Nucleation and epitaxial growth of ZnO on GaN(0001)

Abstract

• Significant ZnO nucleation and MBE growth was enabled on Zn, N plasma and Ga pre-treated air-exposed GaN(0001)/Al 2 O 3 -templates. • ZnO nucleation was reduced and growth was completely inhibited on un-treated air-exposed GaN(0001)/Al 2 O 3 -templates. • Significant ZnO nucleation and growth was enabled on untreated in situ MBE grown GaN/4HSiC(0001) buffer layers not exposed to air. • Removal of Ga x O y sub-oxides was necessary for achieving ZnO growth on air-exposed GaN(0001)/Al 2 O 3 -templates. • No removal of Ga x O y sub-oxides was necessary for achieving ZnO growth on GaN(0001)/4H-SiC(0001)-layers not exposed to air. Plasma-assisted molecular beam epitaxy was used to grow ZnO(0001) layers on GaN(0001)/Al 2 O 3 templates and GaN/4H-SiC(0001) layers. The GaN(0001)/Al 2 O 3 template surfaces were subjected to various pre-treatment procedures (Zn, Ga or N pre-exposure or none) prior to the ZnO growth. We studied the impact of these pre-treatment procedures on the initial growth conditions of ZnO(0001). These layers were compared to ZnO layers deposited on 4H-SiC utilizing a GaN(0001) buffer layer that was grown in situ on the 4H-SiC substrate and immediately before the growth of ZnO. The GaN buffer layers were not pre-treated or exposed to ambient. Atomic force and scanning electron microscopy as well as secondary ion mass spectroscopy revealed that the pre-treatment procedures resulted in a very high density of islands. The islands coalesced into films as the growth progressed. In contrast, no ZnO growth occurred on the untreated GaN(0001)/Al 2 O 3 template surfaces. Our main finding is that Ga x O y sub-oxides residing on the surface of the as-received GaN-templates, drastically reduced the ZnO nucleation rate and completely inhibited subsequent coalescence and growth. Our various surface pre-treatment procedures aimed at removing the sub-oxides were necessary for achieving ZnO growth on the GaN-templates. No surface pre-treatment was needed to enable ZnO growth on the in situ grown GaN(0001)/4H-SiC layers.