bluuebunny/crossref_metadata_2025 huggingface dataset:
Role of interfacial compound formation associated with the use of ZnO buffers layers in the hydride vapor phase epitaxy of GaN
Authors: Gu Shulin , Zhang Rong , Sun Jingxi , Zhang Ling , Kuech T. F.
Published: July 2002

ZnO buffer layers have been used in the hydride vapor phase epitaxy of GaN in order to improve the initial nucleation and growth of the GaN and hence the subsequent materials properties. The specific role of the ZnO buffer layer was investigated by x-ray photoelectron spectroscopy and x-ray diffraction. The improvements in the GaN growth behavior are attributed to the formation of a thin surface layer of ZnAl2O4 that results from a reaction-diffusion process between the ZnO and Al2O3. This layer can provide an improved lattice match to GaN as well as change in surface energy affecting the initial growth of the GaN.
Improved temperature performance of Al0.33Ga0.67As/GaAs quantum-cascade lasers with emission wavelength at λ≈11 μm
Authors: Kruck P. , Page H. , Sirtori C. , Barbieri S. , Stellmacher M. , Nagle J.
Published: July 2002

The pulsed operation of a GaAs/AlGaAs quantum-cascade laser is reported up to 258 K. These devices emit at 11.3 μm and are based on a plasmon-confinement waveguide. To optimize the material gain, the active region is designed to diminish electron escape to continuum states. Gain and threshold measurement show evidence of better carrier confinement and improved thermal behavior compared to λ≈9 μm GaAs quantum-cascade lasers. The maximum peak-collected power at 77 K is 520 mW per facet and still 27 mW at 258 K. The temperature dependence of the threshold current density is characterized by a T0=128 K.
Charge collection mapping of the back-transfer process in Er-doped silicon
Authors: Castaldini A. , Cavallini A. , Fraboni B. , Pizzini S.
Published: July 2002

Charge collection analyses have been carried out on Er-doped silicon to investigate the excitation and de-excitation mechanisms of the Er3+ ion related to the λ=1.54 μm luminescence. Carrier recombination and trapping at the defective states induced in the material by the presence of Er play a significant role in the excitation of the Er3+ ion and in its nonradiative decay by the energy back-transfer process. We have obtained a two-dimensional map of the lattice sites where the back-transfer process occurs, and provided experimental proof of the cooperation of two different defects in the excitation and de-excitation processes.
Atomic force microscopy study of erythrocyte shape and membrane structure after treatment with a dihydropyridinic drug
Authors: Girasole M. , Cricenti A. , Generosi R. , Congiu-Castellano A. , Boffi F. , Arcovito A. , Boumis G. , Amiconi G.
Published: July 2002

The overall shape and membrane surface of human erythrocytes (RBCs) in the presence of nifedipine (a dihydropyridinic drug used in the clinical treatment of hypertension and angina pectoris) were imaged by contact-mode atomic force microscopy. Nifedipine induces in RBCs relevant morphological changes the extent of which increases as a function of drug concentration and incubation time. The modifications have been interpreted as mainly due to insertion of nifedipine into the outer layer of the RBC membrane. The potential effect of nifedipine as a hemoglobin denaturant has been ruled out by x-ray absorption near-edge structure and optical spectroscopies.
Plane wave excited vortices in photorefractive materials
Authors: Solymar L. , Kalinin V.
Published: July 2002

The flow of spatially periodic current in a photorefractive film is investigated under the conditions when two coherent plane waves are incident and an electric field is applied. It is shown that in contrast to the usually accepted physical picture the major part of the current does not flow in a simple manner from one boundary to the other boundary. Two examples are given: (i) transients in the perpendicular configuration (grating vector perpendicular to the direction of the applied field) with vertical diffusion neglected and (ii) steady state in the parallel configuration (grating vector parallel with the applied electric field) with vertical diffusion included. The flow of current is plotted with the aid of vector diagrams. It is clearly shown that during transients current may flow from one point on the boundary to another point on the same boundary, and that for the parallel configuration circular currents may flow in the steady state.
Effects of heat treatment on diffusion of Cu atoms into CdTe single crystals
Authors: Soo Y. L. , Huang S. , Kim S. , Kioseoglou G. , Kao Y. H. , Compaan A. D. , Grecu D. , Albin D.
Published: July 2002

Angular dependence of x-ray fluorescence and x-ray absorption fine structure techniques have been used to study the diffusion of Cu atoms into the photovoltaic material CdTe. Depth profile, effective valency, and local structure of Cu atoms in a Cu-doped single crystal of CdTe were investigated before and after a second heat treatment. Enhanced Cu diffusion into the CdTe single crystal was observed as a result of heating at a moderate temperature around 200 °C, resulting in a redistribution of the Cu impurities through a broader depth profile. Some of the Cu atoms are believed either to form small complexes with Te or occupy interstitial sites in the host but accompanied by a large local lattice distortion while others substitute for Cd on the cation sites. The results thus demonstrate that these nondestructive x-ray characterization methods are useful for probing microstructural changes in CdTe photovoltaic materials/devices in which some Cu-containing compounds are used as back contacts.
Photoemission spectroscopic investigation on the interface formation of a ladder-type poly(para-phenylene) with aluminum
Authors: Koch N. , Pairleitner R. , Le Q. Toan , Forsythe E. W. , Gao Y. , Leising G.
Published: July 2002

