Synthesis & Characterizations of ZnO Thin Films and Nanostructures by Modified Aqueous Chemical Growth Method for Sensor Applications

Abstract

In recent years, there is a great demand on the development of portable, low cost, low power operated high performance sensors for medical diagnosis, defence, space, agriculture and industrial applications. Remarkable research on the fabrication of the semiconducting metal oxide (ZnO) based resistive sensors are currently being drawn considerable continuous attention for sensor fabrication because of its semiconducting nature, direct and wide bandgap (3.37 eV), ease of fabrication, low cost, non toxic nature,
high temperature stability and CMOS compatibility.
In order to achieve low cost portable sensors, room temperature operated devices are highly preferred. However, lowering of operating temperature significantly reduces sensing activities. In order to enhance the performance of the room temperature operated sensors, the enhancement in the surface area of the active material is an alternate approach. In this research work, ZnO thin films and nanostructures were fabricated on CMOS compatible silicon substrate. Prior to the growth of the nanostructures, growth condition of seed layer was optimized by varying the sputtering parameters. Postdeposition rapid thermal annealing was carried out for various conditions for improving the crystallinity of the seed layer. The films annealed for higher duration has shown porous morphology. Afterwards, various types of nanostructures such as nanowires, nanorods, nanoplatelets and network like porous structure were synthesized by varying the parameters of limited area heating of aqueous chemical method. Nanoplatelets/flakes like structures were appeared as a result of incorporation of aluminium nitrate in the zinc nitrate and hexamethyltetramine precursor. Electrical contacts on the samples were
fabricated by thermal evaporation of aluminium by adopting shadow mask technique. The sensitivity of ammonia and UV on/off current ratio was calculated by performing current-time measurements. The decay time constant of UV detection are estimated from fitting of exponential data. Among different samples ZnO network like porous structures have shown better sensing property.