Growth Control of N-Polar GaN in Plasma-Assisted Molecular Beam Epitaxy

Abstract

Plasma–assisted molecular beam epitaxy (PA MBE) is one of the main growth techniques of group-III nitrides. It has been found, that the growth of GaN with Ga-polarity is preferable for most device applications, since this promotes two-dimensional growth with an atomically smooth surface (ASS). In contrary, most researchers avoid the N-polar GaN layers possessing as a rule the rough surface morphology. On the other hand, there are some fields (InN growth, new types of high-mobility transistors) where N-polar GaN with ASS is needed. However, only a limited data on this topics has been reported [1, 2]. The paper reports on the growth kinetics of N-face GaN layers. The 1-μm-thick films were grown on nitridated c-sapphire substrates. The growth kinetics was studied in-situ using laser interferometry with a wavelength of 532 nm and RHEED at growth temperature range of TS=690-820°C and group III to activated nitrogen flux ratio FGa/FN* varied within 0.7-1.8. The surface polarity was determined by chemical etching in 2M NaOH for 10 minutes at RT. Structural, optical and electrical properties were characterized by atomic force microscope (AFM), photoluminescence (PL) and Hall measurements. Fig.1 shows a growth rate of N-polar GaN versus TS. It begins to drop at TS>750°C, that corresponds to the onset of GaN decomposition. In addition, it has been found an increase of Ga desorption with rising Ts. Therefore, to provide the two-dimensional growth at TS=760°C the value of FGa/FN* should be kept at ~1.8 instead of ~1.1 sufficient at TS=690°C. The inset in Fig.1 shows the AFM image of the high-TS GaN surface with rms of 3.4nm over a 5×5μm area and <0.5nm over a 200×200 nm, which is comparable to GaN(0001) surface quality. In addition the PL at 20K with full width at half maximum of 8 meV was observed at 3.47eV. Hall measurements showed the residual electron concentration and mobility of 2⋅10 17 cm -3