Mechanisms and kinetics of Si atomic-layer epitaxy on Si(001)2×1 from Si2H6

Abstract

Single-crystal Si films have been grown on Si(001)2×1 substrates by ultraviolet (UV) photostimulated atomic-layer epitaxy (ALE) from Si2H6. The ALE deposition rate R per growth cycle remains constant at 0.4 monolayers (ML), 1 ML=6.8×1014 cm−2, over a wide range of deposition parameters: growth temperature (Ts=180 –400 °C), Si2H6 exposure (peak pressure during gas pulse=0.1–5 mTorr), UV laser energy density (ℰ=250–450 mJ cm−2 where ℰmax is determined by Ts ), and number of UV laser pulses per cycle. A film growth model, based upon the results of the present deposition experiments and Monte Carlo simulations, together with our previous adsorption/desorption measurements, is used to describe the reaction pathway for the process. Si2H6 is dissociatively adsorbed on Si surface dimers as two SiH3 radicals which subsequently dissociate to SiH2 and H. Site blocking during the adsorption and surface dissociation steps limits the surface coverage to 2.9×1014 SiH2 cm−2 (hence, R=0.43 ML per cycle). The H terminated silylene-saturated surface is thermally stable and passive to further Si2H6 exposure. ArF or KrF laser pulses (≊20 ns) are used to desorb H, following a Si2H6 exposure, and the growth cycle is repeated until the desired film thickness is obtained. At Ts<180 °C, the growth process becomes rate limited by the surface dissociation step and R decreases exponentially as a function of 1/Ts with an activation energy of ≊0.5 eV. At Ts>400 °C, H is thermally desorbed and pyrolytic growth competes with ALE. Plan-view and cross-sectional transmission electron micrographs together with selected-area and convergent-beam electron diffraction patterns show that the ALE films are epitaxial layers with no observed extended defects or strain.