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Journal of Applied Physics

Issue 2 • Date Jul 1999

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Displaying Results 1 - 25 of 78
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

    Page(s): toc1
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  • Differential gain, differential index, and linewidth enhancement factor for a 4 μm superlattice laser active layer

    Page(s): 713 - 718
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    We describe temporally and spectrally resolved measurements of the material differential gain, differential refractive index, and linewidth enhancement factor for a multilayer superlattice intended for use in midwave-infrared semiconductor lasers. We find good agreement between measured quantities and theoretical predictions based on a superlattice K∙p formalism. The superlattice was designed for suppression of Auger recombination and intersubband absorption, and we find that the strategies employed in this process result in other characteristics that are desirable in a semiconductor laser gain medium. Specifically, for carrier densities and wavelengths appropriate to threshold in an optimized cavity configuration, this structure has a differential gain of approximately 1.5×10-15cm2, a value comparable to that reported for near-infrared strained quantum wells. The peak gain and peak differential gain are nearly spectrally coincident, leading to a small value for the differential index. The large differential gain and small differential index result in a linewidth enhancement factor of less than one. This indicates that filamentation in high-power lasers based on this superlattice should be suppressed and that this structure is attractive for use in midwave-infrared lasers designed for spectrally pure operation. © 1999 American Institute of Physics. View full abstract»

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  • ZnMgSe/ZnCdSe and ZnMgSe/ZnSeTe distributed Bragg reflectors grown by molecular beam epitaxy

    Page(s): 719 - 724
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    This article investigates distributed Bragg reflectors (DBRs) based on two wide-gap II–VI semiconductor alloy combinations: ZnMgSe/ZnCdSe and ZnMgSe/ZnSeTe. Prior to fabrication of the DBRs, a prism coupler technique was used to determine the indices of refraction n of the above ternary alloys of various compositions prepared by molecular beam epitaxy (MBE). Using these values of n, two DBR systems, Zn0.66Mg0.34Se/Zn0.74Cd0.20Se and Zn0.62Mg0.38Se/ZnSe0.56Te0.44, were fabricated, each with a relatively large difference in the indices of refraction between its layer materials. It was found that although a higher reflectivity could be achieved using the ZnMgSe/ZnSeTe combination (since it manifests a larger difference in their indices of refraction Δn=0.35), the number of periods which can be deposited in this DBR system is limited due to growth difficulties that arise when combining ZnMgSe and ZnSeTe. Therefore the ZnMgSe/ZnSeTe DBR system, which was restricted to just 10 periods, yielded a modest reflectivity of 85%. On the other hand, although Δn in the ZnMgSe/ZnCdSe DBR system is smaller (Δn=0.20), it poses fewer growth difficulties, making it possible to grow DBR stacks consisting of a large number of layers without compromising the crystal quality of the structure. By growing 20 periods of the ZnMgSe/ZnCdSe DBR system, we obtained a DBR with a reflectivity as high as 98%. © 1999 American Institute of Physics. View full abstract»

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  • Carbon and lithium spectra from a vacuum spark

    Page(s): 725 - 730
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    Carbon and lithium spectra are obtained from vacuum spark discharges using a grazing-incidence flat-field spectrometer. Hydrogen and helium-like lines are identified in the carbon spectrum, where only the hydrogen-like lines are found in the lithium spectrum. The absence of the helium intercombination line in the carbon plasma indicates that the electron density is greater than 1×1019cm-3. The electron density is found to be 2.8–4.4×1020cm-3 for carbon and 7.3–10.2×1018cm-3 for lithium plasma using the method of Stark width analysis for hydrogen-like carbon and lithium lines of the Lyman β and Lyman δ of each element. This method was developed and implemented earlier by E. V. Aglitskii, P. S. Antsiferov, I. M. Gaisinskii, E. A. Oks, and A. M. Panin (Institute of Spectroscopy Preprint #13, Troitzk, Moskow region, USSR, 1985). The Lyman β and Lyman δ lines are chosen for our analysis because these two lines do not have the central Stark components. A pinhole picture is obtained for the carbon plasma, and the pinch diameter is measured to be 100 μm from the equal density profile scan of the pinhole photograph. The optical depth and the escape factor are incorporated into the intensity calculation of the lines using an effective plasma size of 50 μm. In this calculation, the intensity ratio of the Lyman α to the helium-like γ line (1s4p→1s2) yields an electron temperature of about 90–95 eV for the carbon plasma. No temperature for the lithium was possible due to the unavailability of the helium-like lines. © 1999 American Institute of Physics. View full abstract»

