bluuebunny/crossref_metadata_2025 huggingface dataset:
Texture analysis of CoGe2 alloy films grown heteroepitaxially on GaAs(100) using partially ionized beam deposition
Authors: Mello K. E. , Murarka S. P. , Lu T.-M. , Lee S. L.
Published: July 2002

Reflection x-ray pole figure analysis techniques were used to study the heteroepitaxial relationships of the cobalt germanide CoGe2 to GaAs(100). The alloy films were grown using the partially ionized beam deposition technique, in which low energy Ge+ ions are employed to alter the heteroepitaxial orientation of the CoGe2 deposits. The CoGe2[001](100)∥GaAs[100](001) orientation, which has the smallest lattice mismatch, was found to occur for depositions performed at a substrate temperature around 280 °C and with ∼1200 eV Ge+ ions. Lowering the substrate temperature or reducing the Ge+ ion energy leads to CoGe2(100) orientation domination with CoGe2[100](010)∥GaAs[100](001) and CoGe2[100](001)∥GaAs[100](001). Substrate temperature alone was seen to produce only the CoGe2(100) orientation. For CoGe2(001) films, additional energy was required from Ge+ ions in the evaporant stream.
Molecular stacking in epitaxial crystals of oxometal phthalocyanines
Authors: Yanagi Hisao , Mikami Tatsuo , Tada Hirokazu , Terui Toshifumi , Mashiko Shinro
Published: July 2002

Molecular stacking structures in epitaxial crystals of oxotitanium and oxovanadium phthalocyanines (TiOPc and VOPc) prepared by physical vapor deposition (PVD) and molecular beam epitaxy (MBE), respectively, onto the (001) face of a KBr substrate were investigated by means of x-ray and electron diffraction studies. In thin epitaxial film of VOPc deposited by MBE at a substrate temperature of 20 °C, the flat-lying molecules oriented in the (3×3) commensurate square lattice (a=1.40 nm) were alternately stacked with body-centered head-to-head and a/4-slipped face-to-face dimeric bilayers. The crystal structure determined was the triclinic space group P1̄ with a=b=1.40, c=1.06 nm, α=109.2, β=133.9, γ=90.0°, Z=2. When TiOPc was thickly deposited by PVD at a substrate temperature of 200 °C, islandlike crystals epitaxially grew in a truncated pyramidal morphology and assumed the same orientation. In the pyramidal crystals the thick molecular layer stacking separate from the KBr surface was relaxed and had a square lattice with a shorter constant (a=1.33 nm). The three-dimensional stacking structure explained the observed grain boundary formation and asymmetric pyramidal morphology.
X-ray photoemission spectroscopy studies of conducting polymer-substrate interfaces: Interfacial electrochemical diffusion
Authors: Kato Hitoshi , Takemura Susumu , Nakajima Yasushi
Published: July 2002

Interfaces between conducting polymer polypyrrole (PP) and transparent conductive materials such as indium tin oxide (ITO) and SnO2 were investigated by x-ray photoemission spectroscopy (XPS). XPS lines of In 3d5/2,3/2 and Sn 3d5/2,3/2 were observed in the XPS spectra of electrochemically reduced PP film sides of PP/ITO and PP/SnO2, respectively. The results indicate that electrochemical diffusion of substrate materials into conducting polymer films takes place by applying negative voltage to the substrate. The energy positions of the 3d lines of the diffused In and Sn which are close to those of the metal oxides support the proposition that the diffused species are metal oxide ions. In the electrochemically diffused PP/ITO and PP/SnO2 interfaces, XPS spectra of substrate sides exhibit In 3d and Sn 3d lines associated with satellite peaks, which indicates magnetic splitting of 3d levels or electron transfer typed shake-up. This indicates that electrochemical diffusion affects the substrate electronically.
Interfacial interaction between Al-1%Si and phosphorus-doped hydrogenated amorphous Si alloy at low temperature
Authors: Liao Wen-Shiang , Lee Si-Chen
Published: July 2002

The interfacial interaction between phosphorus-doped amorphous silicon hydrogen alloy ((n+)a-Si:H) and thermal evaporated Al-1%Si layer after furnace annealing in the temperature range from 150 to 250 °C has been investigated in detail. The scanning electron microscope photographs show that many dendrites were formed on the original (n+)a-Si:H surface at annealing temperature higher than 170 °C. Raman spectroscopy and transmission electron diffraction show that the original (n+)a-Si:H film has been converted to polycrystalline Si with the crystalline Si dendrites on top. The drastic increase (∼4 orders of magnitude) of electrical conductivity of the 200 °C annealed (n+)a-Si:H films with the Al-1%Si removed is caused by the formation of polycrystalline silicon percolation channel in the background area between dendrites. Auger spectroscopy also provides evidence that no aluminum is incorporated into the converted film during silicon recrystallization and thus no SiAl alloy is formed.
Effects of evanescent modes and subband mixing in resonant tunneling transistors
Authors: Lee Chomsik , Weichold Mark H.
Published: July 2002

We theoretically investigate a gated resonant tunneling diode as a potential new quantum resonant tunneling transistor. The resonant tunneling transistor is dimensionally controllable in the active channel using a lateral depletion region. Under bias conditions in which the resonant tunneling transistor reaches zero dimensionality in the well region, we consider the attractive and repulsive perturbation potentials (Vpe) of impurity or defect scattering in source and well regions. A transfer matrix formalism is employed to calculate the transmission probabilities and current densities through the double barrier, and we describe the scattering matrices using the presence of evanescent modes in various lateral confinement configurations. We discuss the effect of scattering and subband mixing on the current-voltage characteristics of resonant tunneling transistors.
Micromagnetics of NiFe/Ag giant magnetoresistance multilayer thin films (abstract)
Authors: Fang H. , Zhu J.-G.
Published: July 2002

