Luminescence related to two-dimensional gas in ZnO/ZnMgO heterostructures

In c-oriented (001) wurtzite ZnMgO/ZnO heterostructures the polarization induced electric field is the driving force for the formation of two-dimensional electron gas (2DEG) at the ZnMgO/ZnO interfaces. In this work we report on our results of photoluminescence (PL) studies of single ZnMgO/ZnO heterostructures with different thicknesses of top ZnO layers grown on a-plane sapphire by molecular beam epitaxy. We show that the PL spectrum of ZnO layers depends on their thicknesses. In particular, in thin ~ 5-8 nm ZnO layers a broad emission band is observed at 3.1-3.3 eV, which shifts to higher energies with increasing temperature and excitation power. We suggest that this PL involves 2D electrons confined in the triangular potential well at the ZnMgO/ZnO interface. We also observe an emission near 3.4 eV, which we interpret as emission involving 2D electrons and holes from the ZnMgO barrier. The dependence of the PL on the thickness of the ZnO layer is discussed and we suggest that emission involving two dimensional 2D electrons reflects occupation of states in the triangular potential at the interface.