bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature dependence of resistivity and Hall mobility in floating zone grown bulk silicon-germanium alloys
Authors: Kishore R. , Prakash P. , Singh S. N. , Das B. K.
Published: July 2002

Silicon germanium (Si1−xGex) alloys have been grown by the floating zone method with x varying from 0.5 to 20 wt %. Resistivity and Hall effect measurements have been carried out in the temperature range 80–410 K. The Hall mobility has been found to decrease with Ge concentration varying between 0.5 and 4.5 wt % and thereafter an increase has been observed. The Hall mobility has also been found to decrease with an increase in temperature in these alloys following the AT−y relation, where A is a constant and T is the temperature in K.
Extending the calculation of electron velocity distribution functions for electrical discharges to large values of E/N
Authors: Richley Edward
Published: July 2002

The popular two-moment expansion of the Boltzmann equation is developed for cases involving significant generation of electrons by ionization. In particular, this formulation enhances the determination of electron velocity distribution functions at high values of E/N where ionization becomes significant. The simple two-moment approach allows such calculations to be made with modest computing resources. Examples are given of two such calculations in an inert gas (argon), and the numerical approach for these is outlined. Comparison with published experiments is shown to be very good, in both the low-E/N and high-E/N regimes. In one example, comparison with published results from a very general numerical procedure is found to be excellent.
Admittance spectroscopy of CuInSe2/CdS solar cells prepared by chemical spray pyrolysis
Authors: Strifler W. A. , Bates C. W.
Published: July 2002

CuInSe2/CdS heterojunction diodes prepared entirely by chemical spray pyrolysis are investigated using admittance spectroscopy to obtain information on electrically active trapping centers within the CuInSe2. Because of the relatively large parallel leakage conductance associated with these devices, a frequency-swept admittance technique (stepped in temperature) is employed to differentiate between the loss signal due to trapping centers and the loss due to parallel conductance. The frequency-swept admittance technique is described and compared to the more conventional temperature-swept method. The devices used in this study exhibit one-sun efficiencies of 2.0%–3.0% and a forward current dominated by generation-recombination processes within the space charge region. Admittance spectra indicate a dominant hole trap is located approximately 270 meV above the valence band of the CuInSe2.
The low-temperature catalyzed etching of gallium arsenide with hydrogen chloride
Authors: Dupuie Jeffrey L. , Gulari Erdogan
Published: July 2002

A heated tungsten filament has been used to catalyze the gas phase etching of gallium arsenide with hydrogen chloride at a substrate temperature of 563 K. Rapid etch rates, between 1 and 3 microns per minute, were obtained in a pure hydrogen chloride ambient in the pressure range of 3.3 to 20.0 Pascal. Low flow rates of hydrogen quenched the etching reaction, and resulted in degradation of the quality of the etched gallium arsenide surface. Dilution of the hydrogen chloride to 10.5% in helium reduced the etch rate to 63 nanometers per minute. The removal of 83 nm of gallium arsenide with the helium-diluted gas mixture resulted in a specular surface. X-ray photoelectron spectroscopy indicated that the gallium arsenide surface became enriched in gallium after the etch in helium-diluted hydrogen chloride. No tungsten or other metal contamination on the etched gallium arsenide surface was detected by x-ray photoelectron spectroscopy.
Aging studies of evaporated ZnS:Mn alternating-current thin-film electroluminescent devices
Authors: Davidson J. D. , Wager J. F. , Kobayashi S.
Published: July 2002

A study of the aging characteristics of evaporated ZnS:Mn alternating-current thin-film electroluminescent (ACTFEL) devices is undertaken by monitoring the capacitance-voltage (C-V) characteristics at various temperatures as a function of aging time. Short-term ACTFEL aging is characterized by a rigid shift in the C-V curve to higher turn-on voltage with aging time. Additionally, the insulator and phosphor capacitances are found to be independent of aging time, the internal phosphor threshold voltage increases slightly with aging time, and the conduction and polarization charges are observed to decrease with aging time. The activation energy for ACTFEL aging is estimated to be about 0.2 eV. These experimental observations lead to a picture for ACTFEL aging in which atomic rearrangement at insulator/phosphor interfaces gives rise to the formation of deep level, fixed charge states. Transported electrons subsequently trapped in these deep levels reduce the amount of charge available for conduction with a concomitant reduction in the polarization charge. The reduction in the polarization charge is responsible for the observed increase in the turn-on voltage with aging.
A theoretical study of resonant cavity-enhanced photodectectors with Ge and Si active regions
Authors: Ünlü M. S. , Kishino K. , Liaw H. J. , Morkoç H.
Published: July 2002

An integration of Ge or Si with compound semiconductors is considered for optical detectors at wavelengths ranging from the visible to 1.55 μm. Both Ge and Si are indirect band gap materials with relatively small absorption coefficients but very long carrier lifetimes. Using a resonant cavity formed by large band gap compound semiconductor mirrors surrounding Ge and Si active layers, high photosensitivity for thin (<0.5 μm) absorbing layers can be achieved. Two different methods are discussed and compared for the calculation of the photosensitivity of resonant cavity enhanced detectors. The standing wave effect and its dependence on cavity parameters are analyzed. Design rules for such resonant cavity enhanced photodetectors are presented. The spectral response of some device designs to achieve high quantum efficiency and wavelength selectivity has also been theoretically investigated.
The effect of the flow of silane on the properties of a-Si:H deposited by concentric-electrode radio frequency glow-discharge
Authors: Conde J. P. , Chan K. K. , Blum J. M. , Arienzo M.
Published: July 2002