Granular NiFe/Ag multilayer film exhibits low saturation field and large giant magnetoresistance (GMR) effect, suited as low-field sensor materials.1 However, observed Barkhausen noise in the GMR measurements in these films have been limiting their practical applications.2 Here, we report a micromagnetic modeling study, focusing on the correlation between the micromagnetic properties and film microstructure. It is found that the spatial randomness of intergranular exchange coupling, likely to occur in the film due to its granular microstructure, can cause Barkhausen jumps in GMR response. Figure 1 shows calculated GMR value of a patterned NiFe/Ag/NiFe trilayer with a size of 1.28 μm×1.11 μm, as a function of external fields, applied along its hard axis direction. The Barkhausen jumps can be clearly identified along the GMR response due to its μm size domain switching. In addition to the spatial nonuniformity, effects of various intergranular and interlayer Heisenberg and non-Heisenberg exchange couplings are systematically studied. Figure 2 shows the calculated GMR response for a patterned planar isotropic trilayer film with an interlayer biquadratic exchange coupling. The triangular shape of the GMR response is the characteristics of the biquadratic interlayer exchange coupling.
Effects of cooling field on magnetic relaxation on an iron-nitride fine particle system
Authors: Mamiya H. , Nakatani I.
Published: July 2002

Effects of the external magnetic field Ha applied during cooling on the magnetic relaxation of a fine particle system are studied. We measured the temperature decay of the residual magnetization and the ac susceptibility for frozen iron-nitride magnetic fluids. It was found that the relaxation time distribution of the diluted sample is independent of Ha and coincides with that expected from the particle diameter distribution, as the superparamagnetic model presents. On the other hand, in the concentrated sample, it is clear that the relaxation time in zero field is widely distributed and depends on Ha.
Influence of the cooling rate on the electrical conductivity of coevaporated CuInS2 thin films
Authors: Alt M. , Lewerenz H. J. , Scheer R.
Published: July 2002

We investigated the influence of the cooling rate in a deposition process on the electrical conductivity σ of CuInS2 thin films for solar cells. In situ measurements were used in order to monitor the electrical conductivity of Cu-poor films during growth and cooldown. It is shown that the room-temperature electrical conductivity σRT depends significantly on the cooling rate of the film after deposition. σRT reaches 1×10−3 S cm−1 for a cooling rate of 2.0±0.2 K min−1 whereas for abrupt cooling and higher cooling rates σRT is reduced to 10−6 and 10−5 S cm−1, respectively.
Bulk and surface magnetic study of Fe80B20 crystallization
Authors: Vavassori P. , Ronconi F. , Puppin E.
Published: July 2002

We report a study relating the evolution of coercivity Hc, both in the bulk and in the surface region, to the microstructural changes occurring during the first stage of the crystallization process of Fe80B20 amorphous samples. The coercive field Hc has been studied with a magneto-optical Kerr ellipsometer in the surface and with a vibrating sample magnetometer in the bulk. The observed behavior of Hc can be accounted for by considering its dependence on the number density of grains and gives information on the domain wall dynamic indicating the occurrence of a two dimensional bending of the domain walls. In addition, the evolution of Hc is substantially insensitive to the details of the internal structure of the grains. This study reveals that the surface crystallization occurs via the same processes as in the bulk but it takes place at lower temperature with respect to the bulk. We also measured the Curie temperature of the surface region of the amorphous ribbon and we obtained a lower value with respect to the bulk.
Surface effects in Fe-based nanocrystalline alloys
Authors: Ślawska-Waniewska A. , Roig A. , Molins E. , Grenèche J. M. , Żuberek R.
Published: July 2002

FeZrBCu nanocrystalline materials obtained by controlled crystallization of the amorphous alloy are composed of bcc-Fe grains (10 nm in diameter) embedded in a residual amorphous matrix. In this article it has been shown that in such a mesoscopic system the interfaces exhibit identifiable properties that differ from those of the interior. In the Mössbauer spectra, besides of contributions from bcc phase and that of the residual amorphous matrix, an additional component was observed and attributed to the crystalline grain-amorphous matrix interfaces. The effect of the surface layer on the effective magnetostriction is also presented and discussed.
Formation of c-axis oriented barium ferrite thin films by alternate deposition of a Fe3O4 layer and BaO⋅3Fe2O3 layer
Authors: Hoshi Y. , Kubota Y. , Onodera H. , Shinozaki H. , Shimizu H. , Ikawa H.
Published: July 2002

