bluuebunny/crossref_metadata_2025 huggingface dataset:
Illumination effects in high impedance InSb photodiodes
Authors: Fastow R.
Published: July 2002

Current-voltage characteristics of high impedance (InSb) photodiodes are modeled both numerically and analytically. Numerical calculations show a linear relationship between the inverse dynamic resistance and the current density over a wide range of photon fluxes. This effect is found to originate from an excess diffusion current in the base region. An analytic derivation of the ideal photodiode equation shows that this excess diffusion current is due to the dependence of the space-charge boundary on the bias voltage. The resulting analytical expression provides a good approximation to the numerical calculations, and explicitly describes the dependence of the excess diffusion current and the dynamic resistance on illumination level and applied bias.
Ar+ bombardment of Si(100) in oxygen atmosphere: Room temperature oxide formation studied by x-ray photoelectron spectroscopy
Authors: Terrasi A. , Coluzza C. , Margaritondo G.
Published: July 2002

We report the formation of silicon oxide thin films at room temperature obtained by Ar+ ion bombardment of Si(100) wafers in partial oxygen atmosphere. Samples have been prepared at several ion beam energies (0≤Eb≤400 eV) and characterized by x-ray photoelectron spectroscopy. The oxidation rate, as well as the SiO2/SiOx (x=0.5, x=1, and x=1.5) ratio, have been found to increase with the ion beam energy. For the highest energy bombardment, Eb=400 eV, we observed the formation of a uniform, electrically insulating, SiO2 top layer about 37 Å thick.
Subband quantum scattering times for AlGaAs/GaAs obtained using digital filtering
Authors: Mena R. A. , Schacham S. E. , Haugland E. J. , Alterovitz S. A. , Bibyk S. B. , Ringel S. A.
Published: July 2002

In this study we investigate both the transport and quantum scattering times as a function of the carrier concentration for a modulation doped Al0.3Ga0.7As/GaAs structure. Carriers in the well are generated as a result of the persistent photoconductivity effect. When more than one subband becomes populated, digital filtering is used to separate the components for each of the excited subbands. We find that the quantum scattering time for the ground subband increases initially as the carrier concentration is increased. However, once the second subband becomes populated, the ground subband scattering time begins to decrease. The quantum scattering time for the excited subband is also observed to decrease as the concentration is increased. Our results are consistent with the theoretical results by A. Isihara and L. Smrcka [J. Phys. C 19, 6777 (1986)]. Finally, from the ratio of the transport and quantum scattering times, it is seen that the transport in the well becomes more isotropic as the concentration is increased.
Continuous-wave operation and tuning characteristics of Nd:LuAlO3 at 1083 nm for 3He and 4He optical pumping
Authors: Stephens E. F. , Vandiver R. , Tin Padetha , Kokta M. R.
Published: July 2002

cw laser operation has been achieved from a diode-pumped Nd:LuAlO3 crystal for the first time. The tuning characteristics centered about 1083 nm are presented and the laser’s use for 3He and 4He optical pumping are established. With 1.5 W of output at 1083 nm and a slope efficiency greater than 22%, this crystal represents the first viable alternative to Nd:LMA.
Soft laser sputtering of GaAs semiconductor (100) surface
Authors: Vivet L. , Barthe M. F. , Gibert-Legrand T. , Dubreuil B.
Published: July 2002

We have studied the soft laser sputtering of (100)GaAs with 337 nm photons, starting from the threshold for particle emission (a few tens of mJ/cm2) to some 300 mJ/cm2 fluences. Atoms and molecules sputtered from the irradiated surface are detected, their relative number measured, and their time of flight determined using laser resonant ionization mass spectrometry. The surface after laser irradiation is examined by scanning electron microscopy and electron microprobe analysis.

One observes a significant preferential emission of arsenic in the form of As2. This leads to the formation of perturbed Ga-rich surface structure which appears even at low fluence and after a few tens of laser shots on the same spot. This initial transformation seems to determine the further evolution of the irradiated surface. First, Ga atoms aggregate to form Ga islands on the surface; after a sufficient number of shots, micrometric structures are produced which finally behave as pure Ga metal. This evolution of the surface state after multipulse irradiation appears practically the same for low and medium laser fluences, the only difference being in the number of shots required to obtain the same microscopic structure. The velocity distribution of Ga atoms and As2 molecules is well fitted by half-space Maxwellian distributions. The kinetic temperatures are in broad agreement with the results obtained from a model of laser heating of the surface. The gross features of the experimental results can be interpreted from the particular thermodynamics properties of GaAs which exhibits very large As2 pressure above the solid as soon as the temperature exceeds 950 K. After a few laser shots, corresponding to particle emission from defect sites, the thermodynamics of GaAs appears to govern the further evolution of the laser-sputtered surface. Two sputtering regimes are evidenced: In the low-fluence regime (from threshold to 90 mJ/cm2) sputtering appears to be dominated by surface defect emission, whereas for higher fluences emission is more characteristic of thermal process accompanied by preferential sputtering of arsenide. According to these experimental results, a simple analytical model was developed which relates the quantitative surface to the quantitative sputtered cloud compositions.
Solvent and pressure effects on the photoluminescence in porous Si
Authors: Kuroda Noritaka , Matsuda Yashuhiro , Nakajima Shinya , Taketsu Iori , Ookubo Norio
Published: July 2002

