bluuebunny/crossref_metadata_2025 huggingface dataset:
Energy relaxation of two-dimensional carriers in strained Ge/Si0.4Ge0.6 and Si/Si0.7Ge0.3 quantum wells: Evidence for two-dimensional acoustic phonons
Authors: Song S.-H. , Pan Wei , Tsui D. C. , Xie Y. H. , Monroe Don
Published: July 2002

We performed heating measurements on holes in a strained Ge/Si0.4Ge0.6 quantum well and electrons in a strained Si/Si0.7Ge0.3 quantum well in the temperature range 0.3–5.5 K. While a power law dependence of carrier temperature on current, Te∼Ia, was observed for both samples, the measured values for the current exponent are different: a=0.50±0.02 for the Ge sample and 0.40±0.02 for the Si sample. We attribute this exponent difference to the difference in their phonon dimensionality.
Comparison of experimental and theoretical GaInP quantum well gain spectra
Authors: Chow W. W. , Smowton P. M. , Blood P. , Girndt A. , Jahnke F. , Koch S. W.
Published: July 2002

A microscopic analysis of experimental GaInP quantum well gain spectra is presented for a wide range of excitation. A consistent treatment of carrier collision effects, at the level of quantum kinetic theory in the Markovian limit, is found to be necessary for agreement with experiment.
Influence of a 36.8° grain boundary on the magnetoresistance of La0.8Sr0.2MnO3−δ single crystal films
Authors: Steenbeck K. , Eick T. , Kirsch K. , O’Donnell K. , Steinbeiß E.
Published: July 2002

Epitaxial ferromagnetic manganite films have been sputtered on bicrystal substrates. Their magnetoresistance was measured as a function of magnetic field and temperature. The grain boundary magnetoresistance at low temperature is separated from the intrinsic magnetoresistance near the Curie temperature. The grain boundary magnetoresistance peaks at about 100 Oe and saturates at about 2 kOe. For a La0.8Sr0.2MnO3 film with a grain boundary angle θ=36.8° a field independent component r0=4.1×10−6 Ω cm2 was separated from a field-dependent component which has its maximum rH=2.3×10−6 Ω cm2 for H of order the coercive field.
Guided mode resonance filters using polymer films
Authors: Rochon P. , Natansohn A. , Callender C. L. , Robitaille L.
Published: July 2002

Guided-mode resonance optical filters are made from thin-film structures which consist of azopolymer films deposited on top of slab waveguides. Surface relief gratings can be optically induced on the azopolymer films. The transmission and reflection spectra as a function of the angle of incidence of the probe beam exhibit very sharp resonance features which correspond to coupling into the waveguide modes. Coupling into the overlying azopolymer film is also observed.
Lithium–aluminum contacts for organic light-emitting devices
Authors: Haskal E. I. , Curioni A. , Seidler P. F. , Andreoni W.
Published: July 2002

Organic light-emitting devices have been prepared with multilayer Al–Li–Al cathodes in an ultrahigh vacuum molecular beam deposition system. The optimum device characteristics are obtained when there is a single Al layer separating the Li layer from the organic materials. Ab initio molecular dynamics calculations of the Al–Li interaction clarify the role of Al as a blocking layer to Li diffusion into the organic films as well as the behavior of the device when the thickness of this Al interfacial layer is changed.
The kinetics of intermixing of GaAs/AlGaAs quantum confined heterostructures
Authors: Saher Helmy A. , Aitchison J. S. , Marsh J. H.
Published: July 2002

An atomic-scale model for the kinetics of intermixing of GaAs/AlGaAs, quantum confined heterostructures is presented. It quantifies the effects of the statistical nature of defect diffusion through heterostructures on the Ga/Al interdiffusion across such an interface. The model has been validated by successfully predicting the observed amounts of quantum well intermixing induced by a hydrogen plasma induced defect layer intermixing process. Agreement within 30% of the measurements was obtained for values of the surface release velocity>1 μm s1.
Contribution of current perpendicular to the plane to the giant magnetoresistance of laterally modulated spin values
Authors: Encinas A. , Nguyen Van Dau F. , Sussiau M. , Schuhl A. , Galtier P.
Published: July 2002

Giant magnetoresistance (GMR) effects up to 10% have been observed in Co/Cu/FeNi spin valve structures grown onto step bunched vicinal Si(111) substrates misoriented towards [11-2]. The step bunching is activated using a simple thermal treatment which leads to surfaces where terraces alternate with facets at the nanometer scale. GMR of the spin valve structures is investigated with the current applied parallel or perpendicular to the steps. An in-plane uniaxial magnetic anisotropy is induced in each magnetic layer with the easy axis parallel to the steps. This results in square GMR behavior when the field is applied along the easy axis. Specific features observed when the field is applied along the hard axis are also shown to be the consequence of this anisotropy. When the initial misorientation angle of the substrate becomes higher than 4°, we observe an enhancement of the room-temperature GMR when the current is applied perpendicular to the steps. The origin of this enhancement is discussed based on the temperature dependence of this effect.
Determination of the energy diagram of the dithioketopyrrolopyrrole/SnO2:F heterojunction by surface photovoltage spectroscopy
Authors: Moons Ellen , Eschle Martin , Grätzel Michael
Published: July 2002

The work functions of solid layers of various thicknesses of dithioketopyrrolopyrrole (DTPP), evaporated on SnO2:F, were measured by the Kelvin probe technique. Surface photovoltage (SPV) measurements confirm the p-conductivity type. By front and back side illumination of the thick DTPP layers, two space-charge layers are distinguished, allocated to the air/DTPP and DTPP/SnO2:F interface, respectively. The SPV of thinner layers is a superposition of the front side signal and back side signal, modulated by a filter effect through the organic layer. This permitted the construction of the energy diagram of the DTPP/SnO2:F junction.
High-performance GaInAsSb thermophotovoltaic devices with an AlGaAsSb window
Authors: Choi H. K. , Wang C. A. , Turner G. W. , Manfra M. J. , Spears D. L. , Charache G. W. , Danielson L. R. , Depoy D. M.
Published: July 2002

