  application o f M O S F E T s was retarded by the time needed to develop methods for producing interfaces with sufficiently low densities of surface states . This is especially 7  true in the case o f SiGe M O S F E T s . The possibility o f high mobilities in SiGe structures points to F E T applications. Incorporation o f a buried SiGe layer into a M O S structure could lead to improvements in M O S circuitry speed. SiGe channel F E T s have better performances than versions based on silicon alone. The SiGe M O S F E T appears to have excellent prospects for reaching the market, but considerable work is needed on device physics, design and processing techniques ' ' . 12  15  16  15  A comparison o f the structures in Figures 1.4 and 1.5 reveals the relative simplicity o f the field-effect  transistor in planar technology.  This has  permitted  M O S F E T s to be fabricated with lower cost, higher densities, and higher levels o f integration than bipolar transistors. M O S F E T s are usually operated at lower current and power levels than bipolar circuits, as desired for high levels o f integration.  1.3 Process Technology 1.3.1 Fabrication of a Bipolar Transistor Microelectronics  fabrication  is  the  sequential  application  of  processing  techniques. Bipolar integrated circuits were the first to come into production and remain important.  The most common type o f bipolar junction transistor is the npn, which takes  advantage o f the higher mobility for electrons compared with holes. A simple example of an npn transistor fabrication process is shown in Figure 1.6. Beginning with a p-type Si substrate, an n-type epitaxial layer is grown on top followed by the thermal oxidation o f a thin layer on the surface (Figure photolithography.  1.6a).  A window in the oxide is opened  by  The sample is then placed in a furnace for boron doping, forming the  p-type base region (Figure 1.6b). After re-oxidation, a new window is opened in the oxide for phosphorus doping, forming the n emitter region (Figure 1.6c). After one more +  re-oxidation, windows are opened to the top surface o f the n , p and n regions. +  A l u m i n u m is evaporated onto the wafer.  The final metallization pattern is defined by  photolithography, and the unwanted A l is etched away (Figure 1.6d).  16  -Si0  2  ; n  _  e  P  • •  1  p-Si  bpitaxy Oxidation  , . (a)  W i n d o w opened i n oxide Boron diffusion  n-epi p-Si  n-epi  (b)  ^ 7  p-Si  • • •  Oxidation W i n d o w opened Phosphorus diffusion  • •  Oxidation Window opened for base, emitter and collector Metallization Metal removed except at contact regions  (c)  metal  • •  n-epi p-Si  (d)  Figure 1.6 A n npn transistor fabrication.  17  1.3.2 Fabrication of a MOSFET Figure 1.7 illustrates a simple n-channel M O S F E T fabrication technique . In this particular model the gate is made o f heavily doped polysilicon.  Insulation (Figure 1.7a) A thin initial oxidation is carried out on a P-type Si substrate, followed by a C V D silicon nitride deposition process.  Using a field mask, the silicon  nitride layer is etched in such a way that only isolation areas are exposed. The field oxide is then grown i n these isolation areas. Following oxidation, the silicon nitride layer is stripped off.  Gate fabrication (Figure 1.7b)  The layer of silicon oxide is partially etched away to  grow a thin "gate oxide". The thin gate oxide should be defect-free, o f high-quality and with a minimum o f dangling bonds at the Si/Si02 interface.  Polysilicon, doped with  phosphorus, is deposited over the oxide to serve as a gate material. It is then etched with the pattern o f the "gate mask".  Source/Drain fabrication (Figure 1.7c) Ion-implantation is done directly on the wafer after the gate is formed, using the gate and the field oxide as implant masks. This "selfaligning" characteristic eliminates an additional mask. Contact (Figure 1.7d) Phosphosilicate glass is deposited using a C V D process.  Contact  openings are etched through the glass and oxide at points determined by the contact mask. Finally, a film o f metal, usually A l , is deposited onto the wafer. The interconnect pattern is printed on the wafer through a metallization mask using a photoresist and the pattern is then etched in the aluminum.  18  F i e l d oxide  f  —  p-type substrate  F i e l d oxide  Poly-Si  (a) Isolation  Gate oxide  p-type substrate  Initial oxidation Silicon nitride deposition Selective silicon nitride etch defined by field mask Selective field oxidation Nitride removal  Oxide etch and gate oxidation Polysilicon deposition, doped with phosphorus Selective polysilicon etch defined by gate mask  (b) Gate  Source-drain implantation with phosphorus ions (self-aligned gate) Source n  +  Drain n  (c) Source Drain  PSG  (d) Contact  Phosphosilicate glass (PSG) deposition Selective P S G etch and gate oxide etch defined by a contact mask A l u m i n u m deposition Metal removal defined by a metalization mask  Figure 1.7 A Simplified n M O S F E T fabrication.  19  1.3.3  Oxidation, Plasma vs. Thermal Oxidation Oxidation plays an important role in integrated circuit fabrication process o f both  bipolar and M O S F E T devices. Producing functioning ICs requires the ability to form a good oxide in a controlled and repeatable manner.  Si02 has several uses: to serve as a  mask against implantation or diffusion o f dopant into silicon, to provide  surface  passivation, to isolate one device from another, to act as a component in M O S F E T structures and to provide electrical isolation o f multilevel metallization systems. Several techniques for forming the oxide layers have been developed, such as thermal oxidation, plasma oxidation, wet anodization, and chemical vapor deposition. Thermal oxidation is the principal technique used in IC processing.  In thermal  oxidation, oxide growth is carried out in a quartz diffusion tube, in which the silicon wafer is maintained at a temperature between 900°C and 1200°C.  Wet oxidation is  usually accomplished by flowing a carrier gas through a water bubbler whose temperature is maintained below the boiling point to fix its partial pressure. The carrier gas may be either oxygen or an inert gas (nitrogen or argon), since the oxidation is almost entirely due to the water vapor.  Wet oxidation is more rapid than dry oxidation, but results in  relatively porous poor grades o f silica films because o f the etching and pitting action of the water. These oxides are used for most general-purpose applications such as surface coverage and diffusion masking.  Oxides grown in dry oxygen, i.e. dry oxidation, are  extremely dense, and have a relatively low concentration o f traps and interface states. Consequently gate oxides for M O S F E T ' s are exclusively made by this process, with elaborate precautions taken to ensure a clean, sodium-free system.  It is also extremely  important that the oxygen be truly dry, since as little as 25 ppm o f water w i l l significantly  20  alter the growth rates, as well as the subsequent properties o f the o x i d e . 17  atmospheric-pressure  and high-pressure  oxidation techniques are used  Both  i n thermal  oxidation. There have been several studies o f the growth kinetics and mechanism o f the thermal oxidation o f S i G e ' . 1 8  1 9  Recently, it was reported that the presence o f Ge at the  S i C V S i interface increased the rate o f wet oxidation by a factor o f about 3 i n the linear 20 24 26  regime  ' ' . This growth enhancement is thought to be related to the catalytic action o f  Ge present at the interface and the weak Si-Ge binding energy. However, the rate o f dry oxidation wasn't affected ' . 20 25  Thermal oxidation o f SiGe alloys resulted i n the formation  of a pure S1O2 layer on top o f an enriched SiGe alloy. The formation o f this enriched layer resulted from the Ge being rejected during oxidation and thus piling up at the SiGe/oxide interface. 20,21,22,23,24,25,26,27  The Ge was found to be almost completely i n its elemental form Ge-rich interfacial layers resulted i n a high oxide charge and  j^[  s  interface state densities ' . 24  26  However, the amount o f Ge segregation has been shown to  decrease at lower oxidation temperatures and at higher oxygen pressures indicating that the segregation is diffusion l i m i t e d . In addition to this Ge segregation problem, the 21  thermal oxidation at temperatures around 1000°C can also cause problems such as the generation  o f dislocations, stacking faults and impurity redistribution.  This high  temperature process is particularly critical for SiGe strained layers as it causes relaxation of the atoms i n the lattice into lower energy states that contain additional dislocations at the interface.  Conventional thermal oxidation is therefore not very useful for the SiGe  system.  21  Finding a suitable gate dielectric for the SiGe M O S F E T s has been a problem. Attempts have also been made to grow different gate dielectrics on SiGe strained layers. However, it has not been possible to grow or deposit a good-quality oxide or other dielectric with acceptable properties directly on SiGe strained layers. The difficulty can be avoided by growing a Si capping layer (serving as a sacrificial layer) on the active SiGe strained layer and oxidizing this cap to S i 0 2 ' . 28  29  In view o f the problems caused by high temperature processing, there is a need to decrease the oxidation temperature.  Plasma oxidation is a low-temperature  process,  usually carried out in a pure oxygen discharge. Plasma oxidation can be accomplished by using plasma excited oxygen, most likely atomic oxygen, created by an electrical discharge, as the oxidizing species. This is usually carried out in a low-pressure system, where a plasma can be sustained by direct-current, microwave, or radio excitation.  frequency  A large number o f potentially reactive oxygen ions and free radicals are  formed during this process. The potential advantage o f plasma oxidation methods lies i n the fact that oxidation can be conducted at low temperatures.  