bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoluminescence of gallium impurity in cadmium telluride
Authors: Wrobel J. M. , Dubowski J. J. , Becla P.
Published: July 2002

We have studied low-temperature photoluminescence in CdTe containing Ga impurity in the 1.28–1.61 eV energy region. The impurity was introduced either by implantation of bulk CdTe wafers or by doping CdTe ingots during Bridgman growth. The presence of Ga was discovered and studied in epitaxial layers of CdTe grown on GaAs substrates by pulsed laser evaporation and epitaxy. Extra features caused by the Ga presence were observed in the edge emission and in the excitonic regions. Implantation results in extended mechanical damage, and only a small fraction of the implanted Ga is optically active. Samples doped during growth by the Bridgman technique exhibit changes in the excitonic region. The photoluminescence spectra of CdTe epitaxial films grown on GaAs exhibit features which are similar to those observed in bulk CdTe with Ga impurity.
Magnetic field effects on trap-assisted tunneling in Hg0.78Cd0.22Te metal-insulator semiconductor capacitors
Authors: Waterman J. R. , Wagner R. J. , Perez J. M.
Published: July 2002

The effect of magnetic fields on trap-assisted tunneling in Hg0.78Cd0.22Te metal-insulator semiconductor capacitors has been investigated. Landau-level formation in the two-dimensional electron gas inversion layer has been observed in high-frequency capacitance–voltage measurements in normal and tilted magnetic fields. With the magnetic field normal to the interface, shifts in the gate voltage position of the trap-assisted tunneling structure in capacitance and conductance–voltage data are observed consistent with the magnetic field induced shift in subband energies and Landau-level formation. With the magnetic field parallel to the inversion layer plane, a large reduction in tunneling current and shifts in electric subband energies are observed. In tilted magnetic fields, additional structure is observed, indicative of tunneling into higher-order Landau levels.
Modulation-doped HgCdTe
Authors: Han Jeong W. , Hwang S. , Lansari Y. , Harper R. L. , Yang Z. , Giles N. C. , Cook J. W. , Schetzina J. F. , Sen S.
Published: July 2002

At North Carolina State University, we have recently employed photoassisted molecular-beam epitaxy (MBE) to successfully prepare p- and n-type modulation-doped HgCdTe. The modulation-doped HgCdTe samples were grown on lattice-matched (100) CdZnTe substrates cut from boules grown at Santa Barbara Research Center. In this paper, we report details of the MBE growth experiments and describe the structural, optical, and electrical properties that this new infrared quantum alloy of HgCdTe possesses.
Hg1−xCdxTe based quantum wells for the 3-μm wavelength region
Authors: Feldman R. D. , Cesar C. L. , Islam M. N. , Austin R. F. , DiGiovanni A. E. , Shah J. , Spitzer R. , Orenstein J.
Published: July 2002

HgCdTe multiple quantum wells with transition energies near 3 μm have been grown by molecular-beam epitaxy. We observe photoluminescence from 2 K to room temperature, and the low-temperature peak width is consistent with layer thickness fluctuations limited to one monolayer. High-resolution transmission electron microscopy shows that there is some roughness at the interfaces, but that there is good layer thickness uniformity throughout the structure. The absorption spectra contain sharp, steplike features due to carrier confinement in the wells at energies that can be described by a square-well model, provided that a temperature-independent HgTe/CdTe valence-band offset near 400 meV is assumed.
Photon-stimulated desorption studies of SiF4 adsorbed on Si(111)(7×7)
Authors: Wen C.-R. , Rosenberg R. A.
Published: July 2002

We have investigated the interaction of SiF4 with the Si(111)(7×7) surface at 30 K and after warming to T≥100 K using angular-dependent photon-stimulated desorption. The initial adsorption of SiF4 at 30 K results in the formation of SiF and SiF3 : the SiF species have the Si–F bond directed normal to the surface, while the bonds in SiF3 are directed off-normal. Warming of a 30 K, multilayer dosed SiF4 -covered surface to T>100 K also shows the presence of only dissociatively chemisorbed species, SiF and SiF3. The SiF bond is directed along the surface normal, while the SiF3 bonds are randomly oriented.
The interaction of the polyimide precursors PMDA (1,2,4,5-benzenetetracarboxylic anhydride) and m-PDA (1,3-phenylenediamine) with Ni(110)
Authors: Jones T. S. , Ashton M. R. , Richardson N. V. , Mack R. G. , Unertl W. N.
Published: July 2002

X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy (HREELS) have been used to study the adsorption of polyimide precursor molecules on Ni(110). PMDA (1,2,4,5-benzenetetracarboxylic anhydride) and m-PDA (1,3-phenylenediamine) were vapor deposited in vacuo at substrate temperatures of between 150 and 300 K. Both m-PDA and PMDA adsorb at 300 K without major fragmentation. For m-PDA, adsorption to the surface occurs via its nitrogen atoms. HREELS results suggest that significant deprotonation of the amine groups occurs and that the plane of the aromatic ring is inclined away from the surface normal. For PMDA, XPS results indicate that the primary chemical interaction with the Ni is through at least one of the anhydride oxygens. HREELS results imply that the plane of the aromatic ring is aligned close to the surface normal, therefore it is concluded that adhesion to the surface is through one anhydride unit only.
Optimization of primary beam conditions for secondary ion mass spectrometry depth profiling of shallow junctions in silicon using a Cameca IMS-3f
Authors: Hunter J. L. , Corcoran S. F. , Griffis D. P. , Osburn C. M.
Published: July 2002