The photoluminescence band in porous Si is found to shift monotonically toward lower energy at a rate of −(18±5) meV/GPa if a hydrostatic pressure of up to 5 GPa is applied with liquid argon, in contrast to a convex behavior of the shift observed when a methanol–ethanol mixture is used as the pressure-transmitting medium. The data show that the electronic energy gap relevant to the luminescence is intrinsically reduced by the hydrostatic compression but has a distinct solvatochromic character.
Concentration dependent Zn diffusion in InP during metalorganic vapor phase epitaxy
Authors: Chu S. N. G , Logan R. A. , Geva M. , Ha N. T.
Published: July 2002

Concentration dependent diffusion of Zn during metalorganic vapor phase epitaxy from a Zn-doped InP layer into the adjacent undoped InP buffer layer were studied systematically using secondary ion mass spectroscopy and carrier concentration profiling. Under the condition that the growth rate of the Zn-doped film is faster than the interdiffusion of Zn into the underlying undoped buffer layer, the diffusion problem can be treated as a one-dimensional diffusion couple between two semi-infinite media. Furthermore, Zn diffusion during the optimized growth condition for InP completely eliminates the thermal decomposition problem encountered in the sealed ampoule and open tube diffusions and also maintains all the intrinsic point defects at their thermodynamic equilibrium concentrations. With an optimal growth temperature at 625 °C and a maximum Zn flow below the incorporation limit for substitutional Zn to ensure that the dominant Zn are incorporated substitutionally, the diffusion profiles of Zn across the interface in this simple and clean system are simulated using a concentration dependent diffusivity. A third power concentration dependence of the effective diffusion coefficient has been confirmed, which applies to both Frank–Turnbull and kickout interstitial-substitutional equilibrium mechanisms using an interstitial-substitutional diffusion model. This indicates a +2 charge state of the fast diffusing Zn interstitials. The extrapolated curve into high-concentration diffusion source regime used by sealed ampoule diffusion experiments generally agrees with the published results although the dominant Zn atoms found in the high-concentration diffusion source regime form complexes with phosphorous vacancies in a neutral state. The enhanced diffusion due to excess interstitials is discussed.
The gettering of copper by keV implantation of germanium into silicon
Authors: Barbero C. J. , Corbett J. W. , Deng C. , Atzmon Z.
Published: July 2002

The gettering of copper by keV implantation of germanium into silicon is investigated. Germanium is implanted at a fixed energy with varying doses into the front side of silicon samples. Copper is thermally evaporated on the backside of the samples and then annealed at 900 °C for 1 h and 10 h, respectively, to allow in-diffusion of the transition metal. Rutherford backscattering spectroscopy, secondary-ion-mass spectroscopy, and cross-section transmission electron microscopy are used to demonstrate that gettering of copper is achieved through stacking faults created by heavy dose germanium implantation and solid-phase epitaxy.
Electron paramagnetic resonance of dangling bond centers in vacuum-annealed porous silicon
Authors: Laiho R. , Vlasenko L. S.
Published: July 2002

Electron paramagnetic resonance (EPR) of centers produced in porous silicon (PS) by heat treatment in vacuum is investigated. Annealing in the temperature range of 400–600 °C and vacuum up to 5×10−5 Torr increases the EPR line intensity of Pb centers in a way depending on the time of annealing. This can be explained by hydrogen depassivation of the Pb centers. An isotropic line with g=2.0055, usually attributed to disordered Si dangling bonds, appears after vacuum annealing of some minutes but at a longer heat treatment its intensity decreases. At room temperature this decrease is exponential with a time constant of a few minutes, depending on the level of vacuum. Heat treatment of PS in vacuum leads to amplification of the Pb EPR spectrum after exposition of samples to air at room temperature.
Superconductor GdBa2Cu3O7−δ edge junctions with lattice-matched Y0.6Pr0.4Ba2Cu3O7−δ barriers
Authors: Jia Q. X. , Wu X. D. , Reagor D. W. , Foltyn S. R. , Houlton R. J. , Tiwari P. , Mombourquette C. , Campbell I. H. , Garzon F. , Peterson D. E.
Published: July 2002

High-temperature-superconductor Josephson junctions with an edge geometry of superconductor/normal-metal/superconductor have been fabricated on yttria-stabilized zirconia substrates by engineering the electrode and N-layer material to reduce the lattice mismatches (a, b, and c). With GdBa2Cu3O7−δ as electrodes and Pr-doped Y0.6Pr0.4Ba2Cu3O7−δ as a barrier, the lattice mismatches from electrode and barrier layer are reduced to a very low level. The junctions fabricated with such a design demonstrate resistively shunted junction current-voltage characteristics under dc bias at temperatures in the range of 77–88 K. The quite low specific interface resistivity on the order of 10−10 Ω cm2 indicates that the junction performance is controlled by the normal-metal (N) layer material instead of the interfaces. The use of lattice-matched electrode and N-layer material is one of the key design rules to obtain controllable high-temperature superconductor Josephson junctions.
Wannier states and optical transitions in a ‘‘diatomic’’ semiconductor superlattice
Authors: Guerrero H. M. , Cocoletzi G. H. , Ulloa S. E.
Published: July 2002

