SOI Compliant Substrate For GaN Epitaxial Growth

GaN is one of the most important materials in III-V nitride semiconductor. The GaN, with 3.4eV midgap width, belongs to the wild gap semiconductor, and is wildly used in long time, low consumption, and shot wavelength light emitter diodes, laser diodes, UV detector and high temperature electron devices. One of the key issues in GaN applications is the starting substrate for the GaN growth. Great effort has been made in GaN growth on Si. However, the lattice mismatch and thermal coefficient mismatch between GaN and Si degrade the quality of the GaN epilayer.SOI compliant substrate is one of the novel approaches to get improved GaN epitaxial layer on Si based substrates. In addition to it's applications in IC manufacturing, SOI has been considered as compliant substrate for hetro-epitaxy. SOI wafer consistes of a sandwich structure of "top silicon laeyr / buried oxide insulator / bulk silicon substrate". The top silicon layer acts as the compliant layer for the epitaxy, and the mechanism has been sucessfully used in the epitaxixal growth of SiGe, Strained-Si and so on.In this paper, SOI compliant substrate for the GaN epitaxy was reported. The compliant property of the top silicon layer in the SOI structure has been investigated by comparing the GaN epilayers on various substrates including SOI, bulk Si and engineered SOI. Futher, the influence of the electrical property to the GaN epitaxial layer has been studied, especially in the initial stage of the epitaxial growth of the SOI substrate. The results were expected to extend to compound semiconductor materials growth on SOI compliant substrate.The results shown that the stress caused both by the lattices mismatch and the thermal coefficient mismatch in the GaN epilayer has been reduced and consequently the crystal quality of the GaN epilayer has been improved dut to the compliancy of the structure of the thin SOI layer to the GaN epilayer, which could not happened in the bulk Si case. Thinner top silicon is helpful for utilizing the compliant property of the SOI substrate. It was found that there are fixed positive charges with density of 10¹¹cm^-2 in the SOI substrate, locat at the BOX/top silicon layer interface. These charges repulse the Al atoms with positive charges, hich results in the decrease of the the island density in the intal stage of the epitaxy and then the reduction of the threading dislocation density in the stage of coalescence. The thinner the top silicon layer, the more it effect. Several kinds of engineered SOI substrate, including ions beam modified SOI substrates, patterned SOI substrates and ELOG SOI substrates, all shown the benifets for the GaN epitaxy in some way. Especially, the ELOG SOI substrate may play a very key role in the development of the SOI compliant substrate.