Recent advances in semiconductor technology have allowed the fabrication of Si-based devices with submicron lateral dimensions, thus requiring the concurrent development of the analytical techniques for characterization of the shallow junctions (30–100 nm) associated with such devices. Although secondary ion mass spectrometry (SIMS) provides high dynamic range and good depth resolution, accurate depth profiling of very shallow junctions require that measures be taken to minimize the broadening effects of cascade and recoil mixing caused by the primary ion beam. In this study, low-energy B and As implants into Si were analyzed using a Cameca IMS-3f. Primary ion beam parameters, including species (O+2,Cs+ and Xe+), angles of impact, energies (1.1–15.4 keV effective), and the effect of oxygen flooding, as well as secondary ion useful yields, were investigated to determine the optimal instrumental conditions for analysis of these shallow junctions. The resulting depth profiles were evaluated in terms of both dynamic range and decay length.
Effect of surface impurities on the permeation of hydrogen through iron
Authors: Andrew P. L. , Haasz A. A.
Published: July 2002

The permeation of hydrogen through metal membranes can be strongly affected by surface impurities for conditions where surface processes dominate over bulk diffusion. Hydrogen permeation was measured in both the surface-limited and diffusion-limited regimes between 300 and 700 K using a membrane technique. Molecular hydrogen (1×10−3–3×104 Pa) was used to determine the membrane permeability and the overall surface-limited permeation rate constant. Atomic hydrogen (3×1015–2×1019 H0/m2 s) incident on either side of the membrane was used to determine the contribution of each surface to the overall surface-limited permeation rate. Impurity concentration on one of the membrane surfaces was monitored in situ with an Auger electron spectrometer. This same surface could also be sputter-cleaned by argon ions. Removal of surface carbon, oxygen, and sulphur had no measurable effect on the diffusion-limited permeation, but increased the surface-limited permeation rate constant by orders of magnitude.
Process parameter-growth environment-film property relationships for the sputter deposited yttrium–oxygen system
Authors: Kwok Chee-Kin , Aita Carolyn Rubin , Kolawa Elzbieta
Published: July 2002

In this study, a Y target was sputtered in radio frequency (rf)-excited, rare gas discharges (Ne, Ar) containing 0%–40% O2, operated at cathode voltage from −1.0 to −1.7 kV. In situ optical emission spectrometry was used to monitor two neutral excited Y atom transitions (λ=0.6191, 0.6793 μm) and an excited O atom transition (λ=0.7774 μm) as a function of changing process parameter. Films were grown on fused SiO2 substrates, and their crystallography, optical behavior, and electrical resistivity was determined. A ‘‘phase diagram’’ for Y–O not grown under conditions of equilibrium thermodynamics was constructed, and included hexagonal Y, cubic Y2O3, and Y and Y2O3 that had no long range crystallographic order. Two direct optical transitions across the energy band gap of cubic Y2O3, at 5.07 and 5.73 eV, were identified. Combining discharge diagnostics, growth rate, and film property results, it was concluded that Y2O3 was formed at the substrate concurrent with the complete oxidation of the target surface. Even after target oxidation, the discharge contained atomic Y. On the basis of fundamental optical absorption edge characteristics, cubic Y2O3 that more closely resembled the bulk material was obtained when the Y-oxide molecule/Y atom flux to the substrate was high.
Properties of Hg1−xCdxTe native oxide interfaces
Authors: Helms C. R.
Published: July 2002

The properties of native oxide/semiconductor systems are of obvious importance for all semiconductor technologies in use today. In a few cases, the formation of native oxide/semiconductor interfaces can lead to superior device structures—metal–oxide semiconductor (MOS) devices in Si are a key example. In other cases, the native oxides formed lead to pathological behavior, such as the case for GaAs. The chemical stability of the oxide semiconductor system normally determines its ultimate utility. It is, of course, conceivable that a thermochemically unstable structure may find technological use if the reactions are slow enough. However, the small number of interface defects necessary to affect electrical properties usually dictates that a stable interface is necessary. Hg1−xCdxTe exhibits these types of instabilities through reactions of oxidized Hg and Te with Te bonded Cd. In addition to the above chemical concepts, this paper reviews the kinetics of the oxide Hg1−xCdxTe reactions and the dynamics of subsequently formed electrically active defects (excess Te or Hg). A discussion of the chemical basis for superior properties of sulfides and halogen based passivations will also be presented.
Electron transport and cyclotron resonance in [211]-oriented HgTe–CdTe superlattices
Authors: Hoffman C. A. , Meyer J. R. , Wagner R. J. , Bartoli F. J. , Chu X. , Faurie J. P. , Ram-Mohan L. R. , Xie H.
Published: July 2002

We discuss a magnetotransport and magneto-optical investigation of [211]-oriented HgTe–CdTe superlattices. Measurements were performed on seven n-type samples with well widths in the range 41–125 Å and energy gaps between 0 and 128 meV. Both magnetotransport and magneto-optical results give evidence for an additional electron species besides the superlattice electrons and holes usually observed. We show that although these have quasi-two-dimensional character, their mobilities are quite sensitive to the superlattice well thickness. These findings are interpreted in terms of band bending and charge transfer from the CdTe substrate into the superlattice. We also discuss the first theoretical calculations of the band structure for [211]-oriented HgTe–CdTe heterostructures.
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u growth surface temperature measurement for molecular beam epitaxial growth of CdTe, ZnTe, and Cd1−xZnxTe alloys
Authors: Rajavel D. , Mueller F. , Benson J. D. , Wagner B. K. , Benz R. G. , Summers C. J.
Published: July 2002

