bluuebunny/crossref_metadata_2025 huggingface dataset:
Ratchet device with broken friction symmetry
Authors: Nordén Bengt , Zolotaryuk Yaroslav , Christiansen Peter L. , Zolotaryuk Alexander V.
Published: July 2002

An experimental setup (gadget) has been made for demonstration of a ratchet mechanism induced by broken symmetry of a dependence of dry friction on external forcing. This gadget converts longitudinal oscillating or fluctuating motion into a unidirectional rotation, the direction of which is in accordance with given theoretical arguments. Despite the setup being three dimensional, the ratchet rotary motion is proved to be described by one simple dynamic equation. This kind of motion is a result of the interplay of friction and inertia.
Enhancement of photoluminescence by microdisk formation from Si/Ge/Si single quantum wells
Authors: Choi Suk-Ho , Kim Jung Nam , Kim Hea Young , Hong Young-Kyu , Koo Ja-Yong , Seok JangHyeon , Kim JaeYon
Published: July 2002

A significant enhancement of photoluminescence (PL) intensity is observed in microdisks of 0.5 and 1 μm diam, which have been fabricated from Si/Ge/Si single quantum wells (SQWs) grown by molecular-beam epitaxy. The three major PL peaks found at 0.972, 0.957, and 0.920 eV are identified as a no-phonon transition of localized exciton, associated transverse-acoustical, and transverse-optical phonon replicas in Si, respectively. It is suggested that the formation of microdisks from the Si/Ge/Si SQWs enhances the intrinsic PL transitions significantly by suppressing the impurity-related ones.
Two-dimensional dopant concentration profiles from ultrashallow junction metal-oxide-semiconductor field-effect transistors using the etch/transmission electron microscopy method
Authors: Yoo K. D. , Marsh C. D. , Booker G. R.
Published: July 2002

The etch/transmission electron microscopy (TEM) method for obtaining one dimensional (1D) and two dimensional (2D) dopant concentration profiles has been developed and applied to n+/p silicon metal-oxide-semiconductor field-effect transistor (MOSFET) structures fabricated using 0.25 μm device technology with ultrashallow source/drain junctions obtained by 5-keV arsenic ion implantation. Cross-section thin-foil specimens were dopant selectively etched with 0.5%HF/99.5%HNO3. Local etch rates were determined from TEM thickness fringes and converted to dopant concentrations using an independently determined calibration curve. The profiles indicated a vertical junction depth of ∼51 nm, a lateral junction distance under the gate edge of ∼27 nm, and an effective gate length of ∼161 nm. The etch/TEM 1D dopant concentration profiles agreed well with spreading resistance probe and secondary ion mass spectroscopy 1D dopant concentration profiles obtained from blanket silicon wafers similarly processed.
Effect of annealing on the dielectric parameters of ferroelectric TSiKN65
Authors: Shenoy Devanand , Lavarello Antonio , Naciri Jawad , Shashidhar Ranganathan
Published: July 2002

The ionic conductivity of ferroelectric liquid crystals used for pyroelectric detection is a measure of the noise contribution to the output of the device. This is obtained from a measurement of the dielectric loss of the liquid crystal material. An annealing procedure has been developed that reduces the ionic conductivity and hence the dielectric loss significantly. The procedure consists of subjecting the ferroelectric liquid crystal sample to an electric field in the isotropic phase and cooling it into the smectic phase at a certain rate. After measurement of the dielectric parameters, the sample is reheated into the isotropic phase and then cooled again at a slower rate into the smectic phase. The dielectric measurements are repeated again. The entire process is repeated for successively slower cooling rates. We present the results of the effect of this annealing treatment on the dielectric parameters and explain the origin of this phenomenon.
Structural studies of annealed ultrathin La0.8MnO3 films
Authors: Qian Q. , Tyson T. A. , Dubourdieu C. , Bossak A. , Sénateur J. P. , Deleon M. , Bai J. , Bonfait G.
Published: July 2002

A detailed study of the long-range, nanoscale, and local structure of La0.8MnO3 films of varying thickness was performed. These measurements give insight on the relative volumes of the insulating and metallic regions. A thin metallic surface region is found in all films. The nature of the film growth is also discussed.
Nanoscale tungsten aerosol particles embedded in GaAs
Authors: Åberg Ingvar , Deppert Knut , Magnusson Martin H. , Pietzonka Ines , Seifert Werner , Wernersson Lars-Erik , Samuelson Lars
Published: July 2002

GaAs containing buried nanoscale tungsten particles has been characterized electrically. The particles were produced using a special aerosol process and were embedded in GaAs by epitaxial overgrowth. Two different particle sizes were investigated separately. When the particle concentration was increased, a conductance drop of about 500 times was observed. A simulation model, based on a random distribution of the particles, was developed and used to support our findings. The major advantage of our method is the simplicity and low processing cost.
Photoreflectance spectra of a ZnO heteroepitaxial film on the nearly lattice-matched ScAlMgO4 (0001) substrate grown by laser molecular-beam epitaxy
Authors: Chichibu S. F. , Tsukazaki A. , Kawasaki M. , Tamura K. , Segawa Y. , Sota T. , Koinuma H.
Published: July 2002

