bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth and characterization of GaN on c-plane (0001) sapphire substrates by plasma-enhanced molecular beam epitaxy
Authors: Lin M. E. , Sverdlov B. N. , Morkoç H.
Published: July 2002

GaN films were grown on the c-plane of sapphire substrates by plasma-enhanced molecular beam epitaxy equipped with an electron-cyclotron resonance (ECR) source. Sapphire substrates were cleaned by a hydrogen plasma treatment, obviating the need for perilous elevated temperatures. ECR sources are plagued with high energetic ions, particularly at high microwave power levels, where they cause damage in the growing films. To circumvent this problem, we systematically optimized the growth conditions and other pertinent parameters for optimum layer quality. Among the parameters optimized were the magnetic field strength, microwave power, nitrogen over-pressure, and growth temperature. The quality of the GaN layers were evaluated by electrical and structural measurements as well as observing the surface morphology.
Magnitude of the piezoelectric field in (111)B InyGa1−yAs strained-layer quantum wells
Authors: Moise T. S. , Guido L. J. , Barker R. C.
Published: July 2002

The relationship between the magnitude of the piezoelectric field and the degree of lattice constant mismatch is investigated via low-temperature photoluminescence measurements of the quantum-confined Stark effect for a series of (111)B Al0.15Ga0.85As-InyGa1−yAs pseudomorphic quantum well heterostructures. The experimental strain-induced electric field values agree well with theoretical calculations for indium mole fractions in the range 0.037≤y≤0.09. In addition, an anomalous saturation of the photoluminescence transition energy is observed at values of applied voltage greater than that required to nullify the piezoelectric field, despite the indication from separate electroreflectance measurements that the net electric field within the quantum well reverses polarity under similar electrical biasing conditions.
Particle- and photoinduced conductivity in type-IIa diamonds
Authors: Pan L. S. , Han S. , Kania D. R. , Zhao S. , Gan K. K. , Kagan H. , Kass R. , Malchow R. , Morrow F. , Palmer W. F. , White C. , Kim S. K. , Sannes F. , Schnetzer S. , Stone R. , Thomson G. B. , Sugimoto Y. , Fry A. , Kanda S. , Olsen S. , Franklin M. , Ager J. W. , Pianetta P.
Published: July 2002

Electrical characteristics associated with radiation detection were measured on single-crystal natural type-IIa diamond using two techniques: charged particle-induced conductivity and time-resolved transient photoinduced conductivity. The two techniques complement each other: The charged particle-induced conductivity technique measures the product of the carrier mobility μ and lifetime τ throughout the bulk of the material while the transient photoconductivity technique measures the carrier mobility and lifetime independently at the first few micrometers of the material surface. For each technique, the μτ product was determined by integration of the respective signals. The collection distance that a free carrier drifts in an electric field was extracted by each technique. As a result, a direct comparison of bulk and surface electrical properties was performed. The data from these two techniques are in agreement, indicating no difference in the electrical properties between the bulk and the surface of the material. The collection distance continues to increase with field up to 25 kV/cm without saturation. Using the transient photoconductivity technique the carrier mobility was measured separately and compared with a simple electron-phonon scattering model. The general characteristics of carrier mobility, lifetime, and collection distance at low electric field appear to be adequately described by the model.
Material and electrical properties of highly mismatched InxGa1−xAs on GaAs by molecular-beam epitaxy
Authors: Chang Shou-Zen , Chang Tien-Chih , Shen Ji-Lin , Lee Si-Chen , Chen Yang-Fang
Published: July 2002

The material properties of 2-μm-thick InxGa1−xAs epilayers grown on GaAs with 0.28≤x≤1 were investigated. It was found that for x≥0.5, the material quality of the larger lattice-mismatched heterojunction recovers, as evidenced by cross-sectional transmission electron microscopy (XTEM) and double-crystal x-ray diffraction (DXRD). Magnetophotoconductivity measurements were performed on InxGa1−xAs epilayers with 0.75≤x≤1. The dependence of both the cyclotron resonance linewidth and the carrier relaxation time on the material quality is consistent with the XTEM and DXRD results. The transport properties of InxGa1−xAs epilayers with 0.75≤x≤1 were studied using temperature-dependent van der Pauw measurements. It was found that the electron mobility in the low-temperature range is determined by a combination of ionized impurity and dislocation scatterings. The contribution of dislocation scattering to ternary InGaAs epilayers is larger than that to InAs, although InAs has a larger lattice mismatch with respect to GaAs. These four different measurement techniques confirm that the growth mode rather than lattice mismatch determines the density of dislocation for the heteroepitaxy of highly mismatched InxGa1−xAs on GaAs.
Self-consistent calculation of the electronic structure of semiconductor quantum wires: Semiclassical and quantum mechanical approaches
Authors: Wu Zhiqiang , Ruden P. Paul
Published: July 2002

