Nickel-induced enhancement of photoluminescence in nc-Si–SiOx nanostructures
The effect of nickel silicide interlayer on the intensity of photoluminescence (PL) from Si nanoclusters (nc) in normally deposited and obliquely deposited in vacuum SiOx/Ni/Si structures have been studied using spectral and time-resolved PL measurements. It has been shown that the intensity of PL b...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
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irk-123456789-1184132017-05-31T03:03:15Z Nickel-induced enhancement of photoluminescence in nc-Si–SiOx nanostructures Michailovska, K.V. Indutnyi, I.Z. Shepeliavyi, P.E. Dan’ko, V.A. The effect of nickel silicide interlayer on the intensity of photoluminescence (PL) from Si nanoclusters (nc) in normally deposited and obliquely deposited in vacuum SiOx/Ni/Si structures have been studied using spectral and time-resolved PL measurements. It has been shown that the intensity of PL band in SiOx/Ni/Si samples is essentially higher than that in reference SiOx/Si samples (without the nickel interlayer) with the same characteristics and treatment. The PL intensity enhancement factor is equal to 5.77 for normally deposited samples and 18 for obliquely deposited samples. The unchanged spectral shape of PL bands and similar position of PL maximum in samples with and without nickel silicide interlayer indicates that in the SiOx/Ni/Si structures after annealing no additional emitting centers are introduced to compare with reference one. Time-resolved measurements showed that PL decay rate was decreased from 8.2*10⁴ s⁻¹ for SiOx/Si specimens to 2.86*10⁴ s⁻¹ for SiOx/Ni/Si one. The emission decay rate distribution was determined by fitting the experimental decay curves to the stretchedexponential model. The observed narrow decay rate distribution, decrease of the PL decay rate and enhancement of the PL intensity in SiOx/Ni/Si samples can be assigned to the processes of nickel silicide passivation of the dangling bonds at the interface of Si nanoparticles and the silicon oxide matrix, which is more effective in porous samples. 2014 Article Nickel-induced enhancement of photoluminescence in nc-Si–SiOx nanostructures / K.V. Michailovska, I.Z. Indutnyi, P.E. Shepeliavyi, V.A. Dan'ko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 4. — С. 336-340. — Бібліогр.: 24 назв. — англ. 1560-8034 PACS 78.67.Bf, 78.55.Ap http://dspace.nbuv.gov.ua/handle/123456789/118413 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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The effect of nickel silicide interlayer on the intensity of photoluminescence (PL) from Si nanoclusters (nc) in normally deposited and obliquely deposited in vacuum SiOx/Ni/Si structures have been studied using spectral and time-resolved PL measurements. It has been shown that the intensity of PL band in SiOx/Ni/Si samples is essentially higher than that in reference SiOx/Si samples (without the nickel interlayer) with the same characteristics and treatment. The PL intensity enhancement factor is equal to 5.77 for normally deposited samples and 18 for obliquely deposited samples. The unchanged spectral shape of PL bands and similar position of PL maximum in samples with and without nickel silicide interlayer indicates that in the SiOx/Ni/Si structures after annealing no additional emitting centers are introduced to compare with reference one. Time-resolved measurements showed that PL decay rate was decreased from 8.2*10⁴ s⁻¹ for SiOx/Si specimens to 2.86*10⁴ s⁻¹ for SiOx/Ni/Si one. The emission decay rate distribution was determined by fitting the experimental decay curves to the stretchedexponential model. The observed narrow decay rate distribution, decrease of the PL decay rate and enhancement of the PL intensity in SiOx/Ni/Si samples can be assigned to the processes of nickel silicide passivation of the dangling bonds at the interface of Si nanoparticles and the silicon oxide matrix, which is more effective in porous samples. |
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| author |
Michailovska, K.V. Indutnyi, I.Z. Shepeliavyi, P.E. Dan’ko, V.A. |
| spellingShingle |
Michailovska, K.V. Indutnyi, I.Z. Shepeliavyi, P.E. Dan’ko, V.A. Nickel-induced enhancement of photoluminescence in nc-Si–SiOx nanostructures Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Michailovska, K.V. Indutnyi, I.Z. Shepeliavyi, P.E. Dan’ko, V.A. |
| author_sort |
Michailovska, K.V. |
| title |
Nickel-induced enhancement of photoluminescence in nc-Si–SiOx nanostructures |
| title_short |
Nickel-induced enhancement of photoluminescence in nc-Si–SiOx nanostructures |
| title_full |
Nickel-induced enhancement of photoluminescence in nc-Si–SiOx nanostructures |
| title_fullStr |
Nickel-induced enhancement of photoluminescence in nc-Si–SiOx nanostructures |
| title_full_unstemmed |
Nickel-induced enhancement of photoluminescence in nc-Si–SiOx nanostructures |
| title_sort |
nickel-induced enhancement of photoluminescence in nc-si–siox nanostructures |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2014 |
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http://dspace.nbuv.gov.ua/handle/123456789/118413 |
| citation_txt |
Nickel-induced enhancement of photoluminescence
in nc-Si–SiOx nanostructures / K.V. Michailovska, I.Z. Indutnyi, P.E. Shepeliavyi, V.A. Dan'ko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 4. — С. 336-340. — Бібліогр.: 24 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT michailovskakv nickelinducedenhancementofphotoluminescenceinncsisioxnanostructures AT indutnyiiz nickelinducedenhancementofphotoluminescenceinncsisioxnanostructures AT shepeliavyipe nickelinducedenhancementofphotoluminescenceinncsisioxnanostructures AT dankova nickelinducedenhancementofphotoluminescenceinncsisioxnanostructures |
| first_indexed |
2025-07-08T13:55:58Z |
| last_indexed |
2025-07-08T13:55:58Z |
| _version_ |
1837087287280664576 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 336-340.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
336
PACS 78.67.Bf, 78.55.Ap
Nickel-induced enhancement of photoluminescence
in nc-Si–SiOx nanostructures
K.V. Michailovska, I.Z. Indutnyi, P.E. Shepeliavyi, V.A. Dan’ko
V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine
41, prospect Nauky, 03028 Kyiv, Ukraine
E-mail: indutnyy@isp. kiev.ua
Abstract. The effect of nickel silicide interlayer on the intensity of photoluminescence
(PL) from Si nanoclusters (nc) in normally deposited and obliquely deposited in vacuum
SiOx/Ni/Si structures have been studied using spectral and time-resolved PL
measurements. It has been shown that the intensity of PL band in SiOx/Ni/Si samples is
essentially higher than that in reference SiOx/Si samples (without the nickel interlayer)
with the same characteristics and treatment. The PL intensity enhancement factor is equal
to 5.77 for normally deposited samples and 18 for obliquely deposited samples. The
unchanged spectral shape of PL bands and similar position of PL maximum in samples
with and without nickel silicide interlayer indicates that in the SiOx/Ni/Si structures after
annealing no additional emitting centers are introduced to compare with reference one.
