bluuebunny/crossref_metadata_2025 huggingface dataset:
Energy dependence of transient enhanced diffusion and defect kinetics
Authors: Saleh Hugo , Law Mark E. , Bharatan Sushil , Jones Kevin S. , Krishnamoorthy Viswanath , Buyuklimanli Temel
Published: July 2002

Boron, a p-type dopant, experiences transient enhanced diffusion (TED) via interstitials. The boron TED and {311} dissolution rates are explored as a function of implant energy dependence. Silicon implants of 1014/cm2 at various energies were used to damage the surface of a wafer with an epitaxially grown boron marker layer. Samples were annealed at 750 °C for 15–135 min to observe the diffusion exhibited by the marker layer and to correlate this with the dissolution of {311} type defects. The diffusion enhancement depends strongly on implant energy but the {311} dissolution rate is weakly dependent.
Configurations of misfit dislocations at interfaces of lattice-matched Ga0.5In0.5P/GaAs heterostructures
Authors: Wang Y. Q. , Wang Z. L. , Brown T. , Brown A. , May G.
Published: July 2002

A configuration of misfit dislocation dipoles is observed in a Ga0.5In0.5P heterostructure grown by solid-source molecular-beam epitaxy on GaAs. The dipole dislocations are mostly of 60° type, separated by ∼3.5 nm. The dislocations are not produced by conventional lattice mismatch, rather, they could be the result of lateral compositional modulation in the Ga0.5In0.5P epilayer.
Low-noise and high-gain image amplification by contradirectional two-wave mixing
Authors: Mu Xiaodong , Xu Xinguang , Shao Zongshu , Jiang Minhua , Luo Haosu
Published: July 2002

Contradirectional two-wave mixing (CTWM) is demonstrated to be a perfect configuration for low-noise and high-gain image amplification. Compared with the image amplified by the forward two-wave mixing configuration, the signal-to-noise ratio is increased from 6 to 2×104 in the experiments in Rh-doped BaTiO3 crystal. The gain of the signal beam is also enhanced effectively. We have contrasted the differences between the two experimental configurations and gave a credible explanation for the low noise and high gain that exists in the CTWM configuration.
Strain and composition of capped Ge/Si self-assembled quantum dots grown by chemical vapor deposition
Authors: Patriarche G. , Sagnes I. , Boucaud P. , Le Thanh V. , Bouchier D. , Hernandez C. , Campidelli Y. , Bensahel D.
Published: July 2002

We have investigated the composition and the strain profile of Ge/Si self-assembled quantum dots. The quantum dots, grown by low-or high-pressure chemical vapor deposition, were covered by a silicon cap layer. The composition and the strain were measured by the selected area transmission electron diffraction of a single quantum dot. The self-assembled quantum dots exhibit a quadratic deformation. No lateral relaxation of the lattice is observed from the main part of the quantum dot. An average composition of Ge around 50% is deduced. The average composition is found dependent on the size of the islands. This composition is correlated to the photoluminescence energy.
Size-dependent picosecond energy relaxation in PbSe quantum dots
Authors: Okuno Tsuyoshi , Masumoto Yasuaki , Ikezawa Michio , Ogawa Tomoyasu , Lipovskii Andrey A.
Published: July 2002

We measured the size dependence of the energy relaxation time of PbSe quantum dots at room temperature. The radius of the dots was evaluated by small-angle x-ray scattering. We found a monotonic decrease of the relaxation time from 25 to 1 ps with a decrease of the radius r from 2.9 to 1.4 nm. The 1/r3 dependence of the relaxation rate was indicated.
Charge noise analysis of an AlGaAs/GaAs quantum dot using transmission-type radio-frequency single-electron transistor technique
Authors: Fujisawa Toshimasa , Hirayama Yoshiro
Published: July 2002

Radio-frequency (rf)-operated single-electron transistors (SETs) are high-sensitivity, fast-response electrometers, which are valuable for developing new insights into single-charge dynamics. We investigate high-frequency (up to 1 MHz) charge noise in an AlGaAs/GaAs quantum dot using a transmission-type rf SET technique. The electron capture and emission kinetics on a trap in the vicinity of the quantum dot are dominated by a Poisson process. The maximum bandwidth for measuring single trapping events is about 1 MHz, which is the same as that required for observing single-electron tunneling oscillations in a measurable current (∼0.1 pA).
High-quality phase conjugation even in a highly transient regime of stimulated Brillouin scattering
Authors: Králiková B. , Skála J. , Straka P. , Turčičová H.
Published: July 2002

A high-quality phase conjugation of the stimulated Brillouin scattering has been achieved even in a highly transient regime, as proved by the mask restoration in the backscattered signal. A special two-lens system of laser beam insertion into a scattering medium was used. A reliable onset of the phase conjugation has been reached using a splitted part of the oscillator Q-switched pulse as a prepulse in the Brillouin cell. Results with and without the prepulse are compared. A possibility of the creation of the phase conjugated mirrors even in high-power gaseous lasers with short pulses is thus proved.
Ultraviolet-blue electroluminescence from Gd3Ga5O12:Ag
Authors: Xu Xiulai , Xu Zheng , Hou Yanbing , Su Yanmei , Xu Xurong , Wang Xiaowei , Tong Wusheng
Published: July 2002

UV-blue light was obtained from a thin-film electroluminescence device using Gd3Ga5O12:Ag as a light-emitting layer, which was deposited by using electron-beam evaporation. The crystal composition and structure of Gd3Ga5O12:Ag were studied by x-ray powder diffraction. The Gd3Ga5O12:Ag has a photoluminescence emission which peaked at around 397 and 467 nm, which were attributed to the oxide vacancies and Ag+, respectively. The brightness of 32 cd/m2 was obtained when an alternating voltage of 130 V at 1 kHz was applied.
Thermal wet oxidation of GaP and Al0.4Ga0.6P
Authors: Epple J. H. , Chang K. L. , Pickrell G. W. , Cheng K. Y. , Hsieh K. C.
Published: July 2002