Efficient self-consistent calculations of dynamically one-dimensional electron gas systems using the finite element method with nonuniform mesh of triangular elements have been implemented. Both self-consistent semiclassical and self-consistent quantum mechanical calculations are carried out. In the semiclassical treatment, Poisson’s equation is solved self-consistently with the induced electron density determined by the Thomas–Fermi continuum formalism. Newton’s method is implemented to ensure fast convergence. In the quantum mechanical treatment, quasi-one-dimensional subband levels are determined by solving the effective mass Schrödinger equation and the induced electron density is determined by filling the occupied subbands according to Fermi–Dirac statistics for zero temperature. Self-consistency is achieved using Newton’s method with an approximate Jacobian derived from the Thomas–Fermi approximation. Etched ridge and split gate semiconductor quantum wires based on selectively doped AlGaAs/GaAs heterostructures are studied. The results of the semiclassical and quantum mechanical calculations are compared. We find that the two approaches predict similar lineal densities of induced electrons for a range of cases but that the dependence of electron density on external parameters can differ significantly. The resulting potential shapes and electron distributions can also be quite different. Specifically, the semiclassical approach results in shallower confining potential wells and less spread of the electron distribution in the direction perpendicular to the heterointerface than is found in the quantum mechanical approach.
Mode of growth and microstructure of polycrystalline silicon obtained by solid-phase crystallization of an amorphous silicon film
Authors: Haji L. , Joubert P. , Stoemenos J. , Economou N. A.
Published: July 2002

The structure and the morphology of crystallized amorphous silicon (α-Si) films which were deposited on glass and annealed in a conventional furnace or by rapid thermal process (RTP) are studied using transmission electron microscopy (TEM). The ellipsoidal shape of the grains is attributed to the fast solid-state crystallization along the two mutually perpendicular 〈112〉 and 〈110〉 crystallographic directions. The growth is solely based on the twin formation. The stability of the microtwins was studied by RTP and in situ TEM heating experiments. The effect of the film thickness on the preferred orientation of the grains is discussed. Very thin films exhibit (111) preferred orientation due to the strongly anisotropic rate of growth of the nuclei, which imposes an orientation filtering due to a growth velocity competition. The mode of growth of these films is compared with poly-Si films grown by low-pressure chemical-vapor deposition.
Copper phthalocyanine-titanium oxide multilayers
Authors: Takada J. , Awaji H. , Koshioka M. , Nevin W. A. , Imanishi M. , Fukada N.
Published: July 2002

Copper phthalocyanine-titanium oxide (CuPc-TiOx) heteromultilayers have been fabricated by evaporation and reactive evaporation techniques, and their structural and optoelectronic properties investigated. X-ray diffraction and secondary ion mass spectroscopy confirm the formation of a clear alternating layered structure on a nanometer scale. An interface roughness of 7 Å has been achieved for a multilayer with an artificial period of 40 Å. Atomic force microscopy at scratched edges of the multilayers reveal double-layered structures of TiOx on CuPc, indicating the existence of two kinds of interface. Transverse photoconduction measurements on samples deposited on SnO2-coated glass, using a back electrode of Al, show the occurrence of electron transfer from CuPc to TiOx at the interfaces.
Separation and analysis of diffusion and generation components of pn junction leakage current in various silicon wafers
Authors: Murakami Yoshio , Shingyouji Takayuki
Published: July 2002

An analytical method to separate the diffusion and generation components of pn junction leakage currents is developed. The voltage dependence between reverse current and capacitance in pn junctions is measured, and an approximately linear relationship between current density (J) and depletion width (W) is derived. In this relationship, the diffusion component corresponds to linearly extrapolated value of J at W=0, and the generation component corresponds to the rate at which J increases with W as voltage is applied. This method allows both components of the leakage current to be obtained for Czochralski, epitaxial, and intrinsic gettering wafers. Separated diffusion components strongly depend on silicon wafers mainly due to the change of minority carrier density and the diffusion of minority carriers. On the other hand, the generation component increases with increases in the electric field applied to the junction for all wafers. We found that this electric field effect on the generation component can be explained by the Poole–Frenkel mechanism.
Deep-level transient spectroscopy of HF-cleaned and sulfur-passivated InP metal/nitride/semiconductor structures
Authors: Landheer D. , Yousefi G. H. , Webb J. B. , Kwok R. W. M. , Lau W. M.
Published: July 2002

Deep-level bulk and interface defect states in InP metal/insulator/semiconductor diodes have been investigated using capacitance-voltage measurements and deep-level transient spectroscopy. The InP surfaces were cleaned with an ultraviolet light/ozone/HF process followed by an optional polysulfide passivation and then capped with a layer of silicon nitride produced by remote plasma-enhanced chemical-vapor deposition. Polysulfide treatment reduced the bulk trap concentrations produced during contact annealing, reducing a bulk trap at 0.33 eV below the measurement limits and reducing a 0.45 eV trap to a concentration of 5×1013 cm−3. The density of interface states close to the conduction-band edge also decreased with the polysulfide treatment.
Empirical rule of intrinsic absorption spectroscopy in Hg1−xCdxTe
Authors: Chu Junhao , Li Biao , Liu Kun , Tang Dingyuan
Published: July 2002

An empirical formula for the calculation of the intrinsic optical absorption coefficient at the Kane region is derived as α=αg exp[β(E−Eg)]1/2 from the measured absorption spectra in the whole intrinsic absorption region for Hg1−xCdxTe with different composition x from 0.170 to 0.443. The parameter β depends on composition x and temperature T, β=24−18x at 300 K and β=5.4+11x at 77 K. The calculated results from this simple empirical formula also agree well with those derived from the Kane model.
Carrier screening effects in piezoelectric strained InGaAs/GaAs quantum wells grown on the [111]B axis
Authors: Sale T. E. , Woodhead J. , Rees G. J. , Grey R. , David J. P. R. , Pabla A. S. , Rodriguez-Gíronés P. J. , Robson P. N. , Hogg R. A. , Skolnick M. S.
Published: July 2002

