bluuebunny/crossref_metadata_2025 huggingface dataset:
Mixing and segregation of granular materials in chute flows
Authors: Khakhar D. V. , McCarthy J. J. , Ottino J. M.
Published: July 2002

Mixing of granular solids is invariably accompanied by segregation, however, the fundamentals of the process are not well understood. We analyze density and size segregation in a chute flow of cohesionless spherical particles by means of computations and theory based on the transport equations for a mixture of nearly elastic particles. Computations for elastic particles (Monte Carlo simulations), nearly elastic particles, and inelastic, frictional particles (particle dynamics simulations) are carried out. General expressions for the segregation fluxes due to pressure gradients and temperature gradients are derived. Simplified equations are obtained for the limiting cases of low volume fractions (ideal gas limit) and equal sized particles. Theoretical predictions of equilibrium number density profiles are in good agreement with computations for mixtures of equal sized particles with different density for all solids volume fractions, and for mixtures of different sized particles at low volume fractions (ν<0.2), when the particles are elastic or nearly elastic. In the case of inelastic, frictional particles the theory gives reasonable predictions if an appropriate effective granular temperature is assumed. The relative importance of pressure diffusion and temperature diffusion for the cases considered is discussed.
Diffusion of ion-implanted Ga in SiO2
Authors: van Ommen A. H.
Published: July 2002

We studied the diffusion of ion-implanted Ga in SiO2 by means of Rutherford backscattering spectrometry. We also investigated the doping of Si from a Ga-implanted SiO2 layer. It turned out that Ga occurs in three different chemical states in SiO2. In one chemical form it is virtually immobile, which we relate to Ga-on-Si sites in the network. A second type of Ga exhibits a very high diffusion coefficient. We have found that the formation of this type of Ga occurs by the same mechanism under almost all of the experimental circumstances. An interstitial GaOH molecule is proposed to be the diffusing species in this case. Finally, there is a third form in which Ga diffuses independently of the other diffusing species. The higher activation energy of this diffusion process suggests the diffusion of gallium in some substitutional form.
Investigation of the SiO2-induced substrate current in silicon field-effect transistors
Authors: Weinberg Z. A. , Fischetti M. V.
Published: July 2002

An experimental investigation is presented for the substrate current (holes) appearing in n-channel field-effect transistors having SiO2 as their gate insulator. In these experiments, the gate is biased by a high and positive voltage, causing an electron current to be injected from the device channel into the oxide. This current is accompanied by the substrate current whose origin is not clear. The experiments were performed by application of short pulses (400 μsec) to the gate. It is shown that the substrate current is too large to be explained by simple electron tunneling from the silicon valence band into the oxide. Temperature-dependence measurements, down to 20 K, show that some of the data are not consistent with models for hole transport from the oxide into the silicon valence band. It is argued that the substrate current may be related to the energy loss experienced by hot electrons as they traverse the oxide. It is further argued that the same mechanism responsible for the substrate current may produce positive charge at the injecting electrode and thus lead to breakdown in thin oxide devices.
The acceptor level of vanadium in III–V compounds
Authors: Clerjaud B. , Naud C. , Deveaud B. , Lambert B. , Plot B. , Bremond G. , Benjeddou C. , Guillot G. , Nouailhat A.
Published: July 2002

The positions of the acceptor level of vanadium in GaAs, GaP, and InP are estimated. This level is found to be at about Ec−0.14 eV in GaAs and about Ec−0.8 eV in GaP. It is above the conduction band edge in InP. These positions in the three hosts are in agreement with the trends proposed by Ledebo and Ridley. The finding of a very high acceptor level of vanadium in GaAs does not allow to explain the semi-insulating behavior of GaAs:V. Other possible compensating centers are considered.
Piezoelectricity and related properties of vinylidene fluoride and trifluoroethylene copolymers
Authors: Koga Keiko , Ohigashi Hiroji
Published: July 2002

The piezoelectric properties and ferroelectric transition behaviors of vinylidene fluoride and trifluoroethylene copolymers, P(VDFx−TrFE1−x), were studied as a function of x in the range of x=0.37–0.94. The electromechanical coupling factor kt and piezoelectric constant d31 are strongly dependent on both x and annealing temperature. In the copolymers of 0.65≲x≲0.82, a large kt value of 0.27–0.3 has been found when annealed above ferroelectric-to-paraelectric transition temperature Tc. The large kt is attributable to their high remanent polarization (∼110 mC/m2) which arises from high degrees of crystallinity and preferred orientation of well-grown crystallites. Structural and morphological changes induced by annealing and poling were characterized by SEM, DSC, and x-ray diffraction studies. Phase diagram of the copolymers were obtained by DSC measurements. Tc increases with increase of x and coincides with melting temperture Tm at x≂0.82. In the range of x≳0.82, Tc=Tm.
Temperature dependence of impact ionization coefficients in InP
Authors: Taguchi Kenko , Torikai Toshitaka , Sugimoto Yoshimasa , Makita Kikuo , Ishihara Hisahiro
Published: July 2002

Impact ionization coefficients for electrons and holes in InP were measured experimentally at 25–175 °C in the 400–600 kV/cm electric field range with planar avalanche photodiodes, in which the n-InP avalanche region was separated from the light absorbing InGaAs and/or InGaAsP layers. α and β monotonically decreased with elevated temperatures; β/α slightly decreased with increasing temperature. Comparison of the experimental results with Okuto–Crowell formula on the impact ionization coefficient gave the phonon energy ERO=46 meV and the phonon scattering mean free path λ0=41.7 Å for electron impact ionization and ERO=36 meV and λ0=41.3 Å for hole impact ionization, respectively. Curves calculated by using these parameters agree with the experimental results quite satisfactorily at each temperature.
Molecular-beam-epitaxial growth and selected properties of GaAs layers and GaAs/(Al,Ga)As superlattices with the (211) orientation
Authors: Subbanna Seshadri , Kroemer Herbert , Merz James L.
Published: July 2002

