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Journal of Applied Physics

Issue 6 • Date Mar 2001

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Displaying Results 1 - 25 of 80
  • Issue Cover

    Page(s): c1
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    Freely Available from IEEE
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Spectroscopic properties of Nd-doped glass for 944 nm laser emission

    Page(s): 3079 - 3083
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    A tunable laser operating near 944 nm is important for the detection of water vapor in the atmosphere. In this work, we have carried out a systematic study of Nd doped fiber glass materials which can be used for this purpose. The emission cross sections of these materials at 944 nm have been calculated. The wavelength dependence of emission of these materials has been studied. Among the silica-based materials Nd doped silica fiber glass has the largest cross section for 944 nm laser emission. The codopants reduce this cross section. Some Nd doped nonsilica materials such as Ba0.25Mg0.75Y2Ge2O12 and chloride glass have higher stimulated emission cross section than silica-based materials. © 2001 American Institute of Physics. View full abstract»

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  • Self-consistent solutions to the intersubband rate equations in quantum cascade lasers: Analysis of a GaAs/AlxGa1-xAs device

    Page(s): 3084 - 3090
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    The carrier transition rates and subband populations for a GaAs/AlGaAs quantum cascade laser operating in the mid-infrared frequency range are calculated by solving the rate equations describing the electron densities in each subband self-consistently. These calculations are repeated for a range of temperatures from 20 to 300 K. The lifetime of the upper laser level found by this self-consistent method is then used to calculate the gain for this range of temperatures. At a temperature of 77 K, the gain of the laser is found to be 34 cm-1/(kA/cm-2), when only electron–longitudinal-optical phonon transitions are considered in the calculation. The calculated gain decreases to 19.6 cm-1/(kA/cm-2) when electron–electron transition rates are included, thus showing their importance in physical models of these devices. Further analysis shows that thermionic emission could be occurring in real devices. © 2001 American Institute of Physics. View full abstract»

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  • In situ temperature measurements via ruby R lines of sapphire substrate based InGaN light emitting diodes during operation

    Page(s): 3091 - 3094
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    The temperature of encapsulated green and ultraviolet light emitting diodes (LEDs) in operation has been measured optically via the ruby R lines emitted by the residual Cr3+ contaminations in the sapphire substrate. These two photoluminescent R lines, which are excited by the electroluminescence of the LED itself, show a well-characterized line shift as a function of temperature and pressure. The temperature is found to rise linearly with the applied forward current of the LED at a rate of ≈1 K/mA. This optical temperature measurement based on monitoring the two ruby R lines could qualify as a sensitive method for an in situ temperature sensor for other sapphire substrate based semiconductor devices such as laser diodes and field effect transistors. © 2001 American Institute of Physics. View full abstract»

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  • Vaporization of heated materials into discharge plasmas

    Page(s): 3095 - 3098
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    The vaporization of condensed materials in contact with high-current discharge plasmas is considered. A kinetic numerical method named direct simulation Monte Carlo (DSMC) and analytical kinetic approaches based on the bimodal distribution function approximation are employed. The solution of the kinetic layer problem depends upon the velocity at the outer boundary of the kinetic layer which varies from very small, corresponding to the high-density plasma near the evaporated surface, up to the sound speed, corresponding to evaporation into vacuum. The heavy particles density and temperature at the kinetic and hydrodynamic layer interface were obtained by the analytical method while DSMC calculation makes it possible to obtain the evolution of the particle distribution function within the kinetic layer and the layer thickness. © 2001 American Institute of Physics. View full abstract»

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  • Stability of a magnetized Hall plasma discharge