Time-resolved measurements showed that PL decay rate was decreased from 8.2104 s–1
for SiOx/Si specimens to 2.86104 s–1 for SiOx/Ni/Si one. The emission decay rate
distribution was determined by fitting the experimental decay curves to the stretched-
exponential model. The observed narrow decay rate distribution, decrease of the PL
decay rate and enhancement of the PL intensity in SiOx/Ni/Si samples can be assigned to
the processes of nickel silicide passivation of the dangling bonds at the interface of Si
nanoparticles and the silicon oxide matrix, which is more effective in porous samples.
Keywords: time-resolved photoluminescence, thin film, silicon oxide, nanoparticle,
nickel silicide.
Manuscript received 22.04.14; revised version received 16.07.14; accepted for
publication 29.10.14; published online 10.11.14.
1. Introduction
Thin-film structures containing Si nanoparticles
embedded into SiOx matrix attract attention of many
researchers because of their promising applications in
advanced electronic and optoelectronic devices [1-5].
But in spite of intensive researches over the past years,
the reported efficiencies of nc-Si–SiOx light-emitting
structures are still low and not high enough for practical
application. The most important factors influencing the
characteristics of PL are nanoparticle size and state of
nanoparticle–SiOx interface. The passivation of
nonradiative states and defects at this interface is an
essential requirement in order to increase the intensity of
PL. Hydrogen passivation through standard forming gas
annealing is impractical for device application because
hydrogen is easy-to-dissociate at elevated temperatures,
which leads to the invalidation of hydrogen passivation.
The Si–SiOx interface can be modified by chemical
compounds of necessary composition. Such treating is
the most efficient in porous structures. Recently [6, 7],
we have proposed the method of porous nc-Si–SiOx
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 336-340.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
337
light-emitting structure formation using oblique
deposition of Si monoxide (SiO) in vacuum. The
electron microscopy studies show that, during this
deposition, SiOx films with a porous (column-like)
structure are formed. During high-temperature annealing
of these films, the thermally stimulated formation of Si
nanoinclusions occurs in a restricted volume of the SiOx
column. Because of free space (cavities) between the
oxide column, the structures is more susceptible to
chemical treatments, e.g., to treatment with HF solution
or vapor [8]. As a result of HF vapor treatment,
approximately 200-fold increase in the PL intensity is
observed.
It was reported also [3] that introduction of thin
nickel interlayer between SiOx films and Si substrates
results in enhancing of the PL intensity of SiOx/Ni/Si
structures by a factor of 4 in comparison with samples
without any Ni interlayer. Using the examples of light-
emitting diodes based on SiO1.56/Ni/Si structures, it was
shown that NiSi2 distribution in SiO1.56 film could
improve turn-on voltage and give a benefit to the
electroluminescence efficiency [3]. In this paper, we
report the results of studying the nickel induced
enhancement of PL emission in porous and solid nc-Si–
SiOx light-emitting nanostructures.
2. Experimental
A thin (~12 nm) layer of Ni film was deposited by
thermal evaporation of nickel powder onto (100)-
oriented Si wafer. After deposition, the samples were
annealed in vacuum at 450 °C for 10 min. The annealing
provides the required energy for the system to overcome
the energy barriers for solid state reaction of formation
of intermediate nickel silicides [5]. Then, silicon oxide
(SiOx) film was deposited onto the nickel silicide film by
thermal evaporation of Cerac. Inc. SiO with 99.9%
purity in the vacuum chamber (the residual pressure
Pa102...1 3 ). Before SiOx deposition, the substrates
were oriented at the angles (α) of 60° or 0° (normal
deposition) between the normal to the substrate surface
and direction to the evaporator. Because of the
additional oxidation by residual gases during evapo-
ration of SiO the compositionally nonstoichiometric
SiOx (x ~1.25 for normal deposition and x ~1.54 for 60°)
films were obtained. The thickness of the SiOx films
were monitored in situ by the quartz-crystal-oscillator
monitor system (KIT-1) and measured after deposition
by a microinterferometer (MII-4). The thickness of
normally deposited SiOx films was equal to 400 nm,
deposited at 60° – 700…800 nm. The samples without
Ni interlayer were used as the reference ones in these
measurements. As-deposited samples with and without
Ni film were annealed simultaneously in vacuum at the
temperature 975 °C for different duration ranging from 4
to 15 min. This high-temperature annealing leads to
decomposition of silicon oxide (where x changed from
1.25 to 1.95 for samples obtained under normal
deposition and from 1.54 to 2.0 for 60° ones) and
formation of Si nanoparticles embedded into such oxide
matrix [9, 10]. (The composition of the oxide matrix in
as-deposited and annealed samples (parameter x) were
determined using compositional dependence of the
position of the main IR band in spectra of SiOx layers
within the range of 1000 to 1100 cm–1, as it was ascer-
tained in [11]. This band corresponds to the Si–O–Si
stretching mode. FTIR measurements were carried out
with Perkin-Elmer Spectrum BXII spectrometer.)
The structure of obliquely deposited SiOx films was
studied in our previous paper [12] by SEM apparatus
ZEISS EVO 50XVP. Such SiOx films have a porous
inclined column-like structure with the column
diameters of 10 to 100 nm. The porosity of films
depends on the angle of deposition and equals to ~40%
for α = 60°. High-temperature annealing of these films
does not change the porosity and column-like structure
of the samples [7].