A large increase in the quantum efficiency (QE) and open-circuit voltage Voc of GaInAsSb thermophotovoltaic (TPV) devices is obtained by the use of an AlGaAsSb window layer compared with devices without a window layer. The TPV structure, grown on GaSb substrates by organometallic vapor phase epitaxy or molecular beam epitaxy, consists of a 1-μm-thick n-GaInAsSb base layer, a 3-μm-thick p-GaInAsSb emitter layer, a 100-nm-thick AlGaAsSb window layer, and a 25-nm-thick GaSb contacting layer. The band-gap energy of the lattice-matched GaInAsSb is 0.53–0.55 eV. The peak internal QE of the TPV cells with the window is >90%, compared with less than 60% for those without the window. At a short-circuit current density of ∼1000 mA/cm2, Voc of ∼300 meV is obtained for cells with the window layer, compared with less than 220 meV without the window layer. These increases are attributed to a substantial decrease in the surface recombination velocity with the window layer. Based on a standard calculation, the electron diffusion length in the p-GaInAsSb layer is at least 5 μm.
Scanning probe nanostructuring of YBa2Cu3O7: A corrosion induced abrasion
Authors: Boneberg J. , Böhmisch M. , Ochmann M. , Leiderer P.
Published: July 2002

Nanostructuring experiments were performed on a YBa2Cu3O7 thin film surface with an atomic force microscope (AFM) with a conductive tip. Measurements of the local conductivity with the AFM tip show that corrosion towards a nonconducting surface occurs on a timescale of hours under ambient conditions. The corroded surface can easily be abraded, whereas the clean YBa2Cu3O7 surface is comparably resistive against mechanical forces. The corrosion is promoted by an electric current at positive sample bias. Thus it can be concluded that the nanostructuring process performed in former experiments with the scanning tunneling microscope consists of two steps: corrosion and succeeding abrasion.
Ge concentration in regrown GaAs for ohmic contacts
Authors: Kim T. J. , Holloway P. H. , Kenik E. A.
Published: July 2002

Dissociation and solid phase epitaxial regrowth of GaAs in Ti/Ge/Ni/GaAs were investigated using the transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS) of x-rays with spatial resolution approaching 2 nm. A ternary Ni2.4GaAs phase, ∼130 nm thick, was formed by 300 °C in situ anneals of 65 nm Ni film on GaAs. After this in situ anneal, films of 30 nm Ge and 20 nm Ti were deposited in sequence. The EDS analysis showed that Ni2.4GaAs transformed into Ni–As and Ni–Ga binaries after annealing at 500 °C for 5 min, while ∼30 nm of GaAs regrew by solid phase epitaxial regrowth from decomposition of the binary phases. High spatial resolution microanalysis allowed detection of ∼1×1020 cm−3 Ge in the regrown GaAs. This confirms that Ge is incorporated into GaAs during regrowth for ohmic contact formation.
Spatially resolved luminescence studies of defects and stress in aluminum gallium nitride films
Authors: Robins L. H. , Wickenden D. K.
Published: July 2002

Aluminum gallium nitride (AlxGa1−xN) films with x=0 to x=0.36, grown by metal–organic chemical vapor deposition on sapphire, were characterized by cathodoluminescence (CL) imaging and spectroscopy, photoluminescence (PL) spectroscopy, and optical microscopy. In each film, the CL and PL spectra excited from the top surface show a narrow band-edge peak and a broad deep-level peak. In some films, the PL spectrum excited from the film–substrate interface (through the sapphire substrate) shows an additional narrow peak below the band edge, which is attributed to interfacial defects. CL imaging and optical microscopy reveal two types of large-scale defects: networks of lines, which are probably microcracks, and roughly hexagonal raised areas. The hexagonal areas occur only in the lower-x films, while the microcrack density increases with x. Spatially resolved CL spectra taken near the large-scale defects show large peak shifts attributed to stress relaxation, and below band-edge peaks attributed to localized states.
High-frequency properties of SrTiO3 thin-film capacitors fabricated on polymer-coated alloy substrates
Authors: Sugii Nobuyuki , Yamada Hiroji , Kagaya Osamu , Yamasaki Matsuo , Sekine Kenji , Yamashita Kiichi , Watanabe Mitsuhiro , Murakami Shirou
Published: July 2002

Low-temperature (200 °C) sputter deposition of SrTiO3 (STO) thin films by using ultrahigh-density ceramic target enabled us to fabricate capacitors on polyimide-isoindroquinazoline dione (PIQ)- or benzocyclobutene (BCB)-coated alloy substrates. Complex impedances of the capacitors were analyzed. Capacitances of the 0.01 mm2 area capacitors with 200-nm-thick STO films were around 22 pF and were not dependent on frequency up to 8 GHz. Leakage-current densities of the films were lower than 10−6 A/cm2 at an applied voltage of 10 V and dielectric loss tangents were lower than 10−6.
Wide-band suppression of photon-number fluctuations in a high-speed light-emitting diode driven by a constant-current source
Authors: Kobayashi M. , Kohno M. , Kadoya Y. , Yamanishi M. , Abe J. , Hirano T.
Published: July 2002

