bluuebunny/crossref_metadata_2025 huggingface dataset:
Relaxation and recovery of highly strained InGaAs/GaAs quantum wells
Authors: Price G. L. , Usher B. F.
Published: July 2002

The growth of In0.4Ga0.6As on GaAs has been studied with a novel multiquantum well technique which amplifies the sensitivity of reflection high-energy electron diffraction and x-ray diffraction sufficiently to determine the quantum well critical thicknesses for very thin layers. The relaxation and recovery of the overlayer for growth between the single heterostructure and quantum well critical were measured and compared with theory.
Epitaxial growth of thin films of BaTiO3 using excimer laser ablation
Authors: Davis G. M. , Gower M. C.
Published: July 2002

Excimer laser ablative sputtering of sintered pellets of BaTiO3 has been used to epitaxially grow thin ∼1-μm-thick films of this material of LiF substrates.
Is the observed quantized conductance on small contacts due to coherent ballistic transport?
Authors: Escapa L. , García N.
Published: July 2002

This letter revises the interpretation of recent experiments showing quantized conductance on GaAs/GaAlAs constrictions or point contacts in terms of coherent ballistic and sequential transport. Our calculations show by using a first iterative self-consistent procedure that the experimental data can also be reconciled with sequential transport. The mean free path concept in the constriction is discussed in comparison with that of the reservoirs. We conclude that, for the higher modes, the former is much smaller than the latter, implying that in the constriction the transport may be sequential.
Quantum field-effect directional coupler
Authors: del Alamo Jesús A. , Eugster Cristopher C.
Published: July 2002

We propose a new quantum-effect electronic device which consists of two one-dimensional electron waveguides which, over a certain interaction length, come in close proximity to each other so that coherent quantum mechanical tunneling can take place between them. The degree of coupling between the two waveguides is controlled by modulating, through the field-effect action of a gate, the height of the potential energy barrier which separates them. If an electron wave packet is injected into this device through one of the waveguides, then the probability density of the electron wave function will oscillate back and forth between the two waveguides as the packet advances. The gate voltage can be adjusted to achieve complete electron transfer at either of the two waveguides at the output of the device. The device, therefore, behaves as a current switch. In analogy with the optical dual-channel directional coupler and the microwave dual-guide multihole coupler, we call this device a quantum field-effect directional coupler. First-order calculations indicate that this device can be fabricated with state-of-the-art nanolithography.
Magneto-optical studies of Co/Au ultrathin metallic films
Authors: Ferré J. , Pénissard G. , Marlière C. , Renard D. , Beauvillain P. , Renard J. P.
Published: July 2002

We report on static and dynamic magneto-optical measurements of the magnetization in ultrathin cobalt films deposited on Au (111). Their large Faraday rotation associated to the squareness of their magnetic hysteresis loop makes these samples very promising as magneto-optical storage media.
Confinement of charge carriers and molecular excitons within 5-nm-thick emitter layer in organic electroluminescent devices with a double heterostructure
Authors: Adachi Chihaya , Tsutsui Tetsuo , Saito Shogo
Published: July 2002

Organic electroluminescent devices with a double-heterostructure indium-tin-oxide substrate/hole transport layer/emitter layer/electron transport layer/MgAg have been fabricated by vacuum vapor deposition. The organic carrier transport and emitter layers were composed of amorphous films. In the double-heterostructure devices, the luminance continued to lie in high level, even when the emitter thickness was 50 Å. The confinement of charge carriers and molecular excitons within a narrow emitter layer was achieved.
Photorefractive effect in ferroelectric lead germanate
Authors: Królikowski Wiesl/aw , Cronin-Golomb Mark , Chen Bo Su
Published: July 2002

The photorefractive effect is demonstrated for the first time in ferroelectric lead germanate crystal (Pb5Ge3O11). An intensity coupling constant of Γ=2.3 cm−1 was measured at wavelength λ=514.5 nm in two-beam coupling experiments.
Single-crystal Sn/Ge superlattices on Ge substrates: Growth and structural properties
Authors: Wegscheider W. , Eberl K. , Menczigar U. , Abstreiter G.
Published: July 2002

Short-period strained-layer α-Sn/Ge superlattices lattice matched to Ge(001) substrates have been synthesized for the first time. The thin, tetragonally distorted α-Sn layers are stabilized by a modified molecular beam epitaxy technique with large modulation of substrate temperature during growth. Optimization of growth conditions is achieved via in situ Auger electron spectroscopy and low-energy electron diffraction. This new kind of strained-layer superlattice is characterized by transmission electron microscopy, x-ray diffraction, and Raman scattering. Distinct superlattice effects are observed in the structural and phonon properties of the samples.
Studies of hydrogen ion beam cleaning of silicon dioxide from silicon using in situ spectroscopic ellipsometry and x-ray photoelectron spectroscopy
Authors: Hu Y. Z. , Conrad K. A. , Li M. , Andrews J. W. , Simko J. P. , Irene E. A.
Published: July 2002

The removal of a thin oxide layer from a silicon substrate without significant damage has been achieved at temperatures as low as 500 °C using a low-energy hydrogen ion beam produced by a high-intensity and low-energy ion source in a high-vacuum system. In situ spectroscopic ellipsometry was found to be a sufficiently sensitive and nondestructive method for simultaneously monitoring silicon surface cleaning and ion-induced substrate damage. This letter reports the optimum cleaning parameters for silicon (i.e., minimum ion-induced damage with maximum etch rate of SiO2) to be 300 eV ion beam energy, 60° beam incidence, and 500 °C substrate temperature.
Impact excitation of the erbium-related 1.54 μm luminescence peak in erbium-doped InP
Authors: Isshiki Hideo , Kobayashi Hitoshi , Yugo Shigemi , Kimura Tadamasa , Ikoma Toshiaki
Published: July 2002

