Growth on c-plane sapphire

The effects of the substrate offset angle (vicinal substrates) on M BE growth of ZnO on c-plane sapphire were investigated by Sakurai et al. [91]. Twin-crystal RHEED patterns from both (1010) and (1120) planes and surface faceting observed on on-axis c-plane substrates were suppressed by extending the offset angle from near-zero to 2.87°, tilted from c-plane toward o-axis direction. As a result, the crystal structure and the surface morphology were improved. 

Consequently, the resultant ZnO film is O polarity. However, if the MgO layer, which would be under compressive strain in the wurtzitic from, is greater in thickness than a critical value, the wurtzite structure gives way to (111) rocksait owing to relaxation. Eventually, for MgO layer thickness exceeding 3 nm, MgO islands with rocksait 

Structure cover the entire surface. Owing to the topmost O atoms in rocksalt, MgO having three dangling bonds, each Zn atom in bonding with those O atoms would have a single dangling bond up along the c-axis, which is characteristic of Zn polarity. 

In short, MgO on sapphire is initially of wurtzitic nature for 1 nm or so, which gives way to the rocksalt symmetry as deposition continues. If ZnO is grown on wurtzitic MgO, O polarity ZnO layer is obtained. On the contrary, if the rocksalt phase of MgO is allowed to form, ZnO grown on it is of Zn polarity [97]. The in-plane epitaxial relationship between ZnO and sapphire in the latter case is [2il0]2 o// 

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