bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic transport in periodically δ-doped GaAs layers
Authors: Egues J. C. , Barbosa J. C. , Notari A. C. , Basmaji P. , Ioriatti L. , Ranz E. , Portal J. C.
Published: July 2002

We investigate periodically silicon δ-doped GaAs sample grown by molecular-beam epitaxy using magnetoresistance measurements. Transport measurements give evidence for quantum confinement of the electronic gas in the layer. Thomas–Fermi approximation is used in order to assess the various subband densities.
Evaluation of erbium-doped silicon for optoelectronic applications
Authors: Xie Y. H. , Fitzgerald E. A. , Mii Y. J.
Published: July 2002

Best-case evaluations are made for potential optoelectronic applications of erbium-doped silicon (EDS). The objective is to find the upper limit of performance when EDS is used as light-emitting diodes, amplifiers/modulators, and lasers. Every effort is made to use intrinsic parameters whose values are determined by physics rather than by factors such as material quality and processing quality. Consequently, the result is expected to be overly optimistic, and should be regarded as a feasibility study only. It is shown that Er-doped Si is not suitable for light-emitting-diode applications because of the low emitted power (microwatts). The intensity amplifiers/modulators made of Er-doped Si can only be expected to provide a very modest gain (<6 cm−1). For laser applications, the threshold population inversion can be achieved in principle (assuming proper design and processing of the laser structure); however, a very efficient pumping mechanism is necessary for the laser to provide reasonable power output (of the order of mW/facet). Finally, a view on the direction of future research in this field is presented. Since the rare-earth ion luminescence is known to be fairly independent of the host materials, the results obtained from this study are expected to be applicable to most of the other rare-earth-doped semiconductors.
Modeling of dopant diffusion in silicon: An effective diffusivity approach including point-defect couplings
Authors: Mathiot Daniel , Martin Serge
Published: July 2002

We present a simplified model for dopant diffusion in silicon in the presence of nonequilibrium point defects. In this model, the dopant flux is expressed with an effective diffusivity, taking into account the various couplings arising from the presence of the defect gradients. The point-defect concentrations are calculated by the resolution of the corresponding continuity equations. This model allows fast and accurate simulations for the usual dopants (P, As, and B), and it is thus well adapted for use as a process-simulation tool.
Dislocations in mismatched layers of GaAsxP1−x in between GaP as observed by low-temperature cathodoluminescence: Part II. Grown on (111) oriented substrates
Authors: Gustafsson A. , Pistol M.-E. , Gerling M. , Samuelson L. , Titze H.
Published: July 2002

Spectrally resolved low-temperature cathodoluminescence (CL) imaging has been performed on thin, 250 Å, mismatched layers of GaAsxP1−x in between bulk GaP. The layers were grown on (111) oriented substrates by metalorganic vapor phase epitaxy, with layers ranging from perfectly strained to totally relaxed. CL imaging has proven to be a very sensitive technique for the study of the onset of the formation of misfit dislocations and is therefore useful for determination of the critical thickness [A. Gustafsson, M.-E. Pistol, M. Gerling, L. Samuelson, M. R. Leys and H. Titze, J. Appl. Phys. 70, 1660 (1991)]. For the use of perfectly strained layers, growth on (111) oriented substrates can be of interest since the critical thickness predicted by the mechanical equilibrium theory [J. W. Matthews and A. E. Blakeslee, J. Cryst. Growth 27, 118 (1974)] is about twice that for growth on (001) oriented substrates. In this work we show that the dislocations involved in the strain relief for the growth of mismatched layers on (111) oriented substrates are of the perfect 60° type and that the experimental critical thickness agrees well with the value expected from the mechanical equilibrium theory.
The characteristics of an In0.5Ga0.5P and In0.5Ga0.5P/GaAs heterojunction grown on a (100) GaAs substrate by liquid-phase epitaxy
Authors: Lee Jong Boong , Kwon Sang Deok , Kim In , Cho Yong Hoon , Choe Byung-Doo
Published: July 2002

The growth of a high-quality In0.5Ga0.5P/GaAs heterostructure on a (100) GaAs substrate by liquid-phase epitaxy is demonstrated. This has been achieved by controlling the vaporizing time of phosphorus after the melt saturation procedure. The photoluminescence spectra of In0.5Ga0.5P/GaAs heteroepitaxial layers show that the major residual acceptor impurity is either carbon or silicon. The measured values of the conduction-band discontinuity ΔEc and the fixed interface charge density σi for a In0.5Ga0.5P/GaAs heterostructure are 110 meV and 1×1011 cm−2, respectively. Only one electron trap with a thermal activation energy of Ea=0.32 eV, which is thought to be related to the anion vacancy, is found in Sn-doped In0.5Ga0.5P (n∼1×1017 cm−3) layers.
On the reduction of hole mobility in strained p-SiGe layers
Authors: Manku T. , Jain S. C. , Nathan A.
Published: July 2002

Using the secondary-ion mass spectrometry profile of the SiGe-base heterojunction bipolar transistor of Patton et al. [IEEE Trans. Electron. Dev. 11, 171 (1990)] we retrieve the hole drift mobility for strained Si1−xGex (SiGe) layers up to Ge fractions of 10% and for dopant levels in the range 1016–1019 cm−3. In contrast to theoretically calculated values, the results show a significant reduction in the hole mobility.
Structural and optoelectronic properties and their relationship with strain relaxation in heteroepitaxial InP layers grown on GaAs substrates
Authors: Olego D. J. , Okuno Y. , Kawano T. , Tamura M.
Published: July 2002