The Er-related 1.54 μm luminescence peak has been observed in erbium-doped InP layers by impact excitation of Er atoms with energetic carriers accelerated by electric field. Er ions were implanted into n-type InP and Au/Sn ohmic contacts were formed on top of the surface. The Er-related sharp peak at 1.543 μm was observed by only applying dc voltages between the electrodes over the temperature range from 77 to 360 K. Neither band-edge emission nor impurity-related emission were observed, although they were intense in the photoluminescence spectra of the same sample. The fine structure of the 1.54 μm peak was also different from that of photoluminescence. This 1.54 μm emission was related to erbium atoms excited through collisions with energetic electrons accelerated by the electric field.
Transit time limited response of GaAs metal-semiconductor-metal photodiodes
Authors: Klingenstein M. , Kuhl J. , Rosenzweig J. , Moglestue C. , Axmann A.
Published: July 2002

The response of GaAs metal-semiconductor-metal (MSM) photodiodes at low temperatures has been investigated in the time domain by photoconductive sampling. The dependence of the response time on temperature for T≳50 K can be described by phonon mediated intervalley and intravalley scattering. The pulse width drops from 10.8 ps at 300 K to 5.6 ps at 70 K and then grows rapidly with decreasing temperature below 50 K. These results demonstrate that the response is limited by the electron/hole transport in the semiconductor rather than by external capacitances or inductances.
Second-order susceptibility in asymmetric quantum wells and its control by proton bombardment
Authors: Yoo S. J. B. , Fejer M. M. , Byer R. L. , Harris J. S.
Published: July 2002

We report measurements of extremely large mid-infrared second-order susceptibilities due to intersubband transitions in an asymmetric quantum well, and selective suppression of these susceptibilities by proton bombardment of the sample. The measured second-order susceptibility for second-harmonic generation of 9.25–10.78 μm is in good agreement with theoretical calculations based on measured infrared absorption spectra, which show the lowest two intersubband transitions. The peak value of the susceptibility is 58 nm/V, 320 times larger than the bulk nonlinear susceptibility of GaAs.
Raman microprobe study of narrow InxGa1−xAs stripes on patterned GaAs(100) substrates
Authors: Tang W. C. , Rosen H. J. , Guha S. , Madhukar A.
Published: July 2002

The strain and structural variations across InGaAs ridges (∼1 μm) and valleys (∼3 μm) grown on patterned GaAs(100) substrates are examined via Raman microprobe spectroscopy. High spatial resolution scans across these structures show significant variations in the Raman features (phonon frequency, linewidth, and intensity). Variation in the GaAs-like longitudinal optical mode frequency suggests a large difference in strain found in the InGaAs on top of the 1 μm ridges and in the valleys. Concomitant measurement of phonon linewidth and Ar+ laser-induced heating are also indicative of the same. These results, in conjunction with transmission electron microscope studies, demonstrate that Raman microprobe spectroscopy is a powerful tool for probing patterned semiconductor structures on a microscopic scale.
Direct deposition of polycrystalline diamond films on Si(100) without surface pretreatment
Authors: Rudder R. A. , Hudson G. C. , Posthill J. B. , Thomas R. E. , Markunas R. J.
Published: July 2002

Dense nucleation of small-grain polycrystalline diamond films on Si(100) substrates has been accomplished without the use of any surface pretreatment such as abrasive diamond scratching, surface oil treatments, or diamond-like carbon predeposition. Diamond depositions occurred in a low-pressure rf plasma-assisted chemical vapor deposition system using mixtures of CF4 and H2. Films deposited at 5 Torr and 850 °C on as-received silicon wafers show dense nucleation, well-defined facets, and crystallites which ranged in size from 500 to 10 000 Å. X-ray photoelectron spectroscopy and electron energy loss show the films to be diamond with no major impurity and no detectable graphitic component. Raman spectroscopy shows a pronounced 1332 cm−1 line accompanied with a broad band centered about 1500 cm−1.
All-optical switching in a nonlinear periodic-waveguide structure
Authors: Sankey N. D. , Prelewitz D. F. , Brown T. G.
Published: July 2002

We report what we believe is the first observation of all-optical switching in a nonlinear periodic-waveguide structure. Measurements on the silicon-on-insulator waveguides were taken at 1.064 μm and at energies ranging from 1 to 18 μJ/pulse using a Nd:YAG pulsed laser. The measurements confirm theoretical predictions of bistability at low intensities with unstable dynamics at higher pulse energies.
Visible photoluminescence from porous silicon: A quantum confinement effect mainly due to holes?
Authors: Voos M. , Uzan Ph. , Delalande C. , Bastard G. , Halimaoui A.
Published: July 2002

We present results of photoluminescence experiments performed at 2 and 300 K on porous silicon layers with different porosities. The samples are obtained by electrochemical dissolution of (100) silicon wafers in hydrofluoric solution. The energy of the photoluminescence and its variation with porosity are found in good agreement with a theoretical model of quantum confinement in Si quantum wires. Electrons have to be taken into account, but it is shown that this quantum effect is mainly due (≊60%) to the large confinement of holes.
Bilayer interference enhanced Raman spectroscopy
Authors: Bacsa W. S. , Lannin J. S.
Published: July 2002