This technique allows oxides to be  grown at temperatures below 500°C, so that it is attractive in V L S I fabrication where dopant diffusion must be kept at a minimum during the oxide growth step. The oxide films formed i n such processes have been found to have properties equivalent to those obtained by thermal oxidation and can be used effectively as an insulating layer for ICs ' ' . 3 0  3 1  3 2  Some success has been achieved by low temperature oxidation o f SiGe with plasmas.  L o w temperature (25-500°C) electron cyclotron resonance ( E C R ) microwave  22  plasma oxidation  o f Sii_ Ge (x=0.2 and 0.5), microwave plasma oxidation x  x  with x=0.09, 0.18, and 0.26 and radio frequency (RF) plasma oxidation  on Sii_ Ge x  x  on SiGe with  10% Ge content have produced fully oxidized Si and Ge without any Ge-rich SiGe layer. -IA  Mukhopadhyay et.al.  -i/r  *in  ' '  studied the effect o f a microwave oxygen plasma on  SiGe (x=0.09, 0.18, 0.26) at temperatures ranging from 150 to 200°C and compared the results with thermally oxidized samples. without any Ge segregation.  Ultrathin SiGe oxides were found to grow  The plasma grown oxides were found to have lower  interface trap densities than thermally grown oxides.  Furthermore an optimized post-  oxidation and post-metal annealing cycle after plasma oxidation resulted i n even lower interface trap densities.  A controlled, in situ, hydrogen-plasma treatment o f the SiGe  prior to oxide growth has been found to be useful in improving the electrical properties of the oxide .  1.4 Oxide/Semiconductor Interface Properties As  technology moves toward thinner gate oxides the  oxide/semiconductor interface becomes ever greater.  importance of the  Associated with the oxidized  silicon system are several types o f electrically charged centers which can adversely affect the device performance, reliability, and yield.  One o f these species is the interface  trapped charge (Qj ), the charge per unit area stored in traps. The interface trapped charge t  is located at the Si-Si02 interface, with either a positive or negative charge. Historically, these states are called fast surface states.  The origin o f these states is thought to be  primarily the disruption of the periodicity of the lattice at the Si-Si02 interface . O n clean  23  Si surfaces i n ultrahigh vacuum, Q j  t  is very h i g h  17  (about 1 0  15  cm" ), i.e. comparable to 2  the density o f atoms at the surface. For well-prepared SiC>2 on S i , most o f the interface trapped charge can be neutralized by low-temperature (450°C) hydrogen annealing. The value o f Q j t can be reduced to as small as 1 0  10  cm" . A t this level, the interface trapped 2  charge has a negligible effect on device performance . 3  Surface states are also frequently referred to as interface traps, since they effectively trap free carriers at the Si-Si02 interface. They are electrically active defects and  can degrade  the  electrical properties  o f the  MOSFET  device.  Degraded  characteristics include changes i n threshold voltage, reduced inversion layer mobility, increased minority carrier generation, and increased noise.  Electron spin resonance  (ESR) experiments have demonstrated that these interfacial defects have one dangling bond, and are called Pb centers when they occur at the S i 0 / S i (111) interface ' . 2  Carriers in the silicon semiconductor interact with these defects, leading to degraded electrical characteristics.  It was found that molecular hydrogen can passivate these  dangling bonds at about 450°C where the following reaction o c c u r s ' , 40  H +P 2  41  • P H+H  b  b  (1.8)  where PbH is the hydrogen-passivated silicon dangling bond. This is a first order kinetic process  42  with an activation energy of 1.66 e V . The dissociation reaction,  P H  •  b  was also postulated  43  P +H b  (1.9)  to happen under vacuum thermal annealing with an activation  energy o f 2.56 e V .  24  It has also been suggested that atomic hydrogen produces interface defects via the reaction:  P H +H b  • H +P 2  (1.10)  b  which is the reverse passivation reaction of (1.8), while the reaction (1.11) (which is the reverse o f (1.9)) removes interfacial defects.  P +H b  • P H  (1.11)  b  Both reaction (1.10) and (1.11) are exothermic chemical reactions, probably requiring very little thermal activations. O n the (100) face o f silicon, two related defects called Pbo and P i are observed. b  Pbi behaves i n a manner analogous to the Pb center on (111), but the Pbo center behaves entirely differently. The Pbo center is present in the "as-grown" oxide, and it cannot be passivated by the same thermal treatment that passivates Pb(l 11) and Pbi(100) centers. A continuous slow growth o f the Pbo center is observed during the thermal cycle. It is likely that the relevant ingredient in the annealing o f Pbi(100) and Pb(l 11) is water vapor. The different behavior o f Pbo vs the other Pb center may be a matter o f kinetics rather than energetic. For example, the Pbo center may lie deeper in the silicon, two or three atomic layers away from the interface. In this position it would be less susceptible to passivation by water vapor, since water w i l l diffuse into the silicon less readily than through S i 0  4 4 2  .  The density o f interface traps (Dj ) can be reduced by hydrogen annealing at low t  temperatures (< 500 ° Q 4 1 ' 4 2 ' 4 5 ' 4 6 . A very detailed study on the chemistry o f the S i - S i 0 interfacial trap annealing was carried out by M . L . Reed, et al. . 47  25  2  In view o f this annealing behavior of SiC>2, annealing o f SiGe oxide was also studied i n an attempt to improve the interface properties. It was found by D . Tchikatilow,  48 et. al.  that l o w temperature H 2 O vapor annealing o f the SiGe oxide (oxidized from  Sio.85Geo.15) decreases the interface state density. It is proposed that changes, which occur as a result o f annealing in forming gas and H 2 O vapor, result from the ability o f hydrogen species to react with unsaturated silicon atoms at the oxide/semiconductor interface forming S i - H bonds.  These workers considered that atomic hydrogen diffuses to the  interface and reacts with unsaturated silicon to eliminate interfacial states. The atomic hydrogen diffusion constant value is D ~ 2x10" length is therefore l=2(Dt)  1/2  7  cm /s at 300°C for S i C V 2  The diffusion  ~ 1CJ cm (at 300°C, t=ls), which is three orders of 3  magnitude larger than the oxide thickness. The post-oxidation and post-metal annealing cycle was also found to result in a low interface trap density . 37  1.5 Objectives of Present Research It is essential to use low temperature processing techniques in fabricating SiGe M O S F E T devices i n order to ensure the structural integrity o f the strained SiGe layer is maintained. Conventional gate oxide growth is a major burden in this respect.  An  alternative strategy is to use plasma enhanced oxide growth which can involve lower temperatures. The objective o f this work is to grow a thin gate oxide on SiGe with a minimum density o f interfacial carrier traps. The approaches we have used are: (1) Obtain 4 types o f samples (i) SiGeo.025 epitaxial layer capped with an intrinsic S i .  26  (ii) SiGeo.3 epitaxial layer grown on a Si wafer. (iii) intrinsic Si epitaxial layer on a Si wafer. (iv) SiGeo.025 epitaxial layer grown on a S i wafer. A S i capped SiGe is an alternative substrate for growing high-quality SiGe oxide. Our  initial work, therefore, was on this sample.  The oxidation o f uncapped SiGe  samples, with low and high G e % , was also considered worth studying to see i f the Si cap is really necessary.  (2) Oxidize these samples in a remote O2 plasma system. L o w temperature is preferred in SiGe oxidation. The oxidation system used in this work is a remote plasma system to take advantage o f its low temperatures which can be as low as 25°C.  (3) Expose surfaces to H 2 and/or H-atoms as a post-oxidation treatment. H 2 annealing has been shown to reduce the interface trap density o f S i / S i 0 at ~ 2  450°C. It has been suggested that this effect involves atomic hydrogen. The question that arises is what w i l l happen in the case o f the SiGe oxide during H 2 annealing and H-atoms exposure?  W i l l these processes also improve the SiGe oxide?  These measurements  should provide answers to these questions.  (4) Monitor the carrier traps with a remote RF-probe during the oxidation and subsequent hydrogen treatment. Attempt a kinetic analysis o f these processes.  27  It is noted that the loss o f photo-generated charge carriers occurs principally on dangling bonds (carrier traps) at the surface.  The steady-state carrier concentration,  therefore, can be used as a technique for following the formation and loss o f dangling bonds.  Since Si and Ge are  indirect band  gap  semiconductors, the  radiative  recombination o f carriers, i.e. photoluminescence, cannot be used for monitoring their concentrations. A novel RF-probe was developed in this lab. W i t h this unique probe, we can continuously monitor the carriers and thus monitor any changes in the carrier trap density.  (5) Characterize the oxide surface and interface structure with X P S . X-ray photoelectron spectroscopy ( X P S ) is a powerful surface analysis technique. The thin SiGe oxide grown by plasma oxidation can be characterized with X P S .  With  this technique, the nature and thicknesses of the oxide can be determined.  28  Chapter 2  Experimental  2.1 Materials 2.1.1 Silicon and Silicon Germanium Samples Four kinds o f semiconductor samples were used in this work. A l l of them were grown i n a "Sirius" C V D reactor at the Institute for M i c r o structural Sciences ( N R C , Ottawa). The "Sirius" C V D hot wall reactor from Leybold A G with a quartz reactor was operated at 525°C and a base pressure o f 10" Torr. Ultra-Large-Scale-Integration grade 9  silane and germane from Matheson were used as precursors at a deposition pressure of ~ 10" Torr. Details o f the growth conditions can be found in the reference . 