Recently, ZnO based thin film transistor is attracting great interests for transparent and flexible displays. So far, ZnO active layers have been mostly deposited by physical vapor deposition such as sputtering or pulsed layer deposition. However, atomic layer deposition (ALD) can be a good deposition technique for this purpose since it can produce films with good uniformity over large area at relatively low growth temperature. Additionally, other components of TFT such as gate insulator and source/drain/gate can be also deposited by ALD. However, the ALD ZnO film from diethyl Zn for precursor and water or ozone for reactant, respectively, has been reported to have intrinsically high carrier concentration (> 10¹⁷ /cm³). This is a significant drawback for the application to TFT channel material, since large off current is expected. In this research, we developed an efficient way to control the carrier concentration in the ALD ZnO film. By using NH₄OH as a source for reactants, nitrogen doping has been achieved in situ during ZnO ALD. The nitrogen concentrations of the films were controlled by changing the ammonia concentrations in NH₄OH solution. By nitrogen doping, the microstructures of the ZnO films were changed to amorphous at high nitrogen concentration and, more importantly, the carrier concentrations were reduced to < 10¹⁵ /cm³.. Inverted staggered type TFTs were fabricated with N:ZnO as a channel at different ammonia concentrations from 0 to 29 % and ALD Al₂O₃ as a gate insulator at low process temperature below 150 °C. By this process, very good electrical properties were achieved; m_sat = 7 cm²/Vs and I_off < 3 × 10^-12 A, I_on /I_off > 10⁸. Also, the hysteresis and subthreashold swing of the TFTs were low, about 0.5 V and 0.7 V/dec, respectively.

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