The surface morphologies of ZnO thin films grown on Al2O 3 (112̄0) (a-plane sapphire) substrate by plasma-assisted molecular beam epitaxy (PAMBE) were systematically investigated by in situ reflection high energy electron diffraction (RHEED) and ex situ atomic force microscopy (AFM) as a function of Zn beam flux intensity and substrate temperature. The ZnO films are uniquely (0001̄) oriented with no trace of secondary orientation. A phase diagram for ZnO growth was established, which described the ZnO growth mode transition at fixed O2 flow rate of 0.3 seem. For the growth at 750°C with Zn beam flux of 3.6 × 10 -7 Torr, the RHEED showed a (3 × 3) pattern and a smooth surface represented by atomically flat terraces and half unit cell high steps (∼0.26 nm, a charge neutral unit of ZnO) was observed corresponding to a monolayer or bilayer thickness of Zn-O along the c-axis of (0001̄) ZnO.
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