We report on essential aspects of the growth of InGaN / GaN p-n junctions by gas-source molecular beam epitaxy (MBE) and present the first blue and green electroluminescence from such structures grown entirely by MBE.A study of the growth conditions for a GaN nucleation layer on sapphire and for the subsequent growth of undoped GaN points out the necessity for Ga-stabilized growth. Unintentionally doped GaN grown at 1 μm/h shows background doping levels below 1017 cm-3 and mobilities up to 320 cm2/Vsec (at 300K). Narrow photoluminescence with very low intensity in the yellow spectral range is observed. N- and p-type doping of GaN with Si and Mg yields layers with high mobilities (220 and 10 cm2/Vsec, respectively at 300K) at carrier densities typical for devices.Although incorporation of Indium is strongly temperature-dependent, InGaN-layers with In-contents of over 40% are obtained routinely. The optical properties of our InGaN layers typically exhibit the commonly observed, broad deep level luminescence. Finally, we present electroluminescence in the visible spectral range up to 540 nm from InGaN / GaN double-heterojunctions.

Excitation spectra of different donor-acceptor pair emission bands and selective pair luminescence spectra were measured on cubic ZnSe crystals with different impurity contents. For the Na and Li acceptor we found the excited states with s symmetry up to 4S, as well as the four 2P excited states. These experiments further demonstrate that the acceptor involved in the A pair band reported in the literature is substitutional Li. The observed excited acceptor states are compared to results of the effective-mass theory of Baldereschi and Lipari, after the theoretical state energies have been scaled by the experimentally determined Rydberg energy of the respective acceptor.

Selective pair luminescence has been used to measure the exicted states of the shallow acceptors C, Zn, Si, and Ge in GaAs. Data for Ge are presented here for the first time while the sets of excited state energies of C, Zn, and Si are completed. These results show that the energies of the p-symmetric impurity states are influenced by the impurity potentials. The germanium excited states are found to be shifted by 3 meV with respect to the valence band compared to the corresponding states in C, Zn, and Si.

Epitaxial layers of GaN on c-plane sapphire are analyzed by continuous-wave and time-resolved photoluminescence at 4K and by X-ray diffraction. Besides the well-known emissions from hexagonal GaN we observe luminescence bands at 3.279 and 3.15 to 3.21 eV which are identified as the transition of the donor bound exciton and the donor-acceptor pair recombination in cubic GaN, respectively. Measurements of the luminescence decay times are essential for the clarification of the emission processes. Due to the probing depth of about 200 nm in PL we find that the fraction of cubic phase typically decreases with layer thickness. In our best samples, however, we do not detect the cubic phase at all.