We present the first detailed study of the molecular-beam-epitaxial growth and the properties of GaAs layers and GaAs/(Al,Ga)As superlattices with the (211)A and (211)B orientations rather than the (100) orientation. There are significant differences in growth morphology of the two (inequivalent) (211) surfaces of GaAs, as well as in their low-temperature photoluminescence. Electron diffraction patterns taken during the growth indicate some submicroscopic faceting on (211)B growth, but much less on (211)A. The visual microscopic morphology of both orientations is improved drastically by first growing a GaAs/(Al,Ga)As superlattice buffer layer, especially for the (211)A growth, which becomes essentially featureless. Doping studies show that silicon acts as an essentially uncompensated donor on GaAs (211)B layers, down to the lowest As:Ga flux ratios, but on (211)A layers, it is amphoteric: At low As:Ga flux ratios, it behaves as a partially compensated acceptor, at high flux ratios, as a partially compensated donor. A bonding model for this incorporation behavior is given. The photoluminescence spectra of the (211)A layers are comparable to (100) spectra, the (211)B spectra are somewhat broadened, correlating with the difference in faceting. Superlattices with well widths from 2 to 12 nm exhibited integrated photoluminescence intensities about a factor of 10 higher than identical (100)-oriented superlattices grown simultaneously side-by-side. The photoluminescence linewidths of the (211)A superlattices were comparable to (100) linewidths, while those of (211)B superlattices were again larger.
Fast response quantum well photodetectors
Authors: Coon D. D. , Karunasiri R. P. G. , Liu H. C.
Published: July 2002

Fast, real time quantum well infrared photodetector concepts are developed. Device parameters are discussed for applications as a high data rate receiver for use with long wavelength, nonsilica infrared optics and fibers. The device relies on photoemission of charge stored in a quantum well and replacement of depleted charge by tunneling from a doped layer.
Structural analysis of Au–Ni–Ge and Au–Ag–Ge alloyed ohmic contacts on modulation-doped AlGaAs–GaAs heterostructures
Authors: Higman T. K. , Emanuel M. A. , Coleman J. J. , Jeng S. J. , Wayman C. M.
Published: July 2002

Both secondary ion mass spectrometry and transmission electron microscopy have been used to characterize Au–Ni–Ge and Au–Ag–Ge ohmic contacts to modulation-doped GaAs/AlGaAs heterostructures. The time periods for alloying the contact were kept to a minimum (<25 s) in order to obtain controlled diffusion of Ge into the semiconductor. Structural analysis shows that a stable alloy contact is formed after 3 s of alloying at temperatures above the Au–Ge melting point. There were no major structural or compositional changes observed with longer alloying times. The Ge-rich region has a tendency to regrow epitaxially on the GaAs after alloying, as shown by high-resolution electron microscopy.
Influence of local heating on current-optical output power characteristics in Ga1−xAlxAs lasers
Authors: Todoroki Satoru
Published: July 2002

Available optical output powers from the Ga1−xAlxAs lasers are limited by the catastrophic optical damage or output power saturation due to local heating on the facet. Especially, the saturation of current-optical output power characteristically occurs during the operations at relatively low output power. This report describes the degradation process in an active region for lasers with the striped window as an electrode with respect to current-optical output power characteristics, electroluminescence image observation, and measurement of temperature rise at the facet and the cavity inside using laser Raman spectroscopy. The nonluminescing dark region observed in rapidly degraded lasers develops from the facet to the cavity inside. The dark region is so nonradiative that electron-hole recombination in that region produces heat rather than light. The measured local temperature on the facet drastically rises and the area influenced by local heating extends into the cavity inside. Although the threshold current increases by only 16%, the optical output power is saturated and finally the initial optical output power (20 mW/facet) is hardly maintained. It is found that the change in current-output power characteristics almost corresponds to the calculated results which incorporated temperture dependence of local absorption loss in the dark region.
Composition and lattice-mismatch measurement of thin semiconductor layers by x-ray diffraction
Authors: Fewster P. F. , Curling C. J.
Published: July 2002

X-ray diffraction methods for determining alloy composition and mismatch have been used for many years by measuring the separation of peaks in a high-resolution diffractogram. This method can still be used, but not for layer thicknesses below 1–2 μm. The diffractogram may appear simple to interpret, for example, two diffraction peaks from a single layer on a substrate, but simple measurement of peak separation can lead to significant errors in determining the mismatch and hence alloy composition. This paper gives examples of peak shifting with thin layers and shows how, by simulating the diffraction profiles, a reliable determination of mismatch, and hence alloy composition can be made.
Electronic processes in silicon nitride
Authors: Manzini S.
Published: July 2002