Thermal wet oxidations of GaP and Al0.4Ga0.6P at 650 °C for various times have been performed. Comparisons are made on oxidation rates and post oxidation morphology. Transmission electron microscopy shows that when oxidizing GaP, polycrystalline monoclinic GaPO4⋅2H2O forms without noticeable loss of phosphorus. Oxidation for 6 h or more leads to poor morphology resulting in cracks and detachment. A thickness expansion of about 2.5–3 times is noticed as a result of oxidation. In contrast, oxidized Al0.4Ga0.6P exhibits much better morphology without cracks or detachment from the substrate. The oxide has an almost amorphous-like microstructure. The oxidation process shows typical diffusion-limited reaction at long anneals. Preliminary work on the oxidation of AlP indicates that the reaction leads to formation of Al2O3 and possible volatile P2O5 diffusing out of the specimen. Thus, from the structural viewpoint, AlGaP forms a better oxide suitable for device needs.
Staircase band gap Si1−xGex/Si photodetectors
Authors: Lo Zhiyun , Jiang Ruolian , Zheng Youdou , Zang Lan , Chen Zhizhong , Zhu Shunming , Cheng Xuemei , Liu Xiabing
Published: July 2002

We fabricated Si1−xGex/Si photodetectors by using a staircase band gap Si1−xGex/Si structure. These devices exhibit a high optical response with a peak responsive wavelength at 0.96 μm and a responsivity of 27.8 A/W at −5 V bias. Excellent electrical characteristics evidenced by good diode rectification are also demonstrated. The dark current density is 0.1 pA/μm2 at −2 V bias, and the breakdown voltage is −27 V. The high response is explained as the result of a staircase band gap by theoretical analysis.
Strong photoluminescence of Sn-implanted thermally grown SiO2 layers
Authors: Rebohle L. , von Borany J. , Skorupa W. , Fröb H. , Niedermeier S.
Published: July 2002

The photoluminescence (PL) and PL excitation (PLE) properties of Sn-implanted SiO2 layers thermally grown on crystalline Si have been investigated and compared with those from Ge- and Si-implanted SiO2 layers. In detail, the violet PL of Sn-implanted SiO2 layers is approximately two and 20 times higher than those of Ge- and Si-implanted SiO2 layers, respectively. Based on PL, PLE, and decay time measurements, the violet PL is interpreted as due to a triplet–singlet transition of the neutral oxygen vacancy typical for Si-rich SiO2 and similar Ge- and Sn-related defects in Ge- and Sn-implanted SiO2 films. The enhancement of the blue–violet PL within the isoelectronic row of Si, Ge, and Sn is explained by means of the heavy atom effect.
Procedure for preparing epitaxial Tl-2201 films on single crystal LaAlO3
Authors: Chen H. Q. , Johansson L.-G. , Ivanov Z. G.
Published: July 2002

Epitaxial Tl-2201 films are prepared by treating precursor films deposited on single crystal LaAlO3 by laser ablation with Tl2O vapor at high temperature. Here, a procedure is described that includes thallization in two separate steps. The first thallization at 720 °C resulted in good epitaxy as well as in smooth morphology. The second thallization at 820 °C resulted in an improvement of the superconducting properties and in film crystallinity without affecting epitaxy or morphology. The films were characterized by x-ray diffraction (φ scan, Ω scan, θ–θ scan), and by resistance and susceptibility measurements. The films are tetragonal. In a typical film, the full width at half maximum of the Ω scan of the (0,0,10) reflection was 0.27°. A 5 μm wide bridge patterned in one film had a Tc of 84 K. The critical current density reached 1.4×106 A/cm2 at 77 K and 1.4×107 A/cm2 at 4.2 K.
Temperature dependence of capacitance/current–voltage characteristics of highly (0001)-oriented YMnO3 thin films on Si
Authors: Yi Woo-Chul , Seo Chang-Su , Kwun Sook-Il , Yoon Jong-Gul
Published: July 2002

Highly (0001)-oriented YMnO3 thin films on Si(100) substrates were obtained by using a stable precursor solution and rapid thermal annealing at a low temperature of 650 °C in a chemical solution deposition process. Temperature-dependent capacitance–voltage (C–V) and current–voltage (I–V) characteristics were discussed in a metal-ferroelectric-semiconductor structure. At 300 K, the voltage-dependent increase of the memory window (ΔV) in the C–V curve and the asymmetric I–V curve were attributed to the formation of positive interfacial charges by field- and thermal-excited electron transport. On the other hand, at 220 K, the voltage-independent ΔV was attributed to ferroelectric polarization switching.
Photoluminescence from sub-nanometer-thick GaN/Al0.8Ga0.2N quantum wells
Authors: Someya T. , Hoshino K. , Harris J. C. , Tachibana K. , Arakawa Y.
Published: July 2002

Photoluminescence (PL) spectra were measured for sub-nanometer-thick GaN quantum wells (QWs) with Al0.8Ga0.2N barriers, which were grown by atmospheric-pressure metal–organic chemical-vapor deposition. The thickness of the GaN QW layers was systematically varied from 1 to 4 ML. We clearly observed a PL peak at room temperature at a wavelength as short as 247 nm (5.03 eV) from 1-ML-thick QWs. The effective confinement energy, or difference between this recombination energy and the band gap of bulk GaN, is as large as 1.63 eV.
Effect of Sn doping on the electronic transport mechanism of indium–tin–oxide films grown by pulsed laser deposition coupled with substrate irradiation
Authors: Adurodija F. O. , Izumi H. , Ishihara T. , Yoshioka H. , Motoyama M.
Published: July 2002

