Epitaxy of Germanium on SI(001) Grating Templates
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EXPERIMENTAL METHODS The experiments were conducted in a UHV chamber with a base pressure of 1.5x10-10 torr. Square-wave gratings of repeat spacing 2 gim and amplitude -1500 A were fabricated using standard lithography and etching techniques, with the grating lines oriented along the [110] direction. Prior to deposition the gratings were flashed to -1250' C and then annealed at 600' C. As previously determined by use of scanning tunneling microscopy, the shape of the gratings is a truncated sinusoid. 13 Ge was deposited from a conical tungsten wire basket filled with 40 ohmcm Ge. The depositions were monitored with a quartz crystal thickness monitor. Deposition rates ranged between 0.5 and 2.0 ML/min. The relative Si and Ge Auger intensities were used to determine the coverage, and this was checked against direct coverage determination by in-situ STM measurements at low coverages and by Rutherford backscattering measurements on samples removed from the vacuum chamber. RESULTS AND DISCUSSION Growth On Nearly Flat Surfaces Figure l(a) is a large-area STM image of 1.0 ± 0.2 ML of Ge deposited on a nearly flat Si(001) surface at 600' C at a rate of -0.5 ML/min., followed by a 5 min. anneal at 600' C. The steps on the Si substrate are separated by -500 A and oriented roughly along the [110] direction: hence the steps can be roughly characterized as A-type or B-type by their average orientation. Large islands are evident on the surface, elongated along the direction of the island's dimer rows. Figure l(b) shows the typical atomic structure of the surface, i.e. buckled dimers separated by missing dimer rows, forming, on average, the 2x8 reconstruction; there is a broad distribution of dimer row lengths. The B-type step shows large-scale wandering because the step is attached to the elongated islands. The islands could either have nucleated at the steps or have been merged with it subsequently. The upper terraces associated with the A-type step exhibit large holes one layer deep, which could arise from islands nucleating at the A-type step and growing parallel to it, finally coalescing with other islands. The large wandering of the B-type step perpendicular to its average orientation causes it to approach very close to the lower A-type step. Initially the substrate steps are equidistant; the steps on the Ge-covered surface appear to remain, on average, equidistant, indicating that the Atype and B-type steps are advancing at equal rates, either through step flow or by merging with islands. The observed growth morphology appears to be dominated by the anisotropic islands. The presence of islands at this temperature and growth rate is consistent with previous experiments.1° In that work a much lower deposition rate was used at 600' C, with no islands being observed. The island aspect ratio of approximately 2 to 3 is similar to that observed on coarsened islands of Si deposited on Si.14 Ge deposition at lower temperatures and submonolayer coverages has produced very small islands of larger anisotropy. 9 Whether the elongated sha
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