Heteroepitaxial layers of InP with thickness D ranging from 0.1 to 6.0 μm were grown by low-pressure metalorganic chemical vapor deposition on (001) surfaces of GaAs substrates. Their dislocation structure, induced strains, and nature of the radiative recombinations were investigated as a function of D with transmission electron microscopy, x-ray diffraction, and photoluminescence spectroscopy. For D<2 μm, the films are highly dislocated with a tangle of interfacial and threading dislocations above the heterointerface. The spatial extent of the interfacial dislocations and the density of threading dislocations increase with increasing D. For D≳2 μm the portion of the layers away from the heterointerface by more than 1.5 μm shows a decrease in the density of threading dislocations and a dramatic improvement in the crystalline quality with increasing D.

Typical dislocation densities in the neighborhood of the top surface are in the mid 107 cm−2 range when D surpasses 4.0 μm. Concomitant with the improved crystalline quality, the following observations are made. Firstly, the full width at half maximum of the x-ray rocking curves diminishes from values larger than 500 arcsec for D<1.0 μm to about 200 arcsec for D≳4.0 μm. Secondly, the near-band-edge photoluminescence transitions, which for D<2.0 μm are predominantly determined by defect-induced band tailing, display excitonic character. Thirdly, below-band-gap transitions due to interfacial defects decrease in intensity. Biaxial compressive strain is present in the layers because of lattice mismatch and differences in linear thermal expansion with the substrate.

The strain removes the degeneracy between the light- and heavy-hole states at the top of the valence band, and consequently with increasing temperature above 12 K recombinations from the conduction to the split valence bands are observed in the photoluminescence spectra for all D. The identification of such transitions follows from their temperature dependence and the activation energy yield for the thermalization of the holes. The measured valence-band splitting decreases from 12.5 meV for D=0.3 μm to saturation values of 5.6 meV for D≳3.0 μm, indicating strain relaxation with D in qualitative agreement with x-ray determinations. Quantitative differences between both methods are realized and are attributed to a temperature dependence of the differential linear thermal expansion. The contribution to the strain from the lattice mismatch is much larger than expected from equilibrium models. The dislocation generation at different stages during the growth is inferred from the strain relaxation against D and the observed location of the dislocations throughout the layers.
Intersubband relaxation dynamics in ternary/binary quantum wells: Role of the electron-optical phonon interaction
Authors: Joshi R. P.
Published: July 2002

Ensemble Monte Carlo calculations of the intersubband dynamics in binary-ternary double-heterostructure systems are presented. The presence of a ternary alloy has been explicitly incorporated to account for complexities arising from the multimode nature of phonons in real heterostructures. Electronic scattering rates are derived as a function of energy and quantum-well width for both confined and interface modes on the basis of a continuum model. Results of Monte Carlo simulations yield an intersubband time constant that is in reasonable agreement with the experimental value, but only when details of the phonon modes and their dispersion, spreading of the electronic wave functions due to poor confinement, and the phonon amplification effects are comprehensively included.
Differential photocurrent spectroscopy: A novel technique for semiconductor characterization
Authors: Tober Richard L. , Li W. Q. , Bhattacharya P. K.
Published: July 2002

Differential photocurrent spectra of various InxGa1−xAs diodes demonstrate that modulated photocurrent techniques can easily be used to characterize p-i-n and Schottky diode structures. The spectra were obtained in such a way as to produce a first derivative with respect to either the photon energy or an electric field. A simple model yields an expression for the differential photocurrent spectra in terms of the derivative functional form normally used to fit modulated spectra. Therefore, the sharp spectral features produced with these techniques can be used to determine not only band-gap energies and spectral linewidths but built-in electric fields as well.
Simulation of the step coverage for chemical vapor deposited silicon dioxide
Authors: Wille H. , Burte E. , Ryssel H.
Published: July 2002

A simulation procedure describing the low-pressure chemical vapor deposition (LPCVD) of silicon dioxide on structured wafer surfaces is introduced. Deposition and step coverage of LPCVD films can be understood by adsorption and desorption of a deposition precursor. Experiments are evaluated to verify the simulation and to determine the sticking coefficient at different conditions of deposition.
Two-dimensional numerical analysis of current blocking mechanism in InP buried heterostructure lasers
Authors: Seki Shunji , Yamanaka Takayuki , Yokoyama Kiyoyuki
Published: July 2002

The current blocking mechanism in InP buried heterostructure (BH) lasers is analyzed with a self-consistent, two-dimensional numerical analysis method. It has been clarified that quasi-Fermi potentials which are kept constant in the active region under lasing oscillation play a key role in determining the current blocking capability. Due to this potential pinning effect, the junction voltages at each constituent p-n junction in the blocking regions are maintained lower than the electrical turn-on voltage, which realizes an excellent current blocking in BH lasers.
Hall mobility and carrier concentration versus temperature for type IIa natural insulating diamond doped with boron by ion implantation
Authors: de la Houssaye Paul R. , Penchina Claude M. , Hewett Charles A. , Zeidler James R. , Wilson Robert G.
Published: July 2002

