Ge nanostructures fabricated on Si substrates have been extensively studied aiming at Si-based optoelectronics application. Ge nanodots formed on an ultrathin (0.3 nm) SiO2-covered Si surface have attracted a much attention owing to their high density (> 1012 cm-2) [1] and tunable gap-width by quantum size effect [2,3]. Two types of Ge nanodot are yielded on the oxidized Si surfaces depending on the growth condition; epitaxial and non-epitaxial. The former has sub-nanometer-sized voids on the interface oxide layer and the nanodots and the substrate contact directly, whereas the latter has no voids and the dots and the substrate are separated. Recently, we successfully evaluated the actual potential barrier height of the holes confined into the Ge nanodots, which indicated reduced confining potential for the epitaxial dots compared to the non-epitaxial ones due to the voids [4]. Such difference in interface barrier is expected to modify carrier transport properties among the nanodots. In this context, we conducted surface electrical conductivity measurements both on the epitaxial and non-epitaxial nanodot arrays and compared each other by means of the micro-four-point probe [5] and the four-tip STM [6].
The epitaixal nanodots exhibited higher conductivity compared with the non-epitaxial ones even when the average dot-size is the same. In addition, for the epitaxial ones, dot-size dependent variation of nominal activation energy of the carriers, which can be evaluated from temperature dependence of the conductivity, shows good accordance with energy difference between the valence band maximum of the Si substrate and the lowest-unoccupied state of the Ge dots. These results strongly suggest that the thermally activated carriers at the voids make dominant contribution on the conductivity on the epitaxial dot arrays. Carrier transport nature on the non-epitaxial nanodot arrays will also be discussed.
[1] A. A. Shklyaev et al., Phys. Rev. B, 62, 1540 (2000).
[2] A. Konchenko, et al., Phys. Rev. B, 73, 113311 (2006).
[3] Y. Nakamura, et al., Appl. Phys. Lett., 87, 133119 (2006).
[4] Y. Nakayama, et al., Appl. Phys. Lett., 87, 253102 (2006).
[5] T. Tanikawa, et al., e-J. Surf. Sci. Nanotechnol., 1, 50 (2003).
[6] S. Yoshimoto, et al., Jpn. J. Appl. Phys. 44, L1563 (2005).
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