TOWARDS THE DEVELOPMENT OF DEFECT-FREE GaN SUBSTRATES: DEFECT CONTROL IN HETERO-EPITAXIALLY GROWN GaN BY NEW BUFFER LAYER DESIGN

This study analyzes the influence of the low-temperature grown GaN buffer layer on the properties of the GaN layer for thin film growth on sapphire. Considerable efforts have already been spent during the course of the past ten years on the optimization of buffer layers and progress in the arena has made the realization of commercially available GaN-based opto-electronic devices. We introduce the stoichiometry of the buffer layer as a new parameter determining the quality of the subsequently grown GaN layer. Adequate design of the growth conditions allows to grow GaN films of significantly improved crystalline and electrical quality. The achieved electron mobilities are among the best ever reported for MBE grown GaN films on sapphire. We further establish a qualitative growth model explaining the observed phenomena by the stoichioemtry-dependent ability of the buffer layer to relax stress during growth. In a second closely-related matter, the main causes of stress in hetero-epitaxially grown GaN are- for the first time- analyzed by measuring the temperature dependence of the GaN bandgap in reference to a lifted-off GaN film. Surprisingly, the observed temperature dependence of stress between 0 and 600 K can not be explained under the mere assumption of thermally induced stress caused by the mismatch of the thermal expansion coefficients