A natural IIa diamond was implanted with boron ions at a substrate temperature of 80 K. Ohmic contacts (Mo/Au) were formed using a solid-state reaction process. Van der Pauw resistivity/Hall measurements were taken as a function of temperature from room temperature to nearly 600 °C. Heating to approximately 350 °C was necessary to stabilize resistance values to over 3.4 MΩ/sq and 1.3 MΩ/sq for the unimplanted and implanted sides, respectively, indicating possible hydrogen incorporation into the diamond during annealing of the metallized sample in hydrogen. Comparison of the electrical properties of the implanted and unimplanted sides showed that the boron implant introduced electrically active acceptors.
Experimental observation of particle entrapment and transport in an intense ion beam
Authors: Brown D. A. , Sferlazzo P. , Beck S. E. , O’Hanlon J. F.
Published: July 2002

An experiment is described in which 1.5- to 3.0-μm-diam gold spheres are injected into the central region of a nearly square 27.4-keV, 9-mA argon ion beam with a cross-sectional area of approximately 3.3 cm2, and the results compared to theory. In the experiment, silicon wafers located 30 and 50 cm from the point of introduction served as targets to collect the particles. After injection, large numbers of gold-filled craters were found throughout and surrounding the central implant region on the wafers. Gravity and axial momentum transfer acting alone generate trajectories that would place all the particles well below the horizontal centerline of the implanted area. Consequently, particles landing above the central impact region can only be explained by the presence of an electrostatic confining force. Theoretical calculations based on an electrostatic trapping model predict particle distributions on the target that are in remarkable agreement with our experimental results. Thus, these observations provide conclusive evidence in support of beam particle entrapment and transport, phenomena that have heretofore remained unconfirmed.
Piezoelectric detection of the photoacoustic signals of n-type GaAs single crystals
Authors: Ikari T. , Miyazaki K. , Fukuyama A. , Yokoyama H. , Maeda K. , Futagami K.
Published: July 2002

Piezoelectric photoacoustic (PA) measurements on liquid-encapsulated-Czochralski-grown n-GaAs were carried out at room temperature. A continuous broad band below 1.35 eV and a peak at 1.383 eV were observed in the PA amplitude spectra. By comparing with the optical-absorption spectra, it is concluded that the broad band is due to the electron transition involving the EL2 deep-lying defect levels. For the observed peak at 1.383 eV, the origin is considered to be dislocation related. The possibility that this peak is an apparent one expected from the proposed models for the PA signal generation is denied.
Modeling of a new field-effect resonant tunneling transistor
Authors: Chen J. , Yang C. H. , Wilson R. A.
Published: July 2002

We present quantum mechanical self-consistent calculations on the transfer characteristics of a new resonant tunneling transistor. The model structure consists of a source, a tunneling barrier, a quantum-well drain, a thick insulator, and a backgate. The tunneling barrier consists of a double-barrier structure. We demonstrate that based on energy and momentum conservations, the transistors display oscillatory negative transconductance, as the gate can control the resonant tunneling probability between source and drain. With the inclusion of a realistic energy relaxation time of ∼0.1 ps, the double-barrier resonant tunneling transistor shows an enhancement of the tunneling current density and the negative transconductance feature is only slightly changed. We also find that the quantum-well drain is not able to completely screen the electric field imposed by the backgate bias as a result of limited density of states of two-dimensional systems.
Evaluation of In1−xGaxAs/In1−yGayAs strained layer superlattice structures by x-ray diffraction measurements with a novel discrimination method of the fundamental peak
Authors: Nakashima Kiichi
Published: July 2002

This article presents a simple alternative procedure for evaluating the structure of InGaAs/InGaAs strained layer superlattices (SLSs) by x-ray diffraction measurements. A symmetric reflection configuration is adopted for the scanning mode of (hkl) reflection measurement contrasting to the commonly used asymmetric configuration for SLS. In order to determine the average lattice constants for the SLS under the scanning mode, an analytical formula is derived with respect to the symmetric reflection configuration. A new discrimination method of the fundamental peak is also proposed in which a simple experimental method is useful especially for the SLS case because the fundamental peak is usually not the most intense. This method works also as a simple criterion of coherent lattice deformation. The analytical procedure is applied to the evaluation of InGaAs/InGaAs SLS structures. The lattice deformation and composition of well and barrier layers are estimated by parameter fitting to the satellite peak intensity profile based on the obtained average lattice constant. The results reveal that the sample is coherently deformed as designed.
Elastic analysis of screw dislocations and two collinear cracks of different length
Authors: Shiue Sham-Tsong , Lee Sanboh
Published: July 2002

The elastic interaction between screw dislocations and two collinear internal cracks of different length has been investigated by the dislocation modeling method. The dislocation distribution to simulate the cracks and the stress field are obtained. From the stress field, the stress intensity factor at the crack tip and the image force on the dislocation have been derived. The dislocations inside the crack also play an important role in fracture. Newton’s third law is satisfied in this system. Finally, three special cases are discussed.
Investigation of optical damage mechanisms in hafnia and silica thin films using pairs of subnanosecond laser pulses with variable time delay
Authors: Chase L. L. , Hamza A. V. , Lee H. W. H.
Published: July 2002