Low-resistivity indium–tin–oxide (ITO) films (8.9×10−5–2.3×10−4 Ω cm), 80±20 nm thick grown by combining pulsed laser deposition and laser irradiation of the substrate were studied in relation to tin (Sn) doping content. Films with Sn doping content over the range 0–10 wt % were deposited at room temperature (RT) and 200 °C at a fixed oxygen pressure of 1×10−2 Torr. The laser beam with energy density of 70 mJ/cm2 was directed at the middle portion of the substrate during growth. At RT, the laser-irradiated and nonirradiated parts of the films exhibited crystalline and amorphous phase, respectively. The amorphous films indicated a steady resistivity, carrier concentration, and Hall mobility of ∼2.4×10−4 Ω cm, 8×1020 cm−3, and ∼32 cm2/V s, respectively, and showed no significant change over 0–10 wt % Sn doping content. The crystalline films deposited at RT by laser irradiation and 200 °C indicated a strong dependence of the resistivity, carrier concentration, and Hall mobility on Sn doping content over the range 0–10 wt %. The Hall mobility of the ITO films steadily decreased with increasing Sn doping content. Study on the scattering mechanisms based on the experimental data and theoretical models showed that the scattering of free carriers in the films was caused mainly by ionized centers.
Valence band offset and hole injection at the 4H-, 6H-SiC/SiO2 interfaces
Authors: Afanas’ev V. V. , Stesmans A.
Published: July 2002

The valence band offset between 4H-, 6H-SiC, and grown-on thermal oxide is directly measured using internal photoemission of holes. The obtained value ΔEV=2.9±0.1 eV in combination with the earlier reported barrier for valence electrons yields an oxide band gap width of 8.9 eV, close to the intrinsic value for amorphous SiO2 which suggests an abrupt SiC/SiO2 interface. Hole tunneling from SiC into SiO2 is mediated by oxide defect states distributed in an energy range of ∼1 eV above the SiO2 valence band.
Sensitive Josephson magnetometry of flux quantization in a normal conducting hole in a narrow YBa2Cu3O7 line
Authors: Barthelmess H.-J. , Krey S. , Ostertun S. , Schilling M.
Published: July 2002

A small YBa2Cu3O7 Josephson junction on a 24° symmetric SrTiO3 bicrystal is used as a sensitive magnetometer with micrometer spatial resolution in magnetic fields of up to 1 mT. The dependence of its critical current on the external magnetic flux is used to measure the local magnetic field. In the narrow line of 4 μm width leading to the Josephson junction we prepared a normal conducting area of about 2.5 μm diameter. This was achieved by heating the YBa2Cu3O7 locally with a focused laser beam to lower the oxygen content and thus suppress superconductivity at 77 K. We investigate the flux quantization in this normal conducting “hole” by cooling the whole device in different magnetic fields, reducing this external field to zero, and measuring the resulting flux. This way, superconducting properties of a hole in a superconducting film have been determined, which are important for the operation of hole-patterned magnetometers based on direct current superconducting quantum interference devices in static magnetic fields.
Determination of average demagnetizing fields in longitudinal magnetic recording using nanosecond field pulses
Authors: Moser A. , Weller D. , Rizzo N. D. , Silva T. J.
Published: July 2002

The demagnetizing field across bit transitions in longitudinal recording media is measured quantitatively as a function of linear density using a quasi static write/read tester. Bit transitions with varying linear densities in the range 500–6000 flux changes per mm are recorded on CoCr10Ta4 media films. The media are deposited onto a coplanar waveguide structure, which is used to generate reversal field pulses of well-defined amplitude and 10 ns width to minimize thermally activated processes. The demagnetizing field is extracted from the measured reversal field, which is the sum of the external waveguide field and the internal, density-dependent demagnetizing field. The experimental results are qualitatively consistent with those predicted by a simple magnetostatic model that assumes finite transition widths.
Near-field surface photovoltage
Authors: Shikler R. , Rosenwaks Y.
Published: July 2002

A phenomenon called near-field surface photovoltage is presented. It is based on inducing photovoltage only at a semiconductor space-charge region using near-field illumination. The photovoltage is obtained by measuring the contact potential difference between an optical near-field force sensor and a semiconductor surface under illumination. It is shown that the near-field illumination induces photovoltage at the surface which is principally different from photovoltage induced by far-field illumination. The mechanisms that govern the different far-field and near-field photovoltage response are discussed.
Effect of grain boundaries on carrier lifetime in chemical-vapor-deposited diamond film
Authors: Yoneda Hitoki , Tokuyama Kazutatsu , Yamazaki Riichi , Ueda Ken-ichi , Yamamoto Hironori , Baba Kazuhiro
Published: July 2002

Using the reflective measurements, the effect of grain boundaries on the lifetime of photogenerated carriers was investigated for chemical-vapor-deposited (CVD) diamond films. To investigate the effect of grain boundaries on the carrier dynamics, photons having energy lower than the band gap energy of a single crystal of diamond were used for pumping. For a 4.8 eV photon, the measured lifetime was several tens of picoseconds, and that was consistent with photoconductive current measurements. However, a dramatic decrease of the carrier lifetime was observed in the case of 3.2 eV irradiation. The variation of the lifetime inside the single grain was measured by a microscopic pump–probe method. The carrier lifetime near the grain boundary decreased from 5 to 8 ps at the center to 0.35–0.5 ps. This decreased lifetime and the carrier generation efficiency with lower energy photon had a negative correlation. To explain this mechanism, we considered the decrease in lifetime to be related to the density of the imperfection or mid-band gap states inside the single-CVD-polycrystalline grain.
On the use of total reflection x-ray topography for the observation of misfit dislocation strain at the surface of a Si/Ge–Si heterostructure
Authors: McNally Patrick J. , Dilliway G. , Bonar J. M. , Willoughby A. , Tuomi T. , Rantamäki R. , Danilewsky A. N. , Lowney D.
Published: July 2002

