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Heavily Ga-doped ZnO layers are grown on bulk ZnO wafers by molecular beam epitaxy. The layers grow in a two-dimensional pseudomorphic mode with high structural quality under increase of the c-lattice constant up to free-electron concentrations of cm. Formation of Zn-polar inversion domains in the O-polar ZnO matrix is identified as the limiting factor for the incorporation of electrically active Ga. The domain formation can be inhibited by growing with larger excess of Zn. This results in a shift of surface plasmon frequency of meV and allows for covering the telecommunication wavelength range.

In polyfluorenes it is generally accepted that (photo)degradation leads to fluorenone type defects that accept the excitation energy and emit green-to-yellow light with rather low efficiency. Although initial spectroscopic studies suggest the same to hold true for ladder-type poly(para-phenylene)s (LPPPs), kinetic studies of the degradation process are not compatible with the established mechanism. In general, the observed green emission can be caused by the introduction of carbonyl groups; however, only if associated with an additional disruption of the backbone rigidity and hence planarity of the entire π-system. This is clearly shown by comparison with synthesized model compounds, which are bearing the fluorenone motif yet possess very different optical properties as compared to the defects, which are actually formed. Degradation can be caused by solvent specific, yet substrate nonspecific aromatic formylation but mainly originates from reaction with in-situ generated singlet oxygen, both in solution as well as in thin films. Time-dependent photoluminescence measurements on thin films show that green emission is enhanced by energy transfer from intact molecules to defect centers.

We investigate the coupling of ladder-type quarterphenyl to the photon modes of a dielectric ZrOx /SiOx microcavity at ultraviolet wavelengths. For a relatively long cavity (≈10 μm) with high-reflectivity mirrors (0.998), optically pumped laser action is demonstrated in the weak-coupling regime. We observe single-mode operation with a threshold of 0.4 mJ cm(-2) . Strong coupling is achieved by using a short λ/2 cavity. We find pronounced anti-crossing features of the molecular (0,0) and (0,1) vibronic transitions and the cavity mode in angle-dependent reflectivity measurements providing Rabi splittings of (90±10) meV. All these features occur spectrally resonant to the exciton transition of ZnO demonstrating the potential of ladder-type oligo(p-phenylene)s for the construction of inorganic/organic hybrid microcavities.

Growth of ZnO by molecular beam epitaxy on submonolayer p-sexiphenyl (6P) films consisting of differently shaped and terminated nanocrystals is studied. The morphology and crystallographic structure of ZnO are analysed by cross-sectional transmission electron microscopy (TEM). The inorganic-on-organic interface is remarkably abrupt and the 6P nanocrystals remain unimpaired as revealed by TEM and complementary photoluminescence measurements. The wurtzite ZnO overlayer shows a distinct columnar structure with a preferential c-axis alignment. The influence of the ZnO growth direction and 6P crystal termination on the structural properties is discussed.

The ability of using onchip microcoils to control the electron–nuclear spin system in semiconductors is demonstrated. Electrically generated magnetic fields of several tens of mT can be obtained on a micrometer length scale, which are switchable on a sub-ns time scale due to the low complex coil impedance. This allows one to electrically (i) manipulate the nuclear spins by means of nuclear magnetic resonance in n-GaAs and (ii) control the hyperfine flip-flop rate in CdSe/ZnSe quantum dots.