Optical damage thresholds of submicron-thick, electron beam deposited HfO2 and SiO2 films on BK-7 substrates have been measured by monitoring the emission of neutral constituents during excitation with time-delayed pairs of 70-ps laser pulses at a wavelength of 1064 nm. The dependence of the optical damage threshold on time delay provides evidence of the optical damage mechanism. For SiO2, linear absorption is the mechanism for energy deposition into the films by the laser beams. The data for HfO2 are less definitive, although linear absorption is the most likely damage mechanism. The behavior of the single-layer films is compared to multilayer HfO2-SiO2 high-reflector coatings, for which a ‘‘conditioning’’ effect causes an increased optical damage threshold due to multiple pulse laser excitation at fluences below the single-pulse optical damage threshold.
Oxide defects originating from Czochralski silicon substrates
Authors: Itsumi Manabu , Nakajima Osaake , Shiono Noboru
Published: July 2002

Oxide defects originating from Czochralski silicon substrates are characterized. It is shown that oxide defect density is significantly large for the oxide thickness range of 20–200 nm. This noteworthy feature is valid for dry oxidation and HCl oxidation alike. It is considered that small defects in the silicon substrate surface layer are introduced into the growing oxides during thermal oxidation and that the small defects in oxides provide conductive paths across the oxides. The origin of the small defects is not known at present. A simple model is proposed in an effort to show the relation between the size and density of the small defects. The oxide thickness dependence data of oxide defect density are characterized in terms of the small-defect behaviors. The relationship between the small-defect size L and the density ρ in the Cz-Si substrates is shown as ρ=ρ0 exp(−L/L0), where ρ0=107 cm−3 and L0=48 nm. In addition, oxide surface etching of thick oxides followed by oxide defect evaluation of the thinned oxides reveals that even thick oxides with seemingly low oxide density contain many small defects. The small defect density in 130 nm oxides is estimated as 120 cm−2.
Photoemission characterization of the H2 plasma-etched surface of CdS
Authors: Nelson Art J. , Frigo Sean P. , Rosenberg Richard
Published: July 2002

The effects of H2 plasma exposure of CdS as a function of substrate temperature was studied by synchrotron radiation soft x-ray photoemission spectroscopy and optical emission spectroscopy. The low-power H2 plasma was generated with a commercial electron cyclotron resonance plasma source using pure H2 with the plasma exposure being performed at ambient temperatures of 100 and 200 °C. Plasma species were identified with optical emission spectroscopy. In situ photoemission measurements were acquired after each plasma exposure in order to observe changes in the valence-band electronic structure as well as changes in the Cd 4d and S 2p core lines. The results were correlated in order to relate the plasma species and characteristics to changes in surface chemistry and electronic structure. These measurements indicate that the H2 plasma exposure type converts the CdS surface to an n-type surface and that the magnitude of the band bending is dependent on substrate temperature during plasma exposure.
Semiconducting iron disilicide films on Si(111): A high resolution electron energy loss spectroscopy study
Authors: Stuhlmann Ch. , Schmidt J. , Ibach H.
Published: July 2002

Thin films of β-FeSi2 on Si(111) have been studied by low energy electron diffraction (LEED) and high resolution electron energy loss spectroscopy (HREELS). The observed LEED pattern is consistent with two possible epitaxial orientations. HREELS measurements demonstrate the semiconducting character of the films. The energy gap is determined to Eg≂0.92 eV–0.33 meV/K T(K). Additionally a number of optical phonons is found in the range between 200 and 500 cm−1. Theoretical spectra are calculated with optical phonon parameters obtained from infrared data and are compared with the measured spectra.
Cyclic deposition of diamond: Experimental testing of model predictions
Authors: Cline B. , Howard W. , Wang H. , Spear K. E. , Frenklach M.
Published: July 2002

The vapor deposition of diamond by cycling growth and etchant mixtures was investigated as a function of cycle time, tcycle, the fraction of the cycle at which growth occurs, τgrowth, and the atomic fraction of carbon in the growth mixture xC. A kinetic model that incorporates diamond growth through a new interdependent methyl and acetylene reaction sequence was used to predict both deposition rates and the carbon deposit sp2 fractions. The important influence of aromatics on the predicted sp2 fractions is thoroughly discussed. The results from the kinetic model were tested experimentally with a microwave-activated deposition system equipped to allow reactant gas cycling between growth (CH4 in He) and etchant (O2 in H2) mixtures. The deposits were characterized by their Raman spectra and scanning electron micrographs. Both the kinetic model and the experimental results show an increase in deposit quality (lower sp2 fraction) with decreasing tcycle and with decreasing τgrowth. Linear growth rates estimated from deposit particle dimensions were typically on the order of 1 μm/h. The kinetic model does not address nucleation rates, but experimental results indicate a trend toward lower rates at shorter tcycle and decreasing τgrowth. The deposit character was less sensitive to xC changes in the growth gas, but renucleation was more apparent at higher carbon fractions. Both the model and experiments show a critical cycle time t*cycle; at shorter cycle times the deposit quality is always high, while at higher times significant fractions of sp2 carbon are present. The t*cycle value is a function of experimental conditions, and in the present study was found to be strongly dependent on the fraction of carbon used in the reactant gas and total pressure, and slightly less dependent on τgrowth. Previously reported cyclic deposition studies were found to be consistent with predictions expected from the kinetic model used in this study.
The initial stages of the thermal oxidation of Si(001) 2×1 surface studied by scanning tunneling microscopy
Authors: Udagawa Masaharu , Niwa Masaaki , Sumita Isao
Published: July 2002