Synchrotron x-ray topography was used in total reflection topography (TRT) mode to observe strain-induced surface bumps due to the presence of underlying misfit dislocations in strained-layer SiGe on Si epitaxial heterostructures. In these experiments, the x rays approached the sample surfaces at grazing incident angles below the critical angles for total external reflection for a number of reflections, and hence, surface strain features nominally less than a few tens of angstro/ms from the sample surface have been observed. These are similar to the surface bumpiness observed by atomic force microscopy, albeit on a much larger lateral length scale. The fact that TRT mode images were taken was confirmed by the observation of conventional backreflection topographic images of misfit dislocations in all samples when the grazing incidence angle became greater than the critical angle.
Use of multiple electrical pulses to study charge transport in polymer light-emitting diodes
Authors: Pinner D. J. , Friend R. H. , Tessler N.
Published: July 2002

The method of applying two consecutive voltage pulses is presented as a method to study the dynamics of injected charges in the “off-state,” i.e., after the removal of the applied voltage pulse. Pulses of different widths are used to selectively probe the dynamics of holes and electrons. The use of multiple (>2) pulses is also described as an additional carrier monitoring technique.
The writhe distribution in DNA plasmids as derived from the free energy of supercoiling
Authors: Tobias Irwin
Published: July 2002

In theoretical work on the molecule, DNA is often treated, approximately, as a naturally straight, inextensible, isotropic elastic rod of circular cross section. It is shown that, consistent with this level of approximation, there exists a general connection between the free energy of supercoiling of plasmids formed by the DNA, and the writhe distribution in plasmids having a given value of the linking number difference, ΔLk. In particular, the writhe distribution in a collection of torsionally relaxed (ΔLk=0), but non-nicked, plasmids is completely determined once the free energy of supercoiling as a function of ΔLk is known. The writhe distribution in the supercoiled plasmids characterized by any other value of ΔLk, we shall also show, is simply related to the distribution in the relaxed plasmid, and, therefore, it, too, is completely determined. These general results are illustrated for two cases: Large plasmids for which the measured free energy of supercoiling, a quadratic function of ΔLk, implies a normal writhe distribution, and miniplasmids for which a theoretical expression for the free energy of supercoiling involving the frequencies of the normal modes of vibration of a circular elastic ring has recently become available. In this latter case, the writhe distribution for supercoiled plasmids is not normal, but shows a skewness related to a property of elastic rings, namely, the loss of stability of the circular equilibrium configuration of the rings when they are twisted beyond a critical value. Such a skewed writhe distribution for miniplasmids is, according to the model, associated with a free energy of supercoiling which is not, as has been assumed, a rigorously quadratic function of ΔLk.
Contribution of ions and radicals in etching of Si1−xGex epitaxial films using an electron-cyclotron-resonance chlorine plasma
Authors: Takeuchi Hajime , Matsuura Takashi , Murota Junichi
Published: July 2002

Etching of undoped Si1−xGex epitaxial films (x=0−1) has been investigated using an electron-cyclotron-resonance chlorine plasma. It is found that the etch rate of Si1−xGex increases with increasing Ge fraction and decreases with increasing Cl2 pressure. From the comparison of the etch rate with the number of the incident ions toward the wafer and the relative radical density in the plasma, it is also found that the etching of Si1−xGex with low x is induced mainly by chlorine ions, and for high x, the contribution of radicals to the etching becomes larger. From the angular resolved x-ray photoelectron spectroscopy analysis, it is found that the etching by chlorine radicals causes surface segregation of Si with the concentration of about 1–1.5×1014 cm−2 on the etched Si1−xGex surface.
Influence of electrode-size effects on plasma sheath expansion
Authors: Uhm Han S. , Kim Gon H. , Kim Young W. , Kim Gun. W. , Han Seung H. , Hong Mun P.
Published: July 2002

Influence of the electrode-size effects on plasma sheath expansion is investigated for a negative voltage at the cathode with maximum amplitude of V0, assuming that the cathode is a thin, flat, conducting, circular disk of radius R. It is shown that propagation of the sheath front is proportional to the one-third power of the combination, 2V0R/π. Experimental measurements have been carried out to verify theoretical predictions. The experimental data agree remarkably well with theoretical results in every aspect.
Photonic band gaps in dusty plasma crystals
Authors: Keskinen M. J. , Fernsler R.
Published: July 2002

A two-dimensional model for electromagnetic wave propagation in a dusty plasma crystal is solved using plane wave expansion techniques. We show that photonic band gaps, where propagation is forbidden, can occur for a range of Debye length scale sizes with respect to the dust particle size. We find that band gap features are dependent on the plasma sheath characteristics of the dusty plasma crystal. In addition we compute the damping due to the conductivity of the dusty plasma crystal.
Coulomb blockade in a single tunnel junction directly connected to a multiwalled carbon nanotube
Authors: Haruyama Junji , Takesue Izumi , Sato Yuki
Published: July 2002