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  • Resonance radiation transport in inhomogeneous media: Cylindrical glow discharges

    Page(s): 731 - 737
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    Numerical simulations of radiation transport in cylindrical geometry are used to determine the effect of radially symmetric inhomogeneities. The focus of this study are inhomogeneities such as may be produced by radial cataphoresis or temperature gradients in cylindrical glow discharges, i.e., a quadratic profile of absorbing atoms which has a minimum on the axis of the cylinder. A propogator function analysis of the Holstein–Biberman equation and a Monte Carlo simulation of resonance photon scattering are simultaneously used to examine three limiting cases of interest: (i) a pure Doppler broadened atomic lineshape, (ii) a pure Lorentz atomic lineshape produced by foreign gas broadening, and (iii) a pure Lorentz atomic lineshape produced by resonance collision broadening. The fundamental mode distribution of excited atoms, the fundamental mode trapped decay rate, and the volume-averaged escape rate for a homogeneous production rate per unit volume are calculated for each of these cases. The trapped decay rates are found to change modestly (depending upon lineshape) as the degree of inhomogeneity is increased, if the volume integral of the absorbing atom density remains fixed. Correction factors for the fundamental mode trapped decay rate are reported. © 1999 American Institute of Physics. View full abstract»

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  • Self-organization of surface wave sustained discharges in the pressure range from 10 to 200 Torr

    Page(s): 738 - 745
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    Experiments showing the dynamics in the self-organization of surface wave sustained discharges are presented. Microwave (2.4 GHz) discharges maintained in an argon gas in a continuous wave regime at a constant applied power and varying gas pressure are studied. The evolution of the discharge from a stationary plasma column at comparatively low pressure (p≤10 Torr) to a plasma torch at atmospheric pressure passes through different stages of self-organization of the wave-field↔plasma nonlinear structure showing evidence of the general trends of behavior of nonequilibrium dissipative systems. The measurements are carried out at the stage of the discharge self-organization into a filamentary structure with an azimuthal rotation. Macroscopic characteristics (number, size, velocity of rotation) of the filaments and their dependence on the gas pressure and its time variation are given. The total light emission of the plasma considered as giving information about the plasma density is measured and different methods of signal processing (including correlation-spectrum analysis) are applied. Oscillations of the filament ends are also observed. The different types of interrelation between plasma density and field intensity, registrated in the different pressure ranges, call for variety in the instability mechanisms. Although the scenario of the discharge self-organization is stressed in the discussions, the observations are important with their relation to the discharge applications, which require avoiding conditions of development of instabilities. © 1999 American Institute of Physics. View full abstract»

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  • Effectiveness of AlN encapsulant in annealing ion-implanted SiC

    Page(s): 746 - 751
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    Aluminum nitride (AlN) has been used as an encapsulant for annealing nitrogen (N), arsenic (As), antimony (Sb), aluminum (Al), and boron (B) ion-implanted 6H-SiC. Atomic force microscopy has revealed that the AlN encapsulant prevents the formation of long grooves on the SiC surface that are observed if the AlN encapsulant is not used, for annealing cycles up to 1600 °C for 15 min. Secondary ion mass spectrometry measurements indicated that the AlN encapsulant is effective in preserving the As and Sb implants, but could not stop the loss of the B implants. Electrical characterization reveals activation of N, As, Sb, and Al implants when annealed with an AlN encapsulant comparable to the best activation achieved without AlN. © 1999 American Institute of Physics. View full abstract»

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  • Ion-implantation in bulk semi-insulating 4H–SiC

