MODULATED MOLECULAR-BEAM SCATTERING OF DISILANE ON SILICON

Decomposition and scattering of disilane on Si(111) has been studied by modulated molecular beam spectrometry over the temperature range of 50-850°C and with beam fluxes from 1015-1016 cm-2 s-1. Disilane exhibits a surface residence time as an intact molecule of ∼ 50 micros at room temperature, which we attribute to a molecular precursor (physisorbed) state. The residence time decreases with increasing temperature and cannot be experimentally observed (< 20 micros) above 250 ° C. The reactive sticking coefficient increases with surface temperature (Ts) from 0% at room temperature on a surface passivated with SiHx species (no reactivity) to 30% at 850 °C. Rapid evolution of monosilane occurs for Ts > 500 °C. Unusually slow desorption of H2 is observed (time constant ∼ 1 s) at temperatures as high as 850 ° C. The rate of hydrogen evolution increases with the incident disilane flux and the substrate temperature. A model is used to fit the experimental data. The model is based on a surface reaction mechanism developed in the next paper (S.K. Kulkarni et al., Surf. Sci. 239 (1990) 26, ref. [1]). According to this model, adsorbed disilane decomposes producing SiH4 and chemisorbed SiH. The SiH migrates to active sites on the surface where hydrogen is produced by a second-order mechanism which has an activation energy of about 20 kcal/mol. Si film growth rates are predicted from the proposed model and these agree reasonably well with experimental growth rate data in the literature. The main channel of Si growth from Si2H6 at high Ts (500-900 °C) is predicted to be by decomposition of disilane emitting SiH4 and chemisorbing SiH2, which rapidly decomposes to SiH. The chemisorbed SiH can react with incident disilane emitting SiH4. This process is more active at H removal from the surface than is the recombination of 2 SiH to desorb H2. Both SiH species and bare Si sites are proposed to be active sites for Si2H6 decomposition. © 1990.