Far-infrared and selective photoluminescence studies of shallow donors in GaN hetero- and homoepitaxial layers

Shallow donors in GaN epilayers grown by metalorganic chemical vapor deposition on sapphire and GaN substrates have been studied by selectively excited photoluminescence (SPL) and by far-infrared (FIR) spectroscopy. A comparison of FIR and SPL results reveals a small splitting between the $2s$ and $2p$ donor states, interpreted as partly being due to the nonspherical crystal symmetry. The utilization of the selective excitation of neutral donor bound excitons $I2$ allows the identification of the $I2$ rotational excited states. An interpretation of two-electron spectra in GaN involving excited states of $I2$ is proposed.