ZnO single crystals, epilayers, and nanostructures often exhibit a variety of narrow emission lines in the spectral range between 3.33 and 3.35 eV which are commonly attributed to deeply bound excitons (Y lines). In this work, we present a comprehensive study of the properties of the deeply bound excitons with particular focus on the Y 0 transition at 3.333 eV. The electronic and optical properties of these centers are compared to those of the shallow impurity related exciton binding centers (I lines). In contrast to the shallow donors in ZnO, the deeply bound exciton complexes exhibit a large discrepancy between the thermal activation energy and localization energy of the excitons and cannot be described by an effective mass approach. The different properties between the shallow and deeply bound excitons are also reflected by an exceptionally small coupling of the deep centers to the lattice phonons and a small splitting between their two electron satellite transitions. Based on a multitude of different experimental results including magnetophotoluminescence, magnetoabsorption, excitation spectroscopy (PLE), time resolved photoluminescence (TRPL), and uniaxial pressure measurements, a qualitative defect model is developed which explains all Y lines as radiative recombinations of excitons bound to extended structural defect complexes. These defect complexes introduce additional donor states in ZnO. Furthermore, the spatially localized character of the defect centers is visualized in contrast to the homogeneous distribution of shallow impurity centers by monochromatic cathodoluminescence imaging. A possible relation between the defect bound excitons and the green luminescence band in ZnO is discussed. The optical properties of the defect transitions are compared to similar luminescence lines related to defect and dislocation bound excitons in other II-VI and III-V semiconductors.

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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.

Cu 2 O thin films were grown on sapphire (0001) and MgO (100) substrates by chemical vapor deposition. The crystalline, vibrational and electrical properties of the layers and the amount of incorporated background impurities have been examined. X-ray diffraction measurements revealed, that the polycrystalline films grew in (111) and (100) orientation on sapphire and in (100) orientation on MgO. Raman measurements indicated the presence of CuO inclusions in the films. The electrical properties are dominated by an acceptor level located 150 meV above the valence band. This level may originate from unintentionally incorporated silicon impurities.

We reconsider acceptor doping of ZnO with Li and Cu published nearly 40 years ago by comparing it with the behaviour of nitrogen in ZnO. While Cu plays an exceptional role due to the d-shell configuration (acceptor level at 190 meV below conduction band) Li and N as single acceptors give rise to deep distorted acceptors with binding energies between 700 and 800 meV above valence band. With these binding energies no hole conductivity at room temperature can be expected, having in mind that typical background donor densities in ZnO are between 10 16 and 10 17 cm À3 . We propose a defect model to explain the role of lithium and nitrogen doping in ZnO which is based on two acceptor-one donor pairing as proposed by theory in the framework of co-doping. Hydrogen is a key candidate for the donor role. The luminescence properties in ZnO:N can be understood on the basis of the calculation of the hole densities without and with compensation assuming pair formation. We compare our experimental findings with published results on nitrogen doped ZnO explaining the limitations of p-type doping of ZnO with nitrogen.

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ZnO:N thin films have been deposited on oxygen and zinc terminated polar surfaces of ZnO. The nitrogen incorporation in the epilayers, using NH3 as doping source, was investigated as a function of the growth temperature in the range between 380 °C and 580 °C. We used Raman spectroscopy and low temperature photoluminescence to investigate the doping properties. It turned out that the nitrogen incorporation strongly depends on both, the surface polarity of the epitaxial films and the applied growth temperatures. In our CVD process low growth temperatures and Zn‐terminated substrate surfaces clearly favour the nitrogen incorporation in the ZnO thin films. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)