The blue Mg induced 2.8 eV photoluminescence (PL) band in metalorganic chemical vapor deposition grown GaN has been studied in a large number of samples with varying Mg content. It emerges near a Mg concentration of 1x10(exp 19) cm(-3) and at higher concentrations dominates the room temperature PL spectrum. The excitation power dependence of the 2.8 eV band provides convincing evidence for its donor-acceptor (D-A) pair recombination character. It is suggested that the acceptor A is isolated Mg(Ga) while the spatially separated, deep donor (430 meV) D is attributed to a nearest-neighbor associate of a Mg(Ga) acceptor with a nitrogen vacancy, formed by self-compensation

The efficient room-temperature photoluminescence bands of wurtzite GaN, which are peaked in the red (1.8 eV), the yellow (2.2 eV), and the blue (2.8 eV) spectral range, have been studied as a function of doping (species and concentration) and excitation power density (PD). It is shown that the yellow and the blue band are induced by Si and Mg doping, respectively, while codoping with Si and Mg generates the red band. At high-doping levels, the yellow and the blue band reveal strong peak shifts to higher energy with increasing PD providing very strong evidence for their distant donor-acceptor (DA) pair recombination character. The deep centers involved in DA recombination having electrical activity opposite to that of the shallow level of the dopant, are suggested to arise from self-compensation and to be vacancy-dopant associates. Self-compensation is found to be weak in the case of Si doping, but significant for Mg doping. A recombination model is presented, which accounts for the ess ential properties of all three bands in deliberately doped GaN. These results also suggest that the yellow and the blue bands in nominally undoped GaN arise from distant DA pairs involving residual Si and Mg impurities, respectively, as well as their respective vacancy associates

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We review the status of InGaAsN-based vertical-cavity surface-emitting lasers (VCSELs) emitting in the wavelength range 1.2-1.3 µm and compare them with similar devices that have been realized using other approaches. To prove the potential of InGaAsN-based VCSELs, we present our results for monolithically MBE-and MOVPE-grown and electrically pumped VCSELs on GaAs substrates. Our MBE-grown devices emit at a wavelength of up to 1305 nm with cw output power at room temperature exceeding 1 mW and a threshold current of 2.2 mA. With an oxide-confined current aperture of about 5 µm diameter, they emit up to 700 µW in single-mode operation at room temperature. Bit-error rates of less than 10 −11 are achieved for transmission over 20.5 km of standard single-mode fibre at 2.5 Gbit s −1 . Our MOVPE-grown VCSELs with a similar device structure emit single mode at a wavelength of 1293 nm with a cw output power of 1.4 mW and a threshold current of 1.25 mA at room temperature. In back-to-back transmission, we reach a data rate of 10 Gbit s −1 , proving the feasibility of high-speed data transmission using InGaAsN VCSELs.

Exciton photoluminescence (PL) in a GaInNAs/GaNAs quantum well was measured in the temperature range from 15 K to 300 K. Two striking features of the PL were observed: the nonmonotoneous temperature dependence of the Stokes shift and the abrupt increase of the PL linewidth in a rather narrow temperature range. These features are known to be strong indications of the hopping relaxation of excitons via localized states distributed in space and energy. Computer simulations of the hopping relaxation of excitons were carried out. Comparison between the simulation results and the experimental data provides an important and reliable information on the energy shape of the density of states and also on the energy range, in which localized states for excitons are distributed.