Chemistry Reference
In-Depth Information
10.3.3 Formation of GeSn Nanodots
Ge
1
−
x
Sn
x
alloys have been intensively studied as promising materials for use
in light-emitting devices made of group IV semiconductors since some studies
reported on direct transition semiconductor Ge
1
−
x
Sn
x
alloys [
25
-
28
] with Sn com-
position,
x
larger than critical composition,
x
c
of
∼
0
.
12 [
27
,
28
]. The conduction
band minimum at the
point preferentially decreases with the increase of
x
, sug-
gesting the type I band alignment in Si films. Despite the limited mutual solid solu-
bility of Ge and Sn
1 %, supersaturated solid solutions of Ge
1
−
x
Sn
x
alloys can be
formed using nonequilibrium growth methods such as low-temperature molecular
beam epitaxy, magnetron sputtering, chemical vapor deposition [
27
-
32
]. However,
direct transition semiconductor Ge
1
−
x
Sn
x
alloys with
x
<
x
c
have an energy band
gap,
E
g
, of less than 0.3-0.5 eV [
27
,
28
], which is too small to apply to light-
emitting devices with wavelengths of
>
∼
1
.
5
μ
m(
∼
0
.
8 eV) for use in optical fiber
communication systems.
We have expected a direct transition semiconductor Ge
1
−
x
Sn
x
nanodots with
E
g
∼
.
of
8 eV. The quantum confinement effect in the nanodots enables us to control
the value of
E
g
by changing the nanodot size [
16
,
17
,
33
,
34
], in addition to the
large optical oscillator strength of the nanodots.
We formed ultrasmall voids in the ultrathin SiO
2
films by predeposition of Ge
in two monolayer (ML) (1ML
0
amounts at 650
◦
C. For
suppression of Sn surface segregation, codeposition of Ge and Sn was performed on
these ultrathin SiO
2
films with ultrasmall voids at
10
14
atoms/cm
2
=
7
.
8
×
)
200
◦
C to form hemispherical
∼
10
12
cm
−
2
). The epitaxial
growth of the Ge
1
−
x
Sn
x
nanodots with a Si(111) crystallographic orientation was
confirmed by RHEED observation. We annealed the Ge
1
−
x
Sn
x
nanodots at 500
◦
C
for 3min to improve the Ge
1
−
x
Sn
x
nanodot crystallinity. The flux ratio of Ge to
Sn during codeposition was kept at 0.85-0.15. The deposition amount of GeSn was
changed in a range from 4 to 24ML to control the nanodot diameters from 4 to
35 nm. We confirmed that
x
was about 0.15 by analyzing Ge
1
−
x
Sn
x
lattice images
[
35
] and also measuring photoemission spectra from Ge
1
−
x
Sn
x
nanodots [
36
].
epitaxial Ge
1
−
x
Sn
x
nanodots with ultrahigh density (
∼
1
×
10.3.4 Electronic and Optical Properties of GeSn Nanodots
To remove the surface state contribution in the energy band gap, we performed
atomic hydrogen termination of the nanodot surfaces by introducing hydrogen
molecules up to 2
10
−
4
Pa for 70min at room temperature after putting the samples
in front of heated W filaments. Scanning tunneling spectroscopy (STS) experiments
were conducted at room temperature using chemically polished PtIr tips cleaned by
electron beam heating in UHV chamber. We considered that epitaxial Ge
1
−
x
Sn
x
nanodots were in contact with Si substrates through the ultrasmall (
×
1nm)voids
in the SiO
2
films beneath the nanodots. We found that the electric potential dif-
ference between Si substrates and nanodots was negligible because of the contact
between the epitaxial nanodots and Si substrates, similar to Ge nanodots [
16
].
<
∼