Doping of ZnO by Ga allows for generation of free electrons up to concentrations of about 1021 cm–3 without significant deterioration of the crystal structure. In this way, a metallic dielectric function is formed with a negative real part tuneable from the mid infrared up to telecommunication wavelengths. The losses are about one order of magnitude lower than in traditional metals. We demonstrate surface plasmon polaritons with dispersion relations that can be engineered in a unique way by utilizing epitaxial multi-layer structures of different doping level. In contrast to the standard air/metal excitations, the surface-plasmon-polariton frequency approaches a finite value in the zero-wavevector limit at metal/metaltype interfaces. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present a systematic non-resonant Raman study of the vibrational properties of the spirobifluorene derivative 2,7-bis(biphenyl-4-yl)-2’,7’-di-tertbutyl-9,9’-spirobifluorene (SP6) on three different substrates: ZnO(000-1), ZnMgO(000-1), and Al2O3(11-20). This investigation of few monolayer coverages is facilitated by the high scattering cross section of this molecule. The vibrational response of SP6 is unaffected by the choice of substrate, showing that the vibrational properties of the molecules remain unchanged by adsorption. Furthermore, we can directly compare the experimental spectra to our density functional theory calculations of the Raman cross sections of SP6 in the gas phase.

We demonstrate the capability of two molecular organic acceptors [1,4,5,8,9,12-hexaazatriphenylenehexacarbonitrile and 2,2\ensuremath{'}-(perfluoronaphthalene-2,6-diylidene)dimalononitrile] to tune the work function (\ensuremath{\Phi}) of intrinsically doped GaN(0001) and for comparison of intrinsically doped ZnO(0001). With ultraviolet photoelectron spectroscopy we determine the accessible \ensuremath{\Phi} range as 4.2--6.0 eV for GaN and 3.9--6.3 eV for ZnO. The contribution to the \ensuremath{\Phi} change (\ensuremath{\Delta}\ensuremath{\Phi}) of acceptor-induced surface band bending within GaN was significantly smaller than in ZnO (0.35 versus 1 eV), which we attribute to surface gap states. We introduce a model that takes these surface states into account and thus allows quantifying the impact of the semiconductor bulk doping level and surface state density on \ensuremath{\Delta}\ensuremath{\Phi}. The lower the doping level is, the lower the surface state density needs to be to reduce the band bending contribution to \ensuremath{\Delta}\ensuremath{\Phi}. This limits the maximum value of \ensuremath{\Phi} tuning for GaN with molecular acceptors, whereas, on the other hand, it renders the method robust against surface structural disorder.

Time-delayed feedback control (TDFC) is transferred to the optical domain and applied to complex multisection semiconductor lasers as used in optical communication. Pyragas-type control is provided by purely optical feedback from an external Fabry-Perot interferometer. This all-optical setup needs no time-consuming signal processing and, thus, has practically no speed limit. Proof-of-principle experiments demonstrate noninvasive stabilization of unstable steady states, chaos control, and suppression of noise-induced oscillations on picosecond time scales. A Lang-Kobayashi type model with optical TDFC is investigated taking into account the dynamic details of the device as well as all-optical time-delayed feedback. The parameter regimes that allow for stabilization of stationary emission are mapped out in good agreement with the experiments. Including noise, an analytical expression for the power spectral density is derived, which is confirmed by the experimental all-optical suppression of noise-induced relaxation oscillations.

We report on our efforts to cultivate the ternary compound ZnCdO as a semiconductor laser material. Molecular beam epitaxy far from thermal equilibrium allows us to overcome the standard solubility limit and to fabricate alloys with band gaps ranging from 3.4 down to 2.1 eV. Optimized structures containing well-defined quantum wells as active zones are capable of low-threshold lasing under optical pumping up to room temperature. The longest lasing wavelength achieved so far is 510 nm.

We demonstrate that fringe fields stemming from nanostructured ferromagnets (FMs) provide the opportunity to manipulate spins in an underlying dilute magnetic semiconductor (DMS) heterostructure, both coherently and incoherently. Fringe fields from Fe/Tb FMs with a remanent out-of-plane magnetization induce a local out-of-plane magnetization in a ZnCdMnSe DMS. Due to the sp-d exchange interaction, optically generated electron-hole pairs align their spin along the DMS magnetization. We obtain a local, remanent spin polarization probed by both spatially resolved, polarization sensitive magneto-photoluminescence spectroscopy and Faraday rotation technique. Fringe fields from in-plane magnetized Co FMs allow us to modify the precession frequency of Manganese (Mn 2+ ) magnetic moments in an in-plane external magnetic field, which is directly probed by time-resolved Kerr rotation technique.

