Influence of buffer layers on the structural properties of molecular beam epitaxy grown GaN layers

The influence of low temperature buffer layers on the structural characteri stics of GaN grown by molecular beam epitaxy on sapphire (0001) substrates was investigated. Layers grown on GaN and AIN buffers were studied by high- resolution X-ray diffraction and transmission electron microscopy (TEM). Fo r both buffer materials, the variation of the buffer parameters, like their thickness and growth temperature, is reflected in a clear change of the Ga N (0002) rocking curve width. For strongly decreased as well as for increas ed Bragg reflection width a deterioration of optical and electrical propert ies of GaN layers grown on buffers with respect to reference samples withou t buffer layers was observed. Moreover, layers grown on thin GaN buffer lay ers show extremely narrow omega scans and layer thickness interferences in 2 theta/omega direction, while TEM reveals a high defect density throughout the entire layer. Therefore, not only the width of the rocking curves but also their shape has to be considered for the estimation of the defect dens ities by X-ray diffraction. (C) 1999 Elsevier Science S.A. All rights reser ved.