Excitation-power-dependent blue shifts in photoluminescence and electroluminescence are studied in piezoelectric strained layer multiple quantum wells (MQWs) incorporated in p-i-n diodes. By investigating MQWs with different geometric structures and controlling external bias it is demonstrated that, in contrast to previous studies, these blue shifts cannot always be attributed to long-range screening across a MQW and that screening must take place due to charge redistribution within individual wells. The results provide design rules to ensure this latter screening mechanism, which is subject to fast recovery.
Growth by molecular-beam epitaxy and photoluminescence of InGaAs/GaAs quantum wells on GaAs (111)A substrates
Authors: Vaccaro Pablo O. , Takahashi Mitsuo , Fujita Kazuhisa , Watanabe Toshihide
Published: July 2002

In0.18Ga0.82As/GaAs single quantum wells (SQWs) were grown on GaAs (111)A just, 1° and 5° off toward [110]- and [001]- and (100)-oriented substrates. Photoluminescence (PL) peaks from SQWs grown on GaAs (111)A just and misoriented substrates show a large full width at half-maximum and redshift as compared with calculated values. The quantum-confined Stark effect due to strain-induced electric fields is the main cause of this redshift in samples grown on GaAs (111)A 5° off toward [001] substrates. The larger redshift observed in samples grown on the other GaAs (111)A substrates is due to strain relaxation. A strain relaxation mechanism that consists of coherently grown islands when InGaAs growth begins and the generation of misfit dislocations when these islands coalesce gives a qualitative explanation of the observed results.
White light from an electroluminescent diode made from poly[3(4-octylphenyl)-2,2′-bithiophene] and an oxadiazole derivative
Authors: Berggren M. , Gustafsson G. , Inganäs O. , Andersson M. R. , Hjertberg T. , Wennerström O.
Published: July 2002

We report on an electroluminescent diode emitting red, green, and blue light simultaneously. The device is based on a thin polymer layer, poly[3-(4-octylphenyl)-2,2′-bithiophene] and a thick molecular layer, 2-(4-biphenylyl)-5-(4-tertbutyl-phenyl)1,3,5-oxadiazole. The quantum efficiency for light conversion is 0.3% and the turn-on voltage for light emission is 7 V. In this article we present electric and spectroscopic characterizations. A mechanism for the light emission, based on electron and hole recombination between the two organic layers, is proposed.
Mobility anisotropy of two-dimensional hole systems in (311)A GaAs/AlxGa1−xAs heterojunctions
Authors: Heremans J. J. , Santos M. B. , Hirakawa K. , Shayegan M.
Published: July 2002

We have measured the low-temperature mobility of high-quality two-dimensional hole systems confined at the (311)A GaAs/AlxGa1−xAs interface. Variables were the thickness of the spacer layer separating the carriers from the Si dopants, and the carrier sheet density. A large anisotropy in mobility is found between the [2̄33] and [011̄] directions. While the high mobility [2̄33] direction yields results analogous to the two-dimensional electron case, we conclude that transport along [011̄] is almost entirely determined by anisotropic interface roughness scattering.
Field orientations and sweep rate effects on magnetic switching of Stoner–Wohlfarth particles
Authors: Lu Jing Ju , Huang Huei Li , Klik I.
Published: July 2002

Field angles and sweep rate dependence of coercivity of the Stoner–Wohlfarth (S-W) particles are studied based on the decay-rate model. An analytic expression of the hysteresis loops based on the model is obtained as a function of applied field for various frequencies at various field angles with respect to the easy axis. Variation of the coercivities versus the sweep rate (measurement frequency), field angles, anisotropy constant, activation volume, and the combination thereof, and the distribution of easy axis, etc., have been obtained. Comparison with the thermal-equilibrium and S-W hysteresis loops are made. Coercivities of such a system generally increase with increasing sweep rate, anisotropy constant and activation volume, etc., hence decreases with increasing sweeping period. As a function of the field angles, the coercivity curve turns flat with increasing field angles and sweeping period.
Analysis of complex 1H nuclear magnetic resonance relaxation measurements in developing porous structures: A study in hydrating cement
Authors: Laganas E. , Papavassiliou G. , Fardis M. , Leventis A. , Milia F. , Chaniotakis E. , Meletiou C.
Published: July 2002

1H nuclear magnetic resonance (NMR) is widely used for the study of water kinetics into various porous systems like sandstones, zeolites, clays, and various cementitious materials. In the case of hydrating cement pastes the evolution of the cement structure ultimately leads to a broad pore size distribution, and consequently to a multiexponential magnetization recovery represented in many cases by a Kohlraush–Williams–Watts ‘‘stretched exponential’’ curve. However, it has been proven an extremely difficult task to fully clarify and exploit the information contained in the multiexponential magnetization recovery of cementitious materials. This difficulty is due to the fact that spin-lattice relaxation times T1 exhibit broad multimodal distributions, which sometimes extend over more than two orders of magnitude. In this work, a numerical routine using a combination of Monte Carlo and Simplex iterations (MCS) is examined which appears to estimate faithfully broad and complex spin-lattice relaxation time distributions. First, the reliability of the routine has been tested on simulated data and then the algorithm has been applied to the analysis of 1H NMR time domain measurements performed on different kinds of cement paste. The development of the pore structure is directly reflected in the time evolution of the T1 distribution function.
Bismuth substituted iron garnet thin films deposited on silicon by laser ablation
Authors: Le T. M. , Huang F. , Stancil D. D. , Lambeth D. N.
Published: July 2002

