bluuebunny/crossref_metadata_2025 huggingface dataset:
Band-edge detection in insulators by tunneling spectroscopy
Authors: Overhauser A. W.
Published: July 2002

Tunneling spectroscopy on a thin metal film, having a thickness less than a conduction-electron mean free path, can reveal band-structure features of an insulating substrate upon which the film has been overlaid. Virtual transitions which couple energy-band states of the substrate with conduction-band states of the metal create subsidiary structure in the metal’s density of states. It is shown that band edges of the insulator should cause sharp peaks in the (differential) tunneling conductance (versus bias voltage) between the metal and, for example, the tip of a scanning tunneling microscope.
Self-pumped phase conjugation in InP:Fe
Authors: Bylsma R. B. , Glass A. M. , Olson D. H. , Cronin-Golomb M.
Published: July 2002

We report here for the first time the demonstration of self-pumped phase conjugation in photorefractive semiconductors using 1.32 μm light. Using an ac field technique to enhance the gain coefficient in InP:Fe and a single input pump beam, phase conjugate reflectivities of 11% were measured using an input beam intensity of less than 1 mW/mm2. These results open up many possibilities for using photorefractive semiconductors in applications with low-power infrared diode lasers.
Optical detection of high-field domains in GaAs/AlAs superlattices
Authors: Grahn H. T. , Schneider H. , Klitzing K. v.
Published: July 2002

We investigated the formation of high-field domains in undoped GaAs/AlAs superlattices under strong illumination. The photocurrent-voltage characteristics showed an oscillatory-like behavior which is attributed to the existence of high-field domains. We used photoluminescence (PL) spectroscopy to identify the existing domains as a function of electric field perpendicular to the layers. A total of five PL lines was observed and assigned to different domains through their Stark shift.
Improvement of the critical current density in the silver sheathed Bi-Pb-Sr-Ca-Cu-O superconducting tape
Authors: Mimura M. , Kumakura H. , Togano K. , Maeda H.
Published: July 2002

The critical current density (Jc) at 77 K and in 0 T of more than 3000 A/cm2 has been obtained in the silver sheathed Bi-Pb-Sr-Ca-Cu-O tape fabricated by the combination process of sintering and intermediate cold working. The decrease of the Jc with increasing magnetic field is much smaller in the tape prepared by this process than that prepared by sintering only. The improvement of the Jc is attributed to the improvement of the coupling between the grains of the high Tc phase. The process enhances the formation of the high Tc phase and produces a highly oriented grain structure. These are considered to be responsible for the large improvement of the coupling.
Calculation of the electron wave function in a graded-channel double-heterojunction modulation-doped field-effect transistor
Authors: Mui D. S. L. , Patil M. B. , Morkoç H.
Published: July 2002

We investigate theoretically three double-heterojunction modulation-doped field-effect transistor structures with different channel composition. All of these transistors have an InxGa1−xAs channel sandwiched between two doped Al0.3Ga0.7As barriers with undoped spacer layers. In one of the structures, x varies from 0 from either heterojunction to 0.15 at the center of the channel quadratically and in the other two, constant values of x of 0 and 0.15 are used. We solve Poisson’s and Schrödinger’s equations self-consistently for the electron wave function for all three cases. The results showed that the two-dimensional electron gas (2DEG) concentration in the channel of the quadratically graded structure is higher than the x=0 one and slightly lower than the x=0.15 one, and the mean distance of the 2DEG is closer to the center of the channel for this transistor than the other two. These two effects have important implications on the electron mobility in the channel.
Absorptive electro-optic spatial light modulators: Effect of well profile on device performance
Authors: Sanders G. D. , Bajaj K. K.
Published: July 2002

We report the effect of well profile on the performance of absorptive electro-optic spatial light modulators based on multiquantum well structures. In particular we calculate the variation in exciton oscillator strength and the absorption coefficient at the signal wavelength as a function of applied electric field. For a monochromatic source at 1572 meV, for example, we consider modulators based on square, parabolic, and asymmetric triangular GaAs-AlxGa1−xAs quantum wells and find that at zero field the exciton oscillator strengths are comparable but that superior performance is obtained using asymmetric triangular wells. These conclusions hold even assuming fairly large exciton linewidths in triangular wells.
Transmission electron microscopy of short-period Si/Ge strained-layer superlattices on Ge substrates
Authors: Wegscheider W. , Eberl K. , Cerva H. , Oppolzer H.
Published: July 2002

Structural investigations of high quality Si/Ge strained-layer superlattices (SLSs) on [001] oriented Ge substrates prepared by molecular beam epitaxy are presented. Cross-sectional transmission electron microscopy reveals that a defect-free superlattice is achieved for a structure composed of a 20-period sequence of 3 monolayers (ML) Si and 9 ML Ge. High-resolution lattice images and electron diffraction patterns show that the whole structure is matched to the Ge substrate. Experimental values for the tetragonal deformation of the Si layers within the SLS are in good agreement with theory. An equivalent sample containing 120 periods exceeds the critical thickness for pseudomorphic growth of the SLS and shows the formation of twin lamellae.
Formation of crystalline diamond from amorphous diamond-like carbon films by pulsed laser irradiation
Authors: Harshavardhan K. Solomon , Yalamanchi R. S. , Rao L. Kameswara
Published: July 2002

