Optical properties of nominally undoped n-type MOVPE GaN epilayers

Nominally undoped GaN epilayers have been grown by the metalorganic vapour phase epitaxy (MOVPE) technique on (0001) sapphire substrates. Various growth conditions result in different concentration of defects, which is strongly correlated with the electron concentration. For epilayers selected to these investigations the electron concentration changes from 5×1015 cm–3 to 5×1018 cm–3. The optical methods like photoluminescence (PL), reflectance (R) and photoreflectance (PR) have been applied to define a correlation between quality and electron concentration of the GaN epilayer. It has been found that an improvement of optical properties, which is always associated with the improvement of the sample quality, appears to be connected with the decrease in electron concentration. The existence of free excitons has been observed for epilayer with the electron concentration lower than 1017 cm–3.