Evidence for non-hydrogen desorption limited growth of Si from disilane at very low temperatures in gas source molecular beam epitaxy?

Abstract

We have studied the growth of Si from disilane on Si(001) in gas source molecular beam epitaxy (GSMBE). The growth rates were measured in situ from reflection high energy electron diffraction (RHEED) intensity oscillations. Various experiments at different substrate temperature and disilane fluxes were carried out. At low temperatures the rate limiting step during the deposition of Si from disilane is the desorption of hydrogen from the Si surface. The hydrogen stems from the dissociation of the disilane molecule and ties up surface dangling bonds, which in turn block adsorption sites for incoming disilane molecules. Therefore, when measuring the growth rate at a given temperature against the supplied disilane flux, one expects to observe saturation of the rate for high absolute surface hydrogen coverages (θ ∗≈1). This behaviour was indeed found. Interestingly, even at growth temperatures as low as 468°C, the growth rate still increases slightly with increasing flux. According to the literature, at such low temperatures, the steady state absolute surface hydrogen coverage is already larger than 1 for the applied fluxed. It has been suggested that an additional reaction path for the growth from disilane on hydrogen terminated surfaces must exist, albeit that it contributes only weakly to the overall growth rate. For example, a possible alternative reaction path, proposed by Kulkarni et al. for the decomposition of disilane on a Si(111) surface: Si 2H 6(g)+H(s)→SiH 4(g)+SiH 3(s), could occur for Si(001) as well (s and g denote species at the surface and in the gas phase, respectively). However, it will be shown in this paper that an additional reaction mechanism is not at all necessary to describe the growth rate dependence on the flux even at low temperatures.