N. Oleynik

Local p-type conductivity in zinc oxide dual-doped with nitrogen and arsenic

A doping approach for p-type ZnO is reported which is reproducible and long-time stable. For p-type doping the zinc oxide layers were doped simultaneously with nitrogen and arsenic in metal organic vapor phase epitaxy. The conductivity type of the layers was investigated by scanning capacitance microscopy, a technique based on local capacitance-voltage analysis (C-V) with submicron spatial resolution. Depending on the growth parameters, largely extended p-type domains were observed, surrounded by n-type regions. The differences in local conductivity type are directly correlated to the topography as measured with atomic force microscopy revealing p-type for smooth, two-dimensional surfaces and n-type signals in the case of three-dimensional island growth or structural defects, i.e., microcracks or surface pits.

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.

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.

Growth of ZnO Layers by Metal Organic Chemical Vapor Phase Epitaxy

Metal organic chemical vapor phase epitaxy-grown ZnO layers using iso-propanol and DMZn as oxygen and zinc precursors, respectively, are presented. The layers were grown on GaN templates in the temperature range 400-475°C at a reactor pressure of 300-500 mbar with VI/II ratios around 45. A columnar structure with an average column size of 6.5 μm in diameter and a roughness of 70 nm (r.m.s.) is observed by atomic force microscopy measurements. In X-ray diffractometry reciprocal space maps the ZnO peak splits up indicating differently strained layers. An approximately 100 nm wide distorted transition region between the GaN and ZnO layers is found in these measurements. The low-temperature cathodoluminescence spectra are dominated by narrow near-band-edge emission and almost no deep defect-related luminescence is found. A pronounced spectral blue shift is found in the growth direction, which is attributed to the relaxation of the tensile stress and associated with inner fields. A strong microscopic correlation between the surface morphology and the emission of the acceptor bound excitons is observed.

High resolution X-ray diffraction of MOVPE-grown ZnO/GaN/sapphire layers

Abstract High quality ZnO is an interesting material for electronic and optoelectronic applications. It belongs to the wide gap semiconductors (bandgap = 3.3 eV). In this paper we present ZnO layers grown by MOVPE (metalorganic vapor phase epitaxy). Several growth parameters like growth temperature and thickness of the layer were varied. For comprehensive investigations of the crystalline quality we employed different X-ray fine structure methods.

Properties of Dislocations in Epitaxial ZnO Layers Analyzed by Transmission Electron Microscopy

The dislocation configuration in epitaxial ZnO layers grown by metal organic vapor phase epitaxy (MOVPE) was analyzed by transmission electron microscopy. Misfit dislocations and the majority of threading dislocations are characterized by Burgers vectors of the type 1/3 . First results on the electrical activity of dislocations are presented which are obtained by electron holography in a transmission electron microscope. The evaluation of the phase change of the electron wave in the vicinity of a dislocation yields a negative line charge of approximately 3 e/nm.

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.