Optical and structural properties of MgZnO/ZnO hetero- and double heterostructures grown by pulsed laser deposition

MgZnO thin films, MgZnO/ZnO heterostructures (HS) and double heterostructures (DHS) have been prepared on a-plane sapphire substrates by means of pulsed laser deposition (PLD). A linear blueshift of the MgZnO emission with increasing Mg content is observed in photoluminescence spectroscopy (PL) at 2 K. Cathodoluminescence measurements verify the spatial homogeneity of the emission properties of the MgZnO films. The film roughness is evaluated from atomic force microscopy scans. In MgZnO/ZnO HS the ZnO grows on all appearing MgZnO facets. PL investigations of such PLD-grown heterostructures show the high optical quality of thin ZnO films (d≤100 nm) grown on MgZnO. Capping those structures with a thin MgZnO layer further improves their luminescence intensity and enhances the emission of free-exciton luminescence from the ZnO layers. MgZnO/ZnO/MgZnO DHS with nominal ZnO layer thicknesses of dnom≤6 nm show a clear intensification of the ZnO PL. Temperature dependent PL and transmission measurements between 4.4 and 300 K prove the dominating emission to be due to the recombination of excitons localized in the ZnO. At 2 K, due to confinement effects, their emission energy is blueshifted up to 51 meV compared to free excitons in bulk ZnO.