Reaction paths for self-organized surface roughening of silicon-germanium alloys during hydride gas-source molecular beam epitaxy

Compressively-strained Si_0.7Ge_0.3 layers were grown on Si(001) by gas-source molecular beam epitaxy (GS-MBE) from Ge ₂H₆/Si₂H₆ mixtures in order to probe the effect of surface hydrogen on the evolution of strain-induced roughening and misfit-dislocation-induced surface roughening (crosshatch).; Fully coherent layers were grown at T_s = 450–550°C, where strain-induced roughening is observed in solid-source MBE (SS-MBE). Three-dimensional strain-induced growth mounds are obtained in samples deposited at T_s = 550°C for which the steady-state hydrogen coverage &thetas; _H is 0.11 ML. However, mound formation is decreased at 500°C (&thetas; _H = 0.26 ML) and completely quenched at 450°C (&thetas;_H = 0.52 ML). The large differences in surface morphological evolution are due primarily to effects of &thetas;_H on film growth rates R_SiGe and asending step-crossing probabilities. Growth at 450°C remains two-dimensional, since both R_SiGe and the rate of mass transport across ascending steps are low. Raising T_s to 500°C increases R_SiGe faster than the diffusivity leading to shorter mean surface diffusion lengths and the formation of extremely shallow, rounded growth mounds. The low ascending step crossing probability at 500°C results in mounds that spread laterally, rather than vertically, due to preferential attachment at mound edges. At T_s = 550°C, the ascending-step crossing probability increases due to both higher thermal activation and lower hydrogen coverages. The mounds height increases by more than a factor of ten, while their areal density remains constant. This leads to the formation of self-organized 3D {lcub}105{rcub}-faceted pyramids at 550°C similar to those observed during SS-MBE.; Si_0.7Ge_0.3(001) layers were grown at T_s = 450°C, where strain-induced roughening is completely quenched, to thicknesses greater than the critical value for misfit dislocation formation (t_c ≃ 100 nm) to probe the effect of &thetas;_H on cross-hatch formation. At t slightly larger than t_c, surface roughness is dominated by single- and multiple-atomic-height steps associated with the interfacial misfits. The surface steps are preferential H desorption sites, thus the increased total step length results in a decrease in &thetas;_H on terraces which increases R_SiGe and allows higher adatom ascending step crossing probabilities which, driven by the inhomogeneous strain fields around the misfit dislocation clusters, contributes to the growth of periodic ridges.