bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature dependence of the energy gap in GaN bulk single crystals and epitaxial layer
Authors: Teisseyre H. , Perlin P. , Suski T. , Grzegory I. , Porowski S. , Jun J. , Pietraszko A. , Moustakas T. D.
Published: July 2002

We performed optical-absorption studies of the energy gap in various GaN samples in the temperature range from 10 up to 600 K. We investigated both bulk single crystals of GaN and an epitaxial layer grown on a sapphire substrate. The observed positions of the absorption edge vary for different samples of GaN (from 3.45 to 3.6 eV at T=20 K). We attribute this effect to different free-electron concentrations (Burstein–Moss effect) characterizing the employed samples. For the sample for which the Burstein shift is zero (low free-electron concentration) we could deduce the value of the energy gap as equal to 3.427 eV at 20 K. Samples with a different free-electron concentration exhibit differences in the temperature dependence of the absorption edge. We explain the origin of these differences by the temperature dependence of the Burstein–Moss effect.
Electrical characterization and modeling of wide-band-gap porous silicon p-n diodes
Authors: Chen Zhiliang , Lee Tzung-Yin , Bosman Gijs
Published: July 2002

The current through porous silicon p-n junctions is measured as a function of bias voltage and temperature. The ideality factor m=4 value observed under forward bias indicates that channels are present at the unpassivated porous silicon junction surfaces. The generation-recombination current stemming from the space-charge regions between the surface channels and the bulk silicon regions dominates the measured diode current. From the thermal activation energy of the diode saturation current a value of 2.2 eV is calculated for the band gap of porous silicon, which is in good agreement with the value of 2.0 eV found from the photo- and electroluminescence spectra emitted by these diodes.
Optical band-gap shrinkage in highly transparent and conducting ZnO thin films deposited by the Pyrosol process
Authors: Tiburcio-Silver A. , Joubert J. C. , Labeau M.
Published: July 2002

An optical band-gap shrinkage was found in highly transparent and conducting thin films deposited by the Pyrosol process. The narrowing appears at carrier concentrations which are consistent with a metal-semiconductor transition. The gap-shrinkage dependence on carrier concentration is consistent with the model proposed by P. E. Schmid [Phys. Rev. B 23, 5531 (1981)] for Si:As and Si:B.
Local thermal conductivity in chemical-vapor-deposited diamond
Authors: Graebner J. E. , Jin S. , Herb J. A. , Gardinier C. F.
Published: July 2002

The thermal conductivity has been measured in the temperature range 5–400 K for a series of chemical-vapor-deposited diamond samples differing only in thickness. By analyzing the conductance, a local conductivity is extracted as a function of height z above the substrate on which the samples were grown. An analysis of the temperature dependence of the conductivity at any given height yields the phonon scattering mechanisms as a function of z. Point defects and extended defects of approximately 1.5 nm diam appear to be the most important phonon scattering entities at room temperature, but their scattering strengths decrease with height above the substrate. At a height z≊300 μm, the material is sufficiently perfect that scattering from extended defects at room temperature is negligible and scattering from point defects is only slightly higher than that attributable to naturally occurring 13C. The observed anisotropy of the thermal conductivity is consistent with aggregation of point defects and extended defects at or near grain boundaries. The z dependence of the scattering from these defects suggests that the quality of the grain boundaries increases with z. For z≳300 μm, the conductivity is equal to that of single crystals of gem-quality type IIa diamond.
Spatially resolved analysis of high-T
c grain boundary Josephson junctions and arrays
Authors: Gerdemann R. , Husemann K.-D. , Gross R. , Alff L. , Beck A. , Elia B. , Reuter W. , Siegel M.
Published: July 2002

Using Low Temperature Scanning Electron Microscopy (LTSEM) we have studied the spread of the critical current values in one- and two-dimensional arrays of high-Tc grain boundary Josephson junctions (GBJs). For series arrays the critical current values and the magnetic field dependences of all GBJs within the array could be imaged by LTSEM. Our measurements showed that part of the spread of the critical current values derived from measurements of the current-voltage characteristics is caused by variations of the local magnetic field at the junction position. For bicrystal GBJs a Gaussian distribution of the critical current values with a spread as small as ±20% was found. For the investigated step-edge and biepitaxial GBJs the spread of the critical current values was considerably larger. The LTSEM technique also was used to study the spatial homogeneity of the critical current density of the different types of YBa2Cu3O7−δ grain boundary Josephson junctions with a spatial resolution of 1μm. Whereas the critical current density of bicrystal GBJs showed only small variations on this length scale, strong inhomogeneities were found for step-edge and biepitaxial GBJs.
Method for the simultaneous determination of anisotropic residual stresses and texture by x-ray diffraction
Authors: Ferrari Mauro , Lutterotti Luca
Published: July 2002

A method is developed that yields the residual stress, the orientation distribution coefficients, the average crystallite dimension, the microstrain, and the crystal structure parameters from x-ray diffraction data in a single-step procedure. To this end, a general approach is introduced that combines the equations of micromechanics with the harmonic description of texture. All relationships are cast into a Rietveld-like format, which incorporates a microstructure model derived from line-broadening methods. In this manner, data collected over the whole x-ray-diffraction pattern at different tilting of the sample can be fitted directly. The associated fitting parameters are the crystal structure and microstructure, the texture coefficients, and the micromechanical properties and fields.
Giant magneto-impedance and magneto-inductive effects in amorphous alloys (invited)
Authors: Panina L. V. , Mohri K. , Bushida K. , Noda M.
Published: July 2002

