ABSTRACT
GaN-based semiconductors have attracted much attention in the past decade as one of the most important semiconductor materials. With high direct band-gap, these materials can be applied to high power and high frequency electronic devices as well as light emitting diodes and laser diodes. Conventional GaN-based epitaxial layers were grown either on sapphire or on SiC substrates. However, sapphire substrates are insulators with poor thermal conductivity while SiC substrates are much more expensive. Thus, growth of GaN on Si substrate has attracted considerable attention. Silicon substrate is also relatively low cost and available in large size with good thermal conductivity. However, large mismatches in thermal expansion coefficient and lattice constant between Si and GaN often lead to the formation of crack networks even with a relatively small GaN thickness (∼1 μm). In this paper, we report the growth of
with Al composition of 70%, 60%, 50%, 40%, 30%, 20% and 10%, respectively, were then deposited on top of the AlN layer sequentially. Finally, a 2 μm thick GaN layer was deposited onto the Al0.1Ga0.9 N intermediate layer. By gradually decreasing the Al composition ratio in the AlGaN intermediate layers, we should be able to maintain two-dimensional growth, minimize the strain and thus achieve a smooth GaN surface. For comparison, samples without the step-graded AlGaN were also prepared. For these samples, we deposited a 2 μm thick GaN layer directly on top of the AlN layer. Schematic diagrams of the samples with and without the step-graded AlGaN intermediate layers are shown in Figures 1(a) and (b), respectively.