3  49  Sample 1. SiGe, Si-capped: 12.5 A intrinsic S i (i-Si) on top o f 1000 A  SiGeo.025-  P-type,  boron-doped float-zone S i substrate with a resistivity o f 30-60 Q c m .  i-Si, 12.5 A  SiGe,'0.025  IOOO A  Si Wafer  30-60 Qcm  29  Sample 2. SiGe, uncapped, high Ge%: 70 A  SiGeo.3 on top o f 150 A intrinsic S i .  70 A  SiGe,ILL i^Si  150A  Si Wafer  Sample 3. Intrinsic Si: 1200 A intrinsic S i grown on Czochralski (CZ) Si(100) P-type, boron-doped substrate with a resistivity o f 13-18 Q c m .  1200 A  13 -18 Qcm  Sample 4. SiGe, uncapped, low Ge%: 1050 A SiGe .o25 on top o f a 150 A intrinsic S i 0  buffer. P-type, boron-doped float-zone S i substrate with a resistivity o f 30-60 Qcm.  1050 A  SiGeo. 25 0  ,«feSfcv..'. Si Wafer  ; "  150 A 30-60 Q c m  30  2.1.2  Chemicals The oxygen, hydrogen and argon used are all ultra high purity standard from  Praxair.  The quoted purity o f oxygen is 99.993%, hydrogen 99.999% and argon  99.999%. ppm.  The maximum moisture in oxygen, hydrogen and argon are all less than 3  Nitrogen dioxide was supplied by Matheson with a quoted purity o f 99.5% in  liquid phase.  The hydrofluoric acid used was 48% H F , A . C . S . reagent grade from Aldrich with an assay o f 48.0-51.0%. De-ionized water with a resistivity o f 18 M Q c m was supplied by the Advanced Materials and Process Engineering Laboratory ( A M P E L ) at the University of British Columbia.  2.2 Apparatus A diagram o f the reactor and associated equipment is shown i n Figure 2.1.  It  mainly consists o f a remote microwave plasma, a heated reaction chamber, an RP-probe to monitor the substrate during the reaction, and a mass spectrometer chamber to monitor the gaseous species.  31  He-Ne laser Light chopper Microwave cavity  Thermocouple  Mass spectrometer  Mechanical pump  Molecular drag pump  2.2.1 Reaction Chamber The reactor was constructed mostly o f Pyrex.  Only the discharge region was  made o f quartz which can withstand the high plasma temperature.  The quartz discharge  region had a 10.5 m m inner diameter and was 100 m m in length. The tube in the reaction region had an inner diameter o f 24 m m and was 160 m m long.  T w o light traps were  added between the discharge region and the reaction region to prevent the radiation from the plasma from impinging on the sample. The plasma was created by a quarter wave cavity attached to an E . M . S . Microtron 200 microwave power generator (incident power 0-200 W ) . Compressed air around the cavity was used to cool down the plasma region when the plasma was on. The sample was placed flat on the bottom o f the Pyrex reactor tube.  A 6 mm  diameter Pyrex sample holder was bent to touch and hold the sample. A heating wire was ' wrapped around the sample region to heat the sample when necessary.  A Variac  transformer, connected to the heating wire, was used to control the temperature o f the sample by adjusting the current "flowing through the heating wire.  A 1 m m diameter  flexible temperature probe (Cole-Parmer H-08514-96, K-type) was inserted into the sample holder for measuring the temperature, which was displayed on a digital readout (Omega, model 115 K C ) . The sample holder was connected to the loading cap by an Oring. A Cajon fitting was used to attach the cap to the reaction chamber. Gases were introduced into the system through % inch copper tubing.  The  pressure o f gases was set by flow control valves and monitored by a capacitance manometer (Edwards Barocel Series 600) with a pressure range o f 10" -10 Torr. A cold  33  cathode pressure gauge (HPS M o d e l 421) was used to measure the pressure in the range of 10" - 10" Torr. In the case o f l o w pressure (10" - 10" Torr) operation, the molecular 2  10  6  1  drag pump (Alcatel M D P 5010, 7.5 liters/second nitrogen o f pumping speed) backed by a rotary pump (Sargent Welch M o d e l N o . 1375, 300 liters/minute o f pumping speed) was used. When a pressure higher than 100 mTorr was needed, the molecular drag pump was closed, leaving only the rotary pump running. Periodic maintenance was necessary for the reaction chamber in order to maintain the high vacuum, to obtain high concentration o f atomic species and keep the experiments repeatable.  For this purpose, the reaction chamber could be disconnected from the  system. The Pyrex and quartz parts were cleaned with dilute H F solution, rinsed with DI water and dried with air. Gaskets and O-rings were checked periodically and replaced when necessary.  2.2.2 RF-Probe A n RP-probe, shown in Figure 2.2, provides a "contactless" technique for monitoring the steady-state carrier concentration o f the sample. The probe was pressed against the outside wall o f the reactor where the sample was placed to maximize the sensitivity o f the probe to the changes in the conductivity o f the sample which is determined by the steady-state carrier concentration.  34  He-Ne laser Light chopper  Sample RF generator  Coupler  RF-probe  Splitter  -±  x.  M ixer  Lock-in amplifier  Computer  Figure 2.2 A n RF-probe and RF-circuit.  The operation o f the RF-probe is described in detail by L i and O g r y z l o ' . The 50  51  RF-probe consists o f a helical resonator made o f a copper coil, which is inductively coupled to the sample. The RF-circuit contains a splitter, coupler and mixer connected as shown i n Figure 2.2.  A n R F signal from an R F generator (HP model 3200B V H F  Oscillator, frequency range 10 to 500 M H z , R F output > 150 m W in band 130 to 260 M H z ) passed into a splitter ( M i n i Circuits ZFSC-2-1). The splitter produces two signals. One goes to a coupler ( M i n i circuits ZFDC-10-2), and then into the C u coil from which it is reflected with an altered magnitude and phase into the mixer ( M i n i circuits Z F M - 2 H ) . The other part o f the signal from the splitter goes directly into the same mixer to act as a reference.  The phase shift and magnitude o f the reflected signal are affected by the  conductivity o f the sample.  The signal from the mixer is then detected by a lock-in  amplifier ( E G & G Princeton Applied Research model 5102). A 10 m W , 633 nm He-Ne laser beam, chopped at a reference frequency by a chopping unit (Grubb Parsons), hits directly on the sample. Free electron and hole pairs are formed since the photon energy is larger than the band gap o f the substrate.  The  change in sample conductivity due to the injected electron and hole concentrations is proportional to the photo-generated  minority carrier concentration.  The chopping  frequency o f 200 H z was chosen so that it was (a) rapid enough to measure exposure times o f less than 1 s, but (b) at least two orders of magnitude slower than the longest carrier lifetime.  The lock-in amplifier measures only the carriers generated by the  chopped laser beam at 200 H z . The output o f the lock-in amplifier was connected to both a D C voltage meter and a computer so that the data could be collected for further analysis.  36  B y varying the laser intensity we have determined that, at least for our samples, the output from the lock-in amplifier is proportional to the change in carrier density. Under constant laser intensity conditions the steady state carrier concentration is a direct measure o f the surface or interface defect density, since carrier losses at the interfaces are several orders o f magnitude greater than in the bulk o f the semiconductor. Consequently under all operating conditions used i n this work the probe output was a direct measure o f the steady state carrier concentration and the reciprocal of this quantity was proportional to the interfacial trap density.  A l l the data collected were  plotted and fitted with a fitting program called Microcal Origin on a Pentium P C .  2.2.3 Mass Spectrometer The mass spectrometer vacuum chamber, shown schematically i n Figure 2.3, was constructed o f stainless steel. The system could be pumped down to 10" Torr with a turbomolecular pump 7  (Edwards E X T 7 0 , pumping speed o f 52 liters/second nitrogen) backed up by a rotary pump (Sargent-Welch M o d e l N o . 1402, pumping speed o f 160 liters/minute).  A gate  valve connected the reaction chamber to the mass spectrometer chamber. A "differential gasket" was employed between the gate valve and flange on the side o f the reaction chamber with a 100 um pin hole in it to leak a small amount of gases from the reaction chamber into the mass spectrometer chamber.  37  Pressure gauge  Sensor  Chamber —r L  Turbo pump Computer Mechanical pump  Figure 2.3 Mass Spectrometer.  38  The mass spectrometer was a quadruple mass analyzer ( M K S partial pressure transducer (PPT) residual gas analyzer) with a mass range o f 1 to 200 atomic mass unit. It consists o f a compact ion-source quadrupole sensor, an electronic control unit ( E C U ) , and interactive P P T software connected to a P C computer to control and monitor the mass spectrometry.  The maximum operating pressure o f the sensor is l x l 0 " Torr. 4  A  total system pressure reading was continuously available in the range o f l x l O " to 2 x l 0 " 4  9  Torr. The pressure o f the M S chamber was also measured with an ionization vacuum gauge (Leybold-Heraeus Combitron C M 330).  2.3 Experiment Procedure 2.3.1 Sample Cleaning A 20x10 m m sample was cut from the wafer supplied by N R C .  It was then  cleaned in 2% H F for 60 s to remove the native oxide, followed by rinsing in DI water and drying with N . 2  The HF-treated S i surface is less reactive and more stable against  oxidation in room air since the surface is passivated by H-termination o f silicon dangling bonds, forming S i - H bonds and protecting the surface from chemical a t t a c k ' ' . 52  53  54  2.3.2 Generation of O-atoms and Plasma Oxidation Generation Oxygen atoms were generated in microwave plasma located 20 cm upstream from the reaction chamber.  