Recent experiments have discovered giant and sensitive magneto-impedance and magneto-inductive effects in FeCoSiB amorphous wires. These effects include a sensitive change in an ac wire voltage with the application of a small dc longitudinal magnetic field. At low frequencies (1–10 kHz) the inductive voltage drops by 50% for a field of 2 Oe (25%/Oe) reflecting a strong field dependence of the circumferential permeability. At higher frequencies (0.1–10 MHz) when the skin effect is essential, the amplitude of the total wire voltage decreases by 40%–60% for fields of 3–10 Oe (about 10%/Oe). These effects exhibit no hysteresis for the variation of an applied field and can be obtained even in wires of 1 mm length and a few micrometer diameter. These characteristics are very useful to constitute a highly sensitive microsensor head to detect local fields of the order of 10−5 Oe. In this paper, we review recently obtained experimental results on magneto-inductive and magneto-impedance effects and present a detailed discussion for their mechanism, developing a general approach in terms of ac complex impedance in a magnetic conductor. In the case of a strong skin effect the total wire impedance depends on the circumferential permeability through the penetration depth, resulting in the giant magneto-impedance effect.
A new method for the determination of polymer optical anisotropy
Authors: Osaki Shigeyoshi
Published: July 2002

A new method for determining the optical anisotropy of transparent polymer sheets is developed and applied to commercial polypropylene and nylon 6. The method allows the retardation, birefringence, orientation angle, and minimum-to-maximum light intensity ratio to be determined in as short a time as ∼10 s. It also reveals anisotropy that depends on draw ratio and position of the film, and is considered practically useful for its accuracy and speed.
Structure of stacking faults formed in pairs in a ZnSe epitaxial layer on a GaAs(001) buffer layer
Authors: Tanimura J. , Wada O. , Ogama T. , Endoh Y. , Imaizumi M.
Published: July 2002

Structure of stacking faults formed in pairs in a ZnSe epitaxial layer grown by gas source molecular beam epitaxy on a GaAs(001) buffer layer was determined with transmission electron microscopy. Extrinsic type stacking faults were formed on (111) and (1̄1̄1) planes with the same polarity, which was determined by convergent-beam electron diffraction. The two stacking faults meet at a point which is a few atomic layers away from the interface between ZnSe and GaAs. Partial dislocations at the edge of the stacking faults were found to be the Shockley type ones with a Burgers vector of 1/6〈211〉. Probable formation processes of the stacking faults have been discussed.
Photoluminescence of quaternary GaInAsSb/AlGaAsSb strained multiple quantum wells
Authors: Shen W. Z. , Shen S. C. , Tang W. G. , Zhao Y. , Li A. Z.
Published: July 2002

We report the dependence on the excitation power and temperature of the photoluminescence emission from a quaternary Ga0.75In0.25As0.04Sb0.96/Al0.22Ga0.78As 02Sb0.98 strained multiple- quantum-well structure grown by molecular-beam epitaxy. Sharp exciton resonances are observed up to room temperature and have been attributed to localized excitons for temperatures ≤80–100 K and to free excitons at higher temperatures up to room temperature by a comparative study with temperature-dependent absorption spectroscopy. We conclude that the dominant luminescence quenching mechanism in this quaternary system is mainly due to the trapped excitons thermalizing from the localized regions below 100 K, and the thermal carrier activation from the first electron and heavy-hole subbands to the second electron and heavy-hole subbands at higher temperatures.
Edge-induced stress and strain in stripe films and substrates: A two-dimensional finite element calculation
Authors: Jain S. C. , Harker A. H. , Atkinson A. , Pinardi K.
Published: July 2002

Finite element calculations of stresses and strains in substrates and stripe films of thickness h and width 2l are reported. Both variation of the stress in the vertical direction (away from the interface) in the film and distortion of the substrate are taken into account. The calculations show that both the horizontal and vertical lattice planes are curved in the film as well as in the substrate. If the thickness of the substrate is infinite, the curvature in the substrate is maximum near the interface and decays rapidly with depth. Furthermore, the edges near the top are over-relaxed, i.e., if the film is originally under compression, the stress becomes tensile because the free edge surfaces affect the relaxation. For a film with h/l=1 or greater, the stress is reversed throughout the top layer. The change from compression to tension takes place partly because of the Poisson effect and partly due to the bending of the lattice planes. The approximations made in the existing analytical models were examined and the conditions under which the models describe the stresses in the film or in the substrate were determined to a good approximation. Our finite element calculations agree with the available experimental data. Ours are the only theoretical results with which measured substrate stresses agree qualitatively.
A model for oxygen precipitation in silicon including bulk stacking fault growth
Authors: Senkader S. , Esfandyari J. , Hobler G.
Published: July 2002

This paper describes a model that simulates the precipitation kinetics of oxygen and the evolution of the precipitate density in annealed Czochralski-silicon wafers. A discrete rate equation representation combined with Fokker-Planck equations are used to treat precipitation of oxygen and simultaneous formation of bulk stacking faults. This approach allows one to describe both statistical clustering during nucleation and diffusional transport during growth. The model considers explicitly influences of self interstitials and stress on precipitation of oxygen and thus is able to accurately predict the experimental observations published previously. In particular, we compare simulation results of precipitated oxygen concentration in 2-step, 3-step, complementary metal oxide semiconductor type multi-step, and rapid thermal anneals with experimental results taken from the literature and discuss them.
Statistical thermodynamic foundation for photovoltaic and photothermal conversion. I. Theory
Authors: Badescu Viorel , Landsberg Peter T.
Published: July 2002

