Hydrogen-mediated quenching of strain-induced surface roughening during gas-source molecular beam epitaxy of fully-coherent Si[sub 0.7]Ge[sub 0.3] layers on Si(001).

Fully-coherent Si[sub 0.7]Ge[sub 0.3] layers were deposited on Si(001) by gas-source molecular beam epitaxy (GS-MBE) from Ge[sub 2]H[sub 6]/Si[sub 2]H[sub 6] mixtures in order to probe the effect of steady-state hydrogen coverages θ[sub 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[sub 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[sub s]=550 °C for which θ[sub H] is small, 0.11 monolayer (ML). However, mound formation is dramatically suppressed at 500 °C (θ[sub H]=0.26 ML) and completely eliminated at 450 °C (θ[sub H]=0.52 ML). We attribute these large differences in surface morphological evolution primarily to θ[sub H](T[sub s])-induced effects on film growth rates R, adatom diffusion rates D[sub s], and ascending step-crossing probabilities. GS-MBE Si[sub 0.7]Ge[sub 0.3](001) growth at 450 °C remains two dimensional, with a surface width <w><0.15 nm, at all film thicknesses t=11–80 nm, since both R and the rate of mass transport across ascending steps are low. Raising T[sub s] to 500 °C increases R faster than D[sub s] leading to shorter mean surface diffusion lengths and the formation of extremely shallow, rounded growth mounds for which <w> remains essentially constant at =0.2 nm while the in-plane coherence length <d> increases from =70 nm at t=14 nm to 162 nm with t=75 nm. 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[sub s]=550 °C, the ascending step crossing probability increases due to both higher thermal activation and lower hydrogen coverages. ... [ABSTRACT FROM AUTHOR]