Deposition of c-axis perpendicularly oriented barium ferrite (BaM:BaFe12O19) thin films was attempted by means of an alternate periodic deposition of S (spinel Fe3O4) and R (BaO⋅3Fe2O3) layers. The period of the layers was fixed at 1.15 nm, which corresponds to the period of hexagonal barium ferrite crystals in the c-axis direction. Films 115 nm thick were deposited on a c-axis oriented ZnO underlayer by using a dc sputtering system with three facing target sputtering sources. This layer deposition method was expected to deposit a film with much better crystallite orientation at a lower substrate temperature than needed to deposit a film when using a conventional sputter deposition method (i.e., sputter deposition using a stoichiometric BaM sintered target). The films deposited at temperatures above 540 °C had a hexagonal crystal structure and showed c-axis orientation, but this deposition method did not result in a marked reduction of substrate temperature needed for the deposition of a BaM film with hexagonal crystal structure. The films deposited at 650 °C had a clear terraced surface due to the growth of disk shaped hexagonal crystallites. The saturation magnetization of the film was about 220 emu/cc and the coercive force of the films was about 2.2 kOe.
Role of film roughness and interdiffusion in the formation of nonferromagnetic fcc Fe in the Fe/Co(100) system
Authors: Escorcia-Aparicio Ernesto J. , Kawakami R. K. , Choi Hyuk J. , Qiu Z. Q.
Published: July 2002

The fcc Fe/Cu(100) and Fe/Co(100) systems are characterized by a wide range of magnetic and structural phases. In particular, a nonferromagnetic fcc phase with a live layer has been observed for room temperature growth Fe films in the ∼5–11 ML thickness range. This nonferromagnetic phase is not present for low temperature (∼120 K) grown films even when the film temperature is raised to room temperature. Annealing the film at 475 K, however, will recover the room temperature phase. Two effects that could account for these features are interdiffusion and surface smoothening. In order to determine which of these effects is responsible for the occurrence of the nonferromagnetic phase, we performed experiments on the Fe/Co(100) system to separate these two effects. An artificially roughened sample grown at room temperature exhibits a ferromagnetic phase only. A sample grown at low temperature in which the first few layers are alloyed to simulate interdiffusion also shows the ferromagnetic phase. Experiments in which a few layers of varying thicknesses are grown at room temperature and the rest of the film is grown at low temperature exhibit different magnetic phases depending on the thickness of the room temperature grown layer. These results indicate that film roughness is the main factor that determines whether the nonferromagnetic phase of fcc Fe will be present in the ∼5–11 ML thickness range.
CF
4
/O
2
plasma induced damage in Si-implanted n-type GaAs crystals
Authors: Wada Jun , Matsukura Yusuke , Ogihara Toshihiro , Ueda Osamu
Published: July 2002

The CF4/O2 or CF4 plasma induced damage in Si-implanted n-type GaAs crystals has been investigated by using Hall measurement and secondary ion mass spectrometry (SIMS) analysis. We found that sheet resistance degraded after plasma exposure and postannealing. The degradation of the sheet resistance during postannealing at 200–400 °C, is mainly associated with the reduction of carrier density in the crystal. From SIMS analysis, fluorine contamination was observed in the surface region of GaAs crystals after CF4/O2 plasma exposure. The internal diffusion of fluorine atoms and the localization of fluorine in the Si-implanted n-type GaAs layer, were observed after postannealing at 400 °C. Based on these findings, a fluorine passivation related model is proposed, where Si–F bonds are formed in the Si-implanted n-type GaAs crystals.
Picosecond lasing dynamics of gain-switched (Zn,Cd)Se/Zn(S,Se)/(Zn,Mg)(S,Se) quantum well lasers
Authors: Michler P. , Neukirch U. , Wundke K. , Gutowski J. , Behringer M. , Hommel D.
Published: July 2002

We present the temporal and spectral evolution of stimulated emission from (Zn,Cd)Se/Zn(S,Se)/(Zn,Mg)(S,Se) multiple quantum well lasers at 2 K after picosecond optical excitation. For pumping well above the threshold, ultrahigh-frequency oscillations of up to 100 GHz are observed. These oscillations are generated by a multimode emission through mode beating caused by the coherent superposition of longitudinal ground modes and higher lateral modes. The modelling of the experimental results shows that the modes are coupled to some extent.
Photoelectron and infrared spectroscopy of semi-insulating silicon layers
Authors: Trchová M. , Zemek J. , Jurek K.
Published: July 2002

X-ray induced photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, and electron microprobe analysis (EMA) were used to study semi-insulating polycrystalline silicon layers obtained by chemical vapor deposition from SiH4 and N2O gases. A mean “bulk” oxygen concentration determined by EMA ranged from 16 to 50 at. %. Photoelectron spectra excited by Mg Kα and Al Kα radiation were used to find surface composition and bonding information of as-received layers. Beneath the native oxide with a stoichiometry close to the SiO2, there is a heterogeneous material consisting of silicon (Si–Si) and silicon oxide (Si–O) regions. A drop in the Si 2p peak position differences with the mean bulk oxygen concentration indicates a differential charging of the silicon islands surrounded by a silicon oxide phase. A spectral band of an asymmetric Si–O–Si stretching vibration mode around 1030 cm−1 was used to characterize the samples. An assumption that the samples were a homogeneous a-SiOx phase leads to a mean oxygen content much higher than that determined by the EMA technique. On the other hand, oxygen concentration deduced from the number of (Si–O) bonds, calculated by the normalized integrated absorption intensity, were very close to the results of EMA. This leads us to the conclusion that the samples consist of two phases, a-SiOr and Si. The values of the refractive index obtained by the spectrophotometric method from reflectance spectra and the results of XPS measurements strongly support the two-phase model.
Oscillation of the cantilever in atomic force microscopy: Probing the sample response at the microsecond scale
Authors: Bouhacina T. , Michel D. , Aimé J. P. , Gauthier S.
Published: July 2002

