bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermomagnetic annealing of soft amorphous metallic alloys monitored by in situ resistance and magnetoresistance measurements
Authors: Gerard P. , Suran G.
Published: July 2002

This study concerns structural relaxation in sputtered Co-metal amorphous films (here, Co8.15Zr8.5Pt10) taking place during isochronal and isothermal thermomagnetic treatment. The aim is to improve the stability of the films and to reduce their induced magnetic anisotropy. In these materials, the topological short-range ordering (TSRO) directly connected to the amorphous film stability is irreversible, whereas the chemical short-range ordering (CSRO), which has to do with magnetic anisotropy, is reversible. With a view to maintaining strict control over both TSRO and CSRO and to treat them separately, equipment was constructed to perform thermomagnetic anneals up to 900 K combined with quenches with initial rates of 500 K s−1. In the course of these treatments, resistance and magnetoresistance measurements were used to follow accurately, with a sensitivity better than 10−5, the evolution of TSRO and of induced anisotropic field HK without resorting to any other characterization than those made with this apparatus. Under these conditions several isochronal and isothermal anneals were carried out which show that: (1) TSRO and CSRO can easily be differentiated, and (2) HK can be reversibly changed or controlled at will.
Thermal conversion of semi-insulating GaAs in high-temperature annealing
Authors: Ohkubo N. , Shishikura M. , Matsumoto S.
Published: July 2002

High-temperature annealing of semi-insulating GaAs has been studied. Thermal conversion induced by annealing at 1060–1200 °C can be explained by the causes of both the reduction of EL2 concentration and the generation of deep acceptors during high-temperature annealing. Both of them can be rationalized by the supposition that the antisite defect, AsGa, breaks into AsI and VGa, and the latter is a deep acceptor.
Zinc doping in gallium antimonide grown by low-pressure metal-organic chemical vapor deposition
Authors: Su Y. K. , Kuan H. , Chang P. H.
Published: July 2002

Zn-doped GaSb epilayers grown on GaAs and GaSb substrates by low-pressure metal-organic chemical vapor deposition were studied. Triethylgallium and trimethylantimony were used as Ga and Sb sources, respectively. The carrier concentrations of p-type GaSb epilayers were affected by V/III ratio and growth temperature. Diethylzinc(DEZn) was used as the p-type dopant. The relationship between carrier concentration P and mole fraction [DEZn] is P=K[DEZn]2.3. Photoluminescence for different carrier concentration was compared. There exist two different regions for the carrier concentration versus growth temperature.
Thermal analysis of incandescent lamp zone-melting recrystallization of thin silicon films
Authors: Robinson Richard D. , Miaoulis Ioannis N.
Published: July 2002

A two-dimensional numerical simulation of incandescent lamp zone-melting recrystallization was developed to investigate the critical thermal processing parameters. Parametric studies examined the thermal effects of lamp intensity profile, susceptor emissive power, and the ambient reflectivity of the system, on the melt zone. Results, presented in graphical form, indicate that the melt width increases nonlinearly with peak intensity due to an increase in reflectivity of silicon upon melting. Melt width increases linearly with susceptor emissive power due to an increase in emissive-dominated effects. Moreover, the ambient reflectivity of the system (often overlooked in thermal analyses) is found to have a significant effect on the process.
Shubnikov–de Haas oscillations on as-grown and annealed molecular-beam-epitaxy-grown Hg1−xCdxTe alloys doped with indium
Authors: Rafol S. B. , Wijewarnasuriya P. S. , Sou I. K. , Sivananthan S. , Faurie J. P.
Published: July 2002

Transverse resistivity ρxx, longitudinal resistivity ρzz, and Hall resistivity ρxy on heavily indium-doped as-grown and annealed Hg1−xCdxTe (0.24<x<34) alloys in a magnetic field up to 12 T and temperature range 1.2–25 K have been measured. The as-grown and annealed samples show good quality Shubnikov–de Haas oscillations in which the effective masses m*, free-electron densities nSdH, and Dingle temperatures TD are extracted. The deduced m* and nSdH from the as-grown and annealed samples are in good agreement within 12%. However, there are considerable improvements in the low field Hall mobility μH (a maximum 54% increase) and a reduction in TD (a maximum 65% decline) in the annealed samples. Analysis at low field seems to indicate that broadening due to an inhomogeneous impurity distribution contributes to TD in addition to the partial cancellation produced by different frequencies of oscillations. An estimate of the average values of an electron density fluctuation Δn/n and its spatial extent ΔL based only on TD and Hall mobility temperature Tμ is presented. The deduced ΔL is in the order of mean free path l. This, therefore, can limit μH and the values of l and ΔL and this indicates that if there is an inhomogeneous distribution of indium donor, high-temperature annealing tends to homogenize the distribution of indium donors by diffusion.
Nonlinear diffusion in Cu-Au multilayer thin films
Authors: Menon E. S. K. , Huang P. , Kraitchman M. , Hoyt J. J. , Chow P. , de Fontaine D.
Published: July 2002

