close

Вход

Забыли?

вход по аккаунту

?

j.spmi.2017.10.009

код для вставкиСкачать
Superlattices and Microstructures xxx (2017) 1e7
Contents lists available at ScienceDirect
Superlattices and Microstructures
journal homepage: www.elsevier.com/locate/superlattices
Enhancement of the luminescence by the controlled growth
of silicon nanocrystals in SRO/SiO2 superlattices
~ as-Tay b, T. Díaz-Becerril a, G. García-Salgado a,
A. Coyopol a, **, S.A. Caban
c
nchez b, *
L. Palacios-Huerta , F. Morales-Morales b, A. Morales-Sa
n en Dispositivos Semiconductores, Universidad Auto
noma de Puebla, 14 Sur y Av. San Claudio, San Manuel,
Centro de Investigacio
72000, Puebla, Mexico
b
n en Materiales Avanzados SC, Unidad Monterrey-PIIT, 66600, Apodaca, Nuevo Leo
n, Mexico
Centro de Investigacio
c
nica, INAOE, Puebla, 72000, Mexico
Instituto Nacional de Astrofísica Optica
y Electro
a
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 September 2017
Received in revised form 5 October 2017
Accepted 6 October 2017
Available online xxx
This work reports the study of highly-luminescent silicon nanocrystals (SieNCs) in silicon
rich oxide (SRO)/SiO2 multilayers (MLs). Parameters such as silicon excess (Si-excess) and
SRO-thickness were modified to evaluate the structure and composition and their effect on
the photoluminescence (PL) response of the different superlattices. SRO monolayers with
the same silicon excess were also deposited for comparison. Both, monolayers and MLs,
emit a broad emission band in the red-orange region (1.45e2.1 eV). The PL of SRO
monolayers strongly increases as Si-excess decreases from 10.2 to 5.2 at.%. Nevertheless,
SRO/SiO2 MLs allow up to 14-fold PL enhancement as compared to SRO monolayers. A
silicon diffusion from SRO nano-layers towards the SiO2 ones reduces the Si content within
the SRO allowing the SieNC size reduction (thus increasing the SieNC density) as
compared to SRO monolayers. Therefore, the high luminescence is correlated with the
SieNCs formation with a mean size below 3 nm where the surface defects (Si]O bonds)
are strongly active.
© 2017 Elsevier Ltd. All rights reserved.
Keywords:
Silicon rich oxide
Silicon nanocrystals
Photoluminescence
SRO/SiO2 multilayers
1. Introduction
Nanostructured materials such as silicon nanocrystals (SieNCs) embedded in a non-stoichiometric dielectric matrix (SiOx),
also known as silicon rich oxide (SRO), have been widely investigated in the last decades with the aim of providing complementary metal-oxide-semiconductor (CMOS)-compatible Si-based light sources [1e3]. Unfortunately, the electro-optical
efficiency in electroluminescent devices is still low, mainly due to the following reasons: a) Since the band-gap value of
dielectric material (SiOx) is high (4e9 eV), high voltage levels are required to promote the flow of carriers [4,5], b) Poor quality
of the SieNCs:SiOx composite yields parasitic current paths, while low density of nanocrystals due to low Si-excess makes
direct charge injection into the nanocrystals difficult [4], and c) the control of the emission energy in SRO monolayers is
complicated because of the low control on the size distribution and density of SieNCs, which produce a broad band emission
[6,7].
* Corresponding author.
** Corresponding author.
nchez).
E-mail addresses: acoyopol@gmail.com (A. Coyopol), alfredo.morales@cimav.edu.mx (A. Morales-Sa
https://doi.org/10.1016/j.spmi.2017.10.009
0749-6036/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: A. Coyopol et al., Enhancement of the luminescence by the controlled growth of silicon
nanocrystals in SRO/SiO2 superlattices, Superlattices and Microstructures (2017), https://doi.org/10.1016/j.spmi.2017.10.009
2
A. Coyopol et al. / Superlattices and Microstructures xxx (2017) 1e7
Some alternatives have been proposed to improve the electro-optical efficiency in the SRO films such as SRO/SiO2
multilayered (ML) structures [8e10] and Si-rich SiNx/SRO MLs in distributed Bragg reflector (DBR)-based architectures [11,12].
In such structures, the SRO-thickness, Si-excess and annealing temperature are parameters that promote the formation of
SieNCs and radiative defects, affecting the optical and electrical properties of the ML. Some techniques with high degree of
accuracy in thickness and homogeneity films have emerged for the design of these SRO/SiO2 ML structures. Among them: low
pressure chemical vapor deposition (LPCVD), plasma-enhanced chemical vapor deposition (PECVD) and sputtering are the
most common techniques [8e13].
