In this contribution we present a critical review of experimental data and theoretical considerations concerning the stimulated emission in ZnO. The main processes discussed in literature leading to stimulated emission at RT are inelastic exciton-exciton scattering (P-band) and recombination of an inverted band-to-band transition in an electron-hole plasma (EHP). While the latter process becomes dominant at RT for densities close to 10(19) cm(-3) theory predicts a lower threshold for inelastic exciton-free carrier scattering compared to the P-band [1]. In addition to that the exciton-nLO phonon process might also have a lower threshold than the P-band. A detailed analysis of the temperature dependence of the band gap and of the homogenous broadening of the exciton resonance allows us to distinguish between the various processes but also casts some doubt on the frequently given claim of excitonic RT lasing in ZnO. Therefore we argue that inelastic scattering processes with carriers, phonons or plasmons in a still nondegenerate and strongly coulomb correlated carrier gas are relevant processes for stimulated emission at RT.

By means of time resolved spectroscopy we compare two samples of ZnO nanorods with respect to their suitability as stimulated emitters. In the case of narrow nanorods their wave guiding quality causes a suppression of exciton–exciton scattering whereas no laser emission is detectable. Unlike their narrow counterparts, wide nanorods not only benefit from a larger overlap of the guided mode with the gain medium but a variation in VLS growth results in gold nanoparticles being present at the bottom of nanorods. Consequently, laser emission from single wide rods is evidenced up to 150 K. In addition to experimental studies we carry out 3D numerical simulations of the electric field distribution to evaluate the influence of gold nanoparticles at the nanorod/substrate interface. This finite element analysis confirms that gold leads to an enhancement of confinement within the resonator. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on an experimental study of the temperature dependent band gap shift and the homogeneous line broadening of the excitonic luminescence in ZnO. This data is derived from photoluminescence (PL) measurements at temperatures from T = 10 - 290 K and fitting the PL line shape with a theoretical model for the exciton line and its LO-phonon replica. Additionally, transmission measurements are utilized to extend the data of the temperature dependent band gap into the temperature range from 290 K up to 800 K. The experimentally determined shift is compared with one that is obtained numerically from the phonon density of states, the pressure dependent band gap, the thermal expansion coefficient and the bulk modulus with only a single fit parameter. The interaction of excitons with phonons leads to a homogeneous broadening of the excitonic emission which reaches values of 40 meV at room temperature. This mechanism results in a substantially increased threshold for stimulated emission from every process involving excitons. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We investigate the lineshape of the zero-phonon luminescence of the A-exciton polariton and of the LO-phonon replicas in ZnO and their dynamics. The lineshape verifies details of the A-exciton polariton model and confirms the inverted band structure (AΓ7, BΓ9, CΓ7). The dynamics give information on relaxation, diffusion and recombination processes. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In the first chapters we give some short comments on the history of ZnO research and on growth, doping, transport, and deep centers, topics which will be covered in more detail by other contributions to this symposium "ZnO-rediscovered". Then we concentrate in a first main chapter on the band-structure, excitons, and polaritons in the regime of linear optics. The second main chapter deals with many particle effects and lasing. It is followed by a short conclusion and outlook.

We report on an experimental study of macroscopic energy transfer in ZnO bulk crystals. We observe the ultraviolet photoluminescence (PL) emission from the near band edge (3.4eV at room temperature), a green emission band from a deep center transition, and an orange emission band. Unusually, the orange PL is not only visible at the excitation spot and at crystal edges, but we also observe a slow spatial expansion of the light emission in the orange band up to a distance of several millimeters. In contrast to that, the green and ultraviolet emission originate only from the excitation spot or are scattered from the crystal edges. We investigate the temporal dynamics of the orange PL as a function of the distance from the laser spot, by turning on and off the laser excitation. We describe the results with a two-dimensional diffusion model and discuss some possible mechanisms that may cause this energy transfer, such as Förster/Dexter transfer or a thermally induced hopping process.

