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The magneto-optical response of type-II tensily strained GaAs self-assembled quantum dots in GaSb was investigated in magnetic fields up to 14 T. By depositing different GaAs amount, the dot sizes and the corresponding emission energies were varied. We analyzed the carrier wave function extent of different dots using the diamagnetic shift results. It was found that, with the increase in the energy (the reduction in the dot size), the diamagnetic coefficient first rises quickly and then saturates at around 21 μeV/T2. Based on a simple calculation model, this unusual tendency is attributed to the electrons gradually spilling out of the quantum dot to the wetting layer as the dots get smaller. This delocalization effect is enhanced in this material system due to the tensile strain relaxation within the dots, which raises the conduction band edge over that in the wetting layer. [ABSTRACT FROM AUTHOR]

The output power of InGaN multiple-quantum-well laser diodes (LDs) emitting at 405 nm wavelength is compared for several Al composition in the AlGaN n-cladding layer. The Al composition has been varied from 2% to 6% to study the effect of n-cladding refractive index on threshold current and slope efficiency of the LDs. As the Al composition in the AlGaN n-cladding layer increases, both threshold current and slope efficiency decrease. This behavior can be explained by the change in optical field distribution with refractive index of the AlGaN n-cladding layer. It is found that the Al composition of ¿4% would be advantageous for achieving more than 100 mW output power and high level of catastrophic optical damage. [ABSTRACT FROM AUTHOR]

In this study, transparent indium tin oxide (ITO) deposited by sputtering was applied to laser lift-off (LLO) GaN-based vertical light-emitting diodes (VLEDs) and the electrical and optical properties of ITO films were measured as a function of annealing conditions. The measured minimum resistivity of ITO film was about 3.78×10-4 Ω cm and the measured optical transmittance at 460 nm was 96.8% after the annealing process. In this condition, about 1×10-5 Ω cm2 of ITO contact resistance to LLO n-GaN could be obtained. By applying the transparent ITO layer to the LLO GaN-based VLEDs, a significant decrease of the forward operating voltage from 3.3 to 3.8 V at 20 mA could be obtained. [ABSTRACT FROM AUTHOR]

Containerless solidification of Nb was carried out by the electrostatic levitation process in which a bulk sample was electrostatically levitated and melted by laser irradiation in high vacuum. A t(T) function in a time-temperature profile was analytically determined to describe the radiative cooling curves of Nb in the liquid state to estimate the ratio of constant pressure specific heat to hemispherical total emissivity, c[sub P](T)/ε[sub T](T), and further to derive ε[sub T](T) and c[sub P](T). The cooling behavior of liquid Nb was found to be well described by an inverse biquadratic polynomial of t(T). With the analytical t(T) function, the derivative dt/dT was easily obtained and the c[sub P](T)/ε[sub T](T) ratio was readily estimated from the radiative cooling curve. The estimated c[sub P](T)/ε[sub T](T) and the derived ε[sub T](T) and c[sub P](T) agreed well with the reference data in the literature. This implies the rationale behind the analytical approach taken in this study was reasonable and can be applied to estimating c[sub P](T)/ε[sub T]( T) and further deriving c[sub P](T) or ε[sub T](T) of materials in the liquid state. [ABSTRACT FROM AUTHOR]