A combination of the interference of incident and reflected coherent beams from a metal surface with a dielectric overlayer is calculated to yield an enhancement of the Raman signal of ultrathin adsorbed layers. The thickness of the dielectric layer is determined by optimizing the interference effect of the incident and reflected beams to enhance the electric field. In the case of SiO2/Al an interference enhancement factor of 27 is found. An experimental confirmation of this enhancement is obtained by comparing the in situ multichannel Raman signals of one monolayer C60 on an Al substrate and on a SiO2/Al bilayer. The use of the same bilayer substrate for a range of ultrathin films makes the metal-dielectric bilayer a versatile tool to investigate a number of nanoscale systems.
Room-temperature diphenyl-diacetylene liquid crystals
Authors: Wu Shin-Tson , Margerum J. D. , Meng H. B. , Dalton L. R. , Hsu C.-S. , Lung S.-H.
Published: July 2002

Thirty new asymmetric diphenyl-diacetylene (PTTP) liquid-crystal homologs are synthesized and their physical properties characterized. In addition to the observed high birefringence and relatively low viscosity, several dialkyl PTTP homologs also possess a low melting point, wide nematic range, and low heat of fusion enthalpy. These properties are highly desirable for forming eutectic mixtures with entirely PTTP components. A binary eutectic showing a nematic range from 10 to 98 °C is studied. These new liquid-crystal compounds and mixtures should be particularly useful for infrared applications.
Epitaxy-induced phase of near-stoichiometry PbTiO3 films prepared by metalorganic chemical vapor deposition
Authors: Bai G. R. , Chang H. L. M. , Kim H. K. , Foster C. M. , Lam D. J.
Published: July 2002

Ferroelectric PbTiO3 thin films have been prepared by a low-pressure MOCVD process. Epitaxial films with a crystal structure of a bulk equilibrium tetragonal phase were readily obtained within a range of growth conditions. However, by providing excess Pb in the growth process, we have observed an epitaxy-induced phase for near-stoichiometry PbTiO3 which possesses a perovskite-like, but possible, orthorhombic structure.
Interstitial carbon-oxygen complex in near threshold electron irradiated silicon
Authors: Shinoda K. , Ohta E.
Published: July 2002

A new electron trap at Ec-0.06 eV is detected in n-type silicon irradiated with 200 keV electrons at room temperature using deep-level transient spectroscopy. The annealing behavior of this defect level shows that the level arises from an interstitial carbon-interstitial oxygen complex that is a configurational precursor of the EPR G15 center. We propose a simple model of defect formation that is consistent with the dependence of the defect level concentration on the electron fluence.
Pulsed excimer laser ablated barium titanate thin films
Authors: Roy D. , Krupanidhi S. B.
Published: July 2002

Thin films of BaTiO3 were deposited on platinum coated silicon substrates by excimer laser (248 nm) ablation at 600 °C or ex situ crystallized at about the same annealing temperature. Films deposited at 600 °C showed good crystallinity and were characterized for ferroelectricity, dielectric constant, dielectric loss, leakage current, and C-V characteristics. The films showed a dielectric constant of 220, a dissipation factor of 0.02, a leakage current of 1.8×10−6 A/cm2 at a bias of 5 V, and a charge storage density of about 40 fC/μm2 at a field of 0.15 MV/cm.
Hard x-ray phase zone plate fabricated by lithographic techniques
Authors: Lai B. , Yun W. B. , Legnini D. , Xiao Y. , Chrzas J. , Viccaro P. J. , White V. , Bajikar S. , Denton D. , Cerrina F. , Di Fabrizio E. , Gentili M. , Grella L. , Baciocchi M.
Published: July 2002

A Fresnel phase zone plate with an unprecedented focusing efficiency of 33% was fabricated using an x-ray lithographic technique and was tested using synchrotron x rays. Contributions by the zeroth-order x ray to the focus were minimal. Spatial resolution in the micrometer range was achieved. The measured spot size was dominated by geometric demagnification of the source. It should be possible to obtain submicrometer resolution by aperturing the source. Experimental results of focusing efficiency measurements, intensity distribution at the focal plane, and spatial resolution tests are reported.
Minimal-data approaches for determining outer-layer dielectric responses of films from kinetic reflectometric and ellipsometric measurements
Authors: Aspnes D. E.
Published: July 2002

For stratified samples where material is being uniformly deposited or removed at a known rate, the dielectric response εo of the outermost few Å is exactly determinable from the value and thickness derivative of the complex reflectance without any knowledge of the underlying structure. For ellipsometric data the equivalent analysis is not exact, but a common-pseudosubstrate approximation is developed, that in applications to semiconductor crystal growth, is accurate to better than 0.1%.
Novel structure of organic electroluminescence cells with conjugated oligomers
Authors: Hosokawa Chishio , Higashi Hisahiro , Kusumoto Tadashi
Published: July 2002

Organic electroluminescence (EL) cells with a novel structure, ITO/semiconducting oligothiophene layer/oligophenylene layer/emitting layer/Mg:Ag, were studied. The oligothiophene layer enhanced hole injection and hole transport into the emitting layer (EML), while the oligophenylene layer achieved electron confinement in EML by electron blocking. In addition, we found that the oligophenylene layer inhibited energy transfer from EML to the oligothiophene layer and played an important role in increasing the luminous efficiency. For our fabricated EL cell with tris(8-hydroxyquinoline)aluminum as an emitting molecule, we succeeded in obtaining bright emission over 2000 cd/m2 at a voltage of about 10 V.
Relationship among mobility, recombination kinetics, and optimized solar cell performance
Authors: Fortmann C. M. , Fischer D.
Published: July 2002