Thin films of bismuth substituted iron garnet are fabricated on silicon (100) substrates using the pulsed laser deposition technique. There is a narrow temperature range in which good polycrystalline garnet films can be grown on silicon; it must be higher than 575 °C for the films to crystallize and lower than 700 °C to avoid chemical reaction between the silicon substrate and the deposited materials. Film microstructure and magnetic properties are compared for films grown at different growth conditions and annealing procedures. The films heated during deposition are smooth and dense in contrast to the post-annealed films, which are characterized by clusters of many smaller grains separated by large cracks and a very small coercivity.
Direct measurement of lateral elastic modulations in a zero-net strained GaInAsP/InP multilayer
Authors: Ponchet A. , Rocher A. , Emery J-Y. , Starck C. , Goldstein L.
Published: July 2002

A zero-net strained GaInAsP/InP(001) multilayer grown by gas source molecular beam epitaxy exhibits large interfacial undulations and an orthorhombic modulated distortion, which are attributed to elastic relaxation of tensile layers. It is examined by high resolution transmission electron microscopy in order to directly determine the strain distribution in the (11̄0) plane. The interplanar spacings are found to be laterally modulated along the [110] direction within tensile and compressive layers. The interplanar spacing modulation is perfectly correlated to the interfacial morphology. Therefore, the strain is not homogeneous but concentrated along [11̄0] oriented lines. Large variations of (110) interplanar spacings up to 3.5% have been measured, while the lattice mismatch between tensile and compressive layers is only 2%. The observed strain distribution is qualitatively consistent with an elastic relaxation mechanism of the tensile layer. Different other effects are reviewed. Surface relaxation effects due to sample thinning are qualitatively discussed. It is also shown that nonplanar interfaces between tensile and compressive layers strongly account for the stress distribution.
Effects of biaxial strain on the intervalence-band absorption spectra of InGaAs/InP systems
Authors: Afzali-Kushaa A. , Haddad G. I.
Published: July 2002

The effects of biaxial strain on the intervalence-band absorption spectra of p-doped InGaAs/InP bulk layers are investigated. The study is performed by calculating and comparing the absorption coefficients corresponding to the direct transitions between the heavy and light hole bands, between the heavy hole and split-off bands, and between the split-off and light hole bands in both the lattice matched and the strained layers. The valence-band structures of these layers are neither isotropic nor parabolic and hence the k⋅p approach is utilized to calculate the band structures and their corresponding wave functions. The quantities are then invoked in the calculation of the (joint) density of states, the Fermi energy, and the momentum matrix element, which are needed in the evaluation of the intervalence-band absorption coefficients. These calculated results show that the intervalence-band absorption coefficients depend on the strain in the layer. The dependence is determined by the bands involved in the intervalence transition, the polarization of the incident light, and the type of the strain (compressive or tensile).
Optical properties of ZrTiO4 films grown by radio-frequency magnetron sputtering
Authors: Chang De-An , Lin Pang , Tseng Tseung-Yuen
Published: July 2002

Thin films of zirconium titanate on quartz glass were prepared using radio-frequency magnetron sputter deposition at various substrate temperature (Td from 25 to 450 °C). Crystallinity of the films sets in either at Td=400 °C or after annealing in air up to 650 °C. The refractive index showed monotonic increase with Td from 2.22 to 2.37 at 550 nm while the extinction coefficient ranging from 1.0×10−3 to 4.7×10−3 increased with decreasing Ti/Zr stoichiometric ratio (from 1.08 to 1.03) of the films. The influence of the Ti/Zr ratio on the oxygen deficiency and absorption properties of the films was attributed to the stronger attraction of the Ti4+ ions compared with Zr4+ ions to the condensing oxygen atoms or ions on the surface of growing film. The optical band gap was found to be 3.4 eV of indirect-transition type. A single-oscillator model was used to fit the dispersion of the refractive index with the wavelength, indicating that the amorphous and crystalline ZrTiO4 films are similar in short-range-order structure. The inhomogeneity and packing density of the films were analyzed using well-known relations.
Si80Ge20 thermoelectric alloys prepared with GaP additions
Authors: Cook B. A. , Harringa J. L. , Han S. H. , Vining C. B.
Published: July 2002

Controlled amounts of GaP and P were added to a Si0.8Ge0.2 matrix by a powder-metallurgical technique in order to evaluate the optimum composition for thermoelectric applications. Bulk determination of the gallium and phosphorus content in fully dense, hot pressed compacts was performed by inductively coupled plasma atomic emission spectroscopy. The transport properties of the compacts were characterized by Hall effect measurements at room temperature and by measurements of electrical resistivity, Seebeck coefficient, and thermal diffusivity to 1000 °C. Considerable variation in the electrical transport properties were found to accompany changes in the Ga/P ratio, in the total amount of dopant, and changes in other preparation conditions. Alloys with gallium phosphide additions exhibit carrier concentrations higher than those obtained in alloys doped only with phosphorus. Alloys with a nominal phosphorus content greater than 2.0 at. % were found to be overdoped and those containing less than 0.6 at. % phosphorus were found to be underdoped relative to the material’s maximum figure of merit. Room temperature electron mobilities greater than 40 cm2/V s were commonly obtained in samples with a carrier concentration of 3×1020 cm−3 or greater. A 300–1000 °C integrated average figure of merit (Z=S2/ρΛ) of 0.93×10−3 °C−1 was achieved within an optimum composition range of 0.8–1.2 at. % P and 0.6–0.8 mol % GaP, which is 20% higher than current n-type Si-Ge alloys doped with 0.59 at. % P as used in the Voyager, Galileo, and Ulysses missions.
The erbium-impurity interaction and its effects on the 1.54 μm luminescence of Er3+ in crystalline silicon
Authors: Priolo F. , Franzò G. , Coffa S. , Polman A. , Libertino S. , Barklie R. , Carey D.
Published: July 2002

