bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of an azimuthally symmetric helicon wave high density plasma source
Authors: Tynan G. R. , Bailey A. D. , Campbell G. A. , Charatan R. , de Chambrier A. , Gibson G. , Hemker D. J. , Jones K. , Kuthi A. , Lee C. , Shoji T. , Wilcoxson M.
Published: July 2002

A cylindrically symmetric (azimuthal mode number m=0) resonant inductive (MO/RITM) radio frequency (rf) helicon wave high density plasma source is described. The source consists of an antenna and bell jar generator immersed in a diverging magnetic field. Plasma is generated in this upstream region and then is transported along the field lines into the low-field downstream processing chamber. A propagating wave is observed in the plasma with rf spatial distribution and propagation characteristics that obey the theoretical m=0 helicon wave dispersion relation. By varying the divergence of the source magnetic field, the wafer etch rate and etch uniformity can be controlled. Spatially resolved optical emission spectroscopy shows that molecular gases are almost completely dissociated near the plasma center and have a uniform radial distribution. Highly uniform plasma and neutral distributions are then produced at the wafer location, and have been used in a variety of etch applications.
In situ conductivity characterization of oxide thin film growth phenomena on microhotplates
Authors: DiMeo F. , Cavicchi R. E. , Semancik S. , Suehle J. S. , Tea N. H. , Small J. , Armstrong J. T. , Kelliher J. T.
Published: July 2002

Through the use of silicon micromachining, we have developed a microhotplate structure capable of reaching temperatures in excess of 500 °C, onto which thin films have been selectively grown via metalorganic chemical vapor deposition. The microhotplate structure contains surface electrical contacts which permit conductance measurements to be made on films during and after deposition, and therefore presents some unique opportunities for the in situ characterization of growing films as well as for novel gas sensing approaches. We have investigated the deposition of conducting oxides such as SnO2 and ZnO on these microhotplate platforms for gas sensing applications. The conductance of the deposited films has been measured in situ as a function of time, and used in combination with postdeposition thickness measurements to provide insights into the growth rate of the oxide films. Results indicate that our conductance measurements are sensitive, in certain cases, to changes in the film thickness on the order of an angstrom. Conductance oscillations observed during the growth of ZnO thin films have been attributed to variations in the precursor concentration, and were detected as a gas sensor response by the growing films.
Optimization of alternating-current thin-film electroluminescent displays
Authors: Krasnov Alex N. , Bajcar Robert C. , Hofstra Peter G.
Published: July 2002

Different factors affecting the performance of alternating-current thin-film electroluminescent (ACTFEL) devices are discussed. The Chen-Krupka model of electron transport was applied to examine the case of non-uniform distribution of the dopant concentration in the active layer. Based on the results, a ZnS doping profile is proposed which incorporates higher doping at the interfaces than in the bulk. Also, the asymmetry in the excitation efficiency of electrons near the two insulator-semiconductor interfaces (ISIs) of ACTFEL devices was studied and was ascribed to the difference in the interface quality arising from the choice of insulating material and to the selection of the annealing process. The role of the space charge stored near the ISI and the influence of drive conditions on the operational characteristics of the device are discussed.
Conventional and dynamic actinometry of glow discharges fed mixtures of tetramethylsilane, sulfur hexafluoride, and helium
Authors: Durrant Steven F. , Bica de Moraes Mário A.
Published: July 2002

Conventional and dynamic actinometric optical emission spectroscopies were used to investigate the composition of discharges of mixtures of tetramethylsilane, sulfur hexafluoride, and helium. Argon was present in a small proportion as an actinometer. Trends in the plasma concentrations of the species H, CH, Si, F, and CF2 were obtained as a function of the proportion of SF6 in the feed, Rs. Dynamic actinometry, which involves monitoring the relative intensities of plasma species following interruption of one of the principal feed gases, gave clues as to the relative importance of gas-phase and plasma/polymer-surface reactions in the production of the measured plasma species. Based on these data, reaction pathways and mechanisms for the production of these species are suggested.
Comparison of inductively coupled plasma Cl2 and Cl4/H2 etching of III-nitrides
Authors: Cho Hyun , Vartuli C. B. , Donovan S. M. , Abernathy C. R. , Pearton S. J. , Shul R. J. , Constantine C.
Published: July 2002

A parametric study of etch rates, selectivity, surface morphology and etch anisotropy has been performed for Cl2 and CH4/H2 inductively coupled plasma (ICP) patterning of GaN, AlN, InN, InAlN, and InGaN at low dc self-biases (typically ⩽−100 V). Controlled etch rates in the range 500–1500 Å min−1 are obtained for all materials. Surface morphology is a strong function of ion energy and plasma composition in both chemistries, while vertical sidewalls are obtained over a wide range of conditions because of the ion-driven nature of the etch mechanism.
Scanning tunneling microscopy of equilibrium crystal shapes
Authors: Surnev S. , Arenhold K. , Coenen P. , Voigtländer B. , Bonzel H. P. , Wynblatt P.
Published: July 2002