Multilayered films with an average concentration of Cu-16 at. % Au and a layering periodicity of 3.31 nm were produced by vapor deposition on sodium chloride substrates. Diffraction studies were carried out at Stanford Synchrotron Radiation Laboratory, thus allowing the observation of up to three orders of satellites, the evolutions of which were measured as a function of annealing time at 515 K. It was found that the first-order satellite intensity decayed nearly exponentially with time, whereas intensities of both second- and third-order satellites decreased very rapidly at first, then increased before decaying exponentially. These results could be explained by following the evolution of satellites during annealing of a one-dimensional modulated system governed by a nonlinear diffusion equation. The diffusivity in this system was assumed to be of the form D0+D1u+D2u2, with u=c−c0, where Di’s are the coefficients, c is the concentration, and c0 represents the average composition of the alloy. The nonlinear diffusion equation was solved numerically to obtain one-dimensional concentration profiles at successive annealing times. Satellite amplitudes, obtained by Fourier transforming the concentration profiles, were found to exhibit a remarkable phenomenon: The mth-order satellite amplitude changes its sign (m−1) times before decaying asymptotically to zero. By comparing the evolution of the square of the amplitude with that of the satellite intensities, a set of Di’s was determined that produced realistic concentration profiles and reproduced the experimentally evaluated satellite intensities reasonably well.
Characterization of Si-TaSi2 in situ composites by synchrotron white beam topography and by double axis diffractometry
Authors: Stock S. R. , Chung Y. H. , Huang P. C. , Rek Z. U. , Ditchek B. M.
Published: July 2002

In situ Si-TaSi2 composites are studied by synchrotron white beam topography and by double axis diffractometry. These results show that the single crystal Si matrix is of excellent quality: Rocking curve widths are between 40 and 60 s of arc, and the topographs do not exhibit asterism. Diffuse radial streaks in the Laue patterns originate from diffraction by the TaSi2 rods in the matrix: The K-absorption edge of filters placed in the incident x-ray beam produces a sharp change in contrast in the streaks, and this is used to determine the d spacings present in the streaks and to show that considerable preferred orientation exists between the TaSi2 rods and the Si matrix.
A new methodology for separating Shockley–Read–Hall lifetime and Auger recombination coefficients from the photoconductivity decay technique
Authors: Pang S. K. , Rohatgi A.
Published: July 2002

A new methodology involving the measurement and integration of photoconductivity decay (PCD) of carriers is developed to separate surface recombination velocity (S), Shockley–Read–Hall lifetime (τSRH), trap-assisted Auger (Bt), and band-to-band Auger (C) recombination coefficients in semiconductors. Conventional differentiation methodology used in the past not only suffers from the potential errors due to the differentiation of the digitized injection level versus decay time transient, N=f(t), but also from the difficulty in assessing errors in the recombination coefficients because the lifetime models or equations are used to fit the differentiated data, τeff(N), rather than the raw experimental data [N=f(t)]. In this new methodology, first a mathematical equation is derived by integration of the general lifetime expression with various recombination coefficients to express the carrier decay time as a function of injection level, t=f(N). The Levenberg–Marquardt numerical method is then used to fit this mathematical equation to the raw experimental data (N,t) for rapid and accurate assessment of τSRH, Bt, and C. Finally, the PCD lifetime measurements are performed on a few silicon samples to demonstrate the validity and advantages of the integration approach over the differentiation methodology in separating and quantifying recombination mechanisms in silicon.
Bistable saturation in coupled quantum-dot cells
Authors: Tougaw P. Douglas , Lent Craig S. , Porod Wolfgang
Published: July 2002

Model quantum dot cells are investigated as potential building blocks for quantum cellular automata architectures. Each cell holds a few electrons and interacts Coulombically with nearby cells. In acceptable cell designs, the charge density tends to align along one of two cell axes. Thus, a cell ‘‘polarization,’’ which can be used to encode binary information, is defined. The polarization of a cell is affected in a very nonlinear manner by the polarization of its neighbors. This interaction is quantified by calculating a cell–cell response function. Effects of nonzero temperature on the response of a model cell are investigated. The effects of multiple neighbors on a cell are examined and programmable logic gate structures based on these ideas are discussed.
Spectroscopy and laser performance of Nd doped gadolinium lithium fluoride
Authors: Zhang X. X. , Villaverde A. B. , Bass M. , Chai B. H. T. , Weidner H. , Ramotar R. I. , Peale R. E.
Published: July 2002

A comprehensive investigation on the spectroscopic properties and laser performance of Nd3+ doped GdLiF4 (GLF), a new laser crystal, is reported. Our high resolution absorption and emission spectra for GLF are nearly identical to those of Nd:LiYF4 (YLF), a well known laser crystal, strongly suggesting that the two crystals are isostructural. The laser performance of Nd:GLF is very similar to that of Nd:YLF. A maximum laser-pump-laser slope efficiency of 68% and 67% was obtained for low (1.0 at. %) and high (4.0 at. %) Nd concentration GLF respectively. Concentration quenching of the fluorescence decay time was observed and appears to be due to the dipole-dipole interaction between the isolated Nd3+ ions and Nd3+ ion pairs.
Photo-assisted tuning of luminescence from porous silicon
Authors: Koyama Hideki , Koshida Nobuyoshi
Published: July 2002

A method is proposed to control the wavelength of visible photoluminescence (PL) from porous silicon (PS). This is based on the post-anodization illumination technique which employs a filtered light with various maximum cutoff photon energies. As the illumination time increases, the PL spectra shift toward the higher energy side. This blue shift tends to stop at the time when the PL peak energy becomes close to the cutoff photon energy. The emission wavelength can be controllably tuned in a wide spectral range by changing the cutoff filter. These results give us a strong indication that the visible PL of PS is closely related to interband excitation in Si nanocrystallites, not to some surface chemical compounds.
Hierarchical scaling: An analytical approach to slow relaxations in spin glasses, glasses, and other correlated systems (invited)
Authors: Souletie J.
Published: July 2002