Direct measurements of the growth surface temperatures of (001) CdTe, ZnTe, and Cd1−xZnxTe (x=0.1,0.25) epilayers were performed in situ by means of the condensation of Te2. Measurements were made over the technologically important 160–220 °C range for the molecular beam epitaxial growth of these materials. Condensation of Te2 was observed when the incident flux of Te2 exceeded the flux of Te2 desorbing from the substrate as calculated at the temperature of the substrate. The surface temperature for the onset of condensation of an incident beam of Te2 depends on the magnitude of the flux of the incident Te2 molecules. The epilayer surface temperature was calculated by equating the expressions for the incident Te2 flux and the desorbing Te2 flux. Experimentally, the condensation of Te2 was detected when transmission diffraction spots due to Te precipitates appeared, superimposed on the streaked reflection high-energy electron diffraction pattern of the substrate. The Te2 condensation temperature measurements were reproducible to within 3 °C. The technique is accurate to better than 5 °C in absolute terms for measurements on (001) CdTe surfaces and 8 °C for (001) ZnTe and Cd1−xZnxTe surfaces.
Low temperature scanning tunneling topography of YBa2Cu3Ox superconducting films
Authors: Gallagher M. C. , Adler J. G.
Published: July 2002

We have performed both topographic and spectroscopic measurements at 4.2 K on thin films of YBa2Cu3Ox with a scanning tunneling microscope. The films all had zero resistance temperatures in excess of 75 K. We found that, even at low temperatures, most of the surface of the films were conducting. Images were obtained at tunneling voltages as low as 2 mV with little effect on the resultant topography. Noisy patches, which are observed in some images, and result from unstable feedback response, indicate regions of reduced surface conductivity. Current–voltage curves obtained at different points indicate that superconductivity can extend out to the surface of the films. Despite excessive broadening resulting from charging effects, gap values as high as 20 meV were observed.
A two-pyrometer method of measuring substrate temperatures
Authors: Inoue M. , Ozaki Y. , Tsuchikawa H.
Published: July 2002

We have developed a new method to measure the temperature of substrates during thermal treatments at elevated temperatures. In this method the emissivity of the substrate is calibrated automatically. The accuracy of the method has been investigated systematically. This method has been used for the in situ monitoring of substrate temperature during the sputter deposition of aluminum thin films. The temperature profiles during the depositions at various temperatures of the heating stage were examined.
Morphological changes in silver foil in microwave generated oxygen and argon plasmas
Authors: Chou C. H. , Phillips J.
Published: July 2002

The morphological changes which take place in silver foils placed in microwave generated argon plasmas can be fully explained on the basis of earlier developed models of ionic sputtering. In contrast, morphological changes in oxygen plasmas were found to be complex and not readily explained by earlier models. Morphological changes were found to be a function of the foil position within the plasma, and thus were clearly influenced by plasma species concentrations and temperature. In environments containing oxygen atoms and no charged species the only change which occurred was a very slow rate of silver oxide growth. In environments containing modest concentrations of charged particles and oxygen atoms extremely rapid oxidation took place. In environments containing high concentrations of radical and charged particle species several unprecedented phenomena were observed. First, silver oxide films were found to be stable at temperatures far above the conventionally calculated decomposition temperature. Second, extremely high etch rates were sometimes observed. Third, the nature of the morphological changes could not be fully predicted, as results varied from sample to sample.
Rastered laser light scattering studies during plasma processing: Particle contamination trapping phenomena
Authors: Selwyn Gary S. , Heidenreich John E. , Haller Kurt L.
Published: July 2002

The distribution and transport of particles in materials processing plasmas has been studied with a rastered laser light scattering technique. Contrary to expectation, the distribution of particles in a plasma processing tool is rarely random. Instead, structured clouds of particles form at the plasma/sheath boundary. The effect is attributed to trapping of the particles by weak electric field nonuniformities and the characteristic negative charge of isolated particles in a plasma. Field nonuniformities appear to be influenced by the topography and material design of the tool. For example, the presence of a Si wafer often induces significant particle trapping. Examples of particle trapping in a laboratory system are given, and similar phenomena are also verified in a manufacturing sputter deposition tool operating in a class 100 cleanroom. The implications of particle trapping in plasma processing are discussed.
The relative effect on the oxygen concentration in YBa2Cu3O7−δ of atomic and ionic oxygen fluxes, produced by a small compact electron cyclotron resonance source
Authors: Yamamoto K. , Hammond R. H. , Harris J. S.
Published: July 2002

The relative effect of atomic and ionic oxygen for the oxidation of YBa2Cu3O7−δ thin films is investigated. To this goal, the characteristics of an oxygen plasma produced by a small, compact, permanent magnet electron cyclotron resonance plasma source were investigated as a function of a bias applied to the plasma. With no bias, a plasma potential of 23 eV was measured by an energy analysis of the ions. With increasing positive plasma bias voltage, the ion current decreases and its energy increases above the 23 eV by the amount of applied bias voltage. When a negative voltage is applied to the plasma, both electron and ion current drastically decrease, while the atomic flux remains high. The atomic oxygen flux was measured by the oxidation rate of Ag and the etching rate of a standard photoresist as a function of distance from the source and the bias voltage. We find that the resistivity of a YBa2Cu3O7−δ film in the plasma is well characterized by the flux of atomic oxygen and that there is no significant effect of oxygen ions.
Vacuum ultraviolet radiation damage in electron cyclotron resonance and reactive ion etch plasmas
Authors: Buchanan D. A. , Fortuño-Wiltshire G.
Published: July 2002

