Plasma Impedance Analysis: A Novel Approach for Investigating a Phase Transition from a-Si:H to nc-Si:H

Abstract

In the present article we report the transition regime of hydrogenated amorphous (a-Si:H) to nano-crystalline (nc-Si:H) silicon thin films in Silane (SiH₄) plasma using 27.12 MHz assisted plasma enhanced chemical vapor deposition process with the approach of plasma diagnosis. The observed transitions occur within a narrow range of diverse deposition process window and hence plasma diagnosis was vital towards envisaging this variation. Impedance Analyser (V/I probe) was used to monitor plasma characteristics during growth at various process pressure (0.03–0.4 Torr) and applied power (4–20 W). Efforts were made to understand the radicals’ formation and plasma-substrate interaction by evaluating the discharge parameters such as electron density, bulk field, and sheath voltage. From the result of plasma characterizations, highest bulk field (5.7 V/cm) in combination to low sheath voltage (0.1 V) observed on 0.2 Torr pressure at 15 W power which thus provides a clear signature of transition from a-Si:H to nc-Si:H. The structural characterizations also validate the results of observed transition where in particular it was found that the mean crystallite size (4.2 nm) with high crystalline volume fraction (42%) and wider band gap (2.01 eV) with higher hydrogen content (35%) signifies the existing nano-crystalline phase. On account of these results, an empirical relation between plasma impedance and phase angle was established in terms of expansion and contraction of two distinct discharge zones (bulk and sheath) to diagnose the phase transition.