Correlation of sapphire off‐cut and reduction of defect density in MOVPE grown AlN

Abstract

X‐ray diffraction and TEM investigations of MOVPE grown AlN on sapphire with small off‐cuts to a‐ and m‐plane reveal the influence of the off‐cut direction and angle on the reduction of threading dislocation density by annihilation during growth. Higher off‐cut angles as well as off‐cut to a‐plane seem to facilitate the annihilation, with the main reduction taking place within the first 300 nm layer thickness. On planar substrate the thickness is limited by cracking to below 2 μm which also limits the ability to further reduce the defect density. By epitaxial lateral overgrowth on stripe patterned substrates the crack‐free thickness is increased and further reduction of the defect density is possible. This process is effective up to 3–5 μm layer thickness. Using templates with off‐cuts ≥0.2° to m‐plane, step bunching perpendicular to the stripe direction occurs and bends the vertically directed threading dislocations into inclined grain boundaries starting from the point of coalescence. These partially block/incline threading dislocations over the ridge areas and thus further reduce the dislocation density. The dislocations are concentrated in stripes over the ridges and the coalescence areas. For smaller off‐cut to m or especially for off‐cut to a‐plane, the dislocation distribution is more homogeneous but nevertheless stripe‐like with alternating densities of low 108 cm−2 in the laterally overgrown areas and low 109 cm−2 in the areas over the ridges and the coalescence lines.