    Page(s): 3099 - 3107
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    Using recent experimental data on the time-averaged, spatially varying plasma properties within a Hall discharge plasma, we present in this article, a theoretical study of the response of this plasma to small (linear) perturbations in its properties. As a starting point for this analysis, we assume a two-dimensional fluid description that includes a simplified equation for the electron energy, and constrain the azimuthal wave vector such that we excite only the dominant (m=1) azimuthal modes. The growth rate and frequencies of predominantly axial and azimuthally propagating plasma disturbances are obtained by numerical solution of the resulting eigenvalue problem under a quasiuniform plasma condition, along the entire discharge channel. The results identify the persistence of a low frequency instability that is associated with the ionization process, concentrated largely in the vicinity of the exit plane, where the magnetic field is at its maximum value, consistent with experimental observations for the relatively low operating voltages (∼100 V) considered in this study. © 2001 American Institute of Physics. View full abstract»

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  • Experimental study of energy coupling and plasma breakdown in a pulsed high frequency resonant cavity

    Page(s): 3108 - 3114
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    An experimental study of the electromagnetic behavior of a pulsed high frequency resonant cavity, operating in its fundamental TE111 mode, is presented. Typical pulse characteristics are a pulse width of 10 μs, microwave frequency of 1 GHz, and microwave power of a few kW. The various parameters (characteristic time τ, coupling factor β, incident power Pi) that determine the temporal variation of the electrical characteristics (power entering in the cavity, electric field) during the pulse are defined. In particular, the influence of the coupling factor on the transition stage to a steady state is discussed for the cavity without plasma. The temporal behavior of the cavity in the presence of a gas (argon and hydrogen) is also studied. For pressures ranging from 0.5 to 100 Torr, the discharge breakdown parameters (time τb, electric field Eb) in hydrogen are deduced from the experiments (tb∼0.1–1 μs, Eb∼300–3000 V/cm). The conditions to reach a quasisteady state plasma are derived from the qualitative analysis of the cavity behavior. © 2001 American Institute of Physics. View full abstract»

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  • In situ determination of absolute number densities of nitrogen molecule triplet states in an rf-plasma sheath

    Page(s): 3115 - 3120
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    A laser induced fluorescence technique (LIF) in combination with optical emission spectroscopy (OES) and Rayleigh scattering (RS) was applied to investigate absolute number densities of the population of the three nitrogen triplet states C 3Πu, B 3Πg, and the metastable A 3Σu+ in an asymmetric low pressure rf discharge. Primary targets of this investigation were the three lowest vibrational levels (v=0,1,2) of each triplet state and additionally v=8 of A 3Σu+ which can be populated very efficiently in the plasma sheath. Calibration of LIF intensities to absolute densities of A 3Σu+ and B 3Πg has been realized by comparison with the signal of the RS experiment done in pure nitrogen gas. Calibration of C 3Πu which we could not detect by LIF but rather only by OES was achieved after comparing OES and LIF signals of the B 3Πg state. Excitation energies of the analyzed states range from 6 up to 11.5 eV and the measured number densities differ by about seven orders of magnitude from as much as 1012cm-3 down to almost 105cm-3. In addition to the interpretation of the results this article describes the used calibration methods which are partly based on a suggestion made by P. Bogen (Proceedings of the XVI International Conference on Phenomena in Ionized Gases, Invited Papers, edited by W. Bötticher, H.- Wenk, and E. Schultz-Gulde, Düsseldorf, 1983, pp. 164–173). © 2001 American Institute of Physics. View full abstract»

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  • Effects of ultraviolet and thermal pretreatment on the formation of self-written χ(2) gratings and optical damage

    Page(s): 3121 - 3124
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    In this article we report the effects of thermal and ultraviolet light pretreatments of a variety of optical fibers with different core dopants. Except for fibers codoped with phosphorus, a negative correlation was found between the onset of optical damage to the fibers and the formation of seeded second-harmonic generation (SHG). From our results and from those of other workers, we conclude that there is a link between susceptibility to optical damage and the formation of seeded SHG in germanosilicate optical fibers. © 2001 American Institute of Physics. View full abstract»

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  • Determination of processing damage in thin polycrystalline Ir films using Bragg-peak fringe analysis