Photoreflectance spectra of a high-quality ZnO epilayer on the ScAlMgO4 (0001) substrate grown by laser molecular-beam epitaxy exhibited clear excitonic resonances due to three excitons associated with uppermost valence bands (A, B, and C excitons). The oscillator strengths of the B and C excitons are larger than that of the A exciton. Their broadening was interpreted to be due to the contribution by exciton–polaritons in terms of large longitudinal–transverse splitting of respective excitons. Dependence of the exciton energy on temperature was well fitted assuming the Bose–Einstein statistics giving the Einstein characteristic temperature ΘE of 380 K (33 meV).
Influence of substrate bias voltage on surface morphology and nanocluster arrangement of gold containing amorphous hydrogenated carbon
Authors: Videnović Ivan R. , Thommen Verena , Oelhafen Peter , Mathys Daniel , Düggelin Marcel , Guggenheim Richard
Published: July 2002

Gold containing amorphous hydrogenated carbon (a-C:H/Au) thin films are studied by means of ex situ atomic-force and scanning electron microscopy, and in situ x-ray photoelectron spectroscopy. Introducing a dc substrate bias voltage during magnetron plasma-assisted chemical-vapor deposition of a-C:H/Au results in significant changes of surface morphology and nanocluster arrangement, compared to films deposited on electrically grounded substrates. Grounded samples are rather flat (below 1 nm of roughness), with topmost Au clusters covered with a very thin, nanometer-range layer of a-C:H. On the other hand, the biased sample surface is characterized by large bump structures (up to 13 nm in height) and bald gold clusters.
Large tunability driven by low electric field in oxygen-isotope-exchanged strontium titanate at cryogenic temperatures
Authors: Wang Ruiping , Itoh Mitsuru
Published: July 2002

The substitution of heavy isotope 18O for 16O in SrTiO3 simultaneously results in a significant enhancement of tunability and a large reduction of driving electric fields at cryogenic temperatures. For a 36% oxygen isotope exchanged SrTiO3 at 2 K, the driving electric field for the tunability of 0.82 is about half an order lower than that for an unexchanged one. The tunability and the figure of merit could be tailored by the oxygen isotope exchange rate. It is considered that the phenomena are closely related to the appearance of ferroelectric microregions and the “domain state” in this system [R. Wang and M. Itoh, Phys. Rev. B 62, R731 (2000)].
Electronic excitation and optical cross sections of methylamine and ethylamine in the UV–VUV spectral region
Authors: Hubin-Franskin M.-J. , Delwiche J. , Giuliani A. , Ska M.-P. , Motte-Tollet F. , Walker I. C. , Mason N. J. , Gingell J. M. , Jones N. C.
Published: July 2002

High resolution UV–VUV photon absorption spectra of methylamine and ethylamine have been recorded between 5.0–9.0 eV (250–140 nm) using synchrotron radiation. In methylamine, the energies of the absorption bands are confirmed as centered at 5.7, 7.2, and 8.7 eV, respectively. In ethylamine the band centers are 5.8, 7.0, and 7.9 eV, respectively; the last band is seen here for the first time. Most of the transitions exhibit rich fine structure dominated by vibrational progressions involving excitation of an amino wagging vibration. The absolute photoabsorption oscillator strengths have been measured by photon absorption over the 5–9 eV range and by dipolar electron energy loss spectroscopy from 5–14 eV (250–90 nm).
Size-dependent oxidation of hydrogenated silicon clusters
Authors: Zhang R. Q. , Lu W. C. , Lee S. T.
Published: July 2002

We show explicitly the size-dependent chemical reactivity of hydrogenated silicon clusters reacting with water. A unique trend of decreasing reactivity with decreasing cluster size has been deduced from reaction energetics, frontier orbital analysis, and chemical reaction rates determined by the transition state theory in conjunction with ab initio calculations at Hartree–Fock and Møller–Plesset perturbation levels of theory, for water reaction with both dihydride and trihydride silicon configurations. This study indicates the possibility of fabricating stable hydrogenated silicon structures by reducing their size to nanometers.
Density functional theory calculations of the energy and free energy of anisotropic multicomponent mixtures
Authors: Chernoff Martin P. , Aranovich Gregory L. , Donohue Marc D.
Published: July 2002

A lattice density functional theory for the nonrandom energy for multicomponent mixtures containing monomers with directional interactions is presented. This theory is a simplification and generalization of a lattice density functional theory developed by Aranovich and Donohue (AD) for two-dimensional and three-dimensional mixtures based upon ideas originally proposed for one-dimensional systems by Ono and Kondo. While quite accurate and general, the AD equations could not be integrated analytically to give expressions for the free energy. With an algebraic rearrangement of this model into a sum of a random mixing internal energy and the deviations from random mixing, an expression is obtained that is both accurate and integrable. Comparisons with Monte Carlo simulations confirm the accuracy of the theory. Unusual phase stability boundaries are predicted.
Probing the SiGe virtual substrate by high-resolution channeling contrast microscopy
Authors: Seng H. L. , Osipowicz T. , Sum T. C. , Tok E. S. , Breton G. , Woods N. J. , Zhang J.
Published: July 2002

Relaxed, epitaxial SiGe layers with low densities of threading dislocations are grown by linearly grading the Ge composition. However, such compositionally graded SiGe layers (virtual substrates) often result in a cross hatch surface morphology which affects subsequent device processing. Here, we report on high-resolution channeling-contrast-microscopy (CCM) measurements on such virtual substrates grown by gas-source molecular-beam epitaxy and low-pressure chemical vapor deposition. A two-MeV He+ beam focused to a submicron spot is used in these CCM measurements to obtain both lateral and depth-resolved information on the cross hatch features observed and their association with a slight lattice tilt.
Design of short-cavity, high-brightness 980 nm laser diodes with distributed phase correction
Authors: Zhang Y. , Lim J. J. , Benson T. M. , Sewell P. , Dods S. , Larkins E. C.
Published: July 2002

