  Growth of narrow-neck, epitaxial as well as non-epitaxial and nearly
spherical Ge islands on air-exposed Si(111)-(7\times7) surfaces has been
investigated by in-situ scanning tunnelling microscopy (STM) and ex-situ high
resolution cross-sectional transmission electron microscopy (HRXTEM). A thin
oxide is formed on Si(111)-(7\times7) surfaces via air exposure. Ge islands are
grown on this oxide. STM measurements reveal the growth of very small (~2 nm
diameter) Ge islands with a high number density of about 1.8\times10^12 cm-2.
The island size has been found to depend on the amount of deposited Ge as well
as the substrate temperature during Ge deposition. HRXTEM micrographs reveal
that the islands are nearly spherical in shape, making narrow-neck contact with
the substrate surface. At 520{\deg}C growth temperature both epitaxial and
non-epitaxial islands grow. However, at 550{\deg}C, Ge islands predominantly
grow epitaxially by a narrow-contact with Si via voids in the oxide layer.
Growth of vertically elongated Ge islands is also observed in HRXTEM
measurements with a very small diameter-to-height aspect ratio (~0.5-1), a
hitherto unreported feature of epitaxial Ge growth on Si surfaces. In addition,
stacking fault and faceting are observed in islands as small as 5 nm diameter.
Ge islands, not even in contact with the Si substrate, appear to be in
epitaxial alignment with the Si substrate. The island size distribution is
essentially monomodal. As the contact area of Ge islands with Si through the
voids in the oxide layer can be controlled via growth temperature, the results
indicate that tunability of the potential barrier at the interface and
consequently the tunability of electronic levels and optical properties can be
achieved by the control of growth temperature.