Scanning tunneling microscopy (STM) has been used to image the equilibrium shape of small 2d and 3d crystallites supported on single crystal substrates. Due to the high resolution of STM, highly accurate shape information is obtained. Concerning 3d particles, special attention was given to the step structure of facets and the curved portions near facets. The shape of the (111) facet of Pb particles, prepared and equilibrated on Cu(111) under ultrahigh vacuum conditions, exhibits a threefold symmetry. The facet edge can be determined with atomic resolution. Hence an accurate analysis of the Pb equilibrium crystal shape (ECS) near facets is possible yielding shape exponents. Expected universal exponents of 3/2 or 2 are not found. Instead, shape exponents exhibit an azimuthal dependence, with values ranging between 1.4 and 1.7. This variation, consistent with the threefold symmetry of the (111) facet, suggests a structure dependent step interaction behavior and, furthermore, that the facet to vicinal surface transition does not belong to the Pokrovsky–Talapov universality class. By comparison, the ECS of 2d islands on Pb(111) is also threefold symmetric and yields orientation dependent relative step energies. Thus, the data of 2d Pb islands on Pb(111) supplement those obtained from the (111) facet of the ECS of 3d Pb particles.
Surface cleaning with hydrogen plasma for low-defect-density ZnSe homoepitaxial growth
Authors: Ohno Tetsuichiro , Ohki Akira , Matsuoka Takashi
Published: July 2002

This article investigates in depth the conditions for cleaning of the (100) ZnSe substrate surface for ZnSe homoepitaxial growth. Wet etching with a K2Cr2O7-based etchant as a pre-growth treatment is found to result in a Se-rich ZnSe surface that retains its original flatness. Reflection high energy electron diffraction patterns show that plasma irradiation during thermal cleaning is indispensable for removing the oxide layer on ZnSe substrate and keeping the flatness. In hydrogen plasma cleaning, the etch pit density (EPD) of the homoepitaxial film increases at rf power higher than 265 W, indicating the optimum cleaning temperature is between 260 and 280 °C. As for the dependence of the EPD on the cleaning time, a minimum EPD of 2.7×104/cm2 can be obtained for a plasma cleaning time of 20 min. A correlation between the interfacial residual oxygen concentration and the defect density in an epitaxially grown ZnSe film is also found.
Ellipsometric investigation of nucleation sites for chemical vapor deposition of Si on SiO2 and Si3N4 surfaces
Authors: Basa C. , Irene E. A.
Published: July 2002

The nucleation of polycrystalline silicon by chemical vapor deposition on stoichiometric and silicon-rich silicon dioxide and silicon nitride films was studied using real time ellipsometry. The time of the appearance of the first stable Si nuclei, the incubation time (tinc), was found to vary with the Si content within a film type (oxide or nitride) and also tinc varied for different film types with the shortest tinc for Si-rich silicon nitride films. Differences between Si bound (to O or N) and free Si (bound to Si) were observed with free Si being more effective at promoting nucleation. Anion screening effects might also yield second order reactivity differences.
Self-limiting growth conditions on (001) InP by alternate triethylindium and tertiarybutylphosphine supply in ultrahigh vacuum
Authors: Otsuka Nobuyuki , Nishizawa Jun-ichi , Kikuchi Hideyuki , Oyama Yutaka
Published: July 2002

Alternate injections of triethylindium (TEI) and tertiarybutylphosphine (TBP) without precracking were used to realize self-limiting growth of an InP layer on a (001) InP substrate in ultrahigh vacuum. Self-limiting growth, in which the growth rate is independent of the injection time and the pressure of TEI and TBP, was achieved at a growth temperature as low as 320 °C. The growth rate of around 0.75 ML per cycle in self-limiting growth was assumed to be due to stable surface reconstruction of (2×4)-β. A specular surface morphology was obtained in the self-limiting growth condition. By analyzing the limitation of the growth rate as well as the surface morphology of the layer, self-limiting mechanisms were proposed with respect to suppression of the TEI decomposition on the indium surface during TEI injection, suppression of the phosphorus dissociation from the InP surface during TBP evacuation, and complete reaction between TEI and TBP during TBP injection. The growth condition range for self-limiting growth was expanded with the increase in TBP dosage which was defined as the multiplication of injection time and injection pressure.
Studies of heteroepitaxial growth of thin II–VI semiconductor layers by sequential ultrahigh vacuum dosing
Authors: Luo Y. , Han M. , Slater D. A. , Osgood R. M.
Published: July 2002

An in situ molecular-level study of material growth using a binary reaction sequence of hydride and metalorganic precursors is presented. The study used a model material system of CdS/ZnSe(100) and focused on the material chemistry of heteroepitaxy growth. In the growth process, dimethylcadmium and H2S precursors were sequentially dosed onto a c(2×2) ZnSe(100) substrate under high-vacuum conditions. At temperatures of ∼300 K, saturated chemisorption of a Cd and a S monolayer occurred during each cycle of the binary reaction sequence. Characterization of the growth surface was accomplished in the growth chamber using Auger electron spectroscopy, x-ray photoelectron spectroscopy and low-energy ion scattering spectroscopy for probing surface chemical composition and low-energy electron diffraction for determining surface order. These measurements showed layer-by-layer growth at a substrate temperature of ∼300 K, yielding an ordered stoichiometric CdS film. Strong variations in the composition of the grown surface layer were observed at different substrate temperatures; these variations were found to be related to the temperature dependence of the precursor reactions with the growth surfaces.
Formation of Ni silicides on (001)Si with a thin interposing Pt layer
Authors: Cheng L. W. , Cheng S. L. , Chen L. J. , Chien H. C. , Lee H. L. , Pan F. M.
Published: July 2002