We demonstrate experimental results on the generation of sub-Poissonian photon fluxes emanating from an AlGaAs light-emitting diode, which manifest a wide-band (0–100 MHz) noise suppression below the standard quantum limit level despite low current density (∼38 A/cm2) operation at room temperature. The experimental noise power spectrum is well fitted in terms of the theoretical curve estimated with the quantum mechanical Langevin equations.
Excess silicon at the Si3N4/SiO2 interface
Authors: Gritsenko V. A. , Petrenko I. P. , Svitasheva S. N. , Wong Hei
Published: July 2002

Using electron energy loss spectroscopy, X-ray photoelectronic spectroscopy, and ellipsometry measurements, a large number of Si-Si bonds at the Si3N4/thermal SiO2 interface is confirmed. After etching away the surface SiO2 of reoxidized Si3N4, we found at the Si3N4/SiO2 interface that the plasmon energy on the surface is 20 eV which is smaller than the bulk plasmon of either Si3N4 (24.0 eV) or SiO2 (23.0 eV). From ellipsometric measurement, a large value of the refractive index (n=2.1) in the Si3N4/ wet SiO2 interface layer was obtained. The effective width of the Si-rich interfacial layer is estimated to be in the range of 6–8 Å. We propose that the excess silicon at the Si3N4/SiO2 interface is created by replacing nitrogen atoms with the oxygen atoms during the oxidation of Si3N4. Based on these observations and on numerical simulation, a hypothesis is proposed to explain the abnormally large electron capturing at the Si3N4/SiO2 interface observed previously and the accumulation of positive charge at the top interface of the nitrided oxide under ionizing irradiation.
Magnetostriction in twin-free single crystals TbyDy1−yFe2 with the addition of aluminum or manganese
Authors: Du J. , Wang J. H. , Tang C. C. , Li Y. X. , Wu G. H. , Zhan W. S.
Published: July 2002

Magnetostriction at room temperature under various conditions of compressive prestress and applied fields of TbyDy1−y(Fe1−xTx)2 (T=Al, Mn) twin-free single crystals was investigated. The substitution of Al or Mn for Fe lowers the magnetostriction under ordinary temperature and pressure, and decreases the saturation field, which enables these materials with potential benefits for applications. Moreover, Tb0.5Dy0.5(Fe0.9Mn0.1)2 shows negative magnetostriction at room temperature under zero prestress, due to the rotation of domains 109.5° away from 〈111〉. Under appropriate compressive stress, a quite large magnetostriction of 2160 ppm with a saturation field of 900 Oe and high d33 of 4.8 ppm/Oe can be obtained.
Quality improvements of ZnxCdyMg1−x−ySe layers grown on InP substrates by a thin ZnCdSe interfacial layer
Authors: Zeng L. , Yang B. X. , Tamargo M. C. , Snoeks E. , Zhao L.
Published: July 2002

The quality of lattice-matched ZnxCdyMg1−x−ySe epitaxial layers grown on (001) InP substrates with a III-V buffer layer has been improved by initially growing a ZnCdSe interfacial layer (50 Å) at low temperature. The widths of double crystal x-ray rocking curves for ZnxCdyMg1−x−ySe epilayers with band gaps as high as 3.05 eV were reduced to about 70 arcsec. The defect density evaluated from etch pit density and plan-view transmission electron microscopy measurements was reduced by two orders of magnitude, to 106–107 cm−2. The photoluminescence band edge emission became more symmetric and slightly narrower. It is proposed that an initial two-dimensional growth mode has been achieved by incorporating such a lattice-matched ZnCdSe layer.
Relationship between the charging damage of test structures and the deposited charge on unpatterned wafers exposed to an electron cyclotron resonance plasma
Authors: Cismaru C. , Shohet J. L. , Nauka K. , Friedmann J. B.
Published: July 2002

The correlation between the nonuniformities of plasma parameters (i.e., floating potential) and the induced charging onto the surface of oxide-covered unpatterned 4 in. Si wafers exposed to O2 electron cyclotron resonance (ECR) plasma is investigated. Wafers covered with a 1000 Å oxide layer were exposed to the ECR plasma under nonuniform conditions, and the induced surface charge was mapped on the wafers using contact potential difference technique. Floating potential profiles were monitored using a Langmuir probe. Experimental data indicate that the magnitude of the surface charge is proportional to the deviation of the floating potential from its surface-averaged potential. These results were compared to location of the damage of metal-oxide-semiconductor capacitor test structures exposed to same plasmas.
Spatial resolution of capacitance-voltage profiles in quantum well structures
Authors: Moon C. R. , Choe Byung-Doo , Kwon S. D. , Lim H.
Published: July 2002

The temperature dependence of the spatial resolution of capacitance-voltage (C−V) profiles in the compositional quantum well (CQW) is investigated. The apparent carrier distribution (ACD) peak in the In0.2Ga0.8As/GaAs single QW is observed to show a strong temperature dependence, compared to that in Si δ-doped GaAs. The ACD peak in CQW is wider (narrower) than the spatial extent of ground-state wave function at high (low) temperatures. The self-consistent numerical simulations on the carrier distribution show that the full width at half maximum of ACD peak in CQW is mainly affected by the debye averaging process at high temperatures and the change in the position expectation value of the two-dimensional electrons at low temperatures. This change in the position expectation value is found to be much smaller than the spatial extent of ground-state electron wave function.
Electrical characteristics of plasma oxidized Si1−x−yGexCy metal–oxide–semiconductor capacitors
Authors: Ray S. K. , Bera L. K. , Maiti C. K. , John S. , Banerjee S. K.
Published: July 2002