Excitons play a key role for the optoelectronic properties of hybrid systems. We apply near-field scanning optical microscopy (NSOM) with a $100\,\text{-nm}$ spatial resolution to study the photoluminescence of surface excitons (SX) in a $20\,\text{nm}$ thick ZnO film capped with a monolayer of stearic acid molecules. Emission from SX, donor-bound (DX), and - at sample temperatures $T>20\,\text{K}$ - free (FX) excitons is separated in steady-state and time-resolved photoluminescence spectra. The $4\,\text{meV}$ broad smooth envelope of SX emission at $T, 14 pages, 3 figures

We propose a new approach for calculating the change of the absorption spectrum of a molecule when moved from the gas phase to a crystalline morphology. The so-called gas-to-crystal shift Δm is mainly caused by dispersion effects and depends sensitively on the molecule’s specific position in the nanoscopic setting. Using an extended dipole approximation, we are able to divide Δm= −QWm in two factors, where Q depends only on the molecular species and accounts for all nonresonant electronic transitions contributing to the dispersion while Wm is a geometry factor expressing the site dependence of the shift in a given molecular structure. The ability of our approach to predict absorption spectra is demonstrated using the example of polycrystalline films of 3,4,9,10-perylenetetracarboxylic diimide (PTCDI).

Time-resolved near-field optical microsopy maps exciton transport in a hybrid system. Within the 100 ps photoluminescence lifetime, an equilibrium distribution of surface and bound excitons displays lateral diffusion on a 50 nm length scale.

The fundamental limits of inorganic semiconductors for light emitting applications, such as holographic displays, biomedical imaging and ultrafast data processing and communication, might be overcome by hybridization with their organic counterparts, which feature enhanced frequency response and colour range. Innovative hybrid inorganic/organic structures exploit efficient electrical injection and high excitation density of inorganic semiconductors and subsequent energy transfer to the organic semiconductor, provided that the radiative emission yield is high. An inherent obstacle to that end is the unfavourable energy level offset at hybrid inorganic/organic structures, which rather facilitates charge transfer that quenches light emission. Here, we introduce a technologically relevant method to optimize the hybrid structure's energy levels, here comprising ZnO and a tailored ladder-type oligophenylene. The ZnO work function is substantially lowered with an organometallic donor monolayer, aligning the frontier levels of the inorganic and organic semiconductors. This increases the hybrid structure's radiative emission yield sevenfold, validating the relevance of our approach., Hybrid inorganic-organic structures can overcome the limits of inorganic semiconductor light emitting devices but the energy level offset is an obstacle. Here, Schlesinger et al. lower the ZnO work function with an organometallic donor monolayer and enhance the radiative emission of the hybrid structure.

We investigated a hybrid structure consisting of microstructured Fe/Tb multilayer ferromagnets (FMs) and a ZnCdMnSe dilute magnetic semiconductor quantum well (DMS QW) by means of superconducting quantum interference device (SQUID) magnetometry and micro-photoluminescence (µPL) spectroscopy. The Fe/Tb multilayers show a strong perpendicular magnetic anisotropy (PMA) with a temperature dependent coercive field Hc reaching values of 2.5 T /μ0 at 4 K. The out-of-plane magnetic fringe field stemming from the Fe/Tb microstructures induces a locally varying magnetization in the underlying DMS QW, which polarizes the spins of optically excited charge carriers in the DMS. We demonstrate that the coupling between these magnetic systems enables us to control the local carrier spin polarization in the DMS via the magnetization of microscale Fe/Tb ferromagnetic wires. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A hybrid of nanostructured Fe/Tb multilayer ferromagnets and ZnCdMnSe dilute magnetic semiconductor (DMS) quantum well was studied by magneto-micro-photoluminescence (µPL) spectroscopy. When the sample is magnetized in an external magnetic field an additional fringe field from the nanomagnets effectively modulates the local exciton spin states in the quantum well (QW) due to the giant Zeeman effect. Observation of 1.5% of remanent PL polarization degree from the QW gives a direct proof of remanent exciton spin polarization in the DMS. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

