Dielectric function spectra of GaN, AlGaN, and GaN/AlGaN heterostructures

The pseudodielectric function spectra (epsilon) of wurtzite GaN, AlGaN, and GaN/AlGaN heterostructures were determined for photon energies ranging from 2 to 5 eV, using variable angle spectroscopic ellipsometry (SE). Samples were grown by low-pressure metalorganic chemical vapor deposition on c-plane sapphire substrates. The experimental (epsilon) spectra were analyzed using a multilayer approach, describing the dielectric functions of the individual layers by a parametric oscillator model. In this way, parametric dielectric function spectra of GaN and Al(x)Ga(1-x) N(x<=0,16) were derived, as well as the composition dependence of the Al(x)Ga(1-x)N band gap energy. The SE band gap data were found to be consistent with a bowing parameter close to 1 eV. Finally, the GaN and Al(x)Ga(1-x)N parametric dielectric functions were used to quantitatively analyze the pseudodielectric function spectrum of GaN/AlGaN modulation doped field effect transistor structures, demonstrating the potential of SE in combination with a multilayer parametric dielectric function model for nondestructive ex situ control of GaN/AlGaN device structures.