The formation of crystalline diamond films from amorphous diamond-like carbon films by pulsed laser irradiation with a 300 μs non-Q-switched Nd:YAG laser has been established by a combined study of transmission electron microscopy, x-ray photoelectron spectroscopy, and electrical resistivity. The films have been prepared by glow discharge decomposition of a mixture of propane, n-butane, and hydrogen in a rf plasma operating at a frequency of 13.56 MHz. Prior to laser irradiation, the films have been found to be amorphous by transmission electron microscope studies. After irradiation, the electron diffraction patterns clearly point out the formation of cubic diamond structure with a lattice spacing of 3.555 Å. However, the close similarity between diamond and graphite electron diffraction patterns could sometimes be misleading regarding the formation of a diamond structure, and hence, x-ray photoelectron spectroscopic studies have been carried out to confirm the results. A chemical shift in the C 1s core level binding energies towards higher values, viz., from 286.5 to 287.8 eV after laser irradiation, and a high electrical resistivity >1013 Ω cm are consistent with the growth of diamond structure. This novel ‘‘low-temperature, low-pressure’’ synthesis of diamond films offers enormous potential in terms of device compatibility with other solid-state devices.
Far-infrared photovoltaic effect in a Landau level diode
Authors: Liu C. T. , Kane B. E. , Tsui D. C. , Weimann G.
Published: July 2002

We have investigated the far-infrared (FIR) photoresponse of a Landau level diode. A negative photovoltaic current is observed. It results from carriers generated by cyclotron resonance transitions in the two-dimensional depletion region at the gate edge of the device and subsequently swept out of the depletion region by the strong built-in electric field. The potential use of this device as a tunable FIR photodetector is discussed.
Metal/(100) GaAs interface: Case for a metal-insulator-semiconductor-like structure
Authors: Freeouf J. L. , Woodall J. M. , Brillson L. J. , Viturro R. E.
Published: July 2002

Various models have been developed to address the problem of ‘‘Fermi level pinning,’’ i.e., why the barrier height varies much less than the Schottky metal work function limit. The most widely accepted mechanism is some variant of the metal-induced gap state model. However, recent experimental data on (100)InxGa1−x As surfaces and interfaces (0≤x≤1) suggest that the surface or interface Fermi level can assume values which lie well outside the variance associated with Fermi level pinning; in fact, recent data suggest the achievement of the Schottky limit. Furthermore, studies of epitaxially grown layers where dopant incorporation is dependent on an interface Fermi level suggest that such Fermi level positions are not always pinned. In order to account for these recent results, along with the pinned values, we introduce the concept of an insulating layer like surface reconstruction. Recent calculations suggest that reconstructions of the GaAs(100) surface are insulating. Thus, we suggest that the GaAs(100) metal interface is often a metal (M)/surface reconstruction layer (I)/GaAs bulk (S), or MIS-like. This approach attempts to reconcile disparate models of interface behavior by showing the limits of validity of these models with respect to the actual physical structure of the interface in question.
Temperature dependence of the electronic coherence of GaAs-GaAlAs superlattices
Authors: Mendez E. E. , Agulló-Rueda F. , Hong J. M.
Published: July 2002

We have shown that the coherence length of electrons in a 55-Å-period GaAs-GaAlAs superlattice does not depend strongly on temperature in the range 5–292 K, varying from 17 periods at 5 K to a minimum of nine periods at room temperature. The quantum coherence was determined by photocurrent spectroscopy experiments that exploit the formation of Stark ladders in superlattices under electric fields.
Femtosecond gain dynamics in InGaAsP optical amplifiers
Authors: Hall K. L. , Mark J. , Ippen E. P. , Eisenstein G.
Published: July 2002

We have studied ultrafast gain dynamics in InGaAsP optical amplifiers by means of pump-probe and cross-correlation measurements using 180 fs optical pulses. The data show strong gain nonlinearities due to nonequilibrium carrier distributions and differ significantly from those observed in AlGaAs amplifiers.
Strongly enhanced flux pinning by high-energy proton irradiation of a Tl2Ca2Ba2Cu3Oy single crystal
Authors: Venturini E. L. , Barbour J. C. , Ginley D. S. , Baughman R. J. , Morosin B.
Published: July 2002

Flux creep data are reported which clearly demonstrate a dramatic enhancement in the pinning potential for magnetic vortices (fluxoids) in a Tl2Ca2Ba2Cu3Oy single crystal upon irradiation with 4.5 MeV protons. The creep rate is markedly reduced between 5 and 80 K, and the effective pinning potential is quadrupled from ≊100 to ≊400 meV at 20 K in a field of 50 mT. This stronger pinning enhances the magnetization hysteresis and hence the inferred low-field critical current density by a factor of 5 at 40 K.
Positive and negative ‘‘resistless’’ lithography of GaAs by electron beam exposure and thermal Cl2 etching
Authors: Clausen E. M. , Harbison J. P. , Chang C. C. , Lin P. S. D. , Craighead H. G. , Florez L. T.
Published: July 2002

Positive and negative lithographic patterns have been produced in epitaxial layers of GaAs, achieved by electron beam writing and subsequent etching by Cl2 gas at elevated temperatures. A latent image is formed in the native oxide which is either less resistant to thermal Cl2 etching (positive pattern) or more resistant to thermal Cl2 etching (negative pattern), depending on the electron beam dose. The pattern is stable in air for at least several weeks. The tone of the pattern also depends on the electron beam accelerating voltage, the etching conditions, and the thickness and initial state of the native oxide. Significant changes in the resulting lithography are due to changes in an oxide only a few monolayers thick. Both positive and negative patterns can be produced in adjacent areas with high contrast by variation of the electron-beam does. Initial Auger analysis suggests that chemical rearrangement of the native oxide occurs with electron beam exposure. The discovery that the native oxide on GaAs acts as both positive and negative resists opens tremendous possibilities for in situ processing and device fabrication.
Energy levels of very short-period (GaAs)n-(AlAs)n superlattices
Authors: Ge Weikun , Sturge M. D. , Schmidt W. D. , Pfeiffer L. N. , West K. W.
Published: July 2002