We report on Coulomb blockade in a single tunnel junction directly connected to a multiwalled carbon nanotube (MWNT) by utilizing a nanoporous alumina film. The MWNT exhibits a weak localization effect with strong spin flip scattering. Experimental results and analysis suggest that a high-impedance external environment caused by the weak localization in the MWNT can yield Coulomb blockade, in accordance with phase correlation theory in a single junction system. It is also revealed that the Coulomb blockade is very sensitive to phase modulation in the MWNT, which also acts as a high-impedance transmission line.
Direct lateral epitaxy overgrowth of GaN on sapphire substrates based on a sparse GaN nucleation technique
Authors: Zhang X. , Li R. R. , Dapkus P. D. , Rich D. H.
Published: July 2002

A sparse nucleation process on sapphire (0001) substrates has been developed for the growth of GaN thin films. The density of nucleation sites is reduced to only 4×104 cm−2. Based on this process, we performed direct lateral epitaxial overgrowth (LEO) of GaN by metalorganic chemical vapor deposition on patterned SiO2/sapphire (0001) substrates. An aggregate lateral to vertical growth rate ratio of around 2:1 was achieved after the coalescence of the GaN stripes. Cathodoluminescence imaging shows strong and uniform near-band gap luminescence from LEO regions and confirms the improved quality of LEO GaN, which is further supported by atomic force microscopy analysis.
A synchrotron radiation study of high-lying excited states of matrix-isolated atomic magnesium
Authors: Kerins P. , Healy B. , McCaffrey J. G.
Published: July 2002

Previous steady-state and time-resolved luminescence spectroscopy of 3p1P1 atomic magnesium, isolated in thin film samples of the solid rare gases, is extended to include the higher energy 4p1P1 excitation. Well-resolved site splittings are recorded in Mg/Ar samples for excitation to the 4p1P1 level. A small red shift in the absorption energy to the 4p1P1 level for Mg/Ar contrasts with a small blue shift on absorption to the 3p1P1 level. Direct emission from the 4p1P1 level is not observed in any of the rare gas matrices; instead, intense emission from the low energy 3p1P1 level is. Measurements of the emission decay curves in Mg/Ar have revealed slow rates in the steps feeding the 3p1P1 level following 4p1P1 excitation. The reason for the differential shifting of the 4p1P1 and 3p1P1 levels as well as the lack of direct 4p1P1 emission is thought to be related to the strong binding interaction between Mg in the 4p1P1 state and the rare gases.
AlGaAs hot-electron optical modulator
Authors: Ryvkin B. S. , Walker A. C. , Avrutin E. A.
Published: July 2002

We propose and analyze a promising surface-normal optical modulator for use in optical interconnects. The device is based on n-AlGaAs materials, operating with red light, and is therefore fully compatible with silicon photodetectors. The operation principle is based on current-induced electron heating, with redistribution of carriers between conduction band valleys playing a significant part. Calculations predict efficient absorption modulation with operating voltages <1 V and switching times of units of picoseconds.
Light emission from Er at the As-terminated Si(111) surface
Authors: Evans P. G. , Golovchenko J. A.
Published: July 2002

Erbium atoms at an arsenic-terminated Si(111) surface can be made to emit light at the 1.55 μm wavelength associated with an internal transition in the Er3+ ion. The As-terminated surface prepared under ultrahigh vacuum conditions has a surface recombination velocity of 50 cm s−1 and partially suppresses competing nonradiative recombination mechanisms. Following the deposition of Er, its characteristic light emission is observed only after oxygen reacts with the surface. The intensity of the light emitted by Er increases significantly upon cooling from 310 to 215 K. No light emission was observed from Er atoms deposited on 7×7 or H-terminated surfaces.
Semiconductor sieves as nonlinear optical materials
Authors: Tiginyanu I. M. , Kravetsky I. V. , Monecke J. , Cordts W. , Marowsky G. , Hartnagel H. L.
Published: July 2002

Electrochemical etching techniques were used to fabricate semiconductor sieves of gallium phosphide, i.e., two-dimensionally nanostructured membranes exhibiting an enhanced optical second harmonic generation (SHG) in comparison with the bulk material. The SHG rotational and fundamental polarization dependencies studied under sample excitation by a 1064-nm Nd-YAG laser beam indicate optical homogeneity and uniaxial symmetry of the membranes. The artificial anisotropy and the enhanced nonlinear optical response induced by nanotexturization make semiconductor sieves very promising for use in all-optical devices.
Effects of seed layer on the realization of larger self-assembled coherent InAs/GaAs quantum dots
Authors: Liu H. Y. , Xu B. , Chen Y. H. , Ding D. , Wang Z. G.
Published: July 2002

The size and shape evolution of self-assembled InAs quantum dots (QDs) influenced by 2.0 ML InAs seed layer has been systematically investigated for 2.0, 2.5, and 2.9 ML deposition on GaAs(100) substrate. Based on comparisons with the formation of large incoherent InAs islands on single-layer samples at late growth stage, the larger coherent InAs quantum dots at 2.9 ML deposition has been observed on the second InAs layer. A simple model analysis accounting for the surface strain distribution influenced by buried islands gives a stronger increment of critical QD diameter for dislocation nucleation on the second layer in comparison with the single-layer samples. Additionally, the inhibition of dislocation nucleation in InGaAs/GaAs large islands can also be explained by our theoretical results.
Pressure dependence of the blue luminescence in Mg-doped GaN
Authors: Ves S. , Venkateswaran U. D. , Loa I. , Syassen K. , Shahedipour F. , Wessels B. W.
Published: July 2002

