Catalytic precursor dissociation in Hot-Wire CVD and comparing a-Si:H growth under continuous and pulsed silane flow conditions

Abstract

Hot-wire Chemical vapor deposition (HWCVD) of hydrogenated amorphous silicon (a-Si:H) thin films utilizes the dissociation of silane (SiH4) precursor over heated tungsten or tantalum filaments (\geq 1600 °C). In this work, assuming catalytic dissociation mechanism, we present kinetic model for SiH4 dissociation and the resulting a-Si:H film growth. Our model calculations showed that for an identical dose of the introduced SiH4 precursor, a-Si:H thickness was considerably higher for the pulsed SiH4 flow as compared to the continuous SiH4 flow. The pulsed SiH4 flow is represented by time intervals t_ON and t_OFF, where the SiH4 flow rate (F_(SiH_4)) is at the set-point and zero, respectively. In agreement with our model calculations for an introduced 75 cm^3 (STP) SiH4 dose, the resulting a-Si:H film thickness was 175 \pm 5 nm under continuous precursor flow, whereas it considerably increased to 425 \pm 8 nm when this SiH4 dose was split into 15 shorter pulses (t_ON =15s ; t_OFF = 60s). Moreover, these a-Si:H films deposited using pulsed SiH4 flow exhibited improved electrical properties, with a dark conductivity ({σ_d}) of 1.1 \times 10^-11 S/cm and a photoconductivity ({σ_ph}) of \sim 5.8 \times 10^-5 S/cm, compared to films deposited under continuous SiH4 flow (σ_d \sim 2.5 \times 10^-10 S/cm and σ_ph \sim 3.5 \times 10^-6 S/cm).