    Page(s): 3125 - 3131
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    We present an analysis of Bragg–Brentano diffraction spectra from blanket thin (10–20 nm) films of polycrystalline Ir, sputter deposited on thermally oxidized Si wafers. We observed that postdeposition annealing sharpened the Ir diffraction peaks, produced enhanced texture, and resulted in the formation of well-defined thickness fringes that were symmetric around the 111 Ir reflection. Scanning electron microscopy confirmed that the fringes were caused by the increased coherence lengths of the annealed grains. Annealed samples subsequently subjected to reactive ion etching exhibited asymmetric fringe patterns, with lower intensity fringes on the high angle side of the 111 Ir peak. Reannealing these samples restored the symmetric fringes. Analysis of the fringe patterns using simple equations and modeling programs in the public domain yielded valuable structural information about the film and the changes caused by processing. These results were verified by x-ray reflectivity measurements and modeling. We conclude that thickness fringe analysis using standard x-ray systems and simple programs can provide cost-effective process diagnostics for high atomic number thin film structures. © 2001 American Institute of Physics. View full abstract»

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  • Determination of Ir consumption during thermal oxidation and PbZrxTi1-xO3 processing using Bragg-peak fringe analysis

    Page(s): 3132 - 3137
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    Thickness interference fringes can be seen around the Bragg peaks of a variety of polycrystalline thin (10–20 nm) films using standard x-ray diffraction techniques in a conventional Bragg–Brentano geometry. In this article, thickness fringe analysis is used to investigate oxidation and roughening in thin Ir films with and without overlayers of the ferroelectric PbZrxTi1-xO3 (PZT). Changes in fringe spacing were used to determine the Ir thickness consumed by oxidation. Fringe contrast degradation (indicating roughening) was observed both after oxidation anneals (which formed a surface layer of IrO2) and after subsequent reduction anneals (which converted the IrO2 back to Ir). Film overlayers were found to have a protective effect against oxidation and roughening, as evidenced by comparison of postoxidation fringe patterns for bare and PZT-coated Ir films. Overall, our results demonstrate that thickness fringe analysis can be used as a simple, quantitative probe of processing-induced thin film thickness and morphology changes. © 2001 American Institute of Physics. View full abstract»

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  • Photoluminescence and photo-modulated reflectance spectra of ion-implanted GaAs/AlGaAs coupled quantum wells

    Page(s): 3138 - 3142
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    Ion-implantation effects on a series of comparable asymmetrically-coupled GaAs/AlGaAs quantum wells processed by implantation method have been studied by photoluminescence (PL) and photo-modulated reflectance (PR) spectroscopy measurements. The peaks in the PL and PR spectra originate from the intersubband transitions between the ground states of electron state and hole states. Corresponding peak positions in the PL and PR spectra shift to higher energies following the increase of the ion-implantation dose due to the enhanced Al diffusion across the GaAs/AlGaAs and GaAs/AlAs heterointerfaces in the system. The observation of the reduction and finally the quench of the PL and PR peaks due to the increased implantation dose is explained by the reduced lifetime of carriers in the system due to the increased scattering processes. © 2001 American Institute of Physics. View full abstract»

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  • Correlation of contact resistance with microstructure for Au/Ni/Al/Ti/AlGaN/GaN ohmic contacts using transmission electron microscopy

    Page(s): 3143 - 3150
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    Ohmic contacts to AlGaN/GaN heterostructures which have low contact resistance and good surface morphology are required for the development of commercial high power, high frequency transistors in the GaN system. The development of such contacts would be helped by a better understanding of the effect of microstructure on electrical behavior, which is studied here. Au/Ni/Al/Ti/AlGaN/GaN ohmic contact structures were rapid thermal annealed in argon for 60 s at temperatures in the range 550–900 °C. The variation of contact resistance with anneal temperature was correlated with the phase distribution observed by transmission electron microscopy (TEM). A combination of TEM techniques was required to determine the resulting microstructure, including energy filtered TEM, high resolution electron microscopy and energy dispersive x-ray spectrum imaging. Contacts with the lowest resistance were formed after 700 °C annealing. Very little consumption of the 30 nm AlGaN layer was observed. An unidentified phase containing Al, Ti and Au is present at the interface in the samples with low specific contact resistance. The identification of the observed thin interfacial phases (including TiN and AlN) is discussed, along with the effects of oxidation and possible mechanisms of ohmic contact formation. © 2001 American Institute of Physics. View full abstract»

