bluuebunny/crossref_metadata_2025 huggingface dataset:
Morphological defects arising during MBE growth of GaAs
Authors: Bachrach R. Z. , Krusor B. S.
Published: July 2002

The growth by molecular beam epitaxy of GaAs and GaAlAs is reviewed, with special attention given to morphological defects which can occur. Although the allowed stoichiometric growth regime for this system is quite wide, more restricted conditions are required when overall perfection is important. The paper characterizes the growth process within the framework of achieving material suitable for GaAs–GaAlAs double heterostructure lasers. This includes substrate preparation, growth initiation, and growth evolution. Specific morphological defects which are found to be observed quite generally are characterized and some explanations will be ventured. Achieving featureless growths is important for integrated optic devices and this is possible over large area substrates.
Effects of the environment on point-defect energy levels in semiconductors
Authors: Dow J.D. , Allen R. E. , Sankey O. F. , Buisson J. P. , Hjalmarson H. P.
Published: July 2002

Deep impurity energy levels within the band gap of a semiconductor can be altered and manipulated by changing the environment of the impurity. The effects of a second impurity, an interface, and a surface have been evaluated for substitutional deep levels in a variety of semiconductor hosts.
Reactive interdiffusion and electronic barriers at metal–CdS and metal–CdSe interfaces: Control of Schottky barrier height using reactive interlayers
Authors: Brucker C. F. , Brillson L. J.
Published: July 2002

Interfaces between UHV-cleaved CdS and CdSe (101̄0) surfaces and metals spanning a wide range of chemical reactivity have been investigated using UPS and XPS. We have also probed the formation of Schottky barriers via J-V and C-V analysis, thus enabling a correlation of measured electrical transport properties with observed features of chemical reaction and interdiffusion. The experimental results reveal a systematic relationship between contact barrier heights and certain critical aspects of the reacted interface region, such as the stoichiometry of semiconductor outdiffusion and the extent of Cd cation dissociation from the reacted interface region. The use of reactive Al interlayers at noble metal contacts to control these aspects of contact formation, and hence the consequent Schottky barrier height, is demonstrated.
Refractory metal silicides for VLSI applications
Authors: Sinha A. K.
Published: July 2002

Results of a recent program undertaken at Bell Laboratories on silicide gate technology are reviewed with particular reference to the TaSi2/n+ poly-Si structure. This high-conductivity refractory gate structure greatly alleviates the layout and routing problems due to large RC time constants of fine-line poly-Si interconnects. This review contains a discussion of the metallurgical effects, associated with sintering metal and metal–silicon alloy films on polycrystalline silicon, oxidation behavior of silicides, their MOS (process) compatibility, the generic reliability, and trade-offs involved in the selection of a silicide from the group TiSi2, TaSi2, MoSi2 and WSi2.
Control of variable beam trapezoid delineator (VL–R2)
Authors: Sumi M. , Chiba F. , Wada H.
Published: July 2002

A new variable shaped beam system has been developed for high speed delineation of LSI patterns composed of polygons. The slant pattern data format, the pattern generating circuit, and the electron optics are presented. To test the capability, an actual 16 K RAM pattern was delineated at ×1/4∠×8 scales. The average beam size (width) was 2.44 μm for the ×1 scale. The equivalent writing speed was 780 Mbit/s.
Failure analysis of LSI using a scanning low-energy electron probe
Authors: Mizuno F. , Mori S. , Satoh K. , Kanda K. , Todokoro H.
Published: July 2002

The voltage contrast mode of SEM (scanning electron microscope) operation is a powerful diagnostic tool of LSI devices which have very small geometry of metal wirings and transistors. However, a conventional SEM uses a high energy electron probe (10 to 30 keV) which gives rise to radiation damage and charging effects to the LSI devices under investigation. We developed a new SEM which uses a low energy electron probe (0.5 to 7 keV) to overcome these problems. The instrument uses a field emission gun to minimize the deterioration of spatial resolution at low beam energy, and has the facility of mounting a probe card to energize an LSI device on a silicon wafer. Power sources and input signals to energize the LSI device under investigation are controlled by a dedicated computer. The SEM has a voltage resolution of about 0.5 V and a spatial resolution of about 0.05 μm with the probe current of 0.05 nA at 1 keV. We have analyzed many failures of LSI memory and logic circuits with several hundred gates by utilizing the SEM. The instrument can easily identify disconnects and bad voltage levels in LSI devices, while other methods (optical microscope observations, mechanical probings, and higher energy electron probes) cannot identify them. Moreover, the SEM is also useful for other devices such as printed circuit boards where wide metal wirings are covered with very thick insulator films (50 μm thick).
Fabrication of polyimide masks for x-ray lithography
Authors: Gong B. M. , Ye Y. D.
Published: July 2002

The dimensional stability of a polyimide membrane is a key to the successful fabrication of x-ray masks. It depends upon fractionation, purification, and the degradation prevention of polyamic acid. The polyimide membranes thus obtained have a great tensile stress so that the membrane flatness and distortion in a 30×40 mm rectangular window area are less than 1 μm and 0.2 μm, respectively. The advantages of using a polished glass plate as a substrate are good flatness, low cost, ease of fabrication, and also that the masks’ dimensions can be made large enough at will.
A model for optimizing CPU time for proximity correction
Authors: Molzen W. W. , Grobman W. D.
Published: July 2002

