Vapor phase epitaxy growth of GaN on pulsed laser deposited ZnO buffer layer

Vapor phase epitaxy (VPE) is a promising method to produce GaN substrates due to its high growth rate. In this paper, we first describe ZnO buffer layer deposition by pulsed laser deposition (PLD) on sapphire substrates for subsequent GaN VPE growth. Previously, ZnO buffer layers for GaN growth were RF-sputtered films with only poly or highly oriented crystal structure. PLD-grown ZnO buffer layers are single crystalline with streaky RHEED patterns and exhibit a sharp band-edge peak in photoluminescence. We examined the effect of ZnO buffer layer thickness on film quality of VPE-grown GaN layers on c-plane sapphire substrates. The VPE-grown GaN surfaces on ZnO buffer layers exhibit terrace-like flat surfaces, whereas three-dimensional growth with sharp rock-like structure occurs without the buffer layer. X-ray rocking curve (XRC) measurements showed that inserting the ZnO buffer reduced the GaN peak width by more than a factor of two, suggesting better crystalline quality. From the XRC measurement, buffer layers upto 50 nm thickness improve the GaN growth, while the optical properties measured by photoluminescence (PL) remain unchanged. With a 200 nm thick ZnO buffer layer, cracks occur in the subsequent GaN layer, resulting in a broader XRC peak width. In addition, the GaN film on a thick ZnO buffer shows strong peaks from donor - acceptor pair recombination and deep acceptor level from 2.6-3.2 eV in the PL spectra which are associated with Zn-doping of GaN. This implies that a thick ZnO buffer results in Zn diffusion from the buffer layer into the VPE-grown GaN film.