Controllability of structural and electrical properties of silicon films grown in atmospheric-pressure very high-frequency plasma

Amorphous silicon (a-Si) and microcrystalline silicon (µc-Si) films were deposited in atmospheric-pressure (AP) He/H2/SiH4 plasma excited by a 150 MHz very high-frequency (VHF) power at a temperature of 220 °C. The variations of thickness and film crystallinity in the gas flow direction were studied using two electrodes (length = 16 and 5 mm). The electrical properties of the deposited Si layers were evaluated by fabricating bottom-gate thin film transistors (TFTs). The results showed that the chemical reactions both in gas phase and on the growing film surface were significantly enhanced in AP-VHF plasma, promoting phase transition from amorphous to microcrystalline on a time scale of the order of less than 0.1 ms. The performance of bottom-gate TFTs showed that a-Si layers formed in the upstream portion of the plasma zone had good enough electrical property (field-effect mobility of 1-1.5 cm2 · V−1 · s−1) despite very high deposition rates (several tens of nm · s−1). Meanwhile, µc-Si layers deposited in the downstream portion of the plasma zone did not acquire their original superiority over a-Si films with reference to electrical properties, although the use of the shorter electrode greatly improved the layer quality. The precise control of gas residence time is primarily important for the deposition of µc-Si films with desired qualities through sufficiently passivating grain boundaries with a-Si tissues.