Various Techniques to Optimize the Quality of Schottky Contacts on ZnO Thin Films: A Short Review

Abstract

INTRODUCTIONZinc oxide based Schottky diode is used for fabricating many important electronic and optoelectronic devices like photo-detector, transistors, gas sensors, etc. Mead [1] in 1965 reported the first Schottky contact to ZnO. Till now a significant amount of work highlighting the important issues has been reported on Schottky contacts on ZnO. A good Schottky contact is charac-terized by very low series resistance with the semiconductor, a high rectification ratio, high thermal stability and a high barrier height. In general, the high work function metal such as Pd, Pt, Au, etc., are used for Schottky contact on ZnO thin films. Ideally, the Schottky barrier height ΦSB should be equal to (ΦM -χZnO), but in real the measured ΦSB differs from ideal one. It is also found that barrier height is almost independent of the type of metal used for the contact. Most of the authors have reported the Schottky barrier heights in the range of 0.6-0.8 eV for the Schottky contacts fabricated on the n-type ZnO material [2]. The barrier height highly depends on the crystal quality, defects, surface treatments and deposition techniques. Also, the long-term stability of Schottky contacts to ZnO is one of the biggest challenges that are faced by the researchers. The poor performance of the contact is mainly due to the defects in the film that occurs due to the improper chemical bonding between Zn-O during the deposition. The surface defects are even induced by the Zinc oxide is a promising material for fabrication of electronic and optoelectronic devices. ZnO based Schottky diode is an important device that can be used for variety of applications. For a high quality Schottky contact on ZnO, it is essential that the quality of the ZnO film should be very good. In this paper, various techniques that can be used to improve the quality of ZnO thin film and hence the quality of Schottky contacts on ZnO have been reviewed. Techniques like surface treatments of ZnO, annealing of the film before and after deposition of the contacts, use of buffer layer and proper selection of deposition method greatly affects the quality of Schottky contact on ZnO. These methods are reported by various research groups that are discussed in this paper. MINI REVIEW Various Techniques to