A study on MOCVD growth window for high quality N-polar GaN for vertical device applications

Abstract Lateral N-polar GaN devices have outperformed Ga-polar GaN counterparts in mm-wave applications. A vertical N-polar GaN device structure with higher breakdown voltage is expected to further improve the output power density. This paper is aimed at understanding the growth conditions for such a vertical N-polar GaN device stack. A vertical device stack requires high-quality n-GaN layer with low unintentional dopant concentration and high electron mobility for better ON resistance–breakdown voltage performance. A systematic study by varying reactor temperature, pressure and NH₃ flow rate is performed in the metal organic chemical vapor deposition growth of N-polar GaN for vertical device structure. Structural quality of the sample is increased with increase growth temperature and higher NH₃ flow rate, yet an increase in surface roughness is observed for highest temperature used. The N-polar GaN grown at the optimized conditions exhibited a x-ray diffraction rocking curve full width half maximum of symmetric (0002) and skew-symmetric (20\overset{ˉ}{2} \overset{ˉ}{1}) scans of 358″ and 569″ respectively with a low oxygen and carbon concentration of 6 × 10¹⁶ cm⁻³ and 5 × 10¹⁶ cm⁻³, respectively. The optimized sample exhibited a low screw dislocation density of 2.5 × 10⁸ cm⁻² on sapphire substrate. A gradual increase in the bulk mobility of electrons with decrease in screw dislocation density in N-polar GaN is also reported.