In situ measurements of temperature-dependent strain relaxation of Ge/Si(111)

We have measured strain relaxation and clustering in Ge films grown by molecular beam epitaxy on Si(111) at substrate temperatures between 450 and 700 °C in real time with reflection high energy electron diffraction (RHEED). At 450 °C, we observe an oscillation of the surface lattice constant for the first 3.5 bilayers [(BLs) thicknesses were calibrated by Rutherford backscattering spectrometry], followed by a sharp two-dimensional–three-dimensional (2D–3D) growth mode transition, when transmission diffraction features appear in RHEED. The surface lattice constant then begins to relax at an initial rate of about 0.5%/BL. The mechanisms of island growth and strain relaxation change with growth temperature. At 500 °C, the surface lattice constant begins to relax after only 1 BL, and at 550 °C relaxation begins immediately. At both temperatures, however, 3D spots do not appear until after 3.5 BL. The initial rate of strain relaxation decreases with increasing temperature until, at 700 °C (when 3D spots never...