Transmission electron microscopy: A guide to improved semiconductor structures, gallium nitride-based in particular

GaN-based devices are attracting much interest due to its excellent properties for use in various electronic applications. Among others there is the blue laser effect and a very high frequency response in field-effect transistors. Optimisation of solid-state devices in general requires a perfect knowledge of the microstructure in order to give appropriate feedback to the fabrication process. In this work, transmission electron microscopy has been used intensively to characterise various GaN-based structures and devices, which has lead to a profound understanding of reactions appearing in the growth process.; Several topics have been treated. Due to the lack of a suitable substrate, defects are common in GaN. As a function of varying growth parameters, the defect structure of plain GaN films has been studied. Experimental evidence of the role of certain growth parameters has been provided. Different secondary techniques to reduce the defect density have been explored and the mechanisms lying at the base of the defect filtering have been retrieved. It has for example been shown that low-temperature AlN interlayers, and not low-temperature GaN interlayers, are able to filter threading dislocations. The requirements to obtain an effective filtering layer are given.; Also devices have been studied. As a function of growth parameters, the In concentration and In distribution in InGaN/GaN LEDs has been obtained. The focus has been put on the study of the growth temperature, the use of a growth interrupt and the effect of post-growth thermal annealing. Based on the obtained results, an InGaN growth model has been proposed. For AlGaN/GaN HEMT structures, the relation between the microstructure and electrical properties has been studied. This basically comes down to determine the relaxation mechanisms as function of several growth parameters and the use of interlayers. Also metal contacts on GaN and AlGaN/GaN have been studied. In here, the role of the Al on the Ohmic contact formation has been retrieved. Al, both in the metal layer as in the AlGaN, decreases the reaction between Ti and GaN. The mechanism is however dependent on where the Al has been incorporated.; In a second part, GdSi1.7 thin films on (111)Si have been examined. These structure are especially attractive because GdSi1.7 form the lowest known Schottky barrier on n-Si, which makes it a suitable candidate for use in low barrier Schottky contacts and Ohmic contacts on Si. Different growth methods have been compared and physical differences in for example the strain-state have been explained. The observation of a strain-stabilised new phase has been reported.