We present the design of a 980 nm short-cavity, high-brightness, laterally graded-index (GRIN) laser diode with distributed phase correction. The proposed laser consists of a feed waveguide section coupled to a GRIN waveguide region with a discretized hyperbolic-secant index profile. Detailed wide-angle two-dimensional finite-difference beam propagation method simulations show that this lateral GRIN laser will exhibit significant performance improvements over comparable tapered lasers.
Tunable, monochromatic x rays using the internal beam of a betatron
Authors: Kaplin V. V. , Uglov S. R. , Bulaev O. F. , Goncharov V. J. , Voronin A. A. , Piestrup M. A. , Gary C. K. , Nasonov N. N. , Fuller M. K.
Published: July 2002

Tunable, monochromatic x rays from thin radiators mounted inside a betatron have been observed. Parametric x-ray radiation (PXR) was generated by 33-MeV electrons passing multiple times through three radiators: a 43-μm-thick Si crystal, a 400-μm-thick graphite crystal, or a 310-layered-pair (W and B4C, d=14.86 Å) multilayer. The pulse-height spectrum of the radiation (5 to 30 keV) was obtained and was tuned by rocking the crystal or multilayer relative to the electron-beam direction. The experimental results appear to follow theoretical predictions for PXR emission with some modification required for the curved trajectory of the electrons.
Maximum current in nitride-based heterostructure field-effect transistors
Authors: Koudymov A. , Fatima H. , Simin G. , Yang J. , Khan M. Asif , Tarakji A. , Hu X. , Shur M. S. , Gaska R.
Published: July 2002

We present experimental and modeling results on the gate-length dependence of the maximum current that can be achieved in GaN-based heterostructure field-effect transistors (HFETs) and metal–oxide–semiconductor HFETs (MOSHFETs). Our results show that the factor limiting the maximum current in the HFETs is the forward gate leakage current. In the MOSHFETs, the gate leakage current is suppressed and the overflow of the two dimensional electron gas into the AlGaN barrier region becomes the most important factor limiting the maximum current. Therefore, the maximum current is substantially higher in MOSHFETs than in HFETs. The measured maximum current increases with a decrease in the gate length, in qualitative agreement with the model that accounts for the velocity saturation in the channel and for the effect of the source series resistance. The maximum current as high as 2.6 A/mm can be achieved in MOSHFETs with a submicron gate.
Zero-field spin splitting in In0.52Al0.48As/InxGa1−xAs metamorphic high-electron-mobility-transistor structures on GaAs substrates using Shubnikov–de Haas measurements
Authors: Cui L. J. , Zeng Y. P. , Wang B. Q. , Zhu Z. P. , Lin L. Y. , Jiang C. P. , Guo S. L. , Chu J. H.
Published: July 2002

Shubnikov–de Haas measurements were carried out for In0.52Al0.48As/InxGa1−xAs metamorphic high-electron-mobility-transistor structures grown on GaAs substrates with different indium contents and/or different Si δ-doping concentrations. Zero-field (B→0) spin splitting was found in samples with stronger conduction band bending in the InGaAs well. It was shown that the dominant spin splitting mechanism is attributed to the contribution by the Rashba term. We found that zero-field spin splitting not only occurs in the ground electron subband, but also in the first excited electron subband for a sample with Si δ-doping concentration of 6×1012 cm−2. We propose that this In0.52Al0.48As/InxGa1−xAs metamorphic high-electron-mobility-transistor structure grown on GaAs may be a promising candidate spin-polarized field-effect transistors.
Depoling a ferroelectric capacitor
Authors: Higgins M. J. , Krishnan A. , Treacy M. M. J. , Bhattacharya S.
Published: July 2002

A “shrinking hysteresis loop” method, analogous to degaussing a ferromagnet, is used to depolarize a ferroelectric thin film and reach the origin, nominally the zero polarization-zero applied field state. Scanning piezoresponse imaging of the depoled state shows fragmentation into domains of up and down polarization that constitute the zero polarization state. Furthermore, the depoling method improves the left–right symmetry of the coercive field in “imprinted” samples.
Hybrid time-resolved spectroscopic system for evaluating laser material using a table-top-sized, low-jitter, 3-MeV picosecond electron-beam source with a photocathode
Authors: Suzuki Yuji , Kozeki Toshimasa , Ono Shingo , Murakami Hidetoshi , Ohtake Hideyuki , Sarukura Nobuhiko , Nakajyo Terunobu , Sakai Fumio , Aoki Yasushi
Published: July 2002

Hybrid time-resolved spectroscopy of laser media comparing electron-beam excitation and optically excited cases is performed using a newly developed, table-top-sized, low-jitter, 3-MeV picosecond electron-beam source with a photocathode. The properties of an electron-beam-pumped Ce3+:LiCaAlF6 (Ce:LiCAF) ultraviolet laser medium significant differ from those of an optically pumped medium.
Gain trimming of the resonant characteristics in vertically coupled InP microdisk switches
Authors: Djordjev Kostadin , Choi Seung-June , Choi Sang-Jun , Dapkus P. D.
Published: July 2002