The PL spectra of obtained SiOx/Ni/Si and SiOx/Si
samples were recorded at room temperature within the
wavelength range 440 to 900 nm by using a system
based on ZMR-3 monochromator equipped with a
photomultiplier and detection system. The PL spectra
were normalized to the spectral sensitivity of the
experimental setup. The 337-nm line of a nitrogen laser
with a spot size of about 2 mm in diameter was
employed to excite the PL. Decay curve measurements
were performed using the same N2 laser with the pulse
duration 9 ns, which was short as compared to PL
average lifetimes of our samples (approximately tens of
microseconds). The PL lifetime was measured at
different emission wavelengths. The time trace was
recorded with a resolution of 0.5 μs.
3. Results and discussion
Fig. 1 presents the PL spectra of normally deposited
SiOx/Ni/Si samples after annealing at 975 °C for 15 and
4 min (curve 1 and 2, accordingly). Fig. 1 shows also PL
spectra of the reference normally deposited SiOx/Si
sample (curve 3) under annealing at the same
temperature for 15 min. At room temperature, all the
samples exhibited a broad band with a maximum of
emission centered within 760…780 nm. This strong
near-infrared luminescence band within the wavelength
range 600…900 nm is associated with quantum
confinement effects, that is with electron-hole pairs (or
exciton) recombination in nc-Si. The intensity of
emission from SiOx/Ni/Si samples IPL(Ni) is enhanced
significantly in comparison with the reference sample
(IPL) and depends on the annealing time. So, the PL
enhancement factor η = PLPL II Ni measured at
780 nm increases from 2.45 to 5.77 for 4- and 15-min
annealing time, accordingly.
Fig. 2 shows the PL spectra for the SiOx/Ni/Si
(curve 1) and SiOx/Si (curve 2) samples deposited at 60°
and annealed at 975 °C for 15 min in vacuum. The
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 336-340.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
338
emission spectrum of both samples exhibit a broad band
with the peak position at 630 nm. A spectral shift of PL
peaks to shorter wavelengths in obliquely deposited
samples in comparison with normally deposited ones can
be caused by decrease in nc-Si dimensions. The size of
nc-Si in these systems depends on the silicon content in
deposited SiOx layers [13]: decrease in the Si content
(i.e., increase of x) results in decreased nc-Si sizes and
blue shift of PL spectrum. Besides, during high-
temperature annealing of porous films with column-like
structure the thermally stimulated formation of nc-Si
occurs in a restricted volume of the SiOx columns. It can
also cause the reduction of the nc-Si size and blue shift
of the PL peak.
As seen from Figs 1 and 2, addition of Ni interlayer
results in enhancement of the PL intensity more
prominent in porous samples. For porous, obliquely
deposited samples, the enhancement factor η measured
at 630 nm is equal to 18.
600 650 700 750 800 850 900
0.0
0.1
0.2
0.3
0.4
0.5
0.6
I P
L
, a
rb
. u
ni
ts
nm
3
2
1
Fig. 1. PL spectra of normally deposited samples: SiOx/Ni/Si
after annealing in vacuum at 975 °C for 15 (1) and 4 min (2)
and the reference SiOx/Si sample after annealing at 975 °C for
15 min (3).
500 550 600 650 700 750 800
0
2
4
6
8
10
I P
L
, a
rb
. u
ni
ts
,nm
1
2
Fig. 2. PL spectra of SiOx/Ni/Si (1) and reference of SiOx/Si
(2) samples obliquely deposited at the angle 60° after
annealing in vacuum at 975 °C for 15 min.
It is known that, in a low excitation regime, the
PL intensity is generally given by the expression IPL ~
(τPL/τR)σφN, where τPL and τR are the photo-
luminescence and radiative lifetimes, respectively, φ is
the photon flux of the laser pump (constant throughout
our experiment), N is the density of nanocrystals in the
film, and σ is their excited cross-section [17]. The
unchanged spectral shape of PL bands and similar
position of PL maximum in samples with and without
Ni interlayer mean that in the SiOx/Ni/Si structures
after annealing no additional emitting centers are
introduced to compare with the reference one. In the
quantum confinement scheme, the light emission from
nc-Si is caused by radiative recombination of electron-
hole pairs (or excitons) confined within nanoparticle
[15]. It is deduced that the radiative lifetimes τR would
be close to each other in SiOx/Ni/Si and SiOx/Si
samples due to the same luminescence scheme and
similar size of nc-Si (similar position of the PL
maximum). One can suppose that nc-Si density (N) and
PL lifetime (τPL) are the main factors changing the PL
intensity in SiOx/Ni/Si samples. More direct
demonstration of enhanced electron-hole pair
recombination involved comparative measurements of
the PL decay rate in the investigated structures.
Time-resolved PL measurements were performed
using the normally deposited SiOx/Ni/Si and SiOx/Si
samples. Fig. 3 shows PL decay curve for SiOx/Ni/Si
(curve 1) and SiOx/Si (curve 2) samples at 780 nm. One
can see that the PL intensity for the SiOx/Ni/Si samples
decayed slower than for the reference samples. The
obtained decay curves of the PL intensity may be
described well by a stretched exponential function:
0
1 exp
t
tCtIPL , (1)
where C, τ0 and β are some constant, decay time and
stretched parameter (0 < β ≤ 1), respectively. It is
known that the stretched exponential function is widely
used for the decay curve analysis of Si nanocrystals
[16]. The least-squares fit of Eq. (1) to experimental
data brings values of τ0 and β. The obtained decay
times τ0 were equal to 35 and 12.2 μs for SiOx/Ni/Si
and SiOx/Si samples, respectively. This range of PL
lifetimes is comparable with those reported in the
literature for silicon nanocrystals at room temperature
[17]. The PL decay rate k 1
0
k at 780 nm is
decreased from 8.2104 s–1 for SiOx/Si to 2.86104 s–1
for SiOx/Ni/Si samples. It was also determined that the
dispersion parameter β is 0.83 for SiOx/Ni/Si and 0.74
for SiOx/Si samples. In general, the parameter β is
related to the stretching of the decay process and is a
direct measure of the width of the decay rate
distribution. In the case of stretched exponential
relaxation function, the PL decay may be analyzed
more thoroughly by recovering the distribution of
recombination rates [16].
