High-Q-Power-GaN - Development of transistors for high-power electronics on low-defect free-standing gallium nitride substrates

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The aim of this research project is to develop AlGaN/GaN based high-power transistors for power conversion on low defect free-standing ammonothermal GaN substrates. The use of ammonothermal grown GaN substrates having a very low dislocation density enables the realization of AlGaN/GaN high power transistors with outstanding electrical properties to exploit the specific physical properties of the GaN material. An important aspect of this proposal is the determination of degradation mechanism of AlGaN/GaN transistors, particularly the breakdown voltages in the GaN buffer structure. Reasons for the reduced breakdown voltages are impurities or structural defects in the GaN, which potentially act as charge carrier traps and generate localized levels in the band gap. Many of the mechanisms for the generation of charge carrier traps are very complex, and as yet incompletely understood. This is because the diversity and density of defects, such as dislocations and point defects, in GaN epitaxial heterostructures is very high due to the deposition process on non-native substrates. In principle, this electron traps can be located at the surface, in the AlGaN barrier layer, at the AlGaN/GaN interface close to the two-dimensional electron gas, or in the GaN buffer layer. In this project, the effects of defects on parameters such as leakage current, threshold voltage, and current-carrying capacity in AlGaN/GaN power transistors will be investigated. A particular object of investigation in this context will be the influence of the carbon doping on the degradation pattern of AlGaN/GaN transistors. Carbon acts as an acceptor in GaN, which is used to compensate the intrinsic n-type conductivity of GaN buffer structures. Recent evidence suggests that certain degradations in the GaN buffer layers of AlGaN/GaN transistors are related to carbon and a high dislocation density. However, the above traps-related effects in heteroepitaxial AlGaN/GaN HEMT are always overlapped by the effects arising from the high defect densities. An unambiguous clarification of whether e.g. a carbon doping may under certain conditions lead to the breakdown voltage of these devices was not possible until now. Using ammonothermal GaN substrates will enable the realization of low defect density AlGaN/GaN power transistors with carbon-compensated GaN buffer layers. With this approach, there is a way to separate the carbonrelated by the dislocation-related effects in AlGaN/GaN based devices. For this purpose, processed power transistors of defect-rich AlGaN/GaN structures grown on non-native substrates (SiC and Si) are systematically compared with power transistors, which were grown homoepitaxially on low-defect ammonothermal GaN substrates.