Reflectivity Studies of Lattice Vibrations and Free Electrons in MBE Grown GaN Epitaxial Layers

We have observed a sharp structure with a peak at the frequency of the E$\text{}_{1}$-TO phonon in the reflectivity of GaN epitaxial layers grown by molecular beam epitaxy on Si substrates. The simulations of the reflection performed show that the observed shape can be explained by assuming both collective lattice vibrations and free carriers contributions to the dielectric function. We assumed the Lorentz oscillator to describe the contribution of the collective lattice vibrations and the Drude-Lorentz model for that of free carriers. Fitting the calculated reflectivity to the spectrum obtained experimentally allowed us to evaluate lattice and free carrier parameters.