Wurtzite GaN films were grown on silicon nitride buffer layers formed on Si (111) substrates by radio frequency plasma-assisted molecular beam epitaxy. Reflection high energy electron diffraction, Auger electron spectroscopy, transmission electron microscopy, and photoluminescence results indicate that the single crystalline wurtzite GaN was grown on the buffer layers of amorphouslike silicon nitride formed on Si (111) substrates by taking the following relationship with the substrate: GaN [0001]//Si [111] and GaN (1̄1̄20)//Si (11̄0). Both faces of the silicon nitride buffer layer were found to be flat and sharp, the thickness of the buffer layer (1–1.5 nm) being constant across the interface. Efficient bound exciton emission was observed at 3.46 eV. The growth technique described was found to be simple but very powerful for growing high quality GaN films on Si substrates.

Mass-separated 56Fe+ ions were implanted into Si(100) at 350 °C using three different energies and doses of 140 keV (1.32×1017 cm−2), 80 keV (6.20×1016 cm−2), and 50 keV (3.56×1016 cm−2). Their optical properties were investigated as a function of subsequent annealing temperature and its duration time. X-ray diffraction analysis revealed that polycrystalline semiconducting β-FeSi2 layers are formed in the as-implanted and annealed samples. From Rutherford backscattering spectrometry analysis, the formation of β-FeSi2 up to the surface was confirmed, and the average thickness and composition of the layers at peak concentration were estimated to be 70–75 nm and Fe:Si=1:2.0–2.2, respectively. The types of optical transition and the inverse logarithmic slope (E0) of the Urbach tail were investigated using room temperature optical absorption measurements. All the synthesized β-FeSi2 layers exhibited a direct transition with direct band-gap energies (Egdir) of 0.802–0.869 eV and with high optical absorption coefficients extending to 105 cm−1 at photon energy above 1.0 eV. The E0 value characteristic of the Urbach tail was observed to decrease from 260 to 100 meV with elevating annealing temperature. Some of the materials having E0<160 meV showed two strong photoluminescence (PL) emissions peaked at 0.805–0.807 eV (No. 1) and 0.840–0.843 eV (No. 2) at 2 K, whereas those with E0≳250 meV exhibited only weak emissions. From the results of the temperature- and excitation power-dependent PL measurements, emissions Nos. 1 and 2 were attributed to the trap-related recombinations related to β-FeSi2, with thermal activation (quenching) energies of 54.7 and 46.7 meV, respectively.

The e6'ect of magnetic field on the excitonic photoluminescence line shape has been studied in a highquality single GaAs-Al, Ga& "As quantum well grown by the molecular-beam-epitaxy technique. An increase of magnetic field from 0 to 6 T has been found to result in (1) a decrease in the Lorentzian contribution I to the line shape from I o(0 T) =0.504+0.01 meV to I o{6 T) =0.336+0.01 meV due to the formation of a quasi-zero-dimensional density of states. This leads, in turn, to an increase in the exciton dephasing time due to the inhibition of the carrier relaxation, and (2) an increase in the Gaussian contribution from o. (0 T) =0.24 meV to o. (6 T) =0.39 meV, attributed to the shrinking of the exciton wave function in real space; the last e6'ect causing the exciton to become more responsive to the statistical potential fluctuations at the quantum-well interfaces.

Optical absorption and electron spin resonance (ESR) spectra have been studied in as-grown, Cr-doped, and annealed in different atmospheres CuAlS2 crystals. It has been found that the major influence on the optical and ESR properties of CuAlS2 crystals, obtained by chemical vapour transport, arises from the transition ions Fe3+ and Cr2+, which are present in the crystal lattice as residual impurities. Thermal treated crystals exhibit drastic changes in their absorption and ESR spectra depending on the annealing atmosphere, which is explained by the process of the Fermi level motion in the vicinity of the Fe2+/Fe3+ and Cr+/Cr2+ demarcation levels. The results obtained show that the valence states of Fe and Cr ions and their relative amounts in each valence state are controlled by the position of the Fermi level in the band gap of the host crystal, and are stoichiometry on dependent.

Photoluminescence (PL) measurements have been carried out at low temperature (77 and 10 K) on CuAlS2 crystals grown by the chemical vapor transport method. Seven sharp PL lines have been observed near the band edge. Based on the photoreflectance measurements, the PL line at 3.550 eV has been assigned to a free exciton emission. The lines at 3.540, 3.532, 3.500 and 3.475 eV are tentatively assigned to the bound excitons, and they are discussed in terms of the crystal composition and the annealing conditions. This study also refers to the PL lines and peaks at about 2.9 eV.