Quantum confinement effect of silicon nanocrystals in situ grown in silicon nitride films

Article abstract of DOI:10.1063/1.1814429

Silicon nanocrystals were in situ grown in a silicon nitride film by plasma-enhanced chemical vapor deposition. The size and structure of silicon nanocrystals were confirmed by high-resolution transmission electron microscopy. Depending on the size, the photoluminescence of silicon nanocrystals can be tuned from the near infrared (1.38 eV) to the ultraviolet (3.02 eV). The fitted photoluminescence peak energy as E(eV)=1.16+11.8/d² is evidence for the quantum confinement effect in silicon nanocrystals. The results demonstrate that the band gap of silicon nanocrystals embedded in silicon nitride matrix was more effectively controlled for a wide range of luminescent wavelengths.

Full text of DOI:10.1063/1.1814429

Quantum confinement effect of silicon nanocrystals in situ grown in silicon
nitride films
Tae-Youb Kim, Nae-Man Park, Kyung-Hyun Kim, Gun Yong Sung, Young-Woo Ok et al.
Citation: Appl. Phys. Lett. 85, 5355 (2004); doi: 10.1063/1.1814429
View online: http://dx.doi.org/10.1063/1.1814429
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Published by the American Institute of Physics.
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APPLIED PHYSICS LETTERS
VOLUME 85, NUMBER 22
29 NOVEMBER 2004
Quantum confinement effect of silicon nanocrystals in situ grown
in silicon nitride films
Tae-Youb Kim, Nae-Man Park, Kyung-Hyun Kim, and Gun Yong Sunga)
Future Technology Research Division, Electronics and Telecommunications Research Institute,
Daejeon 305-350, Korea
Young-Woo Ok and Tae-Yeon Seong
Department of Materials Science and Engineering, Gwangju Institute of Science and Technology,
Gwangju 500-712, Korea
Cheol-Jong Choi
Samsung Advanced Institute of Technology, Yongin Gyeonggi-do 449-712, Korea
(Received 30 April 2004; accepted 2 September 2004)
Silicon nanocrystals were in situ grown in a silicon nitride film by plasma-enhanced chemical vapor
deposition. The size and structure of silicon nanocrystals were confirmed by high-resolution
transmission electron microscopy. Depending on the size, the photoluminescence of silicon
nanocrystals can be tuned from the near infrared ͑1.38 eV͒ to the ultraviolet ͑3.02 eV͒. The fitted
photoluminescence peak energy as E͑eV͒=1.16+11.8/d2 is evidence for the quantum confinement
effect in silicon nanocrystals. The results demonstrate that the band gap of silicon nanocrystals
embedded in silicon nitride matrix was more effectively controlled for a wide range of luminescent
wavelengths. © 2004 American Institute of Physics. [DOI: 10.1063/1.1814429]
Because of its indirect band gap of 1.1 eV, silicon is
characterized as having a very poor optical radiative effi-
ciency and only produces light outside the visible range. Sili-
con nanostructures, however, which show a quantum con-
finement effect have an enhanced rate of electron–hole
radiative recombination.1 In recent years, a great deal of re-
search on silicon nanocrystals embedded in a silicon oxide
matrix has been conducted because of their potential for ap-
plications in silicon-based optoelectronic devices.2,3 How-
ever, a number of groups have reported that when the crys-
tallite size of silicon nanostructures in a silicon oxide matrix
is controlled, the experimental photoluminescence energies
in air are not in good agreement with values that are theo-
retically calculated from quantum confinement effects.4,5
Wolkin et al. proposed that oxygen is related to the trapping
of an electron (or even an exciton) by silicon–oxygen double
bonds and produces localized levels in the band gap of
nanocrystals.6 Therefore, a quantum confinement effect is
not observed in silicon nanostructures, after exposure to
air.7,8 Even when a silicon oxide is used as a typical matrix
material that hosts silicon nanostructures, a silicon oxide ma-
trix may not provide an appropriate emission state for a
quantum confinement effect in small silicon crystallites. Be-
cause of this, the focus of the present study was on an ap-
propriate matrix material for silicon nanocrystals. There ap-
pear to be few localized states that correlate with the optical
process of carriers at a nanocrystal surface in a silicon nitride
matrix, as shown in a previous report related to amorphous
silicon quantum dot structures.9,10 In the present work, we
report on silicon nanocrystals that were in situ grown in a
silicon nitride film by plasma-enhanced chemical vapor
deposition. Typically, silicon nanocrystals are obtained by
the postannealing of a silicon-rich silicon oxide at 1100 °C
for 1 or 2 h.11 The method described here is desirable in
terms of integrating silicon based optoelectronic compo-
nents. This method permits a good match with the quantum
confinement effect in zero-dimensional crystalline silicon by
controlling the crystal size, because this provides a good
emission state in small silicon nanocrystals, when a silicon
nitride matrix is used.