500 mTorr o f 0  2  with a flow o f ~ 100 seem was selected to  maximize the concentration o f O-atoms and minimize their recombination in the  39  connecting tubing. The microwave generator produced a maximum output o f 100 W . A titration experiment, using reactions (2.1) and (2.2) , 55  N0  2  + O -> N O + 0  0 + N O -> N 0 * 2  (2.1)  2  N 0 + hv 2  (2.2)  performed by H . L i in our group, indicating that about 4% o f the O2 was dissociated at the sample position, meaning that the atomic oxygen concentration is about 8%.  Oxidation The substrate, after an H F wash, was put into the flow reactor system and fixed with a sample holder. The system was then closed and pumped down to vacuum. The system was purged with O2 for 30 m i n before igniting the plasma to stabilize the flow and flush out any other gases. During this purging with O2, the system was heated to the desired temperature by the heating wire. The reactor walls, gas, and the substrate were all at the same temperature.  The discharge was ignited with a Tesla coil. Then the sample  was exposed to atomic oxygen from an upstream microwave discharge located 20 cm away from the substrate and separated by two light traps to eliminate all U V radiation, electrons and ions from the stream before it impinged on the substrate.  After the  oxidation was complete, the discharge was shut off by turning off the microwave generator power.  40  2.3.3 H-atoms Generation and Its Exposure to Samples Generation The same microwave discharge method was used to produce H-atoms. 5 mTorr of H diluted i n 30 mTorr o f A r was passed through the microwave plasma which was 2  operated at 40 W . The atomic hydrogen concentration was deliberately kept low to avoid heating o f the sample by recombination o f H-atoms on the surface.  Exposure of H-atoms The H-atoms exposure experiments were carried out in a manner similar to that used for the oxidation experiments discussed in section 2.3.2.  After the sample was  loaded into the reactor, the system was closed and evacuated below 10" Torr. 5  The  desired H / A r flow was then introduced into the system and the sample was heated up to 2  the desired temperature.  When the system was stable, the microwave plasma was then  ignited to produce H-atoms. The sample now was exposed to atomic hydrogen.  After  this process was finished, the H-atoms were terminated by turning off the plasma.  41  2.4 X-ray Photoelectron Spectroscopy (XPS) 2.4.1 Fundamental Principles X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis ( E S C A ) , is a widely accepted and powerful technique for studying surfaces and interfaces to determine the chemical and elemental properties. In X P S analysis, a monochromatic X-ray strikes at a sample with an energy hv > 1 keV.  A s a result o f the absorption o f an X-ray photon, core-level photoelectrons are  emitted from the sample with kinetic energies characteristic o f the target elemental composition and its chemical state. This energy o f the emitted photoelectrons is analyzed by the electron spectrometer. The data are presented as a graph o f intensity (or counts per second) versus electron energy - the X-ray induced photoelectron spectrum.  56  The kinetic energy (Ek) o f the electron is the experimental quantity measured by the spectrometer, but this is dependent on the energy o f the X-ray source employed and is therefore not an intrinsic material property. The binding energy o f the electron (ER) is the parameter which identifies the electron specifically, both in terms o f its parent element and atomic energy level. experiments is as follows:  The relationship between the parameters involved in X P S 57  Ek = hv - E R - W  (2.3)  where Ek is the measured kinetic energy o f the ejected electron, hv is the X-ray photon energy, ER is the binding energy o f the ejected electron, and W is the spectrometer work function.  42  Figure 2.4 illustrates the process o f photoemission . A n electron from the K shell is ejected from the atom.  The photoelectron spectrum w i l l reproduce the electronic  structure o f an element quite accurately as all electrons with a binding energy less than the photon energy w i l l feature in the spectrum. Those electrons which are excited and escape without energy loss contribute to the characteristic peaks in the spectrum. Each photoelectron has a discrete energy representative o f the element from which it was emitted, thus allowing one to identify the atomic species present, chemical state o f the atoms and the elemental composition in a given sample. Although the X-ray photons can penetrate 1-10 um deep into the sample, the electrons generated at that depth simply cannot make it out into the vacuum to be detected.  Relatively low kinetic energy electrons (50 to 2000 e V ) , the type generally  excited by X P S , have inelastic mean free paths (IMFP or X ) which range from about 0.5 to 3 nm. X is defined  59  as the distance that an electron w i l l travel before it suffers an  inelastic collision with the nucleus of an atom in a solid. These collisions w i l l change the direction and the kinetic energy o f electrons, decreasing the peak intensity. This process follows a standard exponential decay (2.4), 56  I (d) = I exp (-d/ X)  (2.4)  0  where 1(d) is the number o f electrons that are ejected without having undergone inelastic collisions from a depth, d, in the sample. I is the number o f electrons emitted from an 0  infinitely thick substrate.  Only 37%, i.e. 1/e, of the electrons that are ejected from an  atom at IX, w i l l reach the surface with their characteristic kinetic energy.  Therefore,  relatively few electrons from depths o f 2X to 3X w i l l leave the bulk with their initial  43  Photoelectron  Figure 2.4 Schematic o f the X P S process.  kinetic energy. Thus, they w i l l not be detected. This accounts for the surface sensitivity of X P S .  X P S can therefore provide a total elemental analysis o f about the first 30 A o f  any solid surface which is vacuum stable . 60  2.4.2 Instrumentation A n X-ray photoelectron spectrometer, schematically shown in Figure 2.5, consists of an X-ray source, a sample support system, an electron energy analyzer and a detection system, all contained within a vacuum chamber.  A data-system is used to convert the  detected current into a readable spectrum. The spectrometer is based on a vacuum system designed to operate i n the ultrahigh vacuum range o f 10" to 10" Torr. 8  10  There are two reasons for this: (1) the l o w  energy electrons are easily scattered by the residual gas molecules and unless their concentrations are kept at an acceptable level the total spectral intensity w i l l decrease whilst the noise present within the spectrum w i l l increase, and more importantly, (2) the high surface sensitivity of the technique requires careful control o f surface composition, which ambient gases can change. X-rays are generated by bombarding the anode materials with electrons. A n ideal X-ray source must be sufficiently energetic to access core levels, intense enough to produce a detectable electron flux, have a narrow line width and be simple to use and maintain. The most common anode materials used are A l and M g which provide A l K a and M g K a photons o f energy 1486.6 e V and 1253.6 e V , respectively.  45  High vacuum system  Filament  Hemispherical electron analyzer  Figure 2.5 X-ray photoelectron spectroscopy.  46  Sample for analysis is first mounted on a sample holder and brought into the forechamber.  It is then transferred into the analysis chamber.  Once the sample is in the  analysis chamber, it needs to be positioned accurately. Angle dependent measurements can be conducted by rotating the sample around the axis to vary the take-off angle of electrons accepted into the analyzer. The kinetic energy o f the ejected electron is measured using a concentric hemispherical electron analyzer which gives the best energy resolution. A s shown in Figure 2.5, two hemispheres are placed concentrically. The incoming electrons, with a pass energy E , go in between these two hemispheres. A potential difference is applied between the inner and outer hemispheres so only electrons in a small energy range (E + AE) w i l l be transmitted to the other end o f the analyzer to the detector. A spectrum can be produced when the voltage between the inner and outer hemispheres is ramped. The analyzer basically measures the number o f electrons with different kinetic energies. The information is processed by a computer to produce a spectrum o f photoelectron intensity as a function o f binding energy.  2.4.3 Spectral Analysis Qualitative A n example o f an X P S spectrum obtained from a S i capped SiGeo.025 sample after a 10 m i n oxidation at 193°C is illustrated in Figure 2.6.  The spectrum is made on a  Physical Electronics Instrument. It is a S i 2p and Ge 2p spectrum with an A l K a source and a "pass energy" o f 50 e V . The spectrum includes several peaks. B y comparing  47  ioooo u  Binding Energy (eV) 78000  76000  Z- 74000  >< > -— * c 72000 (1)  70000 U  68000 U 1224  1222  1220  1218  1216  Binding Energy (eV)  Figure 2.6 X P S spectra of (a) Si 2p and (b) Ge 2p.  1214  binding energies with reference data , individual peaks can be identified and labeled on 61  the spectrum. X P S spectra can provide information on the chemical valence state of an atom from the "chemical shift" in binding energy when the binding changes.  This chemical  shift can arise in several ways including changes in the formal oxidation state and changes in the lattice site. In general it is due to the change in the environment o f an atom. In Figure 2.6a, by comparing X P S peaks from the Si atom i n a silicon crystal and in SiCh, a shift towards higher binding energy is observed for S i C V  The same kind o f  chemical shift was observed in the Ge 2p spectrum shown in Figure 2.6b. It is generally found that the core electron binding energy increases with increasing positive oxidation state. The full width at half-maximum ( F W H M ) is the peak width at half the signal height.  The measured F W H M , E , is a convolution o f contributions from the photon M  source Ep, the electron energy analyzer EA, and the natural line width o f the atomic level E . The sum o f the squares o f these factors is the square o f the measured F W H M . 