Kinetics of Si_1-xGe_x(001) growth on Si(001)2x1 by gas-source molecular-beam epitaxy from Si₂H₆ and Ge₂H₆

Si_1-xGe_x layers with x ranging from 0 to 0.30 were grown on Si(001)2x1 substrates at temperatures ranging from 450 to 950°C by gas-source molecular-beam epitaxy (GS-MBE) from Si₂H₆ and Ge₂H₆. In the low-temperature surface-reaction-limited growth regime, the deposition rate R_SiGe increases with increasing Ge concentration due to an enhancement in the hydrogen desorption rate resulting in a correspondingly higher steady-state dangling bond density. In the high-temperature impingement-flux-limited regime, where the steady-state hydrogen coverage approaches zero, R_SiGe is controlled by the Si₂H₆ and Ge₂H₆ reactive sticking probabilities S which decrease with increasing Ge₂H₆ flux but are not strongly temperature dependent. S_Si2H6 and S_Ge2H6 range from 0.036 and 0.28 on Si(001) to 0.012 and 0.094 during growth of Si_0.82Ge_0.18 at T_s = 800°C. In both growth regimes, large changes in R_SiGe require only modest increases in incident Ge_2H6 to Si_2H6 flux ratios, J_Ge2H6/J_Si2H6, due to Ge segregation which is strongly coupled to the steady state hydrogen coverage. The Ge to Si ratio in as-deposited films increases linearly, while S_Ge2H6/S_Si2H6 remains constant, with increasing J_Ge2H6/J_Si2H6. Hydrogen desorption and Ge segregation rates, together with Si_2H6 and Ge_2H6 reactive sticking probabilities, were quantitatively determined from D₂ temperature-programmed desorption (TPD) measurements. The combined results from film growth kinetics and TPD studies, together with the assumption of linear superposition, were then used 10 develop a predictive model, with no fining parameters, for R_SiGe(T_s,J_Si2H6,J_Ge2H6) during Si_1-xGe_x GS-MBE.