bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct measurements of electron-longitudinal optical phonon scattering rates in wurtzite GaN
Authors: Tsen K. T. , Ferry D. K. , Botchkarev A. , Sverdlov B. , Salvador A. , Morkoç H.
Published: July 2002

Electron-longitudinal optical phonon scattering rates in wurtzite GaN have been directly measured by subpicosecond time-resolved Raman spectroscopy. We find that the total electron-longitudinal optical phonon scattering rate in GaN is about one order of magnitude larger than that in GaAs. We attribute this enormous increase in the electron-longitudinal optical phonon scattering rate to the much larger ionicity in GaN.
Effect of hydrogen dilution on the structure of amorphous silicon alloys
Authors: Tsu D. V. , Chao B. S. , Ovshinsky S. R. , Guha S. , Yang J.
Published: July 2002

We investigate why high levels of hydrogen dilution of the process gas lead to enhanced light soaking stability of amorphous silicon (a-Si) alloy solar cells by studying the microstructural properties of the material using high-resolution transmission electron microscopy (TEM) and Raman spectroscopy. The TEM results show that a-Si alloy (with or without hydrogen dilution) is a heterogeneous mixture of amorphous network and linear-like objects that show evidence of order along their length. The volume fraction of these ordered regions increases with increasing hydrogen dilution.
Crystal growth and fabrication of a 1.3-μm-wavelength multiple-quantum-well laser on a (211)A InP substrate
Authors: Okuno Y. , Tsuchiya T. , Okai M.
Published: July 2002

We demonstrate the fabrication of a long-wavelength laser on a (211) InP substrate, with the expectation of reducing threshold current density. We found that InGaAsP single quantum wells (SQWs) could be fabricated with good optical properties provided the SQW layers were not made too thin. A laser that had an unstrained multiple-quantum-well active layer emitting at 1.3 μm was fabricated on a (211)A InP substrate. Its threshold current density was 900 A/cm2, which is comparable to the value for the same type of laser on a (100) substrate. These results suggest that long-wavelength lasers with satisfactory quality can be fabricated on a (211)A substrate.
Nonuniqueness of time-dependent-dielectric-breakdown distributions
Authors: Jackson J. C. , Robinson T. , Oralkan O. , Dumin D. J. , Brown G. A.
Published: July 2002

The time-dependent-dielectric-breakdown (TDDB) distributions measured on a series of identical oxides at the same voltages have been shown to depend on the resistance and capacitance of the measurement test equipment. The TDDB distributions were shifted to shorter times if the impedance of the test equipment was lowered and/or the capacitance of the test equipment was raised. The lower resistances and higher capacitances allowed the nonshorting early electric breakdowns to develop into shorting, thermal, dielectric breakdowns.
Transient photocurrents in lead zirconate titanate thin films
Authors: Kholkin A. , Boiarkine O. , Setter N.
Published: July 2002

Transient photocurrents are investigated in ferroelectric lead zirconate titanate (PZT) films illuminated by laser pulses in the spectral range of 340–380 nm. At low electric fields, the photocurrent is sensitive to the ferroelectric polarization state and changes sign upon the polarization switching. At higher fields, the polarization-independent current proportional to the applied electric field is dominant. The spectral dependence of polarization-dependent current is controlled by the excitation of charge carriers over the forbidden band of PZT having a maximum near 3.5 eV. Possible mechanisms of the observed effect are discussed and the possibilities of a nondestructive read out of polarization state in the ultraviolet spectral range are demonstrated.
Green phosphor for low-voltage cathodoluminescent applications: SrGa2S4:Eu2+
Authors: Yang S. , Stoffers C. , Zhang F. , Jacobsen S. M. , Wagner B. K. , Summers C. J. , Yocom Neil
Published: July 2002

The optical and physical properties of a low-voltage phosphor, SrGa2S4:Eu2+, are reviewed. This phosphor has excellent chromaticity and high luminous efficiency at excitation voltages <3 kV and at high drive current (∼100 μA/cm2). At high current densities this phosphor was found to have superior saturation properties compared to current phosphors. This is attributed to the fast decay time of this phosphor, which is expected to enhance its resistance to saturation. Recent studies show that this phosphor has good maintenance properties.
A light-emitting device using a lateral junction grown by molecular beam epitaxy on GaAs (311)A-oriented substrates
Authors: Vaccaro Pablo O. , Ohnishi Hajime , Fujita Kazuhisa
Published: July 2002

A lateral p–n junction allows direct injection of electrons and holes in the active layer of devices such as laser diodes and can reduce carrier relaxation time and increase modulation bandwidth. Light-emitting diodes were made on patterned GaAs (311)A-oriented substrates by using a lateral p–n junction formed in GaAs–silicon-doped epilayers grown by molecular beam epitaxy. Good electroluminescence at room temperature was obtained for both GaAs single layers and GaAs/AlGaAs multiple quantum well structures.
Er doping of GaN during growth by metalorganic molecular beam epitaxy
Authors: MacKenzie J. Devin , Abernathy C. R. , Pearton S. J. , Hömmerich U. , Seo J. T. , Wilson R. G. , Zavada John M.
Published: July 2002