The relaxation of an atomic force microscope microlever is investigated after the unsticking instability. We show that the different oscillating behavior can be explained by considering that polymer chains remain stuck at the tip even when the microlever is away from the sample. A simple rheological model is used which satisfactorily describes the different features observed. The approach described in the article provides useful additional information to understand the behavior of a microlever coupled to a polymer network.
Deep level of iron-hydrogen complex in silicon
Authors: Sadoh T. , Tsukamoto K. , Baba A. , Bai D. , Kenjo A. , Tsurushima T. , Mori H. , Nakashima H.
Published: July 2002

Deep levels related to iron in n-type silicon have been investigated using thermally stimulated capacitance (TSCAP) combined with minority carrier injection. The TSCAP measurement reveals two traps of EV+0.31 and EV+0.41 eV. The trap of EV+0.41 eV is a donor due to interstitial iron. The trap of EV+0.31 eV, due to a complex of interstitial iron and hydrogen, is observed in the sample etched chemically with an acid mixture containing HF and HNO3 and annihilates after annealing at 175 °C for 30 min. It is demonstrated that interstitial 3d transition metals such as vanadium, chromium, and iron tend to form complexes with hydrogen in n-type silicon, and the complexes induce donor levels below the donor levels of the isolated interstitial species. This trend is related to the interaction between the metals and hydrogen in the complexes.
Photon effect on radiative properties of silicon during rapid thermal processing
Authors: Chen G. , Borca-Tasciuc T. , Fair R. B.
Published: July 2002

Emissivity is a critical parameter in the rapid thermal processing (RTP) of semiconductors for temperature control and thermal modeling. It is often considered as a material property that depends on the sample temperature and surface finishing. For a silicon wafer placed in a radiation environment such as a RTP chamber, however, the ambient photons emitted from lamps create electron-hole pairs in the wafer. These electrons and holes participate in the thermal emission from the wafer and change its radiative properties. This work studies this photon effect on the radiative properties of silicon and demonstrates, through modeling and experiment, that radiative properties of a silicon wafer may depend on the temperature of the lamps due to free-carrier excitation. Such an photon effect imposes a limit on the accuracy of temperature measurement by the infrared pyrometry method in a RTP environment. It is also an important factor to consider in the measurement of the temperature dependence of silicon’s optical properties and in the thermal modeling of the RTP.
Aluminide formation in polycrystalline Al/W metal/barrier thin-film bilayers: Reaction paths and kinetics
Authors: Bergstrom D. B. , Petrov I. , Allen L. H. , Greene J. E.
Published: July 2002

Polycrystalline bcc W layers, 110 nm thick with 011 preferred orientation and an average grain size of 40 nm, were grown on amorphous-SiO2/Si(001) substrates by ultrahigh vacuum (UHV) magnetron sputter deposition at Ts=600 °C. Al overlayers, 170 nm thick with strong 111 preferred orientation and an average grain size of 120 nm, were then deposited at Ts=100 °C without breaking vacuum. Changes in bilayer sheet resistance Rs were monitored continuously as a function of time ta and temperature Ta during UHV annealing. In addition, area-averaged and local interfacial reaction paths, as well as microstructural changes as a function of annealing conditions, were determined by x-ray diffraction, Rutherford backscattering spectroscopy, transmission electron microscopy (TEM), and scanning TEM in which compositional distributions in cross-sectional specimens were obtained by energy-dispersive x-ray analysis using a 1 nm diam probe beam. The two tungsten aluminides which form, WAl4 and WAl12, are nucleated essentially immediately with no measurable induction time. WAl4 grains, extensively twinned, increase in size during the initial reaction, then stop growing as competitive growth in the diffusion limited regime favors WAl12. Information from microstructural and microchemical analyses was used to model the Rs(Ta,ta) data in order to determine reaction kinetics and activation energies. The results show that WAl12 growth is limited by W diffusion, with an activation energy of 2.7 eV, to the Al/aluminide interface.
Bond-length anomaly in InP1−xAsx monolayers on InP(001) studied by extended x-ray absorption fine structure
Authors: Kuwahara Y. , Oyanagi H. , Shioda R. , Takeda Y. , Kamei H. , Aono M.
Published: July 2002

The bond-length relaxation in InP1−xAsx monolayers epitaxially grown by metalorganic vapor-phase epitaxy on an InP(001) substrate has been investigated using the extended x-ray absorption fine structures on the As K edge. The In–As bond lengths in InP1−xAsx monolayers were determined over a wide range of As composition x (0.08<x<0.80). We find that the In–As bond length RIn–As in InP1−xAsx monolayer shows an anomaly at x∼0.5; RIn–As is compressed at x∼0.5, deviating from the linear interpolation between the values in a dilute limit (x<<1, As:InP) and the strained InAs monolayer (x=1). The As composition dependence of RIn–As coincides with that observed for As atoms incorporated by a surface As–P exchange reaction. The mechanism of anomalous bond-length variation, associated with the change of local structure from the dilute limit to the strained monolayer, is discussed in terms of the elastic energy confined in a strained heterointerface.
Computation of the effective dielectric constant of two-component, three-dimensional mixtures using a simple pole expansion method
Authors: Liu Ce , Wu Hongxu
Published: July 2002

Bergman and Milton proved that the effective dielectric constant or conductivity of a two-component composite material is a function of the ratio of the dielectric constants or conductivities of the components which can be described by a series of simple poles and residues. These poles and residues are determined only by the microgeometry of the composite. In this study, we use a simplified three-dimensional Fourier series expansion method to locate the poles and residues for simple cubic, body-centered, and face-centered lattices in different concentrations. Comparison between the simple pole theory and the Fourier series expansion method shows a good agreement.
Observation of competing etches in chemically etched porous silicon
Authors: Winton M. J. , Russell S. D. , Gronsky R.
Published: July 2002