We intend to cover the need for a general statistical foundation of the thermodynamic models of photovoltaic and/or photothermal conversion. The solid body receiving radiation is considered in considerable generality. This includes (1) nuclei of the lattice and their core electrons, (2) the electrons of the conduction band, (3) the electrons or holes of the valence band, (4) the traps in the forbidden gap. These components are subject to radiation and external fields (such as mechanical stress, gradients of electrostatic potential, or of temperature). The basis of the treatment is the Boltzmann transport equation. Different forms of this equation are considered, as the particles encountered in the body receiving radiation are both fermions (electrons and holes) and bosons (photons and phonons). Three main directions are followed. First, the continuity equations are derived in a rigorous manner. Second, the irreversibilities occurring during the conversion of radiation are emphasized. Third, the differences and similarities between photovoltaic and photothermal conversion are investigated.
Dopant level freeze-out and nonideal effects in 6H–SiC epilayer junctions
Authors: Quintanilla L. , Dueñas S. , Castán E. , Pinacho R. , Pelaz L. , Bailón L. , Barbolla J.
Published: July 2002

Experimental results obtained from current–voltage (I–V) and capacitance–voltage techniques along with admittance spectroscopy have been qualitatively correlated to achieve a more comprehensive picture of dopant freeze-out and conduction mechanisms in a 6H–SiC n+p-type junction. Special attention was paid to the temperature range of 100–200 K. The dependence of the ideality factor, n, on the temperature was obtained experimentally from the I–V measurements. Two contributions have been considered in its evolution. At room temperature, n is very close to 2, indicating that recombination processes dominate the forward conduction mechanism. This result may be related to the SiC sample preparation process: structural defects may be present at the junction interface giving rise to interface states which act as recombination centers. At low temperatures (100–200 K), the Poole–Frenkel effect on the impurity level is the main effect responsible for the nonideal behavior of the junction. We have carried out a quantitative estimation of the n factor predicted by this effect incorporating partial ionization of the dopant. These calculations agree very well with the experimental values. At these temperatures the thermal excitation is low, the traps remain inactive, and their contribution to the conduction mechanisms is negligible. When the temperature increases, traps become thermally activated and then the recombination processes participate in the conduction mechanisms and they become dominant at room temperature. The admittance analysis allows numerical values of the aluminum emission rate to be obtained at different temperatures.
Interface roughness of InAs/AlSb superlattices investigated by x-ray scattering
Authors: Jenichen B. , Stepanov S. A. , Brar B. , Kroemer H.
Published: July 2002

InAs/AlSb short period superlattices grown either with AlAs-like or with InSb-like interfaces are investigated by grazing incidence x-ray scattering and high resolution diffractometry. The superlattices are grown on a relaxed AlSb buffer layer. It is shown that the two possible stackings of layers in the superlattices resulting in a different degree of lattice relaxation lead also to a different height of interface roughness. The lateral and vertical correlation lengths of the roughness decrease with increasing relaxation of the superlattice. The vertical correlation length corresponds to an almost complete correlation of different interfaces in the case of the nearly perfect superlattice with InSb-like interfaces.
A second look at electrokinetic phenomena in boiling
Authors: Snyder Trevor J. , Schneider John B. , Chung Jacob N.
Published: July 2002

In the article by V. Asch [J. Appl. Phys. 37, 2654 (1966)], experiments were performed to study the influence of an electrostatic field on nucleate boiling of Freon-113(R-113). We have found that Asch might not have properly considered the effects of his experimental setup and therefore came to incorrect conclusions concerning electrophoretic and dielectrophoretic forces. Asch's analysis of the electric field distribution led him to conclude that the dielectrophoretic forces were small, however, we show in this article that, in general, there are strong dielectrophoretic forces in the vicinity of the heater wire. This article presents the results from a set of experiments performed with an apparatus similar to that of Asch's with test fluids of R-113 and FC-72. The experimental results show that vapor bubbles can be attracted to either the anode or cathode depending on the potentials with respect to the heater wire on which the boiling takes place. This is contrary to the results obtained by Asch which led him to conclude that the bubbles were always attracted to the anode. The bubble movement appears to be the result of a combination of dielectrophoretic forces (which are very strong but highly localized), electrophoretic forces, and bulk electroconvective flow. Furthermore, the combined effects of these forces away from the wire can lead to unstable behavior. © 1996 American Institute of Physics.
In situ Brillouin light scattering from ultrathin epitaxial Fe/Ag(100) films with Cr and Ag overlayers
Authors: Hicken R. J. , Ercole A. , Gray S. J. , Daboo C. , Bland J. A. C.
Published: July 2002