Time, electric field, and temperature dependence of the flat-band voltage shift and steady-state contact-current versus contact-field characteristics in polycrystalline silicon-oxide-nitride-oxide-silicon structures subjected to high field stress have been studied in detail. Experimental data are compared with exact (numerical) solutions of the Arnett model [P. C. Arnett, J. Appl. Phys. 46, 5236 (1975)] for one-carrier (electron) transport in silicon nitride. It is shown that a simple field-assisted thermal ionization (Poole–Frenkel) detrapping mechanism cannot explain the experimental observations, leading to unphysical values for the attempt-to-escape frequency. An alternative model is proposed based on a thermally assisted tunneling detrapping mechanism. Trap density, effective capture cross section, ground-state energy, and an upper limit for the energy of the first excited state of the electron trap are determined.
Observation of a dose-rate dependence in the production of point defects in quartz
Authors: Mondragon M. A. , Chen C. Y. , Halliburton L. E.
Published: July 2002

Electron-spin resonance (ESR) has been used to monitor the growth of aluminum-associated trapped-hole centers (i.e., [AlO4 ]0 centers) in high-purity cultured quartz during a sequence of irradiations with 1.7-MeV electrons. Production curves were obtained at three dose rates (3.6, 36, and 360 krad/min) for a series of crystals furnished by three commercial growers of quartz. The shape of each curve depended directly on the dose rate and the origin of the quartz; many of them had an initial rapid growth to a maximum concentration followed by a 5%–25% decrease as the number of defects approached an equilibrium value at a higher dose. We suggest that the complex shapes of these defect production curves, and also their variation between samples, is caused by a competition between the formation of [AlO4 ]0 centers and the formation of [AlO4 /H+ ]0 centers. At higher dose rates, in the 4–40 Mrad/min range, no dependence on dose rate was observed.
Compositional study of ultrathin rapidly reoxidized nitrided oxides
Authors: Hori Takashi , Iwasaki Hiroshi , Ohmura Takuichi , Samizo Atsuko , Sato Minoru , Yoshioka Yoshiaki
Published: July 2002

Ultrathin nitrided oxides (7.7 nm) were reoxidized for the first time by lamp-heated rapid thermal annealing in O2 at 900–1150 °C for 15–600 s. Compositions and residual hydrogen contents in various reoxidized nitrided oxides were studied by Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS), respectively. AES analyses show that as reoxidation proceeds, the nitrogen concentration peak near the outer surface decreases rapidly, while that near the Si-SiO2 interface [N]int decreases very slowly. It is newly found that the nitrogen-rich layer near the Si-SiO2 interface moves further into the substrate as reoxidation proceeds, following a movement similar to that of the Si-SiO2 interface. As the starting nitrogen content is lowered or the reoxidation temperature is raised, the distance of the peak movement ΔdAES and the reduction of [N]int are larger. The ΔdAES is found in quantitatively good agreement with the increase of film thickness evaluated by capacitance-voltage measurements. The movements are due to the diffusion-limited interfacial oxidation. In contrast to reoxidation, annealing in N2 of nitrided oxides scarcely reduces the [N]int and does not increase ΔdAES. SIMS analyses show that as reoxidation proceeds, hydrogen concentration [H] in the film decreases monotonically. As the starting nitrogen content is lowered or the reoxidation temperature is raised, [H] decreases more rapidly. Electron trapping was monitored by flat-band voltage shift ΔVFB under high-field stress, and ΔVFB is found to decrease with the reduction of [H] following a proportional relation regardless of the fabrication condition. [H] is also reduced by annealing in N2 and the reduction is comparable to that by reoxidation.
Oxidation of the Si(100) surface promoted by Sr overlayer: An x-ray photoemission study
Authors: Mesarwi A. , Fan W. C. , Ignatiev A.
Published: July 2002

The interaction of strontium overlayers with atomically clean Si(100) surface has been studied by x-ray photoelectron spectroscopy. Also studied were the effects of Sr overlayers on the oxidation of silicon at different Sr coverage (0≤θ≤1.85) and different oxygen exposure. It is found that at low coverage, θ≤1 monolayer (ML), Sr interacts with the Si(100) surface to form a strong ionic bond. Also, Sr is found to significantly increase the oxidation of the Si(100) surface. Silicon oxidation increased with increasing Sr coverage to a maximum at θ≊1 ML. Above 1-ML Sr coverage, the rate of Si oxidation decreased with increasing coverage.
Modeling the memory retention characteristics of silicon-nitride-oxide-silicon nonvolatile transistors in a varying thermal environment
Authors: McWhorter P. J. , Miller S. L. , Dellin T. A.
Published: July 2002

The memory retention characteristics of silicon-nitride-oxide-silicon nonvolatile memory devices are found to be strongly thermally activated. A model is developed based on thermal emission of charge from traps. This model accurately predicts the threshold voltage decay of transistors stored in varying thermal environments. The model is demonstrated to be accurate over 7 decades of time and for temperatures between −40 and 200 °C.
Spontaneous polarization of electrons in quantum dashes
Authors: Bakshi P. , Broido D. A. , Kempa K.
Published: July 2002

We investigate the possibility of generating a spontaneous anti-ferroelectric polarization in an array of elongated quantum dots, which we call ‘‘quantum dashes.’’ We suggest ways in which this phenomenon might be observed and point out possible technological applications resulting from this phenomenon.
HIP beryllium: Thermal expansivity from 4 to 300 K and heat capacity from 1 to 108 K
Authors: Swenson C. A.
Published: July 2002