A model has been developed that associates a hierarchical criterion to the usual renormalization of the distances near a phase transition. At times t smaller than τα∝(1−Tc/T)−zν, which diverges at Tc, it is found that the relaxation develops at a scale s(t), which is a power law of the time elapsed: s ∝ tT/zTc. If one sits in the ergodic regime T≳Tc, the system abruptly reaches an equilibrium at s(τα)=ξ(T)=τα1/z. If T<Tc, by contrast, it deviates to logarithmic behavior. The α relaxation is well approximated by a stretched exponential exp(−t/τα)β(T), which traduces the effect of underlying lognormal statistics. The exponent β(T)∝[1−b ln(1−Tc/T)]−1/2 would cancel at Tc if it was possible to wait for the sample to equilibrate at this temperature. The model has the necessary ingredients to describe effects that have been reported in both the ‘‘activated’’ nonergodic regime and in the critical regime of spin glasses, and of other correlated systems like orientational glasses, glass-forming liquids, and vortex glasses. A possible extension of the model is discussed to describe the ‘‘macroscopic quantum effects.’’
Effects of substrate misorientation and growth rate on ordering in GaInP
Authors: Su L. C. , Ho I. H. , Stringfellow G. B.
Published: July 2002

Epitaxial layers of GaxIn1−xP with x≊0.52 have been grown by organometallic vapor-phase epitaxy on GaAs substrates misoriented from the (001) plane in the [1̄10] direction by angles ϑm, of 0°, 3°, 6°, and 9°. For each substrate orientation growth rates rg of 1, 2, and 4 μm/h have been used. The ordering was characterized using transmission electron diffraction (TED), dark-field imaging, and photoluminescence. The (110) cross-sectional images show domains of the Cu-Pt structure separated by antiphase boundaries (APBs). The domain size and shape and the degree of order are found to be strongly affected by both the substrate misorientation and the growth rate. For example, lateral domain dimensions range from 50 Å for layers grown with rg=4 μm/h and ϑm=0° to 2500 Å for rg=1 μm/h and ϑm=9°. The APBs generally propagate from the substrate/epilayer interface to the top surface at an angle to the (001) plane that increases dramatically as the angle of misorientation increases. The angle is nearly independent of growth rate. From the superspot intensities in the TED patterns, the degree of order appears to be a maximum for ϑm≊5°. Judging from the reduction in photoluminescence peak energy caused by ordering, the maximum degree of order appears to occur at ϑm≊4°.
Influence of carrier energy quantization on threshold voltage of metal-oxide-semiconductor transistor
Authors: Janik Tomasz , Majkusiak Bogdan
Published: July 2002

A new method for calculating free carrier energy quantization in the depletion region is proposed and applied to investigate the influence of this phenomenon on the metal-oxide-semiconductor transistor threshold voltage. This influence is significant when the semiconductor surface region is highly doped. Other phenomena essential for high doping levels such as the energy degeneration, the incomplete ionization of dopants, and the band-gap narrowing are taken into account.
Optical property of InAsP/InP strained quantum wells grown on InP (111)B and (100) substrates
Authors: Hou H. Q. , Tu C. W.
Published: July 2002

InAsP/InP strained multiple quantum wells (MQWs) were grown on InP (111)B and (100) substrates by gas-source molecular-beam epitaxy. Specular surfaces were obtained under optimized growth conditions on InP (111)B substrates miscut 1° to the 〈110〉 direction. Photoluminescence, absorption and photoluminescence excitation spectra were taken for InAsP/InP strained MQWs at low temperature. Distinct optical transitions were resolved. Energy-level calculations were carried out for both (111)B and (100) MQWs by taking into account the differences in elastic deformation, strain-induced band-edge shift, valence-band anisotropy, and the piezoelectric effect. The peaks were successfully assigned as different interband excitonic transitions. The best fit of the energy-level calculation to the experimental spectra suggests that the valence-band offset ratios (Qv=ΔEv/ΔEg) for (111)B and (100) InAsP/InP heterostructures are 0.35 and 0.30, respectively. This dependence on the substrate orientation was accounted for in terms of the strain-induced band-edge shift.
Excitonic polaritons in quantum-confined systems and applications to optoelectronic devices
Authors: Katsuyama Toshio , Ogawa Kensuke
Published: July 2002

An excitonic polariton is a complex quasiparticle that consists of a photon and an exciton. Excitonic polaritons have recently been shown to exist in quantum-confined systems such as GaAs quantum wells. Based on the coherent coupling between the charged electron (hole) and light, the quantum-confined excitonic polariton has the characteristics of large coherence length and large phase modulation under electric fields. Furthermore, because of the inherent large refractive index, the spatial shape of the guided mode of the excitonic polariton transmitted in such quantum-confined waveguides is expected to be squeezed significantly. We discuss the characteristics of such excitonic polaritons and possible applications to ultrasmall optoelectronic devices.
Electro-optical properties of a single domain PbTiO3 crystal
Authors: Fontana M. D. , Abdi F. , Wojcik K.
Published: July 2002

The electro-optic coefficient rc has been measured at low frequency in a single crystal of PbTiO3. Linear and quadratic polarization optic coefficients are derived from this value of rc and from the electric properties measured on the same crystal. Figures of merit for modulator application and the thermo-optic coefficient are determined in PbTiO3 and are compared with those in other electro-optic materials. Results show that PbTiO3 can be interesting for electro-optic applications.
Specific contact resistance measurements of ohmic contacts to semiconducting diamond
Authors: Hewett C. A. , Taylor M. J. , Zeidler J. R. , Geis M. W.
Published: July 2002