The silicon nanocrystals were prepared by plasma-
enhanced chemical vapor deposition, in which argon-diluted
10% silane and nitrogen gas at a purity in excess of
99.9999% were used as the reactant gas sources. (100) crys-
talline silicon wafers were employed as sample substrates.
The total pressure, plasma power, and growth temperature
were fixed at 0.5 Torr, 5 W, and 250 °C, respectively. The
flow rate of silane and nitrogen was used to modulate the rate
of growth of the silicon nitride film and, eventually, to con-
trol the size of the silicon nanocrystals. The flow rate of
silane and nitrogen was in the range from 4 to 12 sccm and
from 500 to 1800 sccm, respectively. No postannealing pro-
cess was required after growing the silicon nitride film. The
size and microscopic structure of the silicon nanocrystals
were confirmed by high-resolution transmission electron mi-
croscopy using a JEOL Electron Microscopy 2010 instru-
ment operated at 200 kV. To demonstrate the quantum con-
finement effect, the photoluminescence of silicon
nanocrystals with various dot sizes were measured. A charge
coupled device detector was used for the photoluminescence
measurements at room temperature, with a He–Cd 325 nm
laser as the excitation source.
Figure 1 shows high-resolution transmission electron mi-
croscopy (HRTEM) images of silicon nanocrystals embed-
ded in a silicon nitride film. The silicon nanocrystals appear
as dark spots and the silicon substrate appears as a dark
region. The average size and dot density of silicon nanocrys-
tals was 4.6 nm and 6.0ϫ1011/cm2, respectively. The stan-
dard deviation for the size distribution was about 0.28 nm.
This sample showed a peak for red-colored light ͑700 nm͒ in
the photoluminescence spectrum. The single dot image in the
a)Electronic mail: gysung@etri.re.kr
0003-6951/2004/85(22)/5355/3/$22.00
5355
© 2004 American Institute of Physics
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5356
Appl. Phys. Lett., Vol. 85, No. 22, 29 November 2004
Kim et al.
FIG. 1. Cross-sectional high-resolution transmission electron microscopic
(HRTEM) images of silicon nanocrystals embedded in a silicon nitride film.
The average size and the dot density of silicon nanocrystals was 4.6 nm and
6.0ϫ1011/cm2, respectively. The insets are the crystal image and the trans-
mission electron diffraction pattern, showing the silicon dot is crystalline.
FIG. 3. Room-temperature photoluminescence spectra of silicon nanocrys-
tals. The peak position can be controlled by appropriate adjustment of the
nanocrystal size.
inset clearly shows that the dark spots are crystallites of sili-
con nanocrystals separated by an amorphous silicon nitride
matrix, and the sharp transmission electron diffraction ring
pattern also confirms the crystal structure of silicon nanoc-
rystals. Diffraction pattern was acquired by the image plate
technology, and the bright spots are originated from silicon
substrate.
between bulk band gap of amorphous silicon ͑1.5–1.6 eV͒.9
This result further verifies that silicon nanoclusters have a
crystal structure, as shown in the HRTEM of Fig. 1. The
fitted confinement parameter of 11.8 is also very large com-
pared with that of an amorphous silicon quantum dot of
2.4.9,12–14 In previous studies, it has been shown that the
confinement parameter by a theoretical calculation can be
changed depending on the calculation method and is about 7
to 14.12,15–18 Our fitted parameters are consistent with the
effective mass approximation. Therefore, the HRTEM mea-
surements and the accompanying photoluminescence results
verify the quantum confinement effect in silicon nanocrys-
tals.
