Chemistry Reference
In-Depth Information
layers to minimize the sum of surface, interface, and strain energies during hetero-
epitaxy. The size and areal density of nanoislands, however, are limited to several
tens of nanometers and
10
11
cm
−
2
, respectively, since they are determined by the
surface diffusion on the wetting layers and growth rate of critical nuclei of deposited
atoms. Many studies have been done to solve the problems so far [
4
]. We have devel-
oped a self-assembled method to form nanodots with
∼
10
12
cm
−
2
density on Si substrates by depositing different materials on Si surfaces covered
with ultrathin Si oxide films of
∼
5 nm size and
∼
3 nm thickness [
5
,
6
]. Several researchers have
afterward developed similar nanodot formation methods on Si surfaces covered with
thin oxide films of thickness thicker than 1 nm [
7
,
8
].
In this chapter, we report a method to form ultrasmall and ultrahigh density
nanodots on Si substrates and focus on electronic properties of the Ge, GeSn, and
GaSb nanodots on Si substrates and optical properties of the nanodots embed-
ded in Si films. We also report an application result to grow GaSb thin films on
Si substrates covered with ultrathin SiO
2
films using GaSb nanodots as seeding
crystals.
∼
0
.
10.2 Experimental Procedure
We used solid source molecular beam epitaxy (MBE) apparatus to formGe nanodots
on Si substrates. After formation of thin Si oxides (
1 nm) by chemical treatment
of Si wafers, the wafer surfaces were cleaned by decomposition of the Si oxides
in the MBE chamber at 830
◦
C under a weak Si flux from an electron beam evap-
orator. Next Si buffer layers of about 100 nm thickness were grown at 550
◦
C. The
ultrathin Si oxides were formed on Si surfaces at 500
◦
C by introducing oxygen in
the chamber at a pressure of 2
∼
10
−
4
Pa for 10min. The thickness of the ultrathin
Si oxide was estimated to be about 0.3 nm and the oxide was mainly composed of
silicon dioxide (SiO
2
)
×
[
9
]. For Ge nanodot formation, Ge with nominal thickness of
several monolayers (ML) was deposited on the surface at the substrate temperature
of about 500
◦
C. This resulted in the formation of Ge nanodots with about 5 nm in
size and the areal density of
10
12
cm
−
2
[
5
]. The layer of Ge nanodots was covered
with about 100 nm thick Si capping layer at 500
◦
C. Some samples were annealed
in a separate lamp furnace at temperatures in the range of 700-900
◦
C for 30min in
dry oxygen atmosphere.
An ultrahigh vacuum scanning tunneling microscope (STM) was used to observe
surface structures and electronic properties of nanodots. A transmission electron
microscope (TEM) and a scanning transmission electron microscope (STEM) were
used to observe cross-sectional structures of the nanostructures. Photoluminescence
(PL) and electroluminescence (EL) spectra of the samples were obtained at tem-
peratures in the range from 4 to
∼
∼
300 K using a standard lock-in technique in con-
junction with an InGaAs photomultiplier detector, a grating monochromator, and a
He-Cd laser with the 325 nm line. The laser beam had about 1mm diameter and
8mWpower.
