Growth and effects of single-crystalline ZnO buffer layer on GaN epitaxy

Single-crystalline ZnO films have been grown on c-cut sapphire substrates by pulsed laser deposition (PLD) using a KrF laser. The optimum growth conditions are 550°C and 10^-2 torr O₂ . We observe a sharp and streaked RHEED (reflection high energy electron diffraction) pattern and atomically smooth surface with a RMS roughness of only 5 Å measured by atomic force microscopy. Photoluminescence at 77 K shows a strong near-band-edge peak at 3.34 eV with no deep level emission. Two dimensional growth of GaN by chloride vapor phase epitaxy (VPE) is greatly enhanced compared to earlier results using RE-sputtered ZnO buffer layers. This is a result of better crystalline structure, optical properties and surface flatness of the PLD films. The properties of the VPE-GaN are strongly dependent on ZnO layer thickness. The optimum thickness of ZnO buffer layer is around 25 nm