Future device technologies will call for the use of very dense and pseudorandom logic patterns. Microfabrication techniques will have to cope with a large number of different designs which are essentially nonrepetitive (e.g., logic). Electron beam technology will also have to be concerned with the large amounts of CPU time it will take to process these designs, especially when considering correction for the scattering of electrons in the resist and substrate (the proximity effect). Proximity correction is divided into the partitioning of design shapes and the dose correction of the resultant shapes. For partitioning, we have evolved a scheme which separates the center from the edges of designed shapes. The centers are given a fixed dose increase of electron exposure to ensure their development and the edges are sent to the proximity correction routine. This will save CPU time for a design which has an abundance of large shape patterns. We have also investigated tradeoffs between the CPU time it takes to set up and solve the set of linear equations for dose correction, and the size of the groups of pattern shapes which are dose corrected at a given time. Since the CPU time needed to solve the matrix of linear equations goes up approximately as the cube of the number of shapes in a group, it is advantageous to know the relationship of group size to total CPU time. For too small groups, the calculations are dominated by overheads incurred in handling the large number of such groups (and their boundaries) comprising the pattern. For large groups, linear equation solution time becomes too large. We develop a polynomial model which chooses the group size for a particular design in order to minimize the total CPU time. Once this is accomplished, the CPU time will become proportional not to a power of ’’N’’ (number of shapes in a chip), but only to ’’N’’ itself.
A multilayer, high resolution, ion-bombardment-tolerant electron resist system
Authors: Hunt B. D. , Buhrman R. A.
Published: July 2002

A multilayer, high resolution electron resist system, which withstands ion bombardment, has been developed. This system consists of four layers which are, from top to bottom: AZ1350B, a thin metal interlayer, PMMA, and a copolymer of PMMA. The bottom two layers define the actual pattern dimensions. Two independent developers have been chosen for these two layers in order to obtain controllably undercut resist profiles ideal for liftoff applications, while maintaining high resolution in the upper PMMA layer. The top two layers of the four-level system serve to provide a protective metal coating which prevents crosslinking of the underlying polymer layer. This allows processing involving ion bombardment, such as ion milling or reactive ion etching. Without this protective metal layer, difficulty is often encountered in liftoff processing after ion bombardment, due to the presence of a thin crosslinked polymer layer which resists solvent penetration. This resist system has been used in conjunction with reactive ion beam oxidation to fabricate high quality, small area, niobium–lead alloy tunnel junctions in an edge geometry. Using a standard Cambridge EBMF-2 microfabricator, junctions with linewidths as small as 0.25 μm have been produced. With the edge geometry, this corresponds to junction areas smaller than 4×10−10 cm2.
Reactive ion etching of aluminum using SiCl4
Authors: Sato M. , Nakamura H.
Published: July 2002

Reactive ion etching of aluminum using SiCl4 is described and compared with that using CCl4. In SiCl4 plasmas, a high aluminum etch rate is maintained which is slightly dependent on gas pressure in the range of 2 to 15 Pa. However, the etch rates of other materials, such as SiO2, Si3N4, polycrystalline silicon, and photoresist decrease with an increase of pressure. This results in high aluminum etching selectivity to other materials at a relatively high gas pressure. On the other hand, when using CCl4 for the etching gas, aluminum etching is abruptly suppressed at gas pressures higher than 5 Pa because of carbonic film deposition on the surface, which limits the etching selectivity. Auger electron spectra indicate that the surface after etching in SiCl4 plasma is almost free from any residual contamination. At a relatively high gas pressure, greater than 3 Pa, the etched figure becomes somewhat isotropic when plasma CVD Si3N4 films are used for etching masks. However, using organic resist masks, lateral etching under the mask is significantly suppressed. As a result, no side etching profile is achieved at a higher gas pressure such as 8 Pa, where high selectivity can be obtained. These characteristics of reactive ion etching using SiCl4 lead to a highly directional and selective aluminum etching method which is free from residual contamination.
Recent developments in electron and photon stimulated desorption
Authors: Menzel D.
Published: July 2002

The realization by Knotek and Feibelman that core ionization can be an important primary mechanism in electron stimulated ionic desorption (ESID) from oxides, and the availability of synchrotron radiation as excitation source for detailed photon stimulated ionic desorption (PSID) studies, have stimulated a number of interesting experiments in the last two years. It is shown that—besides direct valence excitation—adsorbate core ionization is a new channel for ESID and PSID from covalent adsorbates on metals also, while metal core ionization rarely or never causes desorption in such systems. However, detailed threshold studies and comparison to Auger yields show that this cannot be simply explained by treating the core ionization/Auger decay sequence as a composite Franck–Condon transition to multiply valence ionized states. Rather, strong influences of many-body interactions (e.g., shake ups) exist. This is not the case for ionic surface species; in these cases PSID can be used as species-specific EXAFS probe. The investigation of desorption of neutrals, which constitute the overwhelming majority of products at low energies, shows that neutrals can be formed by reneutralization of ions. Additional channels exist below the absolute ionic thresholds; core ionization is not an important primary channel for neutrals production. The consequences of these findings are discussed in context with existing models.
Development of steps on GaAs during molecular beam epitaxy
Authors: Van Hove J. M. , Cohen P. I.
Published: July 2002