A simplified version of the specific contact resistance measurement scheme of G. K. Reeves [Solid State Electronics 23, 487 (1980)] has been developed. Its applicability to semiconducting diamond is demonstrated using four sample types: epitaxial films doped to mid 1019 acceptors/cm3 on 〈100〉 and 〈110〉 type IIa substrates; type IIb diamonds 0.25 mm thick, and type IIb diamonds thinned to 0.035–0.05 mm thick. The ohmic contacts were based on a solid-state annealing process using carbide forming metals. The measured specific contact resistance depends mainly on the doping level in the diamond. Measured values ranged from 8×10−6 Ω cm2 for heavily doped films to 1×10−2 Ω cm2 for lightly doped bulk samples. A simple analysis shows that the contacts to highly doped layers are suitable for device applications.
Microstructures of low-temperature-deposited polycrystalline silicon with micrometer grains
Authors: Wang K. C. , Hwang H. L. , Leong P. T. , Yew T. R.
Published: July 2002

The microstructures of low-temperature polycrystalline silicon grown both on SiO2 and Corning 7059 glass substrate are presented. The silicon was deposited by the hydrogen dilution method using electron-cyclotron-resonance chemical-vapor deposition at 250 °C without any thermal annealing. The hydrogen dilution ratios were varied from 90% to 99%. Transmission electron microscopy images, Raman shift spectra, and x-ray-diffraction (XRD) patterns of the films were obtained. The maximum grain size was about 1 μm and the crystalline fraction which was characterized from Raman shift spectra was near 100%. From the XRD patterns 〈111〉- and 〈110〉-oriented crystalline silicon grains were clearly present in the polycrystalline silicon films.
Raman study of strain relaxation in Ge on Si
Authors: Ichimura Masaya , Usami Akira , Wakahara Akihiro , Sasaki Akio
Published: July 2002

Strain in thin Ge layers grown by molecular beam epitaxy on (100) Si is measured by a Raman technique. When the average Ge thickness is 7 monoatomic layers (ML), Raman results show that the layer is almost coherent to the Si lattice. The strain begins to decrease at an average thickness of 10 ML, i.e., the critical thickness of dislocation generation is 10 ML. On the other hand, the relaxation begins at a thickness of 5 ML, according to reflection high-energy electron diffraction observation during the growth. This initial stage relaxation is due to deformation of islands and not due to dislocation formation. Raman results for thicker layers show that with increasing layer thickness, the misfit strain decreases gradually but more rapidly than predicted by the theory of Matthews and Blakeslee .
Synthesis of epitaxial β-SiC by C60 carbonization of silicon
Authors: Henke S. , Stritzker B. , Rauschenbach B.
Published: July 2002

By deposition of C60 on silicon at temperatures smaller than 1000 °C the formation of thin epitaxial β-SiC films on Si was successful. This low-temperature regime can be important for the application of SiC in microelectronic devices. C60 was deposited on Si(001) and Si(111) in ultrahigh vacuum at constant deposition rates for some hours. The temperature of the Si substrates ranged between 800 and 900 °C. The thickness and the composition of the formed layers are determined by Rutherford backscattering (RBS). The thickness of the layers varied between about 50 nm and 1 μm depending on the deposition parameters. From the shape of the RBS spectra and the x-ray-diffraction (XRD) scans, only β-SiC can be identified. The Θ–2Θ XRD spectra show preferred orientation of β-SiC on Si(001) and Si(111), respectively. Additional XRD pole figure measurements demonstrate the heteroepitaxial growth of β-SiC on Si. The orientational relationships β-SiC(001)[110]∥Si(001)[110] and SiC(111)[1̄10]∥Si(111)[1̄10] can be extracted. The formation of growth defects (twins) can be observed.
Influence of nonuniform doping on the uniformity of current gain, base transit time, and related properties of AlGaAs/GaAs heterojunction bipolar transistors
Authors: Mohammad S. N. , Chyi J.-I. , Chen J. , Morkoç H.
Published: July 2002

An experimental investigation of the effect of doping concentration variation on the uniformity of current gain as a function of collector current of AlGaAs/GaAs heterojunction bipolar transistors (HBT) has been performed. An analysis of the Gummel plots for these HBTs has also been carried out to understand the physics underlying their operations. The study demonstrates that an optimized doping gradient both in the emitter and the base results not only in a uniform current gain with respect to collector current but also leads to a suppression of emitter size effects encountered in the scaling down of transistors. Numerical simulations suggest that the generation of an electric field in the base region of an HBT due to nonuniform base doping leads apparently to no lowering of the base transit time.
Raman spectroscopy of amorphous and microcrystalline silicon films deposited by low-pressure chemical vapor deposition
Authors: Voutsas A. T. , Hatalis M. K. , Boyce J. , Chiang A.
Published: July 2002

