Wide Gap Oxide Research Articles

Luminescence of free and self-trapped excitons in wide-gap oxides

Behaviour of the intrinsic emissions of 7.6, 3.45 and 3.6 eV in Al 2O 3, Y 2O 3 and Sc 2O 3, respectively, has been studied under a crystal excitation by 5–35 eV photons at 8–80 K. In Al 2O 3 and Y 2O 3, electrons and holes do not undergo transformation into a self-trapped state, while self-shrunk excitons are formed at the direct optical creation of excitons or at the recombination of electrons with holes under the conditions of a high excitation density. In line with the theoretical assumption made by Sumi, manifestations of the photocreation of spatially correlated self-trapped d-electrons and p-holes are revealed in the spectra of the tunnel phosphorescence of Sc 2O 3.

Relaxation, self-trapping, and decay of electron excitations in wide-gap oxides

Emission spectra ofMgO, Al 2O3, Y2O3, andSc 2O3 crystals, excited at 8 K by photons with energies 5–40 eV, are measured. The luminescence of free excitons andGe 2+ centers forMgO, the self-trapped exciton emission (STEE) forAl 2O3 andY 2O3, and the tunnel self-luminescence forScO 3 are studied. A distinction between hole polarons in oxides and self-trapped holes in halides leads to a sharp difference in the mechanisms of STEE creation on electronhole recombination for these two classes of solids. The electron-hole, hole-electron, and exciton mechanisms of excitation multiplication are identified. The high radioresistance of wide-gap oxides and the behavior of oxygen interstitials inMgO crystals are discussed.

Spectroscopic and Photoluminescence Studies of a Wide Band Gap Insulating Material: Powdered and Colloidal ZrO2 Sols

Powdered zirconia and colloidal zirconia aqueous sols have been examined by diffuse reflectance and absorption spectroscopies and by photoluminescence methods in solid/gas and solid/liquid systems. The former system was examined following high-temperature treatment in vacuo and under reducing and oxidizing atmospheres. Studies of the influence of H2 and O2 on the photophysics of microparticles (powder) and nanoparticles (colloidal sols) of zirconia at solid/gas interfaces and the effects of free carrier scavengers (CH3OH and O2) on the photophysics at solid/liquid interfaces were undertaken to explore the correlation between surface chemistry and the nature of preexisting or photogenerated defect centers (e.g., F-type and V-type color centers). ZrO2 is an insulating, direct wide-gap metal oxide with an optical band gap of ∼5.0 eV; another optical transition occurs at 5.85 eV. The optical behavior depends on whether zirconia is preirradiated in the intrinsic (hν > 5.0 eV) or extrinsic (hν < 5.0 eV) absorpt...

Photoelectric properties of ITO/p +-p −-InP solar cells in linearly polarized light

Indium phosphide heterostructures and transparent conducting films of wide-gap oxides have previously been used in the development of highly efficient solar cells, making it possible to bring their efficiencies up to 18% [M, M. Koltun, Optics and Metrology of Solar Cells [in Russian], Nauka, Moscow (1985); V. M. Botnaryuk, L. S. Gagara, L. V. Gorchak et al., Geliotekhnika 23, 37 (1990); V. Botnariuc, L. Gagara, E. Negru et al., Solar Energy in Romania 2(1), 53 (1993)]. In the present paper results are reported from the first studies of the photoelectric properties, in linearly polarized light, of solar cells consisting of a heterojunction between single crystal indium phosphide and a mixed indium and tin oxide film (ITO film, E-g congruent to 3.6 eV [G. Check and A. Genis, Solid Stale Techol. 23(1), 102 (1980)]). (C) 1998 American Institute of Physics. [S1063-7842(98)01305-1].

Design of Functional Inorganic Materials by Ion Beams. Approach to High Performance Oxide Glassy and Amorphous Materials by Ion Implantation.

Recently, ion implantation has strongly stimulated efforts for the creation of high performance wide gap oxide materials. This article briefly reviews our recent works on the creation of wide gap oxide glassy and amorphous materials with an optoelectronic function by ion implantation along with the background and expectation for a new extension utilizing properties of photo-active structural defects produced by implantation. The topics cover nanometer-sized colloid embedded glasses for nonlinear optical materials, transparent amorphous oxides with metallic conduction, and fast proton-conducting glasses.

Novel oxide amorphous semiconductors: transparent conducting amorphous oxides

A working hypothesis to find wide gap oxide semiconductors was proposed on the basis of simple considerations. The hypothesis predicts that amorphous double oxides composed of heavy metal cations (HMCs) with an electronic configuration (n − 1)d10s0 are promising candidates for a novel class of amorphous semiconductors. Electrical and optical properties of three amorphous double oxides composed of the HMCs, a-AgSbO3, Cd2GeO4 and Cd2PbO4, were examined, following this hypothesis. It was found that when carrier electrons are generated via the formation of oxygen vacancies or doping of excess cations by ion implantation, these three wide band gap amorphous oxides show high electrical conductivities of 10−1 to 102 S cm−1 at ∼ 300 K and the conductivity remains almost constant down to 77 or 4 K for high carrier concentrations (> 1018 cm−3). This high conductivity originates primarily from a large Hall mobility of ∼ 10 cm2 V−1 s−1, which is higher by several orders of magnitude than that in amorphous transition metal oxides, Si:H and chalcogenides. A variety of chemical compositions for a novel oxide amorphous semiconductor are suggested.

Mechanisms of intrinsic and impurity luminescence excitation by synchrotron radiation in wide-gap oxides

The excitation spectra of intrinsic and impurity luminescence have been measured in the spectral region of 12 to 30 eV for SrO, CaO and YAlO s crystals at 8 K. Processes of exciton selftrapping as well as electron-hole and excitonic mechanisms of multiplication of electronic excitations have been analyzed for wide-gap oxides (BeO, MgO, CaO, SrO, Y 2O 3, YAlO 3).

Photoemission studies on valence band structure of AgSbO 3

Valence band structure of sintered AgSbO 3 with defect pyrochlore structure was examined using ultraviolet photoemission spectroscopy (UPS). The upper valence band has a structure with peaks at the binding energies of 2.5 eV, 3.5 eV, 5.5 eV, and 7.5 eV. Constant initial state (CIS) spectra measured on these peaks demonstrated that the upper valence bands are mainly composed of the mixture of occupied O 2 p and Ag 4 d orbitals. Even the top of valence band with the peak at 2.5 eV includes a significant amount of Ag 4 d component. The present result suggests that this material is a good candidate as a novel wide gap and p-type conductive oxide.

Localized states in wide-gap glasses. Comparison with relevant crystals

In this paper, the electronic properties of localized states in wide-gap oxide glasses such as sodium silicate, sodium germanate, lead silicate, silica and germania are reviewed. Experimental spectroscopic data (optical absorption, as well as luminescence and its excitation) and luminescence decay kinetics at different temperatures were used to determine localized states in these glasses. The corresponding data for crystals of these materials were used for comparison. The spectrum of localized states is determined by many different types of a short-range order in the glass structure. In the glasses studied the localized states create an ensemble of structurally non-equivalent anisotropic luminescence centers. Their concentration is on the level of 1%. These states disappear after glass crystallization. The structural elements of localized states are the precursors of radiation-induced defects. In the case of silica, the optical absorption of the precursor is covered by the excitonic absorption band. Reducing conditions transform the localized states into defects.