Growth and structure of ZnO thin films on polar (√3 × √3)R30° reconstructed and unreconstructed MgO(111) surfaces by atomic layer deposition

Abstract

Polar heterointerfaces of MgO(111) and the II–VI semiconductor ZnO are of technological interest for transparent conducting electrode applications. Growth and structure of thin films on polar surfaces can be different than on nonpolar surfaces due to the large surface energy of polar surfaces. The authors have grown ZnO on unreconstructed MgO(111)-(1 × 1)-OH terminated and reconstructed MgO(111)-(√3 × √3)R30° polar oxide surfaces using atomic layer deposition (ALD). A homemade ultrahigh vacuum-interfaced viscous-flow ALD reactor with in situ quartz crystal monitor was used to grow ZnO thin films on the MgO(111) substrates. In the ALD process temperature window, the growth rate was found to be ∼2.3 Å/cycle. Atomic force microscopy revealed that the surface roughness increases with ZnO film thickness and that reconstructed MgO(111) is a better substrate for production of smooth ZnO films. X-ray diffraction analysis revealed that ZnO thin films grown at 130 °C are polycrystalline, having the wurtzite structure, with preferential growth along the c-axis. ZnO grown on MgO(111)-(√3 × √3)R30° substrates shows strong preferential growth along the (002) direction. In contrast, growth along the (100) and (101) directions is also observed when grown on MgO(111)-(1 × 1) substrates. These observations indicate that the crystal orientation during ALD ZnO growth depends not only on temperature but also on the surface terminations of the substrates. Optical transmittance spectra were used to find the bandgap of 3.27 eV and sharp ultraviolet cutoff of 380 nm for ALD-grown ZnO thin films. The optical transmission was found to be more then 80% in the visible region.