The formation of Ni silicides on (001)Si with a thin interposing Pt layer has been investigated. NiSi was observed to be the only silicide phase for the samples annealed at 500–800 °C with a thin interposing Pt layer. The sheet resistance maintained the same low level in a wide temperature range. Pt addition was found to retard significantly the formation of nickel silicides and enhance the thermal stability of NiSi thin films on (001)Si. For Ni(30 nm)/Pt(1.5 nm)/(001)Si, the process window of NiSi was extended to 500–800 °C. The effects of a thin interposing Pt layer on the formation of Ni silicides on (001)Si are discussed.
Decay rate of photoresist outgassing from ion implantation
Authors: Perel A. S. , Horsky T. N.
Published: July 2002

Ion implantation into silicon wafers coated with photoresist gives rise to large quantities of photoresist outgassing. The purpose of this undertaking was to determine the time dependence of the outgassing rate. Wafers coated with photoresist begin outgassing when implantation begins, but how fast does the outgassing rate decay once implantation stops? The photoresist outgassing decay rate determines the geometry of the gas load on a rotating disk, and is therefore relevant to the chamber design and pump placement to minimize pressure along the path of the beam. The data indicate that an implant of 50 keV boron results in an outgassing rate that depends on time as t−0.8 where t is the time since a wafer was last implanted. Ion implantation “turns on” the outgassing thus allowing for millisecond data collection of outgassing rates.
Quantum transport in GaAs doping superlattices
Authors: Maan J. C. , Englert Th. , Uihlein Ch. , Künzel H. , Ploog K. , Fischer A.
Published: July 2002

The resistance of the n channels in GaAs doping superlattices shows clear oscillations as a function of the magnetic field, and varies with the voltage applied between the n and the p layers and with the orientation of the magnetic field with respect to the doping layer plane. These experimental results are analyzed in terms of the two-dimensional conductivity of the system, and the subband separation is derived from a comparison between the measurements and model calculations.
Chemical reaction at the annealed Si/Yb interface
Authors: Rossi G. , Nogami J. , Yeh J. J. , Lindau I.
Published: July 2002

We present the first evidence of formation of mixed valent compounds at the Si(111)/Yb interface upon annealing. Two stable and uniform compositions were found, these being related to the known bulk silicides YbSi and YbSi2. The mixed valence behavior is seen in the presence of the strong 4f13 (Yb3+) signal and in the shift of the 4f14 (Yb2+) signal to the Fermi level.
Investigation of the chemical bonding of Cr and Ti to silicon nitride
Authors: Orent T. W. , Wagner R. A.
Published: July 2002

The adhesion characteristics of rf-sputtered Cr/Cu and Ti/Cu films to polished silicon nitride substrates were studied. As-deposited samples showed lower rupture strengths than Cr/Cu on unpolished alumina, but Ti samples that had been annealed above 500 °C and Cr films on substrates that had been prefired in air had higher rupture strengths than Cr/Al2O3. The compositions of the metal/substrate interfacial regions were studied by Auger depth profiling and by x-ray photoelectron spectroscopy, and the results were correlated with the adhesion measurements. Chromium did not react extensively with the substrate during deposition, but it did react with the silicon oxynitride surface layer formed during prefiring. On the other hand, Ti reduced the silicon nitride during deposition to produce TiN and free Si. Above 500 °C the free silicon reacted with additional Ti at the Ti/TiN interface to form a silicide.
Plasma enhanced beam deposition of thin films at low temperatures
Authors: Chang R. P. H. , Darack S. , Lane E. , Chang C. C. , Allara D. , Ong E.
Published: July 2002

A plasma enhanced beam deposition technique for thin films is discussed. It is shown that thin films of tailored stoichiometry or amorphous layers can be easily deposited in the temperature range (30–250 °C). The technique uses a combination of active atomic or molecular beams generated by charged particles or photons. Films of SiO2, Al2O3, ZrO3, silicon oxynitride, NbN, etc., have been deposited on metals, semiconductors, and insulators. The interfaces between the deposited films and the substrates are extremely sharp and no native growth of oxides of nitrides occurred on the substrate surfaces during film deposition. Film thickness and composition can be precisely controlled by optical monitoring techniques. For instance, the physical properties of the deposited SiO2 at 100 °C is nearly identical to that of thermal oxides grown on Si at 1100 °C. The deposited SiO2 has an electrostatic breakdown field strength of about 5×106 V/cm, and 1 MHz C–V curves show a hysteresis of 50 mV at a sweep rate of 100 mV/s. The fixed charge density is 3.5×1011 cm−2. The advantages of this process for depositing Al2O3 on InP, GaAs, and Si are discussed. Utilizing the low temperature nature of the technique, patterns of μ-width SiO2 features have been made using photoresist masked substrates and the lift-off technique. Finally, it is proposed that epitaxial growth of compound films should also be possible under UHV conditions.
CF4/silicon surface reactions: Evidence for parallel etching mechanisms from modulated ion beam studies
Authors: McNevin S. C. , Becker G. E.
Published: July 2002

