High deposition rate nanocrystalline and amorphous silicon thin film production via surface wave plasma source

Abstract

A 900MHz surface wave antenna was used for plasma-enhanced chemical vapor deposition (PECVD) of silicon thin films in an H2+SiH4 discharge, with an emphasis on photovoltaic applications. Gas mixtures of 0.7–10% SiH4 at medium pressure (~100mTorr) were tested with an optimal substrate temperature of 285±15°C, producing nanocrystalline hydrogenated silicon (nc-Si:H) at rates up to 3nm/s, while amorphous films were grown in excess of 10nm/s. A sharp transition from crystalline to amorphous growth was seen as SiH4 flowrate increased, as is characteristic of silane PECVD. Increasing both substrate temperature and source power served to move this transition to higher flowrates, and by extension, higher deposition rates for the crystalline phase. Grain size also increased with substrate temperature, ranging from 10±2nm at 200°C up to 15±3nm at 400°C. Electron spin resonance showed that a-Si:H films grown via SWP were of acceptable defect density (~1016cm−3) and conductivity (~10−8S/cm). Conversely, nc-Si:H films were poor quality (~1018cm−3 defect density, 10−3–10−2S/cm conductivity) due to low hydrogenation and small grain size.