We report on the optical properties of nitrogen acceptor-doped ZnO epilayers in the medium and high doping regimes using temperature and excitation power-dependent, as well as time-resolved photoluminescence experiments. The epilayers were doped with ammonia during homoepitaxial growth on ZnO single-crystal substrates with different surface polarities. Significant differences in the optical characteristics of the epilayers are observed between growth on nonpolar a-plane, polar c-plane Zn-face substrates and polar c-plane O-face substrates, which demonstrates different incorporation of the nitrogen acceptor depending on the substrate polarity. The incorporation of nitrogen into the ZnO films ranges between 10 19 and 10 21 cm −3 as determined by secondary ion mass spectrometry. Within this doping range the samples change from lightly compensated to highly doped compensated. We discuss the unique photoluminescence features of nitrogen-doped ZnO epilayers within the concept of shallow donor-acceptor-pair recombinations and at the highest doping level by the appearance of potential fluctuations.

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The intricate relaxation dynamics in compressively strained Ge quantum wells with Ge-rich SiGe barriers is investigated systematically for various excitation conditions. Evidence of efficient ultrafast intersubband relaxation in competition with the intrasubband electron cooling is found in continuous wave photoluminescence spectra measured with different excitation wavelengths. These findings are corroborated by time-resolved pumpprobe absorption measurements yielding an intersubband relaxation time in the range of 100 fs.

ZnO/Zn 1 À x Mg x O single quantum well (SQW) structures with well widths d W between 1.1 nm and 10.4 nm were grown by plasma-assisted molecular beam epitaxy both heteroepitaxially on c-plane sapphire and homoepitaxially on (000 1)-oriented bulk ZnO. A significantly reduced Mg incorporation in the top barrier related to the generation of stacking faults is observed for heteroepitaxial samples. Exciton localization is observed for both types of samples, while an enhancement of the exciton binding energy compared to bulk ZnO is only found for homoepitaxial SQWs for 2 nm d W 4 nm. Consistently, for homoepitaxial samples, the carrier dynamics are mainly governed by radiative recombination and carrier cooling processes at temperatures below 170 K, whereas thermally activated non-radiative recombination dominates in heteroepitaxial samples. The effects of polarization-induced electric fields are concealed for Mg concentrations x < 0.1 due to the reduction of the exciton binding energy, the screening by residual carriers as well as the asymmetric barrier structure in heteroepitaxial wells.