In this work, 10 SRO/SiO2 bilayers were deposited by the co-sputtering method. The atomic concentration ratio x(O/Si) (and
Si-excess) of SRO layers were x ¼ 1.65 (5.2 at.%), x ¼ 1.3(10.2 at.%) and x ¼ 1.1(14.3 at.%). The SRO-thickness was also modified
from 2.5, 5 and 7.5 nm for each atomic concentration and keeping constant the SiO2 layer thickness at about 6 nm. SRO
monolayers with the same Si excess were also deposited for comparison.
Both, SRO monolayers and SRO/SiO2 MLs show a broad emission band in the orange-red region (1.45e2.3 eV). Nevertheless, the SRO/SiO2 MLs emit a stronger PL intensity when compared with SRO monolayers. The most intense PL emission is
observed when the SRO-thickness is 5 nm, and with the highest Si-excess (14.3 at.%), which is important for the design of
electroluminescent devices with low threshold voltage. To the best of our knowledge, there are few reports where an
improved emission was obtained for a high Si-excess. Although multilayer structures with good control in crystal size have
been reported [14], a comprehensive study of SRO/SiO2 MLs as a function of Si-excess (5.2e14.3 at.%) and modulating the SROthickness layer (2.5e7.5 nm) have not been discussed in detail.
2. Experimental details
SRO monolayers and SRO/SiO2 MLs were deposited on p-type (100) Si wafers with resistivity of 2e5 U-cm by using a Torr
International magnetron sputtering system (13.56 MHz). Before deposition, Si substrates were cleaned in ultrasonic bath with
acetone, ethanol, and deionized water successively. Si wafers were immersed in a 10% hydrofluoric acid (HF) aqueous solution
for 2 min to remove the native oxide. After being dried with nitrogen, the substrates were immediately loaded into the
chamber of the sputtering system. Once a base pressure of ~1 106 Torr is achieved, Ar flow of 60 sccm is introduced into the
chamber at a working pressure of 2.4 mTorr.
The SRO monolayers were deposited by the simultaneous co-sputtering of Si and fused quartz (SiO2) targets. The Si content
in the SRO layers was modified by a variation of RF-power applied to Si (Psi) target at 50, 60 and 70 W, keeping constant the
RF-power applied to SiO2 (PSiO2) target at 100 W. Table 1 shows the thicknesses and Si-excess of SRO monolayers.
For SRO/SiO2 MLs, SRO layers with the same atomic concentration that monolayers were used. First, a SiO2 layer was
deposited onto the silicon substrate followed by a SRO film to obtain a SRO/SiO2 bi-layer. 10 periods of SRO/SiO2 bi-layers were
deposited with an additional (upper) SiO2 film (10 nm) to avoid oxidization during high temperature annealing. Each SiO2
layer was about 6 nm in thick while the SRO layer thickness was modified from 2.5, 5 and 7.5 nm (see Table 2). All films were
deposited at 100 C. After deposition, samples were thermally annealed in a conventional tube furnace at 1100 C in N2
environment for 2 h.
Thickness of the films was measured by reflectance using a Filmetrics F20UV equipment. The chemical compositions and
Si-excess in the SRO layers were analyzed by a Thermo Scientific X-ray photoelectron spectroscopy (XPS) Escalab 250Xi
equipment. The Si oxide phase was studied by Fourier transform infra-red (FTIR) spectroscopy using a Bruker Vector 22
spectrometer in the range 400e4000 cm1. The PL emission spectra were measured with a Horiba Fluoromax 3 system. The
samples were excited using a 300 (4.13 eV) nm radiation and the PL emission signal was collected from 400 to 900 nm
(1.37e3.1 eV) with a resolution of 1 nm. Finally, cross-view high resolution transmission electron microscopy (HRTEM)
images were obtained to study the structural properties of the SRO layers and SRO/SiO2 MLs using an electron microscopy
JEOL JEM 2200.
3. Results and discussions
The Si-excess, as analyzed by XPS in depth profile, within SRO monolayers is about 5.2, 10.2 and 14.3 at.% for Rp (PSiO2/
PSi) ¼ 2, 1.66 and 1.4, respectively. These values are very similar to those one obtained in a previous work, indicating the
process is repetitive [15].
Table 1
Thickness and Si-excess obtained for SRO monolayers.
Sample
Sample Topology
Si-excess (at.%)
Thickness (nm)
As-deposited
Annealed
A
B
C
SRO-Monolayer
SRO-Monolayer
SRO-Monolayer
5.2 ± 0.2
10.2 ± 0.3
14.3 ± 0.1
108 ± 1.6
82 ± 1.0
90 ± 1.2
90 ± 0.6
70 ± 0.8
76 ± 1.1
Please cite this article in press as: A. Coyopol et al., Enhancement of the luminescence by the controlled growth of silicon
nanocrystals in SRO/SiO2 superlattices, Superlattices and Microstructures (2017), https://doi.org/10.1016/j