S. Figge

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.

Emission properties of a-plane GaN grown by metal-organic chemical-vapor deposition

We report on the emission properties of nonpolar a -plane GaN layers grown on r -plane sapphire. Temperature-, excitation-density-, and polarization-dependent photoluminescences and spatially resol ...

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.

Strain relaxation in AlGaN under tensile plane stress

Relaxation of tensile strain in AlxGa1−xN layers of different compositions epitaxially grown on GaN/sapphire is investigated. Extended crack channels along 〈211¯0〉 directions are formed if the aluminum content exceeds a critical value, which decreases with increasing layer thickness. This process is found to limit the average strain energy density to a maximum value of 4 J/m2. By calculating the stress distribution between cracks and the strain energy release rate for crack propagation, the relaxed strain as measured by x-ray diffraction is correlated to the crack density, and the onsets of crack channeling and layer decohesion are fitted to a fracture toughness of 9 J/m2. Moreover, the crack opening at the surface is found to linearly increase with the stress. Annealing of samples above the growth temperature introduces additional tensile stress due to the mismatch in thermal expansion coefficients between the layer and substrate. This stress is shown to relieve not only by the formation of additional cr...

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.

High-quality bulk a-plane GaN sliced from boules in comparison to heteroepitaxially grown thick films on r-plane sapphire

Thick GaN bars with [1120] orientation have been sliced from GaN boules grown on freestanding films by hydride vapor phase epitaxy (HVPE) in the [0001] direction. High-resolution x-ray diffraction and transmission electron microscopy have been used to study the structural quality and defect distribution in the material in comparison to heteroepitaxially grown thick HVPE-GaN films grown in the [1120] direction on (1102)-plane sapphire. It is demonstrated that while the heteroepitaxial material possesses a high density of stacking faults and partial dislocations, leading to anisotropic structural characteristics, the (1120)-plane bulk GaN, sliced from boules, exhibits low dislocation density and narrow rocking curves with isotropic in-plane character.

Stress and wafer bending of a-plane GaN layers on r-plane sapphire substrates

The stress and wafer bending of (11 2- 0) a -plane GaN layers of different thicknesses grown on (1 1- 02) r -plane sapphire substrates by hydride vapor phase epitaxy were studied by high-resolution x-ray diffraction and photoluminescence and photoreflectance spectroscopies. The layers are found to be under compression in the growth plane and under tension in the growth direction. The elastic and thermal anisotropies of the GaN and the sapphire crystal result in an in-plane stress and a wafer curvature, both of which are different in the two in-plane directions parallel and perpendicular to the GaN c axis. The GaN unit cell is no longer hexagonal but orthorhombic. The stress relaxes with increasing GaN layer thickness while the curvature of the wafer increases. Different stress relief mechanisms are considered, and the stresses in the layer and the curvature of the wafer are calculated using standard models on wafer bending. The results suggest that the wafer bending is the dominant stress relief mechanism. In addition, the redshift of the near-band-edge photoluminescence and the free exciton photoreflectance peaks with increasing layer thickness is correlated with the strain data determined by x-ray diffraction.

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.