We have studied the magnetic properties of ultrathin epitaxial Fe/Ag(100) films by means of in situ Brillouin light scattering (BLS) in an ultrahigh vacuum chamber equipped with magneto-optic Kerr effect, low energy electron diffraction, and reflection high energy electron diffraction instruments. Our novel in situ BLS system allows us to perform measurements rapidly, in order to avoid surface contamination, and hence to observe the evolution of the spin-wave frequency with increasing film thickness. Good reproducibility is observed between different growth runs. The direction of the applied field within the film plane has been varied revealing that the magnetocrystalline anisotropy has cubic symmetry and that there is no observable in-plane uniaxial anisotropy. From the values of the hard and easy axis spin wave frequencies we have determined the values of the in-plane fourfold anisotropy and effective demagnetizing fields during the growth of 13.9 monolayer (ML) Fe films. The evolution of the fourfold anisotropy is in reasonable agreement with that reported by other researchers. The effect of depositing Cr and Ag overlayers onto the completed 13.9 ML film has also been studied. We have deduced values for the surface anisotropy constants for the Fe/Ag and Fe/vacuum interfaces and we compare these with previously reported values. A qualitatively different evolution of the spin wave frequency with overlayer thickness is observed for the deposition of Cr and Ag, which may be related to the magnetic properties of the Cr overlayer.
Studies of the magnetic anisotropies of Co(11̄00)/Cr(211) and Co(112̄0)/Cr(100) multilayers
Authors: Huang J. C. A. , Tang F. C. , Fang W. W. , Liu R. L. , Hu Y. M. , Lo C. K. , Liou Y. , Yao Y. D. , Yang W. T. , Chang C. P. , Liao S. Y.
Published: July 2002

Co(11̄00)/Cr(211) and Co(112̄0)/Cr(100) multilayers have been simultaneously prepared on MgO(110) and MgO(100) substrates, respectively, by molecular beam epitaxy. They show however distinct magnetic anisotropic behavior which coincides with their magneto–crystalline anisotropy. Magneto–optical Kerr effect shows the existence of a unique easy axis and strong in-plane uniaxial magnetic anisotropy in Co(11̄00)/Cr(211) multilayers, which is induced by the well-defined hexagonal crystalline of the Co(11̄00) layers. For Co(112̄0)/Cr(100) multilayers, on the other hand, an in-plane biaxial magnetic anisotropy is found due to the bicrystalline structure of the Co(112̄0) layers.
Thermal dependence of the refractive index of InP measured with integrated optical demultiplexer
Authors: Gini E. , Melchior H.
Published: July 2002

The temperature dependence of the refractive index of InP in the wavelength range from 1.2 to 1.6 μm has been determined. The method is based on the measurement of the transmission wavelengths of a n−/n+-InP optical demultiplexer as a function of device temperature. The refractive index of InP is found to depend linearly on temperature with a temperature coefficient between 2.3 and 1.9×10−4/K. The measured absolute value and the dispersion of the refractive index of InP are in agreement with data from the literature.
Ferromagnetic resonance and intrinsic properties of La0.67Ba0.33MnOz
Authors: Lofland S. E. , Bhagat S. M. , Ju H. L. , Xiong G. C. , Venkatesan T. , Greene R. L. , Tyagi S.
Published: July 2002

Systematic magnetization and ferromagnetic resonance (FMR) measurements have been performed on thin-film samples of La0.67Ba0.33MnOz (TC∼340 K). It is found from the FMR data that as- grown films show broad lines whose width Γ increases to nearly 1 kOe as T reduced to 77 K. The increase roughly follows the increase in the magnetization suggesting magnetic inhomogeneity as the main cause for the rise in Γ. However, when films were properly annealed in O2, we obtained narrow lines (Γ∼200 Oe at 10 GHz). Γ was effectively independent of temperature and linearly dependent upon microwave frequency, both of which are strong signs that, we have succeeded in obtaining a uniform, homogeneous manganite sample. Having gotten a uniform material, we have measured the intrinsic properties of this system.
Structural anisotropy of Tb/Fe multilayers
Authors: Fujiwara Y. , Yu X. Y. , Tsunashima S. , Iwata S. , Sakurai M. , Suzuki K.
Published: July 2002

We have investigated the local structural anisotropy of Tb/Fe multilayers using linear polarized synchrotron radiation. X-ray absorption measurements were performed at Fe K edge (7.1 keV) using normal and glancing angle incident photon. From the Fe extended x-ray absorption fine structure (EXAFS), the local structural anisotropy was found for all multilayers as a slight difference of oscillation amplitude between the two incident directions. From the Fourier transformed EXAFS spectra, it is found that the local structural anisotropy shows the maximum at bilayer period of about 1 nm, where perpendicular magnetic anisotropy also shows the maximum value in various rare-earth/transition metal multilayers.
Inhomogeneous Schottky barriers at Ag/Si(111) and Ag/Si(100) interfaces
Authors: Weitering H. H. , Sullivan J. P. , Carolissen R. J. , Pérez-Sandoz R. , Graham W. R. , Tung R. T.
Published: July 2002

We have measured current–voltage and capacitance–voltage characteristics of epitaxial Si(111)7×7–Ag, Si(111)(√3×√3)R30°–Ag, Si(100)2×1–Ag, and polycrystalline Ag/Si interfaces, using different doping levels for both n- and p-type silicon wafers. Our data strongly suggest that the Schottky barrier heights (SBHs) are spatially nonuniform. The distribution of local effective SBHs at the epitaxial interfaces is modeled by a summation of a single Gaussian, representing the spread in SBH for the majority of the contact, and two half-Gaussians which represent the high- and low-barrier tails of the full distribution. Despite the fact that the average SBHs of the epitaxial interfaces are hardly structure dependent, the SBH distributions are very broad and markedly different for each interface. The polycrystalline interfaces are characterized by a narrower SBH distribution centered at a substantially smaller mean. We argue that the electrical inhomogeneity is related to structural inhomogeneity at the interface which is a direct consequence of the kinetics and mode of growth of Ag on Si.
Charge collection and trapping in low-temperature silicon detectors
Authors: Penn M. J. , Dougherty B. L. , Cabrera B. , Clarke R. M. , Young B. A.
Published: July 2002