    Page(s): 752 - 758
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    Multiple energy N (at 500 °C) and Al (at 800 °C) ion implantations were performed into bulk semi-insulating 4H–SiC at various doses to obtain uniform implant concentrations in the range 1×1018–1×1020cm-3 to a depth of 1.0 μm. Implant anneals were performed at 1400, 1500, and 1600 °C for 15 min. For both N and Al implants, the carrier concentration measured at room temperature for implant concentrations ≤1019cm-3 is limited by carrier ionization energies, whereas for the 1020cm-3 implant, the carrier concentration is also limited by factors such as the solubility limit of the implanted nitrogen and residual implant damage. Lattice quality of the as-implanted and annealed material was evaluated by Rutherford backscattering spectroscopy measurements. Residual lattice damage was observed in the implanted material even after high temperature annealing. Atomic force microscopy revealed increasing deterioration in surface morphology (due to the evaporation of Si containing species) with increasing annealing temperature. The surface damage is in the form of long furrows running in one direction across the wafer to a depth of ∼25 nm from the surface for samples annealed at 1600 °C for 15 min. We measured room temperature sheet resistivities of ∼70 and 1.2×104 Ω/sq., respectively, for 2×1019cm-3 N (1500 °C annealing) and 1×1020cm-3 Al (1600 °C annealing) i- mplanted samples. © 1999 American Institute of Physics. View full abstract»

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  • Depth distribution of luminescent Si nanocrystals in Si implanted SiO2 films on Si

    Page(s): 759 - 763
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    Depth-resolved measurements of the photoluminescence of Si implanted and annealed SiO2 films on Si have been performed to determine the depth distribution of luminescent Si nanocrystals. Si nanocrystals with diameters ranging from ∼2 to 5 nm were formed by implantation of 35 keV Si ions into a 110-nm-thick thermally grown SiO2 film on Si(100) at a fluence of 6×1016 Si/cm2, followed by a thermal anneal at 1100 °C for 10 min. The photoluminescence spectrum is broad, peaks at λ=790 nm, and contains contributions from both recombination of quantum confined excitons in the nanocrystals and ion-implantation-induced defects. By chemical etching through the SiO2 film in steps and analyzing the changes in the photoluminescence spectrum after each etch step, the depth from which each of the two luminescence features originate is determined. The etch rate of the oxide, as derived from Rutherford backscattering spectrometry data, varies from 1.3 nm/s in the regions of small excess Si to 0.6 nm/s at the peak of the concentration profile (15 at. % excess Si). It is found that the defect luminescence originates from an ∼15-nm-thick near-surface region. Large nanocrystals luminescing at long wavelengths (λ=900 nm) are mainly located in the center of the film, where the Si concentration is highest (48 at. %). This is corroborated by transmission electron microscopy that shows a high density of Si nanocrystals in the size range of 2–5 nm in the center of the film. The largest density of small luminescent nanocrystals (λ=700 nm), not detectable by electron microscopy is found near the SiO2 su- rface and the SiO2/Si interface. This is attributed to either the fact that the surface and the SiO2/Si interface affect the Si nanocrystal nucleation kinetics in such a way that small nanocrystals are preferentially formed in these regions, or an optical interaction between nanocrystals of different sizes that quenches the luminescence of small nanocrystals in the center of the film. © 1999 American Institute of Physics. View full abstract»

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  • Photoluminescence study of the defects induced by neutron irradiation and rapid annealing in semi-insulating GaAs

    Page(s): 764 - 767
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    The rapid annealing behavior of several kinds of defects in semi-insulating GaAs irradiated with various neutron fluences has been characterized using a photoluminescence technique. In this experiment, transmutation impurities form not only donors, but also acceptors, GeAs (the 1.4783 eV peak). The intensity ratio of the 1.4783 eV peak (GeAs) to the 1.4917 eV peak (CAs) increases with the neutron dose. This finding is consistent with the expected increase of GeAs produced by transmutation. We also see that short time heat treatment leads to the increase of antisite defects GaAs and of complex centers IGa-VAs after neutron irradiation. Based on analysis of the rapid annealing process in comparison with the regular annealing process, it is concluded that the two kinds of defects GaAs and IGa-VAs are the products of defect reactions during the annealing process. © 1999 American Institute of Physics. View full abstract»

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  • Photobleaching and photodegradation of photoluminescence in thin films of poly(methylphenylsilane)