Transmission electron microscopy and scanning electron microscopy offer evidence that the purely chemical HF:HNO3:H2O “stain etch” used to form light-emitting porous silicon is actually composed of competing etches. A localized etch forms the porous nanostructure by propagation of a discrete reaction interface into the silicon substrate. An amorphous surface layer (SiO2) that is a significant by-product of this etch has been observed and is believed to be a primary efficiency-limiting factor in attempts to fabricate chemically etched porous silicon devices. A destructive etch competes for ions in solution and removes both the porous silicon and the amorphous layers from the surface of the specimen when it becomes dominant, eventually quenching the luminescence properties.
Effect of capacitive feedback on the characteristics of direct current superconducting quantum interference device coupled to a multiturn input coil
Authors: Minotani T. , Enpuku K. , Kuroki Y.
Published: July 2002

Distortion of voltage versus flux (V–Φ) relation of a dc superconducting quantum interference device (SQUID) coupled to a multiturn input coil is studied. First, resonant behavior of the coupled SQUID due to the so-called input coil resonance is clarified. It is shown that large rf noise flux is produced by the input coil resonance. This rf flux is added to the SQUID, and results in large rf voltage across the SQUID. In the case where parasitic capacitance exists between the input coil and the ground of the SQUID, this rf voltage produces the rf flux again, i.e., a feedback loop for the rf flux is formed. Taking into account this capacitive feedback, we study the V–Φ relation of the coupled SQUID. Numerical simulation shows that the V–Φ relation is distorted considerably by the feedback mechanism. The simulation result explains well the experimental V–Φ relation of the coupled SQUID. The combination of the input coil resonance with the capacitive feedback is the most likely mechanism for the distorted V–Φ curve of the coupled SQUID. The condition for occurrence of the distorted V–Φ curve due to the capacitive feedback is also obtained, and methods to prevent degradation are discussed.
Reply to “Comment on ‘Phase equilibria of the Ga–Ni–As ternary system’ ” [J. Appl. Phys. 82, 493 (1997)]
Authors: Ingerly D. B. , Swenson D. , Jan C.-H. , Chang Y. A.
Published: July 2002

We reply to the comment by Guerin and Guvarc’h concerning the phase equilibria of the Ga–Ni–As ternary system. We refute their position that the quench rate of the phase diagram samples is not important and that the solid-state phase equilibria of the Ga–Ni–As system does not change significantly between the temperatures of 25 and 800 °C. It is also demonstrated that the occurrence of ordered superlattice structures is not inconsistent with our work. We conclude that an adequate number of samples were prepared in our study to justify the modifications proposed to the Ga–Ni–As isothermal section and it is an accurate representation of the phase equilibria at 600 °C.
Residual conductivity of stressed Ge:Ga photoconductors after low-dose gamma irradiation
Authors: Patrashin M. , Fouks B. , Grözinger U. , Lemke D. , Wolf J.
Published: July 2002

Experimental results and a plausible physical model of the γ-induced conductivity changes in stressed Ge:Ga photoconductive detectors are presented. Detector samples with boron-implanted contacts operated under conditions of low-temperatures 1.6−2.3 K and low photon backgrounds 106–108 cm−2 s−1 showed a long-term increase of conductivity and a decrease of the breakdown field after being exposed to 1 MeV γ-quanta for a total dose <4 rad. The radiation-induced changes of the parameters are entirely suppressed by a temperature increase to 4.2 K, while an increase of the electric field above breakdown restores the parameters only partially. The effects are explained in terms of a change of the effective compensation ratio of the samples due to recharging of the donor and acceptor impurities by electrons and holes generated under the γ-irradiation, as well as taking into account that the boron-implanted contacts block minority carriers (electrons) in the semiconductor bulk and prevent their sweepout from the detector.
Free-electron transport in semi-insulating GaAs
Authors: Khirouni K. , Bourgoin J. C.
Published: July 2002

The frequency (ω) response of the admittance (Y) of Czochralski (Cz) and Bridgman (Bg) grown semi-insulating materials have been investigated in the range 300–500 K. For both materials, this conductivity remains constant up to a frequency ω at which it becomes proportional to ω1. A minimum is observed in the Y(ω) characteristics between these two regimes in Cz materials but not in Bridgman ones. It is suggested that the existence of this minimum is related to percolation induced by the presence of space-charge regions located around As precipitates and charged dislocations, which are present in Cz but not in Bg materials. The percolation invalidates the homogeneous conduction assumption made in analyzing the electrical properties of semi-insulating Cz materials.
A comparative study of electrogenerated chemiluminescence in poly(3-hexylthiophene) and poly(2-methoxy-5-dodecyloxy-p-phenylenevinylene)
Authors: Nambu Hidetaka , Hamaguchi Maki , Yoshino Katsumi
Published: July 2002