In this work we used Raman spectroscopy to investigate the structural characteristics of as-deposited amorphous and micro-crystalline silicon films. For amorphous silicon films, the order (or disorder) of the silicon network was quantified using properties of the Raman spectra that were related to key deposition conditions. We found that a strong relationship exists between the structural order of the silicon matrix and the deposition temperature and deposition rate. A quantitative model was proposed relating the intensity ratio of transverse optical phonon peak to longitudinal optical phonon peak to the surface diffusion length, a parameter that was calculated from available data. It was found that optimization of the as-deposited silicon microstructure is possible by selecting deposition conditions yielding peak–ratio values in the vicinity of 0.53. For as-deposited micro-crystalline silicon films, Raman spectroscopy was used to estimate the initial crystalline fraction of the film and monitor the crystallization process during annealing. These data were used to confirm the crystallization mechanism in mixed-phase silicon films and identify the effect of different process parameters on the crystallization time of the annealed films.
Oxygen defect center red room temperature photoluminescence from freshly etched and oxidized porous silicon
Authors: Prokes S. M. , Carlos W. E.
Published: July 2002

Electron spin resonance and photoluminescence experiments have been performed on freshly etched and oxidized porous silicon. Results indicate the presence of oxygen-related centers (nonbridging oxygen-hole center clusters), which consist of similar core structures in as-made and oxidized porous silicon (PSi) samples. A direct correlation exists between the presence of these centers and a red photoluminescence observed in both freshly anodized and oxidized PSi, suggesting that this emission process is the result of optical transitions in the oxygen-hole centers.
Conduction and 1/f noise analysis in amorphous silicon thin-film transistors
Authors: Valenza M. , Barros C. , Dumas M. , Rigaud D. , Ducourant T. , Szydlo N. , Lebrun H.
Published: July 2002

Conduction and low-frequency noise are analyzed in the channel of hydrogenated amorphous silicon (a-Si:H) thin-film transistors. 1/f noise expressions are proposed starting from a simple conduction model describing drain current in the ohmic range. Carrier fluctuations (ΔN model) and mobility fluctuations (Δμ model) are investigated. For long-channel transistors the conduction is quite similar to crystalline metal–oxide–semiconductor field-effect transistors but involving low mobility values. The 1/f noise behavior is analyzed by mobility fluctuations as predicted by Hooge’s theory. For small channel transistors a crowding effect appears and access series resistances affect the conduction. The excess noise is then mainly controlled by these resistances when large gate voltages VGS are applied.
Monte Carlo calculation of noise and small-signal impedance spectra in submicrometer GaAs n+nn+ diodes
Authors: Starikov E. , Shiktorov P. , Gružinskis V. , Varani L. , Vaissiere J. C. , Nougier J. P. , Reggiani L.
Published: July 2002

The time-and-frequency behavior of hot-carrier noise in submicrometer n+nn+ GaAs diodes is investigated theoretically using the Monte Carlo method. We have continuously investigated the noise from current-to-voltage operation mode by calculating the noise-power spectrum at the terminals of a noiseless load-resistance R connected in series with the diode. By varying appropriately the value of R we have calculated the small-signal impedance of the diode and then obtained the full spectrum of the noise temperature. Under voltage-operation mode the current–noise spectrum exhibits two resonant peaks at the transit-time and plasma frequencies, respectively. Under current operation mode, all current oscillations are effectively damped, and the voltage–noise spectrum exhibits a quasi-Lorentzian shape, which vanishes at the transit-time frequency. The behavior of hot-carrier noise closely parallels the frequency dependence of the diode small-signal impedance, which exhibits a dynamic negative differential resistance in the frequency range corresponding to the transit-time resonance.
Aluminum nitride films on different orientations of sapphire and silicon
Authors: Dovidenko K. , Oktyabrsky S. , Narayan J. , Razeghi M.
Published: July 2002

The details of epitaxial growth and microstrictural characteristics of AlN films grown on sapphire (0001), (101̄2) and Si (100), (111) substrates were investigated using plan-view and cross-sectional high-resolution transmission electron microscopy and x-ray diffraction techniques. The films were grown by metalorganic chemical vapor deposition using TMA1+NH3+N2 gas mixtures. Different degrees of epitaxy were observed for the films grown on α-Al2O3 and Si substrates in different orientations. The epitaxial relationship for (0001) sapphire was found to be (0001)AlN∥(0001)sap with in-plane orientation relationship of [011̄0]AlN∥[1̄21̄0]sap. This is equivalent to a 30° rotation in the basal (0001) plane. For (101̄2) sapphire substrates, the epitaxial relationship was determined to be (112̄0)AlN∥(101̄2)sap with the in-plane alignment of [0001]AlN∥[1̄011]sap. The AlN films on (0001) α-Al2O3 were found to contain inverted domain boundaries and a/3〈112̄0〉 threading dislocations with the estimated density of 1010 cm−2. The density of planar defects (stacking faults) found in AlN films was considerably higher in the case of (101̄2) compared to (0001) substrates. Films on Si substrates were found to be highly textured c axis oriented when grown on (111) Si, and c axis textured with random in-plane orientation on (100) Si. The role of thin-film defects and interfaces on device fabrication is discussed.
Enhancement of orbital magnetism at surfaces: Co on Cu(100) (abstract)
Authors: Tischer M. , May F. , Baberschke K. , Hjortstam O. , Arvanitis D. , Dunn J. Hunter , Trygg J. , Johansson B. , Eriksson O. , Wills J. M.
Published: July 2002