We have used reflection high-energy electron diffraction (RHEED) to study the epitaxial growth of GaAs on GaAs (001) and GaAs cut 5 ° off the (001) towards the (11̄0). Ordered arrays of steps were observed on both the singular and vicinal surfaces. The surfaces were prepared by chemically etching and then heating to 900 K to drive off the oxide. The evolution of the RHEED pattern was then followed for growth conditions given with respect to a nonequilibrium phase diagram showing the various surface reconstructions we observe for GaAs (001). Specifying growth conditions in this fashion eliminates instrumental uncertainties. The utility of RHEED for the determination of the step distribution is described and demonstrated. On the high index surface we observe an ordering of a staircase step distribution during growth. We conclude that there is an effective repulsive interaction between steps. On the singular (001) surface we observe a squarewave-like step distribution during growth that has the same periodicity as the four-fold reconstruction. When growth is halted, the squarewave distribution disapppears even though the reconstruction remains. The results are analyzed in terms of current theories that view streak formation in RHEED to be a consequence of finite domain size or thermal diffuse scattering.
Influence of interface composition on Al–GaAs Schottky barriers
Authors: Schwartz G. P. , Gualtieri G. J.
Published: July 2002

Raman scattering and electrical measurements have been performed in order to characterize Al–GaAs structures with abrupt, graded (AlxGa1−xAs), and AlAs-rich interfacial layers. Barrier heights to abrupt and compositionally graded interfaces were equal within experimental error with a value of 0.70 eV. No increase in the barrier height was observed as might have been anticipated for AlxGa1−xAs interposed between the substrate and the metal. Structures with AlAs-rich interfaces showed an increase in barrier height to 0.80 eV, consistent with recent MBE data on Al–AlAs barriers. Reaction-induced substrate doping was observed, but the resultant doping appears to have little effect on the barrier height for Al on (100) GaAs.
Peroxide etch chemistry on 〈100〉In0.53Ga0.47As
Authors: Aspnes D. E. , Stocker H. J.
Published: July 2002

We investigate reactions of peroxide etches on 〈100〉 surfaces of In0.53Ga0.47As by measuring the pseudodielectric function 〈ε〉 with spectroscopic ellipsometry. By comparing these data to calculations in the Bruggeman effective medium approximation, we find that residual 10–80 Å layers of low density oxide, amorphous arsenic, or mixed oxide–arsenic compositions are formed depending primarily on solution pH. The chemical identity of these overlayers is in accord with their known solubility dependence on pH. Etch compositions previously shown to reduce reverse- leakage currents in In0.53Ga0.47As photodiodes correspond to those for which arsenic precipitates at the surface.
Structural studies of Schottky barrier formation by means of surface EXAFS: Pd and Ag on Si(111) 7×7
Authors: Stöhr J. , Jaeger R.
Published: July 2002

The initial stages of Schottky barrier formation for Pd and Ag on Si(111) 7×7 have been investigated by means of surface extended x-ray absorption fine structure (SEXAFS) measurements above the metal L2 absorption edges. For Pd deposition at room temperature (RT) and 1.5 monolayer coverage the local structure around the Pd atoms is found to closely resemble that of a thick (∠300 Å) Pd2Si layer grown on Si(111). The Ag/Si system exhibits a distinctly different interface formation. At RT and around 2.5 monolayers Ag coverage the SEXAFS spectrum is found to be identical to Ag metal. Upon heating to 500 °C a √3×√3 LEED pattern is observed and the polarization dependent SEXAFS signal reveals that the LEED pattern is due to Ag atoms in a threefold site on Si(111) with a Ag–Si distance of 2.45±0.05 Å. At higher Ag coverage the periodic √3×√3 Ag sites on the annealed (500 °C) surface are accompanied by large Ag clusters. The derived structural models are discussed in comparison with various models suggested previously.
Chemical bonding at the Si–metal interface: Si–Ni and Si–Cr
Authors: Franciosi A. , Weaver J. H. , O’Neill D. G. , Chabal Y. , Rowe J. E. , Poate J. M. , Bisi O. , Calandra C.
Published: July 2002

Chemical bonding at the interface of a near-noble-metal (Ni) and a transition metal (Cr) with Si is examined through synchrotron radiation photoelectron spectroscopy studies of in situ formed interfaces, of cleaved bulk silicides, and of disordered surfaces prepared by sputter etching of the silicides. Interpretation of these experimental results is guided by parallel linear combination of atomic orbitals (LCAO) (extended Huckel approximation) calculations of stoichiometric Ni and Cr silicides.
Effects of argon pressure on the structure and properties of dc planar-magnetron-sputtered metal and semiconductor films with implications for solar energy applications
Authors: Craig S. , Harding G. L.
Published: July 2002

The influence of argon gas pressure (0.15–40 Pa) on the microstructure of dc planar-magnetron-sputtered stainless steel and reactively sputtered carbon and silicon films has been investigated for films of thickness ∠1 μm. The carbon and silicon films are amorphous and incorporate hydrogen. Classification of the film structures is in accordance with the general zone model proposed by Thornton, although the voided boundaries characteristic of zone I columnar structure which are observed for stainless steel films do not develop for the carbon and silicon films. The implications of film structure for potential solar energy applications are discussed.
Si incorporation in AlxGa1−xAs grown by molecular beam epitaxy
Authors: Drummond T. J. , Lyons W. G. , Fischer R. , Thorne R. E. , Morkoç H. , Hopkins C. G. , Evans C. A.
Published: July 2002