1.54 μm photoluminescence has been observed from GaN doped with Er during growth by metalorganic molecular beam epitaxy. Strong Er3+-related photoluminescence (PL) was measured at room temperature for GaN:Er doped during growth on c-plane Al2O3 and Si. Experiments to evaluate the effects C and O on the optical activity of Er indicated that these impurities dramatically enhance Er PL in GaN. GaN films doped with Er to a concentration of 3×1018 cm−3 with [O]∼1020 cm−3 and [C]∼1021 cm−3 luminesce at 1.54 μm with an intensity ∼2 orders of magnitude greater than films with oxygen and carbon backgrounds of less than 1019 cm−3. The thermal PL quenching behavior was also markedly different for samples of varying O and C content. Er3+ luminescence from samples with high O and C concentrations quenched by only 10% between 15 and 300 K while the integrated PL signal from the samples with lower [O] and [C] quenched ∼85% over the same temperature range.
High-power λ≈8 μm quantum cascade lasers with near optimum performance
Authors: Gmachl Claire , Tredicucci Alessandro , Capasso Federico , Hutchinson Albert L. , Sivco Deborah L. , Baillargeon James N. , Cho Alfred Y.
Published: July 2002

Quantum cascade (QC) lasers emitting at λ≈8 μm with a power performance equal to short-wavelength (λ≈5 μm) QC lasers are reported. The device improvement is mainly achieved by a design of the injector/relaxation region, which at laser threshold allows resonant carrier injection between the ground state of the preceding and the upper laser level of the subsequent active region. In pulsed operation a peak output power of 1.3 W per facet has been measured at 100 K. At room temperature a record peak power of 325 mW and a record slope efficiency of 180 mW/A have been measured. In continuous-wave operation the maximum power at 30 K was 510 mW per facet and still 200 mW per facet at 80 K. The high values of the output power and slope efficiency demonstrate the validity of the cascading scheme, in which electrons above threshold generate one photon per each active region they successively traverse.
Trap states of tris-8-(hydroxyquinoline) aluminum and naphthyl-substituted benzidine derivative using thermally stimulated luminescence
Authors: Forsythe E. W. , Morton D. C. , Tang C. W. , Gao Yongli
Published: July 2002

The bulk trap state properties of a naphthyl-substituted benzidine derivative (NPB) and tris-8-(hydroxyquinoline) aluminum (Alq3) have been measured using thermally stimulated luminescence (TSL). The TSL spectra for both organic materials show significant trap distributions over the temperature range from 8 to 300 K. Using a general order TSL expression, the four peaks in NPB were modeled with trap states centered from 0.20 to 0.05 eV. Alq3 has three TSL peaks over the temperature range, with the peak at 156 K modeled as a distribution of trap states from 0.25 to 0.13 eV. For both materials, the trapping mechanism involves a combination of first and second order emission. NPB and Alq3 have trap states sufficiently deep to influence the carrier transport and recombination process in bilayer organic based light emitting.
Ballistic electron emission microscopy on biased GaAs–AlGaAs superlattices
Authors: Heer R. , Smoliner J. , Strasser G. , Gornik E.
Published: July 2002

In this work, ballistic electron transport through the lowest miniband of a biased GaAs–AlGaAs superlattice is investigated by ballistic electron emission microscopy (BEEM). In the BEEM spectra the miniband manifests itself as clear peak in the second derivative of the ballistic electron current. Biasing the superlattice results in a shift of the miniband position and the corresponding peak position. It is shown that the measured total transmission of the superlattice is in excellent agreement with the calculated transmission, which makes the superlattice a promising tunable energy filter for studying the energetic distribution of ballistic electrons.
Photoluminescence study of in situ annealed InAs quantum dots: Double-peak emission associated with bimodal size distribution
Authors: Lee Hao , Lowe-Webb Roger , Johnson Thomas J. , Yang Weidong , Sercel Peter C.
Published: July 2002

We report a photoluminescence study of self-assembled InAs islands subjected to in situ annealing prior to the growth of a capping layer. A distinctive double-peak feature is observed in the photoluminescence spectra of annealed samples. The power dependence of the photoluminescence spectra reveals that the double-peak emission is associated with the ground-state transition of islands in two different size branches. This observation agrees with a previous study, which demonstrated that the InAs island size distribution bifurcates during post-growth annealing. The temperature dependence of the photoluminescence intensities from samples with bimodal island size distributions illustrates that different thermal activation energies for carrier emission are associated with islands in different size branches.
Structural properties and Raman modes of zinc blende InN epitaxial layers
Authors: Tabata A. , Lima A. P. , Teles L. K. , Scolfaro L. M. R. , Leite J. R. , Lemos V. , Schöttker B. , Frey T. , Schikora D. , Lischka K.
Published: July 2002

We report on x-ray diffraction and micro-Raman scattering studies on zinc blende InN epitaxial films. The samples were grown by molecular beam epitaxy on GaAs(001) substrates using a InAs layer as a buffer. The transverse-optical (TO) and longitudinal-optical phonon frequencies at Γ of c-InN are determined and compared to the corresponding values for c-GaN. Ab initio self-consistent calculations are carried out for the c-InN and c-GaN lattice parameters and TO phonon frequencies. A good agreement between theory and experiment is found.
B cluster formation and dissolution in Si: A scenario based on atomistic modeling
Authors: Pelaz Lourdes , Gilmer G. H. , Gossmann H.-J. , Rafferty C. S. , Jaraiz M. , Barbolla J.
Published: July 2002

A comprehensive model of the nucleation, growth, and dissolution of B clusters in Si is presented. We analyze the activation of B in implanted Si on the basis of detailed interactions between B and defects in Si. In the model, the nucleation of B clusters requires a high interstitial supersaturation, which occurs in the damaged region during implantation and at the early stages of the postimplant anneal. B clusters grow by adding interstitial B to preexisting B clusters, resulting in B complexes with a high interstitial content. As the annealing proceeds and the Si interstitial supersaturation decreases, the B clusters emit Si interstitials, leaving small stable B complexes with low interstitial content. The total dissolution of B clusters involves thermally generated Si interstitials, and it is only achieved at very high temperatures or long anneal times.
Raman studies on phonon modes in cubic AlGaN alloy
Authors: Harima H. , Inoue T. , Nakashima S. , Okumura H. , Ishida Y. , Yoshida S. , Koizumi T. , Grille H. , Bechstedt F.
Published: July 2002