Charge collection efficiency measurements in silicon detectors at low temperature (T<0.5 K) and low applied electric field (E=0.1–100 V/cm) were performed using a variety of high-purity, p-type silicon samples with room-temperature resistivity in the range 2–40 kΩ cm. Good charge collection under these conditions of low temperature and low electric field is necessary for background suppression, through the simultaneous measurement of phonons and ionization, in a very low event rate dark matter search or neutrino physics experiment. Charge loss due to trapping during drift is present in some samples, but the data suggest that another charge–loss mechanism is also important. We present results which indicate that, for 60 keV energy depositions, a significant fraction of the total charge loss by trapping occurs in the initial electron-hole cloud near the event location which may briefly act as a shielded, field-free region. In addition, measurements of the lateral size, transverse to the applied electric field, of the initial electron-hole cloud indicate large transverse diffusion lengths. At the lowest fields a lateral diameter on the order of 1 mm is found in a detector ∼5 mm thick.
Excitonic optical nonlinearity of CuCl microcrystals in a NaCl matrix
Authors: Yano S. , Goto T. , Itoh T.
Published: July 2002

Absorption and luminescence saturation of confined excitons are measured using a single-beam method for CuCl microcrystals with various radii embedded in NaCl single-crystalline matrices. From the excitation-intensity and microcrystal-size dependencies of their saturation, the effective saturation density of excitons, which is inversely proportional to the magnitude of optical nonlinearity, proves to be inversely proportional to the volume of microcrystals with effective radii of less than 5 nm at 77 K. Therefore, the optical nonlinearity of these microcrystals is not a function of microcrystal radii, but of the number of excitons created in a microcrystal, indicating the characteristic nature of coherent exciton confinement.
Temperature persistent bistability and threshold switching in a single barrier heterostructure hot-electron diode
Authors: Stasch R. , Hey R. , Asche M. , Wacker A. , Schöll E.
Published: July 2002

Bistable current–voltage characteristics caused by competition of tunneling through and field-enhanced thermionic emission across a single barrier are investigated in an n−-GaAs/Al0.34Ga0.66As/n+-GaAs structure. The S-shaped part of the characteristic persists in the whole temperature regime between 4.2 and 300 K in accordance with theoretical predictions. The delay and switching times of the transition from the low- to the high-conducting state are investigated as a function of the applied voltage and compared with simulations. The observed timescales are much longer than expected from theory, which indicates the presence of additional deep centers or interface states.
Enhanced ionic conduction mechanisms at LiI/Al2O3 interfaces
Authors: Lubben D. , Modine F. A.
Published: July 2002

Electrical conduction in LiI thin films has been measured in situ during growth as a function of film thickness and temperature. Films grown at 27 °C exhibited an enhancement of the ionic conduction of ∼40× the bulk value in a region extending ∼100 nm above the interface. The enhanced conduction was not stable and decreased with a log(time) dependence during annealing at the growth temperature. The activation energy for conduction was 0.47±0.03 eV, approximately equal to the value for extrinsic conduction in bulk LiI. Films grown at 100 °C, on the other hand, showed no enhancement near the interface, and a nearly linear increase in conductance with thickness after an initial nucleation stage. The conductivity of these films was stable at and above the growth temperature and exhibited an activation energy of 0.75±0.03 eV, which is comparable to the value for intrinsic LiI. X-ray diffraction measurements on the samples showed that all films were cubic LiI. Films grown near room temperature had a strong (002) orientation while those grown at elevated temperatures were (111) with a mosaic spread of <0.4°. The data show conclusively that there is no enhancement due to space-charge layer formation at the film/substrate interface. On the other hand, the data are all consistent with a model that assumes the conduction enhancement is caused by the overlapping strain fields of dislocations formed during growth.
Intersubband transitions in triple-coupled quantum wells for three-colors infrared detectors
Authors: Huang Danhong , Manasreh M. O.
Published: July 2002

We present a theoretical study of intersubband transitions in multiple-quantum-well structure, which is designed such that the chemical potential lies between the third and fourth electronic energy levels. The subject structure, which may function under a fixed bias voltage, would allow one to observe simultaneously three intersubband transitions that cover different spectral wavelength regions. The current theory is based on self-consistent screened Hartree–Fock calculations for quasiparticle’s energies and wave functions. In this theory we include the effects of the z-dependent electron effective mass and dielectric constant, and the nonparabolic dispersion. The optical absorption spectra were calculated after including the depolarization and excitonlike shifts. These spectra were investigated as a function of temperature, doping concentration, and bias voltage.
Local Ce environments and their effects on optical properties of SrS phosphors
Authors: Warren W. L. , Vanheusden K. , Seager C. H. , Tallant D. R. , Tuchman J. A. , Silliman S. D. , Brower D. T.
Published: July 2002

In this study, we use electron paramagnetic resonance (EPR), optical absorption, and photoluminescence (PL) spectroscopies to determine the various Ce environments in SrS phosphor materials and how these affect absorption and emission properties. As the Ce concentration is increased from 450 to 7500 ppm, the total EPR-active Ce3+ and optical absorption signals increase linearly with Ce concentration; by contrast, the PL intensity saturates at fairly low Ce concentrations (1000 ppm Ce). We suggest that the nonlinear behavior of the PL arises from the presence of nonradiative deexcitation pathways such as defects associated with Ce sites, or Ce–Ce pairs.
The valence band alignment at ultrathin SiO2/Si interfaces
Authors: Alay J. L. , Hirose M.
Published: July 2002

