Formation and Opto-electronic Properties of Nanostructures on Ultrathin SiO2-Covered Si Surfaces - Nanophenomena at Surfaces - page 221

Fig. 10.2 STM images and RHEED patterns after two bilayer (BL) Ge deposition at 390 ◦ C( a )and
( b ), and at 500 ◦ C( c )and( d )

spite of the fact that the Ge was deposited on the amorphous SiO 2 films, the RHEED
pattern (Fig. 10.2 d) shows that Ge nanodots were epitaxially grown on the Si(111)
substrate at higher deposition temperatures higher than 500 ◦ C. The Ge deposition
can create areas of bare Si through the SiO 2 decomposition reaction. These Si
bare areas provided conditions for the epitaxial growth of Ge nanodots. At lower
deposition temperatures lower than 400 ◦ C shown in Fig. 10.2 a, b, bare Si areas
were not created, resulting in the growth of non-epitaxial Ge nanodots. The island
density hardly depended on the deposition rate, indicating that the island density
was mainly determined by the Ge chemical reactions with the ultrathin SiO 2 films.
Stacked structures were made by repeating the process in which the epitaxial Ge
nanodots were embedded in Si films by depositing Si and the Si film surface was
oxidized to form ultrathin SiO 2 films and then Ge was deposited to grow new Ge
nanodots [ 12 ].
The SiO 2 surfaces are chemically inert resulting in small adsorption energy for
Ge atoms on the surfaces. The Si window and Ge nucleated areas are chemically
active resulting in the large adsorption energy on the surfaces. The adsorption differ-
ence produces the potential barriers of about 3 eV between the SiO 2 and Si window
and Ge nucleated areas [ 13 ]. The potential barriers interrupt the atom exchanges
on the surfaces resulting in the selective growth of Ge nanoislands and nanodots as
shown in Fig. 10.1 , and also in uniform size distribution of the nanodots due to the
suppression of the Ostwald ripening.
This formation technology could also be applied to form Si [ 14 ] and

-FeSi 2
[ 15 ] ultrasmall nanodots with ultrahigh densities on Si substrates. We could observe
carrier quantum confinement effects in Ge [ 16 ] and

β

-FeSi 2 [ 17 ] nanodots, and
Coulomb blockade effect in Ge nanodots [ 18 ] even at room temperature due to the
ultrasmall sizes.

β