A link between the observed steady state mobility and the diffusive velocity, the speed at which carriers move to distances greater than a single hop distance (<100 Å), is found to be an important transport consideration for amorphous silicon-based devices. The electron diffusive velocity is a function of material fabrication technique. The diffusive velocity affects device performance through the recombination kinetics as it is this velocity at which carriers move toward interfaces and recombination centers. These new considerations lead to the surprising result that in solar cells, a poor electron mobility can lessen recombination losses.
Structure of S on passivated GaAs (100)
Authors: Lu Z. H. , Graham M. J. , Feng X. H. , Yang B. X.
Published: July 2002

X-ray photoelectron spectroscopy (XPS) and E-polarization-dependent S K-edge x-ray absorption near-edge structure (XANES) are used to characterize the chemical structure and site location of S on the (NH4)2S-treated GaAs (100) surface. XPS studies show that S forms chemical bonds with both Ga and As on surfaces treated only with (NH4)2S. After receiving a sufficient water rinse, only Ga—S bonds remain on the surface. Photon E polarization-dependent XANES studies show that S is bridge bonded to Ga in the [011] azimuth.
Giant recombination centers in Al0.10Ga0.90As grown by metalorganic chemical vapor deposition
Authors: Zhang J. , Keyes B. M. , Asher S. E. , Ahrenkiel R. K. , Timmons M. L.
Published: July 2002

The hole lifetime in n-type Al0.10Ga0.90As is controlled by recombination centers with capture cross sections of about 10−12 cm2. Correlated time-resolved photoluminescence and deep level transient spectroscopy measurements link these centers to well-known impurity oxygen complexes. Film growth at temperatures above about 720 °C eliminate these oxygen complexes from the epitaxial layer resulting in greatly improved electronic properties.
Degenerate electron gas effects in the modulation spectroscopy of pseudomorphic Al0.32Ga0.68As/In0.15Ga0.85As/GaAs high electron mobility transistor structures
Authors: Dimoulas A. , Zekentes K. , Androulidaki M. , Kornelios N. , Michelakis C. , Hatzopoulos Z.
Published: July 2002

The effects of a degenerate two-dimensional electron gas on the interband optical excitations, occurring in the active channel of Al0.32Ga0.68As/In0.15Ga0.85As/GaAs high electron mobility transistor structures, were investigated by using phototransmittance spectroscopy. The ground state transition at room temperature exhibited a characteristic steplike line shape, which was considered to be an effect of the screening of excitons by the degenerate electron gas. A line shape fitting by using a first derivative of the absorption coefficient with respect to the electron sheet concentration ns, allowed an estimation of the latter quantity by phototransmittance. An observed temperature-sensitive excitonlike signal, associated with the second electron subband was attributed to the modulation of the many-body correlation-enhanced excitonic absorption, known as the Fermi-edge singularity.
Thermal stability of strained InxGa1−xAs/InyAl1−yAs/InP heterostructures
Authors: Bennett Brian R. , del Alamo Jesús A.
Published: July 2002

We investigated the thermal stability of strained layers of InGaAs and InAlAs on InP. Epilayer and interface quality was assessed by high-resolution x-ray diffraction and electron mobility measurements as a function of annealing cycle. Both techniques show that pseudomorphic heterostructures retain their high crystalline quality at annealing temperatures of up to 700–800 °C, despite exceeding the Matthews–Blakeslee [J. Cryst. Growth 27, 118 (1974)] critical layer thickness by as much as a factor of 4–8. On the other hand, layers which are partially relaxed (incoherent) as-grown relax further during annealing. These findings demonstrate that layers which are beyond the predicted critical thickness, but coherently strained after growth, are stable to normal device processing and operating temperatures and hence may be suitable for use in device heterostructures.
Electrochromic properties of niobium oxide thin films prepared by radio-frequency magnetron sputtering method
Authors: Maruyama Toshiro , Arai Susumu
Published: July 2002

Electrochromic niobium oxide thin films were prepared by a radio-frequency magnetron sputtering method. Amorphous Nb2O5 thin films deposited at radio-frequency power 100 W showed the electrochromic behavior: Reduction and oxidation of the films in 0.1 M Na2CO3+0.1 M NaHCO3 buffer solution resulted in coloration and bleaching, respectively. Coulometry indicated that the coloration efficiency was 10 cm2/C.
Reflectance anisotropy as a surface science probe of the growth of InAs on (001) GaAs by molecular beam epitaxy
Authors: Armstrong S. R. , Pemble M. E. , Taylor A. G. , Fawcette P. N. , Neave J. H. , Joyce B. A. , Zhang J.
Published: July 2002

Reflectance anisotropy (RA) measurements for the initial stages of the growth of InAs on GaAs (001) by molecular beam epitaxy (MBE) are presented. It is demonstrated that the RA technique is capable of providing information regarding changes of surface In concentration on the 0.1 monolayer level in real time, at high temperatures (∼600 °C), under real InAs MBE conditions. In addition, associated with the detection of subtle coverage variations, surface reconstruction changes detectable by electron diffraction are also readily detectable by the RA technique.
Mössbauer investigations of mechanical alloying in the Fe-Cu system
Authors: Jiang J. Z. , Gonser U. , Gente C. , Bormann R.
Published: July 2002