Electrogenerated chemiluminescence (ECL) from films of the title compounds in an electrolytic solution were studied. Poly(3-hexylthiophene) and poly(2-methoxy-5-dodecyloxy-p-phenylenevinylene) possess ECL predominantly during the anodic and cathodic scan, respectively, in the double potential step electrolysis which alternately produces negatively and positively charged excited species of the polymers. The chronoabsorptometric measurements indicated that the polarity dependence of the ECL is originated from the difference in the kinetics of the electrochemical p- and n-type doping/undoping reactions.
Low temperature three-dimensional thermoluminescence spectra of undoped YVO4 single crystals grown by different techniques
Authors: Erdei S. , Kovács L. , Pető Á. , Vandlik J. , Townsend P. D. , Ainger F. W.
Published: July 2002

Three dimensional (3D) thermoluminescence (TL) spectra of two yttrium orthovanadate (YVO4) single crystals grown from YVO4 melt by the Czochralski (CZ) technique and one additional YVO4 crystal pulled from lithium-metavanadate (LiVO3) flux by the top seeded solution growth (TSSG) method were investigated after x-ray irradiation. The TL spectra were recorded in the 200–800 nm wavelength and 20–300 K temperature ranges before and after annealing of the crystals in an oxygen atmosphere. In spite of the different growth conditions the measured 3D TL spectra of the CZ samples show similar TL characteristics. The main TL emission appears around 450 nm, and between 200 and 250 K. Further weak emissions were also detected at 570, 600, 650, and 710 nm wavelengths, which might be attributed to unwanted impurities. The TL spectrum of the TSSG crystal markedly differs from that of the CZ crystals. On one hand the major TL appears at lower temperatures whereas on the other hand the emission spectrum exhibits a broader band around 500 nm, while the weak bands between 500 and 700 nm could not be detected. Based on these observations the different defect structures in YVO4 crystals are discussed in the article emphasizing that 3D TL measurement is a useful practical tool for the characterization and identification of YVO4 crystals grown by different methods.
Optical spectra and analysis of Er3+ in silicon with C, O, and N impurities
Authors: Wortman D. E. , Morrison C. A. , Bradshaw J. L.
Published: July 2002

Photoluminescence (PL) spectra of Er3+ in an Er-doped silicon film were recorded at 2 and 77 K over the wavelength range from 1.52 to 1.68 μm, which includes optical transitions between the Er3+ I13/24 and I15/24 manifolds. These spectra were then analyzed to determine the nature of the electrostatic field at the site of the Er ion that governs the ion’s optical behavior. The PL spectra were analyzed three different ways: by considering the Er3+ ions to be occupying sites of Td, C3, or D2d point group symmetry. Transition energies and magnetic dipole transition probabilities were calculated for each case by diagonalizing a Hamiltonian representing the free-ion and crystal-field interactions in a Russell–Saunders basis of states spanning the lowest 15 J multiplets of the 4f11 electronic configuration of Er3+. Crystal-field parameters were varied in each case to find the best agreement with the experiment. Our results showed that the site symmetry of the Er3+ ions most consistent with the data is D2d. For this case, the rms deviation between the experimental and calculated energy levels is 3.3 cm−1 with the calculated magnetic dipole transition probabilities in qualitative agreement with the experiment.
A molybdenum disulfide magnetic solid lubricant for ferromagnetic surfaces
Authors: Miremadi Bijan K. , Colbow Konrad , Morrison S. Roy
Published: July 2002

Molybdenum disulfide (MoS2) has been used in industrial applications as a dry lubricant. Although this material initially improves anti-wear properties, as surfaces move against each other the lubricating particles tend to spread out from the asperity points. To reduce this spread of lubricant, MoS2 platelets were made magnetic by replacing the unsaturated or loosely bonded sulfur atoms (located mainly on the edges of the platelets) with magnetic elements such as Fe, Ni, or Co particles. Under appropriate experimental conditions these elements form stable bonds with the partially free Mo atoms of the edges and are protected from oxidation in air by the basal planes of the overlapping neighboring layers. When compared to the friction coefficient of pure MoS2, the incorporation of magnetic Fe particles for example, increases the friction depending on the Fe/Mo ratio, but the overall result is in an improved lubricant with unique magnetic properties when the Fe/Mo ratio is properly optimized. In this application at least one of the surfaces must be ferromagnetic to hold the lubricant in place.
Influence of noise in the route to chaos of directly modulated semiconductor lasers
Authors: Carpintero G. , Lamela H.
Published: July 2002

In this article we carry out a careful investigation of the route to chaos exhibited by directly modulated semiconductor lasers as the modulation current amplitude is varied. The role played by the noise fluctuations in the route to chaos is analyzed. The laser is described by the rate equation model in which the noise is introduced through appropriate Langevin sources. For this analysis, we have first studied the dynamics of the underlying deterministic system. Using time integration, the bifurcation diagrams, when the modulation index is swept forwards and backwards, were obtained. These have provided only the stable solutions of the system. From these diagrams, the well known period doubling route to chaos is shown to coexist at certain intervals of the modulation index with a periodic three solution, showing a hysteresis phenomenon. Thorough explanation of the deterministic dynamics requires knowledge of unstable solutions, which we have found using a continuation method. This method reveals that the periodic three solution appears by a tangent bifurcation and that the hysteresis cycle involves chaotic bifurcations. At this point, we have introduced noise fluctuations, which allow the solutions to probe the surrounding dynamics. The route to chaos is found to be reversible and via period doubling, period quadrupling, and period tripling in agreement with experimental data. From this study, a consistent interpretation of the route to chaos was found in agreement with different experimental results.
Effect of a magnetic field on the stability of the flow of molten material in laser cutting processes
Authors: Chen C. K. , Hung C. I. , Tsai J. S.
Published: July 2002

