Getting high-efficiency photoluminescence from Si nanocrystals in SiO2 matrix

Article abstract of DOI:10.1063/1.1525395

The advancement in the photoluminescence (PL) properties of Si-in-SiO₂ films fabricated by plasma enhanced chemical vapor deposition (PECVD) was discussed. The photoluminescence was found to be improved to 12% when the deposition temperature wa

Full text of DOI:10.1063/1.1525395

Getting high-efficiency photoluminescence from Si nanocrystals in SiO 2 matrix
Y. Q. Wang, G. L. Kong, W. D. Chen, H. W. Diao, C. Y. Chen, S. B. Zhang, and X. B. Liao
Citation: Applied Physics Letters 81, 4174 (2002); doi: 10.1063/1.1525395
View online: http://dx.doi.org/10.1063/1.1525395
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APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 22
25 NOVEMBER 2002
Getting high-efficiency photoluminescence from Si nanocrystals
in SiO2 matrix
Y. Q. Wang,a) G. L. Kong, W. D. Chen, H. W. Diao, C. Y. Chen, S. B. Zhang,
and X. B. Liao
State Key Laboratory for Surface Physics, Institute of Semiconductors, Chinese Academy of Sciences,
Beijing, 100083, China
͑Received 22 April 2002; accepted 7 October 2002͒
Silicon nanocrystals in SiO2 matrix are fabricated by plasma enhanced chemical vapor deposition
followed by thermal annealing. The structure and photoluminescence ͑PL͒ of the resulting films is
investigated as a function of deposition temperature. Drastic improvement of PL efficiency up to
12% is achieved when the deposition temperature is reduced from 250 °C to room temperature.
Low-temperature deposition is found to result in a high quality final structure of the films in which
the silicon nanocrystals are nearly strain-free, and the Si/SiO2 interface sharp. The demonstration of
the superior structural and optical properties of the films represents an important step towards the
development of silicon-based light emitters. © 2002 American Institute of Physics.
͓DOI: 10.1063/1.1525395͔
In the last decade the optical properties of Si nanocrys-
tals have attracted great interest for the potential application
in optoelectronics.1–4 Of all the forms of the material, the
most attention has been paid to porous silicon, mostly be-
cause of its high external quantum efficiency ͑1%–10%͒ of
photoluminescence ͑PL͒.5 Unfortunately, many problems
with porous silicon, including instability of PL efficiency in
ambient condition, inhomogeneous structural, and fragile
mechanical properties, prevent its use in practical applica-
tions. On the other hand, embedding Si in SiO2-matrix has a
number of advantages, such as high stability, the self-
organized quantum well structure, and the compatibility with
microelectronic technology.6–8 However, the reported PL ex-
ternal quantum efficiency of Si in SiO2 matrix was only
0.1%–0.3%.9–11 In the present letter, we report that stable
external quantum efficiency as high as more than 12% can be
realized by optimizing the conditions in producing the mate-
rial from plasma enhanced chemical vapor deposition
͑PECVD͒. The key trick in reaching the superior properties
is actually simple yet very effective. That is, we deposit the
films on RT substrates instead of heating them to above
200 °C as usually accepted in PECVD.
a Philips diffractometer. The local atomic environments and
bonding configurations of silicon oxide matrix in films were
characterized by measurements of infrared absorption, using
a Perkin Elmer 2000 Fourier-transform IR ͑FTIR͒ system.
For excitation of steady-state PL, an Arϩ laser operating at
wavelength of 514.5 nm was used with a low power density
of 0.2 mW/mm2. Detection was achieved with a charge-
coupled detector within the wavelength range from 520 to
900 nm. All PL spectra were taken at RT and corrected for
wavelength-depended sensitivity of the detection.