This paper reports the use of a modulated ion beam system to probe the reaction dynamics occurring in an ion/surface interaction which results in the removal of material from a solid. A modulated ion gun (15 μs rise time) was used to provide pulses of incident ions (primarily CF+3) at a Si surface. The resulting products were detected by a mass spectrometer and signals have been observed at mass 28 (Si), mass 47 (SiF), mass 66 (SiF2), and mass 85 (SiF3, principal cracking fraction of SiF4). A comparison of the amount of signal with the measured cracking pattern of SiF4 indicated that a substantial fraction of the Si left the surface with fewer than four fluorine atoms. By performing the experiments in two different sample positions, we have confirmed that complications due to collisions between the product molecules and the vacuum system walls have been eliminated. The time evolution of these product signals following the start of the ion beam pulse has been measured with signal averaging. The results are consistent with a model where mass 47 (SiF) and mass 66 (SiF2) are produced with ∼1 eV energy in a fast process. Roughly 70% of the Si is removed in this manner. In contrast, mass 28 (Si) and mass 85 (SiF3 and SiF4 cracking fraction) have an appreciably slower time dependence and a model which partially fits this observation assumes that these species are produced at the same time as SiF and SiF2 but have thermal (0.02 eV) energies and thus take longer to reach the mass spectrometer. The observed time dependence is even slower than this model predicts, however, which indicates the possibility of a surface residence time delay due to the formation of a surface complex prior to the desorption of the products.
Electron energy loss spectroscopy from GaAs(110) interfaces
Authors: Bonapace C. R. , Tu D. W. , Li K. , Kahn A.
Published: July 2002

Electron energy loss spectroscopy (EELS) is used to obtain direct information on the morphology of Al, In, Se, and Sb overlayers deposited on GaAs(110). We study particularly the 20.0 eV GaAs surface exciton (SE), a transition which involves the surface Ga empty dangling bond, and correlate its amplitude with the continuity of the deposited layer. We confirm the results obtained for these interfaces with Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) and demonstrate that the SE peak can be used as a highly sensitive indicator of effective surface coverage.
Unpinned Schottky barrier formation at metal–GaP interfaces: A representative III–V compound interface
Authors: Chiaradia P. , Brillson L. J. , Slade M. , Viturro R. E. , Kilday D. , Tache N. , Kelly M. , Margaritondo G.
Published: July 2002

Metal–semiconductor interfaces obtained by deposition of In, Al, Cu, Ge, and Au onto ultrahigh-vacuum cleaved GaP surfaces have been studied by photoemission with synchrotron radiation. The results indicate a wide range of Schottky barrier height. A plot of the Schottky barrier values versus metal work function reveals an almost ideal Schottky-like behavior. Among current alternative models of metal–semiconductor interfaces, only the effective-work-function model is compatible with the experimental data. The GaP data plus the absence of Fermi level pinning recently observed for In-based semiconductors indicate that GaP may be more representative than GaAs of metal/III–V compound interfaces in general.
A GaAs metal-semiconductor field-effect transistor with a mushroom gate fabricated by mixed exposure of focused ion beams
Authors: Morimoto H. , Onoda H. , Sasaki Y. , Mitsui Y. , Ishihara O. , Kato T.
Published: July 2002

The mixed exposure of 200 keV Be++ and 100 keV Be+ focused ion beams was applied to the fabrication of a mushroom gate of a GaAs metal-semiconductor field-effect transistor (MESFET). This method has the advantages of a high resolution and a simplified resist structure. The radiation damage was reduced by the use of RIE-assisted developing. The fabricated FET shows the low gate resistance, which is about 1/4 of the conventional FET.
Alignment errors from resist coating topography
Authors: Bobroff Norman , Rosenbluth Alan
Published: July 2002

The registration difficulty associated with a resist-coated alignment mark depends on the type of optics used to illuminate and view the wafer alignment aid. The interaction of the light with the mark geometry is studied by calculating the outgoing wave front that results when an arbitrary collection of correlated or uncorrelated incident wave fronts is reflected from the nonplanar target structure. This solution is combined with a model of the partially coherent imaging system to predict the alignment signal. Bright-field and dark-field systems typical of those used in optical lithography are studied. Dark-field viewing optics are generally less sensitive to offsets caused by resist coating asymmetries. The statistical alignment precision is related to the shape of the alignment signal and the signal-to-noise ratio. It is shown that the increased contrast in dark-field systems can provide an alignment precision comparable to that of bright-field systems despite a substantially lower photon collection efficiency.
Nanometer-scale structure for the study of flux-line lattice shearing in superconducting double layers of a-Nb3Ge and NbN
Authors: v. d. Drift E. , Radelaar S. , Pruymboom A. , Kes P. H.
Published: July 2002

The fabrication of a nanometer-scale device structure for the study of flux-line lattice shearing in a superconducting double layer of 500-nm amorphous Nb3Ge (bottom) and 50-nm polycrystalline NbN (top) is reported. The device, serving as a model system for technical type-II superconductors, consists of an array of channels in the NbN top layer, each about 100 nm wide and 500 μm long. The preliminary measurements show unique features of flux line shear in a well-defined channel structure.
Macromolecular self-organized assemblies
Authors: Deckman H. W. , Dunsmuir J. H. , Garoff S. , McHenry J. A. , Peiffer D. G.
Published: July 2002

Macromolecular assemblies can be self-organized into patterns with controllable feature sizes ranging from ∼10 Å to ∼100 μm. Smallest features are produced by organizing macromolecules into a Langmuir–Blodgett film. Using this method, we have produced quasiperiodic arrays of metal oxide islands with peroidicities as small as 9 Å. Larger (20–200 Å) extremely monodisperse features can be obtained by causing single polymer chains to self-organize into a coiled spherical ball on the surface of a substrate. By using narrow molecular weight distribution polymer chains, the standard deviation of the ball diameter can be made as small as ∼8 Å. The largest features produced come from self-organization in the assemblage of macromolecular colloidal particles (natural lithography). A wide variety of geometrical packing arrangements can be produced by controlling methods used to coat colloidal particles into a substrate. In particular, the use of surface textures to induce a form of graphoepitaxy in colloidal particle packing is discussed.
Characteristics of a virtual immersion lens spectrometer for electron beam testing
Authors: Aton T. , Garth S. C. J. , Sackett J. N. , Spicer D. F.
Published: July 2002