We have studied the effect of erbium-impurity interactions on the 1.54 μm luminescence of Er3+ in crystalline Si. Float-zone and Czochralski-grown (100) oriented Si wafers were implanted with Er at a total dose of ∼1×1015/cm2. Some samples were also coimplanted with O, C, and F to realize uniform concentrations (up to 1020/cm3) of these impurities in the Er-doped region. Samples were analyzed by photoluminescence spectroscopy (PL) and electron paramagnetic resonance (EPR). Deep-level transient spectroscopy (DLTS) was also performed on p-n diodes implanted with Er at a dose of 6×1011/cm2 and codoped with impurities at a constant concentration of 1×1018/cm3. It was found that impurity codoping reduces the temperature quenching of the PL yield and that this reduction is more marked when the impurity concentration is increased. An EPR spectrum of sharp, anisotropic, lines is obtained for the sample codoped with 1020 O/cm3 but no clear EPR signal is observed without this codoping. The spectrum for the magnetic field B parallel to the [100] direction is similar to that expected for Er3+ in an approximately octahedral crystal field. DLTS analyses confirmed the formation of new Er3+ sites in the presence of the codoping impurities. In particular, a reduction in the density of the deepest levels has been observed and an impurity+Er-related level at ∼0.15 eV below the conduction band has been identified.

This level is present in Er+O-, Er+F-, and Er+C-doped Si samples while it is not observed in samples solely doped with Er or with the codoping impurity only. We suggest that this new level causes efficient excitation of Er through the recombination of e-h pairs bound to this level. Temperature quenching is ascribed to the thermalization of bound electrons to the conduction band. We show that the attainment of well-defined impurity-related luminescent Er centers is responsible for both the luminescence enhancement at low temperatures and for the reduction of the temperature quenching of the luminescence. A quantitative model for the excitation and deexcitation processes of Er in Si is also proposed and shows good agreement with the experimental results.
Photoluminescence study of defects in ion-implanted thermal SiO2 films
Authors: Nishikawa Hiroyuki , Watanabe Eiki , Ito Daisuke , Takiyama Makoto , Ieki Akihito , Ohki Yoshimichi
Published: July 2002

Photoluminescence (PL) study was performed on B or P ion-implanted thermal SiO2 films. Two PL bands at 4.3 and 2.6 eV were observed. For the 4.3 eV bands, two PL excitation (PLE) bands were observed at 5.0 and 7.4 eV. Based on the close similarities of the PL and PLE bands to those observed in oxygen-deficient-type bulk silica, the 4.3 and 2.6 eV PL bands are ascribed to the oxygen-deficient-type defects induced by ion implantation. While the 4.3 eV PL band in the bulk SiO2 decays exponentially, the decay of the corresponding PL band in the implanted thermal SiO2 films follows a power low or stretched exponential, suggesting the distribution of PL lifetimes. This suggests that the oxygen-deficient-type defects induced by the ion implantation in thermal SiO2 films are perturbed by the structural distribution of the surrounding Si-O-Si network, including the concentration of PL quenching centers.
Micro-Raman and infrared reflectivity spectra of YBa2Cu3O7−δ deposited on LaAlO3 and stainless steel/inconel substrates
Authors: Ching-Prado E. , Pérez W. , Katiyar R. S. , Bist H. D. , Soni R. N. , Sathaiah S. , Sinha U.
Published: July 2002

The motivation for this article has been to try to understand the relative phenomenological differences between the behavior of the films of the high-temperature superconductors like YBa2Cu3O7−δ deposited on (a) relatively insulating substrates (like LaAlO3) through sputtering technique and (b) highly metallic substrates (like stainless steel and inconel) through laser ablation method. It is observed that the most prominent Raman bands on LaAlO3 substrate falling near 337, 440, and 503 cm−1 have half widths (defined as the full width at half the maximum intensity) of 22, 42, and 38 cm−1, respectively. The half width gets nearly doubled on the metallic substrates. These results have been attributed to the metal-superconductor interfacial interactions. Further, infrared reflectance measurements and the Kramers–Kronig analysis of the spectra reveal a large difference in the conductivity of the film on LaAlO3 compared to the films on metallic substrates.
Effects of buffer layers in epitaxial growth of SrTiO3 thin film on Si(100)
Authors: Nakagawara Osamu , Kobayashi Masato , Yoshino Yukio , Katayama Yûzô , Tabata Hitoshi , Kawai Tomoji
Published: July 2002

SrTiO3 thin film has been formed on Si(100) substrates with various single buffer layers such as SrO, CeO2, CaF2, CoSi2 and a multibuffer layer, YSZ/Y2O3/YBa2Cu3O7 by ArF excimer laser ablation. The relation of lattice orientation of buffer layers with SrTiO3 layer has been elucidated. The orientation of SrTiO3 film is influenced not only by lattice matching but by crystal structure and chemical bonding of the buffer layers. As well, the multibuffer layer more effectively forms preferential c-axis oriented SrTiO3 film on Si(100), while CoSi2 buffer is more effective for improving the dielectric constant of SrTiO3 than other buffer layers.
Front-illuminated long wavelength multiple quantum-well infrared photodetectors with backside gratings
Authors: Deng Shao-You , Ya-Min Lee Joseph , Lai Jiun-Tsuen , Chih Yue-Der , Sun Tai-Ping , Hong Hong-Ming
Published: July 2002

