Chemistry Reference
In-Depth Information
10.3.6 Optical Properties of GaSb Nanodots
We measured the PL of the 65ML nanodots formed at various codeposition temper-
atures, as shown in Fig.
10.13
. Only nanodots formed at 450
◦
C were kept at 450
◦
C
for 30min in Sb flux after nanodot formation to improve the nanodot crystallinity.
All spectra showed the GaSb-related peak in the low-energy range of 0.75-0.85 eV
in addition to several Si-related sharp peaks in the range of 1.0-1.1 eV. The PL
spectra for the GaSb films were highly dependent on the formation method [
52
,
53
].
Using our formation method, the low-energy peaks were positioned at
76 eV for
nanodots (or films) larger than 200ML nanodots. Hence, the low-energy peak in
the GaSb formed using our method was assigned to the optical transition between
the GaSb conduction band and the acceptor, which fits the known optical properties
of the GaSb bulk at 4.2 K [
54
]. It should also be noted that the nanodots formed
at the higher temperature of 450
◦
C showed higher PL intensity, which had higher
crystallinity but lower density.
Figure
10.14
shows a the cross-sectional TEM image, (b) the high-resolution
transmission electron microscope (HRTEM) image, (c) the RHEED pattern, and
(d) the STM image of the 65ML nanodots formed by codeposition and subse-
quently keeping the sample temperatures at 450
◦
C for 30min in Sb flux, which
corresponds to the upper PL spectrum in Fig.
10.13
. These results demonstrated
that epitaxial nanodots were faceted as various shapes with
L
of
∼
0
.
∼
80 nm and their
aspect ratios (height/base length) were
5. The HRTEM images revealed that
the ultrathin SiO
2
films remained at the interfaces between the nanodots and Si
substrates, as indicated by the arrow. Lines with bright contrast showing lattice
defects were observed in HRTEM image as indicated by “SF” in Fig.
10.14
b. These
were twinning boundaries caused by stacking faults (SF) in the nanodots and do not
influence the strain state in nanodots. The existence of the twin crystal in nanodots
is consistent with the twin crystal spots in the RHEED patterns.
STM images showed the nanodot density to be 5
∼
0
.
10
9
cm
−
2
, revealing that the
process of keeping the sample temperatures at 450
◦
C reduced the nanodot density
×
Fig. 10.13
PL spectra from
65ML nanodots formed at
various codeposition
temperatures