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  • Induced magnetic anisotropy in metallic glasses irradiated by swift heavy ions

    Page(s): 3151 - 3155
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    Effects of swift heavy ion irradiation on ferromagnetic amorphous Fe81B13.5Si3.5C2 ribbons were investigated by transmission and conversion electron 57Fe Mössbauer spectrometry. Irradiation was found to induce changes in the magnetic texture depending on the ion energy and the corresponding projected range. The magnetic anisotropy tends to become randomly distributed in the case of the highest ion energy as the result of the strong level of electronic excitations induced throughout the sample. A perpendicular magnetic anisotropy was evidenced in the case of implantation of the ion beam inside the amorphous alloys. The origin of the induced perpendicular magnetic anisotropy was interpreted in terms of magnetoelastic effects due to strong stresses generated by the reaction of the nonirradiated region to the anisotropic deformation of the irradiated amorphous material. © 2001 American Institute of Physics. View full abstract»

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  • Dependence of ion implantation: Induced defects on substrate doping

    Page(s): 3156 - 3161
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    The characteristics of an ion implantation-induced defect in a silicon substrate are investigated. This defect is considered to be a complex of a point defect and a substrate dopant atom. The experiments are conducted by focusing on the dependence of the substrate dopant species (phosphorus and boron) on defect formation. The characteristics of the defect are investigated by measuring the bulk generation lifetime g) of metal–oxide–semiconductor capacitors, in which Si+ has been implanted to form the dopant-related defects in the substrate (damaging implantation) after gate oxide formation. As a result, it is found that the τg of the boron-doped substrate is one to two orders of magnitude smaller than that of the phosphorus-doped substrate for the same Nsub under the same implantation conditions. The temperature dependence of τg shows that the energy level of the defect is located at the intrinsic Fermi level, independent of the substrate dopant species and the concentration. By measuring the dependence of τg on the temperature of postdamaging implantation annealing, it is shown that the defects vanish at about 800 °C for both types of substrate. Also, it is found that the amount of dopant-related defects depends on the implantation ion species. BF2+ implantation forms more defects than As+ implantation. © 2001 American Institute of Physics. View full abstract»

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  • Electrical and microstructural properties of highly boron-implantation doped 6H–SiC

    Page(s): 3162 - 3167
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    Boron was implanted with four energies and doses at 400 °C into 6H–SiC epilayers to form a 500 nm thick doped layer with a mean concentration between 1×1018 and 1.5×1021 cm-3. Two annealing techniques were used: furnace and flash lamp annealing. The electrical and microstructural effects were investigated using temperature dependent Hall measurements, cross sectional electron microscopy, and secondary ion mass spectrometry. During the annealing two competing processes occurred: boron outdiffusion and growth of boron containing precipitates. The efficiency of these individual processes is different for varying dopant concentrations as well as annealing techniques. After furnace annealing at temperatures between 1550 and 1750 °C and for a mean boron concentration of 5×1019 cm-3 boron containing clusters are found mainly around the region of the three deeper implantation peaks. In the surface region boron outdiffusion is observed adjusting a concentration of 1.5×1019 cm-3. Using flash lamp annealing, the outdiffusion is negligible. For high dopant concentrations (1.5×1021 cm-3) the growth of random distributed boron precipitates is the dominating effect independent of the used annealing techniques. The electrical activation is limited due to the solubility of boron in SiC. After furnace annealing Hall effect measurements show a maximum hole concentration of about 2×1016 cm-3 for the boron concentration of about 5×1018 cm-3. Alternative to the furnace annealing, the electrical properties after flash lamp annealing at about 2000 °C, 20 ms show a slight enhancement of the maximum hole concentration for boron concentrations ≪3×1020 cm-3. © 2001 American Institute of Physics. View full abstract»

