Effects of polarity and surface treatment on Ga- and N-polar bulk GaN

The effects of polarity and surface treatment on the morphological, electrical, and optical behaviors in bulk GaN have been investigated. Kelvin probe, atomic force microscopy (AFM), and photoluminescence (PL) techniques were utilized to examine a set of freestanding, bulk GaN samples, which were grown by halide vapor phase epitaxy. The Ga- and N-polar surfaces were treated with either a mechanical polish (MP) or chemical mechanical polish (CMP), which influences the morphology, surface photovoltage (SPV), and PL behaviors. Topography studies indicate that the CMP-treated, Ga-polar surface is the smoothest of the sample set, whereas the MP-treated, N-polar surface has the highest root mean square roughness. Local current-voltage spectra obtained with conducting AFM reveal a higher forward-bias, turn-on voltage for the N-polar versus Ga-polar surfaces. Using a Kelvin probe, intensity-dependent SPV measurements are performed on samples with CMP-treated, Ga- and N-polar surfaces, and provide band bending values of 0.83 and 0.70 eV, respectively. The restoration of the SPV from CMP-treated surfaces behaves as predicted by a thermionic model, whereas restoration from MP-treated surfaces has a faster rate than expected. This result is possibly due to enhanced electron conduction via hopping between defect states to the surface. The quantum efficiency of the PL from the CMP- and MP-treated surfaces at room temperature is ∼1 and 1 × 10^-5, respectively, suggesting high quenching of the PL for MP-treated surfaces by near-surface defects. Therefore, AFM, PL, and SPV data indicate that the MP-treated surfaces have a significantly higher density of surface defects.