Enhancement of visible photoluminescence in the SiNx films by SiO2 buffer and annealing

Article abstract of DOI:10.1063/1.2408662

The authors report a simple method to significantly enhance the photoluminescence (PL) of Si Nx films by incorporating a Si O2 buffer and annealing treatment under N2 protection. Strong visible PL is achieved with annealing temperature above 650 °C. Optimal PL is obtained at 800 °C. The composition and structure analysis reveal that strong PL is directly related to the content of the Si-O and Si-N bonds in the Si Nx films. These bonds provide effective luminescent centers and passivate the interface between Si core and the surrounding oxide.

Full text of DOI:10.1063/1.2408662

Enhancement of visible photoluminescence in the SiNx films by SiO2 buffer
and annealing
M. Xu, S. Xu, J. W. Chai, J. D. Long, and Y. C. Ee
Citation: Appl. Phys. Lett. 89, 251904 (2006); doi: 10.1063/1.2408662
View online: http://dx.doi.org/10.1063/1.2408662
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APPLIED PHYSICS LETTERS 89, 251904 ͑2006͒
Enhancement of visible photoluminescence in the SiNx films
by SiO2 buffer and annealing
M. Xua͒
Laboratory for Low-Dimensional Structure Physics, Institute of Solid State Physics, Sichuan Normal
University, Chengdu 610068, People’s Republic of China and Plasma Sources and Applications Center, NIE,
Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Singapore
S. Xu, J. W. Chai, and J. D. Long
Plasma Sources and Applications Center, NIE, Nanyang Technological University, 1 Nanyang Walk,
Singapore 637616, Singapore
Y. C. Ee
Chartered Semiconductor Manufacturing Ltd., Singapore 738406, Singapore
͑Received 15 August 2006; accepted 15 November 2006; published online 18 December 2006͒
The authors report a simple method to significantly enhance the photoluminescence ͑PL͒ of SiNx
films by incorporating a SiO2 buffer and annealing treatment under N2 protection. Strong visible PL
is achieved with annealing temperature above 650 °C. Optimal PL is obtained at 800 °C. The
composition and structure analysis reveal that strong PL is directly related to the content of the Si–O
and Si–N bonds in the SiNx films. These bonds provide effective luminescent centers and passivate
the interface between Si core and the surrounding oxide. © 2006 American Institute of Physics.
͓DOI: 10.1063/1.2408662͔
The observation of visible photoluminescence ͑PL͒ in
porous silicon has stimulated researchers to explore effective
silicon-based light-emitting materials.1 Methods such as ion
implantation,2–4 cosputtering,5 laser ablation,6 and reactive
evaporation of SiO powder7,8 have been extensively used in
preparing these materials to form Si+ implanted SiO2 films,
Si-rich SiO2 films, or Si/SiO2 multilayers. Recently, many
research works have focused on the self-assembly of nano-
particle film because of its simple and cost-effective
process.9,10 SiNxOy ͑Ref. 11͒ and SiNx ͑Ref. 12͒ alloys are
regarded as ideal candidates for the investigation of PL in
silicon-based compound, mostly because of their wide band
gap and excellent mechanical properties. Liao et al.13 first
reported the intense PL from C+-implanted SiNxOy. Re-
cently, Qiu et al.14 observed clear enhancement of blue emis-
sion of SiNxOy-capped silicon nanowire arrays. Visible PL
and white electroluminescence from amorphous SiNx films
have also been reported by Molinari et al.15 and Pei et al.,16
respectively.
grown prior to SiNx deposition. The growth rate of SiNx film
is around 7 nm/min.
After deposition, the sample has been cut into small
pieces of a typical size of 10ϫ5 mm2. Annealing is under-
taken in N2 ambient between 650 and 1200 °C for 20 min. A
Renishaw micro-Raman System 1000 with PL capability is
used to measure PL at room temperature. The system uses an
Ar+ laser excitation source with incident radiation of
514.5 nm and incident power of 10 mW. Fourier-transform
infrared transmission ͑FTIR͒ is employed to characterize the
microstructures of the films in the range of 400–4000 cm−1,
with a resolution of 4 cm−1 with a Perkin Elmer Spectrum
One FTIR spectrometer. The chemical bonding state and
composition are analyzed by an x-ray photoelectron spec-
troscopy calibrated using Au ͑4f7/2͒. The structures of the
as-grown and annealed samples are analyzed using high-
resolution transmission electron microscopy.