Linear thermal expansivity measurements from 4.2 to 302 K and heat capacity measurements from 4.6 to 108 K have been made on HIP (hot isostatic pressing) beryllium metal. The heat capacity data are in good agreement with previous data from 1.4 to 30 K for high purity single crystals. Our Θ0 = 1485(5) K also agrees well with that calculated from high-pressure elastic constant results. The linear thermal expansivity decreases rapidly with decreasing temperature from 1.13 × 10−5 K−1 at 293 K, as would be expected for the large Θ0, and becomes very small (< 10−7 K−1) below 50 K. The expansivities below 108 K are consistent with a constant γlat0 = 1.15 and with γlat0 = 0.91 from the high-pressure elastic constant results.
Electronic conduction mechanisms in thin oxynitride films
Authors: Wong H. , Cheng Y. C.
Published: July 2002

We report a new theory for electronic conduction in thin (150 Å) oxynitride films. The present experiment reveals that the electronic conduction mechanism in the oxynitride deviates from that in conventionally grown silicon oxide in the electric field ranging from 6 to 14 MV/cm and suggests that the electronic conduction is governed by three different mechanisms according to the strength of electric field. We suggest that the current conduction is trap-assisted at electric fields lower than 8 MV/cm. Specifically, the conduction is due the tunneling of electrons into the shallow traps in the insulator. In the high-field region (≳10 MV/cm), the Fowler–Nordheim (FN) effect becomes dominant and depends on the dielectrics preparation conditions. In the moderate-field region, traps can be filled by both FN current and direct tunneling of electron into the traps which result in a quasi-saturation in the leakage current. The experiment also shows a turnaround behavior in leakage current level, ledge in current-voltage characteristics, and field dependency of the current as the nitridation proceeds. These observations can be readily explained based on the proposed conduction mechanism.
The optical properties of AlGaAs/GaAs hyperbolic quantum-well structures
Authors: Li E. Herbert , Weiss B. L.
Published: July 2002

The optical properties of a nonsquare quantum well with a hyperbolic potential profile have been analyzed. The quantized energy levels for the confined carriers are determined analytically using the effective mass approximation. The absorption coefficients for an undoped AlGaAs/GaAs hyperbolic quantum-well structure have been calculated based on the one electron density matrix formulation, taking into account broadening effect due to intraband relaxation by carrier–carrier scattering. The results show that the absorption edge as well as its peak value are more sensitive to well shape than well width.
Optical modulation in very thick coupled InxGa1−xAs/GaAs multiple quantum well structures
Authors: Harwit Alex , Fernandez R. , Eades W. D.
Published: July 2002

We report optical modulation results on InGaAs/GaAs coupled multiple quantum well spatial light modulators. The structure consists of an n+-GaAs buffer, an undoped 250-period coupled multiple quantum well layer, and a p+-GaAs/InAs cap. The samples are probed at room- and liquid-nitrogen temperatures using photoabsorption spectroscopy. At room temperature an absorption peak is observed at 969 nm, and this shifts to 982 nm as the field is increased from 0 to 71 kV/cm, in good agreement with theory. For a single pass through the structure, this results in a contrast ratio ≳8:1 at 969 nm. At 80 K, the contrast ratio increases to approximately 60:1 at 914 nm. A second modulator exhibits a contrast ratio ≳8:1 at 1.04 μm at room temperature.
Properties of silicon dioxide films prepared by pulsed-laser ablation
Authors: Slaoui A. , Fogarassy E. , Fuchs C. , Siffert P.
Published: July 2002

Silicon dioxide films have been prepared by ArF pulsed-laser ablation of a silicon monoxide (SiO) target in oxygen atmosphere at various substrate temperatures (20–450 °C). The structural and electrical properties of the SiO2 deposited films have been investigated. The experimental results indicate a strong reduction of the porosity and structural defects in the films for increasing values of the deposition temperature. Film properties are also strongly influenced by the application of post rapid thermal (400–1000 °C) treatments in Ar atmosphere. The refractive index of these films increases, whereas we observe a reduction of their relative dielectric constant which is mainly attributed to the diminution of the amount of hydrogen trapped into the laser grown oxide. A minimum oxide charge density of 1.5×1011 cm−2 with a dielectric breakdown field of 2–8 MV/cm was measured in oxide films deposited at 20 °C and subsequently annealed at 1000 °C by rapid thermal treatment (30 s).
Oxide reliability criterion for the evaluation of the endurance performance of electrically erasable programmable read only memories
Authors: Papadas C. , Ghibaudo G. , Pananakakis G. , Riva C. , Mortini P.
Published: July 2002

The impact of the oxide reliability on the endurance performance of nonvolatile memories [electrically erasable read only memories (EEPROMs)] is analyzed quantitatively. The degradation rate of tunnel SiO2 layers as obtained from EEPROM cells as well as tunnel oxide capacitors subjected to different modes of electrical stress (write/erase operations, static and dynamic stress) are compared and attributed to a specific charge generation mechanism. Furthermore, a reliability criterion for the optimization of the tunnel oxide technology entering the fabrication of EEPROM cells is also proposed.
Thermal interface crack problems in dissimilar anisotropic media
Authors: Chao C. K. , Chang R. C.
Published: July 2002

The fundamental nature of an interface crack between dissimilar anisotropic media under the uniform heat flux is studied. Based on the Hilbert problem formulation and a special technique of analytical continuation, a simple and compact version of general solutions for the thermal field are given. The temperature gradients or heat fluxes are found to possess the characteristic inverse square-root singularity in terms of the radial distance from the crack tip. Due to this singular behavior, the heat flux intensity factor is then introduced to quantify the thermal energy cumulated in the neighborhood of the crack tip. Some numerical examples are given for the application of the final results to the intensity factor of heat flux as well as the full field solutions of temperature. It is very interesting to see that the solutions associated with the dissimilar media can be easily obtained from the corresponding problem associated with the homogeneous media by a simple substitution of their own material properties. In general, the system possesses the symmetric properties of the thermal conductivity coefficients which can make the heat flux intensity factor smaller. Consequently, the thermal energy intensification is diminished.
Influence of refilling effects on deep-level transient spectroscopy measurements in Se-doped AlxGa1−xAs
Authors: Enríquez L. , Dueñas S. , Barbolla J. , Izpura I. , Muñoz E.
Published: July 2002