The influence of the substrate temperature on the PL
intensity is well demonstrated in Fig. 1. Drastic increase of
PL intensity and slight shifts of the peak position of the PL
spectra are observed with the decrease of Td , while the mean
grain size of the silicon nanocrystals for different films is
found to change a little, from 3.4 to 3.8 nm. Thus, the size
changes might be responsible for the shifts of the PL peak
wavelength, but not enough for the improvement of the PL
efficiency. To get a quantitative estimation, the PL efficiency
of our samples was calibrated with a standard lightly doped
ruby ͑0.05% Cr3ϩ) sample under the same conditions of ex-
Silicon oxide films of about 1.6 ␮m thick were deposited
onto both polished and roughened quartz as well as crystal Si
͑100͒ wafers in a conventional capacitance-coupled PECVD
system using a gas mixture of SiH4 , N2O, and H2 . During
deposition, the deposition temperature (Td) was varied from
250 °C to RT, while the flow ratio of SiH4 :N2O:H2 , the rf
power density and the reaction gas pressure was held at
1:3:10, 50 mW/cm2 and 160 Pa, respectively. The films were
then annealed at 1100 °C in N2 ambient for 30 min in order
to form silicon nanocrystals in silicon oxide matrix. The for-
mation of silicon nanocrystals in annealed films was identi-
fied by micro-Raman scattering, using a Renishaw system
1000 Raman imaging microscope. The mean grain size of the
silicon nanocrystals was evaluated by x-ray diffraction, using
FIG. 1. Room temperature PL spectra of the annealed Si-in-SiO2 films fab-
ricated by PECVD with different Td from RT to 250 °C.
a͒
Electronic mail: yqwang@aphy.iphy.ac.cn
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0003-6951/2002/81(22)/4174/3/$19.00 4174 © 2002 American Institute of Physics
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Appl. Phys. Lett., Vol. 81, No. 22, 25 November 2002
Wang et al.
4175
FIG. 2. PL spectra of one of our Si-in-SiO2 samples deposited at RT and a
standard pink ruby ͑0.05% Cr3ϩ) sample. For comparison the PL spectrum
of an independently well-characterized porous silicon sample with an exter-
nal quantum efficiency of 4% is also included.
FIG. 3. Micro-Raman spectra of the annealed Si-in-SiO2 films with different
Td from RT to 250 °C.
This is confirmed by the measurements of micro-Raman
scattering and infrared absorption. Figure 3 shows the Ra-
man spectra of the annealed films with different Td . The
relative redshift of the first-order Raman scattering of silicon
nanocrystals with respect to that of crystalline silicon de-
creases from 4.2 to 0.6 cmϪ1 with reducing Td from 250 °C
to RT, while the line shape of Raman peak remains essen-
tially the same. Thus, it is most possible that the redshift is
due to the elastic strain rather than to the phonon confine-
ment. Then the lattice strain ͑␧͒ can be calculated approxi-
mated by21
citation and detection. Using the well-determined quantum
efficiency of the ruby12,13 and after integration of the emitted
photons over the full emission band, the quantum efficiency
of the samples can be deduced.14 The PL external quantum
efficiency of our samples deposited at RT is found to be
about 12%, far higher than the level ever reached in previous
reports with Si-in-SiO2 films. Figure 2 shows the PL spectra
of one of the Si-in-SiO2 samples deposited at RT and the
standard ruby sample. For comparison the PL spectrum of an
independently well-characterized porous silicon sample with
an external quantum efficiency of 4% is also included. It can
be seen that the PL intensity of the Si-in-SiO2 sample is three
times higher than that of the porous silicon.
Before describing what has happened to our samples, we
need to briefly review how the light-emission process corre-
lates with the material microstructure. It is generally ac-
cepted that the physical mechanism underlying the visible
and near infrared light emission in this kind of silicon mate-
rials is essentially that of the quantum confinement of carri-
ers in a nanometer-scale crystalline structure, and the Si/SiO2
interface is also thought to play an active role in both the
formation of radiative states and the passivation of nonradi-
ative states.15–17 Therefore, a stoichiometric SiO2 matrix and
perfect Si–SiO2 interfaces are of essential importance. Un-
fortunately, in films deposited on substrates at elevated tem-
peratures the oxide matrix is not completely stoichiometric,
at least in the vicinity of Si crystals, and the latter are actu-
ally surrounded by some substoichiometric SiOx (xϽ2)
transition layers with, possibly, changing x.18 The existence
of such transition layers degrades the PL efficiency in two
aspects: It frustrates to a certain extent the quantum confine-
ment and causes many carriers to recombine via the nonra-
diative rather than radiative centers.19 Besides, some lattice
imperfections such as deposition-induced tensile strain inside
silicon clusters are always found in the resulting films, which
cannot be completely released by the high-temperature
post-annealing.20 The lattice strain, especially its inhomoge-
neous component, may degrade the mechanical as well as the
transport and optical properties of the material.
