Sputter epitaxy of Mg-doped ZnO films on sapphire substrates using inverted Stranski-Krastanov mode*

Mg-doped ZnO has a theoretically wide band gap scope from 3.37 eV to 7.8 eV. Its slight lattice mismatch is attributed to the large band gap (7.8 eV) of MgO and the close proximity of ionic radii between Mg²⁺ (0.57 Å) and Zn²⁺ (0.60 Å). Here, we have succeeded in inverted SK growth of Mg-doped ZnO films on 18% lattice mismatched sapphire substrates 1), where buffer layers consisting of 3D islands initially form and relaxed 2D layers subsequently grow on the buffe layers 2). We have realized band gap tuning of Mg-doped ZnO films in the range 3.5–4.3 eV controlling Mg concentration and observed sharp peaks due to exciton absorption in photo-absorption spectra even at room temperature. These results  open a new avenue to make high-performance optoelectronic devices that consist of heterostructures.

1) D. Takahashi, et al., MRS Advances, https://doi.org/10.1557/s43580-022-00234-1. 2) N. Itagaki, et al., Sci. Rep. 10, 4669 (2020).