Photoreflectance (PR) spectra of a GaN thin film and an AlGaN/GaN heterostructure were measured by using a HeCd laser or a mercury lamp as a pump beam. The wavelengths (£f) of the HeCd laser and the mercury lamp are 325 nm and 253.7 nm, respectively. The energy of the HeCd laser is lower than band-gap energy of AlxGa1-xN (x > 0.2) so that electron-hole pairs cannot be generated in the AlGaN layer. Hence, the PR of the AlGaN was measured by using Argon ion laser (£f=300 nm) or quadrupled Nd:YAG (£f=266 nm) rather than HeCd laser in the previous works. In this work, the mercury lamp (£f=254 nm)was used as the pump beam. The problem with using the mercury lamp as the pump beam is because it is a diffused source so that it cannot be focused to a small spot. Nevertheless, defocused pump and probe beams were used in the PR measurement to improve signal to noise ratio. Hence, the diffused property of the mercury lamp is not a hindrance to the PR measurements.

It is known that electroreflectance (ER) of surface-intrinsic-n+ type doped GaAs has exhibited many Franz-Keldysh oscillations to enable the application of fast Fourier transform to separate the heavy and light-hole transitions. However each peak still contains two components, which belong to F+ F/2 and F- F/2 respectively, where F is the built-in field and F is the modulating field of applied voltage (Vac). In this work, we have used a larger Vac to modulate the field, and hence the peaks can be further separated. The peak belonging to heavy hole-transition and F- F/2 can be singled out to compare with Airy function-theory.