VAPOR-PHASE EPITAXY GROWTH OF GAN ON PULSED-LASER DEPOSITED ZNO BUFFER LAYER

Vapor phase epitaxy (VPE) is a promising method to produce GaN substra tes due to its high growth rate. In this paper, we first describe ZnO buffer layer deposition by pulsed laser deposition (PLD) on sapphire s ubstrates for subsequent GaN VPE growth. Previously, ZnO buffer layers for GaN growth were RF-sputtered films with only poly or highly orien ted crystal structure. PLD-grown ZnO buffer layers are single crystall ine with streaky RHEED patterns and exhibit a sharp band-edge peak in photoluminescence. We examined the effect of ZnO buffer layer thicknes s on film quality of VPE-grown GaN layers on c-plane sapphire substrat es. The VPE-grown GaN surfaces on ZnO buffer layers exhibit terrace-li ke flat surfaces, whereas three-dimensional growth with sharp rock-lik e structure occurs without the buffer layer. X-ray rocking curve (XRC) measurements showed that inserting the ZnO buffer reduced the GaN pea k width by more than a factor of two, suggesting better crystalline qu ality. From the XRC measurement, buffer layers upto 50 nm thickness im prove the GaN growth, while the optical properties measured by photolu minescence (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 str ong peaks from donor-acceptor pair recombination and deep acceptor lev el from 2.6-3.2 eV in the PL spectra which are associated with Zn-dopi ng of GaN. This implies that a thick ZnO buffer results in Zn diffusio n from the buffer layer into the VPE-grown GaN film. (C) 1998 Elsevier Science B.V. All rights reserved.