Microwave plasma oxidation (below 200 °C) of partially strain-compensated Si1−x−yGexCy (Ge:C=20:1 and 40:1) with and without a Si cap layer is reported. The electrical properties of grown oxides have been characterized using a metal–oxide–semiconductor structure. Fixed oxide charge density and mid-gap interface trap density are found to be 2.9×1011/cm2 and 8.8×1011/cm2/eV, respectively, for directly oxidized Si0.79Ge0.2C0.01 film. The oxide on samples with low C (0.5%) concentration, exhibits hole trapping, whereas electron trapping is observed for oxides on alloys containing 1% C.
Observation of self-starting phase-conjugate oscillation in a planar nematic liquid-crystal cell
Authors: Chen Shu-Hsia , Shen Yuhren
Published: July 2002

We report the observation of self-starting optical phase-conjugate oscillation in a nematic liquid-crystal cell. A theoretical model based on the continuum theory of liquid crystals and optical theory of degenerate four-wave mixing is developed to explain the observed phenomena.
Correlation between nitrogen concentration profile and infrared spectroscopy in silicon dioxide
Authors: Chang-Liao Kuei-Shu , Lai Han-Chao
Published: July 2002

There exists a strong correlation between the nitrogen concentration profile and infrared (IR) spectroscopy in gate oxynitrides for metal–oxide–Si (MOS) devices. The hot-electron hardness of the MOS device depends strongly on the nitrogen concentration profile in the gate oxide. It is experimentally found that the concentration ratios of nitrogen at SiO2/Si to the maximum amount of nitrogen in the oxide bulk ([N]int./[N]max. bulk) are proportional to the values of IR peak positions of Si–O stretching bonds. A larger value of [N]int./[N]max. bulk indicates a better hot-electron hardness of the MOS device, which can be explained by a reduction of SiO2/Si interfacial strain. The hot-electron hardness in the MOS device can be easily assessed by the IR signals of gate oxynitrides.
Spectrally-resolved near-field investigation of proton implanted vertical cavity surface emitting lasers
Authors: Kim Jeongyong , Pride David E. , Boyd Joseph T. , Jackson Howard E.
Published: July 2002

Emission characteristics of proton implanted vertical cavity surface emitting lasers have been studied by spectrally-resolved near-field scanning optical microscopy (NSOM). Measured spatial intensity distributions of individual transverse modes show a fundamental mode at threshold, and two double-lobed first-order modes excited around aperture edge above 1.3× threshold injection current. The use of high spectral and spatial resolution NSOM enabled the detection of a local resonance wavelength dependence on position across the laser aperture; from these data, a lateral refractive index variation is calculated. As the injection current is increased, these index distributions are found to be consistent with the observed reduction of the spatial width of the intensity distribution and the increase in the spectral mode spacing.
Pinhole formation in solid phase epitaxial film of CoSi2 on Si(111)
Authors: Ruan Like , Chen D. M.
Published: July 2002

The long-standing pinhole problem in solid phase epitaxial growth of a CoSi2 film on Si(111) has been revisited with in situ scanning tunneling microscopy. While the as-deposited film with 5 Å of Co at room temperature shows a smooth granular texture with original substrate terraces remaining intact, annealing at 580 °C produces an epitaxial CoSi2 film with large pinholes enclosed by a thin ring CoSi2, exhibiting a volcano feature. Quantitative analysis shows that the formation of pinholes is a result of rapid Si outward diffusion from bulk to surface, and of the subsequent Si reaction with Co on the outer surface. Evidence suggests that inhibiting the Si diffusion channels during the thermal annealing process is the key to solving the pinhole problem.
Semiconducting Ge clathrates: Promising candidates for thermoelectric applications
Authors: Nolas G. S. , Cohn J. L. , Slack G. A. , Schujman S. B.
Published: July 2002

Transport properties of polycrystalline Ge clathrates with general composition Sr8Ga16Ge30 are reported in the temperature range 5 K⩽T⩽300 K. These compounds exhibit N-type semiconducting behavior with relatively high Seebeck coefficients and electrical conductivity, and room temperature carrier concentrations in the range of 1017–1018 cm−3. The thermal conductivity is more than an order of magnitude smaller than that of crystalline germanium and has a glasslike temperature dependence. The resulting thermoelectric figure of merit, ZT, at room temperature for the present samples is 14 that of Bi2Te3 alloys currently used in devices for thermoelectric cooling. Extrapolating our measurements to above room temperature, we estimate that ZT>1 at T>700 K, thus exceeding that of most known materials.
An investigation of hydrogenized amorphous Si structures with Doppler broadening positron annihilation techniques
Authors: Petkov M. P. , Marek T. , Asoka-Kumar P. , Lynn K. G. , Crandall R. S. , Mahan A. H.
Published: July 2002

In this letter, we examine the feasibility of applying positron annihilation spectroscopy to the study of hydrogenized amorphous silicon (a-Si:H)-based structures produced by chemical vapor deposition techniques. The positron probe, sensitive to open volume formations, is used to characterize neutral and negatively charged silicon dangling bonds, typical for undoped and n-doped a-Si:H, respectively. Using depth profiling along the growth direction a difference was observed in the electronic environment of these defects, which enables their identification in a p-i-n device.
Relationships between hardness, elastic modulus, and the work of indentation
Authors: Cheng Yang-Tse , Cheng Che-Min
Published: July 2002

The work done during indentation is examined using dimensional analysis and finite element calculations for conical indentation in elastic-plastic solids with work hardening. An approximate relationship between the ratio of hardness to elastic modulus and the ratio of irreversible work to total work in indentation is found. Consequently, the ratio of hardness to elastic modulus may be obtained directly from measuring the work of indentation. Together with a well-known relationship between elastic modulus, initial unloading slope, and contact area, a new method is then suggested for estimating the hardness and modulus of solids using instrumented indentation with conical or pyramidal indenters.
Deep-level transient spectroscopy of Si/Si1−x−yGexCy heterostructures
Authors: Stein B. L. , Yu E. T. , Croke E. T. , Hunter A. T. , Laursen T. , Mayer J. W. , Ahn C. C.
Published: July 2002