The initial stages of the thermal oxidation of Si(001) 2×1 surface were studied by scanning tunneling microscopy. The O2 exposure at 600 °C produced Si islands and initial forms of oxides. The oxides appeared as ‘‘dark sites,’’ ‘‘sequence of dots,’’ and ‘‘dots with dark surroundings.’’ Among the oxides, most of the dark sites were removed by the successive heating at 600 °C, while the sequence of dots and the dots with dark surroundings remained. All the oxides were removed by the successive heating at 800 °C. The thermal oxidation of Si(001) 2×1 surface is discussed in terms of these sites.
New model for the shock-induced α-quartz→stishovite phase transition in silica
Authors: Boettger J. C.
Published: July 2002

A new model has been developed to describe the α-quartz to stishovite phase transition in silica under shock-loading conditions. During hydrodynamic simulations, individual global equations of state for the α-quartz and stishovite phases of silica are mixed in accordance with process-dependent constraints on the Gibbs free energy difference between the phases (ΔG). For the shock-induced transition, ΔG is required to equal a simple two parameter function of the mass fraction of stishovite. Unlike previous models, the new constraint equation assumes that the shock induced phase transition begins at the equilibrium phase boundary, but is not completed unless peak stresses on the order of 40 GPa are achieved. On release, the reverse transition is required to satisfy the usual thermodynamic equilibrium condition (ΔG=0). It is shown that hydrodynamic simulations combining this hysteretic phase transition model with a strength model that assumes partial softening after yielding are capable of reproducing experimental Hugoniot and release data for crystalline silica in the mixed-phase region.
Numerical analysis of a step-coupled well: A novel quantum-confined Stark shift structure
Authors: Lin Benjamin S. M. , Lin S. J. , Hwang J.
Published: July 2002

A new quantum-confined Stark shift structure, the step-coupled well, is introduced. The feature of the new structure is using a square well of only one quantum state to enhance the Stark shift of the step well. The design rule of 10-μm modulator is suggested. The transfer matrix technique is used for numerical calculation. Numerical results show that the new structure produces a larger Stark shift (13 meV) than the step well (7 meV) under the same bias (20 kV/cm). A lower electric field is thus needed for the step-coupled well to produce the same Stark shift, so that the leakage problem can be minimized and a high switching speed can be achieved.
ab-oriented YBa2Cu3O7 thin film grown on c-axis layer by radio frequency sputtering
Authors: Wu Sijia , Ng K.-W. , Robertson J. D. , Jacob R. J.
Published: July 2002

YBa2Cu3O7 films can be grown on different substrates by the off-axis ac magnetron sputtering method. Films in the ab orientation tend to grow at a lower substrate temperature than c-axis films. However, as the temperature is lowered, all the substrates we have tested, except for SrTiO3, form amorphous phases that degrade the quality of the films being made. In this manuscript it is shown that if a thin layer of c-axis film is deposited on the substrate first, then it effectively reduces the formation of the amorphous phases and improves the film quality significantly.
Carrier temperature relaxation probed by femtosecond transient grating experiments in CdSxSe1−x semiconductors
Authors: Lap Dao van , Peschel U. , Ponath H. E. , Rudolph W.
Published: July 2002

The relaxation of the electron temperature in CdSxSe1−x was measured by transmission and transient grating experiments using fs light pulses. As compared with ordinary ambipolar diffusion, at high excitation a pronounced delayed rise and a faster decrease of the diffraction signal was observed. A theoretical model that includes one- and two-photon excitation, gap shrinkage, diffusion, and temperature relaxation explains these experimental findings as a result of temperature relaxation with a characteristic time constant of 1±0.2 ps.
Imaging x-ray multilayer structures using cross-sectional high resolution electron microscopy
Authors: Cheng Yuanda , Smith David J. , Stearns Mary Beth , Stearns Daniel G.
Published: July 2002

Mo/Si soft x-ray multilayer films have been imaged in cross section by high resolution electron microscopy and the dependence of important multilayer parameters on imaging conditions has been investigated. Multiple measurements of the widths of the Mo crystalline layers were made from images recorded as part of a through-focal series. The layer thicknesses were found to be insensitive to the focus setting near the Scherzer defocus value (+160 Å to −800 Å). At larger defocus conditions, however, additional Mo fringes were visible so that the Mo crystalline region seemed to become thicker at the expense of the pure Si and interlayer regions. Micrographs recorded with the ML slightly tilted (∼1–2°) from the edge-on orientation suggested thicker Mo and thinner Si and interlayer regions but the apparent bilayer periodicity did not change measurably. The possibility of changes in multilayer structure induced during cross-sectional specimen preparation was also shown. Finally, the results of the image analysis are compared with those obtained from small and large angle x-ray scattering experiments from the same sample.
Band gap shrinkage in GaInAs/GaInAsP/InP multi-quantum well lasers
Authors: Park S. H. , Shim J. I. , Kudo K. , Asada M. , Arai S.
Published: July 2002

