Defect reduction in GaN epilayers and HFET structures grown on (0001)sapphire by ammonia MBE

The quality of GaN epilayers grown by molecular beam epitaxy on substrates such as sapphire and silicon carbide has improved considerably over the pas t few years and in fact now produces AlGaN/GaN HEMT devices with characteri stics among the best reported for any growth technique. However, only recen tly has the bulk defect density of MBE grown GaN achieved levels comparable to that obtained by MOVPE and with a comparable level of electrical perfor mance. In this paper, we report the ammonia-MBE growth of GaN epilayers and HFET structures on (0001)sapphire. The effect of growth temperature on the defect density of single GaN layers and the effect of an insulating carbon doped layer on the defect density of an overgrown channel layer in the HFE T structures is reported. The quality of the epilayers has been studied usi ng Hall effect and the defect density using TEM, SEM and wet etching. The g rowth of an insulating carbon-doped buffer layer followed by an undoped GaN channel layer results in a defect density in the channel layer of similar to2 x 10(8) cm(-2). Mobilities close to 490 cm(2)/Vs at a carrier density o f 8 x 10(16)cm(-3) for a 0.4 mum thick channel layer has been observed. Gro wth temperature is one of the most critical parameters for achieving this l ow defect density both in the bulk layers and the FET structures. Photo-che mical wet etching has been used to reveal the defect structure in these lay ers. (C) 2001 Elsevier Science B.V. All rights reserved.