A fcc-Fe50Cu50 solid solution was prepared by mechanical alloying of elemental Fe and Cu powder blends. The alloying process was studied by using x-ray diffraction and Mössbauer spectroscopy. Initially, the milling process reduced the crystallite sizes of both elemental powders. After 20 h milling, some Fe particles transformed into the fcc structure. Due to the structural similarity of the fcc-Fe and fcc-Cu phases, composites consisting of coherent Cu and Fe regions were formed. The increasing density of interfaces during further milling resulted in an interdiffusion of Cu and Fe. The alloying process was monitored by Mössbauer investigations which showed an increasing Fe concentration in fcc Cu. After 50 h of milling, the Mössbauer spectra consisted of a broadened sextet caused by a hyperfine field distribution, which demonstrates that the Fe and Cu were alloyed on an atomic level. These observations are in agreement with a model proposed by C. Gente, M. Oehring, and R. Bormann [Phys. Rev. B 48, 13244 (1993)] describing the formation of unstable alloys by mechanical alloying.
Schottky barrier on n-type GaN grown by hydride vapor phase epitaxy
Authors: Hacke P. , Detchprohm T. , Hiramatsu K. , Sawaki N.
Published: July 2002

A Schottky barrier on unintentionally doped n-type GaN grown by hydride vapor phase epitaxy was obtained and characterized. Using vacuum evaporated gold as the Schottky barrier contact and aluminum for the ohmic contact, good quality diodes were obtained. The forward current ideality factor was n∼1.03 and the reverse bias leak current below 1×10−10 A at a reverse bias of −10 V. The barrier height φBn was determined to be 0.844 and 0.94 eV by current-voltage and capacitance measurements, respectively.
Analytical method for studying a quasiperiodic disk loaded waveguide
Authors: Schächter Levi , Nation John A.
Published: July 2002

An analytic method to investigate a quasiperiodic disk loaded waveguide is presented. We rely on Cauchy residue theorem to formulate the transmission and reflection from a system composed of radial arms and grooves provided that the inner radius is kept constant; all the other parameters of the system can be arbitrarily changed. This method was successfully utilized to design the input and output section of a high power traveling wave tube which is very sensitive to reflections from both ends. We found this method particularly useful for the design of the output regions where breakdown imposes constraints on the geometry.
Feasibility of room-temperature operation of tunable coupled-quantum-well lasers
Authors: Ogawa M. , Mendez E. E.
Published: July 2002

We have calculated the gain of laser heterostructures whose active regions consists of two strongly coupled quantum wells subjected to an electric field. The results demonstrate that by using two 40-Å GaAs wells separated by an 11-Å Ga0.7Al0.3As barrier a field-induced wavelength tunability of over 7 nm is possible at room temperature.
Optical limiter using a lead phthalocyanine
Authors: Shirk James S. , Pong Richard G. S. , Bartoli F. J. , Snow Arthur W.
Published: July 2002

The performance of an optical limiter based on Pb-tetrakis(cumylphenoxy)phthalocyanine, a robust organic material with a large χ(3) and figure of merit, χ(3)/α0, is described. In an f/5 limiter with a sample transmission of 0.68, the threshold for limiting was 8±2 nJ and the dynamic range was greater than a factor of 103. The threshold for the PbPc(CP)4 limiter was ∼15 times smaller and the high intensity transmission ∼4–5 times lower than an equivalent limiter based on a thermal nonlinearity.
Potential liquid crystal mixtures for Co2 laser application
Authors: Wu Shin-Tson , Margerum J. D. , Meng H. B. , Hsu C. S. , Dalton L. R.
Published: July 2002

Infrared transmission of 25 liquid crystal compounds was evaluated. Based on these results, a liquid crystal mixture consisting of fluoro and dialkyl diphenyl-diacetylene homologs was developed for CO2 laser application. This mixture exhibits a wide nematic range, high birefringence, low viscoelastic coefficient, and relatively small absorption at two CO2 laser lines: 10.59 and 10.77 μm. Its figure of merit is three times higher than the E44 commercial mixture at room temperature.
Growth of (100) oriented CdTe on Si using Ge as a buffer layer
Authors: Bhat Ishwara , Wang Wen-Sheng
Published: July 2002

Epitaxial (100) CdTe layers have been grown on (100) oriented Si substrates by atmospheric pressure organometallic vapor phase epitaxy using thin Ge as a buffer layer. A very thin native oxide layer may be present on Si substrates after etching in a HF solution and this can be removed by passing GeH4 gas over Si at 450 °C. The removal of this oxide takes several minutes depending on the oxide layer thickness and reactor conditions. For the CdTe layers grown on Ge/Si substrates, single-crystal (100) CdTe can be obtained at growth temperatures higher than 420 °C. For growth temperatures below 420 °C, a mixture of both (100) and (111) oriented CdTe was present. The x-ray full width at half-maximum of the (400) peak was 780 arcsec for a 3.1-μm-thick CdTe layer grown at 450 °C with a Ge buffer. Although all the layers had antiphase domains, single-domain CdTe can be grown on (100)Si misoriented towards [110]. This result presents an alternative for nucleating CdTe on Si without a GaAs buffer layer.
Ultraviolet generation at 266 nm in a novel organic nonlinear optical crystal: l-pyrrolidone-2-carboxylic acid
Authors: Kitazawa Manabu , Higuchi Ryoichi , Takahashi Mitsuo , Wada Tatsuo , Sasabe Hiroyuki
Published: July 2002

A novel organic nonlinear optical crystal, l-pyrrolidone-2-carboxylic acid (l-PCA), has been found to be phase matchable for second-harmonic generation (SHG) and sum-frequency mixing down to the UV range at room temperature. Tunable radiation down to 266 nm was obtained. The nonlinear coefficient d14 of l-PCA for the fundamental wavelength of 532 nm was determined to be 0.32 pm/V. The angular acceptance (the full width at half-maximum) of the type II phase-matched SHG has also been measured to be 3.03 mrad cm which agreed with the calculated value. This value is much larger than that of the type I phase-matched SHG in β-BBO.
Temperature-dependent study of spin-dependent recombination at silicon dangling bonds
Authors: Vuillaume D. , Deresmes D. , Stiévenard D.
Published: July 2002

