Dependence of the electrical and optical properties of sputter-deposited ZnO:Ga films on the annealing temperature, time, and atmosphere
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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 N2 + 5%H2. 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 × 10−4Ω cm was obtained by annealing at 200°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°C is less effective. The lowest resistivity of 2.3 × 10−4Ω cm was obtained by annealing at 400°C for 1 h in an N2 + 5%H2 atmosphere. The optical transmittance of the ZnO:Ga film is improved by annealing regardless of the annealing atmosphere. Annealing in N2 + 5%H2 atmosphere widens the optical band gap, while annealing in an O2 atmosphere makes the band gap narrower, which can be explained as a blue shift phenomenon.
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