Showing papers by "Frank Bertram published in 2004"

Bound exciton and donor–acceptor pair recombinations in ZnO

Abstract: The optical properties of excitonic recombinations in bulk, n-type ZnO are investigated by photoluminescence (PL) and spatially resolved cathodoluminescence (CL) measurements. At liquid helium temperature in undoped crystals the neutral donor bound excitons dominate in the PL spectrum. Two electron satellite transitions (TES) of the donor bound excitons allow to determine the donor binding energies ranging from 46 to 73 meV. These results are in line with the temperature dependent Hall effect measurements. In the as-grown crystals a shallow donor with an activation energy of 30 meV controls the conductivity. Annealing annihilates this shallow donor which has a bound exciton recombination at 3.3628 eV. Correlated by magnetic resonance experiments we attribute this particular donor to hydrogen. The Al, Ga and In donor bound exciton recombinations are identified based on doping and diffusion experiments and using secondary ion mass spectroscopy. We give a special focus on the recombination around 3.333 eV, i.e. about 50 meV below the free exciton transition. From temperature dependent measurements one obtains a small thermal activation energy for the quenching of the luminescence of 10 ± 2 meV despite the large localization energy of 50 meV. Spatially resolved CL measurements show that the 3.333 eV lines are particularly strong at crystal irregularities and occur only at certain spots hence are not homogeneously distributed within the crystal contrary to the bound exciton recombinations. We attribute them to excitons bound to structural defects (Y-line defect) very common in II–VI semiconductors. For the bound exciton lines which seem to be correlated with Li and Na doping we offer a different interpretation. Li and Na do not introduce any shallow acceptor level in ZnO which otherwise should show up in donor–acceptor pair recombinations. Nitrogen creates a shallow acceptor level in ZnO. Donor–acceptor pair recombination with the 165 meV deep N-acceptor is found in nitrogen doped and implanted ZnO samples, respectively. In the best undoped samples excited rotational states of the donor bound excitons can be seen in low temperature PL measurements. At higher temperatures we also see the appearance of the excitons bound to the B-valence band, which are approximately 4.7 meV higher in energy. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Self-assembly of ZnO nanowires and the spatial resolved characterization of their luminescence

Abstract: The self-assembled growth of ordered ZnO nanowires on GaN/Si layers has been observed at the low temperature of 500 °C through Zn evaporation and oxidation. The nanowires have a nearly uniform diameter of 40 nm and length of nm. No metal catalyst was used. Interestingly, the nanowires grow on a wetting film of an interconnected vortex-like structure. An empirical model is proposed to explain the growth process. Spatially resolved cathodoluminescence (CL) measurements show a sharp and intense emission (I8 line) from the nanowires, while weak and redshifted luminescence from the wetting layer. The CL indicates the existence of tensile stress in the wetting layer while the nanowires are fully relaxed.

Ostwald ripening and flattening of epitaxial ZnO layers during in situ annealing in metalorganic vapor phase epitaxy

Abstract: ZnO films were grown at 450°C by metalorganic vapor phase epitaxy on GaN∕Si(111) substrate and subsequently annealed in situ at 900°C for times ranging from 0 to 8min. The epitaxial layers were characterized by atomic force microscopy, x-ray diffraction, and cathodoluminescence measurements. The as-grown ZnO layers consist of three-dimensional nanoscale-sized clusters of [0001]-oriented monocrystallites. During the first 1min of annealing, a surface smoothening due to a redistribution of material is observed leading to a decrease of both the island numbers and their heights. After 2min of annealing, the surface morphology has completely changed and is transformed into a nearly two-dimensional cluster-free surface. This phase transition is accompanied by a strong improvement of the crystalline and optical properties. The effect is attributed to Ostwald ripening with a subsequent flattening of the crystal surface.

Localization versus field effects in single InGaN quantum wells

Abstract: The optical properties of InxGa1−xN quantum wells (x=0.13) have been studied by cathodoluminescence (CL) spectroscopy. A blueshift of the quantum well emission is observed with increasing excitation density, which can be explained by considering (a) band filling of in-plane potential fluctuations caused by compositional inhomogeneities, or (b) screening of piezoelectric fields inside the well. We have used time-resolved CL spectroscopy to distinguish between the two effects. The onset and decay of the relaxation and recombination kinetics are measured by using rectangular excitation pulses with ultrafast on and off switching and with pulse lengths sufficiently long to ensure excitation into quasi-steady-state conditions. For well widths of Lz⩽6 nm, a redshift is observed after the electron beam is switched on and a further redshift occurs after the electron beam is switched off. For Lz⩾8 nm, a blueshift is observed after the electron beam is switched on and a redshift is observed after the electron beam i...

Direct evidence for selective impurity incorporation at the crystal domain boundaries in epitaxial ZnO layers

Abstract: A direct correlation of structural properties with the spatial distribution of bound exciton luminescence in ZnO epitaxial layers has been achieved on a microscopic scale using highly spatially and spectrally resolved cathodoluminescence. The morphology of the high quality ZnO layer is characterized by a distinct domain structure. While the laterally integrated cathodoluminescence spectrum shows narrow (full width at half maximum <3meV) I8 luminescence, a pronounced emission line at I0∕I1 emerges in the local spectra taken at domain boundaries. In complete contrast, no I0∕I1 emission is found inside the domains. Monochromatic images further evidence the selective incorporation of impurities at the grain boundaries of domains. Micro mappings of the I8 peak wavelength directly visualize the strain relaxation across the domains toward their very center, where a drop in quantum efficiency indicates enhanced defect concentration.

Feature Article: Bound exciton and donor–acceptor pair recombinations in ZnO

Abstract: The article [1] features the current state of knowledge on the optical properties of excitonic and donor–acceptor pair recombination in bulk n-ZnO, as revealed by recent photo- and cathodoluminescence measurements. The cover picture is a photograph showing an unusual look at the green luminescence of rod-shaped ZnO crystals under ultraviolet excitation. The first author, Bruno K. Meyer, is professor of physics at the Justus Liebig University of Giessen. His scientific interests include synthesis of semiconductor oxides by chemical vapour deposition and magnetron sputtering, in particular with respect to doping issues, materials science of electro- and thermochromic materials, recombination phenomena and defect characterization by magnetic resonance. This issue also contains an Editor's Choice on “Evolution of molecular ordering and phase transitions in C60/C70 solid solutions” by Gabriela Rueda-Morales and Jaime Ortiz-Lopez [2].