In an introductory chapter we shortly review the history of ZnO research and the motivations for the present renaissance of the worldwide interest in this II–VI compound. Then we concentrate on the following topics: band structure, excitons and polaritons, luminescence dynamics, high excitation effects like biexcitons or the transition to an electron–hole plasma, and finally lasing. We finish with a short conclusion and outlook.

We report on an experimental study of the temporal photoluminescence dynamics of high-quality ZnO nanopillars from 10K to room temperature. We find that defect states play an important role in the time evolution of the photoluminescence signal. At low excitation intensities capture into defects dominates the time dependence of the PL, at higher intensities they are saturated and the intrinsic excitation decay is observed. We separate the intrinsic exciton decay from the fast nonlinear M-band with the method of decay associated spectra and obtain the temperature dependence of the intrinsic exciton decay. High excitation measurements show a reduced exciton–exciton scattering in these thin nanorods.

We report on an experimental study of the temporal photoluminescence dynamics of high-quality ZnO nanopillars from 10 K to room temperature. We find that defect states play an important role in the time evolution of the photoluminescence signal. At low excitation intensities capture into defects dominates the time dependence of the PL, at higher intensities they are saturated and the intrinsic excitation decay is observed. We separate the intrinsic exciton decay from the fast nonlinear M-band with the method of decay associated spectra and obtain the temperature dependence of the intrinsic exciton decay. High excitation measurements show a reduced exciton–exciton scattering in these thin nanorods.

The temporal dynamics of the exciton photoluminescence (PL) in ZnO nanorod samples was investigated experimentally as a function of temperature and excitation intensity. Excitonic photoluminescence is observed up to room temperature. The excitation dependence of the PL dynamics reveals a saturable non‐radiative recombination center. Under high excitation conditions the time‐resolved photoluminescence shows two components: the ZnO M‐band which decays with a temperature independent sub‐100 ps time constant, and the intrinsic exciton PL with a time constant of several 100 ps increasing with temperature. Exciton‐exciton scattering effects are notably absent, which is attributed to the reduced polariton phase space resulting from the small nanorod diameter of 50 nm.

We have studied the temperature dependence of the photoluminescence (PL) spectra of molecular beam epitaxy grown ultrathin Zn1-xCdxSe/ZnSe quantum wells with random and inhomogeneous Cd distributions over cation sublattice within the temperature interval 2-300 K. Depending on the Cd concentration, the PL band maximum position E-max(PL)(T) follows either a "normal" or an "anomalous" (known as "S-shaped") temperature dependence. We have analyzed both dependences in detail for a model of an island ensemble which can be characterized by a single-mode distribution of the most important parameters governing the optical properties of the quantum well. We demonstrate that the anomalous behavior arises due to the strong temperature dependence of the lifetimes of a family of metastable states participating in formation of the PL band at low temperatures. The metastablility of some island states is ascribed to a complex topological structure of the islands. The mechanism of the exciton-phonon interaction responsible for the fast decrease of the lifetime of these states with the increase of temperature has the same origin as the mechanism leading to the vanishing of narrow lines in mu-PL. We also present results of time-resolved experiments which yield the shift of the PL band for hot excitons cooling in a cold lattice.

We investigate ZnO epitaxial layers grown by MBE (Molecular Beam Epitaxy) and MOVPE (Metal Organic Vapor Phase Epitaxy) techniques. The samples show similar optical behavior in temperature dependent photoluminescence measurements, reflection and photoluminescence excitation spectroscopy in the low density regime. High excitation measurements show different behavior. While the MBE sample leads to stimulated emission from the exciton‐exciton‐scattering, an electron hole plasma is formed in the MOVPE sample which leads to stimulated emission at higher excitation intensities. The gain value measured by the variable stripe length method is much higher for the MBE grown sample. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

CdS/ZnSe is a quaternary type II system, which allows to tune the luminescence over a considerable fraction of the visible spectrum. We present here theoretical and experimental investigations of the density dependent blue shift of the emission of single quantum wells under cw excitation and on the luminescence dynamics after pulsed excitation as a function of the US well width.

