The very initial stage of the molecular beam epitaxy of Si and Ge on Si͑111͒ −7ϫ 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 Si͑111͒ 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 U͑2, 0͒ 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 width͒ along the lower edge of each U͑2, 0͒ 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 width͒ in the unfaulted halves of the 7 ϫ 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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