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  • Laser induced crystallization of amorphous Ge2Sb2Te5 films

    Page(s): 3168 - 3176
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    The crystallization behavior of Ge2Sb2Te5 thin films has been analyzed by atomic force microscopy and optical reflection measurements on various time scales in order to determine the crystallization kinetics including the crystallization mechanism, the corresponding activation barrier, and the Avrami coefficient. On the minute time scale, thin amorphous films were isothermally crystallized in a furnace under a protective Ar atmosphere. From these measurements the activation energy for crystallization was determined to be (2.0±0.2) eV, in close agreement with previous studies using different techniques. The isothermal measurements also revealed a temperature dependent incubation time for the formation of critical nuclei, which is compared with recent theories. On the nanosecond time scale, Ge2Sb2Te5 was locally crystallized with a focused laser. Either crystalline spots of submicron size were generated in an as deposited amorphous film or amorphous bits in an otherwise crystalline film were recrystallized. For the formation of crystalline spots in an as deposited amorphous film a minimum time of (100±10) ns was found, which is identified as the minimum incubation time for the formation of critical nuclei. In contrast, the complete crystallization of melt-quenched amorphous bits in a crystalline matrix was possible in 10 ns. This is attributed to the presence of quenched-in nuclei inside the amorphous bits. The combination of optical measurements with atomic force microscopy reveals the formation and growth of crystalline bits and shows that the crystal growth in vertical direction strongly affects the reflectivity changes. © 2001 American Institute of Physics. View full abstract»

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  • Analytical model for the disruption of a metal jet penetrating detonating high explosive

    Page(s): 3177 - 3183
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    We investigate the interaction of a constant-velocity, long-rod penetrator with detonating high explosive. An analytical flow-field model extending the Birkhoff model of inert target penetration is developed and compared with numerical simulations. We propose a rod-disruption model, discuss the effect of enhanced “anomalous” rod erosion on penetration history, and estimate the maximum coherent penetration depth. The model is extended to idealized stretching jet penetrators and its predictions are compared with jet-high-explosive (HE) interaction experiments. We predict a strong dependence of the high-explosive penetration on penetrator diameter and a very high “virtual” HE density in agreement with experimental observations. © 2001 American Institute of Physics. View full abstract»

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  • Pressure-induced cubic-to-orthorhombic phase transformation in the negative thermal expansion material HfW2O8

    Page(s): 3184 - 3188
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    The effect of pressure on the crystal structure of HfW2O8 has been investigated by neutron powder diffraction. At a hydrostatic pressure of 0.62 GPa at room temperature the cubic material transforms, with a 5% reduction in volume, to the same orthorhombic phase that is seen in the isostructural compound ZrW2O8 above 0.21 GPa. The transformation is sluggish, requiring about 24 h to complete at constant pressure. Once formed, the orthorhombic phase is retained upon release of pressure. Upon heating to 360 K, the metastable orthorhombic phase transforms back to the cubic phase. The substantially higher pressure for the cubic-to-orthorhombic transition in HfW2O8, compared to ZrW2O8, may be important for the application of this material in composites with controlled thermal expansion because rather large local pressures can occur in such composites. © 2001 American Institute of Physics. View full abstract»

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  • Electromigration of eutectic SnPb solder interconnects for flip chip technology

    Page(s): 3189 - 3194
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    The electromigration of eutectic SnPb solder interconnects between a Si chip and a FR4 substrate was studied at 120 °C for up to 324 h with current stressing of 104amp/cm2. Hillocks were observed at the anode and voids at the cathode. The dominant diffusing species was found to be Pb, confirmed by its accumulation at the anode. Diffusion markers were used to measure the electromigration flux and calculate the effective charge of atomic diffusion in the solder. Extensive microstructural evolution was also observed in the two-phase solder alloy that occurred by a ripening process. © 2001 American Institute of Physics. View full abstract»