Cubic AlxGa1−xN alloy layers have been successfully grown for x=0−1 by gas-source molecular beam epitaxy on cubic-SiC/Si substrates, and the compositional dependence of the transverse-optic (TO) and longitudinal-optic (LO) phonon modes has been studied by Raman scattering. The LO-mode frequency of mixed crystals shows a systematic variation from the pure cubic AlN phase to the cubic GaN phase (one-mode type). On the contrary, there are two branches for the TO mode varying slowly in frequency with the composition (two-mode type). This behavior is explained within a random-element-isodisplacement model including the effect of polarization field. Our result indicates a strong polarization field acting on the cation-nitrogen bonds.
Irradiation-induced growth of nanoquasicrystals from amorphous matrix in the equilibrium immiscible Fe–Cu system
Authors: Yang G. W. , Lai W. S. , Lin C. , Liu B. X.
Published: July 2002

A microstructure of nanosized quasicrystals embedded in an amorphous matrix was formed in the Fe70Cu30 multilayered films upon room temperature 200 keV xenon ion irradiation. The initial Fe and Cu crystalline phases in the as-deposited multilayers transformed into a unique amorphous phase at a dose of 8×1014 Xe+ cm−2 and further irradiation, i.e., up to a dose of 5×1015 Xe+ cm−2, induced the growth of the quasicrystals in some local areas in the amorphous matrix. High-resolution electron microscopy examination revealed that the compositions of the quasicrystals and amorphous matrix were close to Fe50Cu50 and Fe70Cu30, respectively. Apparently, the above microstructure was formed through a two-step phase transition along the increase of ion dose and the amorphous-to-quasicrystal transition was discussed in terms of the similarity in the atomic configuration between the icosahedral and amorphous short-range orders.
Ohmic contact formation mechanism of nonalloyed Pd contacts to p-type GaN observed by positron annihilation spectroscopy
Authors: Lee Jong-Lam , Weber Marc , Kim Jong Kyu , Lee Jae Won , Park Yong Jo , Kim Taeil , Lynn Kelvin
Published: July 2002

The change of the Fermi energy level at the interface of Pd/p-type GaN by surface treatment was investigated using positron annihilation spectroscopy, and the results were used to provide interpretation of the electrical properties of the contact. Changes in the positron parameters at the interface in the aqua regia-treated GaN are more pronounced than that in the HCl-treated one. This provides evidence that the surface treatment with aqua regia prior to Pd metal deposition removes surface oxides, resulting in the shift of the Fermi level position from a middle of the bandgap to near the valence band. Thus, the barrier for hole injection from metal to p-type GaN is lowered, subsequent to the decrease of contact resistivity by two orders of magnitude.
Effects of fluorine dimer excimer laser radiation on the optical transmission and defect formation of various types of synthetic SiO2 glasses
Authors: Hosono Hideo , Mizuguchi Masafumi , Kawazoe Hiroshi , Ogawa Tohru
Published: July 2002

Changes in the optical absorption and the formation of point defects in three types of synthetic SiO2 glasses, wet (OH content; 120 ppm), dry (OH content <1 ppm), and fluorine (F)-doped (∼1 mol %) SiO2 glasses, by irradiation with fluorine dimer (F2) excimer laser light pulses (∼8 mJ/cm2/pulse×3.6×105 pulses) were examined by various spectroscopic methods. Intense optical absorptions were induced in the wet and dry silicas in the range of 4–8 eV, whereas the intensity of absorptions induced in the F-doped silica was smaller by an order of magnitude than that in the F-free glasses. The optical transmission at the wavelength of 157 nm after the irradiation was F-doped silica≫wet silica>dry silica. The dominant electron spin resonance-active defect in the irradiated specimens was the nonbridging oxygen-hole center (NBOHC) for the wet silica, or the E′ center in the dry silica. The concentration of NBOHCs or E′ centers in the F-doped silica was lower by an order of magnitude than that in the wet or dry silica. The present results suggest the possibility of using F-doped silica glasses as photomask materials for F2 laser lithography.
Near-ultraviolet and near-infrared electroluminescence from an indium–tin–oxide film/native Si oxide/p-Si structure
Authors: Wang Y. Q. , Zhao T. P. , Liu J. , Qin G. G.
Published: July 2002

We have deposited indium–tin–oxide (ITO) films on p-Si (100) substrates with native Si oxide layers on their surfaces using the electron beam depositing technique. After the ITO/native Si oxide/p-Si structure was annealed in a N2 ambient, electroluminescence spectra with two peaks at near-ultraviolet (∼360 nm) and near-infrared (∼820 nm) have been measured under a forward bias of 6 V or larger. The experimental results have been interpreted tentatively using the tunneling-luminescence center process. It is considered that the two electroluminescence peaks originate from two groups of luminescence centers in the native Si oxide layers. The luminescence centers responsible for the near-ultraviolet peak have been discussed.
Influence of electric field on the behavior of Si nanoparticles generated by laser ablation
Authors: Muramoto Junichi , Sakamoto Ippei , Nakata Yoshiki , Okada Tatsuo , Maeda Mitsuo
Published: July 2002

The influence of an electric field on particle behavior was investigated to control the transport of Si nanoparticles in a laser ablation plume by an ultraviolet Rayleigh scattering (UV-RS) technique. The majority of the nanoparticles, which could be observed by the UV-RS technique, were transported to the negatively biased electrode, indicating that they were positively charged. The deposition efficiency of nanoparticles onto a substrate was also improved by applying an electric field.
Modeling of Ge surface segregation in vapor-phase deposited Si1−xGex thin films
Authors: Zheng Y.-J. , Lam A. M. , Engstrom J. R.
Published: July 2002