5 8  N  AE  2 M  = AEp + A E 2  2 A  + AE  (2.5)  2 N  Quantitative Quantitative analysis is performed by determining the area under the peaks in question and applying a previously-determined sensitivity factor.  For a homogeneous  sample, the number o f photoelectrons per second, I (peak intensity), in a given peak, assuming constant photon flux and fixed geometry, is given b y  6 2  49  I = K N a A, A T  (2.6)  where K = constant, N = number o f atoms of the element per c m , a = photoionization 3  cross section for the element, X = inelastic mean-free path length for photoelectrons, A = area o f the  sample from  which the photoelectrons emanated  and T = analyzer  transmission function. If we define the sensitivity factor for element x as S = K C T A A T , x  then I = N S . x  X  X  Angle Dependent XPS The surface sensitivity o f X P S can be further enhanced by decreasing the take-off angle from its customary 90°. This effect is demonstrated in Figure 2.7.  If X is the  attenuation length o f the emerging electron then 95% o f the signal intensity is derived from a distance 3X within the solid. However, the vertical depth sampled is clearly given  d = 3  X sin a  (2.7)  and this is a maximum when oc= 90°. In the case o f a substrate (s) with a uniform thin overlayer (o) the angular variation o f intensities is given by: I  s  I  0  d  = I exp (-d/ s  X sin a)  (2.8)  and d  = I (1- exp (-d/ 0  X sin a))  (2.9)  50  2.4.4  Application One very important area o f X P S application is in the semiconductor industry.  64  X P S studies on S i , especially thin oxide/Si interfaces, have been widely carried out because o f the importance o f thin films i n electronic devices. oxidation in the clean-room a i r  65  The initial stage o f  at room temperature and in dry o x y g e n at 300 °C were 66  studied with X P S to determine the growing mechanism and uniformity o f such oxidation. Interfacial states in the Si band gap present at oxide/Si interfaces were investigated  67  by  measurements o f X-ray photoelectron spectra on "biased" samples (i.e. with applied voltages). This method was also employed to determine the effect o f various wet cleaning processes.  It was found that the density of interface states was affected by the interface  roughness . 68  help o f X P S .  K . B. Clark  69  et al. optimized the growth and annealing conditions with the  From a quantitative point o f view, the X P S methodology can provide a  precise determination o f the thickness o f thin film silicon o x i d e s ' 70  71  on S i and silicon  nitride on S i , etc. 72  Recently, X P S has been used to explore SiGe and its oxidation process.  By  identifying the various elements and their chemical shifts i n the oxidized SiGe, the structure o f the oxide can be clearly determined ' ' . 73  74  75  52  Chapter 3  Results and Discussions  3.1 Oxidation of Si Capped SiGe with O-atoms at Different Temperatures The S i capped SiGeo.025 sample was oxidized at three different temperatures: 25°C, 205°C and 440°C. A s shown schematically in Figure 3.1, 1000 A o f SiGe .o25 was 0  grown on a p-type Si substrate wafer, and then a 12.5 A intrinsic S i cap was grown on top.  12.5 A  i-Si  SiGeo.025  1000 A p-Si wafer  Figure 3.1 Sketch o f Si capped SiGeo.025 sample. During  this  oxidation  process,  the  laser-generated  steady-state  carrier  concentration was continuously monitored by the remote RF-probe. It was found that at any temperature i n this range, the carrier concentration drops to a very low level as soon as the surface is exposed to oxygen atoms.  This is illustrated in Figure 3.2.  Further  exposure to O-atoms does not affect the passivation level significantly. Since the probe can not make measurements at temperatures higher than 250°C, the data at 440°C was recorded by lowering the temperature to 25°C before measuring the steady-state carrier concentration with the probe.  53  300  400  Time (s)  Figure 3.2  Change o f carrier concentration for the Si capped SiGeo.025 sample during oxidation at 25°C, 205°C and 440°C.  54  3.2 XPS Analysis of Si Capped SiGe 3.2.1 XPS of Clean and Oxidized Samples Figure 3.3 shows the S i 2p and Ge 2p X P S spectra o f a S i capped SiGeo.025 sample after an H F wash (top half) and after a 50 m i n oxidation at 193°C (the lower spectra). Take-off angles are all 90° (perpendicular to the surface). In the S i 2p spectra, both the H F washed and the oxidized sample showed the S i peak at 99.5 e V due to the underlying 0  Si substrate. However, for the oxidized sample, a broad S i 3.9 e V above the S i peak and a small S i peak 0  +  76  4 +  peak, due to Si02 appeared  appeared ~1.0 e V above the S i . In the 0  Ge 2p spectra , both the H F washed and the oxidized sample show a Ge° peak at 1217.1 eV. In addition, in the case o f the oxidized sample, a G e the Ge° peak.  4 +  peak  33  appeared 3.8 e V above  These results suggest that after 50 m i n o f exposure to O-atoms the  oxidation has extended beyond the S i cap as shown in the next sketch. This implied that we should attempt a shorter oxidation time. We therefore exposed new samples to Oatoms for 10 m i n and also for 30 s. Very similar spectra were obtained for 10 min and 30s oxidation.  Oxidized S i cap partially oxidized SiGe  p-Si wafer  55  S2p HFwash  Ge2p Si°  HFwash  G2>  \  3  50 nin oxidation  50 rrin oxidation  Ge*  +  108  106  104  102  100  98  I.I.I,  96  1226  Binding energy (eV)  Figure 3.3  1224  1222  1220  1218  1216  1214 1212  Binding energy (eV)  S i 2p and Ge 2p spectra from Si capped SiGeo.025 sample after an H F wash and after a 50 m i n oxidation.  Peak intensities, obtained from an X P S peak fitting program, and the relative intensity ratios for peaks are given in Table 3.1 and Table 3.2.  R JO S  and R o o in Table 3.2 e  are relative intensity ratios o f S i / S i ° and G e / G e ° , respectively. x+  4+  56  Table 3.1  Peak intensities for the Si 2p and Ge 2p peaks as a function of plasma oxidation time (take-off angle = 90°)  N.  Sample with  Time 0s  30 s  10 min  10 min  50 m i n  native oxide  Intensity Si°  3.7xl0  Si (x=l&4)  <3.8xl0  Ge°  3.6xl0  Ge  < 1.6x10'  x +  4 +  2.55xl0  3  2  l.OxlO  3  2.35xl0  2  1.5xl0  3  2  2.2xl0  3  3.0xl0  3  2.3xl0  3  3.1xl0  3  7.1xl0  2  1.7xl0  3  1.2xl0  3  8.2xl0  2  1.2xl0  2  2.3xl0  2  6.4xl0  2  1.8xl0  2  4.5xl0  2  1.4xl0  2  2  6.6x10'  1.75xl0  2  Table 3.2 Intensity ratios as a function o f plasma oxidation time (take-off angle = 90°)  0s  Time  30 s  10 min  50 m i n  Ratio^\^^  native oxide  R ,o = S i / S i °  < l.OxlO"  x +  s  R eo = G e  4 +  G  RGeO / R-SiO  /Ge  Sample with  u  <4.4xl0" 4.3x10-'  1  2  4.0x10"'  4.5x10"'  5.1x10"'  2.65x10"'  6.4x10-'  6.6x10"'  6.9x10"'  7.9x10"'  1.6  1.5  1.35  3.0  57  Table 3.1 shows absolute peak intensities obtained after an H F wash (labeled 0 s), after 30 s, 10 min, and 50 m i n oxidation, and with the initial native oxide. The absolute peak intensities are not important, but the relative values are significant. The relative peak intensities, shown in Table 3.2, are the more meaningful data. They revealed that the longer the oxidation time, the higher the S i the S i . The same is true for the G e 0  4 +  4 +  intensity is relative to  /Ge° ratio. This indicates that more oxide was  grown. For the native oxide, Rsio lies in between those o f the H F washed sample and the 30 s oxidized sample.  But RGeo for samples with a native oxide is surprisingly large.  This could be due to some Ge contamination in the capping Si layer. Figure 3.4 shows a plot o f intensity ratios o f S i / S i ° and G e / G e ° for oxidation times o f 0 s, 30 s, 10 min x+  4+  and 50 min. It can be seen from this figure that there is a rapid initial increase o f the oxide layer which reaches saturation i n about 10 min.  3.2.2 Angle Dependent XPS Detailed angle dependent X P S spectra (Figure 3.5) were recorded at take-off angles o f 90° and 45° (relative to the surface). The Si 2p and Ge 2p bands are shown for the S i capped SiGeo.025 sample oxidized for 50 min. The relative intensity ratios of S i S i and G e 0  4 +  x +  /  / Ge° are larger at higher take-off angle, i.e. at the shallower sampling depth,  implying that the oxide region is near the surface as expected.  This behavior is also  clearly shown i n Table 3.3 and Table 3.4 for the fitted peak intensities and their relative ratios, respectively.  58  Figure 3.4 Plot o f relative intensity ratios o f S i / Si and Ge / Ge with oxidation time. x  59  Figure 3.5 Comparison o f S i 2p and Ge 2p spectra obtained at two take-off angles (90° and 45°) for a Si capped SiGe sample after 50 min oxidation, showing the effect  o f increasing the  take-off angle  on the  relative  oxide/element  intensities.  60  Table 3.3 Peak intensities from peak fitting o f Si 2p and Ge 2p as a function o f take-off angle (oxidation time = 50 min)  Angle Intensity  90°  45°  (a.u.j"-\^  Si  0  9.4x10  3  4.9xl0  3  Si  x +  4.8xl0  3  4.4xl0  3  Ge  u  6.4xl0  2  1.2xl0  2  Ge  4 +  4.5xl0  2  2.1xl0  2  Table 3.4 Intensity ratio as a function o f take-off angle (oxidation time = 50 min) Angle  90°  45°  R io = S i / S i °  5.1x10"'  9.0x10-'  Roeo = G e / G e °  6.9X10"  1.7  RGeO / RsiO  1.35  x+  S  4+  1  1.9  61  3.2.3 Determination of the Atomic Sensitivity for Ge 2p and Ge 3d In order to determine the relative atomic sensitivities o f the Ge 2p and Ge 3d bands, a pure Ge sample was analyzed by X P S . Figure 3.6 shows the X P S spectra for Ge 2p and Ge 3d from this pure Ge sample. Peak intensities obtained by integrating over the Ge 2p and Ge 3d peaks, and mean free paths (X) from C . J. Powell's computer calculations ("National Bureau of Standards", U.S.) are given in Table 3.5.  Table 3.5 Integrated Ge 2p and Ge 3d peak intensities for pure Ge and mean free paths Peak  Intensity (I) (a.u.)  MA)  Ge 2p  l.OxlO  6.9  Ge3d  1.25xl0  4  22.5  3  For pure Ge, the following expression holds .  