The formation of the interface between a thin film of a ladder-type poly(para-phenylene) m-LPPP and aluminum was investigated with x-ray and ultraviolet photoemission spectroscopy. The physical properties of this interface are of actual interest as m-LPPP is successfully applied as active material in organic light emitting devices (LED). Almost no changes in the core-level and valence electronic structure of the polymer upon increasing coverage with aluminum (in situ) are found. This gives indication for a weak interaction of Al with m-LPPP, ruling out the formation of chemical bonds between the two materials. The rapid occurrence of metallic aluminum at rather low coverage in the presented experiment is an important finding for the understanding of charge injection and the interfacial electronic structure in organic LEDs.
High-energy He-ion irradiation-induced defects and their influence on the noise behavior of Pd/n-Si1−xGex Schottky junctions
Authors: Mamor M. , Ouacha H. , Willander M. , Auret F. D. , Goodman S. A. , Ouacha A. , Sveinbjörnsson E.
Published: July 2002

We report on the electrical properties of defects introduced by high-energy 5.4 MeV He ions in n-type strained n-SiGe and the impact of this irradiation on the noise properties of Pd/n-Si1−xGex Schottky barrier diodes (SBDs). From the deep level transient spectroscopy measurements, the main defects EA1 and EA2 are observed in both Si and Si0.96Ge0.04 and have energy levels at 0.24 and 0.44 eV, respectively, below the conduction band. EA1 and EA2 have been correlated with the V–V and the P–V pairs, respectively. For both defects EA1 and EA2, the energy level position is found to be the same for x=0 and 0.04, indicating that such levels are pinned to the conduction band. Furthermore, the impact of the high-energy He-ion irradiation on the electrical noise properties of Pd/n-Si1−xGex SBDs is also studied. From the noise experimental data, the main noise source observed in these irradiated diodes was attributed to the generation-recombination noise inducing an abnormal peak in their noise spectra at around f1=180 Hz. This peak is found to be independent of Ge concentration.
Linear and nonlinear optical properties of erbium-implanted coherent array of submicron silica spheres
Authors: Ajgaonkar M. , Zhang Y. , Grebel H. , Sosnowski M. , Jacobson D. C.
Published: July 2002

A coherent array of silica spheres, which was implanted with erbium ions, showed nonlinear behavior at λ=0.532 μm. This was attributed to a large nonlinear refraction effect, which was enhanced by the structure of the opaline matrix. In addition, near infrared photoluminescence measurements showed an overall enhancement as a function of the sphere’s size. A large photoluminescence peak was found around λ=900 nm and attributed to the lensing effect of the opaline matrix.
Optical properties of InAs quantum dots formed on GaAs pyramids
Authors: An Haiyan , Motohisa Junichi
Published: July 2002

Optical properties of InAs quantum dots on GaAs pyramidal structures formed by selective area metal organic vapor phase epitaxy have been studied with low-temperature excitation powerdependent microphotoluminescence (μ-PL) and microphotoluminescence excitation (μ-PLE) spectroscopy. The power-dependent μ-PL spectra show well-defined ground and excited states whose interlevel energy spacings are estimated to be about 21 meV. These experimental results demonstrate the discrete nature of zero-dimensional density of states in position and number controlled InAs quantum dots (QDs) selectively grown on GaAs pyramids. We have also observed relative strong PLE peaks which are ascribed to the multiple longitudinal optical phonon resonance. Excited states and carrier relaxation of InAs QDs formed on GaAs pyramids are discussed.
Spectroscopic measurements on the Andreev reflection probability as a function of temperature
Authors: Appenzeller J. , Jakob M. , Stahl H. , Knoch J. , Lengeler B.
Published: July 2002

The temperature dependence of the Andreev reflection coefficient A(E,T) at a superconductor/normal-metal interface is a key issue for the critical current in a Josephson field-effect transistor at finite temperature. In this letter, we discuss our experimental observations of A(E,T) as a function of temperature determined by point contact spectroscopy. In addition, we point out major discrepancies between our findings and predictions from different theoretical models.
Single-crystalline silicon lift-off films for metal–oxide–semiconductor devices on arbitrary substrates
Authors: Tilke A. , Rotter M. , Blick R. H. , Lorenz H. , Kotthaus J. P.
Published: July 2002

We present a technique to mount single-crystalline silicon thin films on arbitrary substrates. We demonstrate in detail the preparation of a 190-nm-thin silicon metal–oxide–semiconductor field-effect transistor (MOSFET) on a silicon-on-insulator film lifted from its substrate and bonded to quartz. Functioning of this hybrid MOSFET on a rigid surface at room temperature is demonstrated.
Identification of vacancy charge states in diffusion of arsenic in germanium
Authors: Vainonen-Ahlgren E. , Ahlgren T. , Likonen J. , Lehto S. , Keinonen J. , Li W. , Haapamaa J.
Published: July 2002

Diffusion of As into Ge from a GaAs overlayer deposited on p-type Ge substrates has been studied by means of secondary ion mass spectrometry. A concentration-dependent diffusion of As atoms was observed in addition to the concentration-independent diffusion of Ga and As atoms. The concentration dependence is explained by a Fermi-level-dependent diffusion model. Arsenic atoms are shown to diffuse through Ge vacancies with the charge states 2− and 0. No presence of the singly negatively charged vacancies was observed, indicating that Ge vacancy could be a negative U center.
Simple accurate coupled cluster results for the linear E⊗e pseudo-Jahn–Teller effect
Authors: Bishop R. F. , Davidson N. J. , Quick R. M. , van der Walt D. M.
Published: July 2002