Magnetic ‘‘in-the-lens’’ secondary electron spectrometers have shown dramatic improvements over conventional spectrometers for high-resolution voltage measurements on very large scale integrated circuits. In particular, designs incorporating the electron straightening effect of an adiabatic magnetic field have demonstrated considerably improved rejection of local field effects on small geometry devices. A disadvantage of such spectrometers is the need to immerse the sample in a strong magnetic field. This can result in image distortion (due to ferromagnetic lead frame material in the sample) and surface-charging instability (due to localized charging from returned secondary electrons being refocused close to the emission point). We report on an improved spectrometer that maintains the advantages of the adiabatic straightening of the secondary electrons without the need to set the sample in a magnetic field. This ‘‘virtual’’ immersion lens design gives rise to a spectrometer with similar waveform measurement characteristics to the previous lens type but with greatly superior image resolution, field of view, and beam stability characteristics. We show it to be capable of 0.1-μ spatial resolution, voltage errors as small as 2% for 5-V signals on 1-μ lines and spaces, and a ‘‘detector coefficient’’ as good as 1.42 ×10−8 V(A/Hz)1/2. All of these are among the best reported so far in the electron-beam testing literature.
WSix refractory metallization for GaAs metal–semiconductor field-effect transistors
Authors: Lahav A. G. , Wu C. S. , Baiocchi F. A.
Published: July 2002

WSix thin films on GaAs substrates have been investigated for potential use as refractory gates for self-aligned metal–semiconductor field-effect transistor (MESFET) devices. Films with Si/W ratio in the range of 0.19 to 1.76 have been dc magnetron cosputtered. The deposition parameters were optimized to produce adherent films with low stress and minimal impurity content. In order to serve as a refractory gate, the WSix film should perform as a diffusion barrier. The WSi0.45 films satisfied this requirement by having the highest crystallization temperature of 875 °C. Such films remained amorphous following the high-temperature dopant activation annealing (800 °C), thus reducing Ga and As outdiffusion and preventing pit formation in GaAs under the gate. I–V and C–V measurements were used to characterize contact electrical properties such as barrier height, ideality factor, and carrier concentration. A simulated threshold voltage shift for MESFET structure was obtained by modeling diode results. The WSi0.45 films were characterized by the most stable Schottky contact to GaAs (φB =0.75 eV, n=1.1) and showed minimal shift in carrier concentration and threshold voltage after annealing at 800 °C. In addition, the effect of impurities on contact properties was investigated. The presence of Zn at the WSix/ GaAs interface caused drastic reduction in carrier concentration and increased Schottky barrier height up to 0.9 eV.
Properties of WSix using dichlorosilane in a single-wafer system
Authors: Wu T. H. Tom , Rosler Richard S. , Lamartine Bruce C. , Gregory Richard B. , Tompkins Harland G.
Published: July 2002

Chemical vapor deposition of tungsten silicide (WSix) from WF6 and SiH2Cl2 [JB Price, S. Wu, Y.Chow, and J. Mendonca, Semicon West (1986)] at higher deposition temperatures (450–650 °C) than the conventional WF6 and SiH4 (250–400 °C) process has been characterized using a plasma enhanced, single-wafer, cold-wall, radiantly heated system with temperature control utilizing a thermocouple in contact with the backside of the wafer. Film properties such as silicon to tungsten ratio, fluorine and chlorine concentration, resistivity, and film stress were studied as a function of substrate temperature, reactant composition, and flow rates. The film composition was measured by Rutherford backscattering spectrometry. The silicon to tungsten ratio is a function of deposition temperature at a fixed flow (x varying from 2.0–2.8 through the temperature range of 450–650 °C). The as-deposited resistivity is also a strong function of deposition temperature. The chlorine and fluorine distributions in the WSix film were measured using secondary ion mass spectrometry. The fluorine concentration was found to be much lower than levels reported by conventional WF6/SiH4 chemistry with as-deposited values of 9×1015 to 3×1018/cm3 compared to 1.3×1020 cm3 by M. Fukumoto and T. Ohzone [Appl. Phys. Lett. 50, 894 (1987)].
Scanning tunneling spectroscopy of oxygen adsorbates on the GaAs(110) surface
Authors: Stroscio Joseph A. , Feenstra R. M.
Published: July 2002

Spatially resolved measurements of the tunneling current versus applied voltage are obtained for oxygen adsorbed on the GaAs(110) surface, using a scanning tunneling microscopy (STM). Effects with different length scales are observed, arising from two sources of charge in the system: negatively charged adsorbates and a positively charged space charge layer. The space charge layer produces band bending on n-type material, which is observed as a shift in the onsets for tunneling out of and into the valence and conduction bands, respectively. We analyze these shifts using theoretical calculations of the tunneling current through surface space charge layers. Directly above an oxygen adsorbate on n-type material, an enhancement in tunneling is observed near the top of the valence band edge, while a corresponding decrease is observed near the bottom of the conduction band. This local behavior is identified as arising from changes is the surface density-of-states produced by the Coulomb potential of the charged adsorbate. On p-type material, no band bending is observed, and the STM images are indicative of neutral adsorbates. This difference between the n and p-type results is attributed to the existence of acceptor states in the band gap.
Surface kinetic considerations for molecular-beam epitaxy growth of high-quality inverted heterointerfaces
Authors: Newman P. G. , Cho N. M. , Kim D. J. , Madhukar A. , Smith D. D. , Aucoin T. R. , Iafrate G. J.
Published: July 2002