High band gap, device-quality, hydrogenated amorphous silicon (a-Si:H) was deposited from silane at room temperature using concentric-electrode plasma-enhanced chemical vapor deposition (CE-PECVD). Increasing the flow of silane from 15 to 99 sccm resulted in a continuous increase of the optical band gap, Eopt, from 1.7 to 2.1 eV, and changed the dominant bonding configuration from Si-H to Si-H2. The total hydrogen concentration as determined from the integrated absorption of the SiHx stretching bond increased from 7% to 15%. As Eopt varied between 1.7 and 2.1 eV, the photoconductivity, σph, decreased from 10−5 to 10−7 Ω−1 cm−1 and the dark conductivity, σd, dropped from 10−10 to 10−14 Ω−1 cm−1 (σph and σd measured at room temperature after a 1 h anneal at 200 °C). These results are superior to those obtained using a-SiC:H alloys deposited under comparable conditions (i.e., without hydrogen dilution). After annealing, three different conduction paths were identified and correlate with the silicon-hydrogen bonding configuration. The photosensitivity of high band gap a-Si:H films, σph/σd, follows Slobodin’s rule for a-SiGe:H alloys. High band gap a-Si:H deposited by CE-PECVD has significant potential to challenge the role of a-SiC:H as the choice material for films with 1.7 ≲ Eopt ≲ 2.1 eV.
Grain-boundary interfacial states in BeO-doped SiC ceramics: Photoisothermal capacitance transient spectroscopy
Authors: Tanaka Shigeru , Takahashi Ken , Akita Chiyoshi , Okushi Hideyo , Wada Yoshiki , Tanaka Junzo
Published: July 2002

Interfacial states at grain boundaries in BeO-doped SiC ceramics were characterized using the isothermal capacitance transient spectroscopy (ICTS) method. ICTS signals due to the interfacial states were detectable above 160 °C in the dark. By irradiating visible light, the capacitance increased and the behavior of C-V characteristics changed compared to those in the dark; in addition to this, the ICTS signals could be observed at room temperature. The wavelength dependence of ICTS signals showed that the signals disappeared above 950 nm. This result suggested that adiabatic excited states of holes captured at grain boundaries were located at about 1.3 eV above its ground state, which was different from thermally excited states determined by the ICTS in the dark, i.e., 0.9 eV.
Reduction of secondary defects in MeV ion-implanted silicon by means of ion beam defect engineering
Authors: Wang Zhong-lie , Zhang Bo-xu , Zhao Qing-tai , Li Qi , Liefting J. R. , Schreutelkamp R. J. , Saris F. W.
Published: July 2002

A new concept of ion beam defect engineering is proposed. Reduction of secondary defects in 1 MeV As ion-implanted Si(100) has been investigated by Rutherford backscattering/channeling and high resolution electron microscopy. It is found that the additional irradiation of 1.6 MeV Si ions prior to two-step thermal annealing leads to a noticeable reduction of secondary defects. Good recrystallization of a buried amorphous layer is also obtained by irradiation of 1.6 MeV, 2×1015 Si/cm2 into the implanted Si sample held at elevated temperatures.
Polarization Raman microprobe analysis of laser melting and etching in silicon
Authors: Tang Hua , Herman Irving P.
Published: July 2002

Polarization Raman microprobe spectroscopy is used to study crystalline silicon heated to the melting point by a tightly focused cw laser beam, which is either fixed or scanned across the surface. By examining optical phonons in solid silicon, the real-time Raman spectrum monitors the progress of silicon flow during melting and the trench depth during melt-assisted etching. Raman peaks lie between 482 cm−1, which is the Raman shift for silicon uniformly heated to the melting point (1690 K), and ∼510 cm−1, which is the Raman shift for c-Si heated just to the melting point and probed by the same beam. During laser melting with a static laser, the Raman spectrum of scattered light with z(x,y)z̄ polarization has two peaks, while the z(x,x)z̄ spectrum has one peak. This shows that at the beginning of melting in vacuum by a static laser there is a central region with solid silicon floating in the melt, which is surrounded by hot solid material. Because of the flow of the molten semiconductor, the temperature profile changes, causing the Raman spectrum to change rapidly. Laser melting of c-Ge and thin films of silicon in vacuum is also studied, as is the melting of c-Si by a static laser in the presence of an inert buffer gas. The presence of an inert buffer is shown to affect the temperature profile very strongly during melting and also during laser heating at lower laser powers when no melting occurs. During scanned laser melting and etching, the Raman spectrum has a single peak using either polarization configuration. Raman analysis during melting of silicon by a scanning laser shows that the average temperature in the probed region is much higher when there is a gas-phase argon buffer present (and no etching) than when there is an etching mixture of argon/chlorine gas (and etching). Along with these real-time Raman measurements, the reflection of the incident laser was monitored, and post-process Raman analysis and profilometry were also performed to characterize surface changes due to melting and etching.
Shallow ohmic contact formation by sequential deposition of Pd/AuGe/Ag/Au on GaAs and rapid thermal annealing
Authors: Zheng Longru
Published: July 2002