InP, vertically coupled microdisk resonator/waveguide switches with a gain active region are demonstrated. The devices exhibit single-mode operation, large free-spectral range of 10 nm, and a high-quality factor of 5700. The introduction of a quantum-well region inside the cavity provides a way of gain trimming the resonant characteristics. Active switches, routers, and filters with tunable bandwidth based on these devices are envisioned as part of a wavelength division multiplexing system.
Projecting picosecond lattice dynamics through x-ray topography
Authors: Synnergren O. , Harbst M. , Missalla T. , Larssona) J. , Katona G. , Neutze R. , Wouts R.
Published: July 2002

A method for time-resolved x-ray diffraction studies has been demonstrated. As a test case, coherent acoustic phonon propagation into crystalline InSb is observed using a laser plasma x-ray source. An extended x-ray topogram of the semiconductor’s surface was projected onto a high spatial resolution x-ray detector and acoustic phonons were excited by rapidly heating the crystal’s surface with a femtosecond laser pulse. A correlation between the spatial position on the x-ray detector and the time of arrival of the laser pulse was encoded into the experimental geometry by tilting the incident laser pulse with an optical grating. This approach enabled a temporal window of 200 ps to be sampled in a single topogram, thereby negating the disadvantages of pulse-to-pulse fluctuations in the intensity and spectrum of the laser-plasma source.
Steady state diagram of current-carrying layered superconductors
Authors: Artemov A. N. , Medvedev Yu. V.
Published: July 2002

The stability of the nonisothermal state of a layered superconductor carrying a current is investigated in a model based on the heat production in an electrical circuit with nonlinear resistance, which adequately reflects the features of the quasi-two-dimensional behavior of the system. The state diagrams of the layered superconductor are plotted in the following coordinates: heat removal coefficient (which depends on the substrate properties) versus current, and nonlinearity parameter (which characterizes the superconductor material) versus current. The velocity of the resistive-zone motion as a function of the value of the transport current is calculated for different values of the nonlinearity parameter.
Direct evidence for the occurrence of superconductivity in the magnetic compound YFe4Al8
Authors: Dmitriev V. M. , Rybaltchenko L. F. , Wyder P. , Jansen A. G. M. , Prentslau N. N. , Suski W.
Published: July 2002

For the first time we present direct evidence for superconductivity in the ternary magnetic compound YFe4Al8 with the ThMn12 type structure, found via point-contact (PC) experiments on contacts between a silver needle and single-crystal YFe4Al8, which reveal a distinct Andreev-reflection current. The spectra measured prove the existence of a normal–superconducting interface and exhibit a triangular-like shape in the vicinity of zero bias voltage, implying an unconventional type of superconductivity. The derived dependences of the order parameter versus temperature Δ(T) and magnetic field Δ(H) are presented. Δ(T) follows BCS theory, whereas Δ(H) does not satisfy any theoretical predictions. In some cases there exists noticeable superconductivity enhancement by a weak magnetic field. The data obtained imply a very inhomogeneous distribution of superconductivity over the sample volume in spite of its single-crystal structure. We assume that the reason is associated with inherent magnetic inhomogeneities of this material. The highest values for the critical temperature Tc, upper critical magnetic field Hc2, and ratio 2Δ(0)/kTc are 7.4 K, 5 T, and 7.2, respectively.
Tuning characteristics of InAsSb continuous-wave lasers
Authors: Sherstnev V. , Krier A. , Popov A. , Werle P.
Published: July 2002

We present a detailed analysis of the tuning characteristics of InAsSb continuous-wave (cw) single-frequency lasers emitting at 3.3 μm (3050 cm−1). The lasers demonstrate a tuning range of −7.5 cm−1 by changing the current and −3.5 cm−1 by changing the heat sink temperature without mode hopping. The tuning rates are of −0.015 to −0.089 cm−1/mA and −0.11 to 0.27 cm−1/K. The laser tunes to the blue side both with increasing injection current and heat sink temperature. The extended tuning is attributed to the carrier heating effect in the cw operation and the band-filling effect in the presence of strong losses.
Evolution of Rayleigh constant in fatigued lead zirconate titanate capacitors
Authors: Sun J. L. , Chen J. , Meng X. J. , Yu J. , Bo L. X. , Guo S. L. , Chu J. H.
Published: July 2002

The capacitance of Pt/PbZr0.5Ti0.5O3/Pt capacitors versus the strength of applied ac electric field E has been measured for samples that fatigued at different square pulse switching cycles. Based on the in-series capacitor model, the variation of interface capacitance and bulk ferroelectric capacitance at different stages of fatigue were treated separately. A simplified method was derived to calculate the initial dielectric constant and Rayleigh constant for the bulk ferroelectric film from the ac electric field dependence of the sample capacitance. It was found that the initial dielectric constant and Rayleigh constant for the bulk ferroelectric film changed a little when the number of cycles is less than 3×106, whereas it decreased remarkably when the number of cycles is greater than 3×106. The suppression of polarization was attributed to pinning of the domain walls at the electrode/ferroelectric interface.
Preflashover and flashover phenomena of silicon–vacuum system under pulsed excitation
Authors: Zhang Guan-Jun , Yan Z. , Liu Y. S. , Yasuoka K. , Ishii S.
Published: July 2002

Rectangular n+nn+-type silicon blocks with different surface treatments, i.e., one surface is chemically etched and the other is unetched, were used to investigate the characteristics of preflashover and flashover of semiconductor under a high voltage pulse in a vacuum. The metallic contacts were employed to keep a fine contact between the sample and the electrode. For unetched samples, the preflashover current obeyed Ohm’s law, and for etched samples, it was dominated by the space-charge limited current. After flashover shots, two kinds of silicon samples showed quite distinct tracks. It is considered that all the phenomena are attributed to the different density and distribution of surface states for two kinds of samples. The developing process of surface flashover across a semiconductor could be explained as the current filamentation which is induced by thermal effects. It is suggested that the flashover occurs in the interface layer of silicon butted to electrodes, and in the lateral layer of silicon near the vacuum ambient with a depth of ∼2 μm.
Enhancement of the coupling efficiency in optical fibers using a two-beam optical interference
Authors: Amara M. Kamel , Melikechi Noureddine
Published: July 2002

