Direct MOVPE- and MBE-growth of a-plane GaN on r-plane sapphire

We report on the growth of pit-free a-plane GaN on (1 (1) over bar 02) sapphire (r-plane) substrates by metal organic vapor phase epitaxy (MOVPE) using a three step growth method without low temperature nucleation layer. X-ray diffraction omega-scans of the symmetric GaN (11-20) reflex revealed for 1.2 mu m thick crack- and pit-free GaN layers low FWHM of 885 arcsec and 2484 arcsec measured by inclination in c- and m-direction, respectively. To analyze the evolution growth was stopped at different, stages and the samples were measured with AFM, SEM, and XRD. Furthermore, we discuss the overgrowth of MOVPE GaN layer with molecular beam epitaxy (IMBE). For smooth MOVPE templates with closed surfaces the morphology was reproduced, whereas for not coalesced thinner layers a different growth mode and a smoothening of the layers occurred. We will discuss SEM- and AFM-data in detail to-examine this overgrowth procedure.