The incorporation of Si in Al0.25Ga0.75As was studied as a function of Si cell temperature and substrate growth temperature. Within experimental error, the measured electron concentration was equal to the Si atomic concentration as determined by secondary ion mass spectroscopy. For substrate growth temperatures much above 650 °C, the Si concentration decreased with increasing substrate temperature. This is attributed to a possible decrease in the effective group V/III ratio and evaporation of Si. For Si cell temperatures above 1150 °C, stacking faults in the surface morphology indicated the initiation of Si precipitation. Electron mobilities at room temperature were between 1300 and 600 cm2/Vs, decreasing at higher doping levels. The donor activation energy was observed to increase as a function of both Si cell temperature and substrate growth temperature.
Influence of buffer thickness on the performance of GaAs field effect transistors prepared by molecular beam epitaxy
Authors: Su S. L. , Thorne R. E. , Fischer R. , Lyons W. G. , Morkoç H.
Published: July 2002

The influence of buffer thickness on the performance of GaAs metal semiconductor field effect transistors prepared by molecular beam epitaxy is investigated. Little or no looping or light dependence and good transconductances are obtained for devices with buffer thicknesses of 1 μm and greater. For maximum drain saturation resistance and minimum molecular beam epitaxy growth time, buffer thicknesses of 1 to 1.5 μm are indicated. The dependence of field effect transistor performance, as well as observed backgating effects on buffer thickness are attributed to improvements in the quality of both the buffer and active layers. In addition, preliminary results indicate that the saturation characteristics can be improved and looping reduced if the layers are grown at higher temperatures.
CO interactions with copper-covered Ru(0001)
Authors: Bader S. D. , Richter L. , Cao P.-L. , Ellis D. E. , Freeman A. J.
Published: July 2002

Photoemission-thermal desorption studies for saturation coverage CO on submonolayer Cu-covered Ru(0001), and self-consistent total-energy calculations for CO bound to small-cluster analogs of Cu(111), Ru(0001), and Cu/Ru(0001) chemisorption geometries are reported. New Cu-perturbed Ru binding sites are observed experimentally with (∼25%) reduced activation energy for desorption and increased molecular photoelectron binding energies (∼0.3 eV) relative to CO/Ru(0001). The mixed-metal cluster calculation provides a description of a possible precursor intermediate state for CO desorption, in qualitative accord with experiment.
Contact resistance: Al and Al–Si to diffused N+ and P+ silicon
Authors: Faith T. J. , Irven R. S. , Plante S. K. , O’Neill J. J.
Published: July 2002

Prealloy and postalloy contact resistances at [(Al–Si)/bulk (100)Si] interfaces have been measured for a range of Al–Si alloy compositions (0–2 at.% Si), alloying temperatures (325–550 °C), and contact sizes (3–5 μm). Contact doping for n+/p junctions was by diffusion from a POCl3 source; for p+/n junctions, by diffusion from a BN source. Al–Si deposition was by S–Gun■ magnetron sputtering. Statistical distributions of prealloy contact resistance to n+ Si were found to provide highly sensitive monitors of contact cut and deposition-system cleanliness.. Minimum postalloy contact resistance values were independent of Al–Si alloy composition, and silicon pitting in the contact cut was not required to achieve good Ohmic contact. The maintenance of consistent, low-resistance Ohmic contacts throughout postalloy processing was found to depend on the silicon concentration in the aluminum being high enough to keep the aluminum supersaturated during all postdeposition temperature cycling.
Lattice defects in (Hg,Cd)Te: Investigations of their nature and evolution
Authors: Schaake H. F. , Tregilgas J. H. , Lewis A. J. , Everett P. M.
Published: July 2002

Brief summaries of the precipitation of tellurium in (Hg,Cd)Te during the quench from the recrystallization anneal of the solid state recrystallization process and the subsequent dissolution of these precipitates during the postanneal are given. It is shown that if vacancies and interstitials are considered to be the only important point defects in the lattice, the observed mechanisms of precipitation and annihilation require that tellurium in excess of stoichiometry must be accommodated by metal vacancies. Diffusion from the mercury ambient during postanneal then must occur by metal interstitials. Enhanced self-diffusion on the metal sublattice is shown to occur along grain and subgrain boundaries. The most significant source of dark current in CID imagers fabricated on (Hg,Cd)Te is shown to be a sufficiently high dislocation density in a pixel.
Growth of CdTe films on alternative substrates by molecular beam epitaxy
Authors: Bicknell R. N. , Myers T. H. , Schetzina J. F.
Published: July 2002

Growth of epitaxial CdTe films on silicon and sapphire substrates is discussed. The quality of the epitaxial CdTe films was determined from x-ray diffraction, UV reflectance, and photoluminescence measurements. CdTe epitaxy was achieved on both silicon and sapphire substrates using growth rates of 1–3 μm/h and substrate temperatures of 200–400 °C. The CdTe/sapphire epilayers, in particular, exhibit exceptional properties including a large photoluminescence at 77 °K, a large photoconductivity at room temperature, majority carrier (electron) lifetimes greater than 10−5 s at 300 K, and electron mobilities greater than 900 cm2/V s at room temperature. These mobilities are the largest ever reported for CdTe films grown by any technique. On the basis of these results, CdTe/sapphire epilayers appear to offer a viable alternative to bulk CdTe substrates for growth of HgCdTe, an important infrared detector material.
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u cleaning of nickel field-ion surfaces by neon ion bombardment
Authors: Kellogg G. L.
Published: July 2002