I  Ge2p  ^Ge3d  where  Ic 2p, lGe3d e  _  PGe* Ge2p*^Ge2p  (3-1)  A  PGe*  A  G e 3 d * ^Ge3d  are the peak intensities for Ge 3p and Ge 3d from the peak fitting  program. po is the Ge density. e  Ao 2  and Ge 3d X P S peaks. Xoap and  e  P  and  A,Q 3d e  Ao 3d e  are the atomic sensitivities for the Ge 2p  are the electron mean free paths for Ge 2p and Ge  3d photoelectrons, respectively.  62  Figure 3.6  Ge 2p and Ge 3d spectra from a pure Ge sample showing the relative peak intensities.  63  Rearranging the above expression gives the atomic sensitivity ratio o f Ge 2p to Ge 3d (3.2). The results are given i n Table 3.6.  A  A  Ge2p Ge3d  PGe* Ge2p*^Ge3d I  =  PGe* ^Ge3d *  (3-2)  ^Ge2p  Table 3.6 Calculated atomic intensity ratios and sensitivity ratios o f Ge 2p/Ge 3d  lGe2p/lGe3d  8.0  AGe2p/AGe3d  26  3.2.4 Quantitative Analysis The wafer (sample 1) used in this study has a Si cap o f - 1 2 A thick covering - 1 0 0 0 A SiGeo.025 layer. However, since some o f the Si cap may have been oxidized on standing, and hence some o f this oxidized S i was removed when the surface was washed with H F , it was considered important to establish how thick the S i layer was before the oxidation experiment was initiated, and then how thick the oxidized layer was after the O-atom oxidation.  Figure 3.7 illustrated schematically a structure o f S i capped SiGe  sample after an H F wash, " x " is the thickness of Si cap after an H F wash, a is the takeoff angle i n X P S measurements.  64  * I  Si  SiGeo.025  p-Si  wafer  Figure 3.7 Structure o f the H F washed S i capped SiGeo.025 sample.  From a Ge 2p/Ge 3d intensity ratio, the S i layer thickness (x) can be calculated with the following equation , 76  (3 3) I  G « 2 p  P G e * G e 2 p * ^ S i( A  =  E  G e 2 p ) SMI  ^ G e 3 d P G e * G e 3 d * ^ - s i ( G e 3 d ) E  A  s i n  a  PL * e X p ( - X /  \ (E  * exp(-x / X ( E s j  s  G e 3 d  G e 2 p  ) Sin P  ) sin  i )  a)  where Io 2p and Ioe3d are the Ge 2p and Ge 3d peak intensities respectively. po is the Ge e  e  density. Ac 2p and A o 3 are the atomic sensitivities for Ge 2p and Ge 3d lines. Xsi(Eo 2 ) e  e  d  e  , A,si(EGe3d) are the Ge 2p and Ge 3d electron mean free paths in Si. for  P  a is the take-off angle  the spectrometer. Figure 3.8 gives the Ge 2p and Ge 3d spectra at a take-off angle o f 45°  washed sample. W i t h an XPS  for an H F  fitting program for these two spectra, the Ge 2p and Ge 3d  peak intensities were obtained and are listed in Table 3.7, together with their electron mean free paths in S i .  65  G62p  1224  1222  1220  Ge3d  1218  1216  30  1214  32  34  36  38  Brief ng Energy (eV)  BrdrgEnercy(e v) ,  Figure 3.8 X P S spectra o f Ge 2p and Ge 3d from an H F washed sample (take off angle 45°).  Table 3.7 Peak intensities and mean free paths  o f Ge 2p and Ge 3d for an H F washed  sample  Peak  Intensity (a.u.)  * si (A)  Ge 2p  7.2xl0  2  9.3  Ge3d  2.4x10  2  32.6  66  From the data listed in Table 3.7 and the equation above, the S i layer thickness was calculated to be x= 8.3 A . After the H F washed sample was treated with O-atoms, X P S spectra revealed traces o f Ge02. Therefore, we assume that the S i cap is completely oxidized into Si02 (in order to simplify the calculation, S i and S i +  4 +  are all considered as S i 0 ) . The underlying 2  SiGeo.025 was partially oxidized to form a thin layer o f mixed S i and Ge oxides. This assumed structure is illustrated schematically in Figure 3.9.  D  Figure 3.9 M o d e l used to estimate oxide thicknesses from X P S peak intensity ratios.  67  Oxide thickness  can be determined  from X P S spectra  by the following  technique . 70  76  Equation  (3.4) relates the measured intensity ratio Isi02/Isi to the total oxide  thickness (D) i n the above diagram.  Values for other parameters i n this equation are  listed i n Table 3.8. R  _ PSJO * ^s,o s i n a * (1 - e x p ( - D /  =  2  I si  sing))  2  P si * si 1  s  i  na  * e x p ( - D / A.  S l 0 ;  ( - ) 3  4  sin a )  Similarly, to obtain the Si02 cap thickness (D-d), we can use equation  (3.5)  where the parameters are also listed in Table 3.8.  ^GeO  ~  =  y  Psio, * Ko2  s  i  n  a  * e x p ( - ( D - d) / X'  Si0i  Psi * ^si  _ Psio * Ko2 ;  *  e x  P(  d  / ^sio  s  m a  s i n g ) * (1 - e x p ( - d / X'  sina))  * e x p ( - D / X' ^ sin g ) si0  s i n a  ) * d ~ e x p ( - d / X' ^ s i n g ) ) s  2  Psi  ( - ) 3  5  * K  hio, hi, iGeo and Io are the peak intensities o f S i e  SiDi  4 +  (+Si ), S i (from S i 2p), G e , +  0  4+  Ge° (from Ge 2p), respectively. psi02 and psi are the Si02 and S i densities. A-si02, A-si are the electron mean free paths i n Si02 and S i . A,'si02, X's\ are the electron mean free paths for Ge 2p electrons i n Si02 and S i . M e a n free paths are listed in Table 3.8. a is the takeoff angle for the spectrometer.  D and d are the thicknesses o f the Si02 and Si02/Ge02  layers, respectively.  68  B y solving equations (3.4) and (3.5), using X from Table 3.8, the values o f D and d were calculated. The values listed in Table 3.9 are for 3 different oxidation times.  Table 3.8 M e a n free paths (X) used in the calculation  77  Elements  X(A)  X\A)  Si0  37  11.5  31  9.3  2  Si  Table 3.9 Oxide thickness for both Si and Ge as a function o f oxidation time. The last column lists the calculated values for a 45° take-off angle with a 50 min oxidation time.  Oxidation time Substrate  30 sec  10 min  50 min  50 min  S i 0 (D)  18A  20 A  22 A  23 A  7.9 A  8.0 A  8.3 A  10 A  2  S i 0 + G e 0 (d) 2  2  Although the oxide layers become slightly thicker with increasing oxidation time, the variation is small, and it is clear that much shorter reaction times are necessary to stop  69  the oxidation before it reaches the Si/SiGe interface.  The layer thicknesses calculated  from 0° and 45° take-off angles for 50 m i n oxidation are reasonably consistent.  3.3 Effect of Exposure to H-atoms on the Charge Carriers 3.3.1 Unoxidized Samples A s shown in Figure 3.10, hydrogen atoms, when exposed directly to the H F washed S i capped SiGeo.025 sample, lowers the steady state carrier concentration significantly, i.e. the passivation level is adversely affected.  In this experiment, the  sample was first treated with hydrogen atoms for 7 s at 198°C. This caused a dramatic drop o f the steady-state carrier concentration (to almost 0). When the atoms were turned o f f , there was some recovery o f the passivation level. However, longer exposure to H atoms dropped the passivation level even more, from which there was no significant recovery when the H-atoms were turned off.  70  Hen  "nrre(s)  Figure 3.10 Effect o f H-atoms on an H F washed Si capped SiGeo.025 sample at 198°C.  3.3.2 Oxidized Samples When an oxide layer is grown on the surface first, hydrogen atom exposure produces a different effect i f the temperature is high enough. Figure 3.11 and Figure 3.12 illustrate this effect i n terms o f carrier concentration and trap density, respectively. Three samples were initially oxidized for 10 minutes at 207°C and then annealed in H-atoms for 40 m i n at the same temperature. They were then separately exposed to H atoms at 20°C, 83°C and 173°C for 10 m i n and monitored before, during, and after the exposures. The carrier concentrations at all these 3 temperatures were normalized to the same initial value.  It can be seen that at all three temperatures, exposure to H-atoms  dropped the steady-state carrier concentration as it did for the unoxidized sample.  At  room temperature, there was very little recovery of the signal when the H-atoms were removed. However, as the temperature was raised to 173°C, in contrast to the unoxidized sample, there was a large recovery in carrier concentration when the H-atoms were shut off.  72  1.2  173°C  83 °C  ******20 °C 0.0  1  0  500  1000  1500  2000  2500  3000  "[irre(s)  Figure 3.11  Change o f carrier concentration for an oxidized and annealed S i capped SiGeo.025 sample during long exposure to H-atoms at 20°C, 83°C, and 173°C.  73  Figure 3.12  Change o f trap density for an oxidized and annealed Si capped SiGeo.025 sample during long exposure to H-atoms at 20°C, 83°C, and 173°C.  74  3.3.3 Kinetic Analysis and Activation Energy Calculation Although the rise in trap density that occurs when the sample is exposed to H atoms is too rapid to be followed with our instrumentation, the disappearance o f the traps, when the H-atoms are shut off can be subjected to a kinetic analysis.  '  Figure 3.13 shows a plot o f ln(trap density) versus time. It is not a straight line and the process therefore is not first order. If the process is second order then a plot o f l/(trap density) versus time should be a straight line. Figure 3.14 plots the inverse of trap density versus time for 20°C, 83°C, 148°C, 173°C, and 192°C. It can be seen that the fit to second order kinetics is not too bad at the three lower temperatures.  However, for the  two higher temperature the lines are distinctly curved i n the initial region, i.e. the reaction is initially even faster than predicted by the second order rate constant.  75  Figure 3.13 Plot o f the logarithm o f trap density versus time for 20°C, 83°C, 148°C, 1 7 3 ° C , a n d 192°C.  76  50  I -500  1  1  0  1  1  500  I  I  1000  .  I  1500  I  I  2000  I  I  2500  !  I 3000  Time(s)  Figure 3.14 Plot o f the inverse o f trap density versus time and the "best" straight line through the points for 20°C, 83°C, 148°C, 173°C, and 192°C.  77  The second order rate constants (k) extracted from the fitted straight lines at times longer than 250 s (Figure 3.14) are listed in Table 3.10.  According to the Arrhenius  equation,  k =A e  (-) 3  E / k T  6  Therefore, the activation energy can be calculated from the slope o f Arrhenius plot shown i n Figure 3.15. The activation energy calculated is 0.35 e V .  Table 3.10 Tabulated data o f second order rate constants  T(°C)  1/T (1/°K)  k(s-')  20  3.4xl0"  3  9.2x10"  5  -9.3  83  2.8xl0"  3  l.lxlO"  3  -6.8  148  2.4xl0"  3  6.1xl0"  3  -5.1  173  2.2x10"  3  9.2xl0"  3  -4.7  192  2.15xl0"  1.6xl0"  2  -4.1  3  Ink (a.u.)  78  Figure 3.15 Arrhenius plot o f the rate constants for the disappearance o f traps when H atoms are shut off. This process was analyzed by second order kinetics.  