Critical thickness and lattice relaxation of Mg-rich strained Mg0.37Zn0.63O (0001) layers towards multi-quantum-wells

The heteroepitaxy of Mg-rich Mg0.37Zn0.63O layers on ZnO (0001) substrates was carried out using laser molecular-beam epitaxy. Mg0.37Zn0.63O layers changed from a two-dimensional (2D) to three-dimensional growth mode at a layer thickness (tc) between 38 and 100 nm through lattice-strain relaxation. For tc>100nm, hexagonal nanodots with a density in the order of 109cm−2 formed naturally by the Stranski-Krastanov mode. The individual nanodots possessed pyramidal hillocks with lateral sizes raging from 100 to 200 nm, and phase separation to Mg-rich and Mg-poor regions in the Mg0.37Zn0.63O alloys was found from the results of atomic force microscopy and microphotoluminescence spectroscopy. A suitable layer thickness of Mg0.37Zn0.63O concerning quantum barriers was speculated as being 38 nm from a theoretical calculation based on the Matthew and Blakeslee model [J. Cryst. Growth 27, 118 (1974)]. For tc≦38nm, the top surface of the Mg0.37Zn0.63O layer was very flat due to the curtailment of 2D growth. This contributed to the coherent growth of Mg-rich Mg0.37Zn0.63O∕ZnO multi-quantum-well structure (MQWS) with high crystallinity, as characterized from structural analyses using high-resolution x-ray diffraction. The two-dimensional properties of the MQWS were confirmed from the anisotropic optical property and electrical conductivity with 2D electron transport at low temperatures.