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

A one-dimensional Poisson/Schrödinger 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 145cm2∕Vs were predicted for a structure with an average carrier density of 3.8×1018cm−3 and a resistivity of 1×10−2Ωcm; for a comparable resistivity in monolithic ZnO, the mobility would be lower ∼30cm2∕Vs 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 ∼1013cm−2, limiting the lowest calculated resistivity to ∼1.5×10−3Ωcm. The optimal thicknesses to simultaneously achieve high mobility and low resistivity were 2–5nm for both the pure ZnO and ZnMgO:Al layers. For ZnO thicknesses above this range the resistivity steadily increased, and below 2nm the mobility decreased. For ZnMgO:Al thicknesses increased above this range, the mobility rapidly decreased, whereas decreasing below 2nm increased the resistivity. The effect of the pure ZnMgO set-back layer thickness on mobility is discussed and a spacer layer of ∼1.5nm is proposed for ZnO∕ZnMgO multilayers. The effect of ZnO layer thickness on possible intersubband scattering is also discussed.