The very initial stage of the molecular beam epitaxy of Si and Ge on Si (111) -7×7 substrates with atomically straight steps has been studied by scanning tunneling microscopy and spectroscopy. The atomically straight steps have been prepared on a miscut Si(111) substrate by annealing at 830 °C with kink-up direct current. The length of the steps can be maximized by selecting a proper annealing time. The steps have a well-defined U (2, 0) step-edge structure. The growth of both Si and Ge at temperatures between 250 and 400 °C starts with formation of a single-adatom-row nanowire (0.67 nm in width) along the lower edge of each U (2, 0) step. Subsequent growth of Si and Ge at temperatures between 250 and 300 °C results in formation of one-dimensional arrays of nanoclusters (less than 2.0 nm in width) in the unfaulted halves of the 7×7 structure along the upper step edges. Scanning tunneling spectroscopy reveals localized electronic states of the nanoclusters. Differences between the growth of Si and Ge nanoclusters are discussed.
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