By combining MCXD experiments with first principles electronic structure calculations, we demonstrate that the orbital contribution to magnetism can be strongly enhanced at surfaces. This effect is illustrated for Co grown on a Cu(100) surface. The MCXD measurements were performed using the SX 700 plane grating monochromators at BESSY. The Co films were evaporated and characterized in situ, for their cleanness, thickness (1.6–50 ML), and structural order. For a film thickness up to 2.2 ML, measurements of the ac MCXD susceptibility response at a fixed photon energy allowed one to measure the critical temperature, Tc, of the films and their critical properties were characterized in situ. For several samples the MCXD response as a function of the x-ray incidence angle was investigated as well. This allows one to quantify and correct saturation effects that can occur in the measurements. Data were taken in a temperature range between 40 and 350 K, and for many samples measurements were performed at several reduced temperatures, T/Tc. The first layer of Co on the Cu(100) surface shows an enhanced orbital moment, in contrast to the subsequent layers where the orbital moment is bulklike. The lowering of the symmetry, the enhanced spin moment, and the increased value of the density of states at the Fermi level are factors that combine to give the observed enhancements.
First principles calculations of electrical conductivity and giant magnetoresistance of periodic multilayers and spin valves (invited)
Authors: Butler W. H. , Zhang X.-G. , Nicholson D. M. C. , Schulthess T. C. , MacLaren J. M.
Published: July 2002

We used the Layer Korringa Kohn Rostoker technique to calculate the electronic structure of cobalt-copper multilayers and spin valves from first principles within the local spin density approximation. Using this electronic structure together with a phenomenological self-energy which may vary from layer to layer, we calculated the non-local layer-dependent conductivity by means of the Kubo linear response formalism. By calculating the majority and minority conductivities for parallel and anti-parallel alignment of the moments in the cobalt layers we determined the giant magnetoresistance (GMR). Several interesting features emerge from the calculations. When the scattering rates are relatively high, we find that the contributions to the GMR are largely non-local, with the largest contributions arising from changes in the currents carried in a cobalt plane next to copper due to fields sensed in the cobalt layer on the other side of copper. When scattering rates are relatively low (comparable to that of cobalt and copper at room temperature), there are important contributions to the GMR from local conduction in the copper layers. This effect arises from the fact that when the component of the majority spin electron momentum parallel to the layers exceeds a certain value, it gets trapped in the copper layers. If the scattering rate is lower in the copper than in the cobalt there is a significant enhancement in the majority spin conductivity and in the GMR. This effect is analogous to the channeling of light by an optical waveguide.
Lifetime control in silicon devices by voids induced by He ion implantation
Authors: Raineri V. , Fallica G. , Libertino S.
Published: July 2002

A method to control carrier lifetime in silicon locally and efficiently is presented. Voids, formed by high dose He implants, have been characterized by transmission electron microscopy demonstrating they are well localized in depth within layers thinner than 100 nm while their lateral extent is limited only by the masking capability during He implantation. Deep level transient spectroscopy measurements, performed on diodes containing different void densities, revealed the presence of two well defined trap levels, independent of void characteristics, at Ev+0.53 for holes and Ec−0.55 for electrons. These characteristics make them ideal for lifetime control in reducing parasitic transistor gain. Gummel plots on transistors have shown that when voids are formed the gain decreases from 1 to 10−3. The other transistor characteristics are only slightly influenced by the presence of voids.
High-temperature annealings of Sb and Sb/B heavily implanted silicon wafers studied by near grazing incidence fluorescence extended x-ray absorption fine structure
Authors: Revenant-Brizard C. , Regnard J. R. , Solmi S. , Armigliato A. , Valmorri S. , Cellini C. , Romanato F.
Published: July 2002

The local atomic environment of the Sb dopant in 2 and 5×1016 ions/cm2 implanted Si samples has been studied by near grazing incidence fluorescence extended x-ray absorption fine structure at different stages of the Sb deactivation process. The annealings were performed at high temperature (900–1000 °C) during various periods: 30 s–4 h. The Sb out-diffusion and the high percentage of Sb precipitates are put into evidence especially for Sb-only implanted samples. The comparison of the Sb and B codiffusion data with the corresponding ones obtained by the diffusion of Sb alone revealed several anomalous effects due to dopant interaction. Moreover, a simulation program including dopant precipitation and donor–acceptor pairing allows us to foresee most of the anomalous phenomena occurring in high-concentration codiffusion experiments.
Low frequency alternating current conduction and dielectric relaxation in polypyrrole, poly(N-methyl pyrrole), and their copolymers
Authors: Singh Ramadhar , Narula Amarjeet K. , Tandon R. P. , Mansingh A. , Chandra Subhas
Published: July 2002

The alternating current (ac) conductivity [σm(ω)], dielectric constant (ε′) and loss (ε″) of polypyrrole (PPY), poly(N-methyl pyrrole) [P(NMPY)] and their copolymers; poly(N-methyl pyrrole–pyrrole [P(NMPY–PY)] have been measured in the frequency range 102–106 Hz and in the temperature range 77–350 K. At 77 K, the ac conductivity can be expressed by the relation; σac=Aωs where the slope s lies in the range 0.72–0.81 for these three polymers and decreases with increase in temperature. The well-defined loss peaks, whose magnitude decreases with the increase in frequency, have been observed in the temperature region where the measured ac conductivity approaches the direct current (dc) conductivity. These loss peaks have been associated with the movement of charge carriers in these polymeric films. The dc conductivity has also been measured in the temperature range 77–350 K and an attempt has been made to correlate it with dielectric data.
Raman studies of laser-written patterns in YBa2Cu3Ox films
Authors: Li Y. B. , Shelley C. , Cohen L. F. , Caplin A. D. , Stradling R. A. , Kula W. , Sobolewski Roman , MacManus-Driscoll J. L.
Published: July 2002