Using the coupled cluster method (CCM), we present a simple accurate calculation for the energies of the ground- and first excited states of the linear E⊗e Jahn–Teller and pseudo-Jahn–Teller Hamiltonians. From the solution of a single transcendental equation, we obtain results with a maximal error of 1.2%. These results are notably better than previous results obtained both via the CCM and other many-body approximations.
On mathematical structure of effective observables
Authors: Viazminsky C. P. , Vary James P.
Published: July 2002

We decompose the Hilbert space of wave functions into two subspaces, and assign to a given observable two effective representatives that act in the model space. The first serves to determine some of the eigenvalues of the full observable, while the second serves to determine its matrix elements, in any basis in one of the subspaces, in terms of quantities pertaining to the model space. We also show that if the Hamiltonian of a physical system possesses symmetries then these symmetries continue to hold for its effective representatives of the first type. Maximum information about the system can be obtained in terms of two sets of effective representatives. The first set of representatives is complete. Other observables that do not commute with all members of the complete set have only one type of representative.
Viscosity of glasses near and below the glass transition temperature
Authors: Kobayashi Hiroshi , Takahashi Haruyuki , Hiki Yosio
Published: July 2002

The temperature dependence of the viscosity of metaphosphate glasses: (AgI)x(AgPO3)1−x (x=0.0–0.5) with low melting points has been determined near and below the glass transition temperature Tg using a method developed by the authors. In this temperature region, the viscosity–temperature relation of the glasses shows double Arrhenius equations, which have different activation energies near and below Tg, respectively. When an Arrhenius equation is applied to the observed α transition process near Tg for the glasses, the activation energy Eα appears to be proportional to Tg. The relation obtained for the glasses is Eα(kJ/mol)=(154±30)RTg(K) (R is gas constant).
Photoacoustic characterization of n-RuSe2 semiconductor pellets
Authors: Bernal-Alvarado J. , Vargas-Luna M. , Solorza-Feria O. , Mondragón R. , Alonso-Vante N.
Published: July 2002

Using the open photoacoustic cell technique, a set of physical properties for a semiconductor (n-RuSe2) in powder form was measured. According to the Dramicanin theoretical model, charge carriers transport parameters (bulk and surface) and the effective thermal diffusivity were obtained.
Evaluation of two-electron integrals for explicit r12 theories
Authors: Valeev Edward F. , Schaefer Henry F.
Published: July 2002

We present a practical scheme for the evaluation of nonstandard two-electron molecular integrals that appear in ab initio theories employing explicitly correlated wave functions with linear r12 terms (“linear r12” methods). In contrast with previous efforts, the target integrals are evaluated recursively via intermediates formulated solely in terms of Cartesian Gaussian functions. All working equations fit conveniently the framework of highly efficient Head-Gordon and Pople method of evaluation of electron repulsion integrals. Thus, only straightforward modifications of existing codes that employ HGP or HGP–PRISM scheme are necessary to implement our approach. High potential of the pathway is realized in a robust practical implementation.
Isomeric structures and energies of Hn+ clusters (n=13, 15, and 17)
Authors: Barbatti M. , Jalbert Ginette , Nascimento M. A. C.
Published: July 2002

Ab initio calculations have been performed for the Hn+ clusters (n=3–17; odd) at Møller–Plesset second order (MP2)/6-311G(mp), Møller–Plesset complete fourth order (MP4)/6-311G(mp), and coupled-cluster single-double-triple [CCSD(T)/6-311G(1p)] levels of calculations. Such hydrogen clusters are constituted by an H3+ core in which H2 units are bound. In order to understand the features of these bindings, enthalpy and entropy variations upon cluster formation, binding energies, and charge distributions have been computed, and a molecular orbital analysis, based on localized orbital, was performed. Our results show that the way the first three H2 units bind to the H3+ core is fundamentally different from the others, providing an explanation for the binding energies observed for these molecules. For the H13+, H15+, and H17+ clusters, the way in which the external H2 units are distributed around the H3+ plane leads to the formation of different isomers with very close energies, but with a rotational barrier large enough to inhibit the interconversions.
Structural analysis of the cyclic AlO2 and AlS2 systems in doublet and quartet states at density functional theory and the electron correlation levels
Authors: Bu Yuxiang , Song Xinyu
Published: July 2002

The geometries and the bonding properties have been predicted for cyclic AlO2 and AlS2 species in doublet and quartet states using density functional theory, the second, third, and fourth orders Moller–Plesset theory, quadric configuration interaction singles and doubles including a perturbational estimate of the triples and coupled cluster singles and doubles including a perturbational estimate of the triples all-electron correlation methods with 6-311+G* and aug-cc-pvtz basis sets. The geometrical optimizations and the harmonic vibrational frequency analysis are performed using density functional theory and coupled cluster singles and doubles methods. The relevant energy quantities are also determined using several high-order electron correlation methods (the second, third, and fourth orders Moller–Plesset theory, quadric configuration interaction, and coupled cluster theories) at both basis set levels (6-311+G* and aug-cc-pvtz). For the doublet state, each species possesses a A22 ground state with a higher energy level A12 state. The corresponding state–state separations are 11 kcal/mol for AlO2 species and 7.2 kcal/mol for AlS2 species at coupled cluster singles and doubles including a perturbational estimate of the triples and 6-311+G* level. The calculations using quadric configuration interaction and coupled cluster singles and doubles including a perturbational estimate of the triples yield dissociation energies in three dissociation mechanisms of ∼59, ∼190, and ∼294 kcal/mol for AlO2(2A2), and of ∼64, ∼167, and ∼272 kcal/mol for AlS2(2A2), respectively, and other methods [B3LYP, B3P86, B3PW91, Moller–Plesset (n=2,3,4), quadric configuration interaction and coupled cluster singles and doubles] yield dissociation energies within ∼4.5 kcal/mol. For the quartet states, the B14 state is more stable than the B24 state with energy separations of 43.5 kcal/mol for AlO2 and 29 kcal/mol for AlS2. The B14 and B24 states are significantly higher in energy than the ground states by 28.9 kcal/mol (B14) and 57.9 kcal/mol (B24) for AlS2, and 24.2 kcal/mol (B14) and 67.8 kcal/mol (B24) for AlO2. Result analysis has indicated that the cyclic AlO2 in the A22 and B24 states should be classified as superoxides, but they have different spin density distribution. However, AlO2 in the A12 state should not be, while AlO2 in the B14 state may be classified as the dioxide. The AlS2 species in the A22 state should be classified as a supersulfide. Although the A12 state has some supersulfide character, it should not be classified as such. The AlS2 in the B24 and B14 states should be classified as the weak interaction molecular complex and the disulfides, respectively. However, these superoxides and supersulfides are far less ionic than the corresponding alkali metal superoxides.
Use of necked-down areas to control nucleation site and direction of solidification of polycrystalline silicon using excimer laser crystallization
Authors: Hara Akito , Sasaki Nobuo
Published: July 2002