Shallow ohmic contacts to n-GaAs are fabricated by sequential deposition of Pd/AuGe/Ag/Au and rapid thermal annealing at 450–500 °C for 30 s. The metallization displays good electrical properties with a contact resistivity of 2 × 10−6 Ω cm2. The film–substrate reaction is extremely limited and the interface is uniform. A systematic study of various AuGe metallization schemes indicates that the limited interfacial reaction observed in the Pd/AuGe/Ag/Au structure correlates with the stability of Au–Ag solid solutions against GaAs. The diffusion of silver into the AuGe layer upon heating increases the eutectic temperature of the system. As a result, the contact stays in a solid state when the metallurgical reaction takes place to form an ohmic contact.
Reduced intervalley scattering rates in strained Si/SixGe1−x quantum wells and enhancement of electron mobility: A model calculation
Authors: Basu P. K. , Paul Sajal K.
Published: July 2002

In a Si/Si0.5Ge0.5 quantum well grown on a [100] oriented Si0.75Ge0.25 buffer, the four valleys having longitudinal mass normal to the [100] direction are lifted from the remaining two valleys. As a consequence, the intervalley f scattering between these two groups of valleys, the strongest in bulk Si, occurs only when the electrons in the subbands reach a high threshold energy. A high value of mobility limited mainly by acoustic phonon scattering is thus expected and is also obtained from the model calculation described in the present work. It is shown that higher values of the mobility may be obtained for wider wells.
Electrical characteristics of symmetric, compositionally graded triangular heterostructure diodes grown by molecular beam epitaxy
Authors: Giugni Stephen , Tansley T. L.
Published: July 2002

Symmetric, triangular barriers with alloy induced heights of about 250 meV and between 40 and 150 nm thick have been grown in AlGaAs by linear ramping of composition in a conventional solid-source molecular beam epitaxy system and their current–voltage characteristics measured between 55 to 300 K. Characteristics are also calculated, with electron tunneling contributions included by a finite-element Wentzel–Kramers–Brillouin approach which allows an arbitrary composition and/or doping profile to be modeled. Theory and experiment generally coincide, increased tunneling contributions are detected at lower temperatures and in narrower barriers, principally through a reduction in barrier height and an increase in ideality factor. The incorporation of dopants in the barrier produces an ionized-donor space charge with a consequent reduction in real barrier height and width, while anomalous behavior in some samples seems to be associated with trapping within the barrier.
Magnetization correlations and noise in thin-film recording media
Authors: Bertram H. Neal , Arias Rodrigo
Published: July 2002

Magnetization correlations in polycrystalline thin-film recording media are studied numerically via a micromagnetic cellular automaton model. Magnetization reversal processes exhibit self-organized criticality with domain patterns characterized by the intergranular exchange and magnetostatic interactions. Cross-track correlation, or domain width, is shown to be only slightly dependent on the state of magnetization. Noise in magnetic recording is related to an integration of the cross-track correlation. This integrated width is shown to increase dramatically with increasing exchange coupling and only slightly with increasing magnetostatic coupling. With no exchange coupling certain values of magnetostatic coupling yield medium noise that is below that of a system with totally noninteracting grains. For uniformly magnetized media correlations in the recording direction are stationary and found to vary in length with coupling strength. Scaling laws show that noise decreases proportional to the grain area in the film plane only if there is no intergranular exchange coupling and if the film thickness is reduced in proportion to the grain planar diameter. A review of relevant noise power relations and spectral measurement analysis is given.
Automatic trimming technique for multipolar magnets
Authors: Alecci M. , Colacicchi S. , Sotgiu A. , Testa L. , Varoli V.
Published: July 2002

The paper presents a technique for identifying and removing unwanted terms in the field power expansion of multipolar magnets. The technique is based on the measurement of the radial field at the poles of the magnet. The field terms are derived by analytical formulas and used to evaluate the current corrections. This trimming procedure has been tested for a 16-pole magnet designed for electron paramagnetic resonance imaging and permits a continuous control of the field configuration.
Control and conversion of the polarization of the electromagnetic wave
Authors: Kasraian Masoud , Seshadri S. R.
Published: July 2002

Treatments of the three reflector polarizer and polarization control by selective transmission are given. A twist reflector consists of a unidirectionally conducting surface with a parallel, perfectly conducting plane placed behind it. The twist reflector can be used in the development of a device that can rotate the plane of polarization of the incident wave by the rotation of the twist reflector about an axis normal to its surface. The polarization angle of the emerging wave is determined as a function of the orientation angle of the twist reflector. The polarizer design can be modified to enable it to convert the incident linear polarization into either right or left circular polarization.
Optical studies of Cd1−xMnxTe films grown on (001)InSb by pulsed laser evaporation and epitaxy
Authors: Labrie D. , Qiu C. , Wang X. , Dubowski J. J.
Published: July 2002

Photoluminescence, photoreflectance, and photovoltaic measurements have been used to characterize Cd1−xMnxTe (CMT) epilayers, 0.5–1.4 μm thick, grown on (001) InSb by pulsed laser evaporation and epitaxy. The measurements indicate high-quality epilayers comparable to the best bulk CMT or of CMT grown by molecular beam epitaxy. In some cases, a small gradient of Mn composition along the epilayer thickness is indicated by the photoreflectance data.
Growth and characterization of substrate-quality ZnSe single crystals using seeded physical vapor transport
Authors: Cantwell Gene , Harsch W. C. , Cotal H. L. , Markey B. G. , McKeever S. W. S. , Thomas J. E.
Published: July 2002