physica status solidi (c)Volume 0, Issue 5 p. 1544-1547 Original Paper Exciton states and energy relaxation in ZnCdSe nano-islands A. Reznitsky, Corresponding Author A. Reznitsky alexander.reznitsky@physik.uni-karlsruhe.de Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, Germany A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaPhone: +49-721 608-7479, Fax: +49-721 608-8480Search for more papers by this authorA. Klochikhin, A. Klochikhin Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, Germany Petersburg Nuclear Physics Institute, 188350 St. Petersburg, RussiaSearch for more papers by this authorH. Priller, H. Priller Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorB. Dal Don, B. Dal Don Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorG. Schwartz, G. Schwartz Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorH. Zhao, H. Zhao Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorH. Kalt, H. Kalt Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorC. Klingshirn, C. Klingshirn Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorS. Permogorov, S. Permogorov A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorL. Tenishev, L. Tenishev A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorI. Sedova, I. Sedova A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorS. Sorokin, S. Sorokin A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorS. Ivanov, S. Ivanov A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this author A. Reznitsky, Corresponding Author A. Reznitsky alexander.reznitsky@physik.uni-karlsruhe.de Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, Germany A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaPhone: +49-721 608-7479, Fax: +49-721 608-8480Search for more papers by this authorA. Klochikhin, A. Klochikhin Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, Germany Petersburg Nuclear Physics Institute, 188350 St. Petersburg, RussiaSearch for more papers by this authorH. Priller, H. Priller Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorB. Dal Don, B. Dal Don Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorG. Schwartz, G. Schwartz Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorH. Zhao, H. Zhao Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorH. Kalt, H. Kalt Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorC. Klingshirn, C. Klingshirn Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorS. Permogorov, S. Permogorov A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorL. Tenishev, L. Tenishev A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorI. Sedova, I. Sedova A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorS. Sorokin, S. Sorokin A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorS. Ivanov, S. Ivanov A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this author First published: 04 August 2003 https://doi.org/10.1002/pssc.200303227Citations: 6AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract We suggest a new concept of exciton states in nano-islands based on the study of optical spectra, namely the temperature dependence of cw photoluminescence (PL) and PL excitation (PLE), as well as PL spectra and kinetics under ps pulsed excitation. The scheme of the exciton states in nano-islands is as follows: (I) deep spatially isolated ground states, (II) excited meta-stable states the energy relaxation rate of which strongly depends on the temperature, and (III) the states extending over the whole island size. We show that a redistribution of the population between meta-stable and ground states is responsible for the anomalous temperature shift of the PL band in QWs with islands. Citing Literature Volume0, Issue5August 2003Pages 1544-1547 RelatedInformation

Earlier we reported the investigation of the electrical properties of selectively doped and degenerate CdS/ZnSe quantum heterostructures grown by molecular beam epitaxy [V. Kažukauskas, M. Grün, St. Petillon, A. Storzum, and C. Klingshirn, Appl. Phys. Lett. 74, 395 (1999)]. The maximum Hall mobilities in these heterostructures were found to be less than 400 cm2/Vs. In the present work we analyze in detail the scattering mechanisms in order to increase the carrier mobility and to optimize these quantum structures. We demonstrate that the Hall mobility can reach in the CdS quantum wells at low temperatures 2800 cm2/V s for slightly doped structures, having an effective sheet carrier density 2.6×1011 cm−2. In these structures the mobility is mostly limited by interface alloying scattering. At high doping levels carriers become redistributed between the quantum well and the ZnSe doped layer. This causes the parallel conductivity phenomena, which diminishes the effective mobility. Near room temperature the scattering by optical phonons prevails which is superimposed by dislocation scattering.

Photoluminescence of the ground and excited states of type II CdS/ZnSe single quantum wells with different width is investigated for high electron densities and by means of time resolved photoluminescence measurements. For high excitation intensities the filling of states in the QW up to energies of the second excited state can be observed. The PL decay times of the ground state vary from 2-36 ns for different samples and depend strongly on the QW width. This dependence can be explained with the overlap of electron and hole wavefunctions. The PL rise time of the ground state is influenced by inter- and intrasubband relaxation, which is induced by the cooling of hot carriers.