Theoretical work was performed to investigate the geometrical and electronic structures of four new hypothetical thiophene copolymers: poly(thienylene cyclopentadienylene) (PThPD), poly(thienylene silolylene) (PThS), poly(thienylene oxocyclopentadienylene) (PThOPD) and poly(thienylene thiocyclopentadienylene) (PThTPD). AM1 band calculations showed that the ground-state geometries of PThPD and PThS copolymers are of the aromatic forms whereas those of PThOPD and PThTPD are quinoid. Each intraring structure of the copolymers is predicted to be nearly identical to that of their corresponding parent homopolymers and the bond-length alternation (δr) of each copolymer is equal to the average of δr values for the two corresponding homopolymers. Modified extended Hückel band calculations produced that the band gaps (which correspond to the absorption peaks of π–π* band transition) of the copolymers in their ground states are 1.7 for PThPD, 1.55 for PThS, 1.9 for PThOPD, and 2.09 eV for PThTPD. These values, except for PThPD, are quite smaller than the band gaps calculated for the corresponding homopolymers in the ground states. Decomposition of the band gaps reveals that the gaps of the aromatic forms of PThPD and PThS are dominated by the δr contribution and the electronic effect of the bridging groups and that the gaps of the quinoid types of PThOPD and PThTPD correspond primarily to the δr contribution. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Geometrical and electronic structures of new π-conjugated five-membered ring polymers were theoretically investigated. These polymers are analogous to heterocyclic polymers, but adopt as bridging groups >=CH2, >=CF2, >=SiH2, >=SiF2, >=C=CH2, >=C=O and >=C=S moieties instead of heteroatoms. The ground-state geometries of the polymers were predicted to be quinoid from semiempirical band calculations with AM1 Hamiltonian. The electronic properties of these systems were obtained using the modified extended Hückel method. The calculated band gaps (Eg) were analyzed in terms of geometrical relaxations and electronic effect of the bridging groups using the equation of Eg=ΔEδr+ΔE1–4+ΔEel. The effect of bond-length alternation (ΔEδr) amounts to 1.1–1.4 eV for the aromatic forms and 1.8–1.9 eV for the quinoid forms of the polymers. The interactions (ΔE1–4) between C1 and C4 atoms of the cis-PA type backbone tend to decrease the band gaps of the aromatic forms and to increase the gaps of the quinoid forms as much as 0.2–0.5 eV, depending on the size of a bridging atom. It is found that the electronic effect (ΔEel) of these bridging groups is quite small compared to that found in heterocyclic polymers such as polythiophene, polypyrrole, and polyfuran. ΔEel of >=CF2, >=SiH2, and >=SiF2 bridging groups are negligible and that of the other groups amounts to 0.3–1.0 eV. Therefore, the band gaps of these systems almost correspond to the ΔEδr values which arise from the bond-length alternations, except the case of the polymers with >=C=O and >=C=S bridging groups whose π* orbitals strongly interact with the π system of the polymeric backbone. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In order to more accurately predict band gaps, corresponding to π–π* transitions of one-dimensional conducting polymers, the formula for the off-diagonal elements, Hαβij in the extended Hückel (EH) band calculation method was modified according to the form Hαβij=K1(Hααii +Hββjj)exp(-K2Rαβ) Sαβij. Parametrizations for the constants K1 and K2 were performed so as to yield reasonable band gaps for the pure hydrocarbon polymers trans-polyacetylene, poly(para-phenylene), and poly(phenylene vinylene). Since there is a large difference in bond alternations along polymeric chains between ab initio and modified neglect of diatomic overlap optimized geometries, especially for heterocyclic polymers, the valence orbital exponents of oxygen, nitrogen, and sulfur were separately adjusted, depending on the chosen geometry, to reproduce the band gaps of polyfuran, polypyrrole, and polythiophene. It is found that geometrical relaxations in the presence of heteroatoms strongly affect the C1–C4 interactions as well as bond alternations, which in turn affect the band gap. Modified EH band calculations were performed for various polymers. The predicted band gaps had average errors of ca. 10% (less than 0.3 eV) compared to the experimental values, and the method produced band structures consistent with electron-energy-loss spectroscopic observations. [ABSTRACT FROM AUTHOR]

Reports on theoretical study of geometrical and electronic structures of various thiophene-based tricyclic polymers. Fused bithiopene polymers; Thieno-bicyclic polymers; Polydicyanomethylene-cyclopenta-dicyclopentadiene.

BiFeO3-Bi(Zn1/2Ti1/2)O3 (BF-BZT) ceramics were made by a solid state reaction method. Crystal structure and microstructure were observed. Leakage currents were much reduced with BZT doping in BF. Dielectric constants of BF-BZT increased as frequency is decreased. Two distinctive relaxations were observed at about 500 kHz and 3 Hz. Ferroelectriclike hysteresis loops were observed in BF with low electric field and 2.5 and 5 mol % BZT doped ceramics. [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions (MTJ) were fabricated with high oxygen-plasma power and the effects of annealing on the temperature dependence of tunneling magnetoresistance (TMR) were investigated experimentally. As grown, TMR increases, peaks around 160 K, and decreases with increasing temperature from 80 K to 300 K. When MTJs are annealed, T[sub max], the temperature at which maximum TMR is obtained, decreases as annealing temperature increases to the optimal point. In order to explain the temperature dependence of TMR, the difference of conductance between parallel and antiparallel alignments of magnetizations as a function of temperature is also analyzed. The shifts of T[sub max] due to annealing are described phenomenologically with spin-dependent transfer rates of electrons through the barrier. [ABSTRACT FROM AUTHOR]

