Barbara Bastek’s research while affiliated with Otto-von-Guericke-Universität Magdeburg and other places

Zinc nitride films were deposited on Si(100) substrates at room temperature using RF-magnetron sputtering in pure N-2 and in Ar + N-2 atmospheres. Two active phonon modes (270.81 and 569.80 cm(-1)) are observed in Raman spectra for films deposited in Ar + N-2 atmosphere. Atomic force microscopy showed that the average surface roughness of the films deposited in pure N-2 atmosphere (1.3-3.33 nm) was less than for those deposited in a mixed Ar + N-2 atmosphere (10.3-12.8 nm). Low temperature cathodoluminescence showed two emission bands centered at 2.05 eV and 3.32 eV for both types of films.

The temperature dependence of diffusion length and lifetime or diffusivity of the free exciton is measured in a commercial ZnO-substrate and in an epitaxial ZnO quantum well using nm-spatially and ps-time resolved cathodoluminescence (CL) spectroscopy. The characteristic temperature dependence of the exciton mobility is a fingerprint of the underlying excitonic scattering processes. Since excitons are neutral particles scattering at ionized impurities should be not effective. With decreasing temperature diffusion lengths and lifetimes give rise to a monotonous increase of the excitonic mobility. Two different methods for determining the excitonic transport parameters will be presented. On the one hand we are able to perform completely pulsed excitation experiments and on the other hand a combination of cw- and pulsed excitation in two independent measurements are needed.

Resonant coupling of an optical mode confined within a microcavity and an emitter is the basic prerequisite for the observation of Bose-Einstein condensation phenomena and the development of novel optical devices based on cavity polaritons. We demonstrate highly spatially resolved 2” wafer characterization of the reflectivity and emission properties of a nitride based multi quantum well semi microcavity (i.e. structure without top Bragg reflector) to verify resonant regions. Photoluminescence and reflectivity spectra recorded at the same positions on the wafer exhibit a strong spatial dependence of the multi quantum well emission and the center wavelength of the stop band of the bottom Bragg reflector across the sample. Resonance, i.e., matching of the emission and the center wavelength of the stop band, is found in a region 8 mm off the center of the wafer. The thickness profile across the AlInN/GaN Bragg reflector and multi quantum well layers was obtained by x-ray mappings over the full wafer. A perfect correlation between the local optical properties and the x-ray thickness distribution is found. Additional transmission electron microscopy investigations indicate a complete crack free structure and smooth interfaces between the layers within the Bragg reflector making the structure appropriate for strong coupling applications.

A hybrid GaN based microcavity was fabricated to investigate light matter coupling. The sample consists of a lattice matched AnInN/GaN bottom and a dielectric upper Bragg mirror. To increase the coupling strength an InGaN/GaN multi quantum well was embedded inside a GaN cavity. The emission characteristics of the structure without top Bragg mirror were analyzed using photoluminescence spectroscopy. Two emission peaks at nearly fixed energy but changing intensity ratio with respect to the position on the sample arise. A detuning of the cavity mode and the excitonic emission into resonant condition is demonstrated by scanning across the wafer and temperature dependent measurements at about 50 K. Angle resolved photoluminescence investigations at the full hybrid structure exhibit two emission peaks. Predominantly excitonic behavior was found for the high and photonic-like behavior for the low energy peak. No clear anticrossing behavior was observed.

In this paper, we compare the effectiveness of two different methods regarding the reduction of defect densities in heteroepitaxially grown a-plane GaN by heavy Si doping. The insertion of well-established in situ SixNy nanomasks leads to locally heavy Si d-doped GaN. By increasing the SixNy deposition time in the range from 0 to 300 s the full width at half maxima (FWHM) of the X-ray diffraction w-scans at in-plane GaN(1-100) and GaN(0002) Bragg reflections decreases from 0.55° to 0.24° and from 0.45° to 0.16°, respectively. When growing without any SixNy interlayer but instead with continuously heavy Si-doping, these values are further decreased to 0.13° and 0.15°, respectively. By measuring several higher order reflections and detailed evaluation of the w-scan broadening in Williamson–Hall-plots (WHPs) a considerable reduction in defect densities and no hint of basal plane stacking faults (BSFs) were found for the heavy Si doped a-plane GaN sample. To verify this result the micro-structural properties of this sample were additionally investigated by transmission electron microscopy and cathodoluminescence (CL).

Low-strained AlInN/GaN multilayers aimed as Bragg mirrors were grown by metal organic vapour phase epitaxy on GaN/Si(1 1 1). In such structures the upper AlInN/GaN interfaces show a considerable roughening on a nanometre scale whereas the lower ones appear flat as evaluated by cross-sectional electron and transmission electron microscopy. The roughening is attributed to a Stranski-Krastanov transition from two-dimensional layer-by-layer to three-dimensional island growth. In addition, a self-organized wavy-like surface morphology on a micrometre scale is observed in such structures which we discuss in terms of Grinfeld instability.

We present a direct microscopic correlation between local optical properties, characterized by spectrally resolved cathodoluminescence microscopy and the microscopic crystallographic orientation determined by electron backscatter diffraction at identical sample positions of nonpolar and semipolar GaN growth domains simultaneously formed during metal-organic vapor phase epitaxy on the same r-plane sapphire substrate. The luminescence from all nonpolar, (112_0) grown crystallites is dominated by the basal plane stacking fault luminescence, while all crystallites having semipolar (112_6) orientation show a luminescence characterized by pure excitonic emission, i.e., without any contribution of stacking faults, and with an order of magnitude enhanced quantum efficiency.