Effects of surface states on two-dimensional electron gas in ZnMgO/ZnO heterostructures

The band profile and electronic properties of Zn-polar ZnMgO/ZnO heterostructures are investigated using self-consistent calculations of the Schrödinger, Poisson, and charge balance equations for 300K, all based on the assumption that the two-dimensional electron gas (2DEG) originates from donor-like surface states on the ZnMgO (0001) surface. Two models with different surface state distributions are applied to explain the ZnMgO thickness dependence of the 2DEG density. Multi-subband occupation of the 2DEG is taken into account, and surface states are included using Fermi-Dirac statistics. The polarization charge density at the ZnMgO/ZnO interface and the surface donor energy are added as adjustable fitting parameters to optimize the fits of the two models to experimental data. The fitting results indicate that surface states are more likely to be localized at a single energy level. Lastly, the microscopic origin of the surface states is discussed.