A comparative study of vacuum ultraviolet radiation damage has been carried out in an electron cyclotron resonance (ECR) and reactive ion etching (RIE) hybrid chamber. The damage was measured by changes in the density of interface states, the midgap capacitance–voltage shifts, and from photo I–V gate voltage shifts in metal–oxide semiconductor (MOS) capacitors before and after plasma exposure. The results suggest that the damage found in the samples exposed to either ECR or RIE plasmas appears in the form of neutral electron traps in the bulk of the SiO2 film and ‘‘slow’’ interface states. The positive charge found in the exposed samples results from the filling and emptying of these slow interface states and not from trapped holes. Electron trap densities are found to be in the low to mid 1012 cm−2 range for all plasma-exposed samples. Even after a standard post-metalization anneal (PMA), the density of electron traps is only reduced by approximately a factor of 2 and appears relatively independent of the type of the plasma exposure. The density of the slow interface states is found to be in the high 1011 cm−2 range and in the low 1010 cm−2 range, for the ECR plasma samples, before and after the PMA, respectively. The RIE exposed slow state density was initially ≂2×1012 cm−2 and reduced to 2×1111 cm−2 after the PMA, an order of magnitude higher than that of the ECR-plasma exposed samples.
Adsorption of metallic tin on the Au(111) surface
Authors: Zhang Y. , Slavin A. J.
Published: July 2002

The growth mode of metallic Sn deposited onto the Au(111) surface has been investigated using Auger electron spectroscopy, low-energy electron diffraction, electron-energy-loss spectroscopy and work-function measurements for deposition at room temperature and at low temperature (−70 to −100 °C). At room temperature, Sn initially forms an alloy, AuSn, which grows laterally as a double layer, followed by continued growth of the same alloy one layer at a time. This differs from the situation for Pb and Bi on Au(111), which form an overlayer on Au before alloying. However, Sn deposited at low temperature does appear to form one layer of pure Sn followed by the growth of the AuSn alloy underneath. This latter conclusion is different from that of a previous study which suggested that Sn forms one layer of AuSn alloy covered by pure Sn layers at low temperature. The difference may be caused by different deposition rates.
Magnetron enhanced etching of GaAs
Authors: McLane G. F. , Meyyappan M. , Lee H. , Buckwald W.
Published: July 2002

Magnetron reactive ion etching of GaAs in a freon-12 discharge has been studied. Electrical characteristics of the etched samples were investigated by current–voltage and capacitance–voltage measurements on Schottky diodes. The ideality factors were found to be close to that of the control sample. Magnetron etching causes no apparent change in the carrier concentration profile. Magnetron enhancement is shown to yield high etch rates with minimal residual damage to the wafer.
Silicon (2p) surface core-level line shape of Si(111)–B
Authors: Rowe J. E. , Wertheim G. K. , Riffe D. M.
Published: July 2002

Several recent structural studies of the Si(111)–B ((3)1/2×(3)1/2) surface have established that the boron atoms occupy substitutional sites in the second full Si layer and have Si adatoms directly above them. High-resolution (∼80–100 meV) Si(2p) core-level photoemission was used to determine the B-induced perturbation of the surface Si atoms. The samples were prepared by surface segregation from Si(111) wafers (∼1.5 at. % B) after thermally removing the thin protective oxide layer on the surface. Photoemission spectra for photon energies from 110 to 140 eV show three peaks, indicative of at least two well-separated spin-orbit doublet components. Attempts to fit the data with only two components were unsatisfactory unless a Doniach–Šunjić line shape was used for the B-induced component. This suggests that the surface region influenced by the B impurities is metallic, i.e., the Fermi level lies within the B-modified valence band. The intensity of the bulk component goes through a minimum at a photon energy of 130 eV corresponding to a kinetic energy of 26 eV and is 0.19 of the total intensity. This leads to an escape depth of ∼3 Å if the B-influenced region includes three layers below the adatom layer. Improved fits were obtained by using three distinct surface components. These are identified with the Si adatoms, the three B-influenced surface layers, and the Si in the third layer which directly bonded to B.
The thermal oxidation of AlGaAs
Authors: Shin J. , Geib K. M. , Wilmsen C. W. , Chu P. , Wieder H. H.
Published: July 2002

There are many electronic and optoelectronic devices fabricated from Alx Ga1−x As and GaAs epitaxial layers. It is important to characterize the oxides that grow on these materials since the oxides can strongly influence the surface leakage currents and passivation properties of the devices. While all of the oxides of GaAs have been extensively studied, only the anodic oxides of AlGaAs have received much attention. This paper presents x-ray photoelectron spectroscopy results of the chemical composition of thermally grown Alx Ga1−x As oxides grown in O2 in the temperature range 450 °C to 600 °C for x=0.31 and 0.52. All of the oxides were found to have a complex structure consisting of three distinct layers. The outer layer is very thin and contains As, Ga and Al oxides, the middle layer is composed of Ga2 O3 and Al2 O3 in the same ratio as the AlGaAs substrate with a low concentration of elemental As. The interfacial layer contains only Al2 O3 and elemental As. A model is proposed to explain this multilayer growth.
Dual excitation reactive ion etcher for low energy plasma processing
Authors: Goto Haruhiro H. , Löwe Hans-Dirk , Ohmi Tadahiro
Published: July 2002