We investigated a band gap shrinkage in both lattice-matched and compressive strained GaInAs/GaInAsP/InP multi-quantum well lasers. The band gap shrinkage is obtained from the broadening of the low energy side in the spectrum by considering effects of an intraband scattering and a fundamental band edge. It amounts to 20–25 meV at sheet carrier densities of above 1012 cm−2, and shows n1/1.3 dependence at low carrier densities and n1/3 dependence at higher densities. These dependencies agree well with theoretical predictions in the quantum well.
Effect of thickness ratio deviation on soft-x-ray diffraction of multilayers
Authors: Wang Bing , Xiu Lisong , He Xianchang , Wu Ziqin , Red’ko S. V. , Ilyushin A. S.
Published: July 2002

The simulation of soft-x-ray (λ=1.33–14.0 nm) -diffraction behavior at 2θ=90° of W/C and W/Si multilayers with sublayer thickness ratio (composition) deviation in periods is presented. The results show that when the deviation Δx≥0 (which means that the thickness fraction x=d1/d increases from the top to the bottom of the multilayers, where d1 is the thickness of the heavy element sublayer and d the period of the multilayer), (i) in the case of W/Si where the W and Si absorption coefficients are close the diffraction intensities decrease in general, while in the case of W/C where the W and C absorption coefficients are close the increase of the diffraction intensities can reach about 20%; (ii) the full-width at half-maximum of the diffraction peak always decreases; (iii) the peak position tends to move to the small-angle side.
Quantum thermoelectric effects in resonant tunneling real-space transfer
Authors: Bigelow J. M. , Leburton J. P.
Published: July 2002

We present the analysis of a new mechanism for tunneling real-space transfer (TRST) in modulation-doped structures. The conditions for TRST are determined by the quantum thermoelectric properties of the electron systems which results in instabilities. We predict high-frequency oscillations limited only by the tunneling transfer time and thermalization between electron channels. Negative differential resistance and hysteresis are predicted in the I-V characteristics of the modulation-doped structures; recent experiments corroborate the existence of the new mechanism.
Atomic ordering in InGaAsP and InGaAs grown by atmospheric pressure metalorganic chemical vapor deposition
Authors: Chu S. N. G. , Logan R. A. , Tanbun-Ek T.
Published: July 2002

We report on a systematic study of atomic ordering in InGaAsP and InGaAs grown by atmospheric pressure metalorganic chemical vapor deposition. InGaAsP lattice matched to InP, grown in a temperature range of 625–650 °C, reveals atomic ordering on the (111) plane (variant I) and the (11̄1̄) plane (variant II) of the group III sublattice. The extent of atomic ordering increases with decreasing growth temperature and increasing In to Ga ratio. No orderings are observed in InGaAsP grown at 700 °C or in In0.53Ga0.47As lattice matched to InP at all our growth temperatures, in contrast to the commonly believed occurrence of maximum ordering at 1/1 In to Ga ratio. The facts that these conditions differ significantly from the reported conditions for InGaAsP grown by other techniques strongly suggests that the atomic ordering formation is controlled by the surface kinetics and growth environment, i.e., chemistry at the reactive gas-solid surface, fluid dynamics of the reactive gases, and growth temperature, rather than the composition and growth temperature. A fine structure observed in the superspots associated with the atomic ordering in the electron diffraction pattern indicates a possible superlattice structure formed by alternating variant I and variant II ordered layers. Misfit strain as well as sulfur dopant are shown to have no effect on the atomic ordering. Zinc dopant, however, totally eliminates the atomic ordering and shifts the energy band gap to a larger band gap.
Electrical noise and streaming potential generated during capillary flow of poly(ethylene oxide) solutions
Authors: Klason C. , Kubát J. , Quadrat O.
Published: July 2002

An investigation of streaming potential and electrical noise generated during capillary flow of aqueous solutions of poly(ethylene oxide) showed that these effects do not depend on each other. While the former effect arises in the electrical double layer at the capillary wall, the latter is a bulk effect produced by turbulent movement of the flowing liquid.
X-ray photoemission analysis and electrical contact properties of NF3 plasma cleaned Si surfaces
Authors: Delfino M. , Chung B.-C. , Tsai W. , Salimian S. , Favreau D. P. , Merchant S. M.
Published: July 2002

The removal of native silicon oxide on <100≳ silicon with an electron cyclotron resonance (ECR) excited NF3 plasma is demonstrated. In situ x-ray photoemission spectroscopy verifies removal of the oxide and shows that a residue remains on the surface after exposure to the plasma. The residue is about 1.2 nm thick with the approximate formula Si6F8ON2 when analyzed with a uniform overlayer model. X-ray photoemission spectra of the residue show fluorine and oxygen in at least two different bonding states and a unique nitrogen having a diamagnetic bond. Chemical bonding in the residue is ascribed to Fx-Si, Fx-Si-O, Si-O-Si, and N2-O-Si species, where x=1, 2, and 3. A distinct high-energy peak is identified in the quasicore level F 2s transition that is attributed to a small amount of interstitial fluorine having diffused into the silicon lattice. The residue is stable at room temperature in both vacuum and under hydrogen, but when exposed to room ambient, it and the substrate appear to oxidize accounting for a loss of both fluorine and nitrogen. Heating the residue to 640 °C in vacuum causes a significant loss of fluorine and nitrogen also. A possible mechanism accounting for the formation of the residue is proposed assuming that there is a difference in the decay time of the atomic species in the plasma. The ECR cleaning process is integrated into a sputtered TiN/Ti metallization sequence to show the effect of in situ chemical and physical plasma cleaning methods on the electrical contact properties of devices with submicron dimensions and high-aspect ratios. The specific contact resistance of Ti to n+-polycrystalline Si and to TiSi2/n+-, p+-<100≳ Si is found comparable to that achieved with an ECR excited Ar plasma cleaning where a surface residue is not produced.
Photoreflectance spectroscopy of semi-insulating GaAs
Authors: Wang Ruozhen , Jiang Desheng
Published: July 2002