The energy levels of very short-period (GaAs)n-(AlAs)n superlattices (n≤4) were investigated by photoluminescence (PL). The results show that these superlattices are type II but the lowest conduction bands are Xx,y for n≤3 and Xz for n=4, respectively. (Here Xz is the valley with k parallel to the growth axis.) In both cases the X valleys are very close to each other. PL decay, PL excitation, and PL under uniaxial stress confirm this identification. Al0.5Ga0.5As shows very different behavior, showing that even for n=1 our samples are true superlattices.
All-optical switching of picosecond pulses in a GaAs quantum well waveguide coupler
Authors: LiKamWa P. , Miller A. , Park C. B. , Roberts J. S. , Robson P. N.
Published: July 2002

A time-resolved pump-probe configuration employing 10 ps laser pulses has been used to investigate the all-optical switching characteristics of zero-gap directional couplers containing a single quantum well. All-optical switching of the weak probe pulses from one port into the adjacent one was obtained at pump input pulse energies of 150 pJ. The switching takes place within the duration of the laser pump pulses. The recovery time of the device was found to be ∼1.5 ns which indicates that carrier diffusion plays a major role in the device switching speed.
Surface passivation effects of As2S3 glass on self-aligned AlGaAs/GaAs heterojunction bipolar transistors
Authors: Chuang H. L. , Carpenter M. S. , Melloch M. R. , Lundstrom M. S. , Yablonovitch E. , Gmitter T. J.
Published: July 2002

A recently developed As2S3 chemical treatment was used to passivate the perimeters of self-aligned heterojunction bipolar transistors (HBTs). The As2S3 chemical treatment significantly lowered the base current resulting in a two order of magnitude reduction in the collector current density at which dc current gain was observed (β=1). No degradation with time has been observed in the electrical characteristics of the chemically treated HBTs. This absence of degradation is attributed to the impermeability to oxygen of the As2S3 glass which coats the perimeter of the HBT after chemical treatment.
Microwave power absorption by thallium-based superconductors on metallic substrates
Authors: Cooke D. W. , Arendt P. N. , Gray E. R. , Bennett B. L. , Brown D. R. , Elliott N. E. , Reeves G. A. , Rollett A. D. , Hubbard K. M. , Portis A. M.
Published: July 2002

Tl-based superconducting thick films deposited on large-area (11.4 cm2) Ag alloy (Consil 995) substrates have been characterized by surface resistance measurements yielding values of 4 and 14 mΩ at 4 and 77 K, respectively, as measured in an 18 GHz Cu cavity; corresponding Cu values are 8 and 21 mΩ. Utilizing a Nb cavity, the dependence of surface resistance on microwave power was also measured. The surface resistance begins to rise in microwave magnetic fields of 1–10 Oe and saturates in 20–60 Oe, the exact values depending on the degree of c-axis texturing of the polycrystalline films. Enhanced texturing sharpens the high-frequency transition into the superconducting state and also weakens the dependence of surface resistance on microwave magnetic field. These results are significant for high temperature superconductor applications such as microwave cavities.
Examination of the optic tensor structure in the electroclinic effect in liquid crystals
Authors: Elston S. J. , Sambles J. R.
Published: July 2002

The optic tensor configuration of a thin ferroelectric liquid crystal layer exhibiting the electroclinic effect is examined by the excitation of optic modes in the layer. It is seen that the restructuring involves no tilt of the optic axis out of the plane of the cell surfaces. This is interpreted in terms of a quasi-bookshelf structure in the smectic layering of the cell.
Electrochirally controlled bistable surface switching in nematic liquid crystals
Authors: Barberi R. , Durand G.
Published: July 2002

We present an electrically controlled bistable surface switching for nematic liquid crystals, in cells with the usual display geometry. The surface bistability is controlled by means of an electrochiral effect, by doping a nematic with chiral ions. The alternate transient commutation between the two surface states is related to the sign of an applied pulsed electric field. The threshold behavior allows a writing surface time of 32 μs for a field of 21 V/1.3 μm. In the birefringence mode, the optical texture response time is 0.3 ms. The effect, comparable with the one used with ferroelectric smectic liquid crystals, could be used to realize high definition video passive matrix displays.
Temperature dependence of semiconductor band gaps
Authors: O’Donnell K. P. , Chen X.
Published: July 2002

The application of a simple three-parameter fit to the temperature dependence of semiconductor band gaps is justified on both practical and theoretical grounds. In all trials the fit is numerically better than that obtained using the widely quoted Varshni equation. The formula is shown to be compatible with reasonable assumptions about the influence of phonons on the band-gap energy. Approximate analytical expressions are derived for the entropy and enthalpy of formation of electron-hole pairs in semiconductors.
Monolithic integration of GaAs and In0.2Ga0.8As lasers by molecular beam epitaxy on GaAs
Authors: Berger P. R. , Dutta N. K. , Lopata J. , Chu S. N. G. , Chand Naresh
Published: July 2002

Selective area molecular beam epitaxial regrowth of In0.2Ga0.8As lasers through dielectric masks between GaAs laser stripes on a GaAs substrate has been used for the first time to monolithically integrate these two lasers emitting near 1.0 and 0.85 μm, respectively. During regrowth, GaAs laser stripes were protected under a dielectric mask over which polycrystalline material grew, which was later chemically etched away during the fabrication process. The lasers are of the ridge waveguide type and have threshold currents in the 30–35 mA range for cleaved, uncoated facets at room temperature and a T0 value of 100 K. The overall performance characteristics of these lasers selectively regrown on dielectric coated wafers were comparable to lasers grown over a bare substrate.
Kelvin probe force microscopy
Authors: Nonnenmacher M. , O’Boyle M. P. , Wickramasinghe H. K.
Published: July 2002