II–VI quantum dots are used to control and to manipulate single charge and spin excitations in semiconductors. Two-photon excitation enables us to generate an entangled state and to control the emission cascade of the exciton-biexciton system. On charged quantum dots, we demonstrate direct optical manipulation of the resident electron spin. The spin life-times are significantly larger than 10 µs in low magnetic field. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin-lattice relaxation (SLR) is studied in quantum well and quantum dot structures by photo-carrier induced spin heating. For the SLR time constants no qualitative difference is found between quantum well and quantum dot structures. There is no hint for a specific zero-dimensional behaviour.

We report on coherent control of the two-photon transition between the ground state and the biexciton state on a single quantum dot level. The population of the biexciton is monitored by the cascaded spontaneous emission of the biexciton complex via the exciton states. The signal shows interference pattern at the double laser frequency and its contrast decays non-exponentially at the time scale of 10 ps. The contrast decrease is attributed to a dynamical inhomogeneous broadening of the energy levels due to long-term fluctuations in the dot environment.

The trion state is used to monitor spin processes in charged CdSe/ZnSe quantum dots. The single-dot photoluminescence from the singlet groundstate is identified by its magnetic-field pattern. The charged bi-exciton is used to uncover the fine structure of the triplet state. Clear signatures for isotropic as well as anisotropic electron-hole exchange are observed. Photoluminescence excitation measurements enable to determine the energy positions of the excited trion states. Time-resolved measurements provide a single hole spin relaxation time longer than 10 ns at low temperature. The trion emission exhibits negative circular polarization. We present single-dot polarization data that enable to identify the states responsible for this effect.

Time-resolved and time-integrated circularly-polarized photoluminescence of excitons and trions have been studied in external magnetic fields up to 10 T. ZnSe-based quantum well structures of n-type with carrier densities varied from 5×109 to 1011 cm -2 were used in this study. Absence of the chemical equilibrium in the exciton–trion system has been demonstrated at low temperatures (

We have studied the energy spectrum of negatively charged single CdSe quantum dots using two-beam photoluminescence excitation spectroscopy. We observe bleaching of the linear trion spectrum and appearance of induced absorption due to excitation of charged biexciton states. The data embody clear signatures for Pauli blocking, many-particle Coulomb interactions as well as electron–hole exchange fine structure.

We study the non-linear optical properties associated with negatively charged excitons (trions) in II-VI semiconductor quantum wells and single quantum dots. In n-type doped quantum well structures, we observe optical gain and lasing at the trion resonance. The trion momentum distribution spreads out deeper in k-space, while the electrons are closer to the zone center. This results in optical gain at moderate injection levels without inversion in the total number of trions and electrons. The non-linear response of single self-assembled CdSe/ZnSe quantum dots charged with an electron is indeed atom-like. We observe complete bleaching of linear trion absorption spectrum and appearance of induced absorption due to two-pair excitations. The data embody clear signatures for Pauli blocking, few-particle Coulomb correlation, as well as electron-hole exchange finestructure.

X-ray absorption fine structure and photoluminescence measurements are combined to analyze the structure of Cu centers in SrS:Cu thin-film and powder samples. We find that all the data can be understood on the basis of a model involving three kinds of Cu centers: (i) dumbbell-type Cu-Cu complexes (which are the dominant centers), (ii) substitutional centers (CuSr), and (iii) interstitial centers (Cui). The bond lengths in the dumbbell centers are determined from the x-ray absorption data. Two characteristic emission bands peaking at about 460 and 510 nm are attributed to Cui and dumbbell centers, respectively. The dependences of the emission spectrum on temperature and Cu concentration are explained by a dissociation-association reaction of the type Cu-CuCuSr + Cui, assuming center-dependent excitation efficiencies and thermal quenching rates.