Realization of high-quality inverted interfaces has been thwarted by mutually competing surface kinetic requirements. We report a way out of this dilemma based upon reflection high-energy diffraction (RHEED) studies of GaAs/AlxGa1−x As(100) system. Liquid nitrogen mobilities comparable to good GaAs/Al0.3Ga0.7As normal heterojunctions at carrier concentrations ∼5×1011/cm2 have been realized in inverted heterojunctions grown on the basis of RHEED suggested conditions and procedures.
Effects of II–VI epitaxy on III–V surfaces: A study of ZnSe on GaAs
Authors: Olego D. J.
Published: July 2002

Raman scattering and photoluminescence spectroscopies were used in the investigation of the surface properties of GaAs on which ZnSe layers were grown by molecular-beam epitaxy. The results yielded by the optical techniques were correlated with the characterization of the interfaces between GaAs and ZnSe performed with transmission electron microscopy. Heterostructures with ZnSe layer thicknesses ranging from 50 to 5000 nm were investigated. The changes in the surface properties of GaAs were established by monitoring the Raman intensity of the longitudinal optical phonons in the space charge layer and the photoluminescence intensities across the E0 and E0+Δ0 gaps. For pseudomorphic growth (ZnSe layers thinner than ≊160 nm), surface barriers on the order of 0.1 eV and reduced recombination velocities are determined. Above this critical layer thickness misfit dislocations are generated at the interface and a gradual increase in the barrier height is realized. An empirical correlation can be drawn between the magnitude of the surface barrier, the surface recombination velocity and the pinning of the Fermi level in the forbidden gap and the areal density of the dangling bonds associated with the misfit dislocations.
Low-energy electron diffraction investigations of Si molecular-beam epitaxy onto Si(100)
Authors: Horn M. , Gotter U. , Henzler M.
Published: July 2002

The nucleation and growth of Si onto Si(100) has been studied during deposition in ultrahigh vacuum (UHV) by spot profile analysis of low-energy electron diffraction (SPA-LEED). The growth mode and surface atom arrangement have been determined not only by the intensity oscillation of the 00 beam during deposition but also after interruption and cooling by a detailed measurement of the energy dependence of the spot profile. We present two different new evaluation procedures to determine out of the structure factor G(0,K⊥) the layer distributions, i.e., the roughness of the surface, and deposited layers. During deposition a rough surface is always observed with a root mean square value (rms) of Δ=0.75 Å for the fist layer up to Δ=0.9 Å for the 50th layer. In contrast to Si(111), which starts growing islands with double step height (d=6.27 Å), for Si(100) it is found the minimum possible step height of a0/4=1.36 Å for neighboring terraces, i.e., a monoatomic growth mode. The resulting terrace width distribution of the islands on the surface is a geometric distribution with an average terrace length of 16 atoms and an island density of about 6×1011 per cm2 at 415 °C substrate temperature.
Molecular-beam epitaxial growth and characterization of GaAs on epitaxial CoSi2 films on Si(111)
Authors: Chand N. , Phillips J. M. , Lunardi L. M. , Chu S. N. G. , Wecht K. W. , People R.
Published: July 2002

Using molecular-beam epitaxy, we have studied growth of GaAs on epitaxial CoSi2 films on Si(111) substrates. The surface morphology of 1-μm-thick layers was comparable to GaAs grown on GaAs(111) under similar conditions. Also, the quality of GaAs was, in general, a little better on CoSi2 than on Si(111). Secondary ion mass spectroscopy and transmission electron microscopy (TEM) indicated abrupt GaAs/CoSi2 and CoSi2/Si interfaces, suggesting excellent thermal stability of CoSi2. TEM revealed that the defect structure in GaAs was dominated by microtwin plates generated at the interface. The increase in pinhole size in CoSi2 was drastically reduced if the substrate was heated in arsenic flux. The GaAs surface was found to be terminated with As atoms. The photoluminescence intensity of GaAs/CoSi2 was only a factor of 2 lower than that of GaAs/GaAs. The measured Rutherford backscattering spectroscopy minimum channeling yield (χmin) was 0.094. GaAs/CoSi2/Si transistorlike devices were made, but no transistor action was observed due to poor interfacial quality of the GaAs/CoSi2 junction. The ideality factor and barrier heights were 2.5 and 0.45 eV for the GaAs/CoSi2 junction, and 1.25 and 0.635 eV for the CoSi2/Si junction.
Intrawell exciton transport in monolayer-flat GaAs/AlGaAs single quantum wells grown by molecular-beam epitaxy
Authors: Tu C. W. , Miller R. C. , Petroff P. M. , Kopf R. F. , Deveaud B. , Damen T. C. , Shah J.
Published: July 2002