Neon ion bombardment followed by field evaporation is shown to be a simple, effective procedure for cleaning the surface of nickel field-ion samples. The neon ions are generated by reversing the tip polarity and establishing a field emission current in the presence of the neon imaging gas. The procedure removes surface impurities which are resistant to field desorption at the evaporation field of nickel. The experimental conditions required to clean a nickel surface are presented.
GaAs(110)–In: The black sheep in a well-behaved interface family
Authors: Daniels R. R. , Zhao Te-Xiu , Margaritondo G.
Published: July 2002

Photoemission results on the interface formation process for In on cleaved GaAs(110) are completely at odds with respect to those on other III–V/metal interfaces. We do not observe the transition between an ultralow coverage (<0.1 monolayer) ‘‘chemisorption’’ stage and an intermediate coverage (0.1–2 monolayer) ‘‘clustering’’ stage, observed for Al overlayers. Also for Al overlayers, the transition corresponds to the onset of a cation exchange reaction and to the pinning of the Fermi level. The exchange reaction is not observed for GaAs(110)–In, and the Fermi-level pinning occurs at much larger coverages. Furthermore, the pinning position for n-type GaAs is not the canonical one observed for a wide variety of metal, semiconductor, and oxygen overlayers, but 0.2–0.25 eV above it. All of these ‘‘odd’’ results demonstrate that the kinetics of the interface formation is a fundamental factor in the defect model, and clarify its role.
Theoretical study of the adsorption of oxygen on Si(100)
Authors: Batra Inder P. , Bagus P. S. , Hermann K.
Published: July 2002

The geometrical and physical model for adsorption of oxygen on silicon has not been well established. In the literature proposed models have included atomic oxygen, different surface sites and bond multiplicities, and molecular oxygen. We have performed calculations using an ab initio multiconfiguration SCF cluster approach as well as a slab simulation in the framework of an SCF pseudopotential approach. From the results of the cluster and slab calculations we find that oxygen in the first layer bridging positions is more stable than in the on-top configuration. Further, the fourfold center site is shown to be quite weakly bound and thus unlikely to be a stable adsorption site. For O(1×1)–Si(100) the equilibrium bond lengths in the on-top and bridging sites are, from the slab calculations, found to be 1.76 and 2.15 Å. The corresponding values obtained from our cluster calculations are 1.64 and 1.92 Å. We report vibrational frequencies for various sites and suggest a possible interpretation of the EELS data in terms of an atomic chemisorption model.
The preparation and characterization of optical thin films produced by ion-assisted deposition
Authors: Martin P. J. , Netterfield R. P. , Sainty W. G. , Pacey C. G.
Published: July 2002

Inert and reactive gas ion beams have been used in the preparation of high-quality optical thin films for use as dielectric coatings on optical components, solar selective absorbers, and protective films on Ag and Al mirrors. Films are deposited by electron-beam evaporation and simultaneous irradiation of the growing film at the substrate or by ion-beam sputtering with and without ion assistance. Substantial improvement in the film packing density has been achieved, the refractive indices increased, and water penetration through the films was suppressed. TiN has been deposited by Ti evaporation and nitrogen irradiation for use as an element in a high-temperature solar selective absorber. Amorphous hydrogenated silicon (a-Si:H) has also been produced by Si evaporation and hydrogen irradiation or by mixed ion-beam sputtering. Significant changes in the crystal structure of PbS have also been achieved by ion irradiation. Dielectric films of ZrO2, SiO2, and Al2O3 have been deposited by ion-assisted deposition and ZrO2 has been used as a highly successful protective coating on Al and Ag mirrors. The optical properties of the films have been studied and the film composition checked by x-ray diffraction, nuclear reaction analysis (NRA), Rutherford backscattering (RBS), x-ray photoelectron spectroscopy (XPS), and ion scattering spectroscopy (ISS).
Surface transient behavior of the 30Si+ yield with angle of incidence and energy of an O+2 primary beam
Authors: Vandervorst W. , Shepherd F. R. , Newman J. , Phillips B. F. , Remmerie J.
Published: July 2002

The width of the surface transient (w) from Si bombarded with oxygen ions has been investigated as a function of angle of beam incidence (0°≤θ≤75°) and as a function of primary energy (2–10 keV O+2). The actual sputtered depth corresponding to this width has been determined using an internal depth marker. The width of this transient region is seen to increase with angle of incidence for 0°≤θ≤45°. For θ≥45°, w again decreases with increasing θ. The results are explained in terms of a model which calculates the sputtered depths taking into account SiO2 formation (θ≤45°). For greater angles of incidence the sample retention model of the incident ions is seen to predict the angular dependence of the transition width. Computer simulations using Boltzmann transport equations are also performed which illustrate the validity of the above models and which closely agree with the experimental results.
Process control with optical emission spectroscopy in triode ion plating
Authors: Salmenoja K. , Korhonen A. S. , Sulonen M. S.
Published: July 2002

