Predicted electrical properties of modulation-doped ZnO-based transparent conducting oxides

A one-dimensional Poisson/Schrodinger program has been used to predict the effect of layer thicknesses, donor concentration, and band-gap offset on the electrical properties of transparent conducting modulation-doped ZnO/ZnMgO multilayer structures. Mobilities as high as 145 cm(2)/V s were predicted for a structure with an average carrier density of 3.8x10(18) cm(-3) and a resistivity of 1x10(-2) Omega cm; for a comparable resistivity in monolithic ZnO, the mobility would be lower similar to 30 cm(2)/V s and the carrier density would be higher, leading to higher optical absorption. However, it was found that the maximum sheet electron density that could be transferred from the doped to the undoped layers was similar to 10(13) cm(-2), limiting the lowest calculated resistivity to similar to 1.5x10(-3) Omega cm. The optimal thicknesses to simultaneously achieve high mobility and low resistivity were 2-5 nm for both the pure ZnO and ZnMgO:Al layers. For ZnO thicknesses above this range the resistivity steadily increased, and below 2 nm the mobility decreased. For ZnMgO:Al thicknesses increased above this range, the mobility rapidly decreased, whereas decreasing below 2 nm increased the resistivity. The effect of the pure ZnMgO set-back layer thickness on mobility is discussed and a spacer layer of similar to 1.5 nm is proposed for ZnO/ZnMgO multilayers. The effect of ZnO layer thickness on possible intersubband scattering is also discussed. (c) 2005 American Institute of Physics.