Electrical detection of magnetic resonance is used in a large temperature range (150–350 K) to analyze the spin-dependent recombination properties of silicon dangling bonds at the Si-SiO2 interface (created by high-field electron injections) and of silicon dangling bond clusters in bulk silicon (created by electron irradiations). Quite different temperature spin-dependent recombination behaviors are observed for these two kinds of silicon dangling bonds. These features are related to the respective behaviors of their nonradiative capture processes, which are independently determined by deep level transient spectroscopy. Moreover, we show that only the Pb0 center is observed at the Si-SiO2 interface after high-field electron injection. The Pb1 center is not observed by electrically detected magnetic resonance in this large temperature range.
Nitrogen content of oxynitride films on Si(100)
Authors: Tang H. T. , Lennard W. N. , Zinke-Allmang M. , Mitchell I. V. , Feldman L. C. , Green M. L. , Brasen D.
Published: July 2002

The absolute nitrogen concentration in SiOxNy/Si films grown by rapid thermal oxidation in N2O has been determined by nuclear reaction analysis. Compared with conventional surface analysis methods, i.e., Auger electron spectroscopy, x-ray photoelectron spectroscopy, and secondary ion mass spectrometry, the nuclear reaction 14N(d,α)12C provides more accurate depth profiles of 14N due to the quantitative nature of the technique and its high sensitivity, ∼6.0×1013 atoms cm2. Silicon oxynitride films prepared under various conditions, specifically different growing temperatures and times, were analyzed. Nitrogen is observed to accumulate in a narrow region in the oxynitride (within ≲2.5 nm) close to the interface; the total amount of nitrogen increases with increasing temperature and growth time.
Design of three-dimensional photonic crystals at submicron lengthscales
Authors: Fan Shanhui , Villeneuve Pierre R. , Meade Robert D. , Joannopoulos J. D.
Published: July 2002

We present a new class of periodic dielectric structures designed specifically to be amenable for fabrication at submicron lengthscales. The structures give rise to a sizable 3D photonic band gap and can be fabricated with materials widely used today in optoelectronic devices. They are made of three materials and consist essentially of a layered structure in which a series of cylindrical air holes are etched at normal incidence through the top surface of the structure. Our results demonstrate the existence of a gap as large as 14% of the midgap frequency using Si, SiO2, and air; and 23% using Si and air.
Mechanism responsible for the semi-insulating properties of low-temperature-grown GaAs
Authors: Liu X. , Prasad A. , Chen W. M. , Kurpiewski A. , Stoschek A. , Liliental-Weber Z. , Weber E. R.
Published: July 2002

The mechanism responsible for the semi-insulating properties of low-temperature-grown GaAs is investigated by determining the concentration of arsenic antisite (AsGa)-related defects in the material. The concentrations of the defects in neutral and positively charged states, As0Ga and As+Ga, are determined by near-infrared absorption and magnetic circular dichroism of absorption, respectively. Materials grown and annealed at different temperatures are investigated. We find that the defects are abundant in all samples studied, with the concentration of As0Ga higher than that of As+Ga defects. The results indicate that the defects can account for the pinning of the Fermi energy and consequently also the semi-insulating properties of the material.
Deflection microwave amplifier with field-emitter arrays
Authors: Tang Cha-Mei , Lau Y. Y. , Swyden T. A.
Published: July 2002

A new class of microwave and millimeter-wave amplifiers, based on the deflection of collimated microscopic electron beams with high-current density and low voltage from field-emitter arrays, was analyzed. The deflection concept may be applied in two ways; as microelectronic amplifiers or as bunched beam cathodes to power conventional amplifier configurations such as klystrodes and traveling wave tubes. We found that the frequency fT, at which the short-circuit current gain attains unit magnitude, depends only on the electron beam energy, current density, and emittance, and is independent of beamwidth and total current. The results indicate that field-emitter arrays have the potential of making wideband deflection amplifiers at frequencies up to tens of GHz.
Gatable ultrafast field-assisted photoemission to λ=1.55 μm from In0.5Ga0.5As heterostructures
Authors: Parker T. R. , Fawcett A. H. , Phillips C. C. , Majumdar S. , Fawcett P. N.
Published: July 2002

Field-assisted photoemission has been observed from a cesiated metal–semiconductor structure with a fully depleted In0.5Ga0.5As absorption/emission layer. The device showed photoemission at excitation wavelengths up to λ=1.55 μm, with an external quantum efficiency (QE)≊8×10−5 at λ=1.4 μm. Large area Schottky contacts were made and the long-wavelength portion of the photoemissive response was electrically gatable with logic level voltages. A≊14 ps device response time indicates significant potential for its use in long-wavelength high-speed electron-optical imaging applications.
Normal incidence second-harmonic generation in L-valley AlSb/GaSb/Ga1−xAlxSb/AlSb stepped quantum wells
Authors: Xie H. , Wang W. I. , Meyer J. R. , Ram-Mohan L. R.
Published: July 2002

A novel mechanism is proposed for surface-emitting second-harmonic generation at normal incidence using L-valley intersubband transitions in AlSb/GaSb/Ga1−xAlxSb/AlSb stepped quantum wells. The calculations indicate that second-harmonic susceptibilities of at least 9×10−8 m/V should be achievable under double resonance conditions, which is comparable to the best obtainable in GaAs/Ga1−xAlxAs systems for realistic propagation angles (e.g., 45°). Besides the advantage of normal-incidence geometry, the large L-valley conduction band offset between GaSb and AlSb enables doubling to frequencies spanning the entire midwave infrared spectral region.
High-quality GaN heteroepitaxial films grown by metalorganic chemical vapor deposition
Authors: Fertitta K. G. , Holmes A. L. , Neff J. G. , Ciuba F. J. , Dupuis R. D.
Published: July 2002