We successfully controlled the nucleation site and direction of solidification of polycrystalline silicon films by irradiation with an excimer laser. An amorphous silicon island, including a gradually narrowing region, was formed on a quartz glass substrate. The gradually narrowing region intentionally included a necked-down area. The sidewall and top of the amorphous silicon island were covered by a thick polycrystalline silicon film. A single pulse of excimer laser irradiation from the back through the quartz glass substrate was used to melt the island. This method resulted in lateral growth from the inner region of the island toward the outer region, at the edges of the island. When the width of the necked-down area was twice the distance of the lateral growth, only one nucleus was formed in the necked-down region. Solidification from the nucleus toward the region with the narrower width then occurred in an area 2 μm wide and 3 μm long. This method enabled position-controlled formation of large grains.
Crystallization kinetics and structural aspects of TeGaSn amorphous alloys
Authors: Fontana M. , Arcondo B. , Clavaguera-Mora M. T. , Clavaguera N. , Greneche J. M.
Published: July 2002

Cast and rapidly quenched samples of ternary system GaTeSn in the Te rich corner were studied by means of x-ray diffraction, Sn119 Mössbauer spectrometry at 77 K, and differential scanning calorimetry. The crystallization products, the activation energy, the crystallization enthalpy, and the crystallization and glass temperatures of the amorphous phases are reported. The addition of Sn to GaTe increases the glass forming range of the binary system and the thermal stability of the amorphous phase. In the crystalline samples the divalent Sn atoms are always surrounded by Te in octahedrally coordinated environments. It is observed that the addition of Sn, above a threshold concentration, stabilizes the high temperature phase Ga2Te5 at room temperature. The Mössbauer spectra obtained on amorphous samples indicate two kinds of environments at Sn sites: one ascribed to the octahedral coordination, the other one to Sn surrounded by Te with tetrahedral coordination. The Mössbauer results are related to the calorimetric measurements: the role of Sn to stabilize the amorphous structure is discussed. The first steps of the crystallization of pure Te are modeled assuming homogeneous nucleation followed by three-dimensional interface-controlled growth of the crystal.
Anomalous strains in the cubic-phase GaN films grown on GaAs (001) by metalorganic chemical vapor deposition
Authors: Xu D. P. , Wang Y. T. , Yang H. , Li S. F. , Zhao D. G. , Fu Y. , Zhang S. M. , Wu R. H. , Jia Q. J. , Zheng W. L. , Jiang X. M.
Published: July 2002

Strains in cubic GaN films grown on GaAs (001) were measured by a triple-axis x-ray diffraction method. Residual strains in the as-grown epitaxial films were in compression, contrary to the predicted tensile strains caused by large lattice mismatch between epilayers and GaAs substrates (20%). It was also found that the relief of strains in the GaN films has a complicated dependence on the growth conditions. We interpreted this as the interaction between the lattice mismatch and thermal mismatch stresses. The fully relaxed lattice constants of cubic GaN are determined to be 4.5038±0.0009 Å, which is in excellent agreement with the theoretical prediction of 4.503 Å.
Reconstruction of an InP(001) surface grown by metalorganic vapor phase epitaxy in atmospheric hydrogen environment
Authors: Kawamura T. , Watanabe Y. , Utsumi Y. , Uwai K. , Matsui J. , Kagoshima Y. , Tsusaka Y. , Fujikawa S.
Published: July 2002

A reconstructed surface of InP (001) substrate, grown by metalorganic vapor phase epitaxy under atmospheric hydrogen environment, is investigated by using grazing incident x-ray diffraction. Fractional-order diffractions of (n/2 m) were observed, showing the existence of a (2×1) domain on the surface. Calculations based on the P-dimer model suggest that there are P dimers whose bonding is parallel to the [1̄10] direction and indium displacement in the second layer.
Electron cyclotron resonance plasma etching of GaSb-based alloys
Authors: Ahmad R. U. , Nagy G. , Osgood R. M. , Turner G. W. , Manfra M. J. , Chludzinski J. W.
Published: July 2002

