Electronic-transport properties of unhydrogenated amorphous gallium arsenide

Summary Hall mobility, μ_H, and electrical conductivity, σ, of unhydrogenated amorphous-gallium-arsenide films, prepared by r.f. sputtering, have been measured. Conductivity as a function of temperature shows a variable-range hopping mechanism atT<260 K, while at high temperature, conductivity and Hall mobility are both thermally activated. The results are interpreted in terms of the presence of defect complexes due to an excess of Ga. The stoichiometry and the structure of the films are used to explain the behaviour and the values of μ_H. The values of the activation energy of the conductivity seem in agreement with theoretical calculations on the position of electronic states created by defect complexes in the mobility-gap of a-GaAs.     

光声谱研究多孔碳化硅的能带特性

报道了用UV光照射和不用UV光照射条件下形成的p型α PSC以及原始SiC的光声光谱（PAS）.从光声Rosenwaig Gersho理论出发，计算出多孔SiC的吸收系数与能量的关系，得到多孔SiC的能隙低于原始SiC的能隙，并深入分析了能隙的变化原因，同时，对吸收边附近的吸收情况进行了讨论. 关键词： 光声光谱 多孔碳化硅 能隙     

Transparent manganite films as hole injectors for organic light emitting diodes

Organic light-emitting diodes (OLEDs) are nowadays one of the most attractive devices based on organic semiconductors due to their successful application in the display technology. Electroluminescence in OLEDs is mainly governed by the fluorescence from excited singlet states, which have large transition probabilities providing the major radiative pathway. The “forbidden” triplet state emission can be activated by increasing spin–orbit coupling via dye doping. The singlet–triplet exciton formation statistics is usually given by 1:3 partition due to the quantum constrains.

Injection of carriers with finite spin polarisation should influence and modify the recombination statistics and can be used for tuning of the device efficiency. In this context, the development of a new class of electrodes able to guarantee both efficient charge and spin injection becomes of paramount importance. We show that strongly spin polarised colossal magnetoresistance manganite La_0.7Sr_0.3MnO₃ (LSMO) can successfully substitute conventional ITO electrodes in OLEDs. Highly transparent, metallic and ferromagnetic LSMO layers were used in combination with standard Al and spin polarised Co top electrodes. Electrical and optical characterisations of the OLEDs with spin polarised electrodes indicate the applicability of the new manganite electrodes for organic light-emitting devices.     

Photoelectric properties ofp-CuInSe2 crystals

Spectra of the photoelectromotive force of p-CuInSe₂ single crystals obtained by directional crystallization of a stoichiometric-composition melt in an upright ampoule have been investigated by the method of phase-sensitive detection. A difference in the phase shifts between the exciting light and the arising photoelectromotive force in the extrinsic (impurity) and intrinsic regions of optical absorption is determined. The lifetime in the region of optical self-absorption is found to equal τ=15·10⁻⁶ at T=77 K. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 129–131, January–February, 1997.     

A comparative study of nonrelativistic and relativistic 5d atoms as impurities in silicon

A comparison is made between calculations performed nonrelativistically and relativistically for W, Re, Os, Ir, Pt, Au and Hg as substitutional impurities in silicon. The calculations were carried out using the relativistic extended Hückel method. The direct and indirect relativistic effects upon the 5d-like levels and band-gap levels are analysed.     

Energy gaps and elementary excitations for certain VBS-quantum antiferromagnets

It is shown for a class of antiferromagnetic Hamiltonians how one can get lower bounds for the energy gap above the ground state by diagonalizing a finite system. This method is applied to certain spin chains (including a spin-1 chain). Trial wave functons are proposed for the elementary excitations and are tested in the case of the spin-1 chain.     

Cuprite, paramelaconite and tenorite films deposited by reactive magnetron sputtering

Copper oxides films (Cu₂O, Cu₄O₃ and CuO) have been deposited by magnetron sputtering of a copper target in various Ar–O₂ reactive mixtures. The films are characterized by X-ray diffraction, scanning electron microscopy, four-point probe method and UV-Vis spectrometry. The three defined compounds in the Cu---O binary system can be deposited by varying the oxygen flow rate introduced into the reactor. All the films are crystallized with a mean crystal size ranging from 10 to about 35 nm. They are highly resistive and present a direct optical band gap higher than 2 eV. The application of a bias voltage during the deposition phase modifies the texture of the Cu₂O films and also induces a preferential resputtering of oxygen from the Cu₄O₃ ones. This resputtering phenomenon leads firstly to the occurrence of the cuprite phase mixed with the paramelaconite one and secondly to the amorphisation of the films. Finally, the thermal stability in air of cuprite, paramelaconite and tenorite films has been investigated. The results show that the stability of Cu₂O and Cu₄O₃ films in air is influenced by the thickness and/or the texture of the films. Tenorite films with a low optical band gap (1.71 eV) can be formed after air annealing at 350 °C of an unbiased cuprite film.     

A novel method to measure the cell gap and pretilt angle of a reflective liquid crystal display

A powerful method for the phase retardation measurement of a reflective liquid crystal cell is presented. The proposed method is based on the heterodyne interferometer theory for probing cell parameters. Due to common-path optical arrangement, a fast measurement with high accuracy and sensitivity was possible. Both theoretical analysis and experimental procedures are presented. The experimental results agree well with that obtained theoretically.     

Origin of ferromagnetism and nano-scale phase separations in diluted magnetic semiconductors

This paper reviews various origins of ferromagnetic response that has been detected in diluted magnetic semiconductors (DMS). Particular attention is paid to those ferromagnetic DMS in which no precipitation of other crystallographic phases has been observed. It is argued that these materials can be divided into three categories. The first consists of (Ga,Mn)As and related compounds. In these solid solutions the theory built on p–d Zener's model of hole-mediated ferromagnetism and the Kohn–Luttinger kp theory of semiconductors describes quantitatively thermodynamic, micromagnetic, optical, and transport properties. Moreover, the understanding of these materials has provided a basis for the development of novel methods enabling magnetisation manipulation and switching. To the second group belong compounds, in which a competition between long-range ferromagnetic and short-range antiferromagnetic interactions and/or the proximity of the localisation boundary lead to an electronic nano-scale phase separation that results in characteristics similar to colossal magnetoresistance oxides. Finally, in a number of compounds a chemical nano-scale phase separation into the regions with small and large concentrations of the magnetic constituent is present. It has recently been suggested that this spinodal decomposition can be controlled by the charge state of relevant magnetic impurities. This constitutes a new perspective method for 3D self-organised growth of coherent magnetic nanocrystals embedded by the semiconductor matrix.     

Error Bounds of Regularized Gap Functions for Nonsmooth Variational Inequality Problems

We study the Clarke–Rockafellar directional derivatives of the regularized gap functions (and of some modified ones) for the variational inequality problem (VIP) defined by a locally Lipschitz but not necessarily differentiable function on a closed convex set in an Euclidean space. As applications we show that, under the strong monotonicity assumption, the regularized gap functions have fractional exponent error bounds and consequently that the sequences provided by an algorithm of Armijo type converge to the solution of the (VIP). The research of this author was supported by an Earmarked Grant from the Research Council of Hong Kong.