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  • Diffusion, release, and uptake of hydrogen in magnesium-doped gallium nitride: Theory and experiment

    Page(s): 3195 - 3202
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    The diffusion and release of H and its uptake from the gas phase are modeled for Mg-doped, wurtzite GaN using formation energies and vibration frequencies from the density-function theory. Comparison is made with rates of deuterium release and uptake measured by nuclear-reaction analysis of deuterium concentration. Good agreement is found when account is taken of a surface permeation barrier. © 2001 American Institute of Physics. View full abstract»

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  • Effects of contact resistance and film thickness on current crowding and the critical product of electromigration in Blech structures

    Page(s): 3203 - 3208
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    In using Blech structures to study electromigration, the current density has been assumed to be the same in similar sets of short strips deposited on an underlayer. But this is not true owing to the existence of contact resistance between the strips and the underlayer. The contact resistivity may change due to different methods of sample preparation. High contact resistivity reduces the maximum current in the strip. Also, it diffuses the currents at the strip/underlayer interface from a narrow crowding to a wide spreading. In this work, we simulate these phenomena of current reduction and spreading in terms of contact resistivity and film thickness. We explore their effects on back stress and incubation time of void formation in short strips. We demonstrate these effects by the discrepancy in the critical product measurements found in experiments using similar Blech structures. © 2001 American Institute of Physics. View full abstract»

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  • Growth of strained-layer GaAs/Ge superlattices by magnetron sputtering: Optical and structural characterization

    Page(s): 3209 - 3214
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    Strained layer superlattices of GaAs/Ge/GaAs and Ge/GaAs/Ge have been grown by magnetron sputtering of different intercalated layer thickness. The samples exhibited good crystallographic quality, pseudomorphic growth on the substrate, as well as superlattice characteristics. Layer periodicity, concentration profile and the thicknesses of the resultant films were examined by high-resolution x-ray diffraction, secondary ion mass spectroscopy, infrared (IR) optical transmission measurements, and Raman spectroscopy. The heterostructures exhibited IR attenuation peaks in transmission between 0.5 and 1.0 eV, whose energy position was characterized as a function of the thickness of the intercalated thinner layers. The combined results of these techniques reveal that the intended GaAs layers are in fact composed of (GaAs)1-x(Ge2)x alloys with a few percent Ge content. Experimental and theoretical results have been modeled with the transmittance model, which assumes that light hits the surface normally and takes the alternating layer thicknesses as variable parameters. Both, experimental and theoretical results agree to within 3%. © 2001 American Institute of Physics. View full abstract»

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  • Reverse Monte Carlo study of structural relaxation in vitreous selenium

    Page(s): 3215 - 3222
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    Vitreous selenium is used extensively for light detection. Its photoconducting properties are altered by structural relaxation near room temperature. X-ray diffraction, densiometry, and calorimetry are used to determine the change of the glass state during relaxation. Annealing for 24 h at 300 K increases the density by 0.14% and lowers the enthalpy by 105 J/mol. The structure factor of cast samples was measured before and after annealing, and reverse Monte Carlo models of the structure of selenium glass were generated to match the data for the quenched and annealed states. Atoms in amorphous selenium are arranged in randomly oriented chains. In the model clusters annealing effects are found by analyzing inter- and intrachain distances, bond angle distributions, and dihedral angle distributions. The average bond length remains unchanged upon annealing, while the distribution of bond lengths becomes narrower by 1.2%. Distances between atoms in adjacent chains decrease by about 0.03%, i.e., the selenium chains move closer together, and the distribution of interchain distances becomes narrower. Bond angles within the chains are affected slightly. The mean bond angle decreases by 0.016°, which indicates a greater folding of the selenium chains and represents an evolution towards the angles found in crystalline selenium phases. The bond angle distribution for the annealed state is 0.006° narrower. © 2001 American Institute of Physics. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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Editor
P. James Viccaro
Argonne National Laboratory