This paper reports on the observation of optical gain due to charged excitons (trions) in quantum wells. The specific optical coupling of the trion gives rise to stimulated emission on the low-energy wing of the trion photoluminescence line without degeneracy and inversion in the total particle numbers. Gain values as large as 10 4 cm -1 are found for excitation intensities of a few kW/cm. Higher injection levels lead to carrier heating which limits the available gain. A calculation of the absorption-gain crossover based on a kinetically determined equilibrium of excitons, trions and electrons with a common carrier gas temperature describes the experimental data well.

Adjusting the free-electron concentration, the surface plasmon frequency of the semiconductor ZnOGa is tuned into resonance with the molecular vibrations of the n-alkane tetracontane. Closed molecular films deposited on the semiconductor's surface in the monolayer regime generate distinct signatures in total-attenuated-reflection spectra at the frequencies of the symmetric and asymmetric stretching vibrations of the CH2 group. Their line shape undergoes profound changes from absorptive to dispersive and even anti-resonance behavior when moving along the surface- plasmon dispersion by the angel of incidence. We demonstrate that this line shape diversity results from a phase-sensitive perturbation of the surface-plasmon-polariton generation at the molecule/metal interface.

Ga doping of ZnO allows for generation of free electrons up to concentrations of about 10 21 cm -3 without significant deterioration of the crystal structure. In this way, a metallic dielectric function is formed with a negative-to-positive crossover of the real part tunable from mid infrared up to telecommunication wavelengths. The losses are at least one order of magnitude lower than for traditional metals. We demonstrate surface plasmon polaritons with dispersion relations that can be engineered in a unique way by utilizing epitaxial multi-layer structures of different doping level.

Utilizing the anisotropic thermal expansion of $a$-plane sapphire, we study the role of anisotropic in-plane strain on ZnO quantum well structures. X-ray data reveal a deformation of the hexagonal unit cell. The symmetry reduction shows up in the optical spectra by distinct linear-polarization features, both for the neutral and charged exciton transition. We elaborate the excitonic fine structure behind this observation, accounting for spin-orbit coupling and electron-hole exchange in the presence of anisotropic strain. The consequences for optical spin control are discussed.

The fine structure and intensity of resonant exciton-phonon (REP) spectra in a Zn0.87Cd0.13Se/ZnSe single quantum well (QW) have been investigated in detail at tunable laser excitation in the energy region of ground ZnCdSe exciton state at 8?K. Observed REP lines have been shown to go through two different mechanisms. The most intensive component corresponds to Raman scattering through extended exciton states. Other less intensive REP components correspond to hot luminescence of localized excitons. An analysis of optical phonon modes, which can be involved in the formation of the REP spectra in biaxially strained Zn0.87Cd0.13Se QWs, has been carried out.

The influence of high-density optical excitation on n-doped ZnSe/(Zn,Mg)(S,Se) quantum wells is studied by quasi-stationary photoluminescence and pump-probe experiments. In photoluminescence, the trion and exciton transitions saturate weakly with increasing excitation density. This is accompanied by superlinearly growing low- and high-energy wings attributed to collisions in the dense system of excitons, trions and carriers. For non-resonant excitation, a strong heating of the background carrier gas takes place and collisions lead to an effective recombination already during the relaxation of the electron-hole pairs down to the bottom of the band. This process limits reachable exciton and trion densities under quasi-stationary excitation.

Extended x-ray absorption fine structure and photoluminescence studies were performed on epitaxial GaN films implanted with 1×1016 cm-2 Er ions at 80 and 160 keV and, for a part of the samples, co-implanted with oxygen ions at 23 keV, followed by an anneal for 60 min at 900°C. It was shown for the samples both with, as well as without, oxygen co-implantation that Er is incorporated in a six-fold coordination with respect to oxygen, as in the cubic bixbyite structure Er2O3. The oxygen contamination of the non-oxygen-implanted samples is assumed to be due to nitrogen-vacancy-assisted oxygen diffusion from the sapphire substrate during annealing. The Stark level splitting of the 4I15/2 ground state of Er3+ observed in the 1.54 µm photoluminescence at low temperature in both types of samples is consistent with the low symmetry of the Er sites expected in cubic bixbyite Er2O3.

