Effects of buffer layer growth conditions on GaN epilayer quality by MOCVD

We have found that the electrical and optical properties of GaN epilayers strongly depend on buffer TMGa flow rates and NH₃ flow rates. At low flow rate of 4 sccm, the buffer layer quality was good so the concentrations of undoped GaN epilayers decreased and a stronger band-edge emission of 362 nm can be observed in photoluminescence spectra. The carrier density of the films can be reduced from 10¹⁸ to 10¹⁷ cm^-3 by increasing the NH₃ flow rate from 0.5 up to 1 SLM and comparatively increased the near-band-edge emission. So, with a NH₃ flow rate as high as 1 SLM, the GaN epilayers with good optical quality can be obtained. The excitation power density of He-Cd laser influences the photoluminescence property of GaN epilayers. At high excitation power density of 637 W/cm², the near-band-edge luminescence (362 nm) is dominant and the deep level luminescence (near 550 nm) appears very weak. But at low excitation power density, the luminescence from defect yellow band levels is stronger than the near-band-edge transitions.