Electron cyclotron resonance plasma etching is used to fabricate submicrometer-scale GaInAsSb/AlGaAsSb multiple-quantum-well structures. Smooth and anisotropic features at low substrate bias were obtained under appropriate conditions. The etch quality was investigated with photoluminescence spectroscopy; luminescence data from the etched features agree well with a model that assumes a low-damage etching process.
Charge transport in tri-p-tolylamine doped trinaphthalylbenzene glass
Authors: Lin Liang-Bih , O’Reilly James M. , Magin Edward H. , Weiss David S. , Jenekhe Samson A.
Published: July 2002

The charge transport properties of tri-p-tolylamine (TTA) doped trinaphthalylbenzene have been measured as a function of electric field and temperature. The charge mobilities of the composite are comparable to but somewhat lower than that of TTA doped polystyrene, a nonpolar polymeric host, at similar weight fractions. We suggest that the difference is due to inhomogeneity between the host and the dopant. The results suggest that, similar to polymer hosts in molecularly doped polymers, the molecular host only functions as an inert diluter and does not directly participate in the charge transport manifold. The results also substantiate the importance of molecular packing to charge hopping in disordered organic materials. The charge mobility data are analyzed with a disorder model due to Bässler and coworkers and a recently modified expression due to Novikov and coworkers [Phys. Rev. Lett. 81, 4472 (1998)]. Both models provide adequate descriptions of charge transport in organic amorphous materials.
Bias-assisted photoelectrochemical etching of p-GaN at 300 K
Authors: Borton J. E. , Cai C. , Nathan M. I. , Chow P. , Van Hove J. M. , Wowchak A. , Morkoc H.
Published: July 2002

Photoelectrochemical (PEC) etching of p-type GaN has been realized in room temperature, 0.1 M KOH solutions. PEC etching of GaN was achieved by applying a positive bias to the surface of the p-GaN layer through a deposited titanium mask. The applied bias reduces the field at the semiconductor surface, which induced the dissolution of the GaN. The effect of bias on etch rate and morphology was examined. It was found that insulating the Ti mask from the KOH solution with Si3N4 significantly increases the etch rate. The rms roughness of the etched region decreased as the bias voltage increased. Etch rates as high as 4.4 nm/min were recorded for films etched at 2 V.
Intrinsic and coupling-induced elastic nonlinearity of lanthanum-doped lead magnesium niobate–lead titanate electrostrictive ceramic
Authors: Jiang Wenhua , Cao Wenwu
Published: July 2002

The elastic nonlinearity of lanthanum-doped lead magnesium niobate–lead titanate electrostrictive ceramic has been investigated by using ultrasonic second-harmonic generation technique and field-induced strain measurement. A large ultrasonic nonlinearity parameter, β=30, was observed at room temperature, which is more than twice of the values for lead zirconate–lead titanate piezoelectric ceramics. The third-order elastic constant c111 has been calculated from the β value. By introducing an effective nonlinearity parameter βe that includes both the intrinsic and the electromechanical coupling generated elastic nonlinearities, the nonlinearity parameter can be accurately characterized. The strong field dependence of the nonlinearity parameter shows excellent potential for designing field tunable nonlinear devices using this material.
Far-infrared and selective photoluminescence studies of shallow donors in GaN hetero- and homoepitaxial layers
Authors: Neu G. , Teisseire M. , Frayssinet E. , Knap W. , Sadowski M. L. , Witowski A. M. , Pakula K. , Leszczynski M. , Prystawko P.
Published: July 2002

Shallow donors in GaN epilayers grown by metalorganic chemical vapor deposition on sapphire and GaN substrates have been studied by selectively excited photoluminescence (SPL) and by far-infrared (FIR) spectroscopy. A comparison of FIR and SPL results reveals a small splitting between the 2s and 2p donor states, interpreted as partly being due to the nonspherical crystal symmetry. The utilization of the selective excitation of neutral donor bound excitons I2 allows the identification of the I2 rotational excited states. An interpretation of two-electron spectra in GaN involving excited states of I2 is proposed.
Zen diamond-anvil low-pressure cell
Authors: van Uden N. W. A. , Dunstan D. J.
Published: July 2002

A diamond-anvil cell can be operated with only one anvil in order to generate modest pressures in relatively large volumes. We demonstrate it to pressures up to 2.5 GPa with gaskets of steel, brass, and other metals, with a sample chamber 0.25 mm in diameter by 0.25–0.9 mm depth, and with various pressure media. In this form the cell is very simple to operate and is useful for much work on biological systems and soft solids which requires pressures in the 1 GPa range.
Reaction of Co and capping layers and its effect on CoSi2 formation in Si/SiOx/Co system
Authors: Kim Gi Bum , Kwak Joon Seop , Baik Hong Koo , Lee Sung Man , Oh Sang Ho , Park Chan Gyung
Published: July 2002

The role of the reaction between the capping layer and Co on the crystalline nature of CoSi2 films in refractory metal-capped Si/SiOx/Co system has been investigated. The epitaxial CoSi2 film was obtained in the capping layers (Ti, Zr) with high tendency of mixing between Co and the capping layer. Amorphous Ti–Co layer was produced at 450 °C, and its thickness was increased at 550 °C. The formation of amorphous Ti–Co layer during low-temperature annealing may be responsible for the formation of epitaxial CoSi2. Meanwhile, the polycrystalline CoSi2 was formed in the capping layer (Cr, Mo) with low tendency of mixing. These results can be explained by the fact that the mixing layer formed from the reaction between Co and refractory metal control the Co diffusion to the Si substrate as well as the thin SiOx between Co and Si.
Scanning aperture photoemission microscopy for magnetic imaging
Authors: McClelland Gary M. , Rettner Charles T.
Published: July 2002