Figure 3 shows a room-temperature photoluminescence
spectrum obtained from various sized silicon nanocrystals,
where the tuning of the photoluminescence emission from
410 to 900 nm is possible by controlling the size of the sili-
con nanocrystal and, as a result, the emission color can be
changed by controlling the size of the nanocrystal. For ex-
ample, nanocrystal sizes corresponding to red, green, and
blue emission are 4.6, 3.1, 2.7 nm, respectively. The forma-
tion of silicon nanoparticles was also reported in the case
where highly diluted gas plasma conditions were
employed.19,20 An increase in nitrogen gas flow rate or the
decrease in silane gas flow rate is thought to result in an
increase in dangling bonds of silicon atoms and such an in-
crease would facilitate the creation of nucleation sites.
Therefore, the size of the silicon clusters is decreased when
the number of silicon cluster nucleation sites is increased due
to a large increase in the total number of silicon dangling
bonds. The formed silicon nanoclusters are transformed to
nanocrystalline silicon by hydrogen radical diffusion through
the hydrogenated amorphous silicon nanocluster at a sub-
strate temperature of 250 °C, if the growth rate is suffi-
ciently low to permit the amorphous to crystalline
transition.20,21 In this case, the growth rate is very low
͑Ͻ1.7 nm/min͒ compared to that for amorphous silicon
quantum dots ͑Ͼ2.3 nm/min͒. Therefore, a crystalline tran-
sition can occur due to the diffusion of hydrogen after the
formation of amorphous silicon quantum dots. A theoretical
study has shown that the amorphous to crystalline transition
can be controlled through appropriate control of deposition
parameters, such as lattice expansion and crystallite size.22
In conclusion, well-organized silicon nanocrystals were
grown in a silicon nitride film by plasma-enhanced chemical
As the size of a quantum structure decreases, the band
gap of the material increases from the quantum confinement
effect. A blueshift in optical luminescence is the result of this
effect. To demonstrate the quantum confinement effect, we
measured the photoluminescence of silicon nanocrystals with
various dot sizes. The change in the photoluminescence peak
energies with the nanocrystal sizes, as determined by HR-
TEM is shown in Fig. 2. When the crystal size was decreased
from 6.1 to 2.6 nm, the photoluminescence peak energy
shifted toward higher wavelengths from 1.46 eV ͑850 nm͒ to
3.02 eV ͑410 nm͒. This figure clearly shows that the photo-
luminescence peak is blueshifted with decreasing nanocrys-
tal size. Assuming an infinite potential barrier, the energy
gap, E, for three-dimensionally confined silicon nanocrystals
can be expressed as E͑eV͒=Ebulk+C/d2 according to effec-
tive mass theory, where Ebulk is the bulk crystal silicon band
gap, d the dot size, and C the confinement parameter. The
best fit for the data shown in Fig. 2 is to the equation
E͑eV͒=1.16+11.8/d2. Fitted bulk band gap of 1.16 is in
good agreement with reported literature values for bulk crys-
tal silicon ͑1.1–1.2 eV͒, and there exists a great difference
FIG. 2. Photoluminescence peak energy of silicon nanocrystals as a function
of crystal size. The solid line was obtained from the effective mass theory
for three-dimensionally confined silicon nanocrystals. The dashed line was
obtained from the effective mass theory for amorphous silicon quantum dot
structure in Ref. 9.
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Appl. Phys. Lett., Vol. 85, No. 22, 29 November 2004
Kim et al.
5357
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demonstrating the viable potential for the fabrication of ef-
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