We study the effect of several structural parameters on the band structure and Wannier functions of a superlattice with a complex period. Our calculations include nonparabolic-band corrections and different effective masses. As an interesting and physical example, we study the evolution of a ‘‘molecular lattice’’—with two wells per period—with respect to the connectivity between different wells. We find strong dependence of important physical quantities, such as bandwidths, wave functions, and interband oscillator strength. We discuss how these effects should be observable in transport and photoluminescence experiments.
Enhanced flux pinning in HgBa2CuO4+x by neutron irradiation and its relationship to magnetic anisotropy
Authors: Amm Kathleen M. , Schwartz Justin
Published: July 2002

Our previous results indicate that neutron irradiation is an effective method for improving the pinning strength and critical current density of HgBa2CuO4+x, demonstrating that the mercury-based materials are not intrinsically limited to low current density. Here we expand our investigations of random HgBa2CuO4+x powders with a study of the effects of further neutron irradiation and annealing on the magnetization hysteresis and flux creep of powder samples. We also report the effects of irradiation on aligned powder samples and thus understand the relationship between flux pinning and magnetic anisotropy. We find that the critical current density continues to increase with increasing neutron fluence but at a decreasing rate. The irradiation enhancements in ground-then-aligned powders are not as large as in the as-sintered material, which we attribute to small defects introduced during the grinding process that inhibit the formation of defect clusters during irradiation. Lastly, we find that the neutron irradiation decreases and inverts the anisotropy as measured by magnetization hysteresis.
Gallium-implantation-enhanced intermixing of close-surface GaAs/AlAs/AlGaAs double-barrier quantum wells
Authors: Kupka R. K. , Chen Y.
Published: July 2002

The intermixing of close-surface GaAs/AlAs/AlGaAs double-barrier quantum wells by a 50 keV Ga+ implantation has been studied both experimentally and theoretically. It was found that for already low Ga+ implantation doses (≤1012 cm−2) a considerably broadened emission peak with an appreciable luminescence blue shift can be obtained. For medium implantation doses (≊1013 cm−2) very large blue shifts in the range of 200 meV were observed, which still retain a reasonable emission intensity. For high Ga+ implantation doses (≥3×1014 cm−2) total intermixing occurs and no photoluminescence can be recovered due to efficient Γ–X scattering. The photoluminescence blue shifts of lower-dose implantations are not influenced by the annealing temperature, whereas the blue shift of higher-dose implantations depend very much on the annealing conditions. The measured data support a heterogeneously enhanced interdiffusion based on a defect cluster model. The importance of a tight control of the annealing ambient and the sample surface condition is discussed.
Raman spectroscopic studies of proton-exchanged LiNbO3 crystals
Authors: Wu Xinglong , Zhang Ming-sheng , Yan Feng , Feng Duan
Published: July 2002

Raman spectra of LiNbO3 crystals with different proton-exchange time were examined. E(TO) and A1(TO) modes appear in the A1- and E-symmetry spectra, respectively, and their intensities show a dependence on exchange time. We attribute these results to both the internal strains resulting from order-disorder distribution of protons in the sample and the enhanced photorefractive effect. In addition, in the E-symmetry spectra the softening of A1(TO) mode at 630 cm−1 was found about 4%. The phenomenon arises from the existence of ferroelectric and paraelectric phases in the protonated sample at room temperature.
Magneto-photoluminescence of acceptors near the interfaces of AlxGa1−xAs/AlyGa1−yAs heterostructures
Authors: Zhu J. B. , Jeon H. I. , Cha S. S. , Shin Y. G. , Lee B. C. , Lim K. Y. , Suh E.-K. , Lee H. J. , Hwang Y. G.
Published: July 2002

Photoluminescence of AlxGa1−xAs/AlyGa1−yAs heterostructures has been performed in presence of high magnetic fields. Under the magnetic field the broad carbon related peak becomes resolved into several narrow peaks. Experimental results and a theoretical analysis based on the single band model suggest that these peaks are due to the recombination of free electrons with holes bound to neutral carbon atoms distributed in the several atomic layers in the well side of the interfaces of the heterostructures. Magnetic fields reduce the effects of interfacial roughness and make it possible to resolve the carbon related peak into several peaks.
Superconductor-insulator-normal conductor tunnel junctions for frequency mixing around 300 GHz
Authors: Maier D. , Karpov A. , Lehnert T. , Gundlach K. H.
Published: July 2002

Properties of superconductor-insulator-normal conductor tunnel junctions made from Nb-Al/ Aloxide-Al trilayers are described. These devices were successfully employed as quasiparticle mixers. The lowest double sideband receiver noise temperature in the 290–320 GHz band was about 140 K. The Nb wiring, which also defines the tuning circuit of the mixer, induces a small energy gap in the Al counter electrode at a physical temperature of 2.2 K.
Formation of a large grain sized TiN layer using TiNx, the epitaxial continuity at the Al/TiN interface, and its electromigration endurance in multilayered interconnection
Authors: Byun Jeong Soo , Rha Kwan Goo , Kim Jae Jeong , Kim Woo Shik , Kim Hak Nam , Cho Hae Seok , Kim Hyeong Joon
Published: July 2002

