Conduction band modifications by d states in vanadium doped CdO

Abstract

3d transition metal ions have been shown to act as donors and to introduce highly localized d-levels when incorporated in CdO. In this work, we synthesized CdO thin films doped with a 3d metal V (CVO) with a V mole fraction x up to 0.1 by radio-frequency magnetron sputtering. CVO thin films exhibit a monotonic increase in the electron concentration n with x up to a saturation value of ∼1.0 × 1021 cm−3 at a dopant concentration x > 0.045. In contrast to CdO doped with shallow donors such as In where the electron mobility μ is > 100 cm2/V-s even at a high In doping level of >5%, in CVO the μ decreases continuously from ∼100 to <10 cm2/V-s with increasing x. Spectroscopic ellipsometry measurements show a rapid increase in the effective mass in CVO, consistent with the continuous reduction of μ. The electrical and optical properties of CVO are analyzed in terms of the anticrossing interaction of the localized V d-levels with the extended conduction band (CB) states of the host CdO. Such anticrossing interaction splits the CdO CB into two non-parabolic subbands, E+ and E-. The optical absorption edge of CVO with x > 0.045 is consistent with transitions from the valence band to the empty E+ subband while the much reduced μ can be explained by the increase in effective mass due to flattening of the E- subband. Such band anticrossing interaction is a common phenomenon for most transition metal dopants with d electrons in metal oxides and can explain the optoelectronic properties of these materials.