Patterns on YBa2Cu3O6.87 films, laser written in O2 or N2 atmospheres, and with various writing power densities and scanning speeds, have been studied by Raman scattering. For laser writing in O2 at atmospheric pressure, with low power density and fast scanning speed, it is found systematically that the oxygen content of YBa2Cu3Ox can be reduced. Only when the writing power density is high and the scanning speed is low (e.g., 2.3 mW/μm2 and 1 μm/s), do the laser-irradiated lines in an O2 atmosphere approach full oxygenation (x→7). For laser writing in N2 at atmospheric pressure, the oxygen stoichiometry always decreases, rendering the lines nonsuperconducting. The structural phase transition from orthorhombic to tetragonal can be identified with laser writing in a N2 atmosphere with a writing power density of 1.9 mW/μm2 and a scanning speed of 20 μm/s. The oxygen O(4) line (∼500 cm−1) and the Ba line (115 cm−1) disappear, and the intensity of the Cu(2) line (144 cm−1) rises sharply when the phase transition occurs. This study shows how Raman scattering can be used as a routine technique, which is fast, sensitive, and nondestructive, and is able to measure oxygen content on a fine scale (a few μm) during high-Tc superconductor device processing.
Dynamic ductile fragmentation and the damage function model
Authors: Feng Jiapo , Jing Fuqian , Zhang Guanren
Published: July 2002

A new damage function suitable for metals under dynamic loading, based on the physical statistics of microvoids in metals and the law of conservation of energy, is presented in this article. This new damage model has a sound physical or theoretical foundation and fewer adjustable or empirical parameters than most existing models. Several hydrodynamic elastic-plastic numerical modeling experiments that use this damage function combined with the proper constitutional equations have been conducted by us and the results are in good agreement with previously published experimental results on different metals with different geometries.
Steady state model for polymer light-emitting electrochemical cells
Authors: Smith D. L.
Published: July 2002

A model is presented for the steady state operation of polymer light-emitting electrochemical cells (LECs). An LEC consists of a luminescent and ionically conducting polymer, with an ionic salt added to provide ions necessary for p-type and n-type doping, sandwiched between two electrodes. Upon applying a sufficiently large voltage bias, the ions are spatially separated forming an electrical junction. Electrons injected from the n-type side of the junction recombine with holes injected from the p-type side of the junction emitting light. We first describe the LEC at zero bias in which electric fields may occur in charge double layers near the contacts but in which there is a charge neutral, field free region in the device center which has an equal density of anions and cations and essentially no electrons or holes. A threshold voltage for junction formation is found, which depends on the polymer energy gap, the dissociation free energy of the salt, and the added salt density. It is generally somewhat smaller than the polymer energy gap. Below threshold, an applied bias changes the electric fields in the double charge layers near the contacts but the device center remains field free and essentially no current flows. Above threshold, the ions become spatially separated, a junction forms, and current begins to flow. Part of the applied voltage, above threshold, falls in the contact region and is necessary to establish the junction by electrochemical doping and part of the applied voltage falls across the junction. We describe the structure of the junction, which is quite different from that of a conventional p-n junction, including the spatial profiles of the electrons, holes, and ions, and the electrostatic potential. We discuss the current-voltage and capacitance-voltage characteristics of the LECs and show how they depend on the material parameters.
One mode behavior of LO phonon-plasmon interaction in n-type doped In0.5Ga0.5P/GaAs alloys
Authors: Lee Hosun , Klein M. V.
Published: July 2002

The dopant effect of In0.5Ga0.5P/GaAs epitaxial layers has been studied using Raman spectroscopy. Our study of the LO phonon-plasmon interaction shows one mode behavior even though In1−xGaxP alloys are known to have partial two model behavior. We attribute this apparent one mode behavior to enhanced charge transfer from In–P bonds to Ga–P bonds which is induced by the local lattice distortion originating from bond length mismatch. Our work also shows a TOm mode between the InP-like LO mode and the GaP-like LO mode. In terms of this TOm mode, we discuss the valley depth ratio, b/a, which was formerly attributed in the literature to CuPt-type ordering without clear explanation.
Deposition of three-dimensional Ge islands on Si(001) by chemical vapor deposition at atmospheric and reduced pressures
Authors: Kamins T. I. , Carr E. C. , Williams R. S. , Rosner S. J.
Published: July 2002

This report summarizes observations of Ge island formation during growth on Si(001) by chemical vapor deposition from germane in the pressure range from 10 Torr to atmospheric pressure in a conventional epitaxial reactor. A four-step growth process is observed: (1) uniform pseudomorphic overlayer (“wetting’’ layer) formation; (2) three-dimensional island growth with a constant aspect ratio; (3) continued island growth with a constant diameter and increasing height; (4) rapid growth of larger, faceted islands. Ostwald ripening of the islands during continued heat treatment after terminating the deposition is slow compared to island formation and growth during deposition for the experimental conditions used.
Magnetic linear dichroism in electron energy loss spectroscopy
Authors: Yuan J. , Menon N. K.
Published: July 2002

In this article, we demonstrate the effect of magnetic linear dichroism (MLD) in electron energy loss spectroscopy (EELS), as applied to the antiferromagnetic compound α-Fe2O3. The experiments were performed in a high resolution scanning transmission electron microscope (STEM), capable of producing a nanoscale electron probe. The resultant difference spectrum is consistent with the MLD observed in x-ray absorption spectroscopy and the known magnetic structure of Hematite, suggesting that MLD in EELS, coupled with the high spatial resolution attainable in STEM, is a useful technique in the study of magnetic microstructure.
Investigation of possible giant magnetoresistance limiting mechanisms in epitaxial PtMnSb thin films
Authors: Kautzky M. C. , Mancoff F. B. , Bobo J.-F. , Johnson P. R. , White R. L. , Clemens B. M.
Published: July 2002