    Page(s): 768 - 773
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    The photobleaching process has been studied systematically in poly(methylphenylsilane) thin films with thickness comparable to or less than the penetration depth of incident ultraviolet light. It is found out that the photoscission of σ bonds proceeds from longer segments of Si chains to shorter segments, which has been overlooked in an analysis of the photobleaching process so far. Taking into account this phenomenon and its temperature dependence, a thermally assisted photoscission model is proposed. In the model the activation energy ε has a distribution combining two exponential functions of exp(ε/ε1) in a low energy region and exp(-ε/ε2) in a high energy region with the transition energy εT separating the two. The shape of the energy distribution is independent of the length of segments and the photoscission cross section Cr is larger for longer segments. The analysis based on this model is fairly consistent with the experimental results. The parameters of ε1, ε2, εT and Cr are confirmed to be 38 meV, 31 meV, 73 meV, and 5.0×10-17–1.3×10-16 cm2, respectively. The photodegradation of the photoluminescence from the thin films is also well explained on the basis of the proposed model. © 1999 American Institute of Physics. View full abstract»

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  • Crystallization behavior of sputter-deposited amorphous Ge2Sb2Te5 thin films

    Page(s): 774 - 778
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    The crystallization behavior of amorphous Ge2Sb2Te5 thin films were investigated by using differential scanning calorimetry, x-ray diffraction, and optical reflectivity measurements. The analysis of in situ ellipsometry isotherm Ge2Sb2Te5 films based on the Johnson–Mehl–Avrami equation revealed that the crystallization process near 150 °C was a two-step process. In this alloy thin film, the kinetic exponents in the Johnson–Mehl–Avrami equation were about 4.4 for the first stage and 1.1 for the second stage. A kinetic model based on the cascaded crystallization is proposed. The proposed model showed good agreement with the experimental results obtained by transmission electron microscopy and x-ray diffraction. © 1999 American Institute of Physics. View full abstract»

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  • Changes in chemical structure and optical properties of polyimide films induced by ion bombardment

    Page(s): 779 - 781
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    The effect of ion bombardment on the chemical structure and optical properties of polyimide films was investigated. Oxygen and nitrogen ion beams with energies in the range of 0.5–1.0 keV and doses between 1.7×1015 and 8.2×1016ions/cm2 were irradiated on the films, and in situ x-ray photoelectron spectroscopy (XPS) and spectral transmittance analyses were carried out. The in situ XPS analyses showed that the addition reaction of oxygen and nitrogen atoms and desorption of components caused chemical structural changes in ion-bombarded polyimides. The in situ spectral analyses showed that the spectral transmittances of films were decreased by ion bombardment. © 1999 American Institute of Physics. View full abstract»

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  • Analysis of high-resolution x-ray diffraction in semiconductor strained layers

    Page(s): 782 - 790
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    The composition and strain of pseudomorphic epitaxial layers is measured by high-resolution x-ray diffraction. Rocking curves or reciprocal space maps are measured from several reflections from sets of lattice planes, and their peak positions combined to yield the physical parameters of interest. We show here that the peak positions are best evaluated as the centres of gravity of the peaks. These data are then analyzed using least-squares multiple linear regression. This gives best estimates of the values and the errors of the physical parameters. The analysis also provides guidance as to the best sets of reflections to measure. Asymmetric reflections are preferable to symmetric reflections; but if symmetric reflections are used then the asymmetric reflections with high angle of incidence are more valuable than with low angle of incidence. © 1999 American Institute of Physics.   View full abstract»

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  • Diffusion of Cd in GaAs and its correlation with self-diffusion on the Ga sublattice

    Page(s): 791 - 799
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    Cd diffusion into GaAs single crystals has been investigated in the temperature range 756–1201 °C. Penetration profiles measured by secondary ion mass spectroscopy and spreading-resistance profiling agree with each other and are numerically reproduced on the basis of the kick-out diffusion mechanism. A major involvement of vacancies via the dissociative mechanism can be excluded. This enables us to deduce from the Cd profiles effective Cd diffusivities (Dseff) as well as Ga diffusivities (DISD) mediated by doubly positively charged Ga self-interstitials (IGa2+). The Dseff values obtained from As-rich and As-poor ambients are mutually consistent. The same holds true for the DISD data. Reduced to 1 atm of As vapor pressure and electrically intrinsic conditions the Cd diffusion coefficient results as 1.76×104exp(-4.80 eV/kB T) cm2s-1 and the IGa2+-related self-diffusion coefficient as 3.5×104exp(-5.74 eV/kB T) cm2s-1. The present data provide strong evidence that in heavily p-doped GaAs atomic transport related to the Ga sublattice is governed by self-interstitials rather than vacancies. © 1999 American Institute of Physics. View full abstract»