During molecular-beam epitaxy, growth interruption at each heterointerface of single quantum wells results in very narrow multiple peaks in photoluminescence (PL) spectra. These peaks correspond to extended monolayer-flat regions of the wells, differing in well thickness by one monolayer. PL and PL-excitation spectroscopies show that excitons tend to diffuse from narrower-well regions to thicker-well regions before radiative recombination takes place. This is also observed in time-resolved PL. The intensity of the higher-energy peak decreases with time, while that of the lower-energy peak increases with time. The obtained exciton diffusion time of ≂250 ps is in good agreement with the estimated size (≤1 μm)of the monolayer-flat islands from spectrally resolved cathodoluminescence.
Reaction probability for the spontaneous etching of silicon by CF3 free radicals
Authors: Robertson Robert M. , Golden David M. , Rossi Michel J.
Published: July 2002

The spontaneous thermal etching of silicon by CF3 free radicals has been studied in a very-low-pressure photolysis reactor. The radical is produced by infrared multiphoton dissociation of either hexafluoracetone or CF3 I, and is allowed to react with a temperature-controlled silicon sample (560–745 K). Mass spectrometry is used to measure the extent of dissociation of the precursor gas and the formation of product molecules, C2 F6 and SiF4 . The etch rate of the silicon is determined from the SiF4 production. Resonance-enhanced multiphoton ionization of CF3 is used to determine the density and time history of the radical in the reactor. The measurements of the etch rate and CF3 density are combined to derive the reaction probability. CF3 etches silicon much more slowly than F atoms and at a rate comparable to molecular F2 . A carbon layer, that is deposited on the silicon by the radicals, inhibits, but does not stop, further etching. Experiments on the etching of silicon by F2 were performed both to validate the reactor design and to prepare the silicon surface for the CF3 studies.
Controlled-ambient photolithography of polysilane resists at 193 nm
Authors: Kunz R. R. , Rothschild M. , Ehrlich D. J. , Sawan S. P. , Tsai Y. G.
Published: July 2002

Various polysilanes have been examined as self-developed photoresists at 193 nm and the benefits of imaging these materials in controlled, reactive ambients have been determined. Plasticization of the irradiated polymer due to reaction with oxygen is avoided when dry, oxygen-free ambients are employed. In addition, the redeposition of silicon from the ablated polysilane, which is prevalent in a one-pulse imaging mode, is greatly reduced when NF3 or CF2Cl2 are used as exposure ambients. Laser desorption mass spectrometry indicates that thermally activated decomposition of the polysilanes at fluences >100 mJ/cm2 causes more extensive fragmentation of the desorbed molecules. This results in the formation of higher volatility products upon halogenation.
Kinetic theory of bombardment induced interface evolution
Authors: Jurgensen Charles W. , Shaqfeh Eric S. G.
Published: July 2002

An interface evolution equation has been formulated to describe bombardment-induced etching by an axisymmetric angular distribution of energetic particles where the yield per incident particle is assumed to be a function of its energy and its angle relative to the surface normal. These assumptions result in a nonlinear integro differential equation, but this equation reduces to a partial differential equation in several important special cases. At points that are not shadowed by a remote part of the surface, the interface evolution equation reduces to a nonlinear hyperbolic conservation law. Such equations have been applied to bombardment-induced etching by a monodirectional beam with angle-dependent yields; however, this form of equation applies more generally to raised isolated convex regions (e.g., etching masks) regardless of the angular distribution of the incident particles or the angle dependence of the yield. The essential qualitative feature of the solution in these cases is the spontaneous evolution of facet edges (slope discontinuities) from smooth initial conditions. Shadowing by remote parts of the surface may occur in concave regions (e.g., trenches) where it results in proximity effects.
Ballistic electron emission microscopy and spectroscopy of Au/GaAs interfaces
Authors: Kaiser W. J. , Bell L. D. , Hecht M. H. , Grunthaner F. J.
Published: July 2002

The Au/GaAs interface is currently the subject of much investigation. However, a complete understanding of this interface, and the ability to precisely control Au/GaAs interface properties, is still lacking. Previous work has focused primarily on interfaces prepared on the GaAs(110) cleaved surface and on the chemically or thermally prepared GaAs(100) surface. This paper presents the first Schottky barrier results for the Au/GaAs(100) interface prepared completely in situ on GaAs grown by molecular-beam epitaxy. The resulting interface displays unexpected properties which can be interpreted in terms of enhanced electrode interdiffusion. In addition, the capability of molecular-beam epitaxy for in situ processing enables the stabilization of this interface against diffusion, and allows the formation of a Au/GaAs system with nearly ideal properties. Newly developed ballistic electron spectroscopy and imaging techniques demonstrate that the heterogeneity present at the interface of Au/GaAs(100) fabricated on chemically treated GaAs substrates is removed.
Dynamics of growth roughening and smoothening on Ge (001)
Authors: Chason E. , Tsao J. Y. , Horn K. M. , Picraux S. T.
Published: July 2002

We present reflection high-energy electron diffraction measurements of the evolution of surface morphology during molecular-beam epitaxy of Ge on Ge(001) and subsequent annealing. We find that there is a critical ‘‘kinetic roughening’’ temperature (375 °C) above which a smooth surface remains smooth during growth, but below which it roughens during growth. Surprisingly, smooth starting surfaces never appear to roughen without bound, but reach steady-state roughnesses which depend on temperature and deposition rate. The results can be fit empirically with simple phenomenological equations based on a competition between growth roughening and growth smoothening of a ‘‘pseudo-statistical’’ surface. Furthermore, growth-roughened surfaces tend to smoothen, after growth, at a rate consistent with a third-order power-law ripening mechanism.
Assessing thermal Cl2 etching and regrowth as methods for surface passivation
Authors: Clausen E. M. , Harbison J. P. , Florez L. T. , Van der Gaag B.
Published: July 2002