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 336-340.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
339
0 10 20 30 40
0.0
0.2
0.4
0.6
0.8
1.0
I P
L
, a
rb
. u
ni
ts
t, s
1
2
ex
= 337 nm
em
= 780 nm
T= 295 K
Fig. 3. PL decay curves measured at 780 nm for the normally
deposited SiOx/Ni/Si (1) and SiOx/Si (2) samples after
annealing at 975 °C for 15 min.
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
0.0
0.2
0.4
0.6
0.8
1.0
D
is
tr
ib
ut
io
n
Ф
(k
)
Decay rate, s-1
without Ni
2
1
with Ni
Fig. 4. Decay rate distributions of SiOx/Ni/Si (1) and SiOx/Si
(2) normally deposited samples obtained from the stretched-
exponential decay model. The decay rate distributions are
normalized by the peak value of Ф(k) for both samples.
Using the values of τ0 and β measured at λ =
780 nm, we calculated the asymptotic form of the decay
rates probability density function Ф(k) that might be
obtained by the saddle-point method [18]:
aa kk
a
kФ
exp
2
)( 2/1 , (2)
where 11 a and 1/1
0 1
a . Fig. 4
shows the Ф(k) distributions calculated from Eq. (2) for
SiOx/Ni/Si (curve 1) and SiOx/Si (curve 2) samples. The
obtained distribution for the reference SiOx/Si sample is
very broad with a long tail directed towards shorter
lifetimes, which demonstrates the strongly non-single
exponential character of decay curves. In SiOx/Ni/Si
samples, the decay rate distribution Ф(k) is more narrow
as compared with SiOx/Si sample and shifts towards lower
decay rates. To explain the observed feature, it should be
mentioned that the obtained Ф(k) function provides
information about both the radiative (kR) and nonradiative
(kNR) relaxation rates [19]. A very low quantum efficiency
of nc-Si emission suggests that nonradiative processes
should be predominant (kNR >> kR). It allows us to relate
the changes observed in the decay rate distribution Ф(k) to
the different quantity of defect states (dangling silicon
bonds) in the matrix containing nc-Si. We believe that the
interface between nc-Si and the SiOx matrix plays a
crucial role in distributing decay rates [20]. SiO2 is an
ideal matrix for nc-Si, as it can passivate a large fraction
of the dangling Si bonds. There is a remarkable content of
broken silicon bonds in SiOx films, which may act as
nonradiative recombination paths for the excited carriers
and cause the quenching of PL [21]. The observed narrow
decay rate distribution and the increase of the PL decay
time in SiOx/Ni/Si samples can be related to the processes
of nickel silicide passivation of nonradiative
recombination centers. It has been reported previously that
nickel silicide thin films were formed by vacuum thermal
processing (350-750 °C) of Ni thin films deposited onto
(100) p-type Si substrates [4]. Ni as the mobile species
moves through NiSi during the transition from NiSi to the
NiSi2 phase at 750…1000 °C [22]. NiSi2 distributed in
SiOx film passivates the broken bonds on the nc-Si surface
[23]. The model of NiSi2 passivation was proved by
thermodynamic analysis and Fourier transform infrared
spectroscopy in SiO1.56/Ni/Si systems [3].
In the porous SiOx/Ni/Si samples, the nickel
diffusion processes significantly accelerated due to the
presence of voids and the surface of nanocolumns. This
leads to more effective passivation of the nonradiative
recombination centers and more significant increase in
the PL emission intensity.
In conclusion, we can note that the PL intensity
enhancement factor calculated from the experimental
results for SiOx/Ni/Si and SiOx/Si samples annealed at
975 °C is η = 5.77 for normal deposited samples.
Enhancement of PL lifetimes in these samples by the
factor ~2.87 due to the processes of nickel silicide
passivation is not enough for explanation of the increase
in PL intensity. Therefore, we proposed that the PL
intensity enhancement can be caused both by increasing
the PL lifetime and also by increasing the nc-Si density.
The presence of Ni gives an additional driving force to
the separation process of SiOx which was discussed in
the previous paper [24].
5. Conclusion
In summary, we have presented the effect of Ni on the
PL emission from Si nanoparticles embedded in the
silicon oxide matrix. It has been shown that the intensity
of near-infrared emission band in SiOx/Ni/Si samples
was significantly higher than that in the samples without
the Ni interlayer. It was assumed that nickel in SiOx film
may passivate the residual dangling bonds at the
interface of nc-Si and SiOx matrix. Time-resolved PL
measurements showed the decrease in the PL decay rate
in SiOx/Ni/Si samples as compared with the SiOx/Si one.
Decrease in the PL decay rate and increase in the density
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 336-340.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
340
of nc-Si could be the main factors enhancing the PL
intensity in nanostructures with nickel silicide interlayer.
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210 (2010)].
9. V.A. Dan’ko, V.Ya. Bratus’, I.Z. Indutnyi,
I.P. Lisovskyy, S.O. Zlobin, K.V. Michailovska,
P.E. Shepeliavyi, Controlling the photo-
luminescence spectra of porous nc-Si–SiOx
structures by vapor treatment // Semiconductor
Physics, Quantum Electronics & Optoelectronics,
13(4). p. 413-417 (2010).
10. V.Ya. Bratus’, V.A. Yukhimchuk, L.V. Berezhinsky
et al., Structural transformation and silicon
nanocrystallite formation in SiOx films //
Semiconductors, 35(7), p. 821-826 (2001).
11. M. Nakamura, Y. Mochizuki, K. Usami et al.,
Infrared absorption spectra and compositions of
evaporated silicon oxide (SiOx) // Solid State
Communs. 50, p. 1079-1081 (1984).
