Light-Emitting -Fe„ Si
X
Ge
1-X
…
2
Nanodots
on Si
0.8
Ge
0.2
Substrate
Y. L. Chueh, L. J. Chou,
*
,z
S. L. Cheng, J. H. He, W. W. Wu, and L. J. Chen
*
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu,
Taiwan
Uniformly distributed dome-shaped -Fe Si
x
Ge
1-x
2
nanodots synthesized on Si
0.8
Ge
0.2
are reported. A photoluminescence peak at
0.82 eV 1510 nm is observed at 20 K for -Fe Si
x
Ge
1-x
2
nanodots grown epitaxial on Si
0.8
Ge
0.2
substrate.
© 2005 The Electrochemical Society. DOI: 10.1149/1.1896468 All rights reserved.
Manuscript submitted November 10, 2004; revised manuscript received January 18, 2005. Available electronically April 18, 2005.
Silicon-based light-emitting materials, such as SiGe superlattice
structures,
1
porous Si,
2
nanocrystalline Si,
3
erbium-doped Si,
4
and
-FeSi
2
5
have been widely investigated. -FeSi
2
with a bandgap of
about 0.87 eV has the minimum absorption for transmitting in silica
optical fiber at room temperature. It is a silicon-based material,
which has the possibility to be integrated with the well-developed
Si-based semiconductor processes. Previous reports have revealed
that the dot-like shape of -FeSi
2
formed by ion beam synthesis
IBS, reactive deposition epitaxy RDE, or molecular beam epi-
taxy MBE seems to have better emitting performance than that of
the thin film.
6-8
Moreover, Si
1-x
Ge
x
has attracted much attention for its applica-
tions in microelectronics and optoelectronics due to its variable
bandgap and lattice parameter depending on the fraction of Ge at-
oms in the Si layer. The SiGe devices, such as heterojunction bipolar
transistors HBTs, and modulation-doped field-effect transistors
MODFETs or as the template for fabrication of regularly arranged
nanostructures has been hot topics in recent years.
9-11
Previous studies have revealed that agglomeration of germanosi-
licide and Ge-rich SiGe has been found in the reaction of metal thin
film with SiGe.
12-15
This may attribute to the tendency of metal
atoms to react preferentially with Si. Consequently, Ge is precipi-
tated out of the silicide along grain boundary, resulting in the for-
mation of Ge-rich SiGe agglomeration. In the present study, a-Si
and epi-Si layers with different thicknesses were employed to grow
the -Fe Si
X
Ge
1-X
2
nanodots on Si
0.8
Ge
0.2
substrate. The micro-
structures and the PL property are studied by means of HRTEM and
PL spectroscopy. The morphology and the formation mechanisms
are discussed. The approach provided a useful technique for synthe-
sis of a large-area, high-quality light emitting -Fe Si
X
Ge
1-X
2
nanoparticle on SiGe substrate.
Experimental
Single crystal, 1-30 -cm, 001 oriented silicon wafers with a
miscut angle of less than 0.5° were used in the present study. 800
nm thick Si
0.8
Ge
0.2
and 200 nm thick low-temperature Si buffer
layers were deposited on the Si substrates by ultrahigh vacuum
chemical vapor deposition UHV-CVD at 400°C. Surface oxide
layers on SiGe substrate were removed by etching instead of boiling
in acid solutions. The acid solution was composed of
H
2
SO
4
:H
2
O
2
= 5:3.
16
The concentration of the acid solution needed
to be monitored carefully to prevent the surface from serious pitting.
