Several hundred thousands of tons of ZnO are used by per year, e.g. as an additive to concrete or to rubber. In the field of optoelectronics, ZnO holds promises as a material for a blue/UV optoelectronics, alternatively to GaN, as a cheap, transparent, conducting oxide, as a material for electronic circuits, which are transparent in the visible or for semiconductor spintronics. The main problem is presently, however, a high, reproducible and stable p-doping. We review in this contribution partly critically the material growth, fundamental properties of ZnO and of ZnO-based nanostructures, doping as well as present and future applications, with emphasis on the electronic and optical properties including stimulated emission. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

ZnO : From basics towards applications

Deformation Potentials ofk = 0 States of Tetrahedral Semiconductors

Caracterisation optique et physique de ZnSe. Etude des macles et des deformations thermoelastiques d'une couche ZnSe sur un substrat. Etude des excitons libres dans des films de ZnSe, des impuretes et defauts dans la couche de ZnSe epitaxique, et de leurs proprietes optiques non lineaires

Optical Properties of ZnSe Epilayers and Films

The optical properties of strained‐layer ZnSe/Zn0.86Cd0.14Se single quantum wells have been studied. The photoluminescence under direct and indirect excitation is investigated in detail. The temperature dependence of photoluminescence and resonant Raman scattering are investigated. Very strong 2LO‐phonon Raman scattering has been observed with Zn0.86Cd0.14Se quantum wells, where the scattered photon energy is in resonance with an exciton transition. Experimental exciton energies are compared with a finite‐square‐potential quantum‐well model including band nonparabolicity and the strain effect. Based on Hill’s theory [J. Phys. C 7, 521 (1974)] we have computed the band gap of Zn1−xCdxSe as a function of composition x.

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Excitonic and Raman properties of ZnSe/Zn1−xCdxSe strained‐layer quantum wells

A brief description is given of the growth of MOCVD ZnSe and ZnS, contrasted with alternative growth techniques. The general physical properties of these layers, grown as single crystals on GaAs or GaP and as polycrystalline layers on glass, are described; particularly smoothness, crystal structure, and doping by donors and by Mn luminescence activators. Techniques of optical characterization and the general optical properties are discussed. Radiative recombination processes involve shallow donor species, and shallow acceptors when present, and these spectra are used to identify residual donor species. Effects of strain associated with heteroepitaxial growth of ZnSe on GaAs and GaP are evident and provide quantitative estimates of the lattice mismatch-induced strain provided this is relatively small. The spectra contain a variety of low energy luminescence bands, weak in well-prepared MOCVD layers. Some of these bands seem particularly characteristic of the MOCVD growth technique. Amongst these, two bands Y and Z have very unusual aspects of spectral form. A speculative model is offered, of recombinations within extended defects involving a non-central force electronic system. Strong variations in the form and intensity of photoluminescence spectra are described for very thin single crystal layers and attributed to the influence of electric fields on distant donor acceptor pair processes, particularly at the heteroepitaxial interface. These results and variations in the deep centre luminescence observed as a function of deliberate doping by shallow donors, D suggest the strong influence of (DZn–Vzn–Dzn) associates at high D concentrations or high electric fields. Some properties of dead layers in thin film electroluminescence are briefly discussed.



Es wird das Wachstum von MOCVD-ZnSe und -ZnS diskutiert und anderen Wachstumstechniken gegenubergestellt. Dabei werden die allgemeinen physikalischen Eigenschaften dieser Schichten, die als Einkristalle auf GaAs oder GaP und als polykristalline Schichten auf Glas wachsen, beschriebeu, insbesondere Ebenheit, Kristallstruktur und Dotierung mit Donatoren und Mn-Lumineszenzaktivatoren. Die Methoden zur optischen Charakterisierung und die allgemeinen optischen Eigenschaften werden diskutiert. Der Prozes der strahlenden Rekombination schliest flache Donatorarten und, wenn vorhanden, flache Akzeptoren ein, und diese Spektren werden benutzt, um die Restdonatoren zu identifizieren. Spannungseffekte, die mit dem heteroepitaktischen Wachstum von ZnSe auf GaAs und GaP zusammenhangen, werden aufgezeigt und liefern quantitative Ergebnisse der durch Gitterfehlanpassung induzierten Spannung, vorausgesetzt, das diese relativ klein ist. Die Spektren enthalten eine Vielzahl von niederenergetischen Lumineszenzbanden, die in gut hergestellten MOCVD-Schichten schwach sind. Einige dieser Banden scheinen fur die MOCVD-Wachstumsmethode besonders charakteristisch zu sein. Von diesen haben zwei Banden, Y und Z, sehr ungewohnliche Aspekte der Linlenform. Ein spekulatives Modell von Rekombinationen innerhalb ausgedehnter Defekte, die ein Elektronensystem ohne Zentralkraft enthalten, wird vorgeschlagen. Starke Variationen der Form und Intensitat der Photolumineszenzspektren von sehr dunnen Einkristallschichten werden dem Einflus von elektrischen Feldern auf Rekombination zwischen entfernten Donator-Akzeptorpaare, insbesondere an den heteroepitaktischen Grenzflachen, zugeschrieben. Diese Ergebnisse und Aderungen der Lumineszenz tiefer Zentren, die in Abhangigkeit von der Dotierung mit flachen Donatoren D, beobachtet werden, weisen auf den starken Einflus von (DZn–VZn–DZn) -Assoziaten bei hohen D-Konzentrationen oder hohen elektrischen Feldern hin. Einige Eigenschaften von Totschichten bei der Dunnschicht-Elektrolumineszenz werden kurz diskutiert.

Comparison of MOCVD‐Grown with Conventional II‐VI Materials Parameters for EL Thin Films)

The effect of the spin-exchange interaction between electron and hole is investigated for the case of excitons originating from one of the $p$-like valence bands and an $s$-like conduction band, as is the case for IIb-VIb compounds. A general exciton matrix is constructed, starting from the work of Pikus. It includes spin-orbit, crystal-field, spin-exchange, and deformation-potential interactions. Use of this matrix then allows a theoretical fit to our experimental data which describes the shift of exciton levels under uniaxial pressure in ZnO, CdS, and CdSe. This fit results in the determination of six deformation potentials, two spin-orbit parameters, the crystal-field parameter, and the exchange parameter. The general theory, when adapted to the zinc-blende structure, allows us to fit our data on cubic ZnS and ZnSe, resulting in a determination of two deformation potentials and the spin-exchange parameter for each compound.

Takao Koda

Papers

Spin Exchange in Excitons, the Quasicubic Model and Deformation Potentials in II-VI Compounds