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 ZnOZnMgO multilayer structures. Mobilities as high as 145 cm2 V s were predicted for a structure with an average carrier density of 3.8× 1018 cm-3 and a resistivity of 1× 10-2 × cm; for a comparable resistivity in monolithic ZnO, the mobility would be lower ∼30 cm2 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 ∼ 1013 cm-2, limiting the lowest calculated resistivity to ∼1.5× 10-3 × 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 ∼1.5 nm is proposed for ZnOZnMgO multilayers. The effect of ZnO layer thickness on possible intersubband scattering is also discussed.
ASJC Scopus subject areas
- Physics and Astronomy(all)