Deep-level transient spectroscopy (DLTS) and constant-capacitance DLTS (CC-DLTS) techniques have been used to investigate selenium-related DX centers in AlGaAs alloys. The value of the thermal activation energy obtained by both techniques was the same (0.21 eV); however, experimental curves show some important differences. While CC-DLTS curves show only one peak, which reveals that there exists only one DX center in Se-doped AlGaAs, in DLTS curves it is possible to resolve up to two peaks lying at a lower temperature than the one observed by CC-DLTS. This disagreement may be due to the fact that DLTS measurements are strongly affected by refilling effects which occur in the edge zone of the space-charge region during capacitance transients performed at constant voltage. These effects accelerate the capacitance transients and can lead to too high thermal-emission rates. In contrast, these effects do not affect CC-DLTS curves, because in constant-capacitance voltage transients the edge of the space-charge region remains unchanged and refilling effects do not take place. These effects are rather important on DX levels because they exhibit thermally activated capture cross sections and very low ionization factors at the experimental temperatures and, therefore, capture processes are slow and their time constants can be similar to those of the emission processes.
Long wavelength lattice dynamics for quaternary alloys: GaInPSb and AlGaAsSb
Authors: Jaw D. H. , Stringfellow G. B.
Published: July 2002

Long wavelength lattice dynamics of the quaternary alloys Ga1−xInxP1−ySby and AlxGa1−xAsySb1−y have been investigated. The optical phonons show a four mode behavior in Ga1−xInxP1−ySby and a three mode behavior in AlxGa1−xAsySb1−y over the composition ranges investigated. An average random cell model is used to describe the behavior of the optical phonons of both systems. The calculations of the dependence of the long wavelength optical phonon frequencies on solid composition are in good agreement with the experimental results.
Optimization of modulation-doped Ga1−xInxAs/InP heterostructures towards extremely high mobilities
Authors: Hardtdegen Hilde , Meyer R. , Hollfelder M. , Schäpers Th. , Appenzeller J. , Lo/ken-Larsen Hilde , Klocke Th. , Dieker Christel , Lengeler B. , Lüth H. , Jäger W.
Published: July 2002

This paper presents a study of the electrical and structural properties of inverted modulation-doped GaInAs/InP heterostructures grown by low-pressure metalorganic vapor phase epitaxy. First, the thickness of the GaInAs layer was optimized in lattice-matched samples to find the smallest thickness in which high Hall mobility is observed. Next, in a section closest to the InP the In content was varied. A steady increase of mobility with indium composition was observed. A maximum of 450 000 and 15 500 cm2/V s was obtained for a 10-nm-thick Ga1−xInxAs layer with x=0.77 at 6 and 300 K, respectively. Channels with higher indium content exceed the critical thickness and mobility drops off sharply. The decreasing mobility correlates with the formation of misfit dislocations at the interface indicating increasing scattering processes of the GaInAs layer.
A normal-incidence type-II quantum-well infrared photodetector using an indirect AlAs/Al0.5Ga0.5As system grown on (110) GaAs for mid- and long-wavelength multicolor detection
Authors: Wang Y. H. , Li Sheng S. , Ho Pin , Manasreh M. O.
Published: July 2002

A normal-incidence type-II indirect AlAs/Al0.5Ga0.5As quantum-well infrared photodetector grown on (110) GaAs by molecular-beam epitaxy for mid- and long-wavelength multispectrum detection has been developed. The normal-incident excitation of long-wavelength intersubband transition was achieved in the [110] X-band-confined AlAs quantum wells. Six absorption peaks including four from X-band to Γ-band intersubband resonant transitions were observed at wavelengths, λp1−6=2.2, 2.7, 3.5, 4.8, 6.5, and 12.5 μm. The resonant transport from X band to Γ band gives rise to high photoconductive gain, which is highly desirable for focal plane arrays image sensor applications.
Thermally activated real-space-transfer noise in pseudomorphic high-electron-mobility transistors
Authors: van Die A. , Dijkhuis J. I.
Published: July 2002

The low-frequency (0.5 Hz≤f≤100 kHz) noise characteristics of AlGaAs/InGaAs/GaAs pseudomorphic high-electron-mobility transistors at temperatures from 3.8 up to 300 K are reported. Between 100 and 200 K the noise spectra are dominated by an intense thermally activated contribution, due to electron transfer from InGaAs to AlGaAs (real-space transfer) and vice versa. Its spectral shape is significantly broader than a pure Lorentzian and results from a distribution of associated time constants. This is interpreted in terms of an inhomogeneous barrier height at the AlGaAs/InGaAs interface with a half-width at half-maximum of 29.3±0.8 meV. It is proposed that random disorder in alloy composition and the concomitant fluctuations in the conduction-band discontinuity along the channel give rise to such a spread in barrier heights.
Measurement of band offset of a strained-layer single quantum well by a capacitance-voltage technique
Authors: Subramanian S. , Arora B. M. , Srivastava A. K. , Fernandes G. , Banerjee S.
Published: July 2002