Physical vapor deposition (PVD) techniques used to prepare, e.g., hard TiN, HfN, or ZrN coatings include a great variety of processes ranging from reactive evaporation to sputtering and ion plating. In ion plating one effective way to enhance ionization is to use a negatively biased hot filament. The use of an electron emitting filament brings an extra variable to be taken into account in developing the process control. In addition, proper control of the evaporation source is critical in ensuring reproducible results. With optical emission spectroscopy (OES) it should be possible to control the coating process more accurately. The stoichiometry and the composition of the growing coating may then be ensured effectively in subsequent runs. In this work the application of optical emission spectroscopy for process control in triode ion plating is discussed. The composition of the growing coating is determined experimentally using the relative intensities of specific emission lines. Changes in the evaporation rate and the gas flow can be seen directly from emission line intensities. Even the so-called poisoning of the evaporation source with reactive gas can be detected. Several experimental runs were carried out and afterwards the concentration profiles of the deposited coatings were checked with the nuclear resonance broadening (NRB) method. The results show the usefulness of emission spectroscopy in discharge control.
Thermally induced structural and compositional modification of the Cu/Si(111)-7×7 interface
Authors: Chambers S. A. , Weaver J. H.
Published: July 2002

We have investigated the morphology, local composition, and surface stoichiometry of the annealed Cu/Si(111)-7×7 interface using scanning electron microscopy, scanning Auger microscopy, characteristic x-ray emission spectroscopy, and angle-resolved Auger emission. It is found that annealing of intermixed Cu/Si phases with copper thickness between 5 and 20 Å at temperatures of ∼600 °C causes most of the Cu to evaporate. The Si which had been in the intermixed phase recrystallizes in the form of submicron-sized islands. The Cu which does not evaporate forms a very stable Cu3Si-like skin which uniformly covers the surface. This morphology is very different from what has been reported for Ag/Si (Ag island formation) or Au/Si (micron-sized Au islands with silicide skin) and can be related to the kinetics of Cu desorption and the initial intermixed phase.
Oxygen-induced CO reorientation on Cr(110)
Authors: Shinn N. D. , Madey T. E.
Published: July 2002

Studies of CO and CO/O chemisorption on Cr(110) at 120 K using high resolution electron energy loss spectroscopy (EELS), electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES) are reported. On the clean surface, two molecular binding modes are sequentially populated. The first mode (α1CO) consists of molecules bound with the CO axis nearly parallel to the surface and saturates with the formation of a c(4×2) overlayer, whereas the second mode (α2CO) is comprised of CO oriented perpendicular to Cr(110). Predosing Cr(110) with atomic oxygen inhibits the α1CO mode such that for θ0 above ∼1/2 monolayer, only the α2CO mode is populated. More surprisingly, the c(4×2)α1CO overlayer is converted to disordered α2CO by subsequent addition of oxygen, implying a significant reorientation of the adsorbed molecules at 120 K.
Electrical properties of the SiO2:HgCdTe interface
Authors: Wilson J. A. , Cotton V. A.
Published: July 2002

By determining the dependence of flatband bias on insulator thicknesses in MIS devices made with PHOTOX SiO2 on n-type Hg0.7Cd0.3Te, we have for the first time determined the amount of interface charge and the effective work function of the semiconductor surface. Flatband bias was determined from the surface potential as obtained by Berglund integration of quasistatic capacitance measurements at 77 K. Insulator thickness was varied from 500 to 4000 Å. The amount of fixed charge was seen to vary between −0.3×109 cm−2 and +8×1010 cm−2 depending on the type of deposition sequence used. It was also seen that H2O could be absorbed into the layer and removed by vaccum baking at temperatures of 60–100 °C and resulted in a change of interface fixed charge of up to 8×1010 cm−2. This is in agreement with the range of charge variation seen above. The presence of large amounts of H2O also was seen to introduce surface states at 0.06 and 0.14 eV above the valence band edge (Eg=0.24 eV at 77 K). The effective work function difference between the Ti/ Au bilayer gate and the HgCdTe surface was determined to be +0.03 V ±0.05 V.
Molecular beam epitaxy of alloys and superlattices involving mercury
Authors: Faurie J. P. , Boukerche M. , Reno J. , Sivananthan S. , Hsu C.
Published: July 2002

In this review we present a general survey regarding the MBE growth of Hg1−xCdxTe and HgTe–CdTe during the last three years. The quality of the films grown by MBE is now comparable with the best material grown by other techniques. We discuss the limitations of the MBE technique, i.e., the low mercury condensation coefficient at 200 °C where the best layers are grown, and the difficulties arising from the growth and the doping control. The poor crystal quality of bulk CdTe substrates is a limitation of any epitaxial layer grown by LPE, OMCVD, or MBE. Thus use of better substrates like (Cd,Zn)Te, Cd(Te,Se), InSb, GaAs, and sapphire could be a solution to this very important problem. We report here for the first time the successful growth by MBE of epitaxial Hg1−xCdxTe layers onto (Cd,Zn)Te, InSb, and GaAs substrates. We discuss the use of superlattices HgTe–CdTe for IR detection devices.
Photoluminescence of CdTe: A comparison of bulk and epitaxial material
Authors: Giles-Taylor N. C. , Bicknell R. N. , Blanks D. K. , Myers T. H. , Schetzina J. F.
Published: July 2002

