Low temperature fabrication of amorphous silicon thin film transistors by dc reactive magnetron sputtering

We deposit hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon nitride (a-SiN_x:H) films using dc reactive magnetron sputtering at 125 and 230°C substrate temperatures. We characterize the structural properties using infrared absorption, thermal hydrogen evolution, and refractive index measurements, and evaluate the electrical quality using capacitance-voltage and leakage current measurements. Nitride layers deposited at both temperatures have dominant N-H bonding with hydrogen concentration approximately 10²²/cm₃. Metal-insulator-semiconductor devices have been fabricated on c-Si and show electrical leakage of < 5 X 10^-8 A/cm² at 3 MV/cm field, flat band voltage magnitude < 1 V, and hysteresis < 2 V. Low temperature a-Si:H films show monohydride dominant bonding. The photo- to dark conductivity ratio is 5 X 10⁵ for films deposited at 125°C. Inverted staggered thin film transistors have been fabricated with the optimized layers. Thin film transistors deposited at 230°C have a field effect mobility of 0.8 cm²/V s, an I_on/I_off ratio of 5 X 10⁵, a subthreshold slope of 1.2 V/decade, and a threshold voltage of 4 V; those deposited at 125°C have a field effect mobility of 0.3 cm²/V s, an I_on/I_off ratio of 5 X 10⁵, and a threshold voltage of 3 V.