Transition metal elements as donor dopants in CdO

CdO has been shown to achieve a high electron concentration N (>10(21) cm(-3)) and at the same time a high mobility mu (>100 cm(2)/V s) when doped with conventional shallow dopants (In or Ga), and consequently making it a transparent conducting oxide with very low resistivity rho < 10(-4) ohm cm. In this work, the properties of CdO thin films doped with a series of transition metal elements (CdO:TM) with partially filled 3d and 4d shells, including Sc, Ti, V, Cr, Fe, Y, Mo, and W, were investigated. We find that doping with these TM elements can effectively increase the N in CdO to a maximum N (N-max) of similar to(7-12)x10(20) cm(-3) with a dopant concentration x(max) of 4-7 %. However, unlike CdO:In, the mu of CdO:TM films drops rapidly from >100 to <10 cm(2)/Vs as the dopant concentration x increases, so that they can only achieve a minimum rho of similar to(1-2)x10(-4) ohm cm, similar to a factor of 2-3 higher than that in CdO:In. As a result, free-carrier absorption and plasma reflection effects limit their optical transparency to <1200 nm. For most 3d TM dopants, a qualitatively higher d-donor level E-d,E-donor gives rise to higher E-F,E-max or a higher N-max. Although at low x, the optical band gap E-opt of CdO:TM follows the calculated values due to free-carrier effects, as x increases, E-opt values are significantly higher than the calculated values. This is believed to be an effect of the anticrossing interaction of the localized d-levels and the extended CdO conduction-band (CB) states, giving rise to a lower occupied E_ and an upper unoccupied E+ subband. The restructured CBs have much flatter dispersion, which also results in a much higher effective mass m(e)*, hence it can also explain the much lower mu of CdO:TM films with high N.