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 characteristics of GaN grown by molecular beam epitaxy on sapphire (0001) substrates was investigated. Layers grown on GaN and AlN buffers were studied by high-resolution X-ray diffraction and transmission electron microscopy (TEM). For both buffer materials, the variation of the buffer parameters, like their thickness and growth temperature, is reflected in a clear change of the GaN (0002) rocking curve width. For strongly decreased as well as for increased Bragg reflection width a deterioration of optical and electrical properties of GaN layers grown on buffers with respect to reference samples without buffer layers was observed. Moreover, layers grown on thin GaN buffer layers show extremely narrow ω scans and layer thickness interferences in 2θ/ω 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 densities by X-ray diffraction.