Influence of Pulse Atomic-Layer Epitaxy (PALE) AlN Buffer Layer on Quality of MOCVD Grown GaN on Si(111) Substrate.

Abstract

A crack-free GaN-on-Si with a high capability to grow a thick GaN layer is important for the performance of optoelectronic devices. Regrettably, a large lattice and thermal mismatch degraded the quality of GaN-on-Si. Therefore, we introduce the pulse atomic-layer epitaxy (PALE) AlN buffer layer to overcome the surface crack issues and improve the quality of GaN epilayers. The effect of PALE AlN buffer layer with 0, 35, 70, and 140 cycle numbers towards the quality of grown GaN on Si(111) substrate is studied. All samples have been observed using AFM, FESEM, XRD and Raman spectroscopy. AFM imaging revealed a fully-coalesced and clear step flow GaN layer with the lowest RMS roughness of 0.606 nm. FESEM surface morphology shows a crack occurs on a sample without AlN PALE (0 pulse cycle) while crack-free GaN is obtained for samples with AlN PALE layer (35, 70 and 140 cycles). It was observed by XRD that the AlN PALE pulse cycle number greatly affects the structural properties of the top GaN layer where the lowest x-ray rocking curve for (002) and (102) achieved at 70 cycles, indicating the reduction of threading dislocations density in the growth structure. Further, the transition of the GaN E2 (high) from the Raman spectroscopy revealed a tensile strain in the GaN epilayers reduces with the increasing number of pulse cycles.