We report on a modified Kroemer’s analysis [Appl. Phys. Lett. 36, 295 (1980)] for the determination of the band offset ΔEc of a single quantum well from a carrier profile obtained by capacitance-voltage measurement. The procedure is applied to a pseudomorphic GaAs/InGaAs/GaAs strained layer structure.
Investigation of InAs submonolayer and monolayer structures on GaAs(100) and (311) substrates
Authors: Ilg Matthias , Alonso M. Isabel , Lehmann Arno , Ploog Klaus H. , Hohenstein Matthias
Published: July 2002

We demonstrate a new route to the synthesis of InAs monolayer structures in GaAs by bridging the fundamental gap between the requirement of the lowest possible substrate temperatures to suppress In segregation and the necessity to maintain sufficiently high temperatures for the growth of low-defect density material. This mediation between opposing aspects of the molecular beam epitaxy of these InAs structures is achieved by a modulation of the substrate temperature and by a minimization of the amount of GaAs cap material grown at low temperature. High-resolution x-ray diffraction and high-resolution electron microscopy combined with photoluminescence (PL) and PL excitation spectroscopies reveal excellent structural properties for our series of (311) and (100) oriented submonolayer and monolayer structures. A comparison of our PL results with already published data proves our In concentration profiles to be very sharp and from a numerical analysis we deduce an upper limit of 0.2 for the In segregation probability in these structures. In addition we obtain as upper limits for the conduction band offsets Q(100)c≤0.4 and Q(311)c≤0.55 for (100) and (311) orientations, respectively.
Modeling the spectral response of the quantum well solar cell
Authors: Paxman M. , Nelson J. , Braun B. , Connolly J. , Barnham K. W. J. , Foxon C. T. , Roberts J. S.
Published: July 2002

The quantum well solar cell is an alternative to more conventional multiband gap approaches to higher cell efficiency. Preliminary studies have shown that the insertion of a series of quantum wells into the depletion region of a GaAs/AlxGa1−xAs p-i-n solar cell can significantly enhance the cell’s short-circuit current. We present here a model for the spectral response of GaAs and AlxGa1−xAs p-n and p-i-n solar cells, with and without quantum wells, based on a standard solution of the minority-carrier equations. Particular emphasis is placed on modeling the absorption coefficient of the AlxGa1−xAs and of the quantum wells. We find that our model can accurately predict the spectral response of a wide variety of cells: both conventional p-n junctions in GaAs and AlxGa1−xAs, and various geometries of quantum well solar cell in AlxGa1−xAs/GaAs (x∼0.3). We discuss the strengths and weaknesses of the model and its underlying assumptions, and conclude by using the model to design p-i-n quantum well solar cells with higher short-circuit current outputs.
Molecular beam epitaxy of PbTe/EuTe superlattices and their structural investigation by x-ray diffraction using reciprocal space mapping
Authors: Koppensteiner E. , Springholz G. , Hamberger P. , Bauer G.
Published: July 2002

Molecular beam epitaxial growth of PbTe/EuTe superlattices on (111) BaF2 was studied using reflection high energy electron diffraction (RHEED). The surface reconstructions of EuTe, its nucleation and critical layer thickness on (111) PbTe were investigated by in situ RHEED. From these studies, the optimum conditions for the growth of PbTe/EuTe superlattices were derived. PbTe/EuTe superlattice (SL) samples were investigated by high resolution x-ray diffraction. Triple axis x-ray diffractometry was employed to characterize the structural parameters of a PbTe/EuTe SL sample (87.6 PbTe monolayers/5 monolayers EuTe) deposited on a 4 μm thick PbTe buffer layer. Reciprocal space maps around the (222) and (264) Bragg reflections are used to analyze the strain status of the SL layers with very high precision. Using reciprocal space mapping, the small strain gradient present within the SL along growth direction can be determined quantitatively. The full width at half-maximum (FWHM) values of (222) SL x-ray diffraction peaks along and perpendicular to the [111] growth direction were measured. It was found that along the [111] direction the FWHM’s increase with satellite number, which is most probably caused by either lateral superlattice thickness variations of 0.4% for the x-ray spot size of 1×8 mm2 or a thickness variation in growth direction of 1.5%. The broadening of the main superlattice peak perpendicular to the [111] growth direction indicates the presence of mosaicity in the superlattice layers, which obscures due to its magnitude the measurement of lateral interface roughness.
Crystallographic and magnetic properties of CoxFe1−xCr2S4
Authors: Kim Chul Sung , Ha Min Yong , Ko Heung Moon , Oh Young Jei , Lee Heung Soo , Lee Sang Youl , Sur Jung Chul , Park Jae Yun
Published: July 2002

The mixed series CoxFe1−xCr2S4 has been studied by x-ray, Mössbauer spectroscopy, and superconducting quantum-interference device (SQUID) magnetometry. The crystal structure is found to be a cubic spinel, and the lattice constant a0 decreases linearly with increasing cobalt concentration. Mössbauer spectra of CoxFe1−xCr2S4 have been taken at various temperatures ranging from 4.2 to 300 K. The iron ions are ferrous and occupy the tetrahedral sites. The Curie temperature increases linearly with cobalt concentration, suggesting that the superexchange interaction for the Co-S-Cr link is stronger than that for the Fe-S-Cr link. Magnetic hyperfine and quadrupole interactions in Co0.9Fe0.1Cr2S4 at 4.2 K have been studied, yielding the following results: Hhf=80.8 kOe, 1/2e2qQ(1+1/3η2)1/2=2.65 mm/s, θ=15°, φ=75°, and η=1.0. It is notable that, as the temperature decreases below the Curie temperature, quadrupole splitting increases with decreasing temperature, suggesting the presence of an electric field gradient and accompanying relaxation effects. Magnetic susceptibility measurements by SQUID magnetometry show that superexchange interactions between Fe2+ ions are ferrimagnetic.
Study of vibrational properties of InGaAsP by far-infrared reflectivity
Authors: Amiotti M. , Guizzetti G. , Patrini M. , Piaggi A. , Borghesi A. , Colombo L. , Landgren G.
Published: July 2002

