Fine structure of AlN/AlGaN superlattice grown by pulsed atomic-layer epitaxy for dislocation filtering

We report the detailed structure analysis of our AlNAlGaN superlattice (SL) grown by pulsed atomic-layer epitaxy (PALE) for dislocation filtering. Due to the nature of PALE, the AlGaN well material itself in the SL was found to be composed actually of an Alx Ga1-x N Aly Ga1-y N short-period superlattice (SPSL), with the periodicity of 15.5 Å (≈6 monolayer), determined consistently from high-resolution x-ray diffraction and high-resolution transmission electron microscopy measurements. The SPSL nature of the AlGaN layers is believed to benefit from the AlNAlGaN SL's coherent growth, which is important in exerting compressive strain for the thick upper n-AlGaN film, which serves to eliminate cracks. Direct evidence is presented which indicates that this SL can dramatically reduce the screw-type threading dislocation density.