Epitaxy Of GaN Layers And AlGaN/GaN Heterostructures On Silicon Substrates Via MOCVD

Since commercial GaN-based light emitting devices (LEDs), laser diodes (LDs) and high electron mobility transistors (HEMTs) had been launched, excellent GaN-based devices are attracting more attention worldwide. However, the comercializaiton of GaN-based optoelectronic devices is hampered by high cost for production and various bottlenecks in developing high power devices.Due to lack of bulk materials, GaN-based devices are only grown on foreign substates. Thanks to its merits, silicon (Si) is considered as a potential substrate for heteroepitaxy of GaN-based devices with low cost and a good solution to high-power problem. Compared with sapphire and SiC substrates, growth of high quality GaN epilayers on Si faces more difficulties:firstly, strong reactivity between GaN and Si causes serious corrosion on Si substrates; secondly, GaN epifilms are apt to crack and bear higher defect density for the huge mismatch in lattice constants and thermal expansion coefficients between GaN and Si substrates. Facing the problems above, a systematic research on the growth techniques for high quality GaN on Si via MOCVD was carried out, and the main achievements are summarized as follows:For the first, influence of pre-processing of Si, especially the optimization of Al-preseeding and AlN buffer layer growth, on the epitaxy of GaN layers was studied. It was shown that cleaning and baking of Si has noticeable effect on surface morphology of substrates which subsequently influeced the growth of buffer layers. And duration for Al-preseeding was found as a key parameter for AlN buffer growth. It was also concluded that the optimization of AlN buffer growth is essential for separation of Ga and Si and epitaxy of GaN.For the second, three designs of strain-relaxed buffer layer structures were applied to eliminate cracks in GaN epilayers caused by large tensile stress. Several LT-AlN interlayers were optimally designed to achieve crack-free GaN on Si, but crystal quality was still inferior. Crack-free GaN layers over 1.5μm with high crystal quality were achieved by inserting optimal thin continuously composition-graded AlxGa1-xN interlayer between GaN layer and HT-AlN/Si(111) template. And novel supperlattices-contained multiple buffer layers were designed for GaN growth on Si, however, the results were unacceptable due to the rough interfaces.Finally, growth and characterization of AlGaN/GaN heterostructures on Si(111) substrates were carried out based on high-quality GaN epitaxy. Thickness of AlGaN barrier layer and interface roughness were identified as key factors to influence the sheet carrier density and mobility of 2-dimensional electron gas (2DEG).