Measurements of the contact potential difference between different materials have been performed for the first time using scanning force microscopy. The instrument has a high resolution for both the contact potential difference (better than 0.1 mV) and the lateral dimension (<50 nm) and allows the simultaneous imaging of topography and contact potential difference. Images of gold, platinum, and palladium surfaces, taken in air, show a large contrast in the contact potential difference and demonstrate the basic concept.
Practical high T
c Josephson junctions and dc SQUIDs operating above 85 K
Authors: Di Iorio M. S. , Yoshizumi S. , Yang K-Y. , Zhang J. , Maung M.
Published: July 2002

Reproducible high Tc YBa2Cu3O7/Ag/YBa2Cu3O7 Josephson junctions have been fabricated using an entirely in situ step-edge technique. The junction process requires the deposition of a single high Tc film and the patterning is performed using standard photolithography and ion-beam etching. The junctions exhibit a well-defined supercurrent from 4.2 to 87 K and the ac Josephson effect is observed up to 80 K in response to microwave radiation. dc SQUIDs have been fabricated and show the expected periodic modulation of the critical current in a magnetic field at temperatures ranging from 4.2 to 85.7 K. All of the 56 devices fabricated and measured to date show a well-defined supercurrent from 4.2 to over 77 K.
Pulsed laser assisted epitaxy of GexSi1−x alloys on Si 〈100〉
Authors: Lombardo S. , Kramer K. , Thompson Michael O. , Smith Duane R.
Published: July 2002

The epitaxial quality of GexSi1−x films grown on Si〈100〉 by a novel laser-assisted technique has been investigated for compositions in the range x≤0.37 and thicknesses between 50 and 500 nm. Epitaxy is induced during electron beam deposition of GexSi1−x films on Si substrates by pulses of an excimer laser operating at 308 nm with 30 ns pulse duration. Good epitaxial growth is obtained for 50 nm films for x≤0.04 on chemically cleaned Si(100) surfaces even in the presence of substantial fluorine coverage. Using a predeposition/crystallization of a 50 nm pure-Si buffer layer, good epitaxy is then obtained in 50 nm films for x=0.05–0.13. At x=0.07, defect-free alloys have been grown up to thicknesses of 0.5 μm. However, for films with compositions above 19 at.% Ge, dislocations at a nearly constant density of ≊10 μm/μm2 are observed. This uniform density suggests a breakdown mechanism other than simple strain relaxation.
Resonant transmission in the base/collector junction of a bipolar quantum-well resonant-tunneling transistor
Authors: Seabaugh A. C. , Kao Y.-C. , Frensley W. R. , Randall J. N. , Reed M. A.
Published: July 2002

A new transistor effect is demonstrated in a 120 nm base, bipolar quantum-well, resonant-tunneling transistor (BiQuaRTT). In this BiQuaRTT, a strong, multiple negative differential resistance (NDR) characteristic is obtained at room temperature with high-current gain (≳50). The effect is shown to be the consequence of an asymmetric, quantum-well-base heterostructure whose shape is controlled by the base/collector bias. Changes in the quantum-well shape lead to large modulations of the transmission coefficient for quasi-thermalized minority electrons crossing the quantum-well base. In this letter, we describe the transport characteristics of these transistors, including also temperature and magnetic field dependence.
Defect densities and hydrogen diffusion in hydrogenated amorphous Si-based alloys
Authors: Robertson J.
Published: July 2002

The density of Si dangling bond defects in plasma-deposited Si-rich a-Si1−xCx:H and a-Si1−xNx:H is argued to depend primarily on deposition temperature via the hydrogen diffusion rate. The optimum temperature equals about 0.62 of the bulk-bonded hydrogen evolution temperature, or about 500 °C for a-Si3N4:H. A model of hydrogen diffusion in alloys is proposed.
Single-crystal growth of Nb films onto molecular beam epitaxy grown (001)InAs
Authors: Akazaki Tatsushi , Nitta Junsaku , Takayanagi Hideaki
Published: July 2002

Thin Nb films are grown by electron beam evaporation in an ultrahigh vacuum system on molecular beam epitaxy grown (001)InAs epitaxial layers. The Nb on InAs grows as a single-crystalline deposit at a substrate temperature of 200 °C. The orientation relation is (001)Nb//(001)InAs with [110]Nb//[110]InAs, which is different from Nb growth on GaAs. The interface between Nb and InAs features a crystal-disordered layer with a thickness of 1–2 nm, which provides relaxation from lattice mismatch. Critical current measurement of Nb/InAs/Nb junctions shows that the crystal-disordered layer does not affect the superconducting characteristics of the junctions.
Shallow ohmic contacts to n-type GaAs and AlxGa1−xAs
Authors: Zheng L. R. , Wilson S. A. , Lawrence D. J. , Rudolph S. I. , Chen S. , Braunstein G.
Published: July 2002

Sequential deposition of Pd/AuGe/Ag/Au and rapid thermal annealing are used to form shallow ohmic contacts to n-GaAs and n-AlxGa1−xAs. Ion backscattering measurements and cross-sectional transmission electron microscopy show limited metal-substrate reaction and uniform interfaces. The metallization on GaAs displays good electrical properties with a contact resistivity of 2.0×10−6 Ω cm2 at a carrier concentration of 2×1017 cm−3. The contacts formed on Al0.55Ga0.45As have a contact resistivity of 2.1×10−5 Ω cm2 at a carrier concentration of 7.5×1017 cm−3.
Real-time characterization of acoustic modes of polyimide thin-film coatings using impulsive stimulated thermal scattering
Authors: Duggal Anil R. , Rogers John A. , Nelson Keith A. , Rothschild Mordechai
Published: July 2002