Recent experimental results concerning Ge segregation in Si1−xGex epitaxial thin films deposited on Si(100) substrates using Si2H6 and GeH4 cannot be accounted for by a simple two-site model involving surface and bulk states. This is due to Ge enrichment in the subsurface layers. Here, we demonstrate that a simple model based on the regular solution theory, which invokes both nearest, and next-nearest neighbor interactions, can explain the Ge enrichment in the subsurface. A computer simulation using the Monte Carlo method verifies the assumptions made in the model, and both methods show excellent agreement with the experimental data.
High-density InAs nanowires realized in situ on (100) InP
Authors: Li Hanxuan , Wu Ju , Wang Zhanguo , Daniels-Race Theda
Published: July 2002

High-density InAs nanowires embedded in an In0.52Al0.48As matrix are fabricated in situ by molecular beam epitaxy on (100) InP. The average cross section of the nanowires is 4.5×10 nm2. The linear density is as high as 70 wires/μm. The spatial alignment of the multilayer arrays exhibit strong anticorrelation in the growth direction. Large polarization anisotropic effect is observed in polarized photoluminescence measurements.
A surfactant-mediated relaxed Si0.5Ge0.5 graded layer with a very low threading dislocation density and smooth surface
Authors: Liu J. L. , Moore C. D. , U’Ren G. D. , Luo Y. H. , Lu Y. , Jin G. , Thomas S. G. , Goorsky M. S. , Wang K. L.
Published: July 2002

A method to grow a relaxed Si0.5Ge0.5 graded layer with a very smooth surface and a very low threading dislocation density using solid-source molecular-beam epitaxy is reported. This method included the use of Sb as a surfactant for the growth of a 2 μm compositionally graded SiGe buffer with the Ge concentration linearly graded from 0% to 50% followed by a 0.3 μm constant Si0.5Ge0.5 layer. The substrate temperature was kept at 510 °C during the growth. Both Raman scattering and x-ray diffraction were used to determine the Ge mole fraction and the degree of strain relaxation. Both x-ray reflectivity and atomic force microscopy measurements show a surface root mean square roughness of only 20 Å. The threading dislocation density was determined to be as low as 1.5×104 cm−2 as obtained by the Schimmel etch method. This study shows that the use of a Sb surfactant and low temperature growth is an effective method to fabricate high-quality graded buffer layers.
X-ray-absorption studies of boron-doped diamond films
Authors: Hsieh H. H. , Chang Y. K. , Pong W. F. , Tsai M.-H. , Chien F. Z. , Tseng P. K. , Lin I. N. , Cheng H. F.
Published: July 2002

X-ray-absorption near-edge structure (XANES) measurements have been performed for a variety of boron-doped and undoped diamond films at the C K edge using the sample drain current mode. The C K-edge XANES spectra of B-doped diamonds resemble that of the undoped diamond regardless of the B concentration, which suggests that the overall bonding configuration of the C atom is unaltered. B impurities are found to enhance both the sp3- and sp2-bond derived resonance features in the XANES spectra.
Persistent photoconductivity in InGaP/GaAs heterostructures
Authors: Fan J. C. , Wang J. C. , Chen Y. F.
Published: July 2002

Photoconductivity spectra have been investigated in InGaP/GaAs heterostructures, and persistent photoconductivity (PPC) has been observed. Through a detailed study of the dependence on excitation wavelengths and the decay kinetics, we identify that the separation of electrons and holes due to the macroscopic potential barrier between the heterointerface and the substrate is the origin of the PPC effect. In addition, we found that the PPC effect can be reduced and the photosensitivity can be enhanced after hydrogenation. We suggest that this behavior can be interpreted by the fact that the incorporation of atomic hydrogen produces donor neutralization and passivates the activity of defective bonds, thus reducing the potential barrier for the recombination of electrons and holes and enhances the photoresponse.
In situ CdS nanocluster formation on scanning tunneling microscopy tips for reliable single-electron tunneling at room temperature
Authors: Jiang P. , Liu Z. F. , Cai S. M.
Published: July 2002

We report on a chemical approach to fabricating double-barrier tunneling junction (DBTJ) for room-temperature single-electron tunneling studies, by which CdS nanoclusters were directly synthesized on a scanning tunneling microscopy (STM) tip through an organic self-assembled monolayer (SAM) and in situ chemical reaction. The SAM/CdS-modified STM tip directly establishes a DBTJ structure with highly oriented pyrolytic graphite substrate with STM-based current–voltage measurement. Clear Coulomb staircases were observed at room temperature with this experimental approach, and the reproducibility was nearly 90%. When gradually reducing the tip-substrate gap by increasing the setpoint current, the staircase width showed a gradual decrease from 320 to 180 mV. These phenomena are well described by semiclassical model for correlated single-electron tunneling.
Evidence of phase separation in cubic InxGa1−xN epitaxial layers by resonant Raman scattering
Authors: Silveira E. , Tabata A. , Leite J. R. , Trentin R. , Lemos V. , Frey T. , As D. J. , Schikora D. , Lischka K.
Published: July 2002

