Effect of process pressure and temperature on ZnON material properties in reactive sputtering

We have systematically studied Zinc Oxynitride (ZnON) materials for thin film transistors in advanced display applications. TheZnON materialswere depositedusing a reactive sputtering process with a metallic Zn target in a gas mixture ofAr, N2 and O2. We previously optimized the gas mixture and RF powerand found optimized process parameters for O2 and N2 flow rates andRF power. In this contribution, we report the material properties as a function of process pressure and temperature. Weobserved that at a relatively lower pressure of 5 mTorr, the material shows a cubicZn3N2-like structure with a narrow band gap of 1.1 eV, highconductivity, high carrier density, and high carrier mobility; at medium pressure of approximately 13-15mTorr, the material becomes nanostructured or amorphous ZnON (nc-ZnON or a-ZnON)with a band gap ofapproximately 1.3-1.5 eV; and at pressures higher than 20 mTorr, the material shows a hexagonalpolycrystalline ZnO-like structure with a band gap of 3.1 eV. The deposition rate decreases butthe band gapincreases monotonically with increasing pressure; the Hall electron mobility decreases withincreasing pressure in the range from 5 mTorr to 13 mTorr and changes very littlein the highpressure regime; the conductivity and carrier density decrease with theincrease of pressure from 5 mTorr to 17 mTorr and then increase with furtherincrease of pressure, which is related to the material structure changes fromZn3N2-like to a-ZnON, and then to ZnO-likematerials. Thesubstrate temperature has little effect on the material properties. Increasing substrate temperatureslightly increases the bandgap, carrier concentration, and conductivity, but slightly decreases thecarriermobility. Finally, under the optimized conditions, the nc-ZnONfilms are madewith an optical bandgap of 1.3-1.5 eV, electron mobility above 80 cm2/Vs andelectron densityof 1 × 1018 cm-3, which are suitable for high quality TFTs in advanced displayapplications