A method for real-time optical characterization of thin-film acoustic modes is demonstrated. Pseudo-Rayleigh acoustic waves of thin (∼1 μm) polyimide films attached to silicon substrates are optically excited and monitored in real time as they propagate. The results can be used to determine mechanical properties (elastic moduli) and to guide thin-film fabrication and curing procedures.
Rapid thermal chemical vapor deposition of thin silicon oxide films using silane and nitrous oxide
Authors: Xu X. L. , Kuehn R. T. , Wortman J. J. , Öztürk M. C.
Published: July 2002

Thin (80–200 Å) silicon dioxide (SiO2) films have been deposited by low pressure rapid thermal chemical vapor deposition (RTCVD), using silane (SiH4) and nitrous oxide (N2O) as the reactive gases for the first time. A deposition rate of 55 Å/min has been achieved at 800 °C with a SiH4/N2O flow rate ratio of 2%. Auger electron spectroscopy (AES) and Rutherford back scattering spectroscopy (RBS) have shown a uniform and stoichiometric composition throughout the deposited oxide films. Electrical characterization of the films have shown an average catastrophic breakdown field of 13 MV/cm and a midgap interface trap density (Dit) of equal to or less than 5×1010 eV−1 cm−2. The results suggest that the deposited RTCVD SiO2 films using SiH4-N2O gas system may have the potential to be used as the gate dielectric in future low-temperature metal oxide semiconductor (MOS) device processes for ultralarge scale integration (ULSI).
Deposition of amorphous fluoropolymer thin films by thermolysis of Teflon amorphous fluoropolymer
Authors: Nason T. C. , Moore J. A. , Lu T.-M.
Published: July 2002

Thin films (0.3–5 μm) of an amorphous fluoropolymer (AF) derived from the copolymeric material Teflon AF 1600 were deposited on Si (100) wafers by vacuum pyrolysis. Infrared spectroscopy indicated that the composition of the deposited films was similar to the source material. The deposited films were amorphous by x-ray diffraction. The surface morphology contained micropores which did not extend through films deposited at a low rate. The refractive index was ∼1.2 at 633 nm. Comparisons are made to films derived from ordinary Teflon (also by pyrolysis). The mechanism for the repolymerization of the Teflon AF copolymer at the substrate surface is discussed.
Room-temperature recombination of point defects produced in silicon p-n junctions by light ion irradiation
Authors: Germaná R. S. , Campisano S. U.
Published: July 2002

The reverse current in a p+-n silicon junction irradiated by high-energy helium ions was measured as a function of time at room temperature. A rather large exponential decrease of the reverse current has been observed and the measured time constant is 3±0.2 days. Defect profiling by current-voltage characteristics indicates that the damage recombination occurs without long-range migration of defects. The defects responsible for the reverse current of irradiated silicon diodes after the long room-temperature aging are characterized by an energy level 0.11 eV above Si midgap.
Josephson characteristics in a-axis oriented YBa2Cu3O7−δ/PrBa2Cu3O7−δ′/YBa2Cu3O7−δ junctions
Authors: Hashimoto Tatsunori , Sagoi Masayuki , Mizutani Yoshihisa , Yoshida Jiro , Mizushima Koichi
Published: July 2002

A sandwich-type all-oxide Josephson junction consisting of an a-axis oriented YBa2Cu3O7−δ/PrBa2Cu3O7−δ′/YBa2Cu3O7−δ structure has been realized and its Josephson characteristics experimentally investigated. Almost all the fabricated junctions showed the Josephson characteristics, including a resistively shunted junction (RSJ)-like current-voltage characteristic and a Fraunhofer-like magnetic-field dependence of the critical current. The obtained maximum values of the critical-current density jc and the IcRn value were 110 A/cm2 and 80 μV, respectively, for a 50-nm-thick PBCO interlayer at 3 K. Besides, the temperature dependencies of the critical current and the normal resistance were measured. Ic was proportional to exp(− aT1/2), where a is a constant.
Planar, low-loss optical waveguides fabricated by solid-phase regrowth
Authors: Xia W. , Yu L. S. , Guan Z. F. , Pappert S. A. , Yu P. K. L. , Lau S. S. , Schwarz S. A. , Pudensi M. A. A. , Florez L. T. , Harbison J. P.
Published: July 2002

Planar, low-loss AlGaAs/GaAs waveguides have been fabricated using the solid-phase regrowth (SPR) process. Single-mode waveguide with a propagation loss as low as 1.6 dB/cm have been obtained. This process requires only thin-film deposition and low-temperature short-duration annealing (i.e., 650 °C for 30 s), thus making the SPR method a much simplified technique to induce compositional disordering. Simultaneous electrical isolation and compositional disordering are also demonstrated with the SPR process.
Broadband (8–14 μm), normal incidence, pseudomorphic GexSi1−x/Si strained-layer infrared photodetector operating between 20 and 77 K
Authors: People R. , Bean J. C. , Bethea C. G. , Sputz S. K. , Peticolas L. J.
Published: July 2002

Performance characteristics of a pseudomorphic p-type, normal incidence, Ge0.25Si0.75/Si strained-layer quantum well infrared photodetector on (001) Si is described for 20≤T≤77 K. The device shows broadband photoresponse (8–14 μm) which is attributed to strain and quantum confinement induced mixing of heavy, light, and split-off hole bands. Typical device responsivity at λ=10.8 μm is ∼0.04 A/W over the 20–77 K temperature range. A detectivity D*λ=3.3×109 cm √Hz/W was measured at a bias of −2.4 V for a temperature of 77 K at λ=10.8 μm and no cold shield. Room temperature FTIR measurements yield a quantum efficiency η≊3.1% at λp≊8 μm at 300 K.
Superlattice band structure probed by tunneling hot-electron injection
Authors: Kuan C. H. , Tsui D. C. , Choi K. K.
Published: July 2002