We report the growth of high-quality GaN heteroepitaxial films on (0001) sapphire substrates by low-pressure metalorganic chemical vapor deposition. These films have exhibited narrow x-ray-diffraction rocking curves with full-width-at-half-maximum values as low as ΔΘ∼38 arc sec. These are the narrowest x-ray-diffraction rocking curve linewidths reported to date for any III-V nitride film. Electrical and optical measurements further indicate the samples to be of high quality.
Hot-carrier-temperature model for the dark current of quantum-well infrared photodetectors
Authors: Man P. , Pan D. S.
Published: July 2002

A hot-carrier-temperature model is proposed in this letter to calculate the dark current of a p-type GaAs/Ga1−xAlxAs quantum-well infrared photodetector. Hot-carrier effects are incorporated inside both the mobile-carrier concentration (by a two-carrier-temperature energy distribution) and the drift velocity. Results of our calculations are in good agreement with experimental data at four different lattice temperatures and for the entire measured range of applied electric field. The increase of mobile-carrier concentration at a hot-carrier temperature is found to be the prevalent cause of the rise in dark current at higher applied electric fields.
High strain effects evidenced by Raman scattering in arsenic clusters in As-implanted GaAs
Authors: Pizani P. S. , Mlayah A. , Groenen J. , Carles R. , Claverie A.
Published: July 2002

Raman scattering has been used to track structural information on arsenic clusters in GaAs obtained by annealing of high dose As-implanted GaAs layers. Beyond the good crystalline quality of both the clusters and the matrix, high tensile stresses within the precipitates have been originally deduced from the A1g and Eg mode frequency shifts. The results are well explained in terms of the difference between the thermal expansion coefficients of the two materials. The temperature dependence of the stresses corroborates this interpretation.
CCl
4 doping of GaN grown by metalorganic molecular beam epitaxy
Authors: Abernathy C. R. , MacKenzie J. D. , Pearton S. J. , Hobson W. S.
Published: July 2002

Hole concentrations up to 1017 cm−3 are achieved in GaN doped with C from a CCl4 source during metalorganic molecular beam epitaxy at 700 °C. The hole mobility under these conditions is 103 cm2 V−1 s−1 at 300 K. The deposition rate of the GaN is reduced by addition of CCl4 to the growth chemistry even at low flow rates and net etching is observed for a halocarbon flow above 1.1 sccm under our conditions. Annealing up to 800 °C did not increase the hole concentration indicating that residual hydrogen passivation of the acceptors is not significant when employing a He carrier gas for transporting the group III metalorganic precursor (triethylgallium).
Polycrystalline silicon–germanium films on oxide using plasma-enhanced very-low-pressure chemical vapor deposition
Authors: Tsai Julie A. , Reif Rafael
Published: July 2002

Si1−xGex thin films on oxide-coated Si substrates have been formed by plasma-enhanced very-low-pressure chemical vapor deposition. Two modes of deposition, thermal and plasma enhanced, were studied using SiH4 and GeH4 at temperatures ≤600 °C. In both cases, growth rates and grain sizes increase with Ge content, and the polycrystalline-to-amorphous transition temperature is lower for Si1−xGex than Si. Compared to thermal growth of polycrystalline Si1−xGex, plasma-enhanced deposition of poly-Si1−xGex promotes higher growth rates, smaller grain sizes, and direct deposition onto oxide, as well as improved structural properties such as a smoother surface and a more columnar grain texture. Plasma deposition of amorphous-Si1−xGex films followed by a crystallization anneal at 600 °C results in an even smoother surface morphology with grain sizes enhanced by an order of magnitude compared to plasma-deposited poly-Si1−xGex films.
Two-dimensional infrared photonic band gap structure based on porous silicon
Authors: Grüning U. , Lehmann V. , Engelhardt C. M.
Published: July 2002

We have fabricated a two-dimensional photonic band structure based on macroporous silicon with individual gaps for both polarizations in the infrared region between 250 and 500 cm−1 (20–40 μm). A square lattice of circular air rods with a lattice constant of 8 μm was etched 340 μm deep in an n-type silicon substrate by electrochemical pore formation in hydrofluoric acid. The transmission spectra between 50 and 650 cm−1 were in good agreement with the theoretical calculated structure. The pore formation technique should allow the fabrication of photonic lattices with a complete two-dimensional band gap in the middle and near infrared.
Diamond films from combustion of methyl acetylene and propadiene
Authors: Harris Stephen J. , Shin Ho Seon , Goodwin David G.
Published: July 2002

To date diamond films grown with the combustion technique have used either acetylene or, rarely, ethylene as the fuel. However, there are barriers to large scale commercialization of the combustion technique using either fuel. For example, acetylene is relatively expensive and difficult to handle, while the use of ethylene gives relatively low growth rates. In this letter we propose replacing acetylene with MAPPTM gas, a commercial mixture of methyl acetylene and propadiene in liquefied petroleum gas (primarily propylene). MAPP gas is considerably cheaper, safer, and easier to handle than acetylene. Furthermore, the experiments described here suggest that MAPP gas flames are only slightly less efficient than acetylene flames at converting fuel carbon atoms into diamond.
Measurement of Seebeck coefficient using a light pulse
Authors: Wood C. , Zoltan D. , Stapfer G.
Published: July 2002