High resolution x-ray photoelectron spectroscopy has been used to determine the valence band alignment at ultrathin SiO2/Si interfaces. In the oxide thickness range 1.6–4.4 nm the constant band-offset values of 4.49 and 4.43 eV have been obtained for the dry SiO2/Si(100) and the wet SiO2/Si(100) interfaces, respectively. The valence band alignment of dry SiO2/Si(111) (4.36 eV) is slightly smaller than the case of the dry SiO2/Si(100) interface.
Transient ballistic transport in GaN
Authors: Mansour N. , Kim K. W. , Bannov N. A. , Littlejohn M. A.
Published: July 2002

Monte Carlo simulations have been used to study the spatial scales of electron ballistic transport in GaN. The large optical phonon energy (92 meV) and the large intervalley energy separation between the Γ and satellite conduction band valleys (⩾1.5 eV) suggest an increasing role for ballistic electron effects in GaN, especially when compared with most III–V semiconductors such as GaAs. However, the concomitant high polar optical phonon scattering rate in GaN tends to diminish the desirable electron transport properties. The relationships between these two factors have been studied for the range of electric fields up to 140 kV/cm and lattice temperatures between 300 and 600 K. We demonstrate that in most cases electrons in GaN lose their directed average velocity over distances of only 100−200 Å, and ballistic transport occurs only over such short distances. The main cause for the small spatial scales of ballistic transport in GaN is the strong electron–optical phonon coupling which results in rapid relaxation of the directed electron velocity.
Crystal structure and magnetic properties of Fe (111) singe crystal films
Authors: Endo Y. , Okamoto S. , Kitakami O. , Shimada Y.
Published: July 2002

Fe (111) single crystal films have been grown on GaAs (111) by the sputter-beam method. Reflection high energy electron diffraction and x-ray diffraction measurements have revealed excellent aligned epitaxial growth of the Fe (111) film on GaAs. No evidence of preferential diffusion of As atoms into the deposited Fe layers has been found in the depth profile of Auger electron spectra. Magnetic measurements clearly show that the Fe (111) film has nearly ideal threefold symmetric anisotropy and exhibits isotropic initial permeability in the film plane, as predicted by theory.
Formation mechanism of stains during Si etching reaction in HF–oxidizing agent–H2O solutions
Authors: Nahm Kee Suk , Hun Seo Young , Lee Hyung Jae
Published: July 2002

The mechanism of stain formation in the chemical etching reaction of silicon has been investigated in HF–oxidizing agent–H2O solutions. The chemical formula of the stain formed during the silicon etching reaction is K2SiF6. The concentration of holes on silicon surface increases with the increase of redox potential and the concentration of oxidizing agent used in manufacturing the etching solution. The increase in the hole concentration accelerates not only the etch rate but also the formation rate of K2SiF6. The etched silicon surfaces are covered with a K2SiF6 layer when redox potential and concentration of oxidizing agent are great at low HF concentrations. This happens because the formation rate of K2SiF6 is much greater than its dissolution rate by HF. Sufficiently high HF concentration in the etching solution is apparently essential to increase the etch rate without the formation of K2SiF6.
Interdiffusion studies in GaAsP/GaAs and GaAsSb/GaAs superlattices under various arsenic vapor pressures
Authors: Egger U. , Schultz M. , Werner P. , Breitenstein O. , Tan T. Y. , Gösele U. , Franzheld R. , Uematsu M. , Ito H.
Published: July 2002

Interdiffusion coefficients on the group V sublattice of GaAs were determined in GaAsP/GaAs and GaAsSb/GaAs superlattices. Strained GaAs0.86P0.14/GaAs, GaAs0.8P0.2/GaAs0.975P0.025 and GaAs0.98Sb0.02/GaAs superlattices were annealed between 850 °C and 1100 °C under different arsenic vapor pressures. The diffusion coefficient was measured by secondary ion mass spectroscopy and cathodoluminescence spectroscopy. The interdiffusion coefficient was higher under arsenic-rich conditions than under gallium-rich conditions, pointing to an interstitial-substitutional type of diffusion mechanism.
Spin-polarized secondary electron microscopy of soft magnetic films on antiferromagnetic substrates
Authors: Iwasaki Y. , Takiguchi M. , Bessho K.
Published: July 2002

Coupling of soft magnetic films to antiferromagnetic substrates was studied by spin-polarized secondary electron microscopy. Permalloy (Ni80Fe20 alloy) films were deposited on antiferromagnetic NiO(111) single-crystal substrates. A domain structure consisting of fine grains (∼50 μm) was characteristic of the NiFe film deposited without applying magnetic field. The domain image and its response to applied field suggested existance of local easy axis and local unidirectional anisotropy. When macroscopic unidirectional anisotropy was introduced to the NiFe film by depositing under an applied dc magnetic field, many of the fine grains disappeared and the film showed large domains (∼1 mm). In the large domains, small elongated reversed domains (∼20 μm×60 μm) aligned parallel to each other implying local easy axis of magnetization. The type of domain behavior described above was not observed in a NiFe film on MgO(111) substrate that had the same crystal structure as NiO but no magnetic ordering. An x-ray analysis revealed that the local easy axes observed in NiFe/NiO specimens coincided with the [112̄] of the NiO crystal lying in the (111) substrate plane, which supported coupling of the NiFe film with the single-crystal substrate.
Structure and magnetoresistive properties in La–manganite thin films
Authors: Gommert E. , Cerva H. , Rucki A. , Helmolt R. v. , Wecker J. , Kuhrt C. , Samwer K.
Published: July 2002