Tunneling hot-electron injection is used to probe superlattice (SL) minibands and the effect of a magnetic field (B) on a 10-μm infrared (IR) detector. Within each miniband, the data agree with the transmission calculation including nonparabolicity, but outside the miniband, the data show that around 15% of electrons can pass through the SL due to impurity scattering. When B is applied, the injection energy of the hot electrons increases slightly. For the IR detector samples, a Gaussian hot-electron distribution gives a better fit to the data than the Lorentzian one, indicating that inhomogeneous broadening is significant in these devices.
Enhanced second-order optical nonlinearity using inter- and intra-band transitions in low-dimensional semiconductors
Authors: Shimizu Akira , Kuwata-Gonokami Makoto , Sakaki Hiroyuki
Published: July 2002

We propose a novel scheme of enhancing χ(2)(ω3;−ω1,ω2) in asymmetric quasi-low- dimensional (QLD) semiconductors. The ω1 and ω2 are tuned to exciton levels, while ω3=ω2−ω1 is tuned to an intersubband transition energy. Fairly large χ(2) is obtained when a certain asymmetry is introduced in the QLD structure by, e.g., an external electric field F. A possible application is the efficient down conversion of coherent light emitted from a laser diode(s) into an intersubband frequency ω3 via a parametric process or difference-frequency generation. It is also possible to modulate the intensity of the down-converted, long-wavelength light by modulating F at a very high frequency of several tens of GHz.
Correlation between photoinduced absorption and large off-resonant nonlinear refractive index of polydiacetylene
Authors: Quintero-Torres R. , Thakur M.
Published: July 2002

We have calculated the off-resonant nonlinear refractive indices of PTS and TCDU polydiacetylenes using photoinduced absorption data and applying the Kramers–Kronig transformation. The results are in good agreement with the recently published experimental measurements. This is the first time that a correlation between photoinduced absorption and the off-resonant nonlinear refractive index has been established for polydiacetylenes. The calculated values are: n2∼1.3×10−5 cm2/MW for PTS, and ∼2.2×10−6 cm2/MW for TCDU at 1.06-μm wavelength. The sign of n2 is positive at this wavelength.
C60 encapsulation of the Si(111)-(7×7) surface
Authors: Hong Hawoong , McMahon W. E. , Zschack P. , Lin D.-S. , Aburano R. D. , Chen Haydn , Chiang T.-C.
Published: July 2002

The structure of a Si(111)-(7×7) surface capped by a 200 Å film of C60 was studied by grazing-incidence x-ray diffraction. The Si(111)-(7×7) reconstruction prepared in vacuum, including the loosely bonded ‘‘adatoms’’ on the surface, is preserved under the C60 overlayer. This result illustrates that C60 can be used as an inert cap for surfaces and suggests potentially interesting applications in surface science research and electronic device engineering.
Single liquid source plasma-enhanced metalorganic chemical vapor deposition of high-quality YBa2Cu3O7−x thin films
Authors: Zhang Jiming , Gardiner Robin A. , Kirlin Peter S. , Boerstler Robert W. , Steinbeck John
Published: July 2002

High-quality YBa2Cu3O7−x films were grown in situ on LaAlO3 (100) by a novel single liquid source plasma-enhanced metalorganic chemical vapor deposition (CVD) process. The metalorganic complexes M (thd)n, (thd=2,2,6,6-tetramethyl-3-5-heptanedionate; M=Y, Ba, Cu) were dissolved in an organic solution and injected into a vaporizer immediately upstream of the reactor inlet. The single liquid source technique dramatically simplifies current CVD processing and can significantly improve the process reproducibility. X-ray diffraction measurements indicated that single phase, highly c-axis oriented YBa2Cu3O7−x was formed in situ at a substrate temperature 680 °C. The as-deposited films exhibited a mirrorlike surface, had transition temperature Tc0≂89 K, ΔTc<1 K, and Jc (77 K)=106 A/cm2.
Surface-emitting second-harmonic generator by intersubband transition in asymmetric quantum wells with slab waveguide
Authors: Chen Zhenghao , Li Meihua , Cui Dafu , Lu Huibin , Yang Guozhen
Published: July 2002

We report the first experimental evidence of second-harmonic generation due to intersubband transitions in GaAs-Ga1−xAlxAs step multiquantum wells with a waveguiding surface-emitting structure. The second-harmonic generation signal obeys wave vector conservation and propagates perpendicular to the waveguiding surface. As high as 3×10−3 power conversion efficiency has been obtained. The experimental analysis and the severalfold benefit of this novel technique have been given in detail.
Modulation-doped In0.48Al0.52P/In0.2Ga0.8As field-effect transistors
Authors: Kuo Jenn-Ming , Chan Yi-Jen , Pavlidis Dimitris
Published: July 2002

In0.48Al0.52P/In0.2Ga0.8As pseudomorphic modulation-doped field-effect transistors (MODFETs) were realized for the first time by gas-source molecular beam epitaxy on GaAs substrates. Extrinsic transconductances of 173 and 283 mS/mm at 300 and 77 K, respectively, were measured for MODFETs with a 1 μm long and 75 μm wide gate. The devices showed very good pinch-off characteristics, and the output conductance was only 1.3 mS/mm. Extremely high Schottky barrier height (0.92 eV) and low gate leakage current (Irev<250 nA at VGS=−5 V) were achieved. The gate breakdown voltage was −17 V. No I-V collapse was observed at 77 K. Microwave measurements showed that the current gain cutoff frequency fT of the devices was 11.5 GHz and the maximum frequency of oscillation fmax, was 26 GHz. These results demonstrate the promising potential of pseudomorphic In0.48Al0.52P/InGaAs MODFETs as high frequency and high power devices.
Carrier-dependent hydrogen migration in hydrogenated amorphous silicon
Authors: Santos Paulo V. , Johnson N. M.
Published: July 2002