Deep-level transient spectroscopy was used to measure the activation energies of deep levels in n-type Si/Si1−x−yGexCy heterostructures grown by solid-source molecular-beam epitaxy. Four deep levels have been observed at various activation energies ranging from 231 to 405 meV below the conduction band. The largest deep-level concentration observed was in the deepest level and was found to be approximately 2×1015 cm−3. Although a large amount of nonsubstitutional C was present in the alloy layers (1–2 at. %), no deep levels were observed at any energy levels that, to the best of our knowledge, have been previously attributed to interstitial C.
Analysis of composition fluctuations on an atomic scale in Al0.25Ga0.75N by high-resolution transmission electron microscopy
Authors: Neubauer B. , Rosenauer A. , Gerthsen D. , Ambacher O. , Stutzmann M.
Published: July 2002

Composition fluctuations in the Al0.25Ga0.75N layer of an AlGaN/GaN transistor structure grown by plasma induced molecular beam epitaxy on Al2O3(0001) at a growth temperature of 870 °C were studied by digital analysis of lattice images (DALI) of high-resolution transmission electron microscopy (HRTEM) cross-section images. DALI exploits the linear dependence of the lattice parameters on the Al content by applying Vegard’s law. Detecting the distances between intensity maxima positions in the micrograph which can be considered as a fingerprint of the local lattice parameters quantitatively derives composition profiles on an atomic scale. In the HRTEM cross-section image different areas were observed in the Al0.25Ga0.75N layer with either homogeneous or “striped” contrast. In the striped areas the analyses indicate a strong periodic decomposition with a period of 1 nm consisting of 1 ML Al0.8Ga0.2N and about 3 ML Al0.07Ga0.93N. The regions with homogeneous contrast do not exhibit significant composition fluctuations.
“S-shaped” temperature-dependent emission shift and carrier dynamics in InGaN/GaN multiple quantum wells
Authors: Cho Yong-Hoon , Gainer G. H. , Fischer A. J. , Song J. J. , Keller S. , Mishra U. K. , DenBaars S. P.
Published: July 2002

We report temperature-dependent time-integrated and time-resolved photoluminescence (PL) studies of InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition. We observed anomalous emission behavior, specifically an S-shaped (decrease–increase–decrease) temperature dependence of the peak energy (Ep) for InGaN-related PL with increasing temperature: Ep redshifts in the temperature range of 10–70 K, blueshifts for 70–150 K, and redshifts again for 150–300 K with increasing temperature. In addition, when Ep redshifts, the spectral width is observed to narrow, while when Ep blueshifts, it broadens. From a study of the integrated PL intensity as a function of temperature, it is found that thermionic emission of photocarriers out of local potential minima into higher energy states within the wells is the dominant mechanism leading to the thermal quenching of the InGaN-related PL. We demonstrate that the temperature-induced S-shaped PL shift is caused by a change in the carrier dynamics with increasing temperature due to inhomogeneity and carrier localization in the InGaN/GaN MQWs.
An analysis of temperature dependent photoluminescence line shapes in InGaN
Authors: Teo K. L. , Colton J. S. , Yu P. Y. , Weber E. R. , Li M. F. , Liu W. , Uchida K. , Tokunaga H. , Akutsu N. , Matsumoto K.
Published: July 2002

Photoluminescence (PL) line shapes in InGaN multiple quantum well structures have been studied experimentally and theoretically between 10 and 300 K. The higher temperature PL spectra can be fitted quantitatively with a thermalized carrier distribution and a broadened joint-density-of-states. The low temperature PL line shapes suggest that carriers are not thermalized, as a result of localization by band-gap fluctuations. We deduce a localization energy of ∼7 meV as compared with an activation energy of ∼63 meV from thermal quenching of the PL intensity. We thus conclude that this activation energy and the band-gap fluctuation most likely have different origins.
Second-harmonic imaging of ferroelectric domain walls
Authors: Bozhevolnyi Sergey I. , Hvam Jo/rn M. , Pedersen Kjeld , Laurell Fredrik , Karlsson Håkan , Skettrup Torben , Belmonte Michele
Published: July 2002

Domain walls in periodically poled ferroelectric KTiOPO4 and LiNbO3 crystals are observed by making use of second-harmonic (SH) generation enhancement in the transition regions between neighboring domains. SH images of domain walls obtained with various samples for different polarization configurations are presented. The SH generation enhancement is found especially pronounced for the polarization of the SH radiation being perpendicular to the domain walls. The origin and selection rules for the contrast in SH images of domain walls are discussed. The results obtained suggest that the domain walls produce a deteriorating effect on SH generation by quasiphase matching.
Blue SrS:Cu thin-film electroluminescent devices grown by hot-wall deposition using successive source supply
Authors: Ohmi Koutoku , Yamabe Kazuharu , Fukada Haruki , Fujiwara Toshihiko , Tanaka Shosaku , Kobayashi Hiroshi
Published: July 2002

SrS:Cu, Cl thin-film electroluminescent (TFEL) devices have been prepared by hot-wall deposition using a successive supply of source materials. The TFEL devices show blue EL emission with the chromaticity coordinate of (0.19, 0.28). The luminance and the efficiency at 1 kHz drive are 100 cd/m2 and 0.1 lm/W, respectively. EL emission is observed at both the leading and the trailing edge of the applied pulse voltage. The presence of trailing edge emission implies that Cu+ centers are ionized during EL excitation, and then electrons are subsequently captured by ionized Cu2+ centers giving rise to the EL emission.
19.8% efficient “honeycomb” textured multicrystalline and 24.4% monocrystalline silicon solar cells
Authors: Zhao Jianhua , Wang Aihua , Green Martin A. , Ferrazza Francesca
Published: July 2002