A front-illuminated long wavelength multiple quantum-well infrared photodetector (QWIP) with backside gratings is successfully fabricated. The front illumination is achieved through the reflection of incoming light by etched gratings fabricated on the backside of the wafer. The detector is based on bound-to-continuum state transition in the quantum wells. The epitaxial structure is grown by molecular beam epitaxy. There are 30 periods of GaAs/AlGaAs quantum wells. The measured results show a detection peak at 8.7 μm and a cutoff wavelength of 10.3 μm. The dark current of QWIP is studied in the temperature range from 20 to 300 K. The dark current is found to be due to thermionic emission at low bias voltage and in the temperature range above 50 K, and due to tunneling in the low-temperature range below 50 K and at a bias voltage above 1 V.
Optical gain and laser emission in HgCdTe heterostructures
Authors: Bonnet-Gamard J. , Bleuse J. , Magnea N. , Pautrat J. L.
Published: July 2002

A detailed study of stimulated emission mechanisms as well as laser emission capability has been carried out on Hg1−xCdxTe (0.44<x<0.5) separate-confinement heterostructures grown by molecular beam epitaxy. At low temperature, spontaneous photoluminescence (PL) occurs on extrinsic levels below the gap whereas optical gain exhibits a maximum of stimulated emission shifted towards higher energy, close to the gap. As temperature increases, spontaneous PL is shifted from the extrinsic states to the band-to-band transition by a thermally activated detrapping of the carriers. Above 100 K, spontaneous and stimulated emission vary in a similar way with temperature. Laser emission has been observed up to room temperature for all the heterostructures. The use of quantum wells in the active layer and graded index in the barriers has allowed a significant reduction of the excitation density threshold, as compared to a single separate-confinement heterostructure (SCH) of same composition. However, the high-temperature characteristic temperature T0 is found to be similar in the two structures. A SCH with a higher energy gap exhibits a more favorable behavior with temperature. These experimental results have been compared to theoretical models. The experimentally observed T0 can be well simulated by taking into account the Auger effect. From the experimental data, the Auger constant has been determined for each heterostructure.
Model for determining the density and mobility of carriers in thin semiconducting layers with only two contacts
Authors: Lowney Jeremiah R.
Published: July 2002

A new method for determining the carrier densities and mobilities in a thin semiconducting layer was recently described. It is based on fitting the transverse magnetoresistance of the layers as a function of magnetic field, and it requires only two contacts. To improve the accuracy and generalize the procedure to multicarrier systems, a computer code was written to solve for the magnetoresistance as a function of magnetic field and the length-to-width ratio of a rectangular sample. A nonlinear-least-squares fit was made to the results of the computer model for a single-carrier system. The results for multicarrier systems are discussed. This method is especially useful as a monitor for improving the quality control of the electrical characteristics of thin conducting layers in finished devices. The code is also useful for interpreting standard four-terminal measurements as well.
Mechanism of different orientation of CdTe on GaAs by molecular beam epitaxy
Authors: Yin Jingjiang , Huang Qi , Zhou Junming
Published: July 2002

CdTe films have been grown on (001) GaAs substrates by molecular beam epitaxy in a wide range of growth temperature. X-ray double crystal rocking curve has been used to analyze the quality and orientation of the epitaxial films. Our experiments prove that the growth temperature has a great effect on the quality and orientation of the epitaxial CdTe films: Tsub<300 °C, CdTe (111)∥GaAs (001); Tsub≳300 °C, CdTe (001)∥GaAs (001). An explanation is given for this result by considering the lattice mismatch and interface energy.
Atomic bonding in amorphous carbon alloys: A thermodynamic approach
Authors: Efstathiadis H. , Akkerman Z. , Smith F. W.
Published: July 2002

The free energy model previously developed for the prediction of the bonding in amorphous Si-based alloys is extended here to amorphous carbon alloys, a-CxH1−x, containing carbon atoms with sp3 and sp2 hybridization. Predictions have been made for the bonds present in the alloys, with the case of ‘‘chemical’’ ordering at T=0 K corresponding to phase separation into separate C (sp3) and C(sp2) regions. For T≳0 K phase separation is eliminated and there is no evidence for the clustering of graphitic carbon, indicating the importance of the configurational entropy in influencing the bonding in the alloys. Hydrogen atoms are predicted to bond preferentially to C (sp3) atoms for all T. The sp3/sp2 ratio is predicted to increase with increasing H content, as observed experimentally, and also with increasing T due to entropy effects. Predictions have been made for the distribution of bonds in tetrahedral C(sp3)- and planar C(sp2)=C(sp2)-centered units. It is found that essentially no aromatic or graphitic structures are present in typical alloys. The a-CxH1−x alloys have been proposed to consist of five amorphous components: diamondlike, graphitic, polymeric, olefinic, and mixed diamond–graphitic (d–g) components. It is predicted that the polymeric and mixed d–g components dominate in typical plasma-deposited alloy films while the mixed d–g component dominates in hydrogen-free a-C films.
One step electrodeposition of CuInSe2: Improved structural, electronic, and photovoltaic properties by annealing under high selenium pressure
Authors: Guillemoles Jean-François , Cowache Pierre , Lusson Alain , Fezzaa Kamel , Boisivon Frederic , Vedel Jacques , Lincot Daniel
Published: July 2002