Crystallographic and optical characterization techniques were carried out on ZnSe single-crystal samples grown by the seeded physical vapor transport (SPVT) and by high-pressure Bridgman techniques. A comparison of etch pit densities shows much lower values for the SPVT material. The distribution of etch pits across a wafer is uniform in SPVT samples but extremely nonuniform in the Bridgman samples. X-ray topography studies reveal that the SPVT material has few defects and no grain boundaries while the Bridgman material shows both low and high angle grain boundaries. Photoluminescence (PL) data at 12 K on the SPVT material reveal an absence of donor acceptor pair (DAP) emissions. The spectrum is dominated by the Id1 Cu-related line and its phonon replicas and only weak Cug and Cur emissions are observed. No thermoluminescence (TL) is seen from the SPVT samples but they do give thermally stimulated conductivity (TSC) signals due to the release of holes from CuZn centers with activation energies of 0.33 eV and 0.71 eV. Only hole states are seen in the SPVT material. In contrast the Bridgman samples show intense DAP PL lines, as well as Id1 lines. They show Cug and Cur emission, give strong TL and TSC signals, and reveal an array of both electron and hole states.
Experimental observation and simulation of mode-locking phenomena in coupled-oscillator arrays
Authors: York R. A. , Compton R. C.
Published: July 2002

Systems of coupled oscillators possess a wealth of interesting and useful nonlinear dynamical phenomena. Arrays of coupled oscillators have been used to model complicated biological and neural activity, and have proved useful in power-combining applications. The analysis of such arrays is difficult, and has generally been restricted to the case where all oscillators are synchronized to a common frequency through the phenomenon of injection locking. When the natural (or free-running) frequencies are very widely distributed, this mutual synchronization is impossible, and the system can exhibit chaotic behavior. However, by carefully choosing the frequency distribution, a stable mode-locked state can be established in which the collective output of the oscillator array consists of a train of pulses, similar to a mode-locked laser. Due to the different physical mechanisms involved, and due to a spatial as well as spectral separation of the oscillators, the mode-locked array exhibits new phenomena not present in mode-locked lasers. A theory describing the coupled-oscillator dynamics is developed to explore these new effects, and the theory is amply supported with empirical evidence obtained with a linear array of Gunn diode oscillators operating at 11 GHz.
Control of exchange coupling for overwritable magneto-optical media by sputter etching
Authors: Hayashi Kazuhiko , Shimizu Shoji , Okada Osamu
Published: July 2002

It has been found that the interface wall energy density σw for exchange-coupled trilayer (ECTL) media, which is a key characteristic in obtaining overwritable magneto-optical media, is controlled by sputter etching the SiN underlayer. The sputter-etching effect of the SiN underlayer on film surface morphology, magnetic anisotropy, σw, and overwrite characteristics in ECTL media has been investigated. The media consist of a polycarbonate substrate, a SiN underlayer, TbFe memory layer, a NdFeCo exchange coupling control layer, a GdTbFeCo writing layer, and a SiN protective layer. By measuring saturation magnetization and switching field for the writing layer, σw between the memory layer and the writing layer was determined, according to the exchange-coupled double-layer theory. The effective perpendicular anisotropy of the NdFeCo layer decreased with sputter etching of the SiN layer, whereas the interface wall energy density changed from 0.6 to 1.7 erg/cm2 as 50 Å of SiN was sputter etched away. These effects are believed to be related to morphological changes observed in the SiN. For mark lengths greater than 2.3 μm, the primary effect of sputter etching the SiN underlayer was to increase the noise by 10 dB with respect to the non-sputter-etched case, whereas the carrier was the same.
Atomic layer controlled doping by reflection high-energy electron diffraction during epitaxial growth
Authors: Zhu Ziqiang , Kawashima Mitsuo , Yao Takafumi
Published: July 2002

The detailed observation of dynamical behaviors of reflection high-energy electron diffraction (RHEED) patterns during the adsorption processes of Li, Se, and Zn is carried out. It is found that the RHEED intensity variation reflects the Li surface coverage during Li adsorption process on a Se-covered surface. This fact enables one to control the doping of Li ‘‘in situ.’’ A new method for atomic-layer controlled doping of ZnSe layers with lithium is proposed based on the RHEED investigations. The method would allow the incorporation of Li atoms on Zn sites of ZnSe by monitoring the RHEED patterns and intensities, by which the formation of Li interstitials would be suppressed. Photoluminescence spectrum shows the growth of high-quality p-type layers.
Effects of Sb4/Ga ratios on the electrical properties of GaSb Schottky diodes
Authors: Wang Y. H. , Houng M. P. , Sze P. W. , Chen J. F. , Cho A. Y.
Published: July 2002

Au, Cu, Ag, and Al are deposited on Te-doped n-GaSb layers directly grown on lattice mismatched GaAs semi-insulating substrates. Sb4/Ga beam equivalent pressure ratios are found to profoundly influence the electrical properties of the Schottky diodes investigated here. The fact that both breakdown voltage and barrier height decrease with increasing Sb4/Ga ratios is attributed to the increase in surface state densities for samples grown at higher Sb4/Ga ratios. This suggestion is further confirmed by the model of surface state densities employing the relationship of barrier height to metal work function. The surface state densities are in the range of 2.3×1014 to 1.2×1015 states/cm2/eV corresponding to Sb4/Ga ratios of 2 to 9, respectively. X-ray rocking peaks of samples grown at various Sb4/Ga ratios, and subsequently subjected to annealing, indicate different interactions at the interfaces which might support the observations.
Pressure dependence of the thermal conductivity of pyrophyllite to 40 kbar
Authors: Chen Wei , Decker D. L.
Published: July 2002