We report on an experimental technique, based on the launch of the central fringe of a two-beam interference pattern into an optical fiber that allows considerable enhancement of the coupling efficiency. Using helium–neon laser operating at 633 nm and a 3.3 μm wide fiber with a numerical aperture of 0.16, we have obtained a coupling efficiency of about 91%.
Far-infrared absorption in Sb-doped Ge epilayers near the metal–insulator transition
Authors: Bandaru Jordana , Beeman Jeffrey W. , Haller Eugene E.
Published: July 2002

Epitaxial germanium layers doped near the metal–insulator transition were grown by liquid phase epitaxy from a Pb melt. Far-infrared absorption was measured between ∼20 cm−1 and 150 cm−1. Linear optical absorption coefficients were determined for Ge:Sb in the doping range of 9.0×1014 cm−3<ND<6.7×1016 cm−3. The peak absorption was found to increase linearly with dopant concentration as expected. The absorption at ∼50 cm−1 increased superlinearly with Sb concentration as a result of impurity banding. The optimal Sb concentration for extended far-infrared photoconductive response (λ>200 μm) in blocked impurity band detectors is discussed.
Small-signal modulation response of InP/GaInP quantum-dot lasers
Authors: Riedl T. , Hangleiter A. , Porsche J. , Scholz F.
Published: July 2002

We report on investigations concerning the modulation dynamics of InP/GaInP quantum-dot (QD) lasers grown by metalorganic vapor-phase epitaxy. Room-temperature operation of our lasers occurs at threshold current densities (jthr) around 1.8 kA/cm2 and emission wavelengths (λ) between 696 and 710 nm with characteristic temperatures of about 50 K. At −33 °C a differential gain dg/dn≈1.5×10−16 cm2 is determined. Measurements on GaInP/AlGaInP quantum-well lasers with similar λ yield comparable values for dg/dn. The decrease of dg/dn in our QD lasers at elevated temperatures is attributed to a lack of carrier confinement, which foils the benefits expected from the zero-dimensionality of the active medium. A maximum intrinsic modulation bandwidth of 12.7 GHz at −33 °C is determined.
Use of near-surface channel conductance and differential capacitance versus potential measurements to correlate inversion layer formation with low effective surface recombination velocities at n-Si/liquid contacts
Authors: Michalak David J. , Lewis Nathan S.
Published: July 2002

Near-surface channel conductance measurements, differential capacitance versus potential measurements, and surface recombination velocity measurements have been performed on (111)- and (100)-oriented n-type Si samples in contact with nitrogen and/or liquid electrolyte solutions containing I2, I2/I−, ferrocene+/0, or decamethylferrocene+/0 in either methanol or tetrahydrofuran. Si/liquid contacts that displayed a low effective surface recombination velocity S corresponded to those that formed an inversion layer at the solid/liquid contact as indicated by channel conductance measurements or by differential capacitance versus potential measurements. Contacts that did not produce an inversion layer at the Si surface did not produce low effective S values. The observed behavior is consistent with the known energetics of Si/liquid contacts and provides an explanation for the low effective S values observed in these systems.
Observation of dopant-mediated intermixing at Ge/Si Interface
Authors: Takeuchi Hideki , Ranade Pushkar , Subramanian Vivek , King Tsu-Jae
Published: July 2002

Rapid intermixing of Ge deposited onto a Si substrate during 900 °C rapid thermal annealing was analyzed using secondary ion mass spectroscopy. In undoped Ge samples, a 50 nm thick graded Si1−xGex layer was formed in 1 min, consuming 30 nm Ge and 20 nm Si. Negligible profile change was seen after an additional 1 min anneal. With dopants inside the deposited Ge layer, the extent of the intermixing is increased: For B doping, 30 nm Ge and 30 nm Si are consumed; for As doping, 5 nm Ge and 100 nm Si are consumed. In the case of B, Ge–B codiffusion from the Si1−xGex/Si heterojunction edge was also observed. The p-n junction depth difference between the two dopants can be explained by the difference in their solubilities, while the Ge–B codiffusion is attributed to excess vacancies generated during the initial intermixing.
Electrical characterization of Fe-doped semi-insulating InP after helium bombardment at different implant temperatures
Authors: Too P. , Ahmed S. , Sealy B. J. , Gwilliam R.
Published: July 2002

The effect of helium ion bombardment into semi-insulating (SI) InP at room temperature (RT) and 200 °C has been investigated with various ion doses in the range of 4×1012 to 1×1016/cm2 at 600 keV. The sheet resistivity of SI InP decreases by at least an order of magnitude (∼1×107 Ω/□) after RT helium bombardment at different doses. In the case of the 200 °C implant, the sheet resistivity decreases by less than an order of magnitude. We infer that the hot implant creates less donor defects as compared to RT implants. Samples irradiated with a helium dose of 1×1014/cm2 at RT were annealed from 100 °C to 800 °C. The sheet resistivity remains constant up to an annealing temperature of 500 °C but decreases in the temperature range of 600 °C–650 °C and is increased by annealing to higher temperatures. The removal of defects varies with annealing temperature. These results can be used to choose the right implant conditions before the InP substrate becomes conductive for effective electrical isolation of In-based devices.
Dynamics of twisted nematic liquid crystal pi-cells
Authors: Chen Shu-Hsia , Yang Chiu-Lien
Published: July 2002