Experiments demonstrate that hydrogen migration in hydrogenated amorphous silicon is controlled by the concentration of electronic carriers and, therefore, is strongly suppressed when the carriers are removed by an electric field. Furthermore, the few hydrogen atoms migrating in a region depleted of carriers have an increased mean free path for diffusion due to a reduction in the concentration of trapping centers, which are probably silicon dangling bonds.
Surface chemical bonding of (NH4)2Sx-treated InP(001)
Authors: Maeda F. , Watanabe Y. , Oshima M.
Published: July 2002

(NH4)2Sx-treated InP(001) surfaces were analyzed by using synchrotron radiation photoelectron spectroscopy to characterize the S-passivated surfaces and elucidate the solution etching mechanism. Polysulfide chemical states were observed for the first time in both the P 2p and In 4d spectra. Monosulfide and polysulfide states were also evident in the S 2p spectra. Etching models consistent with the experimental results were discussed.
Highly nondegenerate four-wave mixing and gain nonlinearity in a strained multiple-quantum-well optical amplifier
Authors: Zhou Jianhui , Park Namkyoo , Dawson Jay W. , Vahala Kerry J. , Newkirk Michael A. , Koren Uziel , Miller Barry I.
Published: July 2002

Highly nondegenerate four-wave mixing was investigated in a 1.5 μm compressively strained multi-quantum-well semiconductor traveling-wave optical amplifier at detuning frequencies up to 600 GHz. A gain nonlinearity with a characteristic relaxation time of 650 fs was determined from the data, and the nonlinear gain coefficient was estimated to be 4.3×10−23 m3. Dynamic carrier heating is believed to be the major source of nonlinear gain in this device at the wavelengths investigated.
Quantum capture limited modulation bandwidth of quantum well, wire, and dot lasers
Authors: Kan Sidney C. , Vassilovski Dan , Wu Ta C. , Lau Kam Y.
Published: July 2002

We investigate the quantum capture limited modulation bandwidths of various lower-dimensional semiconductor lasers. It is shown that, for buried quantum well, wire, and dot lasers, the maximum bandwidth is proportional to the packing density of the active region. For the quantum wire lasers grown on V-grooved substrates, the maximum bandwidth is enhanced by the precapture of carriers from three-dimensional states to two-dimensional states before the capture into the one-dimensional states.
Origin of Cu-rich precipitate formation on superconducting films: A competition between nucleation, oxidation, and growth kinetics
Authors: Locquet Jean-Pierre , Jaccard Yvan , Gerber Christoph , Mächler Erich
Published: July 2002

We have unraveled the growth mechanism of Cu-rich precipitates on the surface of c-axis-oriented DyBa2Cu3O7 thin films. Despite the strong oxidation conditions present, the surface of c-axis DyBa2Cu3O7 acts as an ideal nucleation site for Cu2O crystallites, whose (110) planes are parallel to the substrate surface. Once nucleated, they act as effective sinks for the rest of the deposited copper and form large crystals that revert to monoclinic CuO upon cooling. This result has important implications for all currently used thin film growth processes and indicates that smooth surfaces can only be prepared under stringent composition control.
Flux flow in high-T
c Josephson junctions
Authors: Filatrella G. , Pedersen N. F.
Published: July 2002

The possibility of achieving fluxon nucleation in nonhysteretic high-Tc Josephson junctions due to the presence of inhomogeneities is investigated numerically. For a large range of parameters the I-V characteristics in presence of such discontinuities show a strong similarity with those obtained experimentally. The spatial inhomogeneities considered are on the scale of the Josephson penetration depth (μm). It is demonstrated that the topic is of interest for the construction of amplifiers. Thus when fluxons are generated the resulting flux flow regime proves to be much more sensitive than the uniform solution to external fields.
Low-temperature ion-induced epitaxial growth of α-FeSi2 and cubic FeSi2 in Si
Authors: Lin X. W. , Behar M. , Desimoni J. , Bernas H. , Washburn J. , Liliental-Weber Z.
Published: July 2002

Ion-beam-induced epitaxial crystallization of amorphous Si implanted with Fe to 18 at. % peak concentration was studied. The structure of the specimen was characterized using transmission electron microscopy and Rutherford backscattering spectrometry. Both cubic FeSi2 and α-FeSi2 were formed in epitaxy with the Si matrix with two types of orientations (fully aligned and twinned). The twins of α-FeSi2 and those of cubic FeSi2 were found to have exactly the same type of epitaxial relationship as for the aligned ones. The thermodynamically stable β-FeSi2 is not formed, demonstrating that ion-beam-induced crystallization can lead to preferential phase formation as well as to epitaxy.
High electron mobility transistor based on a GaN-AlxGa1−xN heterojunction
Authors: Asif Khan M. , Bhattarai A. , Kuznia J. N. , Olson D. T.
Published: July 2002

In this letter we report the fabrication and dc characterization of a high electron mobility transistor (HEMT) based on a n-GaN-Al0.14Ga0.86N heterojunction. The conduction in our low pressure metalorganic chemical vapor deposited heterostructure is dominated by two-dimensional electron gas at the heterostructure interface. HEMT devices were fabricated on ion-implant isolated mesas using Ti/Au for the source drain ohmic and TiW for the gate Schottky. For a device with a 4 μm gate length (10 μm channel opening, i.e., source-drain separation), a transconductance of 28 mS/mm at 300 K and 46 mS/mm at 77 K was obtained at +0.5 V gate bias. Complete pinchoff was observed for a −6 V gate bias.
45-cm long compression-molded polymer-based optical bus
Authors: Chen Ray T. , Tang Suning , Jannson Tomasz , Jannson Joanna
Published: July 2002