Films of Cu–In–Se alloys can be electrodeposited in a wide range of controlled composition. Annealing treatments under Se pressure transform these precursor films in large grain CuInSe2 films with improved electronic properties. These modifications are shown to depend on the Se pressure imposed during the treatment allowing a certain tailoring of the electronic properties of the films. The properties of electrodeposited/selenized films are presented as obtained from luminescence measurements, Hall effect, and photoelectrochemical characterization. An efficiency of 6.5% (total area, without antireflecting coating) is reported for the best CuInSe2/CdS/ZnO solar cell. An analysis of the device is also presented where limitations by interface recombination are shown to be the dominant loss mechanism.
Magnetic and structural properties of Ba–ferrite films prepared by sol-gel processing
Authors: Ishikawa A. , Tanahashi K. , Futamoto M.
Published: July 2002

The magnetic properties and microstructure of barium ferrite films (BaFe12O19) coated by a sol-gel technique involving a spin coating of metal-organic compounds are investigated. The in-plane magnetic properties and the grain size are controlled by a Fe/Ba atomic ratio in the coating solution and annealing conditions. The in-plane coercivity increases as the decrease of the Fe/Ba ratio and the increase of the annealing temperature. Maximum value of 4–6 kOe is obtained at the Fe/Ba ratio of 2–8 with the annealing at 900 °C for 10 min. At these preparation conditions, the c axis of the hexagonal barium ferrite is slightly oriented parallel to the film plane. The grain size decreases as the decrease of the Fe/Ba ratio. The films with the Fe/Ba ratio of 4–6 show a grain size of almost one-tenth of that of the sputtered films without additives such as Cr2O3 or CoTi. The smaller grain size of the sol-gel-coated films is probably due to the high nucleation density at the annealing process.
Magnetic entropy in nanocomposite binary gadolinium alloys
Authors: Shao Yuanzhi , Zhang Jinxiu , Lai J. K. L. , Shek C. H.
Published: July 2002

Investigation on magnetocaloric effects, magnetic entropy, Curie temperature, and specific heat of nanocomposite binary gadolinium alloys Gd–Tb, Gd–Zn, and Gd–Y has been carried out with an applied magnetic field of 1 T and in a temperature range of 233–313 K. Compared with the respective bulk alloys, the as-prepared nanocomposite alloys were found to have higher specific heat and lower Curie temperature. The nanocomposite Gd–Y alloy exhibited distinctive enhancement of both magnetocaloric effect and magnetic entropy over that of the bulk alloy. The finding is of importance for developing new materials for magnetic refrigeration at room temperature. A discussion concerning the enhanced magnetic entropy in nanometer superparamagnetic systems is presented in detail.
Reduction of photoconductive gain in quantum well infrared photodetectors
Authors: Choi K. K.
Published: July 2002

In this work, we show that there is a reduction of photoconductive gain g in quantum well infrared photodetectors from its classical value. The reduction is caused by the quantum nature of electron transport in these structures. On the other hand, the generation-recombination noise is unaffected by the transport model, and remains to be the same as a classical photoconductor. The reduction of g leads to an apparent noise increase in these structures, i.e., the noise gain deduced from the noise measurement is larger than g deduced from the photoconductivity measurements. We compared the present theory with existing experimental data, and found reasonable agreement.
Photoelectric properties of Copper-phthalocyanine/PbTe multilayer
Authors: Lee Heayeon , Kawai Tomoji
Published: July 2002

The transverse current–voltage characteristics of Copper- phthalocyanine (CuPc)/Si, PbTe/Si, and PbTe/CuPc/Si junction have been observed in the dark and under illumination. The PbTe/CuPc/Si junction exhibits a strong photovoltaic response with quantum efficiency of 15.4% and power conversion efficiency of 3.46×10−2. The photocarrier is generated in CuPc layer and the carrier is well separated by the steep incline of the potential near the CuPc/PbTe interface. The CuPc/PbTe multilayers show large photoconduction effect in the in-plane direction.
Monte Carlo simulation of electron transport in 4H–SiC using a two-band model with multiple minima
Authors: Nilsson Hans-Erik , Sannemo Ulf , Petersson C. Sture
Published: July 2002

A Monte Carlo study of the high-field electron transport in 4H–SiC is presented using a new analytic band model. The band model consists of two analytical bands that include band bending at the Brillouin zone boundaries. The band bending is very important in 4H–SiC and 6H–SiC and has to be taken into account in order to have an accurate model at high electric fields. Numerical calculation of the density of states given by the new model has been used in order to model the energy dependence of the scattering mechanisms accurately. The new model predicts a lower saturation velocity in the c direction (peak velocity 1.8×107 cm/s) than in perpendicular directions (peak velocity 2.1×107 cm/s). This is directly related to the strong band bending in the c direction. This effect is also responsible for a much more pronounced velocity peak in the c direction compared to perpendicular directions. In the low-field region the mobility is higher in the c direction (mobility ratio near 0.8), which is in agreement with experimental results.
Measurements of Young’s modulus and the modulus of rigidity of the solid solution of hydrogen in zirconium between 300 and 1300 K
Authors: Ashida Yuh , Yamamoto Masahiro , Naito Shizuo , Mabuchi Mahito , Hashino Tomoyasu
Published: July 2002