A mathematical model for calculating the temperature distribution as a function of power delivered to a line source and the thermal conductivity of the surrounding medium in the pressure cell of a cubic-anvil press was derived. The model will handle anisotropic thermal conductivities. A simple sample assembly consisting of a line source and two or three thermocouple junctions is described. A comparison of measured to calculated temperatures yields the thermal conductivity. Thermal conductivity measurements were made on natural pyrophyllite and baked pyrophyllite to 40 kbar. For the natural pyrophyllite the thermal conductivity parallel to the bedding plane at room temperature increased with pressure from 13 to 15 (mcal/s cm K) over the pressure range but for the baked material it decreased with pressure.
Improvement of GaAs/AlGaAs molecular beam epitaxial growth by a flux-interruption-and-annealing method using phase-locked reflection high-energy electron diffraction oscillation
Authors: Lee Kun-Jing , Chang Yu-Jeng , Ya-min Lee Joseph , Juang Feng-Yuh
Published: July 2002

An exact two-dimensional molecular beam epitaxial growth of GaAs/AlGaAs layers has been achieved by using a flux-interruption-and-annealing method. This growth has been made possible by using phase-locked reflection high-energy electron diffraction oscillation during epitaxial growth. Quality of epitaxial layers grown by this method has been confirmed by photoluminescence and Fourier transform infrared spectroscopy measurements.
In situ preparation of high-T
c TlBaCaCuO thin films by a combination of laser ablation and thermal evaporation
Authors: Betz J. , Piehler A. , Pechen E. V. , Renk K. F.
Published: July 2002

We report on in situ preparation of high-Tc TlBaCaCuO thin films by a combination of laser ablation and thermal evaporation. The films were prepared on MgO〈100〉 substrates and had zero resistance at 110 K. They had polycrystalline structure with grain size of about 1 μm. The critical current density at 77 K was about 0.6×105 A/cm2.
Spatiotemporal-mode structure in spherical dielectric waveguides
Authors: Hayata K. , Koshiba M.
Published: July 2002

A novel physical concept termed ‘‘spatiotemporal mode’’ is presented for ultrashort optical pulse-beam propagation in a saturable Kerr-law nonlinear medium. The mathematical structure of the mode with spherical symmetry is described analytically. Dispersion characteristics are calculated and compared with those of the conventional spatial modes in planar and cylindrical dielectric waveguides. Nonvanishing cutoff frequency is obtained even for the fundamental mode.
Optical gain enhancement in Fabry–Perot microcavity lasers
Authors: Lei C. , Deppe D. G.
Published: July 2002

Fabry–Perot microcavities are analyzed in terms of their light emission characteristics. The analysis considers full output coupling, and we calculate both spontaneous and stimulated emission dependencies on cavity length, mirror design, and spectral characteristics. The cavities correspond to vertical-cavity surface-emitting lasers in the AlAs-GaAs-InGaAs material system, and a GaAs cavity with Bragg mirrors of CaF2/ZnSe. We show that considerable gain enhancement depends on the degree of coherence in the spontaneous emission, the microcavity length, and the Bragg reflector design.
A comprehensive analysis of surface acoustic wave reflections. III. Oblique incidence grooved and metallic gratings
Authors: Robinson H. , Hahn Y. , Gau J. N.
Published: July 2002

First- and second-order reflection effects in oblique incidence surface acoustic wave gratings are examined. A general formulation usable for both metallic and same material grooved overlays is provided. The existence of several important effects, particularly a reflection effect at the second harmonic of the grating and a velocity shift at the first harmonic for grooved gratings, are demonstrated, and the behavior of these effects as a function of incidence angle is examined. For angles of incidence of interest in the production of reflective array compressors, these effects are analyzed as a function of metallization ratio.
Frequency locking of a tunable liquid-crystal filter
Authors: Patel J. S. , Wullert J. R.
Published: July 2002

We describe a technique for frequency locking of a tunable liquid-crystal Fabry–Perot filter. An ac control voltage at frequency ω, used to tune the cavity, also produces a small signal modulation at 2ω. When phase sensitively detected, the 2ω signal results in a first derivative signal. Frequency tracking of the Fabry-Perot resonator is performed by a feedback system that minimizes this derivative signal. We show that this technique compensates for temperature fluctuations that would otherwise cause frequency drifts of the resonator.
Epitaxial YBa2Cu3Ox thin films on sapphire with a PrBa2Cu3Ox buffer layer
Authors: Gao J. , Klopman B. B. G. , Aarnink W. A. M. , Reitsma A. E. , Gerritsma G. J. , Rogalla H.
Published: July 2002

Highly c-axis-oriented YBa2Cu3Ox thin films have been grown on (11̄02) sapphire substrates by a modified off-axis magnetron rf-sputtering technique. To prevent the interaction and to improve the lattice match between film and substrate a thin layer of 10–60 nm of PrBa2Cu3Ox was used as a buffer layer. The diffusion process was effectively blocked and the epitaxy was enhanced. The Jc (77 K) has been measured to be 4×105 A/cm2 which is significantly higher than the Jc of films directly grown on Al2O3 (∼8×104 A/cm2). The surface resistance of such films at 11 GHz and 4.2 K was measured to be 80 μΩ for 50-nm thin films. These results are comparable to the best results reported so far on sapphire with other buffer layers.
Photoluminescence properties of a new 1.951-eV transition in III-V semiconductors
Authors: Olsthoorn S. M. , Driessen F. A. J. M. , Giling L. J.
Published: July 2002