Magnetic imaging has been demonstrated by photoemission microscopy in which spatial resolution is achieved by scanning an aperture across the sample. To achieve magnetic contrast, the difference between photoemission current with right and left circularly polarized light is recorded while holding the aperture-surface distance to 30 nm. We used conical gold tips with apertures as small as 30 nm, drilled by a focused ion beam. Images of Co/Pt multilayer films show a magnetic contrast of ±2% and a resolution (25%–75% of a step) of 30 nm. Resolution better than 10 nm should be achievable with smaller apertures. With this imaging method, there is no magnetic interaction between sample and tip, and ultrafast time resolution should be achievable.
Current mode Schmitt trigger for a phototransistor/resistor detector
Authors: Liao Tai-Shan , Chang Chun-Ming
Published: July 2002

A current mode Schmitt trigger for a phototransistor/resistor detector is presented. It can be successfully operated with a 1.5 V power supply.
Scanning eddy current dynamometer with 100 μm resolution
Authors: Palmer B. S. , Drew H. D. , Decca R. S.
Published: July 2002

We report on a new contactless scanning probe technique to probe thin metallic films. The scanning eddy current dynamometer (SECD) is based on the measurement of the force generated by a magnetic tip oscillating in close proximity to a conducting thin film glued to a high-Q mechanical oscillator. By measuring the resonant motion induced in the mechanical oscillator, the eddy current force on the sample can be determined. The size of the magnetic field profile from our tip limits the spatial resolution of the instrument to ≈ 100 μm. This spatial resolution is demonstrated by scanning the magnetic tip over inhomogeneities in metallic films. For a homogeneous metallic film much larger than 100 μm the force measured by the SECD is directly proportional to the conductance of the film. In this configuration the equivalent noise conductance of the eddy current dynamometer is Δσ□≈30 Ω−1. By exploiting the boundary conditions on the induced current density in the conducting film, cracks much smaller than 100 μm in a metallic film can be detected. To demonstrate this property, a 2.5 μm wide slit is detected using the SECD.
A new device for dynamic sampling of radon in air
Authors: Lozano J. C. , Escobar V. Gómez , Tomé F. Vera
Published: July 2002

A new system is proposed for the active sampling of radon in air, based on the well-known property of activated charcoal to retain radon. Two identical carbon-activated cartridges arranged in series remove the radon from the air being sampled. The air passes first through a desiccant cell and then the carbon cartridges for short sampling times using a low-flow pump. The alpha activity for each cartridge is determined by a liquid scintillation counting system. The cartridge is placed in a holder into a vial that also contains the appropriate amount of scintillation cocktail, in a way that avoids direct contact between cocktail and charcoal. Once dynamic equilibrium between the phases has been reached, the vials can be counted. Optimum sampling conditions concerning flow rates and sampling times are determined. Using those conditions, the method was applied to environmental samples, straightforwardly providing good results for very different levels of activity.
Si/SiGe electron resonant tunneling diodes
Authors: Paul D. J. , See P. , Zozoulenko I. V. , Berggren K.-F. , Kabius B. , Holländer B. , Mantl S.
Published: July 2002

Resonant tunneling diodes have been fabricated using strained-Si wells and strained Si0.4Ge0.6 barriers on a relaxed Si0.8Ge0.2 n-type substrate, which demonstrate negative differential resistance at 298 K. Peak current densities of 5 kA/cm2 with peak-to-valley current ratios of 1.1 have been achieved. Theoretical modeling of the structure demonstrates that the major current peak results from the tunneling of light-mass electrons from the relaxed substrate and not from the heavy-mass electrons in the emitter accumulation layer.
Control of the residual doping of InAs/(GaIn)Sb infrared superlattices
Authors: Bürkle L. , Fuchs F. , Schmitz J. , Pletschen W.
Published: July 2002

Magnetotransport and photoluminescence (PL) measurements on InAs/(GaIn)Sb superlattices (SLs) grown by molecular-beam epitaxy on GaSb substrates at different substrate temperatures are reported. With increasing growth temperature, a transition of the SLs from residual n type to residual p-type doping was observed. For n-type samples, a decrease in the electron concentration leads to a strong increase in the PL intensity. In contrast, the PL intensity of p-type samples is only weakly dependent on the hole concentration. This correlation can be used to control the residual doping of the SLs.
Magnetoresistance effect in the fluctuating-valence BaSmFe2O5+w system
Authors: Nakamura J. , Lindén J. , Karppinen M. , Yamauchi H.
Published: July 2002

The occurrence of negative magnetoresistance (MR) in semiconductive BaSmFe2O5+w double-perovskite samples is demonstrated. A peak in the MR value was observed at the Verwey–type transition temperature. The transition signifies the charge separation of the Fe2.5+ fluctuating mixed valence state into high-spin Fe2+ and Fe3+. The samples were ferrimagnetic with a Curie temperature of ∼710 K. Upon oxidizing/reducing the samples the size of the MR peak and the temperature at which the peak occurred varied. The largest MR value observed was 1.4% at 7 T.
Response to “Comment on ‘Analysis of asymmetric giant magnetoimpedance in field-annealed Co-based amorphous ribbon’ ” [Appl. Phys. Lett. 77, 1727 (2000)]
Authors: Kim Cheol Gi , Jang K. J. , Kim D. Y. , Yoon S. S.
Published: July 2002