␻cϪ␻
Pϩ2Q
n ϭϪ
␧,
͑1͒
ͩ
ͪ
2
␻
2␻
c
c
where ␻ is the phonon frequency of crystalline silicon ͑521
c
cmϪ1͒, ␻ is the measured phonon frequency of silicon
n
nanocrystals, PϭϪ1.43 ␻ and QϭϪ1.89 ␻2c, the phonon
2
c
deformation potentials of silicon. The deduced ␧ upon the
redshift of ␻cϪ␻ decreases from 0.31% to 0.04% when Td
n
reduces from 250 °C to RT. These data demonstrate that de-
creasing the deposition temperature Td indeed renders the
resulting silicon crystals with smaller residual strain.
Figure 4 gives the infrared absorption spectra of the an-
nealed films with different Td . The absence of the absorption
feature related to Si–H vibration indicates the annealed films
The present work reveals that when the deposition tem-
FIG. 4. FTIR spectra of the annealed Si-in-SiO2 films with different Td from
RT to 250 °C.
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perature is reduced the situation can be greatly improved.
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4176
Appl. Phys. Lett., Vol. 81, No. 22, 25 November 2002
Wang et al.
contain no hydrogen. The principal absorption feature in the
spectra can be assigned to the stretching vibration mode of
Si–O–Si bonds. The frequency of the Si–O–Si stretching
vibration shifts to higher wave numbers with decreasing Td
from 250 to RT, in a direction that would be expected from
increasing x in SiOx . This indicates that the chemical com-
position of oxide matrix becomes more and more close to the
stoichiometric SiO2 . The broad shoulder at the low fre-
quency side of the main feature, most probably identified as
the substoichiometric SiOx (xϽ2), is found to gradually
shrink with lowering Td from 250 °C to RT. All these fea-
tures promise a marked decrease of the substoichiometric
SiOx phase with low Td . The quality of the interface region
between Si nanocrystals and SiO2 matrix is thereby expected
to be greatly improved.
PL properties of Si-in-SiO2 films fabricated by PECVD. Due
to the structural quality improvement of inner silicon nanoc-
rystals and Si/SiO2 interfaces, high external quantum effi-
ciency up to 12% has been achieved by reducing the depo-
sition temperature from 250 °C to RT. The much more stable
structure and higher PL efficiency than that of porous silicon
explore the great possibility of the Si-in-SiO2 films in fabri-
cating efficient silicon-based light-emitting devices.
The authors wish to thank Professor D. L. Zhang and
Professor C. C. Hsu for stimulating discussions. This work
was supported by the National Natural Science Foundation
of China ͑69976028, 29890217͒ and China State Key
Projects of Basic Research ͑G2000028201͒.
The earlier facts show that the deposition temperature-
depended ‘‘pristine state’’ of the films is of key importance
for a high quality final structure. The result can be under-
stood in the following way. Because films deposited at el-
evated temperatures are basically substoichiometric SiOx ,
the precipitation of silicon from SiOx is inevitable for nucle-
ation and growth of silicon nanocrystals during annealing
process. When such a chemical reaction is involved, it is
only natural to expect there are regions where the reaction is
incomplete. Low temperature deposition has two advantages.
First, the Si and its oxide phase could be well-separated in
the pristine films and the composition of the oxide matrix
could be very close to stoichiometric SiO2 .22 In this case,
mainly thermal crystallization of the Si nanoclusters is in-
volved in the postannealing process and perfect Si–SiO2 in-
terfaces are expected. Second, a large amount of hydrogen is
contained in as-deposited films of low Td , which is very
helpful in releasing the strains of the Si crystals. Therefore,
low Td should greatly improve the abruptness of Si–SiO2
interfaces, and release the strains in the structure. These
structural improvements result in the remarkable increase of
PL efficiency, as demonstrated in Figs. 1 and 2. The PL ef-
ficiency can be further improved by increasing the composi-
tional ratio of hydrogen ͑H dilution͒ in PECVD, while the PL
peak energy can be controlled through adjusting the compo-
sitional ratio of oxygen in PECVD. With increasing compo-
sitional ratio of oxygen, the PL peak wavelength blueshifts
due to the size reduction of nanocrystals. These points will
be elaborated elsewhere.
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