Clear observations of photoreflectance (PR) spectra due to excitonic transitions in semi-insulating GaAs bulk materials are reported. The modulation mechanism is attributed to the electromodulation induced by the Dember effect. This study indicates that the PR spectroscopy provides an important method for characterizing the crystal quality of high-resistivity GaAs.
GaAs substrate cleaning for epitaxy using a remotely generated atomic hydrogen beam
Authors: Rouleau C. M. , Park R. M.
Published: July 2002

In situ GaAs surface cleaning prior to molecular beam epitaxy using a combined thermal/H-atom treatment, the H-atom flux being derived from a rf (13.56 MHz) plasma discharge, hydrogen free-radical source, has been compared and contrasted to conventional thermal treatment of GaAs surfaces. Surface quality, i.e, morphology, was monitored in situ in real time using conventional reflection high-energy electron diffraction and a diffuse optical reflectivity technique employed simultaneously. GaAs surfaces were found to clean readily at temperatures below 400 °C using the combined thermal/H-atom treatment as opposed to the conventional thermal treatment, which requires temperatures in the vicinity of 600 °C. The atomically clean GaAs surfaces were also found to be specular when prepared using the combined thermal/H- atom treatment in contrast to conventional thermally treated GaAs surfaces, which are considerably rough on the atomic scale, surface roughening in the conventional case being associated with the oxide desorption process.
Switching response of a polymer dispersed liquid-crystal composite
Authors: Jain Sukhmal C. , Thakur Ravindra S. , Lakshmikumar S. T.
Published: July 2002

Control over the switching times of polymer dispersed liquid crystal (PDLC) displays is an area of technological importance. We found that the switching times of a PDLC cell can be effectively controlled by using a dual frequency addressable liquid crystal mixture and driving it with appropriate electric fields. The decay and rise times of such a PDLC cell can be reduced from a few thousand milliseconds to a few hundred milliseconds when driven by a low- and a high-frequency field, sequentially. We have also found that the decay and rise times of PDLC cells depend considerably on the frequency of the driving signal and on the initial light-scattering state of the film (which depends on the sample preparation conditions). Indeed, we found that a PDLC cell having an initial highly scattering state exhibits better switching response times than a PDLC cell with an initially transparent optical state. An attempt is made to elucidate the role of the confinement of the LC to the microdroplets on the switching response of a PDLC cell.
The hysteresis and transient behavior of Si metal-oxide-semiconductor transistors at 4.2 K. I. The kink-related counterclockwise hysteresis regime
Authors: Simoen E. , Claeys C.
Published: July 2002

The experimental hysteresis behavior of Si metal-oxide-semiconductor transistors (MOSTs) operated at liquid-helium temperatures (LHT) is described in detail. It is demonstrated that two different hysteresis regimes can be distinguished: a kink-related counterclockwise and a prekink clockwise regime. The difference in hysteresis sense is related to the different physical mechanisms underlying the phenomena. It is demonstrated that there exists a strong relationship between the hysteresis and the transient behavior of a Si MOST at 4.2 K. The possible transient mechanisms, e.g., self-heating or dopant ionization, are critically reviewed. The transient results presented are in line with the field-assisted ionization of the frozen-out dopant atoms in the Si substrate. For the counterclockwise hysteresis regime an improved analysis is presented, which is based on the original forced depletion layer formation approach. In the companion article the transient behavior in the prekink, clockwise regime is described in detail and discussed in view of the different possible dopant ionization mechanisms at LHT.
Specific heat measurements of pressure-induced reentrant superconductivity in Eu0.9Ho0.1Mo6S8
Authors: Chen X. , Perel A. S. , Brooks J. S. , Guertin R. P. , Hinks D. G.
Published: July 2002

Specific heat measurements of Eu0.9Ho0.1Mo6S8 were performed using an ac calorimetry technique under the extreme conditions of low temperature, high magnetic field, and hydrostatic pressure. The title compound is a Chevrel phase material, which is paramagnetic at ambient pressure, becomes superconducting for pressures above 7 kbar, and has a reentrant upper critical field [dHc2(T)/dT≥0, as T→0 K]. Evidence that this material also has a high magnetic field-induced superconducting phase at low temperatures (H≥5 T, T≤1 K) was previously reported. The present ac calorimetry study carried out for 0.15≤T≤6 K, 0≤H≤20 T, and P≂8 kbar, shows a sharp superconducting transition for T≤1 K in the specific heat, which broadens at higher fields and temperatures. The results are analyzed to obtain a reentrant phase diagram in agreement with previous transport work, but direct evidence for the high field-induced phase was not found. The mechanism for the field and temperature dependence of the superconductivity of Eu0.9Ho0.1Mo6S8 is discussed.
Delayed collection field experiment in amorphous arsenic triselenide
Authors: Naito Hiroyoshi , Okuda Masahiro
Published: July 2002