In a Comment [D. X. Chen, L. Pascual, and A. Hernando, Appl. Phys. Lett. 77, 1727 (2000)] on our recent letter [C. G. Kim, K. J. Jang, D. Y. Kim, and S. S. Yoon, Appl. Phys. Lett. 75, 2114 (1999); 76, 1345 (2000)] Chen et al. claimed that the unidirectional anisotropy due to bias field is unphysical one for the description of asymmetric giant magnetoimpedance (GMI) profiles. The symmetric two peaks of GMI profiles measured in the normal sample with an uniaxial anisotropy, allow one to take the minimum energy condition which assumes a jump of magnetization under the field from a metastable state to a stable one. However, the divergence in a calculated GMI profile should appear even in case of a uniaxial anisotropy of normal sample where there is no jump. Divergence indicates the asymmetry and hysteresis in GMI profile. The analysis of this calculation in Chen et al.’s Comment is simply a matter of hysteresis in GMI profile for the increasing and decreasing field, even in a normal sample with uniaxial anisotropy. Even though the hysteresis is ignored by taking the minimum energy condition, the asymmetric profiles with the negligible hysteresis are well ascribed by the model with two kinds of anisotropy fields, as proposed in our previous letter [C. G. Kim, K. J. Jang, D. Y. Kim, and S. S. Yoon, Appl. Phys. Lett. 75, 2114 (1999); 76, 1345 (2000)]. In this model the bias field is quite physical, and is based on the observed experimental results in a specially prepared sample.
Determination of internal optical mode loss of semiconductor laser diodes
Authors: Wu Linzhang
Published: July 2002

This letter proposes a technique to accurately determine the internal optical mode loss of individual semiconductor laser diode for each given current, which is simple and based on measurements of spontaneous emission spectra. Therefore, it can be used to study the dependence of loss on temperature, current, and carrier density.
A sub-Doppler resolution double resonance molecular beam infrared spectrometer operating at chemically relevant energies (∼2 eV)
Authors: Srivastava H. K. , Conjusteau A. , Mabuchi H. , Callegari A. , Lehmann K. K. , Scoles G.
Published: July 2002

A molecular beam spectrometer capable of achieving sub-Doppler resolution at 2 eV (∼18 000 cm−1) of vibrational excitation is described and its performance demonstrated using the CH stretch chromophore of HCN. Two high finesse resonant power-buildup cavities are used to excite the molecules using a sequential double resonance technique. A v=0→2 transition is first saturated using a 1.5 μm color center laser, whereupon a fraction of the molecules is further excited to the v=6 level using an amplitude modulated Ti:Al2O3 laser. The energy absorbed by the molecules is detected downstream of both excitation points by a cryogenically cooled bolometer using phase sensitive detection. A resolution of approximately 15 MHz (i.e., three parts in 108) is demonstrated by recording a rotational line in the v=6 manifold of HCN. Scan speeds of up to several cm−1/h were obtained, with signal-to-noise ratios in excess of 100. The high signal-to-noise ratio and a dynamic range of 6×104 means that future experiments to study statistical intramolecular vibrational energy redistribution in small molecules and unimolecular isomerizations can be attempted. We would also like to point out that, with improved metrology in laser wavelengths, this instrument can also be used to provide improved secondary frequency standards based upon the rovibrational spectra of molecules.
Coupled-flux nucleation modeling of oxygen precipitation in silicon
Authors: Wei P. F. , Kelton K. F. , Falster R.
Published: July 2002

Experimental data for oxygen precipitation densities in Czochralski-grown silicon following multistep annealing treatments are compared with predictions from a coupled-flux model for time-dependent nucleation. This is a more correct model for diffusion-controlled nucleation processes than is the classical theory of nucleation since it directly couples the two stochastic fluxes of interfacial attachment and long-range diffusion. Quantitative agreement is obtained between the measured and calculated densities for nucleation temperatures greater than 650 °C. Good agreement is obtained for lower temperatures if the oxygen diffusion rate is taken to be larger than is predicted from high-temperature diffusion data. The fit values for the diffusion coefficient from the nucleation data are in good agreement with recent results from dislocation-unlocking experiments. The oxygen loss calculated by coupled-flux nucleation and diffusion-limited growth agrees with the experimental observations. Classical theory nucleation calculations predict a much greater oxygen loss, signaling the failure of the theory to correctly treat nucleation when long-range diffusion is important, true in most solid-state precipitation processes.
High-brightness organic double-quantum-well electroluminescent devices
Authors: Huang Jingsong , Yang Kaixia , Liu Shiyong , Jiang Hongjin
Published: July 2002

An organic double-quantum-well structure electroluminescent device fabricated by a doping method is demonstrated. The device consists of N,N′-bis-(1-naphthl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) used as a hole transporter, undoped tris(8-quinolinolato) aluminum (Alq) as a barrier potential or electron transporter, and Alq doped with 5,6,11,12-tetraphenylnaphthacene (rubrene) as a potential well and emitter, and has the following structure: indium tin oxide/NPB/Alq:rubrene/Alq/Alq:rubrene/Alq/Mg/Al. The maximum brightness and efficiency reach 48 000 cd/m2 and 4.59 lm/W, respectively. The present double-quantum-well structure device shows higher brightness and higher efficiency than those of the common heterostructure devices.
Demonstration of near-field scanning photoreflectance spectroscopy
Authors: Paulson Charles , Ellis A. B. , McCaughan Leon , Hawkins Brian , Sun Jingxi , Kuech T. F.
Published: July 2002