A new technique on the formation of TiN film with large grain size from TiNx is described. The TiNx layer (defined in Sec. II A) was deposited by sputtering in argon and nitrogen. The nitrogen atoms in Ti matrix relaxed the mechanical stress of the deposited film and limited the surface mobility during the deposition process, resulting in a nanocrystalline structure with extremely uniform thickness. After rapid thermal annealing (RTA), the TiNx yielded a bilayer structure of TiN/TiSixOy, in which the TiN showed (111) preferred texture, extremely smooth surface, and large grain size without any columnar structure. In addition, the resistivity of the bilayered TiN was as low as 40 μΩ cm. In the TiN multilayered aluminum structure (i.e., TiN/Al/TiN), epitaxial continuity and the intermetallic compound such as TiAl3 were clearly observed at the aluminum/TiN interface. The multilayered structure showed better endurance against EM in comparison with the same structure using the conventional TiN (also defined in Sec. II A).
Microwave effects on YBa2Cu3Oy Josephson junctions with step-edge, bi-epitaxial, and sandwich geometries
Authors: Yang H. C. , Lu J. H. , Lin S. D. , Ku L. C. , Cho H. M. , Jian W. B. , Chern M. Y. , Horng H. E. , Wu J. M. , Lo J. T. , Chiou C. C.
Published: July 2002

Fabrication and characterization of YBa2Cu3Oy Josephson junctions with step-edge, bi-epitaxial, and sandwich geometries are performed in this study to investigate the effects of microwave irradiation on the I–V characteristics. I–V curves under microwave irradiation reveal Shapiro steps. The width of the constant voltage steps were modulated by the microwave power. The period of the magnetic induction ΔB detected from the V–Φ curves for superconducting quantum interference devices is temperature dependent. Additionally, Tc and Ic of the proximity sandwich junctions are enhanced by the microwave irradiation.
Study of various types of diamonds by measurements of double crystal x-ray diffraction and positron annihilation
Authors: Fujii S. , Nishibayashi Y. , Shikata S. , Uedono A. , Tanigawa S.
Published: July 2002

Annihilation characteristics of positrons in various types of diamonds were studied by measurements of two-dimensional angular correlation of positron annihilation, those of Doppler Broadening profiles and those of lifetime spectra. From the measurements, it was found that there is a small amount of defects in type Ib synthesized diamond and the positron lifetime is 115 ps. On the other hand, it was found that positronium is formed in natural diamonds. Results of the double x-ray measurements suggested that IIa, IIb, and Ia type natural diamonds also may have empty space between crystallites or grain boundaries.
Structural and magnetic properties of Co/face-centered-cubic Mn(001) multilayers
Authors: Wang Q. , Metoki N. , Morawe Ch. , Zeidler Th. , Zabel H.
Published: July 2002

The structural and magnetic properties of Mn in Co/Mn (001) multilayers were investigated by x-ray scattering, by magneto-optical Kerr effect measurements, and with a Faraday balance. The multilayers were deposited by rf-sputtering on MgO(001) substrates. Small angle x-ray reflectivity scans and high angle Bragg scattering confirmed good layer quality and crystal coherence properties. In- and out-of-plane x-ray scattering revealed a coherent growth of face-centered-cubic (fcc) (or fct) Mn on fcc Co(001). The stability limit of the metastable fct Mn structure is about 20 Å or 10 atomic layers. The magnetic exchange coupling appears to be partly antiferromagnetic with a first maximum located between 20 and 30 Å. Saturation magnetization measurements indicate no ferromagnetic moment for the fct Mn layers. Furthermore, the saturation moment per unit volume is reduced as compared to the bulk Co value, suggesting the presence of ‘‘dead layers’’ at the Co/Mn interfaces.
Photoluminescence from modulation doped AlGaAs/ low-temperature molecular beam epitaxy-grown GaAs heterostructures
Authors: Schulte D. , Subramanian S. , Ungier L. , Arthur J. R.
Published: July 2002

Photoluminescence (PL) results from a novel modulation doped AlGaAs/ low-temperature molecular beam epitaxially-grown-GaAs (LT-GaAs MODFET) heterostructure are reported. A new PL line at 1.65 eV is consistently observed in all the LT-GaAs MODFET structures investigated. A spatially indirect transition from a two-dimensional electron gas at the heterojunction interface to the holes in AlGaAs is believed to be responsible for the observed 1.65 eV PL line. LT-GaAs MODFET structures in which LT-GaAs region is grown at 350 °C show additional lines lying in the band edge region as well as deep inside the band gap region of LT-GaAs.
Temperature and concentration profiles in a low pressure methane process plasma
Authors: Hädrich S. , Pfelzer B. , Doerk T. , Jauernik P. , Uhlenbusch J.
Published: July 2002

Coherent anti-Stokes Raman scattering (CARS) is applied to a microwave (2.45 GHz) excited plasma used for plasma enhanced chemical vapor deposition. The applicability of CARS to low pressure (2 Pa) plasmas is verified, and the absence of saturation is carefully checked. Ground state concentration profiles as well as rotational temperatures of CH4 are presented with high spatial and temporal resolution. The measurements show a decrease of the CH4 ground state density under the influence of the plasma down to ≊25% (nCH4=1.0×1020 m−3) of its initial value. The rotational temperature is nearby room temperature across the total discharge volume. A straightforward modeling of the plasma explains the decrease of CH4 ground state densities as an effect of electron collisions and delivers an approximate value of the electron temperature of about 3 eV.
Electrical and mechanical characterization of chemical vapor deposition of tungsten on sputter-deposited TiN layers
Authors: Zhang S. -L. , Palmans R. , Petersson C. S. , Maex K.
Published: July 2002

