Observation of Fermi-edge and excitons and exciton-phonon complexes in the optical response of heavily doped n-type wurtzite GaN

We present the imaginary part of the dielectric function of n-type wurtzite GaN measured by spectroscopic ellipsometry in the spectral range of 3.2-4.5 eV at room temperature for electron concentrations from 1x10(16) to 2.3x10(19) cm(-3). The observed behavior is consistent with the Mott density of about 2x10(18) cm(-3). The comprehensive line-shape analysis demonstrates the importance of excitonic effects over the whole doping range. For low electron concentrations, we observe contributions from discrete excitons, Coulomb enhanced band-to-band (BB) optical transitions, and transitions into exciton-phonon complexes (EPCs). For degenerate GaN, along with the BB transitions, we identify Fermi-edge excitons as well as a significant enhancement of the optical response due to EPCs. States with different numbers of phonons (from one to approximately ten) are active in such optical transitions in heavily doped GaN.