The electroreflectance (ER) spectra of a surface-intrinsic n[sup +]-type doped (100) GaAs have been measured at various polarization angles of the probe beam. Several Franz–Keldysh oscillations were observed above the band-gap energy, thus enabling heavy- and light-hole transitions to be separated by the application of the fast Fourier transform to the ER spectra. From this, the ratios of the amplitudes of the light- to heavy-hole transitions versus angle of polarization were obtained. At a large incident angle (80°), the strength of the field of the probe beam in the normal direction of the sample (F[sub z]) was varied from zero to a larger component. It was found that the ratios increased with increasing F[sub z] which is consistent with the theory that the light-hole transition becomes more enhanced with z-polarized light. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Piezoelectric coefficient (d33) of (Na,K)NbO3 (NKN) is enhanced not only at its morphotropic phase boundary (MPB) composition but also enhanced at its polymorphic phase transition (PPT) temperature between orthorhombic and tetragonal phases (TO-T ). Thus, for NKN-based ceramics, even higher d33 could be obtained if both MPB and PPT are simultaneously optimized. This temperature as well as composition dependence of piezoelectric properties of NKN-based ceramics requires a systematic approach that differentiates factors for MPB and PPT. In this paper, the roles of Li and Ta known to affect d33 and TO-T were identified in relation with lattice parameters. [ABSTRACT FROM AUTHOR]

We report successful development of both discrete ultraviolet (UV) photodiodes and UV imagers consisting of 8×8 backside-illuminated AlGaN p-i-n photodiode arrays. The discreteAlGaN p-i-n photodiodes showed a responsivity of 0.1 A/W, a cutoff wavelength of 350 nm, an external quantum efficiency of 35%, and a response time of 4.1 ns. Then, 8×8 AlGaN p-i-n photodiode arrays were fabricated, followed by hybridizations with readout integrated circuits manufactured using a 0.5 μm 2poly-3metal N-well complementary metal-oxide semiconductor process by gold stud bumping. The 8×8 focal plane array UV imagers operated successfully by sensing radiation in the 300–350 nm spectral bands. [ABSTRACT FROM AUTHOR]

Effects of Bi nonstoichiometry on (Bi0.5+xNa)TiO3 (BNT) ceramics were investigated at x=-1-+2 mol % of Bi0.5 covering Bi deficiency and excess. At all compositions, rhombohedrally symmetric BNT perovskite formed without secondary phases. Increasing x caused smaller grains. Higher piezoelectric coefficient (d33) but lower depolarization temperature (Td) occurred at Bi excess than at Bi deficiency and vice versa. Leakage current at room temperature decreased with decreasing x. Electrical conductivity of the stoichiometric BNT (x=0) from 700 to 900 °C increased with decreasing partial oxygen pressure from 1 to 10-5 atm suggesting n-type conductivity at elevated temperatures. [ABSTRACT FROM AUTHOR]

Structure property relations in (1-x)(Bi0.5Na0.5)TiO3-xBaTiO3 ceramics were investigated at x=0–10 mol %. A solid solution with either rhombohedral or tetragonal symmetry was confirmed with a morphotropic phase boundary (MPB) at x=5–7 mol %. Grains became small with increasing x possibly due to grain boundary pinning by BaBi′–BaNa•. Lattice distortion was a key factor affecting piezoelectric coefficient (d33) and depolarization temperature (Td). Both rhombohedral 90-α and tetragonal cT/aT lattice distortion became smaller toward the MPB causing poling by field and depoling by temperature to become easier subsequently resulting in higher d33 but lower Td, and vice versa. [ABSTRACT FROM AUTHOR]

Effects of K nonstoichiometry and Li addition on piezoelectric and dielectric properties of (Bi0.5K0.5+xLiy)TiO3 ceramics were studied in the range of x=-8–1 and y=0–5 mol % from the stoichiometric K0.5. A tetragonal perovskite structure was maintained at all compositions despite the formation of secondary phases with K deficiency. Piezoelectric coefficient (d33) was improved from 13 pC/N at x=0 mol % to 32 pC/N at x=-5 mol % due to K vacancies facilitating domain walls movement. It was further enhanced showing a peak value of 103 pC/N at x=-5 and y=3 mol % due to the microstructure improved with the tetragonality optimized by Li addition. [ABSTRACT FROM AUTHOR]