Multicrystalline silicon wafers, widely used in commercial photovoltaic cell production, traditionally give much poorer cell performance than monocrystalline wafers (the previously highest performance laboratory devices have solar energy conversion efficiencies of 18.6% and 24.0%, respectively). A substantially improved efficiency for a multicrystalline silicon solar cell of 19.8% is reported together with an incremental improvement in monocrystalline cell efficiency to 24.4%. The improved multicrystalline cell performance results from enshrouding cell surfaces in thermally grown oxide to reduce their detrimental electronic activity and from isotropic etching to form an hexagonally symmetric “honeycomb” surface texture. This texture reduces reflection loss as well as substantially increasing the cell’s effective optical thickness by causing light to be trapped within the cell by total internal reflection.
Single- and multi-wall carbon nanotube field-effect transistors
Authors: Martel R. , Schmidt T. , Shea H. R. , Hertel T. , Avouris Ph.
Published: July 2002

We fabricated field-effect transistors based on individual single- and multi-wall carbon nanotubes and analyzed their performance. Transport through the nanotubes is dominated by holes and, at room temperature, it appears to be diffusive rather than ballistic. By varying the gate voltage, we successfully modulated the conductance of a single-wall device by more than 5 orders of magnitude. Multi-wall nanotubes show typically no gate effect, but structural deformations—in our case a collapsed tube—can make them operate as field-effect transistors.
Optical properties of GaN-rich side of GaNP and GaNAs alloys grown by gas-source molecular beam epitaxy
Authors: Kuroiwa R. , Asahi H. , Asami K. , Kim S.-J. , Iwata K. , Gonda S.
Published: July 2002

GaN-rich side of GaNP and GaNAs layers is grown at 750 °C by gas-source molecular beam epitaxy. Phase separation is observed for the layers with P and As composition of over 1.5% and 1%, respectively. Photoluminescence (PL) spectra for the non-phase-separated GaNP (P composition: 0.37%) and GaNAs (As composition: 0.26%) show redshift of 50 and 40 meV, respectively, from that of GaN, and exhibit Stokes shift of about 80 meV which is smaller than that of GaN (100 meV). On the other hand, the PL spectrum for the phase-separated GaNP shows a large redshift peaking at 2.101 eV. This peak is considered to be an emission from the phase-separated GaP-rich GaPN region. PL excitation spectrum shows two large broad peaks. One at 2.982 eV corresponds to the absorption at the Γ point of GaP-rich region, and the other at 2.308 eV corresponds to the absorption at the isoelectronic band edge of GaP-rich GaPN alloy originated from the X point of GaP. In the case of phase-separated GaNAs, no PL is observed, suggesting that the optical properties are much more sensitive to crystalline quality in GaAs-rich GaAsN than in GaP-rich GaPN.
Three dimensional dislocation substructure in NdBa2Cu3Oy
Authors: Sandiumenge F. , Vilalta N. , Rabier J. , Obradors X.
Published: July 2002

A three dimensional dislocation substructure is reported by transmission electron microscopy in an as-grown directionally solidified (rare earth) Ba2Cu3Oy superconductor. Dislocations with a near or perfect [001] orientation and Burgers vectors lying on the basal plane are found to glide on (010) and {110}. A model is proposed in which cubic-like glide is achieved through the formation of a track of Nd→Ba antisites along the path swept by the dislocations. This finding can be correlated with the enhanced flux pinning under H‖[001] observed in these materials.
Orientation of graphitic planes during the bias-enhanced nucleation of diamond on silicon: An x-ray absorption near-edge study
Authors: Jiménez Ignacio , Mar Garcı́a M. , Albella José M. , Terminello Louis J.
Published: July 2002

The bias-enhanced nucleation of diamond on Si(100) is studied by angle-dependent x-ray absorption near-edge spectroscopy (XANES). During diamond nucleation, a graphitic phase is also detected. The angle dependence of the XANES signal shows that the graphitic basal planes are oriented perpendicular to the surface. Implications of this result on the mechanism of bias-enhanced nucleation are discussed.
Aluminum-induced crystallization of amorphous silicon on glass substrates above and below the eutectic temperature
Authors: Nast Oliver , Puzzer Tom , Koschier Linda M. , Sproul Alistair B. , Wenham Stuart R.
Published: July 2002

The achievement of high-quality continuous polycrystalline silicon (poly-Si) layers onto glass substrates by using aluminum-induced crystallization is reported. The crystallization behavior of dc sputtered amorphous silicon on glass induced by an Al interface layer has been investigated above and below the eutectic temperature of 577 °C. Secondary electron micrographs in combination with energy-dispersive x-ray microanalysis show that annealing below this temperature leads to the juxtaposed Al and Si layers exchanging places. The newly formed poly-Si layer is fully crystallized and of good crystalline quality, according to Raman spectroscopy and transmission electron microscopy investigations. At 500 °C, the time needed to crystallize a 500-nm-thick Si layer is as short as 30 min. By annealing above the eutectic temperatures, layer exchange is not as pronounced and the newly formed Al layer is found to contain a network of crystallized Si.
Retention characteristics of SrBi2Ta2O9 thin films prepared by metalorganic decomposition
Authors: Zhang Z. G. , Wang Y. N. , Zhu J. S. , Yan F. , Lu X. M. , Shen H. M. , Liu J. S.
Published: July 2002

