Free-carrier effects on zero- And one-phonon absorption onsets of n-type ZnO

Epitaxial, n-type ZnO:Ga layers grown by laser molecular-beam epitaxy were investigated by Hall-effect and spectroscopic techniques. We have studied the effects of free carriers on their optical properties in the spectral range close to the fundamental energy gap. To precisely determine the absorption onset energies, line-shape analysis of the absorption spectrum was performed using a model taking the many-body effect due to the presence of high-density electron gas into account. As a result, donor doping gives rise to a systematic blueshift of the fundamental absorption edge, consistent with the Burstein-Moss effect. The Burstein-Moss shifting behavior of ZnO:Ga is compared with that in the case of ZnO:Al. The experimental data for ZnO:Ga agree very well with the theory developed by Sernelius et al. [Phys. Rev. B 37 (1988) 10244], while the data for ZnO:Al do not. The temperature-dependent spectroscopy allowed us to investigate the effect of free carrier also on an exciton-phonon complex. It was found that, at low temperatures, the spectrum had an abrupt onset followed by two asymmetric peaks, including the anti-Stokes phonon sideband, which is explained in terms of optical singularities.