Phase separation effects in cubic InxGa1−xN epitaxial layers were investigated by means of resonant Raman scattering. The alloy epilayers were grown by radio-frequency plasma-assisted molecular beam epitaxy on GaAs (001) substrates. The results, which are confirmed by x-ray diffractometry (XRD) experiments, show the presence of In-rich inclusions in c-InGaN layers with x=0.19 and 0.33. In-rich inclusions were also found by XRD in a lower In-content layer with x=0.07. Compositional inhomogeneity of about 10% was observed through selective resonances of localized regions in the In-rich separated inclusions. We find that the In-rich separated phase has nearly the same composition in all analyzed samples (x≅0.8).
Switching of nanoscale magnetic elements
Authors: Kirk K. J. , Chapman J. N. , McVitie S. , Aitchison P. R. , Wilkinson C. D. W.
Published: July 2002

We have investigated the magnetic properties of ultra-small-patterned elements of Co and NiFe thin films. The elements were rectangular with an aspect ratio in the range 3.75–20. The smallest were 200×40 nm2 with 50 nm gaps between them, corresponding to an areal density of 27 Gbit/in2 if used as discrete-patterned media for magnetic recording. The elements were fabricated by electron-beam lithography and lift-off patterning and high-resolution magnetic images were obtained by Lorentz microscopy in a transmission electron microscope. In situ magnetization reversal experiments showed that the strong dependence of the switching field on element width extended to the smallest elements of both materials. The switching field for 40-nm-wide Co elements was 1200 Oe and for 40-nm-wide NiFe elements was 800 Oe. Element length and aspect ratio had little effect.
Bulk and surface recombination in InAs/AlAs0.16Sb0.84 3.45 μm light emitting diodes
Authors: Kane M. J. , Braithwaite G. , Emeny M. T. , Lee D. , Martin T. , Wright D. R.
Published: July 2002

A study of the light generation efficiency of a series of InAs/AlAs0.16Sb0.84 light emitting diodes with p-type InAs active layers is reported. The bulk low-injection radiative efficiency of the p-type material is shown to be as high as 24% at a hole concentration of 1.5×1017 cm−3 but to decrease with increasing hole concentration in a manner consistent with the dominant low-injection nonradiative processes being Auger processes with two holes in their initial states and a total rate constant of 2×10−28 cm6 s−1. The maximum internal low-drive quantum efficiency achieved in the light emitting diodes is shown to be limited by interface recombination at the InAs/AlAs0.16Sb0.84 heterojunction and reabsorption in the active layer in addition to the bulk nonradiative processes in the InAs. A maximum value of ∼9% is achieved.
Investigation of vertical transport in n-GaN films grown by molecular beam epitaxy using Schottky barrier diodes
Authors: Misra M. , Sampath A. V. , Moustakas T. D.
Published: July 2002

In this letter, the lateral and vertical transport in lightly doped n−-GaN films, grown by plasma assisted molecular beam epitaxy, were investigated in order to explore the role of electron scattering by charged dislocations. Lateral transport constants were determined by Hall effect measurements on n−-GaN films. The doping concentration and mobility of the investigated films was 1–2×1017 cm−3 and 150–200 cm2/V s, respectively. Vertical transport was studied by etching mesa structures and forming Schottky barrier diodes. The diodes exhibit near ideal forward current–voltage characteristics with reverse saturation current densities in the 1–10×10−9  A cm−2 range. The doping concentrations as well as the barrier height of the diodes were determined from capacitance–voltage measurements to be 8–9×1016 cm−3 and 0.95–1.0 V, respectively. The analysis of the reverse saturation current, using the diffusion theory, leads to vertical mobility values of 950 cm2/V s. The significant increase in mobility for vertical transport is attributed to reduction in scattering by charged dislocations.
Ultrahigh-resolution liquid crystal display with gray scale
Authors: Wen Bing , Mahajan Milind P. , Rosenblatt Charles
Published: July 2002

An atomic force microscope was used to write planar alignment patterns on a polyimide-coated glass substrate. Paired with a substrate treated for homeotropic alignment, the resulting hybrid liquid crystal cell produced fixed gray scale images with pixel sizes of order 1 μm. The physics and efficacy of this architecture are compared to a cell having planar alignment at both substrates.
An optical spin micromotor
Authors: Luo Zong-Ping , Sun Yu-Long , An Kai-Nan
Published: July 2002

An optical spin micromotor operated by optical tweezers, described in the present study, converts radiant energy directly to mechanical energy. This conversion is of great practical and fundamental interest. The optical spin micromotor consists of a two-bead linkage. The large bead (2.0 μm in diameter) is trapped by the optical tweezers and rotates around the laser beam axis. The small bead (0.94 μm in diameter) is partially coated with gold/palladium for generating the spin torque through the change of momentum from gradient radiation pressure of the same laser applied on the coated small bead. With a laser power of 29 mW at the sample plane, the spin speed averaged 158.8±155.5 rpm (mean±standard deviation) and the torque was estimated as 101.9±99.8 pN nm. This optical spin micromotor will be useful in (1) providing a fundamental step toward developing optical-operated mechanical devices, (2) twisting macromolecules, and (3) generating vortex and shear force in medium at nanoscale.
Photoluminescence properties of ZnO epilayers grown on CaF2(111) by plasma assisted molecular beam epitaxy
Authors: Ko H. J. , Chen Y. F. , Zhu Z. , Yao T. , Kobayashi I. , Uchiki H.
Published: July 2002

Photoluminescence (PL) properties of ZnO films grown on CaF2(111) with a low-temperature buffer layer by plasma assisted molecular beam epitaxy are investigated, in which lattice misfit tensile strain is expected to be compensated by the thermal compressive strain. The low-temperature buffer layer is further introduced to accommodate lattice strain leading to the growth of almost strain-free and high quality ZnO films. PL spectra of ZnO layers measured at 10 K are dominated by neutral-donor bound exciton emission at 3.366 eV (I4) with a linewidth of 12 meV. Commonly observed deep level emission at around 2.3 eV is negligibly small in intensity. Free exciton emission develops as temperature is raised and eventually dominates at temperatures higher than 70 K. Detailed study on temperature-dependent PL spectra indicates that the energy position of the free exciton emission is located at the same energy as bulk materials suggesting the growth of strain-free ZnO layers. Consequently, stimulated emission due to exciton–exciton scattering is observed at 77 K.
Defects in planar Si pn junctions studied with electrically detected magnetic resonance
Authors: Wimbauer T. , Ito K. , Mochizuki Y. , Horikawa M. , Kitano T. , Brandt M. S. , Stutzmann M.
Published: July 2002

