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 appear), it is only 0.04%/BL. This behavior may be explained by a temperature-dependent roughness length scale. At low temperature, atomic force microscope images show the development of small (∼ 1000 Å), faceted islands with aspect ratios (height/width) on the order of 0.07. With increasing temperature, the formation of well-defined facets is inhibited. At 700°C, islands grow very large from the outset, with aspect ratios less than 0.015. The islands are unable to thicken much, because dislocations can glide in easily at the edges, enabling the islands to grow laterally quickly. The strain in the "new" islands is not substantially less than that in the "old" islands. At the highest temperatures used, diffusion becomes a relevant mechanism for strain relief.