Polycrystalline SrBi2Ta2O9 (SBT) ferroelectric thin films were synthesized on Pt/Ti/SiO2/Si substrates by metalorganic decomposition. Electric measurements demonstrate that the polarization decay increases with increasing the write/read voltage within the first second. This could be attributed to the depolarization fields, which increases with increasing the retained polarization. However, we found that the polarization loss is insignificant with different write/read voltages over a range of 1–30 000 S. Furthermore, experiment indicates that there is weak pinning of domain walls existing in SBT, which plays an important role for SBT thin film over a range of 1–30 000 S with a low write/read voltage.
Radiotracer investigation of deep Ga- and Zn-related band gap states in 6H–SiC
Authors: Grillenberger J. , Achtziger N. , Günther F. , Witthuhn W.
Published: July 2002

To identify Ga- or Zn-related deep levels, deep level transient spectroscopy (DLTS) measurements were performed repeatedly during the elemental transmutation of Ga67 to Zn67. The radioactive isotope Ga67 was recoil implanted into p-type 6H–SiC for radiotracer experiments. The DLTS spectra exhibit one peak of time-dependent height. It describes the increasing concentration of the daughter element Zn with the half life of the nuclear decay. Thus, one Zn-related level at 1.16 eV above the valence band edge is definitely identified. There is no deep level of Ga in the lower part of the band gap.
Intraband absorption in the 8–12 μm band from Si-doped vertically aligned InGaAs/GaAs quantum-dot superlattice
Authors: Zhuang Q. D. , Li J. M. , Li H. X. , Zeng Y. P. , Pan L. , Chen Y. H. , Kong M. Y. , Lin L. Y.
Published: July 2002

Normal-incident infrared absorption in the 8–12-μm-atmospheric spectral window in the InGaAs/GaAs quantum-dot superlattice is observed. Using cross-sectional transmission electron microscopy, we find that the InGaAs quantum dots are perfectly vertically aligned in the growth direction (100). Under the normal incident radiation, a distinct absorption peaked at 9.9 μm is observed. This work indicates the potential of this quantum-dot superlattice structure for use as normal-incident infrared imaging focal arrays application without fabricating grating structures.
Direct tunneling diode structure with a multilayer charge injection barrier
Authors: Dons E. M. , Skowronski C. S. , Farmer K. R.
Published: July 2002

We report the fabrication and testing of a silicon-based tunnel diode structure that incorporates a multilayer tunneling dielectric. The barrier consists of two thermally grown silicon oxide layers of direct tunneling thickness, ∼3.5 nm each, separated by an ultrathin nanocrystalline silicon layer approximately 5 nm thick. The diode displays current–voltage characteristics that are similar to those of a Fowler–Nordheim device, with a strong current turn-on beyond threshold positive and negative biases, and a “window” region between these two levels where charge transport across the barrier is negligible. In contrast to Fowler–Nordheim devices, the barrier does not appear to degrade significantly when biased in either of its conduction regimes. This property is attributed to the intrinsic degradation resistance of direct tunnel oxides. Additionally, capacitance–voltage characteristics show that the structure is of high quality. Because of its current–voltage and endurance capabilities, this structure shows promise for nonvolatile memory and other applications which require improved endurance and charge retention.
Highly efficient electron emission from diode-type plane emitters using chemical-vapor-deposited single-crystalline diamond
Authors: Ito Toshimichi , Nishimura Masaki , Hatta Akimitsu
Published: July 2002

A highly efficient electron emitting diode with a flat emission surface has been fabricated using a 600 nm single-crystalline diamond film homoepitaxially grown on high-pressure synthesized (100) diamond by means of chemical-vapor deposition and ion-implantation techniques. The emitter contains a buried injection electrode layer and a hydrogenated diamond surface. When driving voltages ranging from 0.3 to 1.1 kV were applied between them, very efficient electron emissions were observed. The emission efficiency, defined as the ratio of the emission current to the injection diode current, reached 100% for emission currents on the order of 10−7 A and did not significantly depend on the currents.
Direct spectroscopic measurement of mounting-induced strain in high-power optoelectronic devices
Authors: Tomm J. W. , Müller R. , Bärwolff A. , Elsaesser T. , Lorenzen D. , Daiminger F. X. , Gerhardt A. , Donecker J.
Published: July 2002

Thermally induced strain caused by device packaging is studied in high-power semiconductor lasers by a noninvasive technique. Fourier-transform photocurrent measurements with intentionally strained laser array devices for 808 nm emission reveal spectral shifts of quantum-confined optical transitions in the optical active region. These shifts by up to 7 meV serve as a measure for strain and are compared with model calculations. For a given packaging architecture, about one quarter of the mounting-induced strain is transferred to the quantum-well region of the device. Spatially resolved measurements demonstrate a lateral strain gradient in the devices.
Correlation between barrier height and band offsets in metal/Si1−xGex/Si heterostructures
Authors: Nur O. , Karlsteen M. , Willander M. , Turan R. , Aslan B. , Tanner M. O. , Wang K. L.
Published: July 2002

The variation of barrier height with the band gap in the metal/heterojunction systems is related to how the Fermi level position varies with respect to band edges. If the Fermi level is pinned by the interface states its movement will also correspond to the movement of the neutrality level at the interface. Metal/Si1−xGex/Si heterostructures (0⩽x⩽0.24) for both n- and p-type substrates were studied to understand the relation between Schottky barrier, Fermi level movement, and the band gap variations. It was shown that a correlation exists between Schottky barrier height variation and band-offset values ΔEc and ΔEv. For n-type substrate, measured barrier height differences are almost the same as the band offsets in the conduction band ΔEc. For p-type substrates they were found to be slightly smaller than ΔEv. This shows that Fermi level position relative to the conduction band edge does not change with band gap variation.
Molecular reorientation and deformation of a freely suspended ferroelectric liquid crystal film
Authors: Uto Sadahito , Ozaki Masanori , Yoshino Katsumi
Published: July 2002