We report electrically detected magnetic resonance (EDMR) measurements on planar Si pn junctions which were isolated via local oxidation of silicon (LOCOS). The investigations of the as-fabricated diodes show the presence of various defects. We observe Pb centers at the boundary to the LOCOS isolation and an isotropic Si dangling bond related signal which is assumed to be a consequence of ion implantation. The E′H center—a hydrogen-complexed oxygen vacancy in the SiO2 device isolation—is also detected via EDMR. The EDMR detection mechanism, which is based on resonant changes of the device current, restricts the detected oxide defects to those which are close to the interface.
Minority carrier diffusion, defects, and localization in InGaAsN, with 2% nitrogen
Authors: Kurtz Steven R. , Allerman A. A. , Seager C. H. , Sieg R. M. , Jones E. D.
Published: July 2002

Electron and hole transport in compensated InGaAsN (≈2% N) are examined through Hall mobility, photoconductivity, and solar cell photoresponse measurements. Short minority carrier diffusion lengths, photoconductive-response spectra, and doping dependent, thermally activated Hall mobilities reveal a broad distribution of localized states. At this stage of development, lateral carrier transport appears to be limited by large scale (≫ mean free path) material inhomogeneities, not a random alloy-induced mobility edge.
Microwave spectra of HCN and DCN in He4 nanodroplets: A test of adiabatic following
Authors: Conjusteau A. , Callegari C. , Reinhard I. , Lehmann K. K. , Scoles G.
Published: July 2002

The purely rotational microwave spectrum of HCN and DCN embedded in He4 nanodroplets has been measured. The J=0→1 transitions for both molecules have been recorded at 72.21 and 59.90 GHz, respectively. The increase in moment of inertia due to the presence of liquid helium, which the assumption of adiabatic following of the helium density predicts should be almost identical for both molecules, is found to be 9% smaller for the faster of the two rotors, HCN. This result is interpreted as a breakdown of the adiabatic following approximation, which is valid for the slower rotors. Power-saturation measurements have also been performed, and show that the rotational relaxation time for these molecules is on the order of 10−8 s.
Carrier distribution and relaxation-induced defects of InAs/GaAs quantum dots
Authors: Wang J. S. , Chen J. F. , Huang J. L. , Wang P. Y. , Guo X. J.
Published: July 2002

The carrier distribution and defects have been investigated in InAs/GaAs quantum dots by cross-sectional transmission electron microscopy (XTEM), capacitance–voltage, and deep level transient spectroscopy. Carrier confinement is found for 1.1- and 2.3-monolayer-(ML)-thick InAs samples. For 2.3 ML sample, XTEM images show the presence of defect-free self-assembled quantum dots. With further increase of the InAs thickness to 3.4 ML, significant carrier depletion caused by the relaxation is observed. In contrast to 1.1 and 2.3 ML samples in which no traps are detected, two broad traps and three discrete traps at 0.54, 0.40, and 0.34 eV are observed in 3.4 ML sample. The traps at 0.54 and 0.34 eV are found to be similar to the traps observed in relaxed In0.2Ga0.8As/GaAs single quantum well structures. By comparing with the XTEM images, the trap at 0.54 eV is identified to be the relaxation-induced dislocation trap in the GaAs layer.
Stable nonplanar surface formed on patterned GaAs (311)A substrate by molecular-beam epitaxy
Authors: Gong Q. , Nötzel R. , Schönherr H.-P. , Ploog K. H.
Published: July 2002

The evolution of the growth front during molecular-beam epitaxy on patterned GaAs (311)A substrates is investigated by cross-sectional atomic force microscopy. The growth rate on the sidewalls is enhanced due to the preferential atom migration from the mesa top and bottom to the sidewalls. The growth front evolution is terminated by the formation of a stable, corrugated surface which is composed of (311)A terraces and steps toward the [23̄3̄] and [2̄33] directions. Modulation of island density in strained-layer growth is demonstrated by growing 4 ML InAs on this nonplanar surface.
Extrapolation chamber response in low-energy x radiation standard beams
Authors: Dias Simone K. , Caldas Linda V. E.
Published: July 2002

Measurements of absorbed dose rates in air and tissue produced by low-energy x radiation are often difficult to obtain with accuracy. The recommended instruments for these applications are extrapolation chambers. The performance of an extrapolation chamber, developed at IPEN, was studied in low energy x radiation standard beams in relation to its response linearity, extrapolation curves and energy dependence. The results obtained indicate that the developed chamber is suitable for x radiation measurements.
Ferroelectric silver niobate-tantalate thin films
Authors: Koh Jung-Hyuk , Khartsev S. I. , Grishin Alex
Published: July 2002