The influence of the excitation frequency in the parallel plate cathode-coupling equipment has been investigated and it was found that the self-bias voltage of the cathode becomes a logarithmic function of the excitation frequency in two distinct regions. By selecting appropriate excitation frequencies in a dual excitation system, the study found that the plasma density can be controlled by varying the plasma generating rf power without affecting the dc potential of the electrodes. The effect of the plasma generating rf power on plasma density was determined by monitoring the optical emission spectrum, ion current collected at dc biased electrode, and the SiO2 etching rate when the substrate was rf biased to −50 V. Also, the low ion energy etching of Si and SiO2 was demonstrated in CF4 and SiF4 plasmas utilizing the dual excitation system. Then, applied to the plasma cleaning of the fluorocarbon layer subsequent to CF4/H2 plasma etching, i.e., a typical gas combination used for SiO2 etching. SiF4 plasma cleaning as well as H2 plasma were effective in removal of organic contamination of the silicon.
Diffuse optical reflectivity measurements on GaAs during molecular-beam epitaxy processing
Authors: Lavoie C. , Johnson S. R. , Mackenzie J. A. , Tiedje T. , van Buuren T.
Published: July 2002

In situ measurements of the diffuse and specular reflectivity of GaAs substrates in molecular-beam epitaxy show that the GaAs surface roughens during thermal desorption of the oxide and smooths during the growth of a buffer layer. The diffuse reflectivity increases abruptly over a temperature interval smaller than 0.4 °C as the oxide desorbs. This provides a convenient optical signature for determining when the substrate is clean. The oxide desorption temperature and the amount of surface roughening both increase with the thickness of the oxide. The decrease in the diffuse reflectivity during the growth of the buffer layer is not monotonic, showing instead an oscillatory behavior.
Fabrication and properties of superconductor–insulator–normal metal tunnel junctions
Authors: Lehnert T. , Gundlach K. H.
Published: July 2002

This article first describes the preparation and properties of small area (∼1 μm2)Pb/Bi/In-oxide–Ag junctions made by evaporation through metal masks. These junctions have successfully been used in a 230 GHz receiver. We then describe the preparation and properties of two slightly different types of Nb/Al-based superconductor–insulator–normal conductor (SIN) junctions applying a ‘‘trilayer technique’’ similar to that used in the fabrication of Nb–Al-oxide–Nb superconductor–insulator–superconductor junctions. For one type of the Nb/Al-based SIN junctions the base layer consists of Nb, for the other type, the base layer is made of Al. In practical applications these devices will operate at temperatures where the Al is in the normal state. Junctions with areas down to about 0.5 μm2 have been made.
Generation of high-density O2 supermagnetron plasma for highly uniform plasma etching
Authors: Kinoshita Haruhisa , Nomoto Kentaro
Published: July 2002

The spatial distribution of O2 supermagnetron plasma generated in between two parallel cathodes was measured by optical emission spectroscopy. Uniform distribution was observed for the cathode spacing of about 20 mm, a gas pressure lower than 10 mTorr and a rf phase difference of about 120°. Optical emission intensities of O radicals (777.1 nm) and O ions (464.9 nm) at a rf phase difference of 150°–180° were about 2 times higher than those of a conventional magnetron plasma. The supermagnetron plasma under a static magnetic field at 3 mTorr O2 gas pressure allowed a uniform photoresist etching of 4-in.-diam wafer, with the high etch uniformity of ±5%. Based upon these results, a new diffusive E×B drift model is proposed to explain the generation mechanism of the high-uniformity and high-density plasma.
Spin superlattice formation in ZnSe-based diluted magnetic semiconductor heterostructures
Authors: Jonker B. T. , Chou W. C. , Petrou A. , Warnock J.
Published: July 2002

We have observed spin–polarized confinement of both electrons and holes in ZnSe-based single and multiple quantum well structures. The samples were grown by molecular beam epitaxy with 100 Å Zn0.99Fe0.01Se barriers and 100–200 Å ZnSe wells. In these systems, the band offsets are very small, so that the large spin–splitting of the diluted magnetic semiconductor levels dominates the alignment of both the conduction and valence bands, permitting magnetic field-tunable confinement. Magnetoreflectivity measurements show a pronounced asymmetry of the heavy hole spin–splitting, a signature of spin-dependent quantum confinement. The relative phase and near-unity intensity ratio of the ‘‘spin–up’’ and ‘‘spin–down’’ excitons confirm localization of spin–up carriers to the ZnSe wells and spin–down carriers to the ZnFeSe layers, producing a field-tunable ‘‘spin–superlattice.’’ These results will be compared to those obtained for higher magnetic ion concentrations in the barriers.
X-ray photoelectron study of surface layers on orthopaedic alloys. I. Ti–6Al–4V (ASTM F-136) alloy
Authors: Lewis Gladius
Published: July 2002