This study investigates the structure of perovskite thin films and its influence on their colossal magnetoresistance (CMR) properties. Epitaxial thin films of perovskite manganites La1−xBxMnO3−δ (B=Ca,Sr) were prepared on SrTiO3 (100) substrates using on- and off-axis pulsed laser deposition (PLD) techniques. X-ray diffraction, resistance and magnetoresistance measurements, as well as high-resolution transmission electron microscopy (HRTEM) investigations were carried out. HRTEM observations reveal epitaxial growth for the first few layers of all prepared samples. Thicker on-axis prepared films grow with a large number of defects, whereas off-axis prepared samples grow in a columnar structure. Since the magnetic properties in systems with double-exchange interaction are very sensitive to the local structure it has great influence on the electronic properties.
Observations of exchange resonance modes on submicrometer sized ferromagnetic particles
Authors: Toneguzzo Ph. , Acher O. , Viau G. , Fiévet-Vincent F. , Fiévet F.
Published: July 2002

Spherical, monodisperse, submicrometer-sized particles in the CoxNi(100−x) system have been synthesized by the polyol process over the whole chemical composition range. These fine particles have been insulated by a chemical treatment in solution by a superficial layer of MnO2. The microwave permeability of these passivated particles, which are mechanically compacted, has been investigated in the frequency range 0.1–18 GHz. The dynamic permeability measurements exhibit several narrow resonance bands. This experimental behavior is in qualitative agreement with Aharoni’s calculation [A. Aharoni, J. Appl. Phys. 69, 7762 (1991)] of exchange resonance modes. The influence of the chemical composition of the ferromagnetic particles upon permeability has been investigated in the microwave range. It is shown that the peaks are shifted towards the high frequencies when atomic Co/Ni ratio increases.
The easy magnetization directions in R6Fe23 intermetallic compounds: A crystal field analysis
Authors: Bowden G. J. , Cadogan J. M. , de Leon H. , Ryan D. H.
Published: July 2002

X-ray diffraction patterns on magnetically aligned powder samples of R6Fe23 (R=Dy, Er, Ho, and Tm) show that these compounds all magnetize along a [111] easy direction. At first sight it is difficult to reconcile the common easy magnetization direction of Er6Fe23 and Tm6Fe23 on one hand, with Dy6Fe23 and Ho6Fe23 on the other, since the respective B20 rare earth crystal field parameters of these pairs of compounds are opposite in sign. In this article we show that the crystal field stabilization energy of the [111] direction, relative to either [100] or [110], varies as the square of the crystal field term B20, thereby providing an explanation for the common [111] direction of easy magnetization.
Band metamagnetism and related phenomena in Er(Co1−xSix)2
Authors: Cuong T. D. , Havela L. , Sechovský V. , Andreev A. V. , Arnold Z. , Kamarád J. , Duc N. H.
Published: July 2002

We report on magnetic, transport, and cohesive properties of Er(Co1−xSix)2 compounds in varying external conditions (T,H,p). The localized Er 4f magnetic moments in ErCo2 order ferromagnetically below TC=33 K. The Co 3d band states form no moment above TC, but below TC the majority and minority 3d sub-bands split due to strong 4f−3d exchange interaction. Consequently, Co moments of ∼1 μB appear coupled antiparallel to the Er moments. This is accompanied by a ∼50% resistivity drop and an abrupt lattice expansion. The sharp magnetization, resistivity, and volume anomalies in the critical temperature region indicate a first order magnetic phase transition. Detailed comparative studies reveal that the onset of Co magnetizm appears at T0∼(TC−1) K. Si substitution for Co enhances both, TC (up to 66 K for x=0.1) and ΔT=TC−T0, but the lattice parameter remains unchanged. In opposite, both the magnetovolume and resistivity anomaly at T0 become reduced. Pressure effect on TC(T0) is large and negative, ∂ ln TC/∂p∼−28 Mbar−1 for all x⩽0.1. Also the resistivity drop at T0 and the magnetovolume effect (ωs=3 ΔL/L=4.2×10−3 in ErCo2) below T0 become reduced with increasing pressure. The latter is reflecting reduction of the Co moment gained below T0.
Conductivity conversion of lightly Fe-doped InP induced by thermal annealing: A method for semi-insulating material production
Authors: Fornari R. , Zappettini A. , Gombia E. , Mosca R. , Cherkaoui K. , Marrakchi G.
Published: July 2002

As-grown Fe-doped semiconducting InP wafers (residual carrier concentration ⩽1015 cm−3, estimated iron concentration 5–8×1015 cm−3) were converted to semi-insulating, with high resistivity and good mobility, when annealed under appropriate conditions. This fact is very interesting, since it permits the preparation of semi-insulating InP with an Fe content substantially lower than that of standard as-grown material. In this article, we report the annealing parameters, together with the results of an extensive characterization (Hall effect, C–V, infrared absorption, and photoinduced current transient spectroscopy) of the treated samples. The onset of the semi-insulating regime seems to be primarily due to an annealing-related loss of shallow donors.
On the detection of single optical photons with superconducting tunnel junction
Authors: Peacock A. , Verhoeve P. , Rando N. , van Dordrecht A. , Taylor B. G. , Erd C. , Perryman M. A. C. , Venn R. , Howlett J. , Goldie D. J. , Lumley J. , Wallis M.
Published: July 2002

