Direct insertion ofSiH3radicals into strained Si-Si surface bonds during plasma deposition of hydrogenated amorphous silicon films

Abstract

We investigate the interaction of silyl (SiH 3 ) radicals with hydrogenated amorphous silicon (a-Si:H) films using real time in situ infrared spectroscopy in a mode that can detect as little as ϳ0.2 ML of Si-H bonds. The results are directly relevant to the growth of a-Si:H by plasma-chemical vapor deposition. In this paper, a remote silane plasma source is used to generate a pure SiH 3 beam, without any contribution of H or SiH 2 . Deuterated (a-Si:D) films are exposed to this beam and the change in the IR absorption caused by the loss of SiD groups and the creation of SiH groups is measured in real time. At the beginning of exposure to SiH 3 radicals, a hydrogen-rich layer is deposited on top of the deuterated sample, but no release of deuterium from SiD surface groups can be observed. This is a surprising result, since it has been assumed in the literature that SiH 3 radicals can easily abstract surface-bonded H atoms, and that abstraction must occur in order to create dangling bond sites on which SiH 3 radicals subsequently adsorb. Our results show that the reaction mechanism with the largest rate coefficient must be the direct insertion of SiH 3 radicals into bonding sites at the film surface, which leads to a hydrogen-rich top layer while preserving the preexisting SiD bonds. After completion of one monolayer of surface Si-H bonds, deuterium atoms from the initial surface are released simultaneously with the creation of SiH bulk groups. We propose a reaction scheme based on the direct insertion of SiH 3 radicals into strained Si-Si bonds. This scheme also predicts that the surface-bonding configurations depend on a dynamic balance between the rates of SiH 3 adsorption and thermal desorption, which is confirmed experimentally as a function of SiH 3 flux. We discuss the implications of this reaction mechanism for the growth of a-Si:H from silane discharges, and for the growth of microcrystalline Si in H 2 diluted SiH 4 discharges.

Fundamentals and Applications of Reflection FTIR Spectroscopy for the Analysis of Plasma Processes at Materials Interfaces

“…This postulated reaction scheme has been tested by HD exchange experiments starting with a deuterated a‐Si:D film . This film is exposed to a plasma based SiH 3 source.…”

Plasma processing and chemistry

“…There is some effect of substrate temperature with higher values at higher temperature. Figure 2 also shows some points from RF deposition studies and from ECRH deposition both obtained from Von Keudell [24]. For these conditions, ion effects are important, and the relation between film density and rate is different.…”

“…The good overall correspondence suggests a pure radical mechanism, in which a high flux of radicals is advantageous. The points marked RF and ECRH, obtained from Von Keudell [24], refer to RF plasma and ECRH plasma deposition. In these plasmas, evidently, ions and ion energy play a role and the mechanism is different.…”

The a-Si:H growth mechanism and the role of H abstraction from the surface by SiH3 radicals via an Eley–Rideal mechanism

“…The most-accepted adsorption mechanisms of SiH 3 rely on surface dangling bonds which act as active sites for radical adsorption [1], although recently SiH 3 adsorption mechanisms have also been proposed that do not require surface dangling bonds (e.g, insertion of SiH 3 into strained Si-Si bonds) [2,3]. Dangling bonds can be created at the H-covered a-Si:H surface during deposition by energetic ions from the plasma or by radical species such as H and SiH 3 .…”