Depth dependence of defect density and stress in GaN grown on SiC

Abstract

We report high resolution x-ray diffraction studies of the relaxation of elastic strain in GaN grown on SiC(0001). The GaN layers were grown with thickness ranging from 0.29to30μm. High level of residual elastic strain was found in thin (0.29to0.73μm thick) GaN layers. This correlates with low density of threading screw dislocations of 1-2×107cm−2, observed in a surface layer formed over a defective nucleation layer. Stress was found to be very close to what is expected from thermal expansion mismatch between the GaN and SiC. A model based on generation and diffusion of point defects accounts for these observations.

Crystalline perfection of GaN and AlN epitaxial layers and the main features of structural transformation of crystalline defects

“…In the top part of the layer, the density of these defects is about 2 Â 10 7 cm À2 . The maximum thickness of such ''perfect'' layers (0.5-0.6 mm) is occasionally comparable to the diffusion length of point defects, evaluated from lateral coherence length of diffracted radiation [18]. Increase of the layer thickness up to 790 nm significantly changes the structure of crystalline defects and hence changes the spatial distribution of scattered radiation (sample 2, Fig.…”

“…The interference pattern, symmetrical in both, q z (2yÀo) and q x (o) directions, corresponds to the perfect domain layer and unambiguously shows that the uniformity or coherence of the main part of the epitaxial layer is not deteriorated during epitaxial growth. The main types of crystalline defects in this layer are threading dislocations and dislocation walls, both rising from the initial thin (20-50 nm thick) sublayer, which almost completely accommodates the initial elastic strain caused by lattice mismatch between the substrate and the epitaxial structure [18]. Misfit dislocations occupying the bottom interface attract point defects, which are generated on the growth surface and diffuse inward, to increase their size.…”

“…4b), is typical for layers with considerable gradient of crystalline quality in the growth direction [18,[23][24][25]. The fast transition from a wide base to a narrow central part indicates significant diminution of the density of threading dislocations from the accommodation sublayer to the top surface.…”

“…3). In the main top part of the layer, elastic strain (e c ) is slightly negative (compressed in growth direction) [18], whereas in the transformation sublayer and in the bottom part of the layer, elastic strain gradually changes due to continuous structural transformation of crystalline defects. The top part of this sublayer, corresponding to the maximum concentration of dislocation loops, becomes stretched due to relaxation by edge segments, and then, in downward direction, becomes compressed again due to the structural conversion of small dislocation loops into threading dislocations in the transformation sublayer.…”

Correlation between Physical Defects and Performance in AlGaN/GaN High Electron Mobility Transistor Devices

“…Due to the 4% lattice mismatch between GaN and SiC substrate, high threading dislocation density is present in epitaxially grown GaN films, 10 7 -10 10 cm -2 [11,12]. These highly conductive dislocations are known to provide high leakage current pathway at room temperature [9].…”