We report the detection of individual optical and ultraviolet photons using a different approach to photon detection based on a superconducting tunnel junction. A 20×20 μm2 junction, employing a 100 nm niobium film and operated at a temperature of ∼0.4 K, has been used to detect individual photons with inherently high quantum efficiency (>45%) over a broad wavelength range (between 200 and 500 nm), yielding high temporal (sub-ms) resolution, spatial resolution determined by the junction size, under conditions of minimal dark current, and in the absence of read noise. The quantum efficiency is limited by surface reflection, and could be improved by the deposition of antireflection coatings. The theoretical wavelength response range continues into the far UV and soft x-ray region, and is presently limited beyond 500 nm largely by the available signal processing electronics. The device intrinsically functions at very high incident photon rates—with count rates of order ∼10 kHz or higher being feasible and again currently limited primarily by the signal processing electronics—thus providing a correspondingly enhanced dynamic range by several orders of magnitude compared with previous panoramic photon counting detectors. The measured charge output from the device is highly linear with photon energy resulting in an optical photon detection system with intrinsic spectral resolution, related to the critical temperature of the junction material and, in the current device, providing a limiting spectral resolution of about 50 nm. It is realistic in the future to envisage that these devices could be packaged into arrays, with the resulting system characteristics offering advantages over detectors based on semiconductors.
Schottky barrier formation at metal/n-ZnSe interfaces and characterization of Au/n-ZnSe by ballistic electron emission microscopy
Authors: Coratger R. , Girardin C. , Beauvillain J. , Dharmadasa I. M. , Samanthilake A. P. , Frost J. E. F. , Prior K. A. , Cavenett B. C.
Published: July 2002

Current transport and ballistic electron emission microscopy (BEEM) studies have been carried out on metal contacts fabricated on chemically etched n-ZnSe epitaxial layers grown by molecular beam epitaxy. The contact materials Ag, Sb, Au, Ge/Au, Sn, Ni, and Pd form one or more barrier heights out of the following seven discrete values: 0.90, 1.20, 1.32, 1.50, 1.67, 1.80, and 2.10±0.04 eV observed to date. BEEM work carried out on Au/n-ZnSe systems has identified four levels 1.32 [Morgan et al., J. Appl. Phys. 79, 1532 (1996)], 1.50, 1.67 [Coratger et al., Phys. Rev. B 15, 2357 (1995)] and 1.80 eV to date, confirming Fermi-level pinning at different positions. Schottky barrier formation at metal/n-ZnSe systems cannot be explained by the simple Schottky model. The strong Fermi-level pinning observed could be due to bulk and/or surface defects of the ZnSe material.
Deposition-temperature dependence of texture and magnetic properties of sputtered Ni/Fe multilayers
Authors: Veres T. , Cai M. , Morel R. , Cochrane R. W. , Abdouche R. , Sutton M.
Published: July 2002

We present the effect of deposition temperature (Ts) on the structural and magnetic properties of sputtered Ni/Fe multilayers. As Ts increases above room temperature, a systematic change in texture from (111) to (200) and an increase in grain size are observed. Superlattice modulations are clearly visible in low-angle x-ray reflectivity measurements for all the multilayers deposited up to 250 °C. However, superlattice peaks intensities decrease with increasing Ts indicating enhanced mixing of Ni and Fe across interfaces. Nonmonotonic variations in electrical resistivity (ρs) and coercivity (Hc) with Ts are seen and explained in terms of the interplay between the increase in grain size and alloying at the interfaces. The total anisotropic magnetoresistance (AMR) as well as its field sensitivity increase with Ts.
Optical probing of interface roughness in resonant tunneling structures
Authors: Galvão Gobato Y. , Triques A. L. C. , Rivera P. H. , Schulz P. A. , Guldner Y. , Vinter B.
Published: July 2002

We report on photoluminescence (PL) and photoluminescence-excitation measurements in GaAs/In0.1Ga0.9As/AlAs double-barrier tunneling structures as a function of bias voltage and temperature. We have observed a splitting in the quantum well photoluminescence due to island formation in the quantum well. We have a good correlation between the variation of integrated PL intensity, linewidth, and tunnel current bias for both lines. The temperature dependence of photoluminescence spectra shows that transfer of carrier between islands can be tuned by the applied bias and that states in different islands are populated by electrons in the resonant tunneling process.
A study of ion velocity distribution functions in processing plasmas produced by electron cyclotron resonance discharges
Authors: Cronrath W. , Mayumi N. , Bowden M. D. , Uchino K. , Muraoka K. , Yoshida M.
Published: July 2002

Ion velocity distribution functions were measured using Doppler-shifted laser-induced fluorescence spectroscopy in an electron cyclotron resonance discharge in argon. The influence of the magnetic field configuration on the distributions was studied by making measurements with different magnetic field configurations. Results of a two-dimensional hybrid model of the discharge were used to help interpret the measured data. The results from the experiment and the simulation indicated that the magnetic field configuration had a strong influence on the ion velocity distribution functions. From the simulation it was concluded that the magnetic field configuration determined the axial distributions of plasma potential and electron density, and how these plasma parameters determined the ion velocity distribution function in the downstream region of the discharge.