Effects of Free Carriers on the Optical Properties of Doped CdO for Full-Spectrum Photovoltaics

CdO-based transparent-conducting oxide thin films have great potential applications in optoelectronic devices due their high mobility, low resistivity, and high transparency over a wide spectral range. In this paper, we report the results of a comprehensive study of optical properties of CdO thin films doped with different donors (In, Ga, V, Ti) with a carrier concentration in the range of 10(20) to > 10(21)/cm(3). Variable angle spectroscopic ellipsometry (SE) studies reveal that the complex dielectric function of CdO thin films drastically depends on the carrier concentration. Specifically, with increasing carrier concentration, (1) the net effect of Burstein-Moss shift and band-gap renormalization gives rise to an increase in the optical band gap from 2.6 to 3.2 eV; (2) the free-carrier absorption coefficient at a wavelength of 1200 nm increases from 10(2) to 1 x 10(4) cm(-1); (3) the refractive index decreases from 2.4 to 2.05 at 600 nm; (4) the high-frequency dielectric constant reduces from 5.5 to 4.8. The SE results are analyzed with results from Hall measurements to obtain information on the electron effective mass and optical mobility of CdO thin films. The significantly higher effective mass of V- and Ti-doped CdO thin film is attributed to the modification of the conduction band due to an anticrossing interaction between the localized d levels of V and Ti atoms and the CdO conduction-band extended states. The effective mass of In-and Ga-doped CdO increases with the electron concentration, consistent with the prediction from the nonparabolic conductionband model. We also find that the optical mobility mu(opt) is close to the Hall mobility mu(Hall) when the mu(Hall) < 60 cm(2)/Vs, while mu(opt) < mu(Hall) for materials with higher mu(Hall).