Dependence of the electrical and optical properties of sputter-deposited ZnO:Ga films on the annealing temperature, time, and atmosphere

Effects of annealing process parameters such as annealing temperature, time, and atmosphere on the electrical resistivity and transmittance properties of Ga- doped ZnO ( ZnO: Ga) thin films deposited on glass by rf magnetron sputtering were investigated. The electrical resistivity of a ZnO: Ga thin film is effectively decreased with increasing annealing temperature and time in a reducing atmosphere such as N-2 + 5% H-2. This is attributed to passivation of grain boundaries and zinc ions by hydrogen atoms resulting in increases in carrier concentration and mobility. Also the resistivity of 4.9 x 10(-4)Omega cm was obtained by annealing at 200 degrees C for 15 h in the same atmosphere, which is not bad for a transparent conductor for solar cell applications. However, annealing at a temperature higher than 400 degrees C is less effective. The lowest resistivity of 2.3 x 10(-4)Omega cm was obtained by annealing at 400 degrees C for 1 h in an N-2 + 5% H-2 atmosphere. The optical transmittance of the ZnO: Ga film is improved by annealing regardless of the annealing atmosphere. Annealing in N-2 + 5% H-2 atmosphere widens the optical band gap, while annealing in an O-2 atmosphere makes the band gap narrower, which can be explained as a blue shift phenomenon.