Comment on 'Bandgap and Effective Mass of Epitaxial Cadmium Oxide' [Appl. Phys. Lett. 92, 022101 (2008)]: Article No. 106103

In a recent letter, Jefferson et al. discussed the determination of the bandgap and effective mass (of electrons) in CdO. The authors cited our paper and our interest in thin-film photovoltaics and flat-panel displays. While this comment is correct, the authors neglected to note the fact that we used the method of four coefficients, a transport-based technique, to measure the effective mass of electrons and their dispersion with carrier concentration in CdO thin films. We were also able to estimate the value of the effective mass at the bottom of the conduction band. In addition, we measured the relaxation time of the electrons and the Fermi level as functions of carrier concentration. Furthermore, we confirmed the presence of an optical phonon mode, as noted by the authors in connection with Eq. (1). The authors made no comparison between their estimate of effective mass and our more detailed measurements and nor did they comment that our estimate agreed closely with that obtained by Koffyberg, whereas their value did not agree.