The damage layer that results from ion beam processing ultimately limits the smallest size structure which will emit light and hence prevents the realization of low-dimensional quantum size effects. Effective surface passivation of etched structures requires both layer removal and compensation of surface states. Ultimately these two processes must be made compatible. To address this problem, we have coupled an ultrahigh-vacuum (UHV) dry-etching system to a molecular beam epitaxy (MBE) growth chamber. Material is patterned and masked outside of the combined etching/regrowth system using electron beam lithography. Samples are then loaded into the combined UHV system and vertical structures are produced by chemically assisted ion beam etching (CAIBE). These structures are then subjected to a thermal Cl2 gas etch to remove the damage layer created by the ion beam etching. Samples are subsequently transferred directly into the MBE growth chamber, where a cladding layer of Al0.3Ga0.7As is grown over the etched structure to provide passivation. In this way both the damage removal and surface passivation are achieved in-situ without the deleterious effects caused by exposure to air. We find, by low temperature cathodoluminescence, that submicrometer structures subjected to this in-situ processing display fivefold improved luminescence efficiencies. Structures etched and regrown without the thermal Cl2 etching step show no comparable improvement, indicating the damage removal is the key to improved efficiencies.
Improved resolution of an i-line stepper using a phase-shifting mask
Authors: Terasawa Tsuneo , Hasegawa Norio , Tanaka Toshihiko , Katagiri Souichi , Kurosaki Toshiei
Published: July 2002

Improved resolution of an available i-line (365 nm) stepper using a phase-shifting mask is discussed. The resolution investigated here is not only for periodic lines but also for isolated spaces and hole patterns. To reduce the sizes of isolated space images for printing fine single spaces on a wafer, two additional line apertures with widths smaller than the critical dimension of the stepper lens are placed on each side of the main aperture of the mask. The optical phase of light passing through the main aperture and those through additional apertures are opposite. The additional apertures play a role in reducing the bright feature size to less than the line spread function of the lens. Similarly, printing a fine hole is accomplished by using a main aperture surrounded by four additional apertures. The intensity distribution on the wafer surface is simulated by comparing the images obtained with a phase-shifting mask and those obtained with a conventional transmission mask. Printing fine patterns are performed using an i-line stepper with a numerical aperture (NA) of 0.42. A pattern of 0.3 μm lines and spaces, 0.3 μm isolated spaces, and 0.4 μm hole patterns are resolved using the phase-shifting mask. The process latitudes and the effects of variations in the optical phase of the additional apertures are also investigated. The image simulations and experimental results suggest that the phase-shifting mask improve not only resolution but also exposure latitude and focus latitude. Furthermore, it is found that it is possible to control the position of the best focal plane by changing the optical phases of the additional apertures.
Electronic structure of a quasi-one-dimensional electron gas at finite temperature
Authors: Yu Hong , Hermanson J. C.
Published: July 2002

Self-consistent calculations of the subband electronic structure of semiconducting quantum wires are reported. The sample parameters are chosen to match the GaAs/AlGaAs heterostructure recently fabricated by molecular-beam epitaxy and ion milling by Weiner et al. The Schrödinger and Poisson equations are simultaneously solved in a rectangular waveguide geometry, using Fermi statistics for the electrons at finite temperature. Calculated energy levels, wave functions, charge density, and potential differ from the predictions of the simple square well and harmonic oscillator models because of the influence of accumulation and depletion regions. The subband energies depend on the free-carrier density and the temperature as well as the band bending. We find that the quasi-one-dimensional electron gas occupies three or four subbands whose spacings are in good agreement with optical experiments, when certain depolarization shifts of the resonance frequencies are included.
Surface dielectric functions of (2×1) and (1×2) reconstructions of (001) GaAs surfaces
Authors: Chang Yia-Chung , Aspnes D. E.
Published: July 2002

Theoretical calculations of the surface dielectric function of (001) GaAs with As- and Ga-terminated (2×1) reconstructions are presented. An empirical nearest-neighbor tight-binding model with six orbitals (s,x,y,x2−y2,3z2−r2) per atom is used. The calculated dielectric functions anisotropies for GaAs (2×1) surfaces are analyzed with reference to the electronic structures. The theoretical results are in qualitative agreement with present data, and allow structures observed at 1.8, 2.6, and 4.1 eV in the surface dielectric anisotropy to be related to transitions from Ga bonding dimer to Ga dangling bond, As lone pair to As antibonding dimer, and As bonding dimer to As antibonding dimer transitions, respectively.
Empirical interatomic potentials for compound semiconductors: Application to superlattice stability
Authors: Khor K. E. , Ito Tomonori , Das Sarma S.
Published: July 2002

We present the first systematic attempt to develop an empirical potential for compound semiconductors. Based on our earlier theoretical work on a universal interatomic potential for semiconductors, we obtain an explicit many-body interatomic potential that employs very few parameters. We confirm the validity of our potentials by calculating elastic constants and cohesive energies (as functions of atomic volumes and displacements), showing that our calculated results compare very well with the available experimental data and ab initio calculations. We apply our empirical potentials to the calculation of excess energies, equilibrium structural, and lattice parameters for several different monolayer superlattices (e.g., AlAs–GaAs, GaAs–InAs, GaP–InP, and GaAs–GaSb) getting very good agreement with the available first principles calculational and experimental results. Finally, we report on molecular dynamics simulations of various III–V defects using our potentials.