Origin of high-density two-dimensional electron gas in ZnO/ZnMgO heterostructures

Numerical calculations are performed for determining the polarization charge density a and residual donor concentrations in ZnO/ZnMgO heterostructures. The a value is determined to be 10 mC/m2 for explaining the cathodoluminescence (CL) peak positions in narrow ZnO/ZnMgO multiple-quantum-well (MQW) structures (≤ 4 nm), and is consistent to those obtained from the two-dimensional electron gas (2DEG) density formed in single-hetero and single-quantum-well structures. A significantly high residual donor concentration of 9 x 1018 cm-3 for ZnMgO layers is obtained for explaining the CL energy to level off for wider wells (≥ 4 nm), and is confirmed by predicting the 2DEG density in a thick ZnO layer grown on an MQW buffer layer which is identical to the one of the MQW structures. The calculation also shows that the 2DEG concentrations in the MQW structures are dominated by the high residual donor concentration