Tungsten (W) films are deposited from tungsten hexafluoride on sputter-deposited TiN adhesion layers in a cold-wall chemical vapor deposition reactor. The film resistivity of the W films is found to be thickness dependent. It decreases monotonically with increasing film thickness. Typical resistivity values of 40-nm-thick W films are about 19.3–23.4 μΩ cm, depending on the structure of the underlying TiN layer used. The resistivity of a 980-nm-thick W film is 9.8 μΩ cm. Oxygen and fluorine impurities, as well as structural difference in the W films are found to be the major causes for the resistivity variations. Lowering impurity level and/or increasing W crystallite size can decrease film resistivity. The stress of all the W films is found to be tensile, independent of the structure of the TiN layers. However, the absolute value of the stress is intimately associated with the structure of the TiN layers. The stress values can differ by a factor of more than 2 for the 40-nm-thick W films deposited on the different underlying TiN layers. The amplitude of stress also monotonically decreases with increasing film thickness. Consequently, the difference in stress induced by the difference in the underlying TiN layers gradually disappears as the film thickness increases. A strong correlation between the stress and the film texture is found.
Analysis of quantum superconducting kinetic inductance photodetectors
Authors: Bluzer Nathan
Published: July 2002

Analysis of a quantum superconducting kinetic inductance photodetector (QSKIP) structure is presented for operation in a low background environment. We project the QSKIP’s spectral response to be limited by the Cooper pair binding energy, 2Δ or about 32 μm for YBCO. The QSKIP response and sensitivity are computed from the minimum of the Hamiltonian energy functional and linearized Rothwarf–Taylor equations. Photoresponse and sensitivity expressions are computed in terms of the quasiparticle lifetime and indicate background limited infrared performance at very low photon flux levels. At low temperatures and under background limited infrared performance conditions, the photoresponse is proportional to the number of absorbed photons. Operating the QSKIP in the Meissner state and below 50% of the critical current limits the noise sources to fluctuations in the condensate population. With the noise and photoresponse equations, the QSKIP noise equivalent power is computed in terms of the characteristic lifetimes of: quasiparticle generation; quasiparticle recombination; anharmonic phonon decay; and phonon trapping. The calculated noise equivalent power for a YBCO QSKIP is about 2.5×10−15 W/√Hz at 9 K.
Normal domains in Rutherford-type superconducting cables
Authors: Kovner V. S. , Mints R. G.
Published: July 2002

We study the formation and dynamics of normal domains in Rutherford-type superconducting cables. We use an effective circuit model to account for the electric current redistribution process between the multifilamentary strands in the presence of a normal zone. We obtain and integrate numerically the diffusion equations for the temperature and the current-density distributions in the cable. Our simulations show the formation of stable normal domains propagating along the cable. We derive an analytical expression for the threshold current Id above which the propagating normal domains exist.
Deep-level analysis of n-type GaAs1−xPx alloys
Authors: Ben Salem M. M. , Zaidi M. A. , Maaref H. , Bourgoin J. C.
Published: July 2002

Deep-level transient spectroscopy has been used to study the properties of electron and hole traps present in n-type GaAs1−xPx alloys and their evolution versus the alloy composition. An electron trap labeled E0 is observed for all values of the alloy composition x, while a second electron trap E1 appears only for 1≳x≥0.81. As for hole traps one (H2) appears for 1≳x≥0.75, while two others, H0 and H1, are detected for x≥0.81 and for 1≳x≥0.81, respectively. Their ionization energies have been determined and the barriers, associated with electron capture, have been measured in order to determine the energetic position of the two electron traps relative to the conduction band.
Study of nitrogen location on ZnSe surface using ion scattering and recoiling spectrometry
Authors: Kubo Minoru
Published: July 2002

We have applied ion scattering and recoiling spectrometry to study the nitrogen location on radical nitrogen doped ZnSe surface by molecular beam epitaxy. In the recoiling nitrogen measurements of heavy doped ZnSe:N surface, a two-peak structure is observed corresponding to two recoil processes, i.e., direct recoil and surface recoil. Direct recoil events occur due to the single-collision recoiling from Se site nitrogen. On the other hand, surface recoil events occur due to the nitrogen recoiling scattered by second layers and is highly sensitive to the nitrogen location on the ZnSe:N surface during molecular beam epitaxy growth. The experimental results and calculation suggest that the optimum doped nitrogen atoms locate in the substitutional Se sites but excess nitrogen atoms locate in the interstitial sites.
Nonlinear characteristics induced by accumulation of photogenerated holes in GaAs/GaAlAs multi-quantum well structure under weak photoexcitation
Authors: Matsui Y. , Kusumi Y.
Published: July 2002

The linearity between photoluminescence characteristics and incident laser power is discussed on the basis of an electric field distortion induced by photoexcitation in a GaAs/GaAlAs multi-quantum well structure. Peak intensities and wavelengths at half maxima of PL spectra have been analyzed quantitatively for different incident laser powers. The relation is nonlinear in the case of the cap layer where two contacts are both Al Schottky metals although linear in the case of an Al Schottky metal and a AuGe ohmic alloy. The difference can be explained theoretically considering an electric field distortion induced by the accumulation of photogenerated holes in the GaAs cap layer. The nonlinear relations are due to the large accumulation rate of holes in the cap layer.
Micromagnetics of soft magnetic thin films in presence of defects
Authors: Komine Takashi , Mitsui Yu , Shiiki Kazuo
Published: July 2002