79  3.3.4 Mechanism of the Trap Decay To explain a second order process, we would have to postulate the following model i n which the carrier trap is a bound hydrogen atom (H-Site):  ki  H-Site  •  H + H-Site  (3.7,3.8)  H + Site  •  H  2  + Site  (3.9)  where reaction (3.9) is very much slower than reactions (3.7) and (3.8). When the H atoms are turned on, the trap concentration is determined largely by reactions (3.8) and (3.9) (governed by k and k3). When the discharge-produced H-atoms are removed, the 2  loss o f traps is governed by the reaction (3.7) which produces a small steady-state H-atom concentration at the interface. The rate o f loss o f H-site is given by, d[H-Site]_d[H ] 2  2dt  dt  k [H][H-Site]  (3.10)  3  If the concentration o f H is determined principally by reaction (3.7) and (3.8), kjH-Site]  (3.11)  [H]  which is derived from, k, [H-Site] = k [H] [Site] 2  Substituting (3.11) in (3.10)  (3.12) (3.13)  80  If we assume [Site] ~ constant, i.e. there are a large number o f sites, then the reaction (3.7) becomes second order. The experimental activation energy which we measure is then identified with the composite rate constant k ki/k2. 3  k  2  and kj would be predicted to have very small  activation energies, and therefore k i w i l l have an activation energy o f about 0.35 eV, which is not unreasonable. It is also possible to analyze the trap removal process by a multiple-exponential decay. This can be explained into a two-concurrent first order kinetics. Figure 3.16 plots the trap density versus time when the H-atoms were turned off at 20°C, 83°C, 148°C, 173°C, and 192°C. The trap density at each temperature is fitted with a double-exponential i n the form: T = T + T i exp(-kit) + T exp(-k t) 0  where T is the trap density.  2  (3.14)  2  This double exponential decay fits the experimental data  reasonably well. It is therefore also possible that H-atom exposure generated 3 kinds o f traps. After the H-atoms are turned off, one To remained unchanged, the other two (Ti and T ) decayed with rate constants k i and k . 2  2  The decay constants k i and k  2  are listed in Table 3.11.  temperature is presented as Arrhenius plots in Figure 3.17.  Their change with  The experimental value  obtained for k i at 20°C was ignored when the straight line was drawn through the points in Figure 3.17 because it lies several standard deviations from the value predicted by the other 4 points. The Arrhenius plots yield an activation energy o f 0.21 e V for the rapidly decaying traps, and o f 0.16 e V for the slowly decaying traps.  81  Figure 3.16 Trap density decay when H-atoms are turned off at different temperatures. The dashed lines are fitting lines from a double exponential decay function , y = y + yi exp(-kit) + y exp(-k t). 0  2  2  82  Table 3.11 Trap decay constants k i and k at different temperatures when H-atoms were 2  turned off  T(°C)  1/T (K" )  k, (s" )  20  3.4xl0"  3  4.5x10"  83  2.8xl0"  3  148  2.4x10"  173  2.2xl0"  192  2.15xl0~  1  k (s-')  1  2  3  6.8xl0" 3  2  8.0x10"  2  2  4  -7.7  -7.6  2  1.3xl0"  3  -4.05  -6.65  2  3.4xl0"  3  -3.05  -5.7  4.0xl0"  3  -2.7  -5.5  4.1xl0"  3  -2.5  -5.5  1.75xl0" 4.75xl0"  l n k (a.u.)  4.8xl0"  4  3  lnki (a.u.)  83  Figure 3.17 Arrhenius plot o f trap decay rate constants (ki for a fast process and k for a 2  slow process) when H-atoms are turned off. These processes were analyzed by a two-concurrent first order kinetics.  84  3.4 Comparison of Si Capped SiGe Substrate with Other Substrates  3.4.1 SiGeo.3 In order to identify the effects that may be due to Ge during oxidation and hydrogen passivation, several experiments were performed on a layer o f SiGeo.3 that is very much richer in Ge than the samples used earlier. atoms and then treated with H-atoms at 209°C.  SiGeo.3 was exposed to oxygen  The change o f steady-state carrier  concentration during the processes is shown i n Figure 3.18. The change o f trap density is shown i n Figure 3.19. It can be seen that the exposure to H-atoms after a 10 minutes exposure to O-atoms resulted in very little improvement. It would appear that the oxidation o f silicon with a 30% Ge content produces an oxide that has a poorly passivated interface with the SiGe crystal, and no treatment that we could apply improved the passivation level significantly.  85  0.75 r-  I 3  0.50 h-  8  0.25 h  0.00 h  Time (s)  Figure 3.18 Change o f carrier concentration for SiGeo.3 during O-atom and H-atom treatments at 209°C.  86  Tirre'(s)  Figure 3.19 Change o f trap density for SiGeo.3 during O-atom and H-atom treatments at 209°C.  87  3.4.2 Intrinsic Si and Uncapped SiGeo.025 A n intrinsic S i surface, an uncapped SiGeo.025 surface, and the capped sample treated earlier were subjected to identical treatments.  Figure 3.20, Figure 3.21, and  Figure 3.22 records the changing trap density during the treatment o f these three substrates. The following numbers w i l l be used in these figures to identify the conditions under which the measurements are being made. The samples were kept at a constant T= 209°C. (1) H F washed surface with O2 passing over the surface (2) Surface exposed to O-atoms for 10 min (3) H-atom treatment (4) Discharge shut off but H flow retained 2  88  Time (s)  Figure 3.20 Change o f trap density for an intrinsic Si surface during O-atom and H-atom treatments at 209°C.  89  Oaff,Hai 1Q50A 6h-  150A  2  Hon 0  (Tf^Octi  f  Hoi 2000  km 4000  Hon 6000  Time (s)  Figure 3.21 Change o f trap density for a SiGeo.025 surface during O-atom and H-atom treatments at 209°C.  90  Figure 3.22 Change o f trap density for a S i capped SiGeo.025 surface during O-atom and H-atom treatments at 209°C.  91  Consider first the changes that occurred at the Si surface as recorded in Figure 3.20. The H F wash produced a surface with a very low carrier trap density (region (1)). The oxidation that resulted from exposure to O-atoms for as little as 30 s produced a layer of Si02 o f about 20 A (see Section 3.2). This corresponds to the oxidation o f about 8 atomic layers o f S i . A s soon as the oxygen atoms were introduced, the carrier trap density increased by more than an order o f magnitude, and within a second it settled to a value that remained unchanged during the 10 minute exposure (region (2) in F i g . 3.20). When the hydrogen atoms were passed over this oxidized surface the trap density dropped to about 40% and then did not change a great deal while it was being exposed to H-atoms for a long period (region (3)). However, when H-atoms were removed, i.e. the microwave discharge was shut off (region (4)), the carrier trap density dropped slowly to a level that, with longer exposures to H-atoms, approached the initial trap density o f the H F washed sample. Comparing Figures 3.20, 3.21 and 3.22, it is clear that the behavior o f the three samples is almost indistinguishable. We therefore conclude that the presence o f 2.5% Ge in the S i crystal insignificantly affects the response o f the material to oxidation and subsequent H-atom treatment. It is also clear from the data i n Figure 3.19 that this is not true when the concentration o f Ge in the S i is 30 mole%.  92  3.4.3 Effect of H annealing 2  Since standard industrial practice ( R C A patent 1972) involves the annealing o f "gate oxides" i n H at about 450°C in order to reduce the density o f interfacial carrier 2  traps, the important question is whether the treatment with H-atoms that we have used in this work has the same effect as H annealing. 2  To test this we annealed one intrinsic silicon sample in H  2  at 450°C after  oxidation. A comparison o f the response o f this sample and an unannealed sample to H atoms is shown is Figure 3.23. It can be seen that annealing the oxidized sample in H at 2  450°C for 20 minutes removed about 80% of the carrier traps. When this sample or one that has not been annealed in H was now exposed to H-atoms the carrier trap density 2  changed to the same value (Figure 3.23). The subsequent behavior o f the two samples was very similar and the eventual trap density after the H-atoms were shut off was very similar. Both are better than the original H annealed sample. 2  93  Time®  Figure 3.23 Comparison o f H-atom treatment at 202°C on intrinsic S i samples that (a) have been annealed in H2 for 20 minutes at 450°C, and (b) that have not been annealed i n H2.  94  Chapter 4 Conclusion L o w temperature remote microwave plasma oxidation was employed in this work to grow a gate oxide on SiGe with a minimum density o f interfacial carrier traps. X P S was used to characterize the various layers and their thicknesses.  Plasma oxidation o f  SiGe capped with 12.5 A o f S i completely oxidized the Si cap and generated both S1O2 and GeC>2. The total oxide thickness was about 20 A after as little as 30 s oxidation. The oxide layer became slightly thicker with oxidation time. A novel RF-probe was used to continuously monitor the interfacial trap density. It was found that the trap density changes very rapidly during oxidization and subsequent H-atom exposure.  the  For Si capped SiGeo.025 the trap removal rate when H -  atoms were shut off was subjected to a kinetic analysis.  It was found that this process  could be fitted either as a second order reaction or to 2 or more concurrent first order processes. To explain a second order rate law the following mechanism was proposed,  H-Site  ,  k l  k  > H + Site  (3.7,3.8)  2  H + H-Site  • H  2  + Site  (3.9)  Where the carrier trap is a bound hydrogen atom (H-Site). Reaction (3.9) must be very much slower than reactions (3.7) and (3.8).  Temperature-dependence studies (20°C ~  200°C) gave an activation energy o f 0.35 e V for reaction (3.7).  