In this analysis, the generalized kinematic equation for the thickness of the molten film is derived by perturbation methods and the method of multiple scales is used to investigate the weakly nonlinear stability of the flow of molten material in laser cutting under a transverse uniform magnetic field, taking into account the effect of phase change at the liquid–vapor interface. Analysis of the linear stability of the molten layer shows that the optimum cutting speed can be increased through use of the magnetic field. Nonlinear stability analysis further shows that supercritical stability and subcritical instability exist in the laser cutting system with or without magnetic effect. In a magnetic field, the equilibrium amplitude of high cutting speeds decreases, and the minimum threshold amplitude increases at low cutting speeds. The effect of the magnetic field, measured by the Hartmann number (m) is to stabilize the flow regardless of the cutting speed value or gas velocity. An appropriate applied magnetic field improves the laser cutting process and counteracts surface roughness.
Uniform bookshelf alignment of chiral smectic C films with guided backflow
Authors: Jákli Antal , Saupe Alfred
Published: July 2002

We describe a new technique to prepare cells of smectic C* films with a uniform bookshelf texture that can be bistably switched. This is achieved with a homeotropic surface coating with weak anchoring properties. The surface of one plate was modulated by parallel stripes of a polymer which hinder flow perpendicular to the stripes. This gives bias to field induced flow effects and enhances alignment by periodic switching. The resulting texture is uniform with smectic layers normal to the cover plates and parallel to the deposited stripes. The stripes also help to reheal textures with defects.
Observation of new type resonances in triple barrier resonant tunneling diodes
Authors: Jo J. , Alt K. , Wang K. L.
Published: July 2002

Current–voltage characteristics measured in triple barrier resonant tunneling diodes reveal new features about energy level alignment in a superlattice. Our data indicate that energy levels in the two quantum wells are not aligned at current peaks. Current peaks are observed when one of the energy levels in the two wells becomes resonant with the emitter level. One of the current peak was thermally activated, and showed inverted bistability at 77 K temperature. We explain that this is due to X state assisted tunneling.
Light-induced infrared absorption spectra and properties of impurity levels in doped photorefractive BaTiO3
Authors: Zhang Jiasen , Gao Hong , Dou S. X. , Zhu Yong , Ye Peixian
Published: July 2002

Light-induced absorption spectroscopy, the decay process of light-induced absorption, and its temperature dependence were investigated to study the properties of the photorefractive impurity levels in doped BaTiO3. Light-induced absorption spectra in one Rh-doped and two Ce-doped BaTiO3 crystals from 0.7 to 3.0 μm were measured and it was found that the light-induced absorption of all these crystals approached zero above ∼1.3 μm. A two-shallow-level model is proposed to explain the biexponential behavior of the decay of the induced absorption in BaTiO3:Ce. The activation energies are deduced to be 0.72 eV for the single-shallow level in BaTiO3:Rh, 0.50 and 0.66 eV for the two shallow levels in the 8 ppm Ce-doped BaTiO3, and 0.50 and 0.74 eV for that in the 30 ppm Ce-doped BaTiO3.
Modeling multipeak current–voltage characteristic and hysteresis phenomena for several resonant tunneling diodes connected in series
Authors: Gan Kwang-Jow , Su Yan-Kuin
Published: July 2002

The multipeak current–voltage (I–V) characteristic and hysteresis phenomena based on a series combination of several resonant tunneling diodes (RTDs) are analyzed and simulated. Our analytic model is based on a load-line technique with a piecewise-linear approximation for the I–V curve of RTD. The peak-to-valley current ratio of N RTDs in series and the effects of series resistance on the combined I–V characteristic are investigated. The results provide a useful design and estimation for the multipeak I–V characteristic with series-connected several RTDs before circuit accomplishment.
Low temperature magnetic properties of potassium doped pentacene
Authors: Mori Takao , Ikehata Seiichiro
Published: July 2002

Static susceptibility of the new molecular compound potassium doped pentacene was measured for the first time together with electron spin resonance (ESR) spectra in the dilution temperature region 4.2–0.02 K. Interesting thermal hysteresis and magnetic field dependence were observed in both the ESR spectra and static susceptibility indicating that an unusual kind of magnetic transition is taking place in this system. The present result is consistent with the previous observation of an antiferromagnetic-like transition in ESR data around 0.2 K.
Mapping two-dimensional arsenic distributions in silicon using dopant-selective chemical etching technique
Authors: Neogi Suneeta S. , Venables David , Ma Zhiyong , Maher Dennis M. , Taylor Mitchell , Corcoran Sean
Published: July 2002

Transmission electron microscopy (TEM) image contrast was used to characterize doping-dependent etching of n+/p junctions in silicon. The local variations in crystal thickness give rise to the appearance of thickness fringes which may be interpreted as two-dimensional iso-concentration contours that map the dopant distribution. The etchant solution used for selective chemical etching of TEM samples prepared using wedge technique was modified to reduce the etch rate and maintain high selectivity to the n+ doped region. The two-dimensional dopant profiles were quantified by calibrating against one-dimensional secondary ion mass spectroscopy data and also compared with one-dimensional spreading resistance analysis data.
Measurement of multiple-electron emission in single field-emission events
Authors: Piestrup M. A. , Puthoff H. E. , Ebert P. J.
Published: July 2002