A twisted nematic liquid crystal pi-cell with fast optical response time of 2.2 ms was prepared. We investigated the dynamics of this cell and observed the back-flow-induced optical overshoot phenomena both in homeotropic-to-planar state transition and planar-to-homeotropic state transition. We analyzed the behavior of the director and found that there is a tip-over phenomenon when the field is removed from relatively high voltage (>6 V). More important, the fluid flow effect results in the reverse twist both in the rising process and the decay process. Consequently, the reverse twist increases and decreases the effective phase retardation on the optical rising and decay process, respectively, and thus speeds up the optical response in both stages.
Epitaxial PrBa2(Cu0.8Al0.2)3O7 thin films grown by rf sputtering
Authors: Jin Mingji , Chen Q. Y. , Tipparach Udom , Chen T. P. , Wang C. , Seo H. W. , Yuan Lixi , Chu W. K. , No K. S. , Chen C. L. , Song Y. S.
Published: July 2002

Epitaxial thin films of PrBa2(Cu0.8Al0.2)3O7 have been vacuum deposited by rf sputtering on the LaAlO3 substrates. Though electrically much more resistive, these Al-substituted films are all of orthorhombic structure and epitaxial quality on various oxide substrates, much like YBa2Cu3O7 and PrBa2Cu3O7 grown under similar conditions. The samples appeared to be shiny and dark as observed with the naked eye, but their electrical resistivity, ρ(T), ranged from ∼1 Ω cm at room temperature to about six orders of magnitude higher at T∼30 K. From the ρ(T) functional for both the target and the films, which, by and large, follows Mott’s T−1/4 law with their own relevant material constants, we believe that the mechanism of electrical conduction was mainly through variable range hopping. This suggests that the substitution of Al has caused extensive localization of charge carriers. The localization radius is ∼0.2 nm while the hopping distance is ∼3–9 nm.
Theoretical valence band offsets of semiconductor heterojunctions
Authors: Shim Kyurhee , Rabitz Herschel
Published: July 2002

A theoretical model is presented to predict the valence band offsets of heterojunction semiconductors. The total valence band offsets are split into bulk and interfacial contributions. The bulk contributions are based on the tight binding approach while the localized effects at the interface are described by the bond exchange and the elastic energy due to the strained layer. We calculate the valence band offsets for various types of heterojunctions and compare that with experimental data and the results of other theoretical calculations. Our results are in good agreement with experimental data for most of the systems explored. This suggests that the present model is generic and contains the dominant physical factors operative at the interface to make it applicable to various heterojunctions.
Temperature dependence of the growth front roughening of oligomer films
Authors: Tsamouras D. , Palasantzas G.
Published: July 2002

Growth front roughening characteristics of vacuum deposited pentamer 2,5-di-n-octyloxy-1,4-bis[4-(styryl)styryl]-benzene oligomer thin films, onto silicon substrates, strongly depend on the substrate temperature in the range ∼20 °C–100 °C. The measured roughness exponents H increase from H≈0.4 at low substrate temperatures where growth is dominated by vacancy formation, to H≈0.7–0.8 at elevated temperatures where diffusive growth takes place. Moreover, the root-mean-square roughness amplitude and the correlation length evolve with temperature closely as an Arrhenius process with activation barrier comparable to molecule transnational and rotational barriers on oligomer surfaces.
Fabrication of conductive single-crystal semiconductor nanoscale electromechanical structures
Authors: Yamaguchi Hiroshi , Hirayama Yoshiro
Published: July 2002

As an application of the self-organization growth technique to the fabrication of nanoscale mechanical structures, we selectively etched a GaAs sacrificial layer under InAs wires preferentially grown on bunched steps on misoriented GaAs (110) surfaces, which led to the formation of single-crystal InAs nanoscale cantilevers. Their lengths, widths and thickness are typically 50–300, 20–100 and 10–30 nm, respectively. The structures are expected to be electrically conductive and to be promising for use in fabricating single-crystal nanoelectromechanical systems.
Mechanical properties of high-aspect-ratio atomic-force microscope tips
Authors: Jänchen G. , Hoffmann P. , Kriele A. , Lorenz H. , Kulik A. J. , Dietler G.
Published: July 2002

Focused-electron-beam-deposited carbon atomic-force microscope tips were characterized using flexural vibrations excited with piezoelectric transducers and observed directly inside a scanning electron microscope. Frequencies in the high-MHz range were measured and the elastic modulus was estimated to be 0.54±0.16 TPa.
HIP: Symbolic high-energy physics calculations
Authors: Hsieh Alexander , Yehudai Eran
Published: July 2002

HIP, a set of computer packages for symbolic calculation of Feynman diagrams, is presented. The packages were designed as an aid in calculating tree-level multiparticle electroweak production processes, though they can be used for a much wider class of calculations. The packages have been used to calculate such processes as eγ→Wν and γγ→W+W− at tree level with arbitrary Wγ couplings. The packages are written in the computer algebra language Mathematica, which provides a powerful working environment.
An interactive NeXTSTEP interface to a Fortran code for solving coupled differential equations
Authors: Silbar Richard R.
Published: July 2002