Delayed collection field experiments, which are known to be a method for measuring lifetimes in amorphous semiconductors, are examined for amorphous arsenic triselenide. It is experimentally shown that the effective transit time is approximately equal to the delay time, when the delay time is long compared with the zero-delay transit time, and that the effective transit time is almost independent of collection electric field and temperature. These results are found to be consistent with the prediction by Kastner [Solid State Commun. 45, 191 (1983)], which is based upon the multiple trapping model. Influence of a structured, tail-state distribution on the experimental results has been also numerically examined.
Thermal boundary resistance and diffusivity measurements on thin YBa2Cu3O7−x films with MgO and SrTiO3 substrates using the transient grating method
Authors: Marshall C. D. , Tokmakoff A. , Fishman I. M. , Eom C. B. , Phillips Julia M. , Fayer M. D.
Published: July 2002

Interface selective transient grating experiments are performed on oriented thin films (∼100 nm) of YBa2Cu3O7−x, with MgO and SrTiO3 substrates. The anisotropic YBa2Cu3O7−x thermal diffusivity constants and the thermal boundary resistance between the thin film and substrate are measured. Four different excitation and probe geometries are utilized such that each geometry results in a unique temporal decay. The grating has a significant amplitude on both sides of the film–substrate interface with a grating wave vector parallel to the interface. The four experimental geometries comprise an over-determined system that can be used to confirm the validity of the model assumptions. Numerical fits to the experimental data, using a straightforward diffusive model, are performed to obtain information on thermal diffusivity and to demonstrate the applicability of the technique to monitor anisotropic thermal relaxation processes in thin film–substrate structures.
Soft magnetic properties of Fe multilayered films with amorphous intermediate layers
Authors: Kobayashi Toshio
Published: July 2002

Amorphous intermediate layers are studied to improve soft magnetic properties of Fe-based multilayers, since the interfacial stresses between layers are assumed to be low. Fe multilayers with crystalline intermediate layers (e.g., Ni-Fe alloy) required heat treatment to release interfacial stress to achieve soft magnetic properties. Multilayers Fe/B4C, Fe/SiC, and Fe/C (B4C, SiC, and C layers are amorphous when formed by ion beam sputtering) show relative permeabilities over 2000 and coercivities below 56 A/m (=0.7 Oe), even without heat treatment. In particular, Fe/B4C multilayers with B4C thicknesses over 0.5 nm show clear periodic structures, even smaller crystallite sizes, and relative permeabilities above 3000, while the films have saturation magnetic flux densities above 1.8 T.
Correlation of optical and dielectric properties of water trees in polyethylene
Authors: Viard Jérôme
Published: July 2002

Many experimental works report that the presence of water trees lowers the voltage needed for the initiation of dielectric breakdown in polyethylene. The influence of water trees on the dielectric breakdown is generally related to their length. Here, in addition to this spatial property, we consider the extinction of the light by the water trees due mainly to scattering of the incident light by water microcavities. This effect is described by means of the water tree absorbance. The absorbance itself is related to another main variable: the water content. We also take into account the existence of a structure in the water tree, i.e., of a heterogeneous distribution of water microcavities inside the trees. Water content and spatial water distribution allows us to define a local permittivity for each region in the tree. These elements are the basis of a model for the computation of the disturbance induced by the tree in the spatial distribution of the electric field (reduction of the field inside the tree, enhancement outside). The field enhancement effect is computed as a function of the water concentration. Measurements of the light transmitted by given water trees are performed; they allow us to propose an estimate of the water contained by different water trees. From these concentrations, different field enhancement coefficients are computed from the model. They are then compared with measurements of electrical treeing inception voltage performed with and without water trees. Theoretical values obtained from the model are found in good agreement with experimental data.
Radiative transitions in quaternary In0.52Ga0.18Al0.30As layers grown by molecular beam epitaxy
Authors: Rinaldi R. , Cingolani R. , Ferrara M. , Tapfer L. , Künzel H. , Hase A.
Published: July 2002

The structural and optical properties of molecular beam epitaxy-grown In0.52Ga0.18Al0.30As layers (E300 Kg≂1.18 eV), suitable for waveguide applications, have been studied by means of high-resolution x-ray diffraction, absorption, photoluminescence, photoreflectance, and high-excitation intensity photoluminescence spectroscopy. The combination of these techniques allowed us to study the free-exciton states, the impurity related transitions, and the formation of a dense electron-hole plasma.
A Tougaard background deconvolution study of the compositional depth profile in amorphous a-Si1−xCx:H alloys
Authors: Sastry Murali , Sainkar S. R.
Published: July 2002

A Tougaard background deconvolution analysis of the x-ray photoelectron spectra of an a-Si1−xCx:H film deposited on a silicon substrate by the radio-frequency plasma-enhanced chemical vapor deposition method has been performed. The analysis indicates that carbon and silicon atoms inhabit an exponentially decreasing concentration profile from the surface with different characteristic attenuation lengths. Oxygen has also been incorporated in the film with a similar concentration profile. The differential inelastic scattering cross section for the deconvolution has been obtained from reflection electron energy-loss spectroscopy. This nondestructive depth profile analysis appears to be a powerful tool for determining the uniformity of deposition of such films.