Thermal and field electron emission from a modified electron microscope W source were measured with an energy-dispersive counting system. Thermal-emission spectra were consistent with random emission of single electrons, while field-emission spectra were consistent with multiple-electron emission in random events. As many as 11 electrons were detected in isolated random field-emission events.
Surface crystallization and magnetic properties of amorphous Fe80B20 alloy
Authors: Vavassori P. , Ronconi F. , Puppin E.
Published: July 2002

We have studied the effects of surface crystallization on the magnetic properties of Fe80B20 amorphous alloys. The surface magnetic properties have been studied with magneto-optic Kerr measurements, while those of bulk with a vibrating sample magnetometer. This study reveals that surface crystallization is similar to the bulk process but occurs at a lower temperature. At variance with previous results on other iron-based amorphous alloys the surface crystalline layer does not induce bulk magnetic hardening. Furthermore, both the remanence to saturation ratio and the bulk magnetic anisotropy do not show appreciable variations after the formation of the surface crystalline layer. The Curie temperature of the surface layer is lower with respect to the bulk of the sample. These effects can be explained by a lower boron concentration in the surface region of the as-cast amorphous alloy. Measurements of the chemical composition confirm a reduction of boron concentration in the surface region.
Refractive index of interdiffused AlGaAs/GaAs quantum well
Authors: Li E. Herbert
Published: July 2002

A model is developed for the polarization dependent refractive index of interdiffused AlGaAs/GaAs quantum well at room temperature for wavelengths ranging from 0.6 to 2 μm. The present model is based on a semianalytic and semiempirical method through the Kramers–Krönig transformation. The multi-quantum-well structures, including the exciton effect and above barrier gap contributions, are fully considered in this model. The distinct structures at energies of the E0,E0+Δ0,E1, and E2 critical points are revealed. Moreover, the birefringence at room temperature is also analyzed in the wavelength range varying from 0.7 to 1.0 μm. The calculated refractive index results are in satisfactory agreement with the experimental measurements over the quantum well band edge, i.e., 0.8–0.9 μm. The effect of interdiffusion on the change of refractive index is discussed. The polarization dependent absorption coefficients are also calculated with all the bound excitons, and results agree well with experiments. These results are important, since refraction index in a particular wavelength region of interest, where experimental data are not available, can be determined and thus are very useful in the design of devices.
Comment on “A new approach to optimum design in thermoelectric cooling systems” [J. Appl. Phys. 80, 5494 (1996)]
Authors: Chen Jincan , Schouten Jan A.
Published: July 2002

It is pointed out that there are some errors existing in a recent investigation in this journal [M. Yamanishi, J. Appl. Phys. 80, 5494 (1996)]. The correct results are given so that one can better understand the performance of real thermoelectric cooler systems.
An effective-field approach for the Fowler–Nordheim tunneling current through a metal–oxide–semiconductor charged barrier
Authors: Miranda E. , Redin G. , Faigón A.
Published: July 2002

The tunneling current through the gate insulator of a poly Si–SiO2–Si structure with trapped charge within the oxide is described by a modified Fowler–Nordheim expression, in which the cathode electric field is substituted by the local electric field in the insulator corresponding to the Fermi energy at the injecting electrode. Trap creation in the oxide was induced by a high-field constant current injection, and the flatband voltage and the tunneling current were measured after reaching the steady states corresponding to different applied gate voltages. The trapped charge is considered to be uniformly distributed within the insulator and its value is calculated from the flatband voltage shift. The proposed model applies for positive gate injection regardless of the stress polarity used for sample degradation.
Observation of a geometrical barrier in YBa2Cu3O6.95 single crystals
Authors: Gardner M. W. , Govorkov S. A. , Liang R. , Bonn D. A. , Carolan J. F. , Hardy W. N.
Published: July 2002

The remnant and in-field magnetic field distributions of two crystals of YBa2Cu3O6.95, one with a single twin boundary and one completely untwinned, were observed using a nondestructive magneto-optic technique. Geometrical barrier effects on the flux penetration of both samples were found to be present. Two distinct penetrating field patterns were observed resulting from the different pinning characteristics of the samples. A concentration of penetrating vortices in the center of one sample indicated an extremely low degree of pinning. A comparison was made between experimental flux penetration profiles from both samples and those calculated using a theoretical model for a thin strip superconductor. A single twin boundary in the extremely low pinning crystal was also observed to be a barrier to transverse vortex motion.
Low-noise heterodyne mixing with NbN microbolometers at 800 GHz
Authors: Lehnert T. , Rothermel H. , Gundlach K. H.
Published: July 2002

Mixer performance of NbN transition edge microbolometers was investigated in a frequency range from 795 to 813 GHz. The devices consist of a parallel array of 3 NbN lines with submicron length patterned by e-beam lithography in a self-aligned process. The heterodyne mixer response was proven by means of two superposed oscillator signals at the receiver input and the observation of a discrete line in the output spectrum. Receiver noise temperatures, Trec, were determined by the hot/cold load Y-factor method in a quasioptical receiver at 4.2 K. The accessible intermediate frequency (IF) band was 1000 to 1750 MHz. We have compared devices with a NbN film thickness of 100 and 50 Å. Lowest receiver noise temperatures were obtained at 800 GHz. Uncorrected Trec values around 900 K were measured for both devices at a 1000 MHz IF frequency. For higher IF frequencies, Trec increased much less in the 50 Å device than in the 100 Å device. At 800 GHz and 1750 MHz IF frequency, Trec was only 1180 K. This result indicates that in an optimized NbN bolometric mixer the IF frequency band can extend to reasonable high values for radioastronomical applications.