Thermal stability and hydrogen atom induced etching of nanometer-thick a-Si:H films grown by ion-beam deposition on Si(100) surfaces

Abstract

Amorphous hydrogenated silicon (a-Si:H) films in the thickness range 0.1–4.5 nm were deposited on Si(100) surfaces at 350 K using the ion-beam-deposition method. The thermal stability of these a-Si:H films was studied by temperature programmed desorption spectroscopy. The films are stable up to 500 K, where a-Si:H starts to decompose via evolution of hydrogen (H2) and silane (SiH4). Approximately 99% of the hydrogen initially bound to the Si network was detected in the hydrogen channel. The hydrogen evolution peaks at ∼780 K caused by the decomposition of monohydride groups; the presence of SiH2 groups is indicated by hydrogen desorption below 700 K. The silane desorption states at 625 and 750 K reveal the existence of two different types of silyl (SiH3) groups. Etching of a-Si:H by impinging gas-phase H atoms was investigated in the temperature range from 150 to 700 K by in situ mass spectrometry. Silane was the sole etch product observed. The formation of silane proceeds via direct abstraction of silyl precursor groups by impinging hydrogen atoms, SiH3(a)+H(g)→SiH4(g); the silyl abstraction probability increases by a factor of 6 with increasing substrate temperature between 150 and 525 K. However, the steady-state erosion rate is controlled by the supply of silyl groups by successive hydrogenation of the Si network with the formation of SiH2 as bottleneck of the silyl supply.