95  Alternatively i f the trap removal process is analyzed in terms o f two concurrent reactions o f two independent species. These reactions are first order processes described by a double-exponential decay: T = T + T, exp(-kit) + T exp(-k t) 0  2  (3.14)  2  where there are three types o f traps created by the hydrogen atom exposure, one kind o f trap (T ) remained unchanged, the other two kinds of traps (Ti and T ) are removed with 0  2  rate constants k i and k , that have activation energies o f 0.21 e V and 0.16 eV 2  respectively. B y monitoring the interfacial trap density during the oxidation o f Si, SiGeo.3, Si capped SiGeo.025, and SiGeo.025 with atomic oxygen at ~200°C, we have observed that the presence o f 2.5% Ge in the Si lattice does not result in the formation of larger concentrations o f interfacial carrier traps than is formed in a pure Si lattice. Furthermore, just as exposure o f the oxide layer to atomic hydrogen at ~200°C can result i n the removal of a large portion o f these traps in S i , it can cause an almost identical reduction in the SiGeo.025- However, i n the case o f SiGeo.3, this reduction was quite small. The H-atom treatment at 200°C results in a trap reduction that is comparable with that obtained with a high temperature H anneal which is always used in the fabrication industry to reduce the 2  interfacial traps and further improve the oxide interface quality.  96  References George E . Anner, Planar Processing Primer, V a n Nostrand Reinhold, N e w York, 1990.  I  W . Scot Ruska, Microelectronic  Processing, M c G r a w - H i l l , 1987.  David L . Pulfrey and N.Garry Tarr, Introduction to Microelectronic H a l l , N e w Jersey, 1989.  Devices, Prentice  4  A d i r Bar-Lev, Semiconductors and Electronic Devices, Prentice H a l l , 1984.  5  James W . Mayer and S.S. Lau, Electronic Materials Science: For Integrated Circuits in Si and GaAs, M a c m i l l a n Publishing Company, 1990.  6  Ben G . Streetman, Solid State Electronic Devices, Prentice H a l l , N e w Jersey, 1995. Robert W . Keyes, The Physics of VLSI Systems, Addison-Wesley, 1987. J. D . Cressler, I E E E Spectrum, March, 49 (1995).  8  D . L . Harame, J. H . Comfort, J. D . Cressler, E . F. Crabbe, J. Y . - C . Sun, B . S. Meyerson, and T. Tice, I E E E Trans. Electron Devices, 42, 455 (1995).  9  1 0  G . L . Patton, J. H . Comfort, B . S. Meyerson, E . F. Crabbe, G . J. Scilla, E . D . Fresart, J. M . C . Stork, J. Y . - C . Sun, D . L . Harame, and J. N . Burghartz, I E E E Electron Device Lett. 11, 171 (1990).  I I  S. S. Iyer, G . L . Patton, J. M . C. Stork, B . S. Meyerson, and D . L . Harame, I E E E Trans. Electron Devices, 36, 2043, (1989).  12  T. E . W h a l l and E . H . C. Parker, Journal of Materials Science: Materials in Electronics, 6, 249 (1995). Andres G . Fortino, Fundamentals of Integrated Circuit Virginia, 1984.  Technology, Prentice Hall,  S. M . Sze, Semiconductor Devices: Physics and Technology, John W i l e y & Sons, 1985.  1 4  1 5  P. M . Garone, V . Venkataraman, and J. C. Sturm, I E E E Electron Device Letters, 13, 56 (1992).  1 6  S. C. Martin, L . M . Hitt, and J. J. Rosenberg, I E E E Electron Device Letters, 10, 325 (1989).  97  17  Sorab K . Ghandhi, VLSI Fabrication Wiley & Sons, 1983.  Principles:  Silicon and Gallium Arsenide, John  1 8  A . R. Srivatsa, S. Sharan, 0 . W . Holland, and J. Narayan, J. A p p l . Phys. 65, 4028 (1989).  1 9  G . L . Patton, S. S. Iyer, S. L . Delage, E . Ganin, and R. C . Mcintosh, Mat. Res. Soc. Symp. Proc. 102, 295 (1988).  2 0  J. P. Zhang, P. L . F. Hemment, S. M . Newstead, A . R. Powell, T. E . Whall, and E . H . C. Parker, Thin solid films, 222, 141 (1992).  2 1  D . C . Paine, C. Caragianis, and A . F. Schwartzman, J. A p p l . Phys. 70, 5076 (1991).  2 2  H . K . L i o u , P. M e i , U . Gennser, and E . S . Yang, A p p l . Phys. Lett. 59, 1200 (1991).  2 3  J. Eugene, F. K . LeGoues, V . P. Kesan, S. S. Lyer, and F. M . d'Heurte, A p p l . Phys. Lett. 59, 78 (1991).  2 4  D . K . Nayak, K . Kamjoo, J. S. Park, J. C . S. Woo, and K . L . Wang, A p p l . Phys. Lett. 57,369(1990).  2 5  D . Nayak, K . Kamjoo, J. C . S. Woo, J. S. Park, and K . L . Wang, A p p l . Phys. Lett. 56, 66 (1990).  26  F. K . LeGoues, R. Rosenberg, T. Nguyen, F. Himpsel, and B . S. Meyerson, J. A p p l . Phys. 65, 1724(1989). 2 7  F. K . LeGoues, R. Rosenberg, and B . S. Meyerson, A p p l . Phys. Lett. 54, 644 (1989).  2 8  S. S. Iyer, P. M . Solomon, V . P. Kesan, A . A . Bright, J. L . Freeouf, T. N . Nguyen, and A . C . Warren, I E E E Electron Device Letters, 12, 246 (1991).  2 9  S. C . Jain and P. Balk, Thin Solid Films, 223, 348 (1993).  3 0  V . Q. H o and T. Sugano, I E E E Trans. Electron Device, ED-27, 1436 (1980).  3 1  J. Kraichman, J. A p p l . Phys. 38, 4323 (1967).  3 2  D . L . Pulfrey, F. G . M . Hathorn, and L . Young, J. Electrochem. Soc. 120, 1529 (1973).  3 3  P. W . L i , H . K . L i o u , and E . S. Yang, A p p l . Phys. Lett. 60, 3265 (1992).  98  M . Mukhopadhyay, S. K . Ray, C . K . Maiti, D . K . Nayak, and Y . Shiraki, A p p l . Phys. Lett. 65, 895 (1994).  3 4  3 5  1 . S. G o h , J. F. Zhang, S. H a l l , W . Eccleston, and K . Werner, IOP, 818 (1995).  3 6  M . Mukhopadhyay, S. K . Ray, T. B . Ghosh, M . Sreemany, and C . K . M a i t i , Semicond. Sci. Technol. 11, 360(1996).  3 7  M . Mukhopadhyay, S. K . Ray, C. K . M a i t i , D . K . Nayak, and Y . Shiraki, J. A p p l . Phys. 78,6135 (1995).  3 8  T. Hattori, Critical Reviews in Solid State and Materials Sciences, 20, 339 (1995).  3 9  M . J. Uren, J. H . Stathis, and E . Cartier, J. A p p l . Phys. 80, 3915 (1996).  4 0  E . Cartier and J. H . Stathis, A p p l . Phys. Lett. 69, 103 (1996).  4 1  W . L . Warren, K . Vanheusden, J. R. Schwank, D . M . Fleetwood, P. S. Winokur, and R. A . B . Devine, A p p l . Phys. Lett. 68, 2993 (1996).  4 2  K . L . Brower, Physical Review B 38, 9657 (1988).  4 3  K . L . Brower, Physical Review B 42, 3444 (1990).  4 4  J. H . Stathis i n The Physics and Chemistry of S1O2 and the Si-Si02 Interface 2, (edited by C. R. Helms and B . E . Deal), Plenum, N e w York, 1993.  45  H . Fukuda, M . Yasuda, T. Iwabuchi, S. Kaneko, T. Ueno, and I. Ohdomari, J. A p p l . Phys. 72, 1906 (1992).  4 6  S. C . Vitkavage , E . A . Irene, and H . Z . Massoud, J. A p p l . Phys. 68, 5262 (1990).  4 7  M . L . Reed, and J. D . Plummer, J. A p p l . Phys. 63, 5776 (1988).  4 8  D . Tchikatilow, J. F. Yang, and E . S. Yang, A p p l . Phys. Lett. 69, 2578 (1996).  4 9  50  5 1  H . Lafontaine, D . C . Houghton, D . Elliot, N . L . Rowell, J. - M . Baribeau, S. Laframboise, G . I. Sproule, and S. J. Rolfe, J. V a c . Sci. Technol. B 14, 1675 (1996). H . L i , E . A.Ogryzlo, T. Tiedje, and J. G . Cook, Proceedings o f the 2 2 Conference on the Physics o f Semiconductors, 1, 541 (1995).  nd  International  H . L i , and E . A . Ogryzlo, Can. J. Phys. 74, 685 (1996).  99  3 Z  M Sakuraba, J. Murota, and S. Ono, J. A p p l . Phys. 75, 3701 (1994).  5 3  E . Yablonovitch, D . L M a r a , C . C . Chang, T. Gmitter, and T. B . Bright, Physical Review Letters 57, 249 (1986).  5 4  T. Takahagi, I. Nagai, A . Ishitani, H . Kuroda, and Y . Nagasawa, J. A p p l . Phys. 64, 3517(1988).  5 5  5 6  L . Elias, E . A . Ogryzlo, and H . I. Schiff, Can. J. Chem. 37, 1680 (1959). A . J. Nelson in, Microanalysis of Solids, Edited by B . G . Yacobi, D . B . Holt and L . L . Kazmerski, Plemum Press. N . Y . 1994.  57  J. F. Watts, An Introduction to Surface Analysis by Electron Spectroscopy, Oxford University Press, N . Y . 1990. 58  H . Windawi and F. L . H o , Applied Electron Spectroscopy for Chemical Analysis, Wiley & Sons, U . S . A . 1982. 5 9  John  N . H . Turner i n Investigations of Surfaces and Interfaces-Part B, Edited by Bryant W . Rossiter and Roger C. Baetzold, John Wiley & Sons, 1993.  6 0  L . C . Feldman and J. W . Mayer, Fundamentals of Surface and Thin Film Elsevier Science, North-Holland, 1986.  Analysis,  6 1  J. F. Moulder, W . F. Stickle, P. E . Sobol, and K . D . Bornben, Handbook of X-ray Photoelectron Spectroscopy: a Reference Book of Standard Spectra for Identification and Intepretation of XPS Result. Pekin-Elmer Corporation, 1992.  62  M . H . K i b e l in Surface Analysis Methods in Materials Science, Edited by D . J. O'Connor, B . A . Sexton and R. St. C. Smart, Springer-Verlag, Germany, 1992. 63  Edited by D . Briggs and M . P. Seah, Practical edition), John Wiley & Sons, 1990. 6 4  Surface Analysis,  Vol 1,(second  P. H . Holloway and G . E . M c G u i r e , A p p l . Surface Sci, 4, 410 (1980).  6 5  F. Yano, A . Hiraoka, T. Itoga, H . Kojima, K . Kanehori, and Y . Mitsui, J. Vac. Sci. Technol. A 13, 2671 (1995).  6 6  T. Hattori, T. A i b a , E . Iijima, Y . Okube, H . Nohira, N . Tate, and M . Katayama, Applied Surface Science 104-105, 323 (1996).  100  Y . Yamashita, K . Namba, Y . Nakato, Y . Nishioka, and H . Kobayashi, J. A p p l . Phys. 79, 7051 (1996). 68  Y . Yamashita, Y . Nakato, H . Kato, Y . Nishioka, and H . Kobayashi, Applied Surface Science, 117-118, 176 (1997).  6 9  K . B . Clark, J. A . Bardwell, and J. - M . Baribeau, J. A p p l . Phys. 76, 3114 (1994).  7 0  D . F. Mitchell, K . B . Clark, J. A . Bardwell, W . N . Lennard, G . R. Massoumi, and I. V . Mitchell, Surface and Interface Analysis, 21, 44 (1994).  7 1  H . J. Mathieu, M . Datta, and D . Landolt, J. V a c Sci, Technol. A 3, 331 (1985).  7 2  A . Muto, T. M i n e , and M . Nakazawa, Jpn. J. A p p l . Phys. 32, 3580 (1993).  7 3  Y . R. X i n g , J. A . W u and S. D . Y i n , Surface Science 334, 705, (1995).  7 4  O. Vancauwenberghe, O. C. Hellman, N . Herbots, and W . J. Tan, A p p l . Phys. Lett. 59, 2031 (1991).  7 5  V . Craciun, A . H . Reader, D . E . W . Vandenhoudt, S. P. Best, R. S. Hutton, A . Qndrei, and I. W . Boyd, Thin Solid Films 255, 290 (1995).  76  Joseph D . Andrade in Surface and Interfacial Aspects of Biomedical Polymers, Volume 1, Surface Chemistry and Physics, Edited by Joseph D . Andrade, Plenum Press, 1985. 77  S. Tanuma, C . J. Powell and D . R. Perm, Surface and Interface Analysis, 11, 577 (1988).  101  

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

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"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0061674/manifest

Comment

Related Items