We report the hydrostatic pressure dependence of the blue luminescence band observed at ∼2.8 eV in heavily magnesium-doped GaN (GaN:Mg) epilayers grown on sapphire by metalorganic chemical vapor deposition. Photoluminescence (PL) studies carried out up to 6 and 8 GPa, respectively, at room and low (10 K) temperature show that the pressure-induced energy shift of this PL band is ∼26 meV/GPa. This blueshift is about 40% less than the shift observed for the band edge in GaN. A substantially smaller pressure coefficient suggests that the 2.8 eV PL transition is associated with the recombination via deep centers.
Nature of the highly conducting interfacial layer in GaN films
Authors: Hsu J. W. P. , Lang D. V. , Richter S. , Kleiman R. N. , Sergent A. M. , Molnar R. J.
Published: July 2002

Using several scanning probe techniques to investigate local electronic properties, we show that the GaN/sapphire interfacial region contains ⩾ ten times higher electron density but with the Fermi level being 50–100 meV deeper in the band gap compared to the less-conducting bulk film. This anomalous behavior cannot be explained by transport in the intrinsic conduction band of GaN. Rather, it points to the existence of a partially filled donor impurity band. We relate the presence of this impurity band conduction to excess oxygen in the region and the defective microstructure at the GaN/sapphire interface.
A simple fiber-optic pin for detecting a shock-wave front
Authors: Homae Tomotaka , Moritoh Tatsumi , Ooya Kentaro , Nakamura Kazutaka G. , Kondo Ken-ichi
Published: July 2002

A simple fiber-optic pin for detecting a shock-wave front has been developed. The pin has a simple structure, which consists of a graded index optical fiber and a glass capillary. The performance of the optopin has been confirmed with copper targets at a shock compression of about 70 GPa using a two-stage light gas gun. The shock velocity in copper was determined by these pins within error of about 1% (a few nanoseconds) compared with well-known Hugoniot data.
Raman spectroscopy of porous and bulk GaP subjected to MeV-ion implantation and annealing
Authors: Sarua A. , Irmer G. , Monecke J. , Tiginyanu I. M. , Schwab C. , Grob J.-J. , Hartnagel H. L.
Published: July 2002

Porous layers on (100)-oriented n-type liquid encapsulated Czochralski grown GaP crystals were fabricated by electrochemical etching in a H2SO4 aqueous solution and analyzed by scanning electron microscopy. C+12 ions were introduced at room temperature by 3 MeV energy implantation into porous and bulk samples at two ion doses of 1014 and 1015 cm−2. The prepared samples were annealed in the temperature range between 200 and 600 °C applying rapid thermal annealing (RTA) technique. A comparative micro-Raman study was carried out on the porous and bulk substances. Porosity was found to lead to the violation of the selection rules and to remarkable changes in the optical properties. Additionally, Fröhlich-type modes were observed in the Raman spectra of the porous layers. High energy implantation produces a thin high damaged layer, buried at the depth of the mean projected range. Implantation does not result in a drastic damage of the samples and they undergo a fast recovery after RTA. After this treatment a semi-insulating GaP layer is created, which is thermally stable up to 600 °C.
Fine structure of the H2 5g–4f inter-Rydberg transition revealed by difference frequency laser spectroscopy
Authors: Uy D. , Gabrys C. M. , Oka T. , Cotterell B. J. , Stickland R. J. , Jungen Ch. , Wüest A.
Published: July 2002

The spectrum of the 5g–4f inter-Rydberg band of H2 has been recorded with a difference frequency laser system and analyzed using multichannel quantum defect theory (MQDT). New transitions have been observed; in addition to the singlet–triplet splittings previously observed, the hyperfine structure of the ortho-hydrogen spectrum is partially resolved in the present experiment. MQDT is used to analyze the data in a two stage process. First, the ab initio MQDT predictions were refined by fitting the quantum defect functions over a range of internuclear separation R. Second, 4f singlet and triplet quantum defects are extracted from the para-hydrogen spectra, i.e., those lines without complicating hyperfine structure. This information was then used to calculate the fine structure of a sample ortho-hydrogen line, R3(2)v+=0. While the spectra are predominantly composed of absorption lines, some transitions from high vibrational levels of the 5g triplet manifold to 4f triplet levels are observed in stimulated emission.
Transition from a single-domain to a multidomain state in mesoscopic ferromagnetic Co structures
Authors: Seynaeve E. , Rens G. , Volodin A. V. , Temst K. , Van Haesendonck C. , Bruynseraede Y.
Published: July 2002

We have performed magnetic force microscopy measurements on isolated 35 nm thick rectangular Co structures. The structures have a length L ranging between 0.25 and 10 μm and a width W ranging between 0.25 and 5.5 μm, covering aspect ratios m=L/W between 1 and 40. This enables us to map the transition from a magnetic single-domain state towards a magnetic multidomain state when increasing the size of the structures. This transition depends on the size as well as the aspect ratio of the structures. Our results can be interpreted in terms of the theoretical model developed by A. Aharoni [J. Appl. Phys. 63, 5879 (1988)].
Fabrication of iridium field emitter arrays
Authors: Chalamala Babu R. , Wei Yi , Rossi Gene , Smith Bruce G. , Reuss Robert H.
Published: July 2002

Iridium field emitter arrays were fabricated using Spindt tip process. Ir field emitter cones show an aspect ratio of 0.95, slightly less than Mo field emitter arrays fabricated using the same microfabrication process. When compared to the Mo field emitter arrays, the current–voltage characteristics of the Ir arrays were found to scale with the work function difference between Ir and Mo. Under ultrahigh vacuum conditions, the emission current stability of the Ir arrays measured over 180 h was found to be similar or slightly better than the emission stability of Mo arrays. However, when operated in the presence of O2, Ir field emitter arrays proved to be more robust and showed improved emission current stability versus their Mo counterparts.
Growth temperature dependence for the formation of vacancy clusters in Si/Si0.64Ge0.36/Si structures
Authors: Knights A. P. , Gwilliam R. M. , Sealy B. J. , Grasby T. J. , Parry C. P. , Fulgoni D. J. F. , Phillips P. J. , Whall T. E. , Parker E. H. C. , Coleman P. G.
Published: July 2002