Extended x-ray absorption fine-structure measurements have been performed on ZnSe crystals doped with Er by an addition of ErF3 during growth. The results show that Er exists in these samples in an orthorhombic configuration with nine nearest F neighbours similar to that in orthorhombic ErF3. The energy level splittings observed in optical absorption and photoluminescence investigations in the region of the 4I15/24I13/2 transitions at 1.54 µm are fully consistent with the orthorhombic symmetry of the Er centre.

Relaxation from spatially direct to the spatially indirect exciton through ZnSe barriers of different thicknesses is investigated in (ZnCdMn)Se/ZnSe/(ZnCd)Se asymmetric double quantum wells by use of magneto-optical steady-state photoluminescence (PL) and PL excitation (PLE) experiments. The 1-LO-phonon scattering has been found to be the relevant mechanism for effective electron and hole tunneling.

A study on self-assembled CdSe quantum dots (QDs) on ZnSe is reported. The use of an in situ ultra-high vacuum atomic force microscope (UHV-AFM) provides the morphological evidence for the thermally activated reorganization of an initially two-dimensional CdSe film into an array of QDs. The QDs have heights between 1.5 and 3.5 nm and lateral diameters smaller than 10 nm. UHV-AFM images do not yield any evidence for Ostwald ripening. We assign the ultra-high stability to the existence of a kinetic barrier prohibiting the heterosystem to pass from the metastable QD state into the thermodynamic equilibrium.

We study, both experimentally and theoretically, the influence of polarization-induced electric fields on the optical properties of heavily doped $(7\ifmmode\times\else\texttimes\fi{}{10}^{18}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3})$ $\mathrm{G}\mathrm{a}\mathrm{N}/(\mathrm{A}\mathrm{l},\mathrm{G}\mathrm{a})\mathrm{N}$ multiple-quantum-well structures. To investigate the impact of the strain state on the transition energy, these heterostructures are deposited on either a GaN or an (Al,Ga)N relaxed buffer layer. Furthermore, we show that the recombination dynamics in these heavily doped multiple quantum wells is still controlled by residual electric fields, contrary to the common assumption that flatband conditions are established at this doping level.

The fine structure of the heavy-hole exciton in CdSe quantum dots grown on ZnSe by molecular beam epitaxy is studied. The application of a tilted magnetic field allows us to observe all four levels of the ground state simultaneously. The zero-field separation between the optically forbidden and allowed doublets ranges between 1.7 and 1.9 meV and is markedly reduced due to the existence of a wetting layer and the finite energy barrier at the hetero-interface. The energy splitting within the allowed doublet (200 μeV) is substantially larger than for the forbidden states (⩽20 μeV), proving that a dominant contribution arises from the long-ranged part of the electron–hole exchange interaction. The total set of g -factors is derived. A huge anisotropy for the electron is found.

In this paper, the secondary emission of different (Zn,Cd,Mn)Se/ZnSe and ZnSe/(Zn,Mn)Se quantum structures and (Zn,Mn)Se layers is studied under resonant excitation of the energetically lowest exciton state. A pronounced Stokes satellite is found arising from the exciton-induced spin-flip of anti-ferromagnetically aligned Mn 2+ pairs. From the Stokes shift, the nearest-neighbor Mn 2+ –Mn 2+ exchange constant is precisely determined to J 1 / k B =(−12.8±0.2) K for (Zn,Mn)Se. An incorporation of 15% Cd does not change this value within the experimental accuracy.