To investigate the possibility of using the half-metallic ferromagnet PtMnSb in giant magnetoresistance (GMR) applications, we have examined the surface roughness, structural uniformity, mean-free path, and lattice disorder of sputtered epitaxial PtMnSb(111) films on Al2O3(0001) in the range 85 Å⩽t⩽1100 Å. Atomic force microscopy revealed the films to be very flat, with an rms roughness of ≈3 Å for thicknesses below 100 Å. X-ray diffraction and vibrating sample magnetometry indicated that the entire volume of the single-phase films were ferromagnetic (Ms≈500 emu/cm3), with no evidence for a nonstoichiometric or oxidized surface layer. Hall-effect measurements and fits to resistivity versus thickness data by surface scattering models yielded mean-free-path values of 18.4 and 80–100 Å, respectively. Atomic site disorder was estimated at ≈10% from structure factor analysis of integrated x-ray peak intensities. These results suggest that bulk properties will play the largest role in determining the effectiveness of PtMnSb in GMR applications.
Surface morphology changes in ZnSe-related II–VI epitaxial films grown by molecular beam epitaxy
Authors: Tomiya S. , Minatoya R. , Tsukamoto H. , Itoh S. , Nakano K. , Morita E. , Ishibashi A.
Published: July 2002

Surface morphological changes in ZnSe-related II–VI epitaxial films grown by molecular beam epitaxy have been investigated by atomic force microscopy and transmission electron microscopy. We found that under group-II-rich conditions with c(2×2) surface reconstruction, the process of roughening gives rise to periodic elongated corrugations aligned in the [11̄0] direction. Under group-VI-rich conditions with (2×1) surface reconstruction, rounded grains form instead of corrugated structures. The surface morphology is dependent on the VI/II ratio and growth temperature, but is independent of the film strain. The observed morphological changes are mainly due to growth kinetics and are not stress driven. We propose a model to explain the changes in surface morphology under group-II-rich conditions and group-VI-rich conditions.
Annealing-induced blue shift in luminescence band from Si-implanted SiO2 layer
Authors: Lan A. D. , Liu B. X. , Bai X. D.
Published: July 2002

The SiO2 layers thermally grown on Si wafers were implanted by 130 keV Si ions at liquid nitrogen temperature to a dose of 1×1017 ions/cm2. From the as-implanted samples, a visible photoluminescence band centered around 2.0 eV was observed. After postannealing at 1100 °C for 90 min another visible band in the range of 1.7 eV was detected. Interestingly, with increasing thermal annealing time, a blue shift in peak energy and an intensity variation of the 1.7 eV band were observed. A possible interpretation for the observations was discussed in terms of a so-called three-region model.
Amphoteric native defect reactions in Si-doped GaAs
Authors: Ky Nguyen Hong , Reinhart F. K.
Published: July 2002

Strong evidence for amphoteric native defect reactions is obtained by photoluminescence analysis of Si-doped GaAs samples (n≈1.5×1018 cm−3) annealed under different conditions. Annealing in excess As4 vapor creates a large concentration of Ga vacancies, making possible the transformation of this defect into an As vacancy and an As antisite defect. Similarly, As vacancies generated at high concentration during annealing under Ga-rich conditions are transformed into Ga vacancies and Ga antisite defects. Photoluminescence intensities associated with the corresponding defects are in qualitative agreement with the predictions of the mass action law applied to the amphoteric native defect reactions.
Effect of interstitial nitrogen on the structural and magnetic properties of NdFe10.5V1.5Nx
Authors: Yang Jinbo , Cui Bo , Mao Weihua , Cheng Benpei , Yang Jianian , Hu Bo , Yang Yingchang , Ge Senlin
Published: July 2002

The NdFe10.5V1.5Nx nitrides crystallize in the ThMn12-type structure. The nitrogen atoms occupy interstitial sites, and their most important effects are on the crystal fields around the rare earth ion sites. The variation of anisotropy fields of NdFe10.5V1.5Nx as a function of the nitrogen content x is presented. The crystal field interaction parameters are determined by using single-ion model. In the light of this study, high performance magnetic powders based on NdFe10.5V1.5Nx were successfully prepared.
Influence of oxygen on the thermal stability of amorphous hydrogenated carbon films
Authors: Zhang Qing , Yoon S. F. , Rusli  , Yang H. , Ahn J.
Published: July 2002

Influence of oxygen on the thermal stability of amorphous hydrogenated carbon (a-C:H) films is investigated by annealing the films at 520, 610, and 670 K in vacuum and in O2 flow, respectively. The a-C:H films used for this study were deposited using microwave plasma from a mixture of 10 sccm pure methane and 100 sccm hydrogen in an electron cyclotron resonance chemical vapor deposition system. It is found that the anneals in O2 gas flow lead to much more prominent variation in film thickness, optical band gap, and IR absorption related to C–H stretching vibration than the anneals in vacuum. Raman scattering and optical absorption indicate that O2 greatly promotes the transformation from polymeric to graphitic microstructure with increasing annealing temperature. Possible conversion of bond structure and the role of O2 in the stability of the films are discussed.