We solve numerically the Landau-Lifshitz-Gilbert equation and calculate the magnetization in a magnetic film including a nonmagnetic defect for two initial magnetization states consisting of circular structure with 4 domains and 7 domains. The defect at the vortex wall restrains the magnetization temporarily but the vortex wall comes out of the defect beyond a critical applied field. This critical field increases as the defect size becomes large. The defect at the film side fixes the magnetization at the film edge. The edge defect leads to a reproducible magnetization state inside the film, though a locally stable structure appears for the magnetic thin film without defects. It seems that the defect may improve the stability of the magnetic thin film head.
Analysis of effective coupling strength in high-frequency SQUIDs
Authors: Gottschlich M. , Sodtke E. , Zhang Y. , Portis A. M.
Published: July 2002

Understanding the coupling mechanism between SQUID (superconducting quantum interference device) loop and resonator in high-frequency SQUIDs is essential for the application of these devices in sensitive magnetometers or controllable microwave devices. We have investigated, therefore, two kinds of planar microstrip tank circuit resonators: (1) S-shaped λ/2-resonators with direct or indirect coupling to the SQUID loop with optional flux-focussing pads and (2) conventional rf-washer-SQUID structures for which we have found resonant modes of the washer providing a coupling to the SQUID loop. A lumped element representation of SQUID loop and resonator is presented in order to describe the effective SQUID-resonator coupling in case of direct or indirect coupling. According to this approach, the square of the effective coupling constant k in both cases is equal to the ratio of the magnetic energy stored in the SQUID loop to that stored in the resonator. For the indirect coupled S-shaped λ/2-resonators, k can be described in terms of a geometry-defined mutual inductance. The predictions of the lumped element description are compared with simulations using a standard full-wave analysis computer program in order to quantify the effect of the different layouts on microwave current distribution and on k. A method for the determination of k by simulation is described, leading to useful design rules for the optimization of high-frequency SQUIDs. The simulation results will also be compared with experimental data.
Demonstration of an isolated buried channel field-effect transistor fabricated via in situ patterned electron-beam deposition of Si in GaAs
Authors: Mills A. P. , Hong M. , Mannaerts J. P. , Pfeiffer L. N. , West K. W. , Martin S. , Ruel R. R. , Baldwin K. W. , Rowe J. E.
Published: July 2002

A partial monolayer of silane, SiH4, adsorbed on a GaAs(100) surface at 40 K may be fixed in a desired pattern by irradiation with an electron microbeam, and then covered in situ by GaAs grown by molecular-beam epitaxy. The initial rate of Si coverage under irradiation by 1.5 keV electrons is (0.031±0.005) Si per electron per monolayer of silane. Applications include the in situ fabrication via patterned doping of circuit elements and structures with interesting electronic properties. As an example, we have made an isolated buried channel field-effect transistor and measured its properties.
Dislocation generation and crack growth under monotonic loading
Authors: Cherepanov G. P. , Richter A. , Verijenko V. E. , Adali S. , Sutyrin V.
Published: July 2002

The processes of crack growth and dislocation emission induced by the crack tip are investigated. A crystal with cubic lattice of atoms under plane strain conditions is considered. The main principles of the nanofracture mechanics approach employed in this study are outlined. Both ductile and brittle mechanisms of crack growth in the crystal are examined in nano- or interatomic scale. Only the fundamental constants of the classical theory of dislocations are used which include the interatomic spacing, elastic constants, the Schmid friction constant, and the true surface energy of crystal lattice. The efficient solution of the elastic problem for an arbitrary number of dislocations near the crack tip is obtained in terms of complex potential functions. The equilibrium of dislocation pairs near the crack tip during monotonic loading is investigated. It is shown that dislocation generation at the crack tip occurs at certain quantum levels of external load. The magnitude of external load corresponding to crack growth initiation and emission of the first pair of dislocations is calculated. The mathematical problem for an arbitrary N number of dislocation pairs near the crack tip is reduced to a parametric system of N nonlinear equations, where the stress intensity factor of external load KI plays the role of parameter and N the role of discrete time. The minimum value of KI at which the solution of this system of equations exists corresponds to the stress intensity factor at which the Nth pair of dislocations is generated. The numerical method is presented to determine the minimum value of KI. The approximate method of self-consistent field is employed to reduce the order of the system of nonlinear equations. The approximate method is used to calculate the fracture curve KI(lc) relating the value of KI which maintains the crack growth to the crack length increment lc. The exact solution is also studied, and numerical results are given for a crack in an aluminum specimen and involve the quantum levels of external load corresponding to the moments of dislocation generation and the values of the superfine stress intensity factor up to 150 dislocations.
Study of film growth from excimer laser-induced ablation of CdS
Authors: Calì C. , La Rosa F. , Targia G. , Robba D.
Published: July 2002

This work describes some investigations on the atomic and ionic beams generated by a CdS target under excimer laser irradiation at 308 nm, and on factors influencing the quality of the films obtained. The angular distribution of particles emitted in a vacuum was found to be highly directional, concentrated around a direction normal to the target surface. The angular emission pattern depends on laser fluence. The method used for studying the emission pattern helps in positioning substrates for optimum film quality and uniformity. Measurements of the energy of the emitted particles yielded high values (several tens of eV) for both ionized and neutral particles, suggesting a nonthermal mechanism. Some possible models are discussed. Finally, considerable improvement in both film uniformity and homogeneity was obtained through the use of a laser-triggered plasma discharge in the source region.
Spatially variable reaction in the formation of anodically grown porous silicon structures
Authors: Teschke O. , dos Santos M. C. , Kleinke M. U. , Soares D. M. , Galvão D. S.
Published: July 2002