In this paper a detailed study is presented on the photoluminescence (PL) properties of a recently discovered transition in AlxGa1−xAs (x≳0.42) on GaAs with an energy of 1.951 eV. A transition of this energy has also been observed in Ga0.5In0.5P layers on GaAs. The transition originates from an isolated center in the semiconductor alloy, although excitation occurs indirectly via the semiconductor. The PL intensity is relatively high at room temperature, and the position of the peak is independent of temperature. The origin of the isolated center responsible for this transition is still unknown.
Temperature dependence of the linewidth of shallow impurity spectral lines in lightly doped weakly compensated semiconductors
Authors: Baranovskii S. D. , Thomas P. , Vaupel H.
Published: July 2002

The broadening of spectral lines of intradonor absorption in n-GaAs is determined by random electric fields of charged donors and acceptors. The temperature dependence of the spectral line width reflects a redistribution of electrons over donors. At low temperatures the relative positions of charged and neutral impurities are strongly correlated by the Coulomb interactions in the system, whereas at high enough temperatures the distribution of electrons over impurities is random. This effect was recently suggested for quantitative characterization of semiconductors and a few theoretical attempts were made to describe the temperature dependence of the width of the spectral lines due to the transition from a correlated distribution of electrons to a random distribution in the low compensated material. We report here on the results of a Monte Carlo computer simulation of the transition. It is shown that the available analytical theories are not accurate enough and the reasons for such an inaccuracy are discussed.
Growth of epitaxial Fe/Cr superlattices having extremely flat interfaces
Authors: Kamijo A. , Igarashi H.
Published: July 2002

By introducing an epitaxial Cr(00l) buffer layer, single-crystal Fe(001)/Cr(001) superlattices having extremely flat interfaces were grown on MgO(001) substrates by molecular beam epitaxy. Sharp reflection high-energy electron diffraction patterns and low resistivity at 5 K show that the Fe/Cr superlattice has the highest quality of the interfaces among those so far reported. Results indicate that the interface roughness influences the magnitude of both residual resistivity and magnetoresistance.
Composition dependence of Ag photodoping into amorphous Ge-S films
Authors: Kawaguchi Takeshi , Maruno Shigeo
Published: July 2002

Photodoping kinetic studies of thin sample of Ag/GexS100−x (21≤x≤45) were made through optical transmission measurements. The initial photodoping rate increases with increasing S content, whereas the total amount of dissolved Ag shows a maximum around the stoichiometric composition GeS2. The results are similar to those of the Ag/Ge-Se system but different from the Ag/As-S system. The composition dependencies are qualitatively explained by the structure of Ge-S bulk glass and the glass-forming ability of Ag-Ge-S bulk glass. In this study it was also found that aging effect is considerable within several hours after sample preparation and a reciprocity law holds between the initial rate and illumination power.
Buffer gas effect in a discharge-pumped XeBr excimer laser
Authors: Mizunami Toru , Takagi Keiji
Published: July 2002

Using He, Ne, and Ar buffer gases, lasing performances of a compact Blumlein-type XeBr laser at 282 nm have been studied. The maximum energy of 0.64 mJ and the maximum power of 190 kW have been obtained with 4-atm Xe/HBr/Ne mixture. He-based mixture has been inferior to Ne-based mixture both in the output energy and in the gas lifetime. Atmospheric-pressure operation has been possible only by an Ar buffer gas. These buffer-gas dependences have been similar to those of XeCl lasers.
Power-dependent output properties of a nonlinear grating coupler
Authors: Matsumoto Masayuki , Ohtsuka Manabu
Published: July 2002

A field analysis of a nonlinear grating output coupler is presented. The coupler is assumed to be operated in high-power regime, that is, the guided-wave field profile is dependent on the guided-wave power. Near-field output-intensity distributions are calculated. It is shown that focused intensity distributions are obtained and the focal point moves as the input power is varied.
Temperature dependence of the photoluminescence of Zn-doped In0.32Ga0.68P grown on GaAs0.61P0.39 substrates
Authors: Wu Meng-Chyi , Chen Chyuan-Wei
Published: July 2002

The photoluminescence (PL) spectra of Zn-doped In0.32Ga0.68P epitaxial layers grown on GaAs0.61P0.39 substrates by liquid-phase epitaxy has been investigated in the temperature range of 8–300 K. The radiative recombination processes of the direct In0.32Ga0.68P alloys for which the composition is near the direct–indirect band gap crossover point have been studied at various temperatures. At higher temperatures (≳150 K) only one emission band corresponding to free-electron-to-free-hole transition dominates. Two peaks and one broad band are observed in the PL spectrum when the temperature is below 100 K. The peak denoted by A is due to direct interband radiative recombination. The temperature dependence of the band gap in In0.32Ga0.68P layers can be expressed as 2.25 − [1.79 × 10−3T2/(T + 1236)] eV. The peak denoted by B, exhibited by undoped or moderately Zn-doped (p ≤ 3 × 1018 cm−3) InGaP samples, is attributed to the conduction-band-to-acceptor transition. A third broad band (C) dominates at low temperatures. The maximum shifts toward shorter wavelengths as the temperature is further lowered. It is the indirect donor–acceptor pair recombination from the unidentified deep donor levels associated with indirect Xc minima via Zn acceptor levels.
Luminescence of thulium in III-V semiconductors and silicon
Authors: Pomrenke Gernot S. , Silkowski E. , Colon J. E. , Topp D. J. , Yeo Y. K. , Hengehold R. L.
Published: July 2002