A near-field scanning optical microscope (NSOM) was developed to perform photoreflectance (PR) spectroscopy experiments at high spatial resolution (∼1 μm). Representative PR spectra are shown, along with an image illustrating the capability of observing contrast in images due to the strength of a PR feature. It was found that sufficiently high intensity light from the NSOM tip can produce photovoltages large enough to limit the spatial resolution of the electric field determination by PR. The photovoltage effect is measured as a function of light intensity, and the results are discussed in terms of a simple photovoltage expression.
Stress-induced anisotropy of phosphorous islands on gallium arsenide
Authors: Li C. H. , Li L. , Fu Q. , Begarney M. J. , Hicks R. F.
Published: July 2002

The initial growth of (2×4) phosphorous islands on (4×2) terraces of gallium arsenide (001) has been studied. The islands grow anisotropically in the [110] direction with an aspect ratio of approximately 8 to 1 at moderate coverages. The distribution of island widths in the [1̄10] direction follows a Gaussian function. The mean width increases from 24±6 to 47±11 Å as the phosphorous coverage increases from 0.10 to 0.85 monolayers. Evidently, the island anisotropy is caused by stress imposed on the underlying gallium layer by the smaller, more tightly bound phosphorous dimers.
Comparative study of the analog performance of a vertical-cavity surface-emitting laser under gain and cavity loss modulation
Authors: Hudgings Janice A. , Stone Robert J. , Lim Sui F. , Lau Kam Y. , Chang-Hasnain Connie J.
Published: July 2002

The analog performance of a vertical-cavity surface-emitting laser is compared for gain and cavity loss modulation. Direct gain modulation consistently results in a larger spur free dynamic range (SFDR) than does cavity loss modulation; the maximum SFDR achieved is 96.5 dB Hz2/3. Analysis of the physical origins of variations in the measured SFDR with applied electrical bias indicates potential strategies for improving the analog response of these lasers.
Elliptically-deformed spherulitic crystallization observed in the formation of μm-sized periodic grating structures in photopolymerized prepolymer/liquid crystal material systems
Authors: Karasawa Takeshi , Taketomi Yoshinao
Published: July 2002

This paper reports the effects of an artificially introduced μm-sized periodic grating structure on the growth of spherulites in prepolymer/liquid crystal material systems between two glass substrates. The authors use the technique of forming the volume holographic grating to create well-defined obstacles having periodic structures. Spherulites grown in an ordinary polymer-dispersed liquid crystal (PDLC) environment are compared with spherulites in the holographic grating environment, and differences in shape are presented. Although spherulites grown under ordinary conditions for the formation of PDLC are characterized by a circular shape and well-developed radial striations, those grown in volume holographic gratings are elliptically deformed, oriented so that the long axis is in the direction of the gratings, and the major striations tend to be either parallel or perpendicular to the grating walls. These findings have been observed for different material systems, suggesting that the features are not simply material specific but more common to prepolymer/liquid crystal mixtures. Compared with other factors affecting the growth and shape of spherulites, a μm-sized periodic grating structure made of polymer-based materials acts not as a complete barrier but as a retarder, which causes directional variation in the movement of the growth front and leads to elliptical deformation in the growing spherulite. Based on the results of this study, the authors have proposed a model of the growth process of a spherulite in the grating environment.
Lateral gettering of iron by cavities induced by helium implantation in silicon
Authors: Roqueta F. , Ventura L. , Grob J. J. , Jérisian R.
Published: July 2002

Lateral gettering has been studied by introducing cavities in the periphery of large active devices. Cavities were induced by helium implantation followed by a thermal treatment on samples previously contaminated by iron. Those cavities are known to be efficient to trap metallic impurities in silicon by chemisorption. The iron distribution in samples of 6×6 mm2 area has been monitored by measuring current versus voltage characteristics and interstitial iron concentrations by deep level transient spectroscopy on Schottky diodes uniformly distributed. A symmetrical iron distribution has been observed with a decreasing concentration close to the gettering region. This lateral gettering is enhanced with increasing thermal budget. Extensions of several millimeters can be obtained allowing applications in power device technology.
Stiffness of compressed fiber mats
Authors: Åström J. A. , Mäkinen J. P. , Hirvonen H. , Timonen J.
Published: July 2002

We investigate, using an analytical and a numerical model, the in-plane stiffness of fiber mats. A mat is modeled by randomly depositing thin linear-elastic fibers on top of each other under the influence of an external pressure. The external pressure has the effect of bending the fibers over each other. The fibers are assumed rigidly bonded at contacts. For a low external pressure the stiffness of the mat deviates from that of its two-dimensional projection only by a geometrical factor, and the effective Poisson contraction is close to zero. For higher pressures, stiffness is governed by two competing effects and a maximum appears in the stiffness. The effective Poisson ratio is clearly negative in this range. An approximative analytical description is developed for the stiffness of mats formed under low external pressure. The stiffness is given as a function of only a few parameters: the degree of bonding, the dimensions of the fibers, the elastic constants of the fiber material, and the density of fibers.
Wavelength locking of multiple diode lasers by multiplexed gratings in a photorefractive crystal
Authors: Yang Changxi , Xiao Min
Published: July 2002

We propose a technique to simultaneously stabilize relative wavelengths among multiple diode lasers by using multiplexed photorefractive gratings. Each grating is designed to be Bragg matched for the counter-propagating reflection at a specific diode laser wavelength. Experimental results from stabilization of two diode laser wavelengths by using two photorefractive gratings exhibit excellent short- and long-term wavelength stability. We show that this technique can be extended to stabilize relative wavelengths of many diode lasers. The thermal stability of the photorefractive gratings and the signal-to-noise ratio (or the bit-error rate) are discussed.