A study of the deformation of a freely suspended ferroelectric liquid crystal film induced by an alternating electric field has been carried out. A notable peak of the magnitude of the deformation was observed at the damping frequency of the molecular reorientation. A deformation that depends on the polarity of the applied field was observed in the frequency range higher than the damping frequency of the molecular reorientation.
Degradation of AlGaN during high-temperature annealing monitored by ultraviolet Raman scattering
Authors: Kuball M. , Demangeot F. , Frandon J. , Renucci M. A. , Sands H. , Batchelder D. N. , Clur S. , Briot O.
Published: July 2002

We have illustrated the use of ultraviolet (UV) Raman scattering to investigate the thermal stability of AlGaN layers with high-aluminum content. The degradation pathway of Al0.72Ga0.28N was monitored for high-temperature treatments up to 1200 °C. For annealing temperatures higher than 1150 °C, the Al0.72Ga0.28N film decomposes: a low- and a high-aluminum composition AlxGa1−xN phase emerge. At 1100 °C, prior to the Al0.72Ga0.28N decomposition, UV Raman scattering detects the buildup of a large strain in the Al0.72Ga0.28N film. The crystalline quality of Al0.72Ga0.28N is unaffected up to 1000 °C.
Nonsteady-state photo-EMF effect in photorefractive polymers
Authors: Bittner Reinhard , Meerholz Klaus , Stepanov Serguei
Published: July 2002

The nonsteady-state photo-electromotive force (photo-EMF) effect has been observed in photorefractive (PR) polymer films based on poly(N-vinylcarbazole) (PVK). It was investigated in the typical tilted transmission configuration without and with application of moderate external direct current (dc) electric fields. The dependencies of the photo-EMF signals on the frequency and the amplitude of the phase modulation, the tilt angle, and the externally applied dc field are qualitatively explained using a simple model of the effect, developed earlier for unipolar photoconductive inorganic crystals without saturation of the trapping centers. The absolute value of the photo-EMF signal was two to three times larger than expected from this model and the experimental data on conventional photoconductivity of the polymer film. This was rationalized considering the presence of additional space-charge gratings appearing due to light reflectance from the rear surface of the thin-film device.
Structure and magnetic properties of Gd3(Fe1−xCox)25Cr4 compounds
Authors: Yang Dong , Wang Jianli , Tang Ning , Shen Yuping , Yang Fuming
Published: July 2002

The structural and magnetic properties of Gd3(Fe1−xCox)25Cr4 compounds with x=0–0.6 have been investigated. The structures of all the compounds investigated are found to possess monoclinic symmetry and belong to the A2/m space group. Substitution of Co for Fe leads to a clear contraction of the unit-cell volume. The Curie temperature increases from 492 K for x=0 to 762 K for x=0.6. The composition dependence of the saturation magnetization at 5 K reaches a maximum around x=0.3. It is noteworthy that substitution of Co for Fe results in a significant change of the magnetocrystalline anisotropy of the Co sublattice, and changes the easy magnetization direction of Gd3(Fe1−xCox)25Cr4 compounds from basal plane to easy axis.
Room-temperature deep-ultraviolet-stimulated emission from AlxGa1−xN thin films grown on sapphire
Authors: Schmidt T. J. , Cho Yong-Hoon , Song J. J. , Yang Wei
Published: July 2002

Room-temperature deep-ultraviolet-stimulated emission (SE) has been observed from optically pumped metalorganic chemical vapor deposition grown AlxGa1−xN thin films. SE has been observed for Al concentrations as high as x=0.26, with a resultant SE wavelength as low as 328 nm at room temperature. The results obtained for the AlxGa1−xN layers are compared with InxGa1−xN layers of comparable alloy concentration and GaN reference layers. The incorporation of Al into GaN is shown to result in AlxGa1−xN layers with similar high excitation-density emission behavior as GaN, in contrast to InxGa1−xN layers, which exhibit markedly different SE behavior. The observation of room-temperature SE from AlxGa1−xN layers of significant Al concentration illustrates the suitability of AlxGa1−xN based structures, not only for use in deep-ultraviolet detectors, but also as a potential source of deep-ultraviolet laser radiation.
Optimization of active regions in midinfrared lasers
Authors: Olesberg J. T. , Flatté Michael E. , Brown B. J. , Grein C. H. , Hasenberg T. C. , Anson S. A. , Boggess Thomas F.
Published: July 2002

The ideal performance of bulk, quantum well, and superlattice active regions for III–V interband midinfrared lasers are compared according to the maximum net gain per unit current density. Based on this figure of merit, which is appropriate for high-power as well as near-threshold operation, InAsSb quantum well active regions should have an order of magnitude lower threshold current than bulk InAs at room temperature. Optimized four-layer superlattices based on the InAs/GaInSb material system, however, should have two to ten times lower threshold currents than the quantum well active regions. Optimal thicknesses for these active regions were evaluated assuming a separate confinement region design. For the four-layer superlattices the optimal thickness is substantially thinner than has been commonly grown: 3 periods rather than 40 periods.
Disordered dielectric high reflectors with broadband from visible to infrared
Authors: Li Hongqiang , Gu Guochang , Chen Hong , Zhu Shiyao
Published: July 2002

With proper structure, one-dimensional (1D) binary nonperiodic dielectric multilayers can own compact strucuture and provide widely extended stop band from visible to infrared which is insensitive to the angle of incidence. The striking phenomenon attributes to the combining effect of Bragg reflection and light localization induced by disorder. The characteristic parameters to describe 1D binary nonperiodic dielectric multilayers are well defined. Material parameters of MgF2 and GaP are used to model 1D realistic device. Optimal design is suggested by comparison to binary λ0/4 stack reflectors with the same optical length, number of layers and degree of disorder.