Thulium (Tm) emissions were investigated in 390 keV, Tm-implanted AlGaAs, GaAs, InP, and Si using photoluminescence spectroscopy. The emissions in the different semiconductors were observed in the 1.20 to 1.35-μm region and are attributed to transitions between the crystal-field-split spin–orbit levels 3H5– 3H6 of Tm3+ (4f12). The main emission in GaAs is located at 1.233 μm and shows doublet structure with a width for each component of better than 0.15 meV at 6 K. Anneal temperature dependent studies identify multiple sites as the source of the Tm3+ emissions. The optimum anneal temperatures of Tm-implanted GaAs and AlGaAs are between 725 and 750 °C, for 10- or 15-min anneals, whereas InP:Tm appears to have the strongest emissions in the 575 to 625 °C temperature range. The intracenter Tm3+ emissions for Si:Tm were observed at 850 °C. Sample temperature dependent studies of Tm3+ emissions show that the sharp emissions can be seen to as high a temperature as 240 K for specific samples. Luminescence intensity studies as a function of excitation laser power in GaAs:Tm and AlGaAs:Tm exhibit a sublinear relationship. Lifetime measurements at 10 K of the main 1.233-μm line found two decay components with times of 0.5±0.3 μs and 2.6±0.3 μs. Selective excitation studies indicate that free carriers and/or excitons are involved in the excitation of the Tm centers.
Low temperature epitaxial growth of Ge using electron-cyclotron-resonance plasma-assisted chemical vapor deposition
Authors: Varhue W. J. , Carulli J. M. , Peterson G. G. , Miller J. A.
Published: July 2002

Epitaxial Ge films have been deposited on Si and Ge substrates at 300 °C using electron-cyclotron-resonance plasma-assisted chemical vapor deposition. Helium was fed into the resonance chamber, and a mixture of helium and germane were fed downstream at a location above the substrate. Surface roughness increased with energetic ion bombardment as quantified by the number of ions striking the surface per Ge atom deposited. Surface roughness also increased with increasing substrate temperature. Films with very rough surface morphology were found to be polycrystalline. The large hydrogen content of the films, particularly those deposited on Si, appeared to prevent the reduction of the epitaxial temperature below 300 °C. In the temperature range between 300 and 325 °C, hydrogen bubbles formed at the Ge/Si interface and caused the films to pucker from the surface. Increasing the substrate temperature above 325 °C eliminated this problem by decreasing the surface coverage of hydrogen during deposition.
Analysis of cathode fall from a planar electrode in gas discharges at low E/P low pressure
Authors: Xi D. P.
Published: July 2002

An analytic investigation is reported of cathode fall from a planar electrode in a gas discharge at low electric field to pressure and low gas pressure. In order to avoid complex calculations, a simple function is used to fit the experimental data on Townsend’s first ionization coefficient. Analytic expressions of variation of electric field, cathode fall, and length of the cathode dark space with the ratio of current density to pressure squared are obtained using a first-order approximation. The results show the characteristic current-voltage relation for subnormal and abnormal discharge. The cathode fall, the length of cathode dark space, and the ratio of current density to pressure squared are also estimated for the normal discharge. It is also shown that the length of cathode dark space is a function of the cathode fall. Numerical calculations for several gases are made to demonstrate the model.
Electrical activation of group-IV elements implanted at MeV energies in InP
Authors: Ridgway M. C. , Jagadish C. , Thompson T. D. , Johnson S. T.
Published: July 2002

The electrical activation and carrier mobility of InP implanted with the group-IV elements at MeV energies has been studied as a function of implanted atom (C, Si, Ge, and Sn) and rapid thermal annealing temperature (500–800 °C). In addition, electrical results have been correlated with photoluminescence (PL) measurements. In general, for a dose of 5×1014/cm2 and a projected range of ∼1.0 μm, the electrical activation and carrier mobility increase then saturate with increasing annealing temperature. Similarily, PL emission intensity increases with increasing annealing temperature. At a temperature of 750 °C, the electrically active fraction increases from C, Ge, Si, to Sn, respectively, while carrier mobility and PL emission intensity decreases with increasing atomic mass. Thus, Sn exhibits the highest electrical activation yet lowest carrier mobility with little optically observable, postanneal lattice recovery.
The lattice location of ion-implanted Ga, Ge, and Se in InP
Authors: Kringho/j P.
Published: July 2002

The lattice location of ion-implanted Ga, Ge, and Se in InP has been determined with a combined Rutherford backscattering spectrometry/channeling and proton-induced x-ray emission technique. It is shown that Ga exclusively occupies the In position and Se the P position, whereas the Ge atoms are distributed equally between both sites (the relative population has been found to be 0.50±0.08). Differential Hall/resistivity measurements demonstrate n-type doping for all three dopants; the electrical activity for Ge and Se was found to be 12% and 25%, respectively, whereas the electrical activity for Ga is as low as 0.05%. A significantly lower mobility, however, is observed in the case of Ge doping relative to Se, despite the much lower carrier concentration; this is argued to be due to a higher concentration of ionized scattering centers in the case of Ge, in agreement with the amphoteric behavior observed with channeling.
Parametric investigation of the fission-fragment excited helium/argon laser at 1.79 μm
Authors: Hebner G. A. , Hays G. N.
Published: July 2002