We measured room-temperature reflectivity in the far-infrared region (100–700 cm−1) of In1−xGaxAsyP1−y films grown by metalorganic phase epitaxy on InP substrate. The actual As content of the quaternary alloy (ranging from y=0.29 to y=1) was derived by combining photoluminescence and high-resolution x-ray diffraction results. Over the whole compositional range four different vibrational branches, corresponding to InAs-like, GaAs-like, InP-like, and GaP-like mode, have been observed, thus confirming the attribution of ‘‘four-mode behavior’’ to the quaternary alloy. The frequency position of the four main peaks has been calculated within a valence-force-field model. The frequency shift of these peaks in the alloy with respect to the bulk values has been explained taking into account anharmonicity and strain effects by means of a simplified elastic model.
Direct current magnetron sputtered In2O3 films as tunnel barriers
Authors: Kasiviswanathan S. , Rangarajan G.
Published: July 2002

Insulating films of In2O3 were prepared by sputtering indium in the presence of pure oxygen using dc magnetron sputtering. Transmission electron microscopic investigations showed the films to be single phase and polycrystalline. Analysis of the optical transmittance data showed the films to have an optical band gap of 3.71±0.01 eV. Tunnel junctions were made with high Tc superconductors Bi2Sr2Ca1Cu2Oy and NdBa2Cu3O7−δ using indium oxide as the barrier layer and Pb0.5In0.5 as the counter electrode. The conductance spectra displayed prominent structures attributable to energy gap. The reduced gap parameters for Bi2Sr2Ca1Cu2Oy and NdBa2Cu3O7−δ were found to be 4.0±0.5 and 5.2±0.6, respectively.
Eu-doped KCl phosphor crystals as a storage material for two-dimensional ultraviolet-ray or x-ray imaging sensors
Authors: Nanto H. , Endo F. , Hirai Y. , Nasu S. , Taniguchi S. , Takeuchi N.
Published: July 2002

Intense optically stimulated luminescence (OSL) with a peak at about 430 nm is observed in ultraviolet (UV)-ray or x-ray irradiated europium-doped potassium chloride (KCl:Eu) phosphor crystals. The OSL intensity is increased with increasing UV-ray or x-ray irradiation dose. This suggests that KCl:Eu phosphor crystal is useful as a material for two-dimensional UV-ray or x-ray imaging sensor utilizing OSL phenomenon. The results obtained are consistent with the proposed emission mechanisms of the 420 nm OSL peak, based on the recombination of electrons released from the F centers with Eu3+ ions. The excitation mechanism for the OSL in UV-ray irradiated KCl:Eu phosphor crystals is also discussed.
Laplace transform deep-level transient spectroscopic studies of defects in semiconductors
Authors: Dobaczewski L. , Kaczor P. , Hawkins I. D. , Peaker A. R.
Published: July 2002

A quantitative improvement in deep-level transient spectroscopy (DLTS) resolution has been demonstrated by using Laplace transform method for the emission rate analysis. Numerous tests performed on the software used for the calculations as well as on the experimental setup clearly demonstrated that in this way the resolution of the method can be increased by more than an order of magnitude. Considerable confidence in this approach was gained through measurements of a selection of well-characterized point defects in various semiconductors. The results for platinum in silicon and EL2 in GaAs are presented. For each of these cases conventional DLTS give broad featureless lines, while Laplace DLTS reveals a fine structure in the emission process producing the spectra.
Roughening instability and ion-induced viscous relaxation of SiO2 surfaces
Authors: Mayer T. M. , Chason E. , Howard A. J.
Published: July 2002

We characterize the development of nanometer scale topography (roughness) on SiO2 surfaces as a result of low energy, off-normal ion bombardment, using in situ energy dispersive x-ray reflectivity and atomic force microscopy. Surfaces roughen during sputtering by heavy ions (Xe), with roughness increasing approximately linearly with ion fluence up to 1017 cm−2. A highly coherent ripple structure with wavelength of 30 nm and oriented with the wave vector parallel to the direction of incidence is observed after Xe sputtering at 1 keV. Lower frequency, random texture is also observed. Subsequent light ion (H, He) bombardment smoothens preroughened surfaces. The smoothing kinetics are first order with ion fluence and strongly dependent on ion energy in the range 0.2–1 eV. We present a linear model to account for the experimental observations which includes roughening both by random stochastic processes and by development of a periodic surface instability due to sputter yield variations with surface curvature which leads to ripple development. Smoothing occurs via ion bombardment induced viscous flow and surface diffusion. From the smoothing kinetics with H and He irradiation we measure the radiation enhanced viscosity of SiO2 and find values on the order of 1–20×1012 N s m−2. The viscous relaxation per ion scales as the square root of the ion induced displacements in the film over the range of the ion penetration, suggesting short-lived defects with a bimolecular annihilation mechanism. The surface instability mechanism accounts for the ripple formation, while inclusion of stochastic roughening produces the random texture and reproduces the observed linear roughening kinetics and the magnitude of the overall roughness.
Nonlinear optical response, screening, and distribution of strain in piezoelectric multiple quantum wells
Authors: Huang X. R. , Harken D. R. , Cartwright A. N. , McCallum D. S. , Smirl Arthur L. , Sánchez-Rojas J. L. , Sacedón A. , González-Sanz F. , Calleja E. , Muñoz E.
Published: July 2002