InGaN optical confinement layers (OCLs) were introduced into blue-violet AlInGaN-based laser diodes (LDs), resulting in the drastic improvements of lasing performance. Comparing with conventional LD structure, the lowest threshold current density of 2.3 kA/cm2 has been achieved by adding 100-nm-thick InGaN OCLs which represented maximum optical confinement factor. Additionally, we observed the high quantum efficiency and the uniform emission intensity distribution of InGaN quantum wells grown on lower InGaN OCL than on typical GaN layer. Upper InGaN OCL can reduce Mg diffusion from p-type layers to InGaN active region by separating the distance between InGaN quantum wells and p-type layers. [ABSTRACT FROM AUTHOR]

The origin of the electron memory trap in an oxide-nitride-oxide structure deposited on n-type Si is investigated by both capacitance-voltage and deep level transient spectroscopy (DLTS). Two electron traps are observed near 0.27 and 0.54 eV, below the conduction band minimum of Si and are identified as the nitride bulk trap and the Si–SiO2 interfacial trap, respectively. The trap depth, viz., vertical distribution of the electron trap, in both nitride bulk and Si–SiO2 interface, are also estimated from the bias voltage dependent DLTS. [ABSTRACT FROM AUTHOR]

We demonstrated the long wavelength (485 nm) lasing of InGaN laser diodes under continuous wave condition at room temperature over 10 mW. Two InGaN laser structures were adapted with different indium composition for InGaN optical confinement layers (OCLs) below quantum wells. The blue shift of electroluminescence (EL) was reduced in InGaN laser diodes grown on 3% In concentration in InGaN OCL compared with 1.5% In concentration in InGaN OCL. The EL peak for laser diode with 3% In concentration in InGaN OCL occurs at longer wavelength for all current levels compared to the laser with 1.5% In concentration in InGaN OCL. In addition, the laterally nonuniform InGaN wells grown on 1.5% In concentration in InGaN OCL was verified by the cross-sectional view of InGaN active layer using high-resolution transmission electron microscopy. [ABSTRACT FROM AUTHOR]

The authors report the improvement of InGaN/GaN light-emitting diodes on Al reflectors, commonly used as n-type GaN contacts. A Cu-doped indium oxide (CIO) (5 nm)/indium tin oxide (ITO) (380 nm) interlayer was deposited and annealed at 500 °C, after which an Al (400 nm)/Ti–W (30 nm) layer was sputtered on the ITO interlayer to reflect the light. The reflectance of CIO/ITO/Al/Ti–W was ∼92% at 460 nm, higher than that of the popular Ni/Ag/Pt scheme, and the forward voltage was 3.2–3.3 V, similar to that of the Ni/Ag/Pt contact. Furthermore, the mean leakage current of CIO/ITO/Al/Ti–W was 0.12 μA, much lower than 0.54 μA of Ni/Ag/Pt at -5 V. [ABSTRACT FROM AUTHOR]

The authors report the improvement of GaN light-emitting diodes (LEDs) by applying a ZnNi/indium tin oxide (ITO) (5 nm/380 nm) electrode with high transparency and low resistance to p-GaN. The Pt/ITO (5 nm/380 nm), Ni/Au/ITO (2.5 nm/5nm/380 nm), and Ni/Au (2.5 nm/5 nm) electrodes were prepared and annealed at 400, 500, and 600 °C for 1 min in air. The ZnNi/ITO contacts showed the lowest specific contact resistance of ∼1.27×10-4 Ω cm2 and the highest transmittance of ∼90% at 460 nm. LEDs fabricated with ZnNi/ITO p electrodes showed the best performance with a forward voltage of 3.28 V and a typical brightness of 11. 7 mcd at 20 mA. [ABSTRACT FROM AUTHOR]

We report stable temperature characteristics of threshold current and output power in InGaN blue laser diodes emitting around 450 nm. The threshold current is changed by <3 mA in operation temperature range from 20 to 80 °C, and even negative characteristic temperature is observed in a certain temperature range. This peculiar temperature characteristic is attributed to originate from unique carrier transport properties of InGaN quantum wells with high In composition, which is deduced from the simulation of carrier density and optical gain. In addition, slope efficiency is also maintained well and wall plug efficiency is even improved as temperature increases. [ABSTRACT FROM AUTHOR]