It is now widely acknowledged that the stability, performance, and life of prostheses fabricated from metallic materials are directly related to the compositional details of the layers formed on the surfaces of these components at the time of implantation. The thrust of the present investigation is thus the use of angle-resolved x-ray photoelectron spectroscopy for the identification of the differences in composition and thickness between the layers formed on the surface of a Ti–6Al–4V alloy specimen following exposure for 1 h to either laboratory air or wet steam.
Effect of deposition parameters on the microstructure of ion beam assisted deposition TiN films
Authors: Kheyrandish H. , Colligon J. S. , Kim J-K.
Published: July 2002

The structure of TiNx films formed by low energy ion-assisted deposition has been investigated by electron microscopy techniques. A dual ion beam system was used for the deposition with two Kaufman sources; one producing a 1 keV beam for sputtering Ti, the other a 100–400 eV argon or nitrogen ion beam. The variables of the experiment are the ion energy (100–400 eV), ion species (Ar+ or N+2), ion/atom arrival ratio, the angle of incidence of the ions with respect to the film and the partial pressure of nitrogen during deposition. The structure of the films has been examined using a 200 keV electron microscope for production of electron diffraction patterns and bright and dark field images of the films. It is found that films formed by reactive sputtering (i.e., no ion bombardment) are single phase TiNx which has fcc structure. The grain sizes of the microcrystals, which are randomly orientated, are 10–15 nm. Ion bombardment during deposition generally leads to strong preferential (100) orientation of the microcrystals parallel to the surface of the film and this is coupled with a reduction in the grain size of the microcrystals in the film. Grain growth also appears to take place approximately along the direction of bombarding ions. Results suggest that the degree of orientation depends on the added energy per atom during deposition, indeed there seems to be an energy ‘window’ within which strong, preferred orientations are observed.
Carbon dioxide jet spray cleaning of molecular contaminants
Authors: Hills M. M.
Published: July 2002

Infrared spectroscopic experiments were performed to ascertain the mechanisms of molecular contaminant film removal with a carbon dioxide (CO2) gas/solid jet spray. In the jet spray nozzle, liquid CO2 expands through an orifice and forms a spray of submillimeter-sized solid particles entrained in a gas stream. This spray has been shown to be effective in removing particles and some molecular contaminant films. It has been hypothesized that the spray removes films by one of three methods: abrasion by the solid particles, thermal shock of the film, or formation of a transient liquid film into which the film dissolves, and which then resolidifies and scatters from the surface. We have compared the jet spray removal efficiency of contaminant films of varying solubility in CO2 and measured in real time the cleaning rates of the jet spray for these films. The infrared spectroscopic data show strong correlations between film removal efficiency, the time required to remove a film, and the solubility of the film in liquid CO2. These data, combined with visual observations, indicate that the CO2 jet spray cleaning occurs in two regimes. First, bulk film removal may occur via either thermal shock or abrasion. Second, the residual contaminant film is removed via solvation into the transient liquid CO2 film formed by the impacting CO2 particles. Not all contaminant films can be removed by this technique. Thick films that formed droplets could not be removed by the spray; the drops were simply moved around on the surface. Thin films that were not sufficiently soluble in CO2 were not removed either. These results allow prediction of the jet spray removal efficiency based on identification of the contaminants.
Positron annihilation studies of diamondlike nanocomposite films
Authors: Asoka-Kumar P. , Dorfman B. F. , Abraizov M. G. , Yan D. , Pollak Fred H.
Published: July 2002

Positron annihilation spectroscopy (PAS) as a nondestructive, depth-sensitive probe was applied to study diamondlike nanocomposite (DLN) films. DLN films were deposited from a plasma discharge of polyphenilmetilsiloxane on rf-biased Si substrates. The film properties show systematic changes with variations in the voltage applied to the substrate. The film deposited at an rf bias voltage of 1000 V is stable under a 450 °C anneal. PAS results show that films with the highest breakdown voltage have the highest open-volume regions. Ellipsometric measurements and Raman scattering results from these films are also provided.
Superhard nanocrystalline W2N/amorphous Si3N4 composite materials
Authors: Vepřek S. , Haussmann M. , Reiprich S.
Published: July 2002

Thin films of superhard nanocrystalline nc-W2N/amorphous a-Si3N4 composite have been prepared by plasma chemical vapor deposition at temperatures in the range, 500–550 °C. Maximum hardness of 5200 kg/mm2 (about 51 GPa) and elastic modulus of ≥500 GPa is reached at a silicon content of 7–8 at. %. During the indentation experiment the material shows a large elastic recovery of 80%, which compares favorably with those of hard a-C:H (hardness only about 2000 kg/mm2). The results are similar to those recently obtained with nc-TiN/a-Si3N4 and thus support our concept for the design of novel superhard composite materials with a variety of transition metal nitrides. The question of the upper limit of the strength of these materials is addressed and discussed.
Structural and mechanical properties of carbon nitride CNx (0.2⩽x⩽0.35) films
Authors: Sjöström H. , Hultman L. , Sundgren J.-E. , Hainsworth S. V. , Page T. F. , Theunissen G. S. A. M.
Published: July 2002