The incorporation of vacancy clusters and vacancy point defects during the growth of Si/Si0.64Ge0.36/Si structures has been observed for growth temperatures between 250 °C and 550 °C using positron annihilation spectroscopy. A strong correlation between the electrical characteristics of the structures and the size and concentration of the clusters is observed. For the onset of two-dimensional hole gas behavior, a defect concentration less than 5×1016 cm−3 is required. A further reduction in concentration below 1×1016 cm−3 results in optimum electrical performance. The depth at which defects are observed increases with decreasing growth temperature indicating defect mobility during growth or subsequent annealing.
Investigation of two infrared bands at 1032 and 1043 cm−1 in neutron irradiated silicon
Authors: Londos C. A. , Fytros L. G.
Published: July 2002

We report on infrared (IR) studies of defects in Czochralski-grown silicon (Cz-Si) subjected to fast neutron irradiation and subsequent thermal anneals. We focus mainly on the investigation of the VO4 defect which, in the literature, has been correlated with the pair of bands (1032 and 1043 cm−1) in neutron-irradiated Si and another pair of bands (983 and 1004 cm−1) in oxygen-implanted Si. Semiempirical calculations of the localized vibrational mode frequencies of the VO4 structure support its correlation with the second pair of bands. This correlation is consistent with the ascertainment that the zero point energy of each VOn (n=1,2,3,4) defect is smaller than the zero point energy of the constituent defects VOn−1, Oi, that is: EVOn<EVOn−1+EOi.
Studies on the fabrication and properties of luminescent silicon films on foreign substrates
Authors: Di Francia G. , La Ferrara V. , Lancellotti L. , Morvillo P.
Published: July 2002

Thin luminescent silicon layers are fabricated and placed on various substrates using a purely wet-chemical process at room temperature. To this aim, silicon powder with an average grain size of about 0.1 μm is etched in a HF:HNO3 solution. During the reaction, a coalescence phenomenon is observed resulting in the formation of the photoluminescent film (PLF). This film has been picked up and placed on silicon, gallium arsenide, and glass substrates, respectively. Physical properties of PLF depend on the etching time. Under our processing conditions the film with a thickness between 20 and 14 μm has a porosity ranging between 40% and 50%, respectively, and has very high electrical resistance.
Characterization and minimization of flicker in silicon light valves
Authors: Huang H. C. , Cheng P. W. , Kwok H. S.
Published: July 2002

We have performed systematic characterizations of flicker in silicon light valves. It was found that there were four conduction mechanisms accounting for the flicker. These four mechanisms were residual dc charge on the silicon surface, voltage holding capability of the liquid crystal cell, voltage holding capability of the silicon panel, and parasitic capacitor coupling of the pixel. Major causes of these four mechanisms were identified. Solutions of flicker minimization were obtained for each mechanism. Among these solutions, offset of common voltage was found very useful to compensate for residual dc charge and parasitic capacitor coupling. Frame rate multiplication was found very useful for the minimization of flicker due to low voltage holding capabilities of the liquid crystal cell and silicon panel.
Transient characteristics of AlxGa1−xN/GaN heterojunction field-effect transistors
Authors: Li J. Z. , Lin J. Y. , Jiang H. X. , Sullivan G. J.
Published: July 2002

Transient characteristics of drain-source current in response to picosecond pulsed gate-source voltages in AlxGa1−xN/GaN heterojunction field-effect transistors (HFETs) have been measured. It was found that the switching time constants were of the order of tens of picoseconds and depended strongly on the gate-source bias VGS as well as drain-source bias VDS. Slow transients caused by charge trapping effects such as those observed in AlGaAs/GaAs HFETs were absent in AlGaN/GaN HFETs. Our results suggested that the dependence of the effective electron mobility on the sheet density dictates the overall drain current transient characteristics as well as the device switching speed of AlGaN/GaN HFETs.
Photoluminescence study of InGaAs/AlAsSb heterostructure
Authors: Georgiev Nikolai , Mozume Teruo
Published: July 2002

InGaAs/AlAsSb quantum well structures have been grown by molecular beam epitaxy nominally lattice matched to InP substrates and characterized by photoluminescence. Growth interruptions at interfaces combined with selective group V species exposure were used. Our results indicate that interface quality: interface roughness as well as compositional variations involving group V sublattice species intermixing determine the nature of band alignment at heterointerfaces. The type I band lineup with band-edge discontinuity was estimated to be about 1.6 eV for As-terminated samples, which exhibit the lowest compositional fluctuations across heterointerfaces. The photoluminescence linewidths from InGaAs/AlAsSb quantum wells agree with linewidths calculated on the assumption of 1 ML fluctuations in well width. The inhomogeneous nature of the intermixed layers results in a large broadening of the luminescence spectra line shape for samples grown without termination or Sb termination.
High-dielectric-constant Ta2O5/n-GaN metal-oxide-semiconductor structure
Authors: Tu L. W. , Kuo W. C. , Lee K. H. , Tsao P. H. , Lai C. M. , Chu A. K. , Sheu J. K.
Published: July 2002

