Effects of the thickness of low-temperature AlN interlayers on GaN layers grown on Si(111) substrates by MOCVD

Silicon is a promising substrate for GaN growth due to its low cost, large size and potential applications in the integration of optoelectronic and microelectronic devices. However, a high lattice (~17%) and thermal (~56%) mismatch between Si substrate and GaN epilayer causes large tensile stress and leads to the formation of cracks when the GaN layer thickness is above 1.0 μm during the cooling down to room temperature. Therefore, the growth of high quality and crack-free GaN layers on silicon substrates is still a challenging issue. It has been widely reported that cracking problem can be mitigated by using techniques like low-temperature (LT) AlN interlayers, AlxGa1-xN transition layer, thin SixNy interlayers, AlN/GaN superlattices, etc. In this work, we used the LT-AlN interlayers for strain engineering during the growth of GaN layers by low-pressure metal-organic chemical vapor deposition (LP-MOCVD) on Si(111) substrates. And we investigated the fluence of the thickness of LT-AlN interlayers, which is considered as one of the most important factors in reducing tensile stress and controlling density of micro-cracks in GaN layer. Optical microscopy, atomic force microscopy, X-ray diffraction and Raman spectrum were employed to characterize these samples of GaN epilayers. The results demonstrate that the crystal quality of GaN depends strongly on the thickness of the epilayers, and crack-free GaN layers with thickness exceeding 1.5 μm can be achieved by using LT-AlN interlayers with an optimized thickness.