12. I.Z. Indutnyi, K.V. Michailovska, V.I. Min’ko,
P.E. Shepeliavyi, Effect of acetone vapor treatment
on photoluminescence of porous nc-Si–SiOx
nanostructures // Semiconductor Physics, Quantum
Electronics & Optoelectronics, 12(2), p. 105-109
(2009).
13. D. Nesheva, C. Raptis, A. Perakis et al., Raman
scattering and photoluminescence from Si
nanoparticles in annealed SiOx thin films // J. Appl.
Phys. 92, p. 4678-4683 (2002).
14. C. Garcia, B. Garrido, P. Pellegrino et al., Size
dependence of lifetime and absorption cross section
of Si nanocrysrals embedded in SiO2 // Appl. Phys.
Lett. 82, p. 1595-1597 (2003).
15. C. Delerue, G. Allan, M. Lannoo, Theoretical
aspects of the luminescence of porous silicon //
Phys. Rev. B, 48, p. 11024-11036 (1993).
16. G. Zatrub, A. Podhorodecki, J. Misiewicz et al., On
the nature of the stretched exponential
photoluminescence decay for silicon nanocrystals //
Nanoscale Res. Lett. 6, p. 106 (2011).
17. M. Dovrat, Y. Goshen, J. Jedrzejewski, I. Balberg,
A. Sa’ar, Radiative versus nonradiative decay
process in silicon nanocrystals probed by time-
resolved photoluminescence spectroscopy // Phys.
Rev. B, 69, 155311 (2004).
18. R. Sato, K. Murayama, A universal distribution
function of relaxation in amorphous materials //
Solid State Communs. 63, p. 625-627 (1987).
19. A.F. van Driel, I.S. Nikolaev, P. Vergeer et al.,
Statistical analysis of time-resolved emission from
ensembles of semiconductor quantum dots:
interpretation of exponential decay models // Phys.
Rev. B, 75, 035329 (2007).
20. G. Hadjisavvas, P.C. Kelires, Structure and
energetics of Si nanocrystals embedded in α-SiO2 //
Phys. Rev. Lett. 93, p. 226104 (2004).
21. I. Mihalcescu, J.C. Vial, R. Romestain, Carrier
localization in porous silicon investigated by time-
resolved luminescence analysis // J. Appl. Phys. 80,
p. 2404 (1996).
22. M. Bhaskaran, S. Sriram, T.S. Perova et al., In situ
micro-Raman analysis and X-ray diffraction of
nickel silicide thin films on silica // Micron, 40(1),
p. 89-93 (2009).
23. H.F. Yan, Y.J. Xing, Q.L. Hang, D.P. Yu,
Y.P. Wang et al., Growth of amorphous silicon
nanowires via a solid-liquid-solid mechanism //
Chem. Phys. Lett. 323, p. 224-228 (2000).
24. Y. He, K. Ma, I. Bi, J.Y. Feng, Z.J. Zhang, Nickel-
induced enhancement of photoluminescence from
Si-rich silica films // Appl. Phys. Lett. 88, 031905
(2006).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 336-340.
PACS 78.67.Bf, 78.55.Ap
Nickel-induced enhancement of photoluminescence
in nc-Si–SiOx nanostructures
K.V. Michailovska, I.Z. Indutnyi, P.E. Shepeliavyi, V.A. Dan’ko
V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine
41, prospect Nauky, 03028 Kyiv, Ukraine
E-mail: indutnyy@isp. kiev.ua
Abstract. The effect of nickel silicide interlayer on the intensity of photoluminescence (PL) from Si nanoclusters (nc) in normally deposited and obliquely deposited in vacuum SiOx/Ni/Si structures have been studied using spectral and time-resolved PL measurements. It has been shown that the intensity of PL band in SiOx/Ni/Si samples is essentially higher than that in reference SiOx/Si samples (without the nickel interlayer) with the same characteristics and treatment. The PL intensity enhancement factor is equal to 5.77 for normally deposited samples and 18 for obliquely deposited samples. The unchanged spectral shape of PL bands and similar position of PL maximum in samples with and without nickel silicide interlayer indicates that in the SiOx/Ni/Si structures after annealing no additional emitting centers are introduced to compare with reference one. Time-resolved measurements showed that PL decay rate was decreased from 8.2(104 s–1 for SiOx/Si specimens to 2.86(104 s–1 for SiOx/Ni/Si one. The emission decay rate distribution was determined by fitting the experimental decay curves to the stretched-exponential model. The observed narrow decay rate distribution, decrease of the PL decay rate and enhancement of the PL intensity in SiOx/Ni/Si samples can be assigned to the processes of nickel silicide passivation of the dangling bonds at the interface of Si nanoparticles and the silicon oxide matrix, which is more effective in porous samples.
Keywords: time-resolved photoluminescence, thin film, silicon oxide, nanoparticle, nickel silicide.
Manuscript received 22.04.14; revised version received 16.07.14; accepted for publication 29.10.14; published online 10.11.14.
1. Introduction
Thin-film structures containing Si nanoparticles embedded into SiOx matrix attract attention of many researchers because of their promising applications in advanced electronic and optoelectronic devices [1-5]. But in spite of intensive researches over the past years, the reported efficiencies of nc-Si–SiOx light-emitting structures are still low and not high enough for practical application. The most important factors influencing the characteristics of PL are nanoparticle size and state of nanoparticle–SiOx interface. The passivation of nonradiative states and defects at this interface is an essential requirement in order to increase the intensity of PL. Hydrogen passivation through standard forming gas annealing is impractical for device application because hydrogen is easy-to-dissociate at elevated temperatures, which leads to the invalidation of hydrogen passivation. The Si–SiOx interface can be modified by chemical compounds of necessary composition. Such treating is the most efficient in porous structures. Recently [6, 7], we have proposed the method of porous nc-Si–SiOx light-emitting structure formation using oblique deposition of Si monoxide (SiO) in vacuum. The electron microscopy studies show that, during this deposition, SiOx films with a porous (column-like) structure are formed. During high-temperature annealing of these films, the thermally stimulated formation of Si nanoinclusions occurs in a restricted volume of the SiOx column. Because of free space (cavities) between the oxide column, the structures is more susceptible to chemical treatments, e.g., to treatment with HF solution or vapor [8]. As a result of HF vapor treatment, approximately 200-fold increase in the PL intensity is observed.