A high-temperature (1900 K) Seebeck coefficient apparatus is described in which small thermal gradients are generated in a sample by light pulses transmitted via light pipes. By employing an analog subtraction circuit, the Seebeck coefficient is displayed directly on an X–Y recorder. This technique presents a convenient, accurate, and rapid method for measuring the Seebeck coefficient in highly doped semiconductors as a function of temperature. The nature of the resulting display (X–Y recording) is a valuable tool in determining validity of the data. A straight line results (i.e., a minimum of hysteresis) only if all potential experimental errors are minimized. Under these conditions, the error of measurements of the Seebeck coefficient is estimated to be less than ±1%.
Simple scanner for measuring laser beam size
Authors: Shelton D. P.
Published: July 2002

A simple and inexpensive device for measuring the spot size of a laser beam is described. The device is constructed from a clock motor and scans wires of various diameter through the beam. It is most useful for beams of about 0.1 to 2 mm diameter.
Laser Doppler velocimetry using polarization-preserving optical fibers for simultaneous measurement of multidimensional velocity components
Authors: Nakatani Noboru , Tokita Muneo , Izumi Takao , Yamada Tomoharu
Published: July 2002

Laser Doppler velocimeter (LDV) systems using polarization-preserving optical fibers are developed by the polarization and the color methods for simultaneous measurement of multidimensional velocity components. By the use of fibers the construction of the optical systems are made easy, the systems are made more flexible, and remote measurement is realized. A signal processor for the LDVs is developed using only a single frequency counter, a high-speed electronic scanner, and a computer. Using Karman vortex flow and turbulent premixed flame, it is demonstrated that these systems are powerful tools for fluid mechanic measurements.
Sodium atom distribution within a heat sandwich oven
Authors: Cappelli M. A. , Cardinal P. G. , Herchen H. , Measures R. M.
Published: July 2002

In a heat sandwich oven the metal vapor is confined to be disk shaped and is optically accessible through 360° in the plane of the disk. We have used this feature to optically measure the radial atom density distribution of sodium vapor within this type of oven, under a range of conditions. In particular, we have observed the formation of a donut-shaped atom density distribution when the heat sandwich oven is operated at high temperatures with an under pressure of the argon buffer gas.
Simple instrument for the measurement of photoelectron structure functions
Authors: Basano L. , Ottonello P.
Published: July 2002

It has been recently shown that the structure function S(τ)=〈[I(t)−I(t+τ)]2〉 can often replace with success the correlation function C(τ)=〈I(t) I(t+τ)〉. We show that at high pulse rates a serial structure correlator is much faster than a serial time-delay type correlator and represents, therefore, a significant improvement in the development of low-cost instruments for photoelectron statistics. We describe a simple serial structure correlator and report some preliminary results obtained with this instrument.
An instrument to synchronize Thomson scattering measurements with magnetohydrodynamic activity
Authors: Wintenberg A. L. , Lazarus E. A. , Kindsfather R. R. , Halliwell J. W.
Published: July 2002

We describe an instrument that synchronizes the firing of a ruby laser for a Thomson scattering diagnostic with audio-frequency plasma oscillations. The instrument uses analog signal processing circuits to separate signal components, remove unwanted components, and produce zero-phase synchronization pulses. A digitally controlled, self-adjusting bandpass filter is used for sine wave processing. Additional circuitry produces pulses at the desired phase of the signal and calculates the time to start the laser-firing sequence in advance of the desired phase. A CAMAC-controlled data-acquisition system is incorporated to monitor the synchronization process and to ensure that the laser is fired on the correct phase.
The LIDAR Thomson scattering diagnostic on JET (invited)
Authors: Salzmann H. , Bundgaard J. , Gadd A. , Gowers C. , Hansen K. B. , Hirsch K. , Nielsen P. , Reed K. , Schrödter C. , Weisberg K.
Published: July 2002

By combining the time-of-flight or LIDAR principle with a Thomson backscatter diagnostic, spatial profiles of the electron temperature and density are measured in a magnetically confined fusion plasma. This technique was realized for the first time on the JET tokamak. A ruby laser (3-J pulse energy, 300-ps pulse duration, 0.5-Hz repetition rate) together with a 700-MHz bandwidth detection and registration system yields a spatial resolution of about 12 cm. A spectrometer with six channels in the wavelength range 400–800 nm gives a dynamic range of the temperature measurements of 0.3–20 keV. The stray light problem in the backscatter geometry is overcome by spectral discrimination and gating of the photomultipliers. A ruby filter in the spectral channel containing the laser wavelength allows calibration of the vignetting along the line of sight by means of Raman scattering, enabling the measurement of density profiles. The low level of background signal due to the short integration time for a single spatial point yields low statistical errors (ΔTe /Te ≊6%, Δne /ne ≊4% at Te =6 keV, ne =3×1019 m−3 ). Goodness-of-fit tests indicate that the systematic errors are within the same limits. The system is described and examples of measurements are given.
Microwave multipolar plasmas excited by distributed electron cyclotron resonance: Concept and performance
Authors: Pichot M. , Durandet A. , Pelletier J. , Arnal Y. , Vallier L.
Published: July 2002

Distributed electron cyclotron resonance or DECR, a new plasma excitation technique well adapted to microwave multipolar plasmas, is described. The novel reactor concept is based upon the use of several microwave linear applicators working at the ECR mode along the multipolar confinement magnets. At a microwave frequency of 2.45 GHz, large volumes of highly homogeneous plasmas with an electron temperature of the order of 3 eV and ion densities better than 1011 cm−3 are currently being produced in DECR reactors.