Photoluminescence (PL) studies of bulk and epitaxial CdTe samples obtained from several sources are discussed. Steady state PL measurements were carried out at temperatures ranging from 16–300 K. The effects of surface preparation, substrate temperature, and film thickness were studied in detail for homoepitaxial films grown on the (111)A and (100) planes of CdTe. PL studies of epitaxial CdTe films grown on (0001) sapphire by molecular beam epitaxy (MBE), by hot wall MBE, and by metal-organic chemical vapor deposition (MOCVD), and on the (111)B and (100) planes of GaAs by MBE have also been completed. The CdTe epilayers on sapphire and GaAs substrates typically display a bright PL spectrum dominated by the near edge peak at 1.58 eV (77 K). In addition, a number of films exhibit a near edge peak at 1.503 eV at 300 K, which is indicative of high quality epitaxy and which allowed direct measurement of the room temperature band gap of CdTe. PL studies of epitaxial Cd1−x MnxTe films grown by MBE on 5.0 μm thick CdTe buffer layers on GaAs substrates reveal shifts of the band gap into the visible spectral region with increasing x accompanied by a significant increase in edge peak magnitude.
‘‘Diamondlike’’ carbon films: Optical absorption, dielectric properties, and hardness dependence on deposition parameters
Authors: Natarajan V. , Lamb Joel D. , Woollam John A. , Liu David C. , Gulino Daniel A.
Published: July 2002

An rf plasma deposition system was used to prepare amorphous ‘‘diamondlike’’ carbon films. The source gases for the rf system include methane, ethylene, propane, and propylene, and the parameters varied were power, dc substrate bias, and postdeposition anneal temperature. Films were deposited on various substrates. The main diagnostics were optical absorption in the visible and in the infrared, admittance as a function of frequency, hardness, and Auger and ESCA spectroscopy. Band gap is found to depend strongly on rf power level and band gaps up to 2.7 eV and hardness up to 7 Mohs were found. There appears to be an inverse relationship between hardness and optical band gap.
Measurement of semiconductor heterojunction band discontinuities by x-ray photoemission spectroscopy
Authors: Waldrop J. R. , Grant R. W. , Kowalczyk S. P. , Kraut E. A.
Published: July 2002

Accurate knowledge of the band discontinuities at a heterojunction interface and the factors that affect their magnitude are of both fundamental and practical interest. The application of x-ray photoemission spectroscopy (XPS) to the direct, contactless, and quantitative measurement of the valence band discontinuity (ΔEv) at abrupt heterojunctions is discussed. The topics covered include a description of a method to achieve precise measurement, results of ΔEv measurements for many heterojunction pairs selected from the lattice-matched semiconductor series Ge, GaAs, ZnSe, CuBr, and AlAs, and a comparison of experiment to models that predict ΔEv. Examples are given to illustrate the effect on ΔEv of such preparation-dependent parameters as growth sequence and interface crystallographic orientation.
Broad-beam ion sources: Present status and future directions
Authors: Kaufman Harold R.
Published: July 2002

Operational problems are believed to constitute the major obstacle to the more widespread use of broad-beam ion sources, particularly in production applications. In the category of problems that are primarily the responsibility of ion-source operators: an excessive discharge voltage is undesirable because it causes excessive damage depth and contamination at the target; an excessive ion-beam current is undesirable because it leads to rapid wear of the accelerator grid, increased target contamination, and little or no increase in process rate; and an electron backstreaming condition is undesirable because false indications of ion-beam current are given by the power supply. In the category of problems that are primarily the responsibility of ion-source manufacturers: the need for excessive maintenance has an obvious and direct impact on cost; the lack of reproducibility of both ion-beam profile and direction results in an overall lack of reliability that has required an excessive degree of care by highly skilled operators; and the lack of adequate information from the manufacturer is a problem that, in turn, aggravates all other problems. From examination of ion sources and industrial applications of these sources, most ion sources are misused, as well as being unsuited for production applications. There is an obvious need for improved education of ion-source operators and improved ion-source design.
Stress measurement in sputtered copper films on flexible polyimide substrates
Authors: Entenberg Alan , Lindberg Vern , Fletcher Kurtis , Gatesman Andrew , Horwath Ronald S.
Published: July 2002

We have investigated the dependence of internal stress on argon pressure for 0.25-μm copper films sputtered onto a 1-mil(25-μm)-thick polyimide substrate. A dc planar magnetron was used to deposit the copper at a rate of 2 Å/s onto the flexible substrate which was held flat by top and bottom edges. The stress was estimated directly from the resulting radius of curvature of the relaxed film and substrate. The critical argon pressure at which a transition from compression to tension occurs was found to be about 2.5 mTorr. In addition, the dependencies of film morphology, resistivity, and optical reflectance on argon pressure were studied. The critical pressures at which resistivity starts to rise and reflectance begins to decrease are very close to the stress transition pressure. In general, the data are consistent with the dependencies of physical properties on pressure observed for other sputtered metals.
Shadow-cone enhanced desorption with angle-resolved secondary ion mass spectrometry detection
Authors: Chang Che-Chen , Malafsky Geoffrey , Winograd Nicholas
Published: July 2002

The shadow cone created by an incident keV bombarding ion is observed to enhance the yield of secondary ions sputtered from the target. This effect may be utilized to determine the bonding configuration of surface atoms in a manner similar to that reported for impact-collision ion scattering spectrometry (ICISS). This new method is utilized to study the bonding of Cl to Ag{110} as the coverage is increased from near zero to the p(2×1) low-energy electron diffraction structure. The results show that the Ag–Cl bond length is extended by 0.4 Å in the isolated atom limit when compared to the p(2×1) coverage due to a large amount of charge transfer between the Ag substrate and the adsorbed Cl atom. This bond length becomes shorter as the coverage increases due to dipole–dipole repulsions. Experimental results are compared to computer simulations of the ion-impact event and the possible advantages of this method over ICISS are discussed.
The effect of alkali metals on the interaction of O2 and CO with some transition-metal surfaces
Authors: Kiskinova M.
Published: July 2002