In porous silicon formations there is an increase of dissolution rate at the fluorine-covered sites of the silicon surface due to the presence of excess electrons coming from oxidation of molecular hydrogen at the passivated (hydrogen-covered) sites. The dissolution rate increase in the presence of excess charge at the fluorine-covered sites is experimentally measured and a theoretical investigation is carried out by a semiempirical Hartree–Fock calculation. This spatially variable dissolution generates the porous silicon surface.
Effect of solvent vapor on optical properties of Pr4VOPc in polymethylmethacrylate
Authors: Tang Fulong , Zhu Congshan , Gan Fuxi
Published: July 2002

Dye-in-polymer (DIP) films consisting of soluble Pr4VOPc (vanadyl phthalocyanine having four propyl substituents) dye and polymethylmethacrylate (PMMA) were prepared by spin-coating process. Pr4VOPc precipitates as a glassy phase in pure PMMA matrix during the film formation. Upon exposure of these DIP films to organic solvent vapor, stable crystalline Pr4VOPc, which has stronger absorption in the near-IR region and larger nonlinear refractive index n2 than the glassy phase, forms. The organic solvent vapor promotes the solubilization and crystallization of the dye, plastication of the polymer chains, and migration of the dye. The nonlinear refractive index n2 and optical spectra of the DIP films were measured, and the change rule has been discussed in terms of electron transition and molecular packing of Pr4VOPc.
A hybrid model for particle transport and electron energy distributions in positive column electrical discharges using equivalent species transport
Authors: Huang Fred Y. , Kushner Mark J.
Published: July 2002

A new modeling technique for low-pressure, low-temperature discharges is introduced in which electrons, ions, and excited neutrals are equivalently treated using particle-mesh algorithms. Heavy particles are represented as electrons in bound states having large effective masses and the appropriate charge. An effective density of states is used which has discrete negative energies representing bound states, and a positive continuum representing free electrons. Collisions between continuum electrons and heavy particles are computationally treated as electron–electron collisions. This method has been used as the kinetic portion of a hybrid kinetic-fluid model for positive column electrical discharges. Densities and electric fields are obtained from the fluid portion of the model. Transport coefficients, source functions, and energy distributions for all species are generated in the kinetic module. The hybrid model has been used to examine electron energy distributions and radial electric fields in positive column discharges. Evidence is presented for diffusion heating of electrons in the sheaths.
Ion-bombardment effects on Fe/MgO single crystals
Authors: Al-Hawery A. S.
Published: July 2002

Electron-paramagnetic-resonance linewidths at the M=+1/2↔−1/2 transition of the Fe3+ spectrum of Fe/MgO single crystals have been examined at 9 GHz and 300 K before and after ion bombardment. The nominal ion concentrations ranged from 310 to 4300 ppm. Specimens were bombarded at 150 keV with argon ions for up to 12 min. The ion-bombardment produces reductions in the peak-to-peak linewidth of ΔHpp, which occurred in two stages for all the specimens examined.
Study of the optical properties of In0.52(AlxGa1−x)0.48As by variable angle spectroscopic ellipsometry
Authors: Pan J.-W. , Shieh J.-L. , Gau J.-H. , Chyi J.-I. , Lee J.-C. , Ling K.-J.
Published: July 2002

The optical properties of In0.52(AlxGa1−x)0.48As epilayers with various x values were systematically studied using variable angle spectroscopic ellipsometry in the wavelength range of 310–1700 nm. The refractive indexes were determined and could be given as n(x)=0.12x2–0.51x+3.6 at the wavelength of 1.55 μm. The measured thickness of the epilayers agrees within 5.2% of the nominal thickness. The energies and broadening parameters of the E1 and E1+Δ1 transitions as a function of Al composition were also examined based on the second-derivative spectra of the dielectric function. The comparison between the results and the reported data is presented.
Magneto-optical δM measurements of interactions in electroplated CoPt films
Authors: Callegaro Luca , Puppin Ezio , Cavallotti Pietro Luigi , Lecis Nora , Zangari Giovanni
Published: July 2002

The codeposition of phosphorus in electroplated Co80Pt20 films strongly increases the coercive force, up to values (5000 Oe) comparable with those of bulk samples. The δM method has been applied in order to characterize the magnetic interactions among grains. Longitudinal Kerr effect measurements have been performed onto samples having a different concentration of phosphorus. Our results show that the exchange interaction among grains is absent in samples containing phosphorus; the proposed mechanism of exchange decoupling, which can also account for the increase in the coercive force, is the presence of a nonferromagnetic (or weakly ferromagnetic) phosphorus-rich phase localized at grain boundary.
Effects of rf magnetron plasma on the particle size distribution in laser ablation deposition
Authors: Bao Tien-I , Tsai S. H. , I Lin
Published: July 2002

The effect of the externally introduced rf magnetron plasma on the particle size distribution on films deposited by pulsed laser ablation is investigated. A cw low energy magnetron rf plasma is sustained between the target and the substrate during the laser ablation deposition process. The ablated droplets can be negatively charged and filtered by the plasma. For Si and Al target ablation, the filtering efficiency of the inert Ar plasma is about 20%, and is independent of the ejected particle size in our system. If oxygen is introduced into the plasma to deposit oxide thin films, the oxidation of the target surface in the reactive plasma changes the size distribution of the ejected particles, and largely reduces the average size of the ejected particles.
Principle of operation and carrier distributions of AlGaAs/GaAs in-plane-gated channels
Authors: Armstrong M. A. , Etchin S. , Melngailis J. , Antoniadis D. A.
Published: July 2002