This paper describes a user-friendly frontend to a Fortran program that integrates coupled nonlinear ordinary differential equations. The user interface is built using the NeXTSTEP Interface Builder, together with a public-domain graphical palette for displaying intermediate and final results. The main obstacle was implementing communication between the Objective-C environment governing the NeXTSTEP interface and the Fortran code. This was overcome by breaking up the Fortran into separate subroutines (which compile as C function calls), corresponding to the various control buttons in the interface. In running the code for a given set of equation parameters the user sees a plot of the solutions at each stage of the iterative process. In the case of a successful sequence of iterations, the initially discontinuous curves smooth out as the scale parameters of the solutions are adjusted to achieve a solution to the nonlinear equations. If the iterative process goes astray, as it often does for a poor choice of starting scale parameters, the user has the opportunity to stop and start over with a better choice, guided by the result of the previous run. The ease of interaction with the equations also allows the user to develop an intuition regarding their solutions and to explore the parameter space for the equations much more quickly.
Symbolic symmetry verification for harmonic functions invariant under polyhedral symmetries
Authors: Zheng Yibin , Doerschuk Peter C.
Published: July 2002

Methods for using symbolic computation systems to verify polyhedral symmetries of harmonic functions on the sphere are presented. The particular case of icosahedral symmetry, which is important in the structure of small spherical viruses, fullerenes, and quasi-crystals, is examined in detail. Previous work by the authors, in which general expressions for polyhedral harmonics in terms of spherical harmonics were derived, is verified by checking the symmetry of all icosahedral harmonics up to order 44. © 1995 American Institute of Physics.
Formation of thin silicon oxide films by rapid thermal heating
Authors: Ponpon J. P. , Grob J. J. , Grob A. , Stuck R.
Published: July 2002

Rapid thermal heating of silicon samples in a dry O2 ambient has been used to form thin SiO2 films. Compared to conventional furnace oxidation, an increased growth rate was observed which is linearly dependent on the square root of time. Activation energies of 1.99 and 2.26 eV for 〈111〉 and 〈100〉 orientation, respectively, have been determined in the range 1000–1200 °C.
The regrowth of implantation damage in silicon studied via silver depth profiling
Authors: Wilson R. G.
Published: July 2002

High and low densities of 300-keV Ag atoms were implanted into (100) and (111) crystalline and self-amorphized Si, and annealed at temperatures from 100 to 900 °C and for various times. Silver depth distributions were profiled using secondary ion mass spectrometry. Regrowth locations and rates of the various damaged regions were measured with good sensitivity via the Ag profiles, which resulted from the retention of Ag atoms by defect complexes and their release upon annihilation of the defect complexes during annealing. The following results were obtained for (100) Si: regrowth toward the surface of the 0.3-μm-thick amorphized layer beginning at about 450 °C, up to 650 °C, with a regrowth rate of (18±2) Å/°C or 5.2 Å/min at 530 °C; regrowth of the amorphous-crystalline interface region at 850 °C at a rate of (8±1) Å/min. The presence of 2×1019 cm−3 Ag atoms retards the regrowth rate of (100) Si by a factor of 2. Two separate, but closely spaced regions of residual damage in the amorphous–crystalline interface region were revealed. Regrowth of (111) Si occurs uniformily in depth throughout the amorphized region, consistent with the growth of polycrystalline or microtwinned Si in this region as seen by transmission electron microscopy.
Nonlinear optical properties of thin-film waveguides deposited onto semiconductor-doped glasses
Authors: Patela Sergiusz , Jerominek Hubert , Delisle Claude , Tremblay Réal
Published: July 2002

We report the fabrication procedure and properties of optical nonlinear guided wave structures consisting of a thin film of Corning 7059 glass deposited on the surface of CdSxSe1−x doped glass. The limiting action in power-dependent prism coupling in the structures produced is described. The nonlinear prism coupling is used to evaluate both the sign and the steady-state value of the intensity-dependent refractive index of the substrate. For the glass used the nonlinearity is negative and its value is in the range of 10−10 m2/W.
Diffusion of azimuthal magnetic fields into a cylindrical plasma
Authors: Roderick N. F.
Published: July 2002

Simple analytic models have been developed to describe the current density and magnetic field profiles in a cylindrically symmetric plasma carrying a z-directional current. Solutions are obtained for time varying boundary magnetic fields characteristic of imploding liner and gas puff plasma implosions. The cylindrical models are compared with previously developed planar solutions. Applications to electromagnetically imploded plasma shells show a small variation in the magnetic field profiles at early times and large radius. These variations become more pronounced as the implosion moves to smaller radius. In general, the effects of cylindrical geometry become significant only when the outer radius reaches the same order as the magnetic field gradient scale length.
Aligned, plasma sprayed SmCo5 deposits
Authors: Kumar K. , Das D.
Published: July 2002

Highly aligned SmCo5 deposits were produced using plasma spraying. c-axis alignment, normal to the plane of the deposit, was achieved by depositing the Sm-Co alloys on steel substrates maintained at high temperatures. The substrates were heated by the plasma flame to obtain the high temperatures. The attainment of a range of substrate temperatures was made possible through control over the geometry of the substrate.
Light–heavy-hole mixing in quantum well structures
Authors: Coon D. D. , Liu H. C.
Published: July 2002

