This paper reports on the optical and transport properties of undoped and In-doped CdO films prepared by pulsed laser deposition. Film thickness (around 150 nm) was chosen to allow for an accurate measurement of the absorption coefficient spectrum up to 2-3 eV above the direct bandgap. The imaginary part of the dielectric function, as determined from absorption spectra, is consistent with ellipsometry results in the literature. The optical gap and transition width are determined from a gaussian fit to the first derivative of the absorption spectrum. CdO optical gap so determined increases from 2.3 to 3.4 eV for electron concentrations increasing from 1.8 × 1019 to 1.1 × 1021 cm−3 with a Burstein-Moss shift of 1.1 eV, much larger than those currently found in the literature for similar or larger carrier concentration ranges. We discuss this discrepancy and show that the origin of the current underestimation is related to an extended misuse of α2-versus-hν plots to determine the optical gap. A clear correlation between the optical transition width and electron mobility is also shown and discussed.

The nature of the charge-transfer absorption band in undoped and Ga-doped Zn1−xMxO (M: Co, Mn) thin films is investigated by means of photoconductivity and optical absorption measurements under pressure. Internal transitions in the crystal field split Co 3d shell do not contribute to the photoconductivity spectrum and have very low pressure coefficient. Broad absorption bands at photon energies just below the band gap in both ZnMnO and ZnCoO clearly contribute to the photoconductivity spectra, indicating that they create free carriers and are consequently charge-transfer transitions. Under pressure, charge transfer bands have a pressure coefficient close to or larger than the band gap, in contrast to the expected low or negative pressure coefficient in a valence-band-to-localized level transition. Finally, the expected Burstein–Moss shift in the fundamental edge of heavily Ga-doped samples of ZnMO is associated to a larger shift and intensity decrease in the pre-edge band, confirming that charge-transfer transitions in ZnMO should be ascribed to transitions from the Co or Mn 3d shell to the conduction band.

This paper investigates the electronic structure of wurtzite (W) and rock-salt (RS) Zn1-xCoxO nanoparticles (NPs) by means of optical measurements under pressure (up to 25 GPa), X-ray absorption, and transmission electron microscopy. W-NPs were chemically synthesized at ambient conditions and RS-NPs were obtained by pressure-induced transformation of W-NPs. In contrast to the abrupt phase transition in W-Zn1-xCoxO as thin film or single crystal, occurring sharply at about 9 GPa, spectroscopic signatures of tetrahedral Co(2+) are observed in NPs from ambient pressure to about 17 GPa. Above this pressure, several changes in the absorption spectrum reveal a gradual and irreversible W-to-RS phase transition: (i) the fundamental band-to-band edge shifts to higher photon energies; (ii) the charge-transfer absorption band virtually disappears (or overlaps the fundamental edge); and (iii) the intensity of the crystal-field absorption peaks of Co(2+) around 2 eV decreases by an order of magnitude and shifts to 2.5 eV. After incomplete phase transition pressure cycles, the absorption edge of nontransformed W-NPs at ambient pressure exhibits a blue shift of 0.22 eV. This extra shift is interpreted in terms of quantum confinement effects. The observed gradual phase transition and metastability are related to the NP size distribution: the larger the NP, the lower the W-to-RS transition pressure.