In-plane-gate field-effect transistors were fabricated on a modulation-doped AlGaAs/GaAs heterostructure substrate using focused ion beam implantation. The two-dimensional electron gas was divided up by insulating lines so as to define a coplanar gate and channel. The devices were simulated in two-dimensional cross section including the effects of fringing fields, surface states, and unintentional p-type doping in the nominally undoped GaAs buffer layer. Excellent agreement was obtained between measured and simulated channel conductance as a function of gate bias. The distribution of electrons in the channel was then studied as a function of gate bias. Simulations show that the two-dimensional electron gas in the channel is depleted through a combination of reduction in overall carrier density and effective width. The nonlateral depletion is enhanced by parasitic backgate coupling through the unintentionally doped buffer layer.
Dynamics of exciton relaxation and excitation transfer to donor-bound excitons in CdTe/CdMnTe quantum wells
Authors: O’ Neill M. , Harrison P. , Oestreich M. , Ashenford D. E.
Published: July 2002

Using time-resolved photoluminescence spectroscopy we investigate the dynamics of exciton relaxation in CdTe/Cd1−xMnxTe multiple quantum wells. Unbound excitons are photoexcited and the timescale of formation of the donor-bound excitons varies with excitation intensity from 73 ps to <8 ps and depends on movement of the unbound exciton in the well and the capture process at the donor site. Estimates of the capture cross-section of excitons by neutral donors and of the radiative recombination time of the donor-bound exciton are made. The rate of kinetic energy relaxation of a photocreated exciton from K=0 of the electron-light hole band to K=0 of the electron-heavy hole band is found to be (2.2±0.8)× 108 eV s−1.
Evaluation method for stability of amorphous silicon thin film transistors
Authors: Hwang Chi-Sun , Bae Byung Seong , Kong Hyang-Shik , Lee Choochon
Published: July 2002

An experimental method, the thermally stimulated bias stress (TSBS) method, is presented to evaluate the stability of amorphous silicon thin film transistors (TFTs). TSBS experiment monitors the drain currents under constant gate bias while increasing the temperature at a constant rate. The changes of the drain current during TSBS experiment are analyzed by numerical calculation, adopting the conventional stretched exponential formula which describes the degradation behavior of TFT under bias stress at constant temperature. The TFT stability is characterized by stretched exponential parameters Eτ and T0, which are related to the effective barrier height and the width of the distribution of barriers for bias induced degradation. For +20 V gate bias stress, Eτ=0.88 eV and T0=850 K are obtained. Higher bias stress lowers the effective barrier height Eτ. For three different cases of the initial state of a TFT (annealed, light soaked, and bias stressed), stabilities are investigated by TSBS method. For the initially light soaked case, parameters Eτ and T0 are about the same as those of the annealed case. But for the initially bias stressed case, Eτ increases and T0 decreases. We discuss the effects of gate bias and initial treatment on the bias induced instabilities in terms of charge trapping and defect creation models.
Overcoming the effect of chain disorder on the Seebeck coefficient of Ln1−xCaxBa2Cu2.9Ga0.1O6+z(Ln=Nd,Dy) by substitution of Ca(0<x≤0.2)
Authors: Zelenay I. , Chavira E. , Suryanarayanan R.
Published: July 2002

We report on the Seebeck coefficient (S) of Ln1−xCaxBa2Cu2.9Ga0.1O6+z(Ln=Nd,Dy; 0≤x≤0.2) as a function of temperature. For Ln=Dy, S at 300 K decreased by a factor of 20 as x(Ca) increased from 0 to 0.2. For all the samples, for a restricted range of temperature, S could be described by S=S0−αT where S0, characteristic of each sample, is related to Tc by Tc=82.4−0.64 S0 and α is a constant.
X-ray standing waves as probes of surface structure: Incident beam energy effects
Authors: Kirchner Stephan , Wang Jin , Yin Zhijian , Caffrey Martin
Published: July 2002

The sensitivity to x-ray beam energy of structure measurements using x-ray standing waves (XSW) generated under conditions of total external reflection has been determined. To this end, the optical properties of the system were examined in a theoretical analysis to identify possible energy-dependent components such as surface roughness. The analysis shows that, provided surface roughness is small (Debye–Waller factor less than 10 Å) and the energy range covered in the XSW measurements lies within several keV, its contribution can be accounted for satisfactorily by a simple Debye–Waller factor. In addition, a series of XSW measurements were made on Langmuir–Blodgett films of manganese arachidate (C20:0) on a gold mirror surface at three incident x-ray beam energies in the 7–11.2 keV range. The XSW data were analyzed to account for the Debye–Waller factor. No obvious dependence on incident x-ray energy was found. These results demonstrate that the contribution of surface roughness to the x-ray fluorescence yield profile is minimal under these conditions. Thus, mirrors of the type and quality used in these experiments are useful in XSW measurements where multiple element types are incorporated as structural labels in organic thin films and at surfaces. We also demonstrate that the resolving power of the XSW method is sufficient to distinguish and to locate two separate probe atom layers in a single Langmuir–Blodgett film.