The interactions of O2 and CO with alkali covered Ni, Ru, and Pt are reviewed with emphasis on the author’s own studies. It is shown that the presence of alkalis changes the coadsorbates adsorption rates, heats of adsorption, and dissociative propensity, the strength of the effect depending on the alkali nature and coverage. The differences for the three substrates are interpreted comparing the properties of the clean surface. A detailed analysis of the coadsorbate–modifier interactions suggests that they depend on the adsorption state of the alkali species and the coadsorbate–substrate bond strength. Evidence of the formation of patches of ‘‘surface complexes’’ coexisting with the unperturbed surface is given and discussed in terms of strong direct alkali–coadsorbate interactions.
Roughness effect upon oxygen adsorption on Si(100) surfaces
Authors: Andriamanantenasoa I. , Lacharme J. P. , Sébenne C. A.
Published: July 2002

Si wafers cut along (100) or selected vicinal faces were cleaned under ultrahigh vacuum and then exposed to increasing doses of O2 from 1 to 103 L. At each step, the surface was characterized by low-energy electron diffraction, Auger electron spectroscopy, and photoemission yield spectroscopy. The roughness-dependent shape of the dangling bond surface state band is deduced from the latter, and its changes upon O2 adsorption are analyzed.
Helium leak detection in JET in the presence of high deuterium partial pressure
Authors: Winkel T. , Hemmerich J. L.
Published: July 2002

Leak detection in JET (∼200 m3 volume, ∼8000 m length of welds, ∼300 bolted vacuum connections, base pressure at room temperature 2×10−9 mbar) had originally been based on the use of helium (4He) as a tracer gas. Later, high deuterium (D2) partial pressures due to D2 plasma operation made high-sensitivity leak testing by standard methods with 4He impossible; hence, alternative methods had to be developed. This eventually led to the use of a high-resolution Omegatron mass spectrometer able to separate D2 and 4He. Additional compression and selective pumping provides a sensitivity of 10−9 mbar l s−1. The advantages of the Omegatron system compared with other alternative methods will be presented. The use of a fully tritium compatible Omegatron in conjunction with ∼400 tracer gas tubes during the active phase leak detection will be discussed.
Vacuum outgassing and gas phase thermal conduction of a microgravity physical vapor transport experiment
Authors: Debe M. K. , Cook E. L. , Poirier R. J. , Follett G. J. , Miller L. R. , Spiering S. M.
Published: July 2002

An experiment to study the effects of microgravity on the deposition of organic thin films using physical vapor transport (PVT) was successfully flown and operated on the Space Shuttle mission 51I. Nine PVT ampoules contained three types of organic solids, buffer gases from He to Xe, and special substrates. The sealed ampoules, heated to 400 °C by concentric heater assemblies, were contained in nine evacuated and sealed UHV quality stainless-steel cells. The outgassing of a cell’s contents and resultant heat loss by gas phase conduction are important parameters for the cell power requirements and ampoule boundary conditions. We report the measurement of the absolute partial pressures of nine residual gases generated within the cells and the radial gas phase heat conductivity in the absence of gravity. H2, CH4, and CO2 were the dominant gases, with total pressures ranging from 0.3 to 2.5 Torr. The heat loss was observed to be pressure dependent, ∼15 W/Torr, and thus an important loss mode.
Formation of high-quality, magnetron-sputtered Ta2O5 films by controlling the transition region at the Ta2O5/Si interface
Authors: Seki Shunji , Unagami Takashi , Kogure Osamu , Tsujiyama Bunjiro
Published: July 2002

Magnetron-sputtered Ta2O5 films on Si are analyzed for their applications to semiconductor devices. It is clarified that the transition region formed at the Ta2O5/Si interface plays a significant role in determining electrical characteristics. This transition region formed during the initial stage of deposition consists of a mixture of Ta, Si, and O, and its thickness is in the range of 4 to 6 nm. The composition profile of this transition region and its thickness depend on two dominant factors: (1) oxygen partial pressure in the sputtering gas and (2) the surface oxide thickness on Si prior to deposition. Furthermore, it is initially indicated that 2.5-nm-thick surface oxide formed on Si prior to deposition produces a significant deterioration in leakage current. By reducing the surface oxide thickness to as low as 1.8 nm and adjusting the oxygen partial pressure, an optimum transition region is formed, which makes it possible to obtain high-quality Ta2O5 films with high dielectric breakdown strength exceeding 6 MV/cm and low leakage current of <10−9 A/cm2 (at 1 MV/cm).
Variations of energies and line shapes of the electroreflectance spectra of epitaxial AlxGa1−xAs
Authors: Wrobel J. M. , Bassett L. C. , Aubel J. L. , Sundaram S. , Davis John L. , Comas James
Published: July 2002

The electrolyte electroreflectance (EER) spectra of AlxGa1−xAs have been obtained in the E0 and E1 regions and for Al concentrations ranging from x=0 to 0.65. From the spectra obtained under low field conditions, the variations of the interband energies with Al concentrations have been established. Following Aspnes [Surf. Sci. 37, 419 (1973)] we have made detailed analyses of the complex resonance line shapes of the experimentally observed modulated reflectance spectra. The parameters Γ and Eg have been calculated and these have been used to generate theoretically the spectra for comparison with experimental results. In addition, we have also examined the effects of dc bias on the EER spectra. Variations of E0, E0+Δ0, E1, and E1+Δ1 have been discussed and results on the line-shape parameters have been presented.