High-dielectric-constant Ta2O5 has been grown on the n-GaN epifilm by rf magnetron sputtering. Photoluminescence measurement has been performed to compare the luminescence intensity with and without the dielectrics. Threefold increase in intensity is obtained, and a surface recombination velocity is estimated to be 3×104 cm/s as an upper limit using a modified dead-layer model. A metal-oxide-semiconductor structure has been fabricated with Al on n-GaN as the ohmic contact and on Ta2O5 as the gate metal. Capacitance-versus-voltage characteristics have been measured. The doping concentration obtained from the depletion regime is compared with the result of Hall measurement, which is 7.0×1016 cm−3. The flat-band voltage is obtained from the high-frequency data, and the effective oxide charge number density is calculated as 4.1×1012 cm−2. Indication of strong inversion appears at low reverse bias due to the high dielectric constant of Ta2O5, and matches closely with calculated values. Hysteresis is observed and ascribed to positive mobile charges derived as 2.1×1012 cm−2. The capacitance dependence on the frequency and the leakage current are discussed.
Decrease in gap states at ultrathin SiO2/Si interfaces by crown-ether cyanide treatment
Authors: Kobayashi Hikaru , Asano Akira , Takahashi Masao , Yoneda Kenji , Todokoro Yoshihiro
Published: July 2002

A simple method to passivate interface states at ultrathin SiO2/Si interfaces is developed. In this method, ultrathin SiO2-covered Si is immersed in a KCN solution containing crown-ether, followed by a rinse in water at 25 °C. The conductance–voltage measurements show that the interface state density is decreased to ∼1/10 by this crown-ether cyanide treatment. The capacitance–voltage measurements show that contamination by K+ ions is effectively avoided by the inclusion of crown-ether. These results demonstrate that crown-ether molecules effectively capture K+ ions and consequently CN− ions effectively may react with defect states, probably forming Si–CN bonds. The passivation of interface states by the cyanide treatment improves the electrical characteristics of metal–oxide–semiconductor tunneling diodes.
Grain boundary filtration by selective nucleation and solid phase epitaxy of Ge through planar constrictions
Authors: Tanabe Hiroshi , Chen Claudine M. , Atwater Harry A.
Published: July 2002

We have proposed and experimentally demonstrated a grain boundary filtration technique for unseeded fabrication of single grain semiconductor films on amorphous substrates by using patterned P-doped amorphous Ge films with In-induced selective nucleation and solid phase epitaxy. The patterns consist of a small Ge island seed region including a deposited metal selective nucleation site, a narrow seed region, and a single grain region consisting of a main rectangular island. Transmission electron microscopy revealed lateral epitaxy initiated at the edge of the selective nucleation site and the grain selection process through the narrow seed selection region produced only one grain orientation at the entrance of the main island. Single grain regions as large as 100 μm2 with only a few low-angle boundaries were formed at 400 °C without spontaneous nucleation.
Effect of an oxidized Ni/Au p contact on the performance of GaN/InGaN multiple quantum well light-emitting diodes
Authors: Kim Hyunsoo , Kim Dong-Joon , Park Seong-Ju , Hwang Hyunsang
Published: July 2002

Feasibility of an oxidized Ni/Au p contact on some aspects of device applications for a GaN/InGaN multiple quantum well light-emitting diode (LED) was investigated. For the oxidation of Ni/Au p contact, furnace annealing of a completely fabricated LED was performed at 600 °C for 5 min in an O2 ambient. For the case of LED with an oxidized Ni/Au system, the I–V measurements showed a reduction in series resistance of the diode by 17.2%. In addition, the optical output power of the oxidized LED was increased by a factor of 2. However, a significant degradation in reliability characteristics was observed, which might detract from the direct application of the Ni/Au oxidation process. We also conclude that the improvement of oxidized Ni/Au contact properties is mainly due to the formation of an intermediate NiO layer, rather than an enhancement in p-type activation.
Very high spectral resolution obtained with SU5: A vacuum ultraviolet undulator-based beamline at Super-ACO
Authors: Nahon Laurent , Alcaraz Christian , Marlats Jean-Louis , Lagarde Bruno , Polack François , Thissen Roland , Lepère Didier , Ito Kenji
Published: July 2002

In this article we wish to report on the commissioning of the SU5 undulator-based beamline, whose primary scientific goal deals with high-resolution spectroscopy and photon-induced dynamics in the vacuum ultraviolet range on dilute species. In order to achieve such a scientific program, we have conceived an original optical design, centered around a 6.65 m off-plane Eagle monochromator equipped with two gratings (2400 and 4300 l/mm) illuminated by an astigmatic prefocusing optical system. The different components of the actual beamline, such as the optical elements, their holders/manipulators and the monochromator are described, with a special emphasis on critical aspects such as the mechanical resolution and stability, the vibrations limitations, and the thermal stability. Then, a spectral calibration procedure of the monochromator is described, followed by the presentation of the measured performances of the beamline in terms of ultimate resolution and flux. Owing to a gas phase ion yield experiment on rare gases, we have been able to measure a raw linewidth of 184 μeV at 21.61 eV (18s′ line of Ne) corresponding to a resolving power of 117 000 and a raw (respectively, lifetime-deconvoluted) linewidth of 119 μeV (respectively, 76 μeV) at 15.82 eV (13s′ line of Ar) corresponding to a raw resolving power of 133 000 (respectively, 208 000). The ultimate targeted 105 resolving power is therefore observed on most of the VUV range, which corresponds to an unprecedented performance. The measured photon flux in a 1/50 000 bandwidth is in reasonable agreement with the expected values lying in the 1010–109 photon/s range over the VUV range. These very satisfactory performances, with a slit-limited resolution down to about 15 μm slits aperture, are due to the special care which has been paid at both the conception and construction stages concerning critical issues such as the optical and mechanical design, the vibrations, the driving/encoding system, and the slope errors of the gratings.