Figure 1 shows the effect of the annealing temperature
Tanneal on the PL ͑peak at 2.0–2.1 eV͒ intensity of the
SiO2-buffered SiNx samples. Data have been collected with
laser excitation power of 2.5 mW ͑out power: 25%͒ and de-
Here, we report a simple and relatively rapid method for
fabrication of strong photoluminescent SiNx films on the
Si͑100͒ substrate buffered by
a tetraethylorthosilicate
͑TEOS͒ SiO2 layer grown by chemical vapor deposition us-
ing tetraethylorthosilicate ͑Si͑OC2H5͒4͒ and O2 gaseous
compound. Strong visible PL has been observed from TEOS
SiO2 film after high-temperature annealing.17 The SiNx films
are deposited on the Si͑100͒ substrate using a specially de-
signed low frequency inductively coupled plasma18 assisted
radio frequency ͑rf͒ magnetron sputtering system. A pure Si
target is sputtered under the constant gas flow rate of
N2:Ar:H2=6.4:35.2:9.6 SCCM ͑SCCM denotes cubic cen-
timeter per minute at STP͒, rf power of 500 W, and substrate
temperature of 400 °C. A 630 nm thick SiO2 buffer has been
a͒
Author to whom correspondence should be addressed; also at: Institute of
FIG. 1. ͑Color online͒ Variation of the PL peak ͑at 2.0–2.1 eV͒ intensity
with the annealing temperature. Curve a: SiNx ͑150 nm͒/SiO2 film; curve b:
only TEOS SiO2 film; curve c: Si ͑150 nm͒/SiO2 film.
Solid State Physics, Sichuan Normal University, Chengdu 610068, Peo-
ple’s Republic of China; electronic mail: hsumingគ2001@yahoo.com.cn
0003-6951/2006/89͑25͒/251904/3/$23.00
89, 251904-1
© 2006 American Institute of Physics
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251904-2
Xu et al.
Appl. Phys. Lett. 89, 251904 ͑2006͒
FIG. 3. TEM image at the surface of the SiNx ͑150 nm͒/SiO2 film after
annealing at 800 °C.
at 516.3 cm−1 ͑TO mode͒. The transmission electron micros-
copy ͑TEM͒ results reveal that there are only a few column
structures ͑not crystal͒ in the as-grown samples. After an-
nealing at 800 °C, only few amorphous nanoparticles with
the size less than 10 nm are observed near the surface
͑Fig. 3͒.
Figure 4 shows the FTIR spectra of the samples with the
variation of Tanneal. All the spectra exhibit the characteristic
absorption peak due to the bending motion of Si–O–Si bonds
at about 820 cm−1 and stretching vibration of the Si–O–Si
bonds at around 1080 cm−1. These two absorption peaks are
almost independent of the annealing temperature. In com-
parison to the FTIR spectra of TEOS SiO2, the bending vi-
bration peak of Si–O–Si bonds slightly shifts toward the
higher wave numbers, possibly due to the existence of some
asymmetric stretching vibration of Si–N bonds. Besides, an
absorption peak at around 950 cm−1 is observed for the
samples annealed below 1000 °C, which corresponds to the
vibration frequency of the Si–N bonds. The absorption inten-
sity at around 950 cm−1 decreases monotonously with in-
creasing Tanneal. A strong decrease is seen when Tanneal ex-
cesses 800 °C. The peak disappears as Tanneal is increased up
to 1200 °C.
FIG. 2. ͑Color online͒ Variation of the PL peaks of SiNx ͑150 nm͒/SiO2
film with the annealing temperature: ͑a͒ 650 °C; ͑b͒ 800 °C; ͑c͒ 1200 °C.
tector time of 10 s. The PL intensity is seen to be fairly weak
below 650 °C. Above this temperature, it increases rapidly
before reaching its maximum at Tanneal=800 °C. Further in-
creasing the annealing temperature leads to a significant re-
duction of the PL intensity. The PL intensity drops to ϳ30%
of its peak value at 1000 °C and increases slowly thereafter.
All PL spectra appear to consist multiple peaks. For the
samples annealed at 650 °C, deconvolution performed on
the PL spectrum yields three peaks—a strong emission at
610 nm and two additional weak peaks located at around 575
and 690 nm ͑Fig. 2͒. After annealing at 800 °C, however,
there exist four new peaks at 570, 606, 660, and 703 nm. As
Tanneal is increased to 1200 °C, only two PL peaks at 608 and
684 nm are observed. The PL band peaked at 1.8–2.0 eV is
originated from the silicon dioxide at the Si/SiOx interface,
whereas PL peak at around 1.7–1.8 eV is resulted from
silicon suboxide or quantum size effect of silicon
nanoparticles.19 The 570–575 and 660–690 nm bands are
presumably due to some defects in SiO2 ͑Ref. 8͒ and Si
suboxide, respectively.
In porous silicon, the upper surface layer is usually re-
garded as the source of luminescent. To understand the
SiNx/SiO2 surface, x-ray photoelectron spectroscopy ͑XPS͒
is employed to investigate the chemical bonds and composi-
tion of film. For the sample annealed at 800 °C, the peaks
corresponding to Si͑2p͒ at 101.4 eV and N͑1s͒ at 397.4 eV
The structural evolution of the samples with increasing
annealing temperature is examined by Raman scattering. No
broad peak below 500 cm−1 has been observed, indicating
that there is no substantial contribution of the surface amor-
phous Si͑N͒ to the Raman intensity. The Raman spectrum of
the SiNx/SiO2 sample is very similar to that of the Si sub-
strate, suggesting that there is no Si crystallite in the Si ox-
ide. All the Raman peaks from silicon are identically located
FIG. 4. ͑Color online͒ FTIR spectra of the SiNx ͑150 nm͒/SiO2 films at
different annealing temperatures.