Characteristics of the fission-fragment excited helium/argon laser operating on the 1.79-μm (3d[1/2]0,10−4p[3/2]1,2 argon transition are presented. Laser output occurs for approximately 80% of the 0.9 to 3 ms full width at half maximum thermal neutron pump pulse. Output power efficiency optimizes for a total gas pressure of 760 Torr and argon concentration of 0.3% to 2.0%. Power efficiency was 1.4%±0.4% for instantaneous pump rates of 45 to 230 W/cm3. The small signal gain and saturation intensity for instantaneous pump rates of 30 to 90 W/cm3 are 0.55% to 1.05%/cm and 70 to 110 W/cm2, respectively. The laser threshold as a function of helium pressure and argon concentration will be presented. The advantages of fission-fragment excitation in predominantly helium gas mixtures will be discussed.
Linear and two-photon absorptions of Si-Ge strained-layer superlattices
Authors: Chang Yia-Chung , Chiou A. E. , Khoshnevissan M.
Published: July 2002

Linear and two-photon absorption spectra of Si-Si0.5Ge0.5 strained-layer superlattices grown along [001], [111], [110] directions are calculated within a realistic microscopic model. The temperature dependence of carrier lifetimes due to deformation-potential scattering (including intra- and intervalley scattering) has been taken into account. It is found that superlattices grown along the [110] direction are most promising for applications both in far-infrared detection and in optical limiting. The value for the two-photon absorption coefficient at 77 K for an incident 10.6-μm radiation polarized parallel to the layer can be as high as 150–2000 cm/MW with a linear absorption coefficient around 600–3500 cm−1.
Fabrication and characterization of diamond moth eye antireflective surfaces on Ge
Authors: DeNatale J. F. , Hood P. J. , Flintoff J. F. , Harker A. B.
Published: July 2002

An antireflective coating on Ge which combines a polycrystalline diamond film with a surface relief (moth eye) structure has been designed and successfully fabricated. This progressive gradation in the effective refractive index between air and the substrate has reduced Fresnel reflection losses to below 1%. This provides a means of overcoming the high refractive index and surface roughness considerations that often limit optical application of polycrystalline diamond thin films.
Double grating formation in anisotropic photorefractive crystals
Authors: Li Hsin-Yu , Psaltis Demetri
Published: July 2002

Double gratings form in anisotropic photorefractive crystals when two plane waves intersecting inside the crystal have both extraordinary and ordinary wave components. The effect can be observed by reading out the two gratings with extraordinary or ordinary light. Experiments confirm the theoretical predictions. The relevance of these results in using photorefractive crystals for holographic storage is discussed.
Effects of ionization on the presheath in flowing magnetized plasma
Authors: Chung Kyu-Sun
Published: July 2002

The effect of ionization on the presheath is analyzed kinetically in flowing magnetized plasma with finite ion temperature. Ion distributions along the presheath are obtained self-consistently with Boltzmann electrons. Sheath potential, ion sheath current density, and ratio (R) of upstream to downstream current density are obtained as a function of plasma drift velocity (Vd) and ionization rate (σi). Calibration factor (K) in the form of R=exp[KM] decreases with σi, where M is the drift velocity in units of [(ZTe + Ti∞)/mi]0.5.
Growth of Pnp heterojunction bipolar transistor structures by metalorganic molecular beam epitaxy
Authors: Abernathy C. R. , Ren F. , Pearton S. J. , Fullowan T. , Wisk P. , Lothian J.
Published: July 2002

Fabrication of high-quality Pnp heterojunction bipolar transistors has traditionally been difficult due to the inability to achieve and confine high p- and n-type doping levels using common dopants such as Be and Si. In this paper we discuss how carbon and tin can be incorporated during growth by metalorganic molecular beam epitaxy in order to produce Pnp structures. In particular it has been found that carbon introduced from trimethylgallium can be used to produce abrupt, thermally stable profiles in AlGaAs and that incorporation at concentrations up to mid-1019 cm−3 does not adversely affect the optical or structural quality of the material. In addition, we have found that the use of tetraethyltin (TESn) for tin doping of the GaAs base layer allows for higher doping and better confinement of the dopant than can be obtained with elemental Sn. Consequently, large-area (90-μm diameter) Pnp transistors fabricated from material grown with TESn show higher gain than those grown with elemental tin, in spite of the higher base dopant concentration. The gain obtained with TESn, 45, is the highest yet reported for an abrupt-junction, uniformly-doped Pnp structure. Furthermore, because of the low parasitic resistances which result from the use of carbon and tin doping, the I–V characteristics obtained in this study show superior performance relative to previously published reports.
Bistable behavior of the dark current in copper-doped semi-insulating gallium arsenide
Authors: Roush R. A. , Schoenbach K. H. , Brinkmann R. P.
Published: July 2002

The dark current characteristics of gallium arsenide doped with silicon and compensated with diffused copper were found to have a pronounced region of current controlled negative differential conductivity (ndc) similar to the characteristics of a thyristor. The resistivity of the semi-insulating semiconductor was measured to be 105 Ω cm for applied voltages up to 2.2 kV, which corresponds to an average electric field of 38 kV/cm. At higher voltages, a transition to a stable high current state was observed with a current rate of rise exceeding 1011 A/s. There is evidence of the formation of at least one current filament during this transition. A theoretical model based on drift diffusion and boundary conditions that allows double carrier injection at the contacts has been used to show that the observed negative differential resistance is due to the filling of deep copper acceptors. The model also shows that the ndc curves may be tailored by adjusting the copper concentration. Doping of GaAs with various concentrations of copper was shown to change the dark current characteristics in a way predicted by the model.