We report the formation of an optical bus using compression-molding technique. The linear dimension of such a waveguide is well beyond that of a microlithographically defined waveguide. Theoretical calculation based on the effective index method was used to determine the optimal dimension of the molding tool design for single- and multimode waveguides. A molded photolime gel-based polymer optical bus with a linear dimension of 45 cm was fabricated and then tested at 0.6328-μm wavelength. Waveguide propagation loss from 0.5 to 2 dB/cm was determined using the two prism method. As a result of this long interconnection distance, board-to-board optical interconnects through backplane can be realized using the technology.
Fabrication of high-quality superconductor-insulator-superconductor junctions on thin SiN membranes
Authors: Garcia Edouard , Jacobson Brian R. , Hu Qing
Published: July 2002

We have successfully fabricated high-quality and high-current density (Jc≊4 000 A/cm2) superconductor-insulator-superconductor (SIS) junctions on freestanding thin (∼1 μm) silicon nitride (SiN) membranes. These devices can be used in a novel millimeter-wave and THz receiver system which is made using micromachining. The SIS junctions with planar antennas were fabricated first on a silicon wafer covered with a SiN membrane, the Si wafer underneath was then etched away using an anisotropic KOH etchant. The current-voltage characteristics of the SIS junctions remained unchanged after the whole process, and the junctions and the membrane survived thermal cycling.
Measurement of femtosecond electron bunches from metal photocathodes
Authors: Tsang T.
Published: July 2002

A technique of direct generation and detection of femtosecond electron bunches is reported. These electrons pulses are measured with high accuracy using nonlinear photoemission. In this initial demonstration, n=1 and higher-order femtosecond optical soliton pulses are used to liberate electrons from metals.
Coulomb blockade in silicon based structures at temperatures up to 50 K
Authors: Paul D. J. , Cleaver J. R. A. , Ahmed H. , Whall T. E.
Published: July 2002

Coulomb blockade has been observed in the current-voltage characteristics of structures fabricated in silicon germanium δ-doped material at temperatures up to 50 K. This is consistent with the estimated effective tunnel capacitance of 10 aF which is significantly smaller than the reported capacitances of tunnel junctions made from Al or GaAs/AlGaAs heterostructures.
Dopant and carrier profiling in modulation-doped GaAs multilayers with cross-sectional scanning tunneling microscopy
Authors: Johnson M. B. , Meier H. P. , Salemink H. W. M.
Published: July 2002

We report the direct measurement of dopant and carrier density profiles in GaAs (001) modulation-doped structures using cross-sectional scanning tunneling microscopy. On ultrahigh-vacuum-cleaved cross-sectional (110) GaAs surfaces, individual electrically active dopants are observed as hillocks in the top several surface layers, and the tip-sample separation is found to be sensitive to the carrier concentration. In structures where the conventionally measured p-type dopant concentration varied from 1×1018 to 1×1019 cm−3 the density of such dopant hillocks varies accordingly and the tip-sample separation changes by 0.1 nm.
Optical phase conjugation in InGaAs/GaAs multiple quantum wells at 1.06 μm wavelength
Authors: Zhao Yang , Wu Cunkai , Shah Pankaj , Kim M. K. , Dawson L. Ralph
Published: July 2002

The observation of phase conjugation in InGaAs/GaAs multiple quantum wells at 1.06 μm wavelength is reported on. The effective nonlinearity of the sample used in our experiments was measured to be χ(3)=10−7 esu. The nonlinearity is induced by the saturation absorption due to band filling and exciton bleaching. The saturation intensity is 1.3 kW/cm2.
Photoluminescence excitation spectroscopy yields band gap of Ga0.5In0.5P containing relatively ordered domains
Authors: Fouquet J. E. , Minsky M. S. , Rosner S. J.
Published: July 2002

Photoluminescence excitation spectroscopy at 9 K reveals that the absorption edge of Ga0.5In0.5P containing relatively ordered domains (grown at 670 °C) is near 1.93 eV, representing the band gap of this inhomogeneous material. Photoluminescence (PL) has been observed at energies lower than this absorption edge energy by up to 70 meV. Along with the unusually slow decay times previously measured, the low energy PL indicates spatially indirect recombination. Therefore PL alone cannot reliably determine the band gap of typical ‘‘ordered’’ Ga0.5In0.5P samples. In contrast, the optical properties of relatively randomly ordered (‘‘normal’’) Ga0.5In0.5P (grown at 775 °C) are typical of a normal direct III-V semiconductor.
Incorporation of hyperfine probes into the thin-film superconductor YBa2Cu3O7−δ during deposition
Authors: Tom D. W. , Platzer R. , Gardner John A. , Tate J.
Published: July 2002

Radioactive 111In has been incorporated into oriented thin films of YBa2Cu3O7−δ during a thermal coevaporation process. The hyperfine technique of perturbed γγ-angular correlation spectroscopy shows that 60% of the indium substitutes at a single site in the superconductor. We obtain spectra with reasonable statistics from 2 mCi of starting material, about 1% of which is incorporated into the film. Other methods of incorporating indium into films and bulk YBa2Cu3O7−δ have often resulted in a significant amount of indium being incorporated into impurity phases.
Elastic scattering and the current-voltage characteristics of superconducting Nb-InAs-Nb junctions
Authors: van der Post Niko , Nitta Junsaku , Takayanagi Hideaki
Published: July 2002

Superconducting niobium contacts are attached to a 0.8-μm-long epitaxially grown InAs channel sandwiched between insulating InGaAs layers. The current-voltage characteristics show nonlinearities at submultiples of the superconducting energy gap indicative of multiple-Andreev reflections. We demonstrate that an increase in the elastic scattering rate in the InAs channel, caused by Ar-ion etching, diminishes the order of Andreev reflections and explains the overall shape of the current-voltage characteristics.