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251904-3
Xu et al.
Appl. Phys. Lett. 89, 251904 ͑2006͒
TABLE I. Surface composition of 150 nm SiNx on TEOS SiOx buffer ana-
lyzed by XPS.
the annealing temperature Tanneal. For the as-grown sample,
there is insufficient O content to provide enough luminescent
centers ͑Si oxide layers͒, and hence the PL is weak. When
Tanneal ͑°C͒
͓Si͔ ͑%͒
͓O͔ ͑%͒
͓N͔ ͑%͒
As-grown
650
800
1200
T
annealϾ650 °C, the O content increases with increasing
Tanneal, leading to an increment of the number of the lumi-
nescent centers. Meanwhile, the Si content providing with
photoexcited carriers keeps almost constant. As a result, the
PL intensity increases during this process. It should be noted
that the N content decreases monotonously as Tanneal in-
creases. As Tanneal is further increased above 800 °C, the O
content keeps increasing while the N content decreases
sharply. In this case, N cannot effectively passivate the
Si/SiOx interface. This means that the probability of photo-
excited carriers transferred to the PL emission center are sub-
stantially decreased. As such, the PL intensity starts to de-
crease when TannealϾ800 °C.
53.67
14.00
32.33
53.90
20.21
25.89
54.44
24.38
21.18
51.12
31.60
17.28
are clearly seen. A quantitative composition analysis ͑Table
I͒ shows that below 800 °C the concentration of Si atom
slightly increases with increasing Tanneal. Above this tempera-
ture, this trend is reversed. On the other hand, the concentra-
tion of N decreases monotonously with increasing Tanneal
,
whereas the opposite is true for the concentration of O.
In order to explain the origin of the PL of the SiNx/SiO2
film, a pure Si film is deposited with the same thickness as
that of SiNx film by sputtering a pure Si target with Ar
plasma at a flow rate of 35.2 SCCM. The PL intensity of
Si/SiO2 films ͑see Fig. 1͒ is found to be even weaker than
that in TEOS SiO2 films. Interestingly, the PL spectra of
Si/SiO2 films exhibit a similar shape to that in SiNx/SiO2
films under high-temperature annealing ͑not shown͒. This
fact indicates that after annealing the PL of SiNx/SiO2 films
is most likely originated from the Si oxide layer which ap-
pears to be formed due to the updiffusing of O atoms in the
TEOS SiO2 layer after high-temperature annealing.20 To un-
derstand if there indeed exist PL centers in the SiNx particles,
a pure SiNx film is deposited directly onto the Si substrate
and subsequently annealed under the same conditions as
mentioned above. However, no obvious PL peak in the pure
SiNx film ͑whether as-grown or annealed at a temperature
above 650 °C͒ has been observed. This further excludes the
possibility of forming Si oxide layer with luminescent cen-
ters from SiNx surface. Another comparison with the PL of
TEOS SiO2 films is highlighted in Fig. 1. A significant in-
crease of the PL intensity in the SiNx/SiO2 films is observed
in comparison to Si/SiO2 films or TEOS SiO2 films. As
shown in Table I, there indeed exists rich Si in all samples.
Therefore, the PL mechanism of the annealed samples may
be understood as follows: the photoexcited carriers are first
generated in the rich Si in the SiNx film. These carriers pass
through the surrounding amorphous SiNx layer and move
into the Si oxide to emit light. In fact, Chen et al.21 have
reported that the defect passivation in multicrystalline Si
could be realized by depositing SiO2/SiN coatings, which
significantly increased the bulk lifetime and decreased the
surface recombination velocity. Therefore, the PL intensity is
remarkably enhanced in the SiNx/SiO2 film in comparison to
the other films presented above. In the recent years, there
have been a number of reports on the PL from the silicon
cluster-containing film in SiNx matrix or Si oxynitride, in
which emission has been conveniently assigned to the pres-
ence of nanometric amorphous silicon or Si–N bonds. In our
work, the photoexcited carriers appear to take place in a
Si-rich SiNx film and effectively move to the luminescent
centers in the Si oxide layer, resulting in strong PL emission.
Further work is needed to confirm if there indeed exists a
blue-violet PL in such system and hence to explore the pos-
sibility of strengthening the PL intensity.
In conclusion, we report the PL enhancement of the SiNx
films prepared by inductively coupled plasma assisted rf
magnetron sputtering by using a TEOS SiO2 buffer and high-
temperature annealing in N2 ambient. The PL intensity
reaches a maximum at the annealing temperature of 800 °C.
The composition and structure analyses revealed that the
strong PL is relevant with the content of the Si–N and Si–O
bonds. Sufficient oxidation and moderate nitridation can ef-
fectively enhance the photoluminescence.
This work is supported by Education Bureau of Sichuan
Province, People’s Republic of China ͑Project No.
2005A092͒ and A*STAR, Singapore ͑Project No. 042-101-
0079͒.
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