The wafers were dipped in a dilute HF solution HF:H
2
O = 1:50
for 2 min immediately before loading into UHV electron-beam
evaporation chamber with a base pressure of 1 10
-10
Torr. 4, 6, 7,
and 8 nm thick amorphous- and epitaxial-Si layers were evaporated
by UHV-E-beam at room temperature and UHV-CVD at 600°C,
respectively. Subsequently, a 1.5 nm thick Fe film was evaporated
on top of the amorphous- and epitaxial-Si layers by UHV-E-beam at
room temperature. Samples were annealed at 850°C for 5 h in a
vacuum at a pressure lower than 10
-8
Torr. A JEM-3000F field-
emission transmission electron microscope FETEM equipped with
an energy dispersion spectrometer EDS, operating at 300 kV with
point-to-point resolution of 0.17 nm and a JSM-6500F field-
emission scanning electron microscope FESEM, operating at 15
kV were used to study the microstructures and the surface morphol-
ogy. In the PL measurements, samples were cooled to 20 K using a
cryopump system. The samples were excited by an Ar
+
laser with
the exciting wavelength of 514 nm and the beam diameter of less
than 1 mm.
Results and Discussion
Figures 1a-d show SEMs of Fe 1.5 nm /a-Si
4,6,7,8 nm /Si
0.8
Ge
0.2
samples annealed at 850°C for 5 h. The
surface morphology changes from discontinuous film to dome-
shaped nanodots with decreasing thicknesses of a-Si layers. In the
present study, the dome-shaped nanodots are found to be distributed
uniformly at Fe 1.5 nm /a-Si 4 nm /Si
0.8
Ge
0.2
sample. The corre-
sponding magnified SEM shows the uniformly distributed dome-
shaped nanodots with dimension of 40-50 nm inset in Fig. 1d.
Figures 2a-c show the XTEM images of Fe 1.5 nm /a-
Si 8,6,4 nm /Si
0.8
Ge
0.2
sample after annealing at 850°C for 5 h. A
wetting layer is found at Fe 1.5 nm /a-Si 8 nm /Si
0.8
Ge
0.2
sample, but as decreasing the thickness of a-Si, the morphology
changed from wetting layer to random island on Si
0.8
Ge
0.2
substrate.
While the thickness of a-Si decreased to 4 nm, the dome-shaped
dots were found to be half embedded on the leveling of surface with
dimension of 40-50 nm and length of 20-30 nm. The corresponding
plan-view TEM, as shown in Fig. 2d, displays the nanodots distrib-
uted uniformly on SiGe substrate. From the diffraction pattern
analysis, as shown in the inset of Fig. 2d, the phase is polycrystal-
line disilicide with calculated interatomic spacing of 0.308, 0.247,
0.1978, 0.182, and 0.151 nm, respectively. The corresponding planes
are 202, 222, 313, 204, and 440. From the TEM/EDS mea-
surements, the dots are composed of Fe, Si, and Ge with atomic
concentrations of 30, 65, and 5%, respectively. The phase was iden-
tified as -Fe Si
X
Ge
1-X
2
, where some Ge atoms are involved into
silicidation process. In addition, the surfaces near the -
Fe Si
X
Ge
1-X
2
nanodots were flat, whose composition of Si and Ge
at interface are 80 and 20%, respectively by TEM/EDS measure-
ments, indicating that the segregation of Ge is prevented.
To replace the a-Si by the epi-Si layers, the SEMs reveal the
uniformly distributed dome-shaped nanodots but different in size, as
shown in Fig. 3a and b. Figures 3c and d show the XTEMs of the
dome-shaped nanodots, revealing that no defects are found in both
samples. The diameters of dome-shaped nanodots changed from
40-50 nm to 30-40 nm by decreasing the thicknesses of interfacial
epi-Si from 6 to 4 nm. The dome-shaped nanodots were composed
of Si, Fe, and Ge to be about 64, 33, and 3%, respectively, by
TEM/EDS measurements for Fe 1.5 nm /epi-Si 4 nm /Si
0.8
Ge
0.2
sample, indicating that the phase of the dome-shaped nanodots is
dislicide phase -Fe Si
X
Ge
1-X
2
. Figure 4a shows plan-view TEMs
* Electrochemical Society Active Member.
z
E-mail: ljchou@mx.nthu.edu.tw
Electrochemical and Solid-State Letters, 8 6 G137-G139 2005
1099-0062/2005/86/G137/3/$7.00 © The Electrochemical Society, Inc.
G137