S. Lautenschläger

Photonic properties of ZnO epilayers

The characterization by various experimental techniques of homoepitaxial growth and photonic properties of ZnO epilayers was exhaustively analyzed. The photonic properties of ZnO as promising material for the realization of polariton lasers were investigated by angular dependent reflection spectroscopy. The fitting of the polariton dispersion curve with the experimental results provided us information about the longitudinal-transverse exciton-polariton splitting and damping constants. In addition, the valence band symmetry was examined by angular resolved magneto-optical photoluminescence. From our theoretical and experimental results we extracted evidence that the topmost A valence band possesses Г7 symmetry. Micro-Raman spectroscopy revealed even in homoepitaxially grown samples the existence of compressive or tensile strain which varied not only in the ZnO layers but also in the templates. In contrast, the untreated substrates were uniformly strained. Sporadically crystal perturbations culminating in the formation of separated growth domains were observed. Additionally, resonant Raman scattering was performed, showing a strong enhancement of the 2E1(LO) mode for resonant excitation of the I8 bound exciton complex. We suggest that the resonant Raman scattering led to a longer lifetime of the resonantly excited phonon mode due to a strong exciton-phonon interaction.

Group I elements in ZnO

In this report we focus on the lithium doping of ZnO epitaxial films grown on GaN templates and ZnO substrates. We compare the results with diffusion studies of Li into ZnO single-crystal substrates. The diffused and in-situ doped layers were studied using mass spectroscopy, low temperature photoluminescence and Raman spectroscopy. Li is known to produce a deep acceptor state which takes part in a shallow donor to deep acceptor recombination in the visible spectral region. We will demonstrate that also shallow Li acceptors are introduced depending on the growth/diffusion temperature. The shallow Li-related acceptor has a binding energy around 300 meV. A donor-acceptor pair recombination with its zero phonon line at 3.05 eV followed by LO phonon replica is observed. The intensity ratio of the zero phonon line compared to the replica indicates weak electron phonon coupling, hence small lattice relaxation in contrast to the recombination with the deep Li acceptor.

Photoluminescence investigations on a native donor in ZnO

The shallow donor impurities in ZnO with binding energies between 46 and 56 meV have been studied in great detail in the recent years. They give rise to neutral donor bound exciton recombinations with the A- and B-valence bands, show rotator states and two-electron-satellite transitions. These properties allowed to establish the excited state splittings of the donors as well as confirming Haynes rule in ZnO. So far they all seem to be of extrinsic origin, hydrogen, aluminum, gallium and indium in order of increasing binding energy. For many years it was common sense that intrinsic defects would dominate the n-type conductivity of ZnO. Interstitial zinc as well as oxygen vacancies should be double donors, and in order to contribute to the n-type-conduction they should have shallow levels, and low formation energies to be abundant. In PL-measurements at T∼100 K on various ZnO samples, single crystals as well as thin films, a luminescence around 3.31 eV was detected. Due to its line shape and temperature behaviour it is identified as bound-to-free recombination. If we assume that the 3.31 eV band with its level at EC ∼ 130 meV is the ++/+ level of the zinc interstitial we calculate for the binding energy of the +/0 level ∼ 130 meV, i.e. around 33 meV. Undoped Zn-rich epitaxial films grown by CVD show a dominant I 3 recombination at 3.367 eV which according to Haynes rule is consistent with a shallow donor level at 33 meV. Moreover, they have free n-type carrier densities of 2×10 19 cm −3 and as revealed by SIMS the common donor impurities (Al, Ga, In) cannot account for the high carrier densities.