This article reports on the structure and mechanical properties of CNx thin films deposited by unbalanced reactive magnetron sputtering of C in N2 discharges. The films were grown on Si(001) substrates kept at temperatures (Ts) between 150 and 600 °C. Depending on Ts, the films contained between 15 and 26 at. % N. X-ray photoelectron spectroscopy of samples typically showed two peaks in the C 1s core level spectrum (centered at 284.5 and 286.2 eV) and two peaks in the N 1s core level spectrum (centered at 398.4 and 400.3 eV). This indicates that there are two types of C–N bonds where N is bonded to both sp2- and sp3-coordinated C atoms in the as-deposited films. Transmission electron microscopy showed that the films produced in the temperature range 200≤Ts≤600 °C had a highly textured, turbostraticlike structure with the c axis in the plane of the film. The basal planes in this structure were found to be buckled and bent due to incorporation of N in substitutional sites. By contrast, films produced at Ts=150 °C were generally amorphous, but contained ∼3 nm diamond clusters. The mechanical properties were evaluated in nanoindentation experiments. Load versus deflection curves showed that the 0.3 μm films grown at Ts≥200 °C had very high elastic recoveries with values up to 85% of the total indentation displacement. Analyses of the shapes of the load versus deflection curves, using a modeling approach that takes into account the elastic and plastic properties of both the film and the substrate, indicate that the contact deformation of the CNx films was predominantly elastic and the films’ hardness values were estimated to be in the range 40–60 GPa. The high hardness and elasticity are proposed to be due to the buckling of the graphitic basal planes, resulting in a structure where the sp2-bonded basal planes are terminated with N bonded to sp3-hybridized carbon atoms that result in a dense, three dimensional, covalently bonded network.
X-ray photoelectron spectroscopy studies of alkoxide-derived lithium niobate
Authors: Kaufherr N. , Eichorst D. J. , Payne D. A.
Published: July 2002

X-ray photoelectron spectroscopy was used to study the thermal conversion of Li, Nb alkoxide derived thin layers on Si(100). Changes in the surface composition with temperature indicated a decomposition process involving lithium transport to the surface, lithium carbonate or oxycarbonate formation, and subsequent reaction to form lithium niobate. For comparison purposes, spectra were also obtained for a single crystal of lithium niobate and layers prepared from sol-gel derived lithium ethoxide and niobium ethoxide. Heat treatment of the individual precursors gave lithium carbonate (or oxycarbonate) and niobium oxide, respectively.
Radio frequency matching for helicon plasma sources
Authors: Rayner J. P. , Cheetham A. D. , French G. N.
Published: July 2002

A theoretical and experimental study of the matching network linking the antenna of a helicon plasma system to its rf power source is described. It is shown that the introduction of a rf step-down transformer into the conventional L-section network has a number of benefits. The transformer increases the reflected antenna impedance seen by the network, bringing it closer to the output resistance of the rf source. The resulting low-Q network is shown to be insensitive to changes in the antenna impedance and the values of the matching elements. It is found that in general only one component in this broad-band network needs to be adjusted rather than the iterative adjustment of two components in the conventional network. The importance of this simplification with respect to the automatic control of the matching network is discussed.
Dielectric properties of Zr, ZrN, Zr3N4, and ZrO2 determined by quantitative analysis of electron energy loss spectra
Authors: Prieto P. , Yubero F. , Elizalde E. , Sanz J. M.
Published: July 2002

The dielectric properties of Zr, ZrN, Zr3N4, and ZrO2 in the energy range from 1 to 80 eV were determined by quantitative analysis of electron energy loss spectroscopy in the reflection mode (REELS) using a recently proposed model. Collective excitations and electronic transitions are well characterized in the REELS spectra after analysis of the respective energy loss function, real and imaginary parts of the dielectric constant, and the optical joint density of states for the four compounds studied here.
Temperature dependent electron cyclotron resonance etching of InP, GaP, and GaAs
Authors: Shul R. J. , Howard A. J. , Vartuli C. B. , Barnes P. A. , Seng W.
Published: July 2002

Electron cyclotron resonance etching of InP, GaP, and GaAs in Ar, Ar/Cl 2, Ar/Cl2/H2, and Ar/Cl2/H2/CH4 plasmas is reported for substrate temperatures from 10 to 170 °C. Etch rates increased as a function of temperature for GaP and GaAs in an Ar/Cl2 plasma. With the addition of H2 or H2/CH4 to the plasma, the GaP and GaAs etch rates decreased and were essentially temperature independent. In comparison, InP etch rates showed a strong temperature dependence regardless of plasma chemistry. At 170 °C, InP etch rates were greater than GaP and GaAs in the Ar/Cl2/H2 and Ar/Cl2/H 2/CH4 plasmas. Atomic force microscopy was used to determine the root-mean-square roughness of the etched surfaces. The etched surface morphology for InP was strongly dependent on temperature and plasma chemistry while smooth pattern transfer was obtained for a wide range of plasma conditions for GaAs and GaP.
Atomic force microscopy studies of rubbed polyimide surfaces used for liquid crystal alignment
Authors: Pidduck A. J. , Bryan-Brown G. P. , Haslam S. , Bannister R. , Kitely I. , McMaster T. J. , Boogaard L.
Published: July 2002