It was reported also [3] that introduction of thin nickel interlayer between SiOx films and Si substrates results in enhancing of the PL intensity of SiOx/Ni/Si structures by a factor of 4 in comparison with samples without any Ni interlayer. Using the examples of light-emitting diodes based on SiO1.56/Ni/Si structures, it was shown that NiSi2 distribution in SiO1.56 film could improve turn-on voltage and give a benefit to the electroluminescence efficiency [3]. In this paper, we report the results of studying the nickel induced enhancement of PL emission in porous and solid nc-Si–SiOx light-emitting nanostructures.
2. Experimental
A thin (~12 nm) layer of Ni film was deposited by thermal evaporation of nickel powder onto (100)-oriented Si wafer. After deposition, the samples were annealed in vacuum at 450 °C for 10 min. The annealing provides the required energy for the system to overcome the energy barriers for solid state reaction of formation of intermediate nickel silicides [5]. Then, silicon oxide (SiOx) film was deposited onto the nickel silicide film by thermal evaporation of Cerac. Inc. SiO with 99.9% purity in the vacuum chamber (the residual pressure
(
)
Pa
10
2
...
1
3
-
×
). Before SiOx deposition, the substrates were oriented at the angles (α) of 60° or 0° (normal deposition) between the normal to the substrate surface and direction to the evaporator. Because of the additional oxidation by residual gases during evapo-ration of SiO the compositionally nonstoichiometric SiOx (x ~1.25 for normal deposition and x ~1.54 for 60°) films were obtained. The thickness of the SiOx films were monitored in situ by the quartz-crystal-oscillator monitor system (KIT-1) and measured after deposition by a microinterferometer (MII-4). The thickness of normally deposited SiOx films was equal to 400 nm, deposited at 60° – 700…800 nm. The samples without Ni interlayer were used as the reference ones in these measurements. As-deposited samples with and without Ni film were annealed simultaneously in vacuum at the temperature 975 °C for different duration ranging from 4 to 15 min. This high-temperature annealing leads to decomposition of silicon oxide (where x changed from 1.25 to 1.95 for samples obtained under normal deposition and from 1.54 to 2.0 for 60° ones) and formation of Si nanoparticles embedded into such oxide matrix [9, 10]. (The composition of the oxide matrix in as-deposited and annealed samples (parameter x) were determined using compositional dependence of the position of the main IR band in spectra of SiOx layers within the range of 1000 to 1100 cm–1, as it was ascer-tained in [11]. This band corresponds to the Si–O–Si stretching mode. FTIR measurements were carried out with Perkin-Elmer Spectrum BXII spectrometer.)
The structure of obliquely deposited SiOx films was studied in our previous paper [12] by SEM apparatus ZEISS EVO 50XVP. Such SiOx films have a porous inclined column-like structure with the column diameters of 10 to 100 nm. The porosity of films depends on the angle of deposition and equals to ~40% for α = 60°. High-temperature annealing of these films does not change the porosity and column-like structure of the samples [7].
The PL spectra of obtained SiOx/Ni/Si and SiOx/Si samples were recorded at room temperature within the wavelength range 440 to 900 nm by using a system based on ZMR-3 monochromator equipped with a photomultiplier and detection system. The PL spectra were normalized to the spectral sensitivity of the experimental setup. The 337-nm line of a nitrogen laser with a spot size of about 2 mm in diameter was employed to excite the PL. Decay curve measurements were performed using the same N2 laser with the pulse duration 9 ns, which was short as compared to PL average lifetimes of our samples (approximately tens of microseconds). The PL lifetime was measured at different emission wavelengths. The time trace was recorded with a resolution of 0.5 μs.
3. Results and discussion
Fig. 1 presents the PL spectra of normally deposited SiOx/Ni/Si samples after annealing at 975 °C for 15 and 4 min (curve 1 and 2, accordingly). Fig. 1 shows also PL spectra of the reference normally deposited SiOx/Si sample (curve 3) under annealing at the same temperature for 15 min. At room temperature, all the samples exhibited a broad band with a maximum of emission centered within 760…780 nm. This strong near-infrared luminescence band within the wavelength range 600…900 nm is associated with quantum confinement effects, that is with electron-hole pairs (or exciton) recombination in nc-Si. The intensity of emission from SiOx/Ni/Si samples IPL(Ni) is enhanced significantly in comparison with the reference sample (IPL) and depends on the annealing time. So, the PL enhancement factor η =
(
)
PL
PL
I
I
Ni
measured at 780 nm increases from 2.45 to 5.77 for 4- and 15-min annealing time, accordingly.
Fig. 2 shows the PL spectra for the SiOx/Ni/Si (curve 1) and SiOx/Si (curve 2) samples deposited at 60° and annealed at 975 °C for 15 min in vacuum. The emission spectrum of both samples exhibit a broad band with the peak position at 630 nm. A spectral shift of PL peaks to shorter wavelengths in obliquely deposited samples in comparison with normally deposited ones can be caused by decrease in nc-Si dimensions. The size of nc-Si in these systems depends on the silicon content in deposited SiOx layers [13]: decrease in the Si content (i.e., increase of x) results in decreased nc-Si sizes and blue shift of PL spectrum. Besides, during high-temperature annealing of porous films with column-like structure the thermally stimulated formation of nc-Si occurs in a restricted volume of the SiOx columns. It can also cause the reduction of the nc-Si size and blue shift of the PL peak.
As seen from Figs 1 and 2, addition of Ni interlayer results in enhancement of the PL intensity more prominent in porous samples. For porous, obliquely deposited samples, the enhancement factor η measured at 630 nm is equal to 18.
600650700750800850900
0.0
0.1
0.2
0.3
0.4
0.5
0.6
I
PL
, arb. units
nm
3
2
1
–
Fig. 1. PL spectra of normally deposited samples: SiOx/Ni/Si after annealing in vacuum at 975 °C for 15 (1) and 4 min (2) and the reference SiOx/Si sample after annealing at 975 °C for 15 min (3).