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  • Numerical predictions of oscillatory combustion in reactive multilayers

    Page(s): 800 - 809
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    Current analytical models and recent experimental studies assume that exothermic formation reactions self-propagate in multilayer foils with steady temperature and composition profiles and uniform velocities. The numerical study presented here shows that in addition to steady propagation, oscillatory combustion can also exist in certain parameter regimes. The oscillatory regimes are characterized by superadiabatic temperature excursions and large variations in the reaction zone width, the flame thermal width, and the instantaneous flame velocity. The magnitudes of the oscillations are shown to increase as bilayer period decreases, as premixing in the as-deposited state increases, and as ambient temperature decreases. © 1999 American Institute of Physics. View full abstract»

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  • Nitrogen uptake and rate-limiting step in low-temperature nitriding of iron

    Page(s): 810 - 816
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    Recently, a method to nitride iron in NH3 ambients at low temperature (225–350 °C) has been developed. In this method, the Fe is covered with a thin (∼40 nm) Ni layer, which acts as a catalyst for the nitriding process. From experiments, in which the amount of nitrogen uptake is measured as a function of nitriding time, it is concluded that the decomposition of NH3 at the Ni surface contains the rate-limiting step in this low-temperature nitriding process. From a model calculation, it is further concluded that the reaction step NH3→NH2+H at the Ni surface is the rate-limiting step with an activation energy of ∼1.5 eV. © 1999 American Institute of Physics. View full abstract»

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  • Accurate method for determining adhesion of cantilever beams

    Page(s): 817 - 827
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    Using surface micromachined samples, we demonstrate the accurate measurement of cantilever beam adhesion by using test structures which are adhered over long attachment lengths. We show that this configuration has a deep energy well, such that a fracture equilibrium is easily reached. When compared to the commonly used method of determining the shortest attached beam, the present method is much less sensitive to variations in surface topography or to details of capillary drying. © 1999 American Institute of Physics. View full abstract»

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  • Micro-Raman investigation of stress variations in lead titanate films on sapphire

    Page(s): 828 - 834
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    Using the sol–gel method, PbTiO3 films of 21, 64, 128, 210, 310, and 420 nm thicknesses were obtained on (0001) sapphire. Raman scattering and x-ray diffraction techniques were used to study the correlation between the film thickness and the structural changes on these films. The Raman and x-ray intensities in 21 nm film were too weak to reveal any structural information, while all other films showed tetragonal structure. At room temperature, the variation of lowest E(1TO) phonon mode frequency with film thickness was observed due to compressive stresses in the films. The lattice parameters and the degree of a axis orientation values of tetragonal PbTiO3 have been evaluated as a function of film thickness. The changes in lattice parameters thus obtained were used to estimate the stress at each thickness. An excellent agreement was found between the stress values obtained using Raman and x-ray results. An exponential decrease in stress with increasing film thickness was observed because of the structural changes in the lattice. Accordingly, a downshift in the tetragonal–cubic transition temperature with decreasing film thickness has been interpreted using the conventional Landau–Devonshire approach. © 1999 American Institute of Physics. View full abstract»

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  • Role of relative tilt on the structural properties of GaInSb epitaxial layers grown on (001) GaSb substrates

    Page(s): 835 - 840
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    The extent of relative tilt angle, crystalline quality, and relaxation of GaxIn1-xSb layers grown on (001) GaSb substrates by organometallic vapor phase epitaxy have been investigated, using double-crystal x-ray diffraction and transmission electron microscopy. An unexpectedly large tilt is formed between Ga0.8In0.2Sb epitaxial layers and oriented (001) GaSb substrates which has not been previously reported. Double-crystal x-ray diffraction measurements revealed that the tilt angle between the Ga0.8In0.2Sb epilayers and the substrates increases as the layer thickness increases. A strong correlation has been established between the variation of the tilt angle and the residual strain in the layers. Transmission electron micrographs of Ga0.8In0.2Sb layers revealed that irregular dislocation activities occur in the layer at different distances from the interface which could be related to tilt formation and relaxation. The structural characteristics of the layers as a function of the compositional variation showed that the amount of tilt angle is small when the indium concentration was in the range from 0 to 12%, but increases at higher indium concentrations. Ga1-xInxSb layers with poor crystalline quality and small tilt angle are obtained when the indium concentration was more than 25%. The tilt angle between Ga0.8In0.2Sb epilayer- s and GaSb substrates grown at temperatures ranging from 560 to 620 °C was essentially identical, indicating that the tilt formation is not a kinetic effect. © 1999 American Institute of Physics. View full abstract»