We studied the influence of the inhomogeneity of fringe fields originating from nanostructured ferromagnets on the coherent dynamics of localized Mn2+ spins in a (Zn,Cd,Mn)Se/ZnSe quantum well by time-resolved Kerr rotation. By studying hybrid systems with different geometry, we demonstrate that the spatially inhomogeneous fringe field results in two main consequences for the localized spin system: First, a temporal variation of the ensemble precession frequency is obtained, and second, a reduced ensemble spin dephasing time T2* is observed in comparison to an unstructured reference area of the sample.

We have studied the fine structure of the heavy-hole exciton in CdSe quantum dots grown on ZnSe by molecular beam epitaxy. Applying a tilted magnetic field, all four levels of the ground state are detected simultaneously. The zero-field separation between the optically forbidden and allowed doublets ranges between 1.7 and 1.9 meV and is crucially influenced by the finite energy barrier at the heterointerface as well as the existence of a wetting layer. The energy splitting within the allowed doublet (200 \ensuremath{\mu}eV) is substantially larger than for the forbidden states (\ensuremath{\leqslant} 20 \ensuremath{\mu}eV), demonstrating that a dominant contribution arises from the long-ranged part of the electron-hole exchange interaction. The total set of g factors is derived. A huge anisotropy for the electron is found.

A giant anti-Stokes photoluminescence is observed at low excitation densities on specially designed semimagnetic II--VI asymmetric double quantum well structures by applying an external magnetic field. The signal is due to a two-step absorption process, mediated by a long-living spatially indirect exciton, created via rapid electron tunneling from the primarily excited direct exciton state.

On high-quality (Zn,Cd)Se/ZnSe quantum wells we found that the decay of the exciton spin transients after resonant excitation with circularly polarized light is even faster than the exciton dephasing time. We demonstrate that this fact is a direct consequence of (alloy) disorder, giving rise to a kind of inhomogeneous broadening and associated interference effects for the spin dynamics across the band of localized exciton states.

We investigate theoretically the impact of internal polarization fields on the disorder broadening of excitons in (In, Ga)N/GaN quantum wells considering both uncorrelated alloy fluctuation and in-plane-correlated interface roughness in the frame of a center-of-mass separation approach. Not only the fluctuation of the band edges, but also the fluctuation of the long-range polarization field of spontaneous and strain-induced dipole moments are taken into account. The polarization field is found to induce a narrowing of the alloy contribution of the center-of-mass potential spectrum due to (i) penetration of the carriers into the barriers and (ii) increased exciton volume. Contributions from short-range interface roughness slightly weaken the narrowing, but do not reverse this effect. In case of a long-range interface roughness, a single-line decomposition of the spectrum occurs.

Recent experimental studies have demonstrated localization of biexcitons in quantum wells, providing even optical gain up to elevated temperatures. We present a theoretical treatment of exciton and biexciton states in the weak localization limit using a center-of-mass separation ansatz. An advanced approach based on an extensive numerical solution is compared with a more simple model for the quantum well biexciton wave function. Our explicit results, derived for parameters of the ternary quantum well material (Zn,Cd)Se, yield that the localization of the biexciton is---despite its larger spatial extension---stronger than that of the single exciton state.

Using time-resolved photoluminescence spectroscopy, we determine the temperature dependence of the radiative lifetime for GaN layers with doping levels from ${10}^{16}$ to ${10}^{18}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}.$ The experimental results are analyzed by a coupled rate-equation model taking into account band-to-band, free and bound excitons, as well as donor-to-band recombination. An analytic expression for the radiative lifetime of this coupled system is derived and fit to the data. Over the entire temperature range, radiative recombination is strongly affected by the decay of either bound or free excitons. The temperature dependence and the absolute values for the radiative lifetime are governed by the coupling between the individual populations. This coupling in turn is determined by the binding energies of the respective species. Using a binding energy of 26.4 meV for the free exciton, we obtain best-fit values of $30\ifmmode\pm\else\textpm\fi{}6$ and $36.3\ifmmode\pm\else\textpm\fi{}2\mathrm{meV}$ for the donor and the donor-bound exciton, respectively.