500550600650700750800
0
2
4
6
8
10
I
PL
, arb. units
,nm
1
2
Fig. 2. PL spectra of SiOx/Ni/Si (1) and reference of SiOx/Si (2) samples obliquely deposited at the angle 60° after annealing in vacuum at 975 °C for 15 min.
It is known that, in a low excitation regime, the PL intensity is generally given by the expression IPL ~ (τPL/τR)σφN, where τPL and τR are the photo-luminescence and radiative lifetimes, respectively, φ is the photon flux of the laser pump (constant throughout our experiment), N is the density of nanocrystals in the film, and σ is their excited cross-section [17]. The unchanged spectral shape of PL bands and similar position of PL maximum in samples with and without Ni interlayer mean that in the SiOx/Ni/Si structures after annealing no additional emitting centers are introduced to compare with the reference one. In the quantum confinement scheme, the light emission from nc-Si is caused by radiative recombination of electron-hole pairs (or excitons) confined within nanoparticle [15]. It is deduced that the radiative lifetimes τR would be close to each other in SiOx/Ni/Si and SiOx/Si samples due to the same luminescence scheme and similar size of nc-Si (similar position of the PL maximum). One can suppose that nc-Si density (N) and PL lifetime (τPL) are the main factors changing the PL intensity in SiOx/Ni/Si samples. More direct demonstration of enhanced electron-hole pair recombination involved comparative measurements of the PL decay rate in the investigated structures.
Time-resolved PL measurements were performed using the normally deposited SiOx/Ni/Si and SiOx/Si samples. Fig. 3 shows PL decay curve for SiOx/Ni/Si (curve 1) and SiOx/Si (curve 2) samples at 780 nm. One can see that the PL intensity for the SiOx/Ni/Si samples decayed slower than for the reference samples. The obtained decay curves of the PL intensity may be described well by a stretched exponential function:
(
)
ú
ú
û
ù
ê
ê
ë
é
÷
÷
ø
ö
ç
ç
è
æ
t
-
=
b
-
b
0
1
exp
t
t
C
t
I
PL
,
(1)
where C, τ0 and β are some constant, decay time and stretched parameter (0 < β ≤ 1), respectively. It is known that the stretched exponential function is widely used for the decay curve analysis of Si nanocrystals [16]. The least-squares fit of Eq. (1) to experimental data brings values of τ0 and β. The obtained decay times τ0 were equal to 35 and 12.2 μs for SiOx/Ni/Si and SiOx/Si samples, respectively. This range of PL lifetimes is comparable with those reported in the literature for silicon nanocrystals at room temperature [17]. The PL decay rate k
(
)
1
0
-
t
=
k
at 780 nm is decreased from 8.2(104 s–1 for SiOx/Si to 2.86(104 s–1 for SiOx/Ni/Si samples. It was also determined that the dispersion parameter β is 0.83 for SiOx/Ni/Si and 0.74 for SiOx/Si samples. In general, the parameter β is related to the stretching of the decay process and is a direct measure of the width of the decay rate distribution. In the case of stretched exponential relaxation function, the PL decay may be analyzed more thoroughly by recovering the distribution of recombination rates [16].
0 10 20 30 40
0.0
0.2
0.4
0.6
0.8
1.0
I
PL
, arb. units
t, s
1
2
ex
= 337 nm
em
= 780 nm
T= 295 K
Fig. 3. PL decay curves measured at 780 nm for the normally deposited SiOx/Ni/Si (1) and SiOx/Si (2) samples after annealing at 975 °C for 15 min.
0.000.050.100.150.200.250.300.350.40
0.0
0.2
0.4
0.6
0.8
1.0
Distribution Ф(k)
Decay rate, s
-1
without Ni
2
1
with Ni
Fig. 4. Decay rate distributions of SiOx/Ni/Si (1) and SiOx/Si (2) normally deposited samples obtained from the stretched-exponential decay model. The decay rate distributions are normalized by the peak value of Ф(k) for both samples.
Using the values of τ0 and β measured at λ = 780 nm, we calculated the asymptotic form of the decay rates probability density function Ф(k) that might be obtained by the saddle-point method [18]:
(
)
(
)
[
]
a
a
k
k
a
k
Ф
-
-
-
t
-
×
t
×
pb
t
=
exp
2
)
(
2
/
1
,
(2)
where
(
)
1
1
-
b
-
b
=
a
and
(
)
[
]
1
/
1
0
1
-
b
-
b
t
=
t
a
. Fig. 4 shows the Ф(k) distributions calculated from Eq. (2) for SiOx/Ni/Si (curve 1) and SiOx/Si (curve 2) samples. The obtained distribution for the reference SiOx/Si sample is very broad with a long tail directed towards shorter lifetimes, which demonstrates the strongly non-single exponential character of decay curves. In SiOx/Ni/Si samples, the decay rate distribution Ф(k) is more narrow as compared with SiOx/Si sample and shifts towards lower decay rates. To explain the observed feature, it should be mentioned that the obtained Ф(k) function provides information about both the radiative (kR) and nonradiative (kNR) relaxation rates [19]. A very low quantum efficiency of nc-Si emission suggests that nonradiative processes should be predominant (kNR >> kR). It allows us to relate the changes observed in the decay rate distribution Ф(k) to the different quantity of defect states (dangling silicon bonds) in the matrix containing nc-Si. We believe that the interface between nc-Si and the SiOx matrix plays a crucial role in distributing decay rates [20]. SiO2 is an ideal matrix for nc-Si, as it can passivate a large fraction of the dangling Si bonds. There is a remarkable content of broken silicon bonds in SiOx films, which may act as nonradiative recombination paths for the excited carriers and cause the quenching of PL [21]. The observed narrow decay rate distribution and the increase of the PL decay time in SiOx/Ni/Si samples can be related to the processes of nickel silicide passivation of nonradiative recombination centers. It has been reported previously that nickel silicide thin films were formed by vacuum thermal processing (350-750 °C) of Ni thin films deposited onto (100) p-type Si substrates [4]. Ni as the mobile species moves through NiSi during the transition from NiSi to the NiSi2 phase at 750…1000 °C [22]. NiSi2 distributed in SiOx film passivates the broken bonds on the nc-Si surface [23]. The model of NiSi2 passivation was proved by thermodynamic analysis and Fourier transform infrared spectroscopy in SiO1.56/Ni/Si systems [3].
