Photoreflectance investigations of GaN epitaxial layers

The photoreflectance spectroscopy was used to investigate GaN epitaxial layers. Heterostructures being under study were grown by metalorganic vapour phase epitaxy (MOVPE) and hydride vapour phase epitaxy (HVPE) techniques on sapphire substrate. All measurements were performed at room temperature, what is an advantage in comparison to other spectroscopic techniques. The photoreflectance spectra exhibit resonances, which originate from the direct optical transition between the conduction band and valence band. From the energy position of these resonances, the direct band-gap energies of semiconductor being under study were obtained. Franz–Keldysh oscillations (FKO) above the band gap in doped (HVPE) structure are observed. From the period of Franz–Keldysh oscillations the strength of the surface electric field was determined.