By embedding piezoelectric InGaAs/GaAs multiple quantum wells (MQWs) in specifically designed p-i-n structures, we demonstrate that the nonlinear optical response can be used to identify the dominant screening mechanisms and simultaneously to determine the strain distribution. Furthermore, we show that a knowledge of the screening mechanisms and spatial band structure, in turn, can be used to control the nonlinear optical response. For this demonstration, we fabricate two p-i(MQW)-n samples on [111]-oriented GaAs substrates. The samples are designed such that, if the dominant screening is associated with photogenerated carriers that remain in the wells, a blue shift of the exciton would be expected in each. By contrast, if the screening is associated with carriers that have escaped the wells and moved to screen the entire MQW, one will shift to the blue and the other to the red if the lattice is mechanically clamped, but both will shift to the red if the lattice is mechanically free. The observation of a blue shift and a red shift indicates that, while in-well screening may be present, the dominant screening is out-of-well and that these particular structures are mechanically clamped to the lattice constant of the GaAs. Most importantly, these results illustrate the added flexibility that the piezoelectric field gives in tailoring the nonlinear optical response.
Hole drift mobility measurements in amorphous silicon-carbon alloys
Authors: Gu Qing , Wang Qi , Schiff Eric A. , Li Yuan-Min , Malone Charles T.
Published: July 2002

Hole drift mobilities have been measured using photocarrier time-of-flight for several hydrogenated amorphous silicon-carbon alloy specimens. We find that, as the band gap increases, the hole drift mobility remains essentially constant. The temperature and dispersion properties were broadly consistent with hole multiple trapping in the valence bandtail. In conjunction with previous drift mobility measurements in hydrogenated amorphous silicon-carbon alloys and hydrogenated amorphous silicon-germanium alloys, these hole measurements complete a simple pattern for the effects of band gap modification on drift mobilities: electron mobilities decline as the band gap is increased or decreased from 1.75 eV, but hole mobilities are relatively unaffected.
Investigation of the epitaxial growth mechanism of ZnTe on (001) CdTe
Authors: Tatarenko S. , Jouneau P. H. , Saminadayar K. , Eymery J.
Published: July 2002

The first stages of the growth of highly strained ZnTe on (001) CdTe are investigated by reflection high energy electron diffraction, HRTEM (high resolution transmission electron microscopy), x-ray photoelectron spectroscopy, and x-ray double diffraction. A precise study of the factors influencing the critical thickness is presented, with emphasis on the effect of Zn pre-exposure of the CdTe surface on the subsequent ZnTe growth. Below the critical thickness small lattice distortions attributed to a nontetragonal elastic distortion are detected. An exposure of the (001)CdTe surface to a Zn flux leads to the desorption of the Cd atoms present on the top of the surface and to the formation of a c(2×2) reconstructed surface with half a monolayer of Zn on the top of the surface. Finally, the morphology of an ultrathin strained ZnTe layer embedded in a (001)CdTe matrix will be discussed using results obtained from analysis of the digitized HRTEM image.
Effect of rapid thermal annealing treatment on electrical properties and microstructure of tantalum oxide thin film deposited by plasma-enhanced chemical vapor deposition
Authors: Jeon Seok Ryong , Han Sung Wook , Park Jong Wan
Published: July 2002

Effect of high temperature annealing in the temperature range of 600–900 °C on the electrical properties and microstructure of tantalum pentoxide (Ta2O5) thin film deposited by plasma-enhanced chemical vapor deposition (PECVD) was studied. Leakage characteristics of the Ta2O5 thin film annealed at 600 °C were found to be the best in this study. However, it was observed that the leakage current in the polycrystalline Ta2O5 thin film decreased with increasing the annealing temperature above 800 °C after a peak for 700 °C annealing. The dielectric constant of the annealed Ta2O5 thin film was 26 after annealing at 600 °C, and decreased with the same tendency as the leakage current characteristics. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) analysis indicated that the microstructure of the Ta2O5 thin film annealed above 800 °C was of δ-Ta2O5 with hexagonal crystal structure. Furthermore, TEM and AES observations revealed that Ta–O–Si transition layers were formed between the annealed Ta2O5 thin film and Si substrate. The electrical properties of the Ta2O5 films are discussed in terms of interface modification and film densification due to rapid thermal annealing treatment.
In situ laser reflectometry study of the morphology of Ge/GaAs layers during their heteroepitaxial growth
Authors: Leycuras A. , Lee M. G. , Hausmann A.
Published: July 2002

The morphology of Ge layers, a few micrometers thick, grown by chemical-vapor deposition on GaAs substrates, is very sensitive to the growth conditions. The evolution of the intensity of the dynamic in situ reflectometry reveals different growth features. We present here an analysis of the basic elements from which it is possible to deduce important physical parameters of the Ge layers: the shape of the growth defects, their density, the optical index of the homogeneous layer, and the effective index of the rough layer. The respective contributions of the scattered and refracted intensities to the measured intensity are described semiempirically. Typical experimental curves and their corresponding time-dependent growth rate, optical index, and morphology are given.