Effect of heat treatment on phase transformation of TiO₂ and its reflectance properties

Abstract

Titanium dioxide (TiO₂) anti reflection coating film applied for solar cell application, gains more interest due to the insufficient energy problems today. Titanium dioxide can be found in several forms, and the most stable phase is rutile. This research aims to study effect of heat treatment on phase transformation of TiO₂ and its effect on reflectance property. Titanium dioxide film was coated on silicon wafer by DC magnetron sputtering to obtain 190 nm and 420 nm thicknesses.  Then, it was heat treated at 900°C for 1 and 4 hours under ambient atmosphere. Phases and surface morphologies before and after heat treatment are investigated by X-ray diffractometer (XRD) and atomic force microscope (AFM). Cross-sectional microstructures of all samples were also observed by field emission scanning electron microscope (FESEM).  It can be seen that heat treatment at elevated temperature induces phase transformation of TiO₂ from anatase to rutile. The heat treatment holding time also affects phase transformation.  The thicker film requires longer holding time to completely transform anatase to rutile. The 420 nm thick TiO₂ requires 4 hours to completely transform to rutile, while, only 1 hour is needed to transform the 190 nm thick TiO₂. Besides phase transformation, agglomeration of rutile grain also takes place.  Hence, after heat treatment, rutile TiO₂ film has coarse columnar structure.  The existence of rutile TiO₂ film can reduce reflectance percentage of light from silicon surface by destructive interference. The reflectance percentage can be reduced to less than 10% when combining TiO₂ coating and etched surface.