Silicon (011) and silicon germanium (011) gas-source molecular beam epitaxy: Surface reconstructions, growth kinetics, and germanium segregation

Epitaxial Si and Si1--xGex layers were grown by gas-source molecular beam epitaxy (GS-MBE) on Si(011)"16x2" from Si2H6 and Ge2H6. The Si(011)"16x2" reconstructed unit cell is composed of adatoms and dimers. Si1--x Gex(011) layers exhibit a "16x2" reconstruction when x < xc(Ts) and "2x8" when x > x c(Ts). Both the "16x2" and "2x8" reconstructions transform to 1x1 at Ts = 650--725°C.; Si(011) growth rates RSi were measured as a function of temperature Ts and gas flux. In the surface-reaction-limited regime (T s < 750°C), R decreases exponentially with 1/Ts. At Ts < 550°C, RSi continues to decrease at a slower rate as a new film growth reaction path becomes important. In the impingement-flux-limited regime (750 ≤ Ts < 900°C), RSi is independent of Ts. At Ts > 900°C, RSi increases with Ts due to surface roughening.; Si1--xGex(011) growth rates RSiGe were also investigated. At Ts ≤ 500°C, RSiGe decreases exponentially with 1/Ts in a surface-reaction-limited growth mode. In the impingement-flux-limited regime (Ts = 500--650°C), RSiGe is constant with Ts. At Ts > 650°C, RSiGe increases again due to both a phase transformation to a 1x1 reconstruction and strain-induced roughening. Ge/Si concentration ratios are linearly proportional to the Ge2H6/Si2H 6 flux ratio.; GS-MBE Si(011) and Si1--xGex(011) growth rates are well described by a kinetic model incorporating second-order dissociative chemisorption of Si2H6 onto dangling bonds and second-order hydrogen desorption as rate-limiting steps. For GS-MBE Si(011), Si2H 6 insertion into Si-H surface bonds followed by desorption of SiH 4 becomes important at Ts < 550°C.; Ge surface segregation occurs during Si1--xGex(011) GS-MBE. Ge surface coverages thetaGe increase with x, ranging from 0.27 to 0.74 ML for x = 0.04--0.20. At a given film composition, theta Ge decreases with decreasing Ts due to the corresponding increase in the fraction of H-bonded Si surface atoms, fSi,H. From these data, we find that the Ge segregation enthalpy DeltaHs for Si1--xGex(011){09} varies from --0.18 eV at Ts = 750°C (fSi,H < 0.003) to --0.09 eV at Ts = 475°C (fSi,H = 0.22).