D. Forster

Optical and structural analysis of ZnCdO layers grown by metalorganic vapor-phase epitaxy

The development of ZnO-based semiconductor devices requires band gap engineering. Ternary Zn1−xCdxO allows reduction of the band gap relative to ZnO, which would be necessary for devices emitting visible light. We have analyzed the structural and optical properties of Zn1−xCdxO layers grown by metalorganic vapor-phase epitaxy. A narrowing of the fundamental band gap of up to 300 meV has been observed, while introducing a lattice mismatch of only 0.5% with respect to binary ZnO. Photoluminescence, high-resolution x-ray diffraction, and spatially resolved cathodoluminescence measurements revealed a lateral distribution of two different cadmium concentrations within the Zn1−xCdxO layers.

Direct imaging of phase separation in ZnCdO layers

A direct correlation of structural and optical properties of MOCVD-grown ZnCdO-layers with a systematic variation of Cd-content has been achieved on a microscopic scale using highly spatially and spectrally resolved cathodoluminescence. The ZnCdO layer luminescence measured in cathodoluminescence wavelength images reveals strong lateral fluctuations directly visualizing local band gap fluctuation as a consequence of different local Cd incorporation. We give direct evidence for a chemical phase separation into Cd-rich and Cd-poor nanodomains in ZnCdO.

Metalorganic chemical vapor phase deposition of ZnO with different O-precursors

ZnO is a promising material for light emitters in the UV region. For MOCVD growth no well-suited O-precursor is available. Three different high-purity oxygen precursors, i.e. iso-propanol, acetone, and N 2 O were tested for the growth of ZnO on GaN/Si(1 1 1) templates. For iso-propanol pre-reactions are observed influencing the growth rate and limiting the growth temperature to below 500°. Best layer quality is obtained around 450°C at 300 mbar reactor pressure and a VI-II ratio larger than 40. ZnO grown in a similar growth regime but using acetone as O-precursor exhibits a surface constructed from nanometer sized filaments. Most of the acetone-grown films have growth orientations of (1011). Using N 2 O higher growth temperatures are needed due to the poor decomposition of this gas. However, no prereactions are observed and (0002) oriented layers with good X-ray ω-scans at ZnO-positions can be obtained around 800°C at 300 mbar and a VI-II ratio above 600.

Local luminescence of ZnO nanowire-covered surface: A cathodoluminescence microscopy study

Vertically aligned ZnO nanowires were synthesized on sapphire through vapor deposition using Au as a catalyst. In addition to the nanowires, the sample surface contained a thin wetting layer of ZnO clusters. Spatially and spectrally resolved cathodoluminescence (CL) microscopy was applied to correlate the luminescent properties and the sample structure. Local CL spectra and monochromatic images revealed that the nanowires are the major source for a strong and sharp I8 emission line, while two weak emissions at low energies originate exclusively from the wetting layer. Information on stress is also obtained from the relative position of the I8 line.

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

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.

Metal Organic Vapor Phase Epitaxy of ZnO on GaN/Si(111) Using Tertiary-Butanol as O-Precursor

High-quality ZnO was grown by metal organic vapor phase epitaxy on 1.3 µm thick GaN layers on Si(111) using dimethylzinc and tertiary-butanol as precursors. The variation of the growth temperature shows a strong correlation with the microstructure as observed by atomic force and scanning electron microscopy. With increasing growth temperature we find an increasing size of ZnO crystallites and a transition from 3D to 2D growth. Moreover, increasing the growth temperature leads to a reduction of tensile stress in the ZnO as observed by X-ray diffraction. In highly spatially resolved cathodoluminescence measurements we observe narrow (A0,X) luminescence from the ZnO surface and a strong donator correlated luminescence at macroscopic defects.

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

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.

Microscopic Spatial Distribution of Bound Excitons in High-Quality ZnO

The surface morphology of the ZnO layers is dominated by a distinct hexagonal domain structure. While the laterally integrated cathodoluminescence spectrum shows intense and narrow I8 luminescence, a distinct emission line at spectral position of I0/I1 emerges in the local spectra taken at domain boundaries. In 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. Monochromatic images of the I8 peak wavelength directly visualize the strain relaxation across the domains towards their very center, where a drop in quantum efficiency indicates enhanced defect concentration.