Submicron thick ferroelectric AgTa0.38Nb0.62O3 (ATN) films have been prepared by pulsed laser deposition technique onto Pt80Ir20 polycrystalline and La0.7Sr0.3CoO3/LaAlO3 single crystal substrates. ATN/Pt80Ir20 films have been found to be (001) preferentially oriented, while the epitaxial quality of ATN/La0.7Sr0.3CoO3/LaAlO3 heterostructures has been ascertained. Comparative analysis of the temperature and frequency dependencies of the dielectric permittivity ε′ and loss tan δ in ATN films and bulk ceramics shows that in films: the coupled structural-ferroelectric monoclinic M1-to-monoclinic M2 phase transition occurs at the temperature 60° lower than in ceramics, thus the temperature stability of ε′ and tan δ in films is improved and occurs in extended temperature range. Reliable tracing of the ferroelectric hysteresis polarization versus electric loops indicates the ferroelectric state in ATN films at temperatures below 125 K and yields the remnant polarization of 0.4 μC/cm2 @77 K. Weak frequency dispersion, high temperature stability of the dielectric properties, as well as low processing temperature of 550 °C, are the most attractive features of ATN films.
Diamond nucleation on iridium buffer layers and subsequent textured growth: A route for the realization of single-crystal diamond films
Authors: Schreck M. , Hörmann F. , Roll H. , Lindner J. K. N. , Stritzker B.
Published: July 2002

It is shown that diamond nucleation on iridium buffer layers followed by an appropriate textured-growth step offers a viable way to realize single-crystal diamond films. Bias-enhanced nucleation on iridium layers results in heteroepitaxial diamond films with highly improved alignment. By a subsequent textured-growth step, the mosaicity can be further reduced for tilt as well as for twist in sharp contrast to former experiments using silicon substrates. Minimum values of 0.17° and 0.38° have been measured for tilt and twist, respectively. Plan view transmission electron microscopy of these films shows that, for low thicknesses (0.6 μm and 8 μm), the films are polycrystalline, consisting of a closed network of grain boundaries. In contrast, at the highest thickness (34 μm) most of the remaining structural defects are concentrated in bands of limited extension. The absence of an interconnected network of grain boundaries shows that the latter films are no longer polycrystalline.
Comparison of low confinement mode transport simulations using the mixed Bohm/gyro-Bohm and the Multi-Mode-95 transport model
Authors: Onjun Thawatchai , Bateman Glenn , Kritz Arnold H. , Hannum David
Published: July 2002

Predictive transport simulations using the mixed Bohm/gyro-Bohm (JET) transport model [M. Erba et al., Plasma Phys. Controlled Fusion 39, 261 (1997)] are compared with simulations using the Multi-Mode-95 (MMM95) transport model [G. Bateman et al., Phys. Plasmas 5, 1793 (1998)]. Temperature and density profiles from these simulations are compared with experimental data for 13 low confinement mode (L-mode) discharges from the Doublet III-D Tokamak (DIII-D) [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] and the Tokamak Fusion Test Reactor (TFTR) [D. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)]. The selected discharges include systematic scans over gyro-radius, plasma power, current, and density. It is found that simulations using the two models match experimental data equally well, in spite of the fact that the JET model has predominantly Bohm scaling (proportional to gyro-radius) while the MMM95 model has a purely gyro-Bohm scaling (proportional to gyro-radius squared).
Patterning ferromagnetism in Ni80Fe20 films via Ga+ ion irradiation
Authors: Kaminsky W. M. , Jones G. A. C. , Patel N. K. , Booij W. E. , Blamire M. G. , Gardiner S. M. , Xu Y. B. , Bland J. A. C.
Published: July 2002

We demonstrate that focused Ga+ ion irradiation can comprehensively modify the ferromagnetic properties of Ni80Fe20 thin films. Magneto-optic Kerr effect measurements at room temperature and magnetoresistance measurements at temperatures between 1.5 and 270 K characterized the irradiation effects. Irradiation steadily reduced the films’ room temperature coercivity, and a dose of 1.0×1016 ions/cm2 at 30 keV was found sufficient to cause a loss of ferromagnetism at room temperature in films of thickness up to 15.5 nm. In situ end-point detection and postirradiation atomic force microscopy confirmed that the sputtering which accompanied doses up to 1.0×1016 ions/cm2 did not compromise the protective caps on these Ni80Fe20 films. We therefore conclude that the modification of ferromagnetic properties occurred primarily because of direct Ga+ ion implantation. From these results, we speculate that focused Ga+ ion irradiation could be a convenient tool for the nanoscale patterning of magnetic properties in 3d transition metal thin films.
Computation of large systems with an economic basis set: Ab initio calculations of silicon oxide clusters
Authors: Zhang R. Q. , Chu T. S. , Lee S. T.
Published: July 2002

Silicon oxide plays an important role in many technological areas. Extensive ab initio calculations for related systems are thus desired but are limited due to the requirement of a huge computation resource. We show here that an economic basis set can be used to determine reliable atomic and electronic structures of the silicon oxide system. Binding energies in a single-point energy calculation using a standard higher-level basis set on the geometric structures optimized with the economic basis set found reasonable agreement with reported ones of high-level calculations. The approach has enabled the computation of silicon oxide clusters as large as 16 atoms being performed with considerably high accuracy using a medium level of computation resources.
On the origin of carrier localization in Ga1−xInxNyAs1−y/GaAs quantum wells
Authors: Pinault M.-A. , Tournié E.
Published: July 2002

We have investigated by temperature-dependent photoluminescence (PL) spectroscopy as-grown GaInNAs, InGaAs, and GaAsN quantum wells (QWs) embedded in a GaAs matrix. The evolution of the PL peak position and of the PL linewidth shows evidence of a strong carrier localization for the GaInNAs QWs only. The high delocalization temperature, in the 150 K range, indicates the presence of a high density of possibly deep-localizing potential wells. In addition, a higher density of nonradiative recombination centers appears to result in stronger carrier localization. Transmission electron microscopy reveals well defined, flat interfaces, in these comparatively high N-content (yN∼0.04–0.05) QWs. Our results thus demonstrate that the origin of localization in GaInNAs QWs is the concomitant presence of both In and N, which may result in strain and/or composition fluctuations.
Electronic structure and compositional interdiffusion in self-assembled Ge quantum dots on Si(001)
Authors: Seok J. H. , Kim J. Y.
Published: July 2002

