We present a comprehensive study of the free-exciton and exciton-polariton photoluminescence in wurtzite GaN. Using polarization-dependent measurements we were able to resolve the fine-structure energy splittings in the n51 state of the A exciton and to determine the energy separation between the 1S and 2P61 states as 19.760.2 meV. For the n51 state, the evolution of the emission from two transverse polariton branches G 5T1 and G 5T2, the longitudinal exciton G 5L , and the dipole-forbidden exciton G 6 in magnetic fields up to 15 T have been studied in Faraday configuration. We have estimated the value of the parallel effective g factor of the hole mixed into the 1S state of the A exciton as gA,1S i 52.2560.2. To describe these data a theory is developed for the exciton energy structure in hexagonal semiconductors with wurtzite symmetry in zero and in weak external magnetic fields which takes into account the effect of the hexagonal lattice anisotropy and the coupling of all excitonic states belonging to different valence subbands. The effect of the exciton interaction with polar optical phonons is considered. The theory is very successful in describing the free-exciton emission and magnetoluminescence in wurtzite GaN. The effective mass parameters and the magnetic Luttinger constant as well as the effective Rydberg numbers and binding energies of the A, B, and C excitons are determined from a comparison of the theory with experimental data. DOI: 10.1103/PhysRevB.64.115204

We studied gain and absorption in CdSe quantum dots (QDs) in a ZnSe matrix. It was demonstrated that, caused by a strong excitonic absorption, excitonic waveguiding occurs in edge geometry. No significant change of the gain maximum energy over a range of two magnitudes above the threshold was detected. Magneto-optical investigations demonstrate the strong lateral localisation of excitons at CdSe QDs in agreement with lack of exciton screening in QDs revealed in gain studies.

Results of theoretical and experimental magneto-optical studies of free excitons in wurtzite GaN are presented. The direct photoluminescence from free exciton states of high-quality GaN epilayers was studied by polarization-dependent magneto-optical measurements in magnetic fields up to 15 T. Zeeman splittings and diamagnetic shifts of the n = 1 and n = 2 states of the A exciton were observed for magnetic fields parallel and perpendicular to the hexagonal axis. The experimental data are described by a theory of the exciton energy structure in hexagonal semiconductors with wurtzite symmetry in the framework of the quasi-cubic approximation.

AlGaN on GaN epitaxial films with Al contents between 6% and 76% were investigated by stationary photoluminescence experiments which allows to determine the dependence of the energy gap on alloy composition. The observed increase of the luminescence linewidth as a function of the Al molar fraction can be explained by alloy broadening. The localization energy of the bound exciton increases considerably and reaches a Value of 53 meV for 61% Al. It could imply that the donor binding energy would markedly deviate from its effective mass value, an unexpected result if the residual donor is Si.

Microcalorimetric measurements of small absorption coefficients have been performed on thin GaN–AlGaN quantum wells grown by Reactive Molecular Beam Epitaxy on Al2O3 substrates. In addition to strong absorption at the energy of the GaN buffer and at the energy of the thick AlGaN barrier layers, we could also readily detect transitions associated to the quantum well. These measurements which pave the way to a precise determination of the gap mismatch between the well and the barrier layers are combined with self consistent excitonic and envelope function calculations in the context of a model for the band line-ups which includes the piezoelectric effect.

Layers of GaxIn1-xP (0.47<x<0.54) with thicknesses of 50, 100, and 1000 nm, epitaxially grown by metal-organic chemical vapor deposition on GaAs substrates, were studied by transmission electron microscopy, high-resolution x-ray diffraction, and photoluminescence measurements. The data obtained were used to derive the degree of ordering, eta(x), as a function of the layer composition, x. A strong influence of the layer thickness on the eta(x) functions (thickness effect) was observed. A semiphenomenological model of the order-disorder phase transition in strained layers was developed, which takes account of the free energy changes arising from the atomic rearrangements. On the basis of this model, the thickness effect in atomic ordering is quantitatively explained in terms of concentration fluctuations across the layer. [S0163-1829(99)10239-X].

Photoluminescence, stimulated emission, and gain studies were performed to investigate an AlGaN/GaN/AlGaN heterostructure. A stimulated emission peak at 3.46 eV exhibits a strong superlinear growth with excitation intensity with a slope of 2. Gain values up to 250 cm(-1) are obtained. The specific surface morphology of the heterostructure is suspected to be responsible for an optical feedback in the sample. From AFM measurements an almost perfect periodic corrugation of the surface is observed. The periodicity of the corrugation of lambda=110-120 nm perfectly matches the stimulated emission wavelength and reflects the strong correlation of structural features and optical properties in this structure.

In this paper recent results of optical studies of bound excitons in wide-gap II-VI and nitride semiconductors are surveyed. In the first part latest results about self-compensation mechanisms will be presented. Then, the dynamical characteristics of excitonic transitions are evaluated for various impurities, dopants, and dopant concentrations, and for excitation via particular resonant excitation channels. Relaxation and conversion channels between excitonic systems are analyzed, especially in strained heteroepitaxial systems which show splitting effects of the bands from which the carriers stem. The dynamical behavior of excitons in wide-gap material gives evidence of nonradiative decay channels. Calorimetric spectroscopy at mK temperatures was used to investigate the nonradiative processes and to determine quantum efficiencies. Recent results of transient four-wave mixing experiments at bound-exciton complexes will be shown to demonstrate that shallow defects influence the dephasing in the investigated systems which gives information about the homogeneous and inhomogeneous broadening mechanisms. In the last part negatively charged excitons (trions) were observed in unintentionally donor doped Zn0.90Mg0.10Se/ZnSe single quantum wells by magneto-luminescence experiments. The binding energy of the trion singlet ground state is about 2.7 meV.

We report on investigations of the excitonic quantum efficiency in GaN in dependence of different buffer layer thicknesses and residual oxygen content in the crystal. The quantum efficiency of the free excitons rises with increasing buffer layer thickness and decreasing residual oxygen content. The influence of oxygen on the quantum efficiency is stronger than that of the buffer layer thickness. In general, the observed quantum efficiencies are below 20% indicating the strong impact of nonradiative relaxation and recombination processes in the excitonic range.

We report on investigations of the excitonic quantum efficiency in GaN epilayers as function of buffer layer thickness, buffer layer material, sample thickness and residual oxygen content. These values are compared to that of GaN bulk material. The quantum efficiency of the free excitons rises with increasing buffer layer thickness, increasing sample thickness and decreasing residual oxygen content. The influence of oxygen on the quantum efficiency is stronger than that of the buffer layer thickness. Additionally, the homoepitaxial growth of GaN shows higher quantum efficiencies than the growth with an AIN buffer layer. In general, the observed quantum efficiencies in GaN epilayers are below 20% indicating the strong impact of nonradiative relaxation and recombination processes in the excitonic range. Only, GaN bulk material shows quantum efficiencies of 25 % for the free A-exciton X A and of 50 % for the donor-bound exciton complex D 0 ,X).

We report on highly resolved exciton spectra of GaN films grown by hydride vapor phase epitaxy. Using calorimetric absorption and calorimetric reflection spectroscopy the excitonic transitions originating from the A-, B-, and C-valence bands are precisely determined and the crystal-field and spin-orbit-splitting energies are calculated. We also present the first magneto-optical experiments on the neutral-donor-bound exciton. Electron as well as the hole g-values are obtained by analyzing the Zeeman splittings at 12 T.