Growth of nanostructured ZnO thin films on sapphire

Growth of ZnO nanostructures on c-plane sapphire has been investigated using three different methods. Pulsed laser deposition (PLD) at low incident pulse energies can yield nanorods, the majority of which are aligned at an angle of ∼50° to the substrate plane. Selected area electron diffraction reveals that the nanorods display two distinct epitaxial relationships with the sapphire substrate. Those inclined to the surface normal exhibit the relationships (112̄4)ZnO//(0001)sap; [101̄0]ZnO//[112̄0]sap and (0001)ZnO//(101̄4)sap; [101̄0]ZnO//[112̄0]sap. Members of the second family are aligned along the surface normal, with (0001)ZnO//(0001)sap and [101̄0]ZnO//[112̄0]sap; the relative yield of this latter class increases at higher incident pulse energies. Hydrothermal synthesis and chemical vapour deposition on sapphire substrates that have been pre-coated (by PLD) with a thin ZnO layer result, respectively, in well-aligned ZnO microrod and nanorod arrays, both of which satisfy the relationships (0001)ZnO//(0001)sap; [101̄0]ZnO//[112̄0]sap. In contrast, employing these latter methods with a bare sapphire substrate results in, respectively, poorly aligned structures and localized islands of growth.