Fabrication and characterization of hexagonally assembled ZnO and ZnO:N thin films with buffer layer by pulsed-laser deposition

Abstract

Zinc oxide (ZnO) is a wide band gap (3.37 eV) material and significantly interesting for many applications. Recently, many studies have been directed toward the fabrication of p-type ZnO using the group V elements (N, As, P, Sb). We have fabricated ZnO thin films in nitrogen background gas by the pulsed-laser deposition (PLD), because nitrogen is the most promising dopant. The nitrogen incorporation into the films was confirmed by X-ray Photoelectron Spectroscopy (XPS) analyses for the films grown under the high nitrogen pressures. However, the nitrogen doped films do show the disordered hexagonal microstructures which induce the defects into the crystal resulting from strains and stresses. Therefore, we have introduced the ZnO low-temperature buffer layers (LTBLs) between ZnO thin films and sapphire substrates to reduce the defects. The growth conditions of the ZnO LTBL were experimentally optimized for the first time. Characteristics of ZnO thin films with and without a ZnO LTBL were determined by x-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FE-SEM), and Atomic Force Microscopy (AFM). The electrical properties of the ZnO thin films were measured by the van der Pauw method. As a result, epitaxial lateral overgrowth (ELO) and hexagonally assembled ZnO have been successfully confirmed using LTBL. Nevertheless, the films still show the n-type conductivity, our results clearly demonstrate the advantages of the ZnO LTBL.