T. Böttcher

Microstructure of heteroepitaxial GaN revealed by x-ray diffraction

The mosaicity of GaN layers grown by metalorganic vapor phase epitaxy, on (0001) sapphire and exhibiting different grain diameters is studied using high-resolution x-ray diffraction. The coherence lengths, the tilt, and the twist of the mosaic structure are determined utilizing data taken in different x-ray scattering geometries. The results of different models, which were applied, are then compared and discussed. The dislocation densities, obtained from the x-ray data, are compared with the results of plan-view transmission electron microscopy and atomic force microscopy.

The role of high-temperature island coalescence in the development of stresses in GaN films

The formation of dislocations and stress in GaN layers grown by metalorganic vapor phase epitaxy on sapphire is investigated with regard to the average grain diameter. The grain diameter was determined by monitoring the high-temperature GaN island coalescence process during growth using reflectometry. It is found that the density of edge threading dislocations decreases and the compressive stress measured after cooling to room temperature increases when the coalescence thickness and the grain diameter increase. The data are consistent with models of development of tensile stress due to island coalescence during growth.

In situ and ex situ evaluation of the film coalescence for GaN growth on GaN nucleation layers

The microscopical evolution of GaN grown by metalorganic vapor-phase epitaxy is investigated at all its stages, as nucleation layer growth, recrystallization, epitaxial overgrowth and coalescence, with the help of in situ normal incidence reflectance measurements. Sample morphology was ex situ characterized at each stage by atomic force microscopy. These investigations revealed that the nucleation layer structure, determined by the V/III precursor ratio and the reactor pressure, strongly influences the coalescence of the subsequent grown film. Particularly, in low-pressure growth additional control of the coalescence process can be gained by adjusting the initial V/III ratio of the high-temperature epilayer growth.

INCORPORATION OF INDIUM DURING MOLECULAR BEAM EPITAXY OF INGAN

We report on the incorporation of In during growth of InxGa1−xN by molecular beam epitaxy under varying In/Ga flux ratios and with different film thicknesses. The incorporation efficiency studied by energy dispersive x-ray microanalysis, high-resolution x-ray diffraction and photoluminescence spectroscopy is strongly affected by the chosen fluxes of Ga and N and is limited by the excess of nitrogen compared to gallium. Furthermore, thick films exhibit a decrease of the In content in growth direction. The behavior can be explained by considering the different stabilities of the two binary compounds InN and GaN.

Mosaicity of GaN epitaxial layers: Simulation and experiment

High-resolution X-ray diffraction has been used to analyze GaN epilayers with varying coalescence thickness which were grown by MOVPE on (0001) oriented sapphire. The decrease of the density of edge type threading dislocations with increasing coalescence thickness causes a marked difference in the mosaicity of the samples. As the defects form along the grain boundaries, this corresponds to an increase in lateral coherence length with increasing coalescence thickness. The lateral coherence length has been obtained from simulations of reciprocal lattice points of off-axis Bragg reflections, measured in asymmetric diffraction geometry.

Magnesium segregation and the formation of pyramidal defects in p-GaN

Magnesium doping of GaN was found to generate extended defects with a pyramidal shape. Transmission electron micrographs of layers with different doping levels typically showed a defect-free region at the start of doping and a modulation of the defect density in the subsequent film. We developed a rate equation model based on the segregation of Mg to explain the formation process of these defects. The model explains the dependence of the defect-free thickness on the doping level and yields a criterion to avoid the defect formation. Hall measurements show a significant reduction of the free hole concentration for samples grown at doping levels beyond defect formation.

In as a surfactant for the growth of GaN (0001) by plasma-assisted molecular-beam epitaxy

The influence of indium on the surface morphology of GaN (0001) grown by plasma-assisted molecular-beam epitaxy (MBE) has been investigated. The rough and grain-like surface under nitrogen-rich growth conditions becomes smoother and similar to surfaces grown under gallium-rich conditions when a sufficiently high indium flux is used. However, the use of indium instead of gallium-rich conditions prevents the formation of gallium droplets on the surface which are associated with voids at their edges. Since indium is not incorporated into GaN for growth temperatures above 700 °C, it can be used as a surfactant in MBE growth of GaN.

Thermally induced stress in GaN layers with regard to film coalescence

Thermally induced plane stress in GaN layers of different thicknesses, grown by metalorganic vapour phase epitaxy on sapphire, is investigated. Thin layers, characterized by isolated grains, are found to be stress-free. With increasing layer thickness, however, grains start to coalesce and stress can build up when the samples are cooled down following growth. As soon as the coalescence process is completed and a compact film has been formed, a maximum stress level is reached which does not further increase for still thicker layers. Therefore, it is proposed that grain edges enable non-compact films to elastically relieve in-plane stress.

Compositional inhomogeneities in InGaN studied by transmission electron microscopy and spatially resolved cathodoluminescence

The structural and optical properties of InGaN epilayers grown by different molecular beam epitaxy (MBE) techniques were studied with high spatial resolution. Mappings of the local emission wavelength obtained by cathodoluminescence (CL) spectroscopy indicate lateral and spectral inhomogeneities in the luminescence of InGaN epilayers grown with continuous In and Ga fluxes. These results agree well with variations in the chemical composition in the lateral and in the growth direction seen in mappings of the local composition which were obtained by energy-dispersive X-ray (EDX) microanalysis in cross-sectional transmission electron microscopy (TEM). Possible origins of these variations are discussed. In comparison, epilayers grown with alternating deposition of (In, Ga)N and (Ga)N are more homogeneous.

Spatially modified layer properties related to the formation of gallium droplets on GaN(0001) surfaces during plasma-assisted molecular-beam epitaxy

The surface morphology and the spatial distribution of defect-related luminescence of GaN(0001) layers grown by plasma-assisted molecular-beam epitaxy under gallium-rich conditions has been investigated. Droplets of liquid gallium form on the surface during growth and lead to distinct spiral hillocks under the droplet. The droplets are surrounded by extended voids which point to an incomplete gallium adlayer on the GaN surface during growth at the droplet boundary. Cathodoluminescence spectra indicate an enhanced intensity in the yellow spectral range for the GaN under the droplets which is attributed to a change in the local density of point defects in the layer.