Liquid crystal (LC) surface alignment, which controls off-state contrast in LC displays, is commonly achieved by directional rubbing of polyimide-coated substrates. In order to better understand the alignment mechanism we have used tapping-mode atomic force microscopy to study the dependence of polymer surface morphology on rubbing conditions. Based on results from three polyimides rubbed at different strengths with two cloths, we have identified five surface feature types induced by the rubbing: (1) isolated point defects, (2) occasional deep scratches (≥5 nm), (3) numerous very shallow scratches (mostly ≤1 nm deep), (4) nanoscale surface islands (3–10 nm high, number density ≳109 cm−2), and (5) modified background texture. The last feature confirms that the entire surface has been affected by the rubbing process. The finest resolution achieved, about 10 nm full width, was insufficient to resolve individual polymer chains. Of the features observed, only occasional very deep off-axis scratches clearly affected azimuthal LC alignment. Quasiperiodic features observed under weak rubbing conditions suggest that initial rubbing occurs by a stick-slip mechanism, and the widths of such features gives an estimate of fiber-surface contact size.
High-precision x-ray reflectivity study of ultrathin SiO2 on Si
Authors: Awaji N. , Sugita Y. , Nakanishi T. , Ohkubo S. , Takasaki K. , Komiya S.
Published: July 2002

We have developed a high-precision difference-x-ray-reflectivity technique using intense synchrotron radiation and applied this to evaluate native oxides and ultrathin thermal oxides on Si(100). We have successfully evaluated the density of native oxides. Native oxides formed by HCl and NH4OH solutions have a low density, in contrast to the oxides formed by H2SO4 solution and UV/O3 whose densities are close to those of thermal oxides. By carefully analyzing ultrathin thermally grown oxides with thicknesses of 40 and 70 Å grown at 800–1000 °C, we have revealed the existence of a dense (∼2.4 g/cm3), thin (∼10 Å) layer at the SiO2/Si interface. Oxides grown in O3 or HCl/O2 have a thinner interfacial layer compared to those grown in O2. We have evaluated the effects of ambient and temperature at oxidation on the interfacial layer and the SiO2 layer.
Thermal decomposition reactions of acetaldehyde and acetone on Si(100)
Authors: Armstrong J. L. , White J. M. , Langell M.
Published: July 2002

We have studied the thermal interactions of acetone and acetaldehyde on Si(100), both sputtered and annealed, using high resolution electron energy loss spectroscopy, (HREELS), x-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). There is no carbonyl stretch in HREELS and the C and O(1s) XPS peaks reflect two different carbonyl processes, one involving bond cleavage, the other a reduction of the C–O bond order. Hydrogen TPD gives a peak at 840–850 K which is as much as threefold more intense than from H-saturated Si(100). SiO desorbs near 1050 K and XPS shows total loss of oxygen and retention of carbon. Approximately 34% of the acetaldehyde monolayer and 62% of the acetone monolayer decomposes on annealed Si(100) to produce silicon carbide. In contrast, after sputtering with 500 eV Ar ions, these percentages are reduced to 14% and 25%, respectively. We conclude that Si dimers play an important role in the chemistry of carbonyl groups.
Decomposition of B2H6 on Ni(100)
Authors: Desrosiers Ryan M. , Greve David W. , Gellman Andrew J.
Published: July 2002

The decomposition of diborane (B2H6) has been studied on the Ni(100) surface by means of thermal desorption spectrometry (TDS), x-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). At low gas exposures, decomposed B2H6 on the Ni(100) surface at low temperatures produces several peaks in the H2 TDS spectra. At high gas exposures, B4H10 was observed desorbing from the surface. The TDS spectra and XPS results for low gas exposures indicate that heating to 500 K results in complete decomposition of B2H6, leaving B bonded to the Ni(100) surface. Heating a thick boron film to 700 K results in a significant shift in the B [1s] and Ni [2p3/2] XPS peak positions, indicating that adsorbed B reacts with Ni(100) to form a nickel boride phase. This nickel boride phase is stable on the surface at temperatures up to 900 K. Nickel boride standards (NiB, Ni2B, Ni3B) were used for comparison with the surface nickel boride phase. XPS analysis of the standards led to the determination that the phase forming on the surface at 700 K was Ni2B. As the temperature is increased above 900 K, XPS results indicate that the boron diffuses rapidly into the bulk. It has also been found that high-temperature annealing in the presence of B2H6 results in the formation of a sharp (3×3) LEED pattern produced by 2/9 ML of B on the Ni(100) surface.
Effect of atomic hydrogen on the growth of Ge/Si(100)
Authors: Kahng Se-Jong , Park J. Y. , Booh K. H. , Lee J. , Khang Y. , Kuk Y.
Published: July 2002

Dynamically supplied atomic hydrogen was used for a surfactant growth of Ge on a Si(100) surface. The transition from three-dimensional to two-dimensional growth of Ge was observed as the hydrogen flux was increased to ∼1 ML/s in scanning tunneling microscope images. Layer-by-layer growth was successfully achieved up to 5 ML of Ge in the presence of atomic hydrogen. Observed missing dimer rows, irregular trench structures, and new “pin holes” are believed to be the product of strain relieving mechanism. The layer-by-layer growth can be understood both by energetics and kinetic pathway arguments.
Low temperature deposition of epitaxial titanium carbide on MgO(001) by co-evaporation of C60 and Ti
Authors: Norin L. , McGinnis S. , Jansson U. , Carlsson J.-O.
Published: July 2002

Films of epitaxial titanium carbide were grown on MgO(001) at 400 and 500 °C using a novel method based on the co-evaporation of Ti and C60. Mirrorlike, adhesive films of TiC1-x (0.2<x<0.4) were deposited at growth rates of approximately 0.1 μm/h. X-ray diffraction showed that the crystal orientation relationship between the film and the substrate was TiC(001)//MgO(001) and TiC[100]//MgO[100]. Transmission electron microscopy and low energy electron diffraction were also used to verify the epitaxial growth of the films.