Light–heavy-hole mixing is analyzed using a scattering formalism. An effective potential is found which describes mixing, scattering, and bound states in an effective mass envelope wave-function approximation. Two simple quantum well structures, a single barrier and a single well, are examined. A connection between mixing and strain in thin epitaxial layers is described. The hole tunneling work of E. E. Mendez, W. I. Wang, B. Ricco, and L. Esaki [Appl. Phys. Lett. 47, 415 (1985)] is discussed.
Effects of dc bias on electrolyte electroreflectance spectra
Authors: Wrobel J. M. , Reddy U. K. , Bassett L. C. , Aubel J. L. , Sundaram S.
Published: July 2002

The effects of a constant component of electric field (dc bias) across an electrolyte–semiconductor interface are studied and discussed. The change of bias causes variations in spectral features depending on the type of the semiconducting material and the position of a feature in relation to the interband transition energy. The phenomena are illustrated in experiments carried out on GaAs and HgMnTe. It is experimentally shown that dc bias in conjunction with the electrolyte electroreflectance technique can be used to determine the type of the material, to locate the flat-band potential, and to study the surface states. It has also been shown that dc bias can be used to determine low field conditions.
Eddy current test of steel tube employing electromagnet technique for dc magnetization
Authors: Kim Young-Joo , Yi Jae-yel , Lee Sekyung
Published: July 2002

The impedance of a test coil was measured as a function of the dc current of an electromagnet, and the determination method of magnetization level with the normalized impedance diagram is discussed. An eddy current test for the outer side of a steel tube wall was also performed. Test results show that the impedance change of the test coil is brought about by the eddy current perturbation and the change of magnetization level. These effects on the flaw inspection are also analyzed.
Influence of melt composition on the longitudinal distribution of midgap native donor concentration in semi-insulating liquid-encapsulated Czochralski GaAs crystal
Authors: Katsumata Tooru , Okada Hideo , Kimura Tadashi , Fukuda Tsuguo
Published: July 2002

The longitudinal distribution of midgap native donor concentration was investigated in detail using semi-insulating liquid-encapsulated Czochralski GaAs crystals grown from various melt compositions. The midgap native donor concentration was found to decrease with fractional solidification under Ga-rich conditions, to increase under As-rich conditions, and not to change under stoichiometric conditions. Moreover, it varied with the magnetic field intensity and the crystal-pulling speed depending on the melt composition. These phenomena were consistently explained by assuming that the midgap native donor acts like a dopant with a certain segregation coefficient k, where k<1 under As-rich conditions, k=1 under stoichiometric conditions, and k>1 under Ga-rich conditions. This is in good agreement with the expected segregation of As atoms determined from a phase diagram with a narrow solid-solution region.
Strongly correlated electron ground-state energy approximations for Anderson-like models
Authors: Mancini Jay D. , Potter Charles D. , Bowen Samuel P.
Published: July 2002

We report preliminary results of convergence properties for nonperturbative resolvent approximations to Anderson-like models of magnetic ions in metals. Our study is initially focused on the spin- 1/2 Anderson model for magnetic impurities, but the methods studied can include multiplet and crystal-field effects which are needed for more accurate descriptions of real systems. We will compare the nonperturbative Lanczos method (tridiagonalization) and similar truncation schemes to exact ground-state energies for the impurity model and assess the efficacy of these nonperturbative approaches to understanding the Anderson lattice, heavy fermions, and other strongly interacting electronic systems.
Mobility enhancement in highly doped GaAs quantum wells
Authors: Shur M. S. , Abrokwah J. K. , Daniels R. R. , Arch D. K.
Published: July 2002

We propose the mechanism of the mobility enhancement in highly doped semiconductor quantum wells, based on the large increase in the carrier concentration using the field effect and on the corresponding reduction of the ratio of the ionized impurity concentration to the electron concentration. We observe the mobility enhancement in GaAs quantum wells doped at ∼2×1018 cm−3 at 77 K from 572 cm2/V s under the equilibrium conditions to approximately 2500 cm2/V s when a large carrier concentration is induced into the quantum well in a field-effect transistor. This effect can be used to increase the voltage and current swing in modulation-doped structures and may also allow us to achieve the mobility enhancement in new semiconductor materials with inherently large defect densities.
Dopants redistribution during titanium-disilicide formation by rapid thermal processing
Authors: Pasa A. A. , de Souza J. P. , Baumvol I. J. R. , Freire F. L.
Published: July 2002

Redistribution of implanted As (5×1015 cm−2, 150 keV) and Sb (1×1015 cm−2 , 200 keV) due to TiSi2 formation by rapid thermal processing was investigated by means of backscattering spectroscopy. By choosing properly the rapid thermal processing parameters, low resistance uniform silicides layers (typically 14 μΩ cm) were obtained, and dopant loss by out-diffusion and redistribution of the dopant were made negligible. High peak concentrations of dopant within silicon substrate, beneath the silicide, were obtained for all processing times.
Distributions of activation energies for electromigration damage in thin-film aluminum interconnects
Authors: Schwarz J. A.
Published: July 2002

The distributions of activation energies for electromigration damage in Al and Al-4%Cu thin-film interconnects were determined using temperature-ramp resistance analysis to characterize electromigration. The activation energies for the pure aluminum conductors are well represented by a normal distribution. It is noted that the normality of the distribution of activation energies can account for the log-normal distribution of failure times that has been observed. On the other hand, the population of activation energies for the Al-4%Cu conductors is composed of three subpopulations, each of which is well represented by a normal distribution. This deviation from the results obtained for the Al samples can account for variations from log normality that have been observed in failure time distributions, for aluminum-alloy interconnects. It is proposed that the origin of the subpopulations is the affect of copper on the grain-boundary transport of aluminum.