>Polarity dependent structure and optical properties of freestanding GaN layers grown by hydride vapor phase epitaxy
GaN freestanding substrate was obtained by hydride vapor phase epitaxy directly on sapphire with porous network interlayer. The morphologies of Ga-face and N-face of freestanding GaN substrate were analyzed by a variety of characterization techniques before and after etching in boiled KOH for 1 min. The obtained characteristics of unetched GaN are strongly dependent on the growth polarity. The N-polar GaN layer has high free electron, impurity and point defect concentrations. In the layers grown on the (0 0 0 1) Ga-polar side, these concentrations are very low. After etching, the Ga-polar GaN has identical properties to those of the unetched Ga-polar GaN layer. But the etched N-polar GaN has significant difference with unetched N-polar GaN layer in structure and optical properties. The etched N-polar GaN has a smaller (0 0 0 2) DCXRD width (646″) than the unetched N-polar GaN (1351″). The optical quality of etched N-polar GaN is comparable with that of Ga-polar GaN, and the FWHMs of the D0X line of Ga-face and etched N-face are 9.3 and 12.8 meV, respectively. The LPP mode in the Raman spectra and FERB peak in PL spectra were used to analyze the free carrier concentration of two sides of etched and unetched freestanding GaN substrate.