Abstract
GaN-based heterostructures have been consistently employed into fabricating high-power electronic and short-wavelength optoelectronic devices with utilizing their wide bandgap since 1990s. Usually these heterostructures are grown on foreign substrates with c-plane surface. One of the pressing problems is the existence of a very strong polarization-induced electric field along the c-orientation direction. This innate characteristic is often disadvantageous for the performance of devices. It is widely acknowledged that developing GaN with the non-polar surfaces and in-plane crystal asymmetry could be the ultimate solution for further success of GaN-based materials and functional devices. Therefore, exhaustive research regarding GaN with non-polar surfaces such as m-plane is highly requisite.
In this study, we present a detailed investigation of the band-edge optical transitions involving the interacting exciton-phonon system, especially first-order longitudinal optical (LO) phonon-assisted luminescence of bound and free excitons in m- and c-plane GaN substrates in a low temperature range from 4 K to 40 K. The main luminescence features of all the three kinds of excitons can be well described by the theoretical models that take exciton-LO-phonon coupling into account. The effective Bohr radii of the excitons play a key role in determining the Huang-Rhys factor characterizing the exciton-LO-phonon coupling strength in GaN. An interesting oscillatory structure is found to appear in the low-temperature luminescence spectra of the nonpolar-plane GaN substrate, which needs to be clarified by further investigations.
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