The radiative recombination peaks over a broad energy range of 0.75–0.9 eV have been observed by photoluminescence spectroscopy in self-assembled Ge/Si quantum dot structures. In order to clarify the broadness of the peak distribution, we have investigated the electronic structure of an approximate model by taking the profiles of strain components at atomic positions into account. The three-dimensionally confined levels are then obtained by diagonalizing the Hamiltonian matrix of the Schrödinger equation based on strain-modified potential. Theoretical results compared with the observed recombination energies verify that significant interdiffusion between a Si capping layer and a Ge dot takes place. This is also consistent with recently reported experimental results.
Surface oxidation as a diffusion barrier for Al deposited on ferromagnetic metals
Authors: Egelhoff W. F. , Chen P. J. , McMichael R. D. , Powell C. J. , Deslattes R. D. , Serpa F. G. , Gomez R. D.
Published: July 2002

Grazing incidence x-ray reflectometry has been used to study surface oxidation as a diffusion barrier for Al deposited on ferromagnetic metals (Co, Fe, Ni, and Ni80Fe20). Samples of the form SiO2\10 nm X\4 nm Al with X=(Co, Fe, Ni, and Ni80Fe20) were investigated for X\Al intermixing. Surface oxidation was achieved by exposing the ferromagnetic layer to O2 to oxidize the top two or three atomic layers before depositing the Al layer. Specular x-ray scans were used for the analysis. Samples of the form SiO2\10 nm X\4 nm Au were used to separate topographical roughness from intermixing. Surface oxidation was found to suppress the diffusion of Al into Co, Ni, and Ni80Fe20 but not into Fe.
Magnetostatic interactions in artificial ferrimagnet based magnetic tunnel junctions
Authors: Tiusan C. , Dimopoulos T. , Buda L. , Da Costa V. , Ounadjela K. , Hehn M. , van den Berg H.
Published: July 2002

Magnetostatic interactions between the soft and the hard magnetic electrodes in magnetic tunnel junctions (MTJs) using artificial ferrimagnets (AFis) are analyzed. We attribute these interactions to the dispersion fields associated to magnetic inhomogeneities arising from domain walls due to local anisotropic ordering. These magnetostatic interactions can be controlled by adjusting the net magnetic moment of the AFi to optimize the magnetotransport response of the MTJ devices.
Biquadratic coupling effect on magnetoresistance response curves of spin valves with a synthetic antiferromagnet
Authors: Zhang K. , Kai T. , Zhao T. , Fujiwara H. , Mankey G. J.
Published: July 2002

Both normal and inverse magnetoresistance (MR) response curves were observed for synthetic spin valves with the structure of Si(100)/Ru/Co(t1)/Ru(0.7 nm)/Co(t2)/Cu(3 nm)/Co(t3)/Ru. Under the assumption of a coherent rotation of the magnetization in the three Co layers, the hysteresis loops, magnetization response, and MR response curves were calculated as a function of the parameters of the system. The parameters include antiferromagnetic coupling of Co(t1) and Co(t2) through Ru layer, a weak ferromagnetic coupling of Co(t2) and Co(t3) through Cu spacer, giant magnetoresistance of the Co/Cu/Co and Co/Ru/Co systems, and AMR contribution of each Co layer. The uniaxial anisotropy of each Co layer and a distribution of the coupling strength of Co(t1)/Ru/Co(t2) were also included. To fit the experimental data well, it was necessary to include a biquadratic coupling in the Co(t1)/Ru/Co(t2) trilayer.
Growth and characterization of BeCdSe alloys and BeCdSe/ZnCdMgSe quantum wells on InP substrates
Authors: Maksimov O. , Guo S. P. , Tamargo M. C.
Published: July 2002

We report the molecular-beam-epitaxy growth and characterization of BexCd1−xSe (0.06<x<0.2) epilayers and Be0.08Cd0.92Se/Zn0.32Cd0.25Mg0.43Se quantum-well (QW) structures on InP substrates. Good optical properties and high crystalline quality were established using photoluminescence and x-ray diffraction measurements. Narrow x-ray rocking curves with line widths down to 49 arc sec were obtained for Be0.2Cd0.8Se, closely lattice-matched to InP. A strong luminescence emission with energy of 2.072 eV and a full width at half maximum of 27 meV at 77 K was obtained from a QW structure with a 48-Å-thick QW. Strong room-temperature luminescence was also observed from the QW. A linear dependence of the QW photoluminescence intensity on the excitation laser density and an absence of shift in the emission energy indicates that the QW emission has an excitonic behavior. Based on these results and on the expected lattice-hardening properties of BeSe, we propose that BeCdSe is an attractive quantum-well material for visible light-emitting diodes.
Correlation between current–voltage characteristics and dislocations for n-GaN Schottky contacts
Authors: Shiojima Kenji , Suemitsu Tetsuya , Ogura Mitsumasa
Published: July 2002

We directly evaluated the effect of dislocations on current–voltage (I–V) characteristics of Au/Ni/n-GaN Schottky contacts. A submicrometer Schottky dot array was formed by electron beam lithography, and I–V measurements were conducted using atomic force microscopy with a conductive probe. The sample, which has a free electron concentration of 5.8×1017 cm−3, showed that neither dislocations nor steps affect the I–V characteristics. These results indicate that, in fabricating short-gate FETs, gate Schottky contacts containing dislocations should not be considered a problem with respect to uniformity and reproducibility.