Surface phase transitions on clean Ge(111) studied by spectroscopic ellipsometry

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Abstract
We have studied by ellipsometry the surface optical response of clean reconstructed Ge between room temperature and up to a temperature very close to the melting point. In monochromatic temperature scans, three reversible phase transitions are detected. The first is the well-known transition c(2×8)⇆(1×1) around 250 °C. A second one occurs between 700 and 750 °C and is probably related to the order-disorder transition reported by McRae and Malic [Phys. Rev. Lett. 58, 1437 (1987)]. The third transition, at about 70 °C below the bulk melting point, is found for the first time. The changes of the electronic properties of the surface transition layer during these phase transitions are investigated with use of spectroscopic ellipsometry. The transition c(2×8)⇆(1×1) is also studied by photoemission spectroscopy. On Ge(100) surfaces in the high-temperature range, no surface phase transitions could be detected.