Study On The Preparation And Properties Of Ti-doped ZnO Transparent Conductive Thin Films

Transparent conductive oxides are an important optoelectronic material for a variety of applications in the fabrication of various devices such as electro-optical devices, thin-film solar cell, surface acoustic wave devices, sensors, flat panel liquid crystal display devices, infrared (IR) reflector, etc. Among the different materials belonging to this category, intrinsic zinc oxide (ZnO) is a n-type semiconductor material with a bandgap of 3.37 eV and a promising alternative to indium tin oxide (ITO) in TCO applications, due to its ease in doping, abundance in nature, low cost, and relatively low deposition temperature and stability in hydrogen plasma compared to ITO and SnO2.To realize these applications on photoelectric devices, an imperative issue is to fabricate high quality n-type ZnO films. Most of the previous studies related to ZnO-based transparent conductive thin films were carried out using metallic cation dopants such as Group IIIA (e.g., Al, Ga and In). However, theoretical studies indicate that titanium could be a promising n-type doping candidate. There are many techniques for preparing Ti-doped ZnO films (TZO), such as atomic layer deposition (ALD), chemical bath deposition (CBD), RF and DC magnetron sputtering (MS) and pulsed laser deposition (PLD). Among these methods, PLD has several advantages. Firstly, the films deposited by PLD usually have a very similar chemical composition to that of the targets, and therefore controlling the composition of films is easier. Moreover, the high kinetic energy of atoms and ionized species produced by laser irradiation allows for the crystallization of films under a lower substrate temperature, which is an important advantage when fabricating crystallized oxide film on a flexible substrate, and the crystalline thin films often have a superior electrical conductivity to the amorphous thin films.In this study, Ti-doped ZnO thin films were fabricated on glass substrate by PLD using TiO2-doped ZnO ceramic targets. The effects of Ti doping concentration, substrate temperature, oxygen pressure and deposition time on structure, composition, surface morphology, electrical and optical properties of the films were studied systematically. The structure and surface morphology were characterized by XRD and AFM, the composition was investigated by XPS, the electrical and optical properties were studied by Hall system and UV-vis spectrophotometer.It was found that all the TZO films had the wurtzite hexagonal structure with a preferred orientation along the c-axis. Under the optimized deposition conditions of Ti content of 1 at% in the target, substrate temperature of 200℃, oxygen pressure of 2.5 Pa and deposition time of 1.5 h, a film would display an electrical resistivity as low as 6.34x10-3 Ω·cm and also had a 93.3% mean transmittance in the visible region.