Thermal stress analysis and structural design of Al x Ga 1−x N/GaN epilayers based on Si substrate

A finite element model based on coupled field was established to analyze and design high quality GaN epilayers grown on Si substrate with thin variable composition graded Al x Ga 1−x N interlayers. Theoretical simulation shows that Al x Ga 1−x N interlayers can effectively reduce the tensile stress which generated in the cooling process between Si and GaN for thermal mismatch. When the Al x Ga 1−x N is single component, with the proportion of Al is larger, the thermal stress will decrease. In this case, the maximum stress reduces 13.8%, but the distribution uniformity of stress make worse. When the Al x Ga 1−x N is variable composition graded, the stress of GaN epilayers reduced significantly. Further analysis showed that bigger x at the lower interlayer (near the Si substrate) and smaller x at the upper interlayer (near the GaN epilayers) of Al x Ga 1−x N interlayers simultaneous can reduce the risk of cracks and make the GaN quality better. The calculation results show that without considering the effect of thermal mismatch and the lattice mismatch, before the experiments if we carefully design the interlayers structural through finite element model can effectively reduce the residual thermal stress. It is an effective way to save cost and time.