Fully-coherent Si 0.7Ge 0.3 layers were deposited on Si(001) by gas-source molecular beam epitaxy (GS-MBE) from Ge 2H 6/Si 2H 6 mixtures in order to probe the effect of steady-state hydrogen coverages θ H on surface morphological evolution during the growth of compressively strained films. The layers are grown as a function of thickness t at temperatures, T s=450-550°C, for which strain-induced roughening is observed during solid-source MBE (SS-MBE) and deposition from hyperthermal beams. With GS-MBE, we obtain three-dimensional (3D) strain-induced growth mounds in samples deposited at T s=550°C for which θ H is small, 0.11 monolayer (ML). However, mound formation is dramatically suppressed at 500°C (θ H=0.26ML) and completely eliminated at 450°C (θ H=0.52ML). We attribute these large differences in surface morphological evolution primarily to θ H(T s)-induced effects on film growth rates R, adatom diffusion rates D s, and ascending step-crossing probabilities. GS-MBE Si 0.7Ge 0.3(001) growth at 450°C remains two dimensional, with a surface width 〈w〉<0.15nm, at all film thicknesses t=11-80nm, since both R and the rate of mass transport across ascending steps are low. Raising T s to 500°C increases R faster than D s leading to shorter mean surface diffusion lengths and the formation of extremely shallow, rounded growth mounds for which 〈w〉 remains essentially constant at ≃0.2nm while the in-plane coherence length 〈d〉 increases from ≃70nm at t=14nm to 162 nm with t=75nm. The low ascending step crossing probability at 500°C results in mounds that spread laterally, rather than vertically, due to preferential attachment at the mound edges. At T s=550°C, the ascending step crossing probability increases due to both higher thermal activation and lower hydrogen coverages. 〈w〉(t) increases by more than a factor of 10, from 0.13 nm at t=15nm to 1.9 nm at t=105nm, while the in-plane coherence length 〈d〉 remains constant at ≃85nm. This leads, under the strain driving force, to the formation of self-organized 3D 105-faceted pyramids at 550°C which are very similar to those observed during SS-MBE.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)