Growth of Si1−xGex(011) on Si(011)16×2 by gas-source molecular beam epitaxy: Growth kinetics, Ge incorporation, and surface phase transitions

Abstract

Single crystal Si1−xGex(011) layers with x⩽0.35 have been grown on double-domain Si(011)“16×2” surfaces from Si2H6/Ge2H6 mixtures at temperatures Ts=400–950 °C. D2 temperature programmed desorption was used to show that the structure of the Si(011)“16×2” surface unit cell, more correctly written as [217 21] since the unit cell vectors are nonorthogonal, is composed of 16 adatoms and eight π-bonded dimers with a dangling bond density half that of the 1×1 surface. Si1−xGex(011) overlayers are “16×2” when x<xc(Ts) and “2×8” with x>xc(Ts). The value of xc decreases from ≃ 0.10 at Ts=475 °C to 0.08 at 550 °C to 0.06 at 650 °C. Both the “2×8” and “16×2” Si1−xGex(011) surface reconstructions gradually and reversibly transform to 1×1 at Ts between 650 and 725 °C. Film growth kinetics exhibit three distinct regimes. At low temperatures (Ts≲500 °C), the film deposition rate RSiGe decreases exponentially with 1/Ts in a surface-reaction-limited growth mode for which the rate-limiting step is hydrogen desorption from Si and Ge monohydride phases. RSiGe becomes essentially constant with Ts in the intermediate impingement-flux-limited range, Ts=500–650 °C. At Ts>650 °C, RSiGe increases again with Ts due initially (Ts≃650–725 °C) to an increase in the steady-state dangling bond coverage as the surface reconstruction gradually transforms to 1×1. The continued increase in RSiGe at even higher Ts is associated with strain-induced roughening. Ge/Si ratios in as-deposited films are linearly proportional to the incident Ge2H6/Si2H6 flux ratio JGe2H6/JSi2H6 and nearly independent of Ts indicating that the reactive sticking probabilities of Si2H6 and Ge2H6 have very similar temperature dependencies. RSiGe(JGe2H6/JSi2H6,Ts) in both the surface-reaction-limited and flux-limited regimes is well described by a simple kinetic model incorporating second-order dissociative chemisorption and second-order hydrogen desorption as rate-limiting steps.