In the porous SiOx/Ni/Si samples, the nickel diffusion processes significantly accelerated due to the presence of voids and the surface of nanocolumns. This leads to more effective passivation of the nonradiative recombination centers and more significant increase in the PL emission intensity.
In conclusion, we can note that the PL intensity enhancement factor calculated from the experimental results for SiOx/Ni/Si and SiOx/Si samples annealed at 975 °C is η = 5.77 for normal deposited samples. Enhancement of PL lifetimes in these samples by the factor ~2.87 due to the processes of nickel silicide passivation is not enough for explanation of the increase in PL intensity. Therefore, we proposed that the PL intensity enhancement can be caused both by increasing the PL lifetime and also by increasing the nc-Si density. The presence of Ni gives an additional driving force to the separation process of SiOx which was discussed in the previous paper [24].
5. Conclusion
In summary, we have presented the effect of Ni on the PL emission from Si nanoparticles embedded in the silicon oxide matrix. It has been shown that the intensity of near-infrared emission band in SiOx/Ni/Si samples was significantly higher than that in the samples without the Ni interlayer. It was assumed that nickel in SiOx film may passivate the residual dangling bonds at the interface of nc-Si and SiOx matrix. Time-resolved PL measurements showed the decrease in the PL decay rate in SiOx/Ni/Si samples as compared with the SiOx/Si one. Decrease in the PL decay rate and increase in the density of nc-Si could be the main factors enhancing the PL intensity in nanostructures with nickel silicide interlayer.
References
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7.
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8.
I.Z. Indutnyi, E.V. Michailovskaya, P.E. Shepeliavyi and V.A. Dan’ko, Visible photoluminescence of selectively etched porous nc-Si–SiOx structures // Fizika tekhnika poluprovodnikov 44(2), p. 218-222 (2010) (in Russian) [Semiconductors, 44(2), p. 206-210 (2010)].
9.
V.A. Dan’ko, V.Ya. Bratus’, I.Z. Indutnyi, I.P. Lisovskyy, S.O. Zlobin, K.V. Michailovska, P.E. Shepeliavyi, Controlling the photo-luminescence spectra of porous nc-Si–SiOx structures by vapor treatment // Semiconductor Physics, Quantum Electronics & Optoelectronics, 13(4). p. 413-417 (2010).
10.
V.Ya. Bratus’, V.A. Yukhimchuk, L.V. Berezhinsky et al., Structural transformation and silicon nanocrystallite formation in SiOx films // Semiconductors, 35(7), p. 821-826 (2001).
11.
M. Nakamura, Y. Mochizuki, K. Usami et al., Infrared absorption spectra and compositions of evaporated silicon oxide (SiOx) // Solid State Communs. 50, p. 1079-1081 (1984).
12.
I.Z. Indutnyi, K.V. Michailovska, V.I. Min’ko, P.E. Shepeliavyi, Effect of acetone vapor treatment on photoluminescence of porous nc-Si–SiOx nanostructures // Semiconductor Physics, Quantum Electronics & Optoelectronics, 12(2), p. 105-109 (2009).
13.
D. Nesheva, C. Raptis, A. Perakis et al., Raman scattering and photoluminescence from Si nanoparticles in annealed SiOx thin films // J. Appl. Phys. 92, p. 4678-4683 (2002).
14.
C. Garcia, B. Garrido, P. Pellegrino et al., Size dependence of lifetime and absorption cross section of Si nanocrysrals embedded in SiO2 // Appl. Phys. Lett. 82, p. 1595-1597 (2003).
15.
C. Delerue, G. Allan, M. Lannoo, Theoretical aspects of the luminescence of porous silicon // Phys. Rev. B, 48, p. 11024-11036 (1993).
16.
G. Zatrub, A. Podhorodecki, J. Misiewicz et al., On the nature of the stretched exponential photoluminescence decay for silicon nanocrystals // Nanoscale Res. Lett. 6, p. 106 (2011).
17.
M. Dovrat, Y. Goshen, J. Jedrzejewski, I. Balberg, A. Sa’ar, Radiative versus nonradiative decay process in silicon nanocrystals probed by time-resolved photoluminescence spectroscopy // Phys. Rev. B, 69, 155311 (2004).
18.
R. Sato, K. Murayama, A universal distribution function of relaxation in amorphous materials // Solid State Communs. 63, p. 625-627 (1987).
19.
A.F. van Driel, I.S. Nikolaev, P. Vergeer et al., Statistical analysis of time-resolved emission from ensembles of semiconductor quantum dots: interpretation of exponential decay models // Phys. Rev. B, 75, 035329 (2007).
20.
G. Hadjisavvas, P.C. Kelires, Structure and energetics of Si nanocrystals embedded in α-SiO2 // Phys. Rev. Lett. 93, p. 226104 (2004).
21.
I. Mihalcescu, J.C. Vial, R. Romestain, Carrier localization in porous silicon investigated by time-resolved luminescence analysis // J. Appl. Phys. 80, p. 2404 (1996).
22.
M. Bhaskaran, S. Sriram, T.S. Perova et al., In situ micro-Raman analysis and X-ray diffraction of nickel silicide thin films on silica // Micron, 40(1), p. 89-93 (2009).
23.
H.F. Yan, Y.J. Xing, Q.L. Hang, D.P. Yu, Y.P. Wang et al., Growth of amorphous silicon nanowires via a solid-liquid-solid mechanism // Chem. Phys. Lett. 323, p. 224-228 (2000).
24.
Y. He, K. Ma, I. Bi, J.Y. Feng, Z.J. Zhang, Nickel-induced enhancement of photoluminescence from Si-rich silica films // Appl. Phys. Lett. 88, 031905 (2006).
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
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