ZnO Thin Films Deposited on Sapphire by High Vacuum High Temperature Sputtering

Abstract

Zinc oxide is widely recognised as a material suitable for applications in transparent electronics, photonics, sensors, and high power electronics due to its semiconductor and oxide properties and today is one of the most studied semiconductors worldwide. One of the challenges for ZnO thin film fabrication is obtaining layers of high crystalline quality. Due to high prices of free standing high-quality ZnO substrates, most of the scientific work on ZnO deposition is performed on sapphire substrates, which however has an 18% lattice mismatch to ZnO. To cope with this problem, thin MgO buffer layers are widely used in MBE ZnO growth. The closer fit of the atom positions on [111] MgO and ZnO [0002] surfaces than on sapphire [0001] and ZnO [0001] surfaces leads to less strain and defects in the ZnO layers. Thus MgO layers can promote the correct crystalline growth of the ZnO thin films [1–4]. They can also be used to control the polarity of ZnO films grown on them [5, 6]. Ease of implementation and low costs in comparison to MBE or metal-organic chemical vapor deposition (MOCVD) make sputtering one of the methods of choice for commercial ZnO thin film fabrication. Furthermore, recent improvements in the technology of sputtering systems allow it to perform better than widely expected in the areas of layer purity, homogeneity and physical properties’ control, which can lead to obtaining very high quality crystals. These reasons motivated us to undertake the attempt at transferring the MgO buffer layer technology from MBE to sputtering, the results of which are reported herein.