Phosphorus incorporation during Si(001):P gas-source molecular beam epitaxy: Effects on growth kinetics and surface morphology

Abstract

The effects of P doping on growth kinetics and surface morphological evolution during Si(001):P gas-source molecular beam epitaxy from Si2H6 and PH3 at temperatures Ts=500–900°C have been investigated. With increasing PH3∕Si2H6 flux ratio JP∕Si at constant Ts, we observe a decrease in the film growth rate R and an increase in the incorporated P concentration CP, both of which tend toward saturation at high flux ratios, which is accompanied by increased surface roughening and pit formation. At constant JP∕Si, R increases with increasing Ts, while CP initially increases, reaches a maximum at Ts=700°C, and then decreases at higher growth temperatures. We use in situ isotopically tagged D2 temperature programed desorption (TPD) to follow changes in film surface composition and dangling bond density θdb as a function of JP∕Si and Ts. Measurements are carried out on both as-deposited Si(001):P layers and P-adsorbed Si(001) surfaces revealing β1 and β2 peaks due to D2 desorption from Si monohydride and dihydride species, respectively, as well as the formation of a third peak β3 corresponding to D2 desorption from mixed SiP dimers. Dissociative PH3 adsorption on Si(001) results in a decrease in θdb and an initial increase in P surface coverage θP with increasing Ts. Saturation θP values reach a maximum of ∼1 ML at Ts=550°C, and decrease with Ts>600°C due to the onset of P2 desorption. Comparison of θP(Ts) results obtained during film growth with postdeposition CP(Ts) results reveals the presence of strong P surface segregation. From measurements of θP versus CP in Si(001):P layers grown as a function of Ts, we obtain a P segregation enthalpy ΔHs=−0.86eV. By using the combined set of results, we develop a predictive model for CP versus Ts and, JP∕Si incorporating the dependence of the PH3 reactive sticking probability SPH3 on θP, which provides an excellent fit to the experimental data.