Young’s modulus (E) and the modulus of rigidity (G) have been measured for zirconium between 300 and 1300 K and for zirconium hydrides ZrHx (0<x<0.9) at 941 and 1001 K, using an apparatus capable of making in situ measurements under ultrahigh vacuum. Values of E and G have been obtained from resonance frequencies for bending and torsion vibrations of a polycrystalline zirconium wire. As the temperature increases, E and G of zirconium decrease in the α phase of an hcp structure and in the β phase of a bcc structure with an abrupt decrease at the α→β transition temperature 1135 K. As the hydrogen concentration increases, E and G of ZrHx decrease in the α phase and increase in the β phase.
Electrical resistivity of TiB2 at elevated pressures and temperatures
Authors: Li Xiaoyuan , Manghnani Murli H. , Ming Li-Chung , Grady Dennis E.
Published: July 2002

The electrical resistivity of TiB2 has been measured using a DIA-6 cubic anvil apparatus at pressures up to 8 GPa and temperatures up to 800 K. The ambient-condition resistivity is determined to be 13.3 (±0.9) μΩ cm. The resistivity decreases with increasing pressure. At pressures above 2 GPa, the pressure dependence of the resistivity is about −0.36 μΩ cm/GPa. On heating, the resistivity increases linearly with temperature. The measurements at simultaneously high pressure (3.2 GPa) and high temperatures yield a temperature dependence of 46 (±5) nΩ cm/K for the resistivity.
Passivation of light-emitting porous silicon by rapid thermal treatment in NH3
Authors: Li Gubo , Hou Xiaoyuan , Yuan Shuai , Chen Huajie , Zhang Fulong , Fan Honglei , Wang Xun
Published: July 2002

The light-emitting porous silicon is treated by the rapid thermal process at 900–1100 °C under NH3 environment. The infrared absorption spectra and Auger electron spectra show that the surface of porous silicon is covered with a nitride-containing layer. From the electron spin resonance, the density of dangling bonds is found to be quite low. The photoluminescence intensity shows a slight decay under the laser illumination and remains almost unchanged after three months storage in the ambient air. All of these results illustrate that the nitride could be an effective passivation film on the surface of porous silicon.
Investigation of photoluminescence and photocurrent in InGaAsP/InP strained multiple quantum well heterostructures
Authors: Raisky O. Y. , Wang W. B. , Alfano R. R. , Reynolds, Jr. C. L. , Swaminathan V.
Published: July 2002

Multiple quantum well InGaAsP/InP p-i-n laser heterostructures with different barrier thicknesses have been investigated using photoluminescence (PL) and photocurrent (PC) measurements. The observed PL spectrum and peak positions are in good agreement with those obtained from transfer matrix calculations. Comparing the measured quantum well PC with calculated carrier escape rates, the photocurrent changes are found to be governed by the temperature dependence of the electron escape time.
Electron–impurity scattering in free-standing quantum wires: Effect of dielectric confinement
Authors: Vagner Pavel , Moško Martin
Published: July 2002

We calculate the impurity-scattering limited mobility of the one-dimensional electron gas in a rectangular GaAs quantum wire confined in the vertical (growth) direction by n-modulation doped AlGaAs layers and free-standing along the transverse direction. The Fourier-transformed scattering potential of the ionized impurity is obtained analytically by solving the Poisson equation with z-dependent electrostatic permittivity. An abrupt permittivity change at the GaAs(AlGaAs)/air interfaces gives rise to the image charge effect which strongly modifies the unperturbed scattering potential. We show that the “image impurity” scattering tends to drastically reduce the electron mobility for sufficiently small (∼10 nm) transverse wire widths.
Microstructural development in MnAl-base permanent magnet materials: New perspectives
Authors: Hoydick D. P. , Palmiere E. J. , Soffa W. A.
Published: July 2002

The MnAl-base permanent magnets continue to loom on the horizon as potentially important technological materials. Marginal enhancement of their technical magnetic properties and improved processing could markedly change the relationship of these materials to the current market. MnAl-base magnets derive their attractive properties essentially from the formation of a metastable τ phase (L10) after cooling from the high-temperature ε-phase (hcp) field. For many years the conventional wisdom has been that the τ phase forms via a displacive or martensitic transformation. In this paper the results of a new electron microscopy investigation into the nature of the ε→τ transformation and microstructural development are reported. It is shown that over the temperature range 450–700 °C the transformation involves a compositionally invariant, diffusional nucleation and growth process rather than a shear mechanism. Importantly, elucidating the mechanism of the phase transformation has important practical implications since a diffusional mechanism involving the migration of incoherent or partially coherent interfaces points to very different thermomechanical processing routes and alloying strategies.
A simple method for calculating strain distributions in quantum dot structures
Authors: Downes J. R. , Faux D. A. , O’Reilly E. P.
Published: July 2002

A simple method is presented for calculating the stress and strain distributions arising from an initially uniformly strained quantum dot of arbitrary shape buried in an infinite isotropic medium. The method involves the evaluation of a surface integral over the boundary of the quantum dot and is therefore considerably more straightforward to implement than alternative stress evaluation techniques. The technique is ideally suited to calculating strain distributions within disordered arrays of pyramidal quantum dots prepared by Stranski–Krastanow growth. The strain distribution for a cuboidal quantum dot is presented and compared to that of a rectangular quantum wire.