The thermophysical ratio of constant pressure heat capacity to hemispherical total emissivity cP(T)/εT(T) of liquid Mo (Tm=2895 K) was analytically estimated from the radiative cooling curve measured during containerless melting and solidification via electrostatic levitation. cP(T) and εT(T) were then derived to describe the temperature-dependent cooling behavior of liquid Mo before nucleation. Spherical Mo liquids by surface tension kept their spherical shapes during containerless solidification becoming single crystals confirmed by the truncated octahedron bound by eight {111} planes exposed by preferential macroetching and the spotty Laue patterns by backreflection. [ABSTRACT FROM AUTHOR]

Spherical Nb (T[sub m]= 2750 K) single crystals were grown via containerless electrostatic levitation (ESL). Samples became spherical at melting in levitation and undercooled typically 300-450 K prior to nucleation. As-processed samples were still spherical without any macroscopic shape change by solidification showing a uniform dendritic surface morphology. Crystallographic {111} planes exposed in equilateral triangular shapes on the surface by preferential macroetching and spotty back-reflection Laue patterns confirm the single crystal nature of the ESL-processed Nb samples. No hysteresis in magnetization between zero field and field cooling also implies a clean defect-free condition of the spherical Nb single crystals. [ABSTRACT FROM AUTHOR]

Presents an erratum for the article 'Deriving thermophysical properties of liquid Nb by containerless electrostatic levitation,' published in the March 2003 issue of the 'Journal of Applied Physics.'

The effects of carbon on TlBa2Ca2Cu3Ox phase formation in the TlF–Ba2Ca2Cu3Oz system were investigated by varying the carbon contents in Ba2Ca2Cu3Oz precursor powders that were mixed with TlF and reacted to form the superconducting phase. It was found that carbon induced Tl2Ba2Ca2Cu3Oy phase formation resulting in superlattices formed by c-axis intergrowths between TlBa2Ca2Cu3Ox and Tl2Ba2Ca2Cu3Oy. The superlattices were highly regular and uniform throughout the samples. While TlBa2Ca2Cu3Ox was the main phase at low carbon content (0.231 wt %), Tl2Ba2Ca2Cu3Oy became the dominant phase at high carbon content (0.472 wt %) even with a 1:2:2:3 (Tl:Ba:Ca:Cu) starting composition. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents a correction for an article published in the 1996 issue of the 'Journal of Chemical Physics,' which focuses on the theoretical study of geometrical and electronic structures of pi-conjugated thiopene copolymers.

A- and B-site ratios in (Na0.5+y+zK0.5-y)(Nb1-xTax)O3 ceramics with ABO3 structure were investigated to differentiate the effects of polymorphic phase transition (PPT), morphotropic phase boundary (MPB), and Na nonstoichiometry on piezoelectric coefficient (d33). MPB was intrinsically determined by the ratio between Na and K, whereas PPT was extrinsically controlled by Ta substitution. PPT temperature between orthorhombic and tetragonal phases (TO-T) was reduced to room temperature at x = 0.45. d33 was increased to 310 pC/N at y = 0.03 and z = 0 by the combined effects of PPT and MPB and was further enhanced to 333 pC/N by the Na nonstoichiometry at z = 0.015 with improved density. [ABSTRACT FROM AUTHOR]

A reliable Ir diffusion barrier was prepared on polysilicon plugged substrate with a contact size of 0.6 μm. Using a Ti adhesion layer and stress-relief process, it was possible to integrate the Ir barrier into a high density 4 Mb ferroelectric random access memory device. After heat treating sol-gel derived Pb(Zr[sub 1-x]Ti[sub x])O[sub 3] (PZT) films at 700 °C, the Ir barrier contact displayed an ohmic behavior and showed a low resistance of 130 Ω per contact in 1k serial contact array. The PZT films on Pt/IrO[sub 2]/Ir poly-plugged substrate exhibited excellent ferroelectric properties such as remnant polarization and coercive voltage of 25 μC/cm2 and 1.15 V, respectively. Auger depth profile and transmission electron microscopy analyses confirmed that no appreciable oxidation was formed between the Ir barrier and the polysilicon plug. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]