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  • Atomic structural study of a Br-chemisorbed Si(111)-7×7 surface by using scanning tunneling microscopy

    Page(s): 841 - 844
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    The atomic structure of a Br-chemisorbed Si(111)-7×7 surface is investigated by scanning tunneling microscopy. At low coverage, Br-reacted adatoms are distinguishable from unreacted adatoms in topographic images. The bias-dependent imaging suggests that the lowest tail of the antibonding state of the Br–Si bond is located about 1.5 eV above the Fermi level. At saturation coverage, a 7×7 structure is perfectly retained at room temperature and most adatoms remain after annealing at 400–630 °C. These results indicate that additional Br atoms cannot insert themselves into the Si–Si backbond between a Br-bonded adatom and a rest atom; therefore, SiBr is the only surface species present even at saturation coverage, which is not the case for F- or Cl-saturated Si(111) surfaces. © 1999 American Institute of Physics. View full abstract»

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  • Strain-balanced Si/SiGe short period superlattices: Disruption of the surface crosshatch

    Page(s): 845 - 849
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    We have studied the effects of inserting Si/Si0.6Ge0.4 strain-balanced superlattices (SLs) into Si0.8Ge0.2 (001) virtual substrates. The SiGe SL layer thickness chosen was larger than the critical thickness for elastic relaxation and generated numerous hemicylindrical features oriented along the <100> directions. These features lead, when covered by Si0.8Ge0.2, to a disruption of the well-ordered surface crosshatch along the <110> directions, and to a significant lowering of the surface roughness. There is also evidence for some filtering of the threading dislocations by the SL. © 1999 American Institute of Physics. View full abstract»

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  • Ellipsometric characterization of anisotropic porous silicon Fabry–Pérot filters and investigation of temperature effects on capillary condensation efficiency

    Page(s): 850 - 858
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    Porous silicon Fabry-Pérot optical filters with reflectivity peaks in the visible and near infrared spectral range have been manufactured electrochemically and characterized with variable angle spectroscopic ellipsometry. Generalized ellipsometry and the Bruggeman effective medium approximation were employed to study the anisotropy of the samples including determination of the tilt of the optic axis relative to the sample normal. At a wavelength of 700 nm, the difference between the real parts of the ordinary and extraordinary indexes of refraction of the low and high index sublayers were 0.05 and 0.07, respectively. In addition, the effect of temperature on the spectral redshifts caused by capillary condensation of water and toluene vapors in the porous material are investigated. It is demonstrated that a temperature decrease causes the spectral shifts to occur at lower partial pressures, indicating a more effective vapor capture at lower temperatures. Finally, it is shown that this phenomenon can be used to increase the ellipsometric response to very low vapor concentrations and thus improve the sensitivity of the technique in gas sensor applications. © 1999 American Institute of Physics. View full abstract»

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  • Hydrogenation and annealing effects on the trapping times of the minority carriers in In-doped CdTe epitaxial layers grown on p-CdTe (211) substrates

    Page(s): 859 - 862
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    Photoconductivity (PC) measurements on as-grown, annealed, hydrogenated, and hydrogenated and annealed In-doped CdTe epitaxial films grown on p-CdTe (211) substrates by molecular beam epitaxy have been performed in order to investigate the behavior of the trapping times of minority carriers in In-doped CdTe films due to annealing and hydrogenation. The results of the PC decay curve showed a slow component with a time constant of a few milliseconds and this behavior was related to the existence of deep trap levels corresponding to minority carriers. The activation energies of the traps, as determined from the temperature dependence of the PC decay times, were (Ev+0.35) and (Ev+0.43) eV for the as-grown and hydrogenated In-doped CdTe epilayers, respectively. The trapping times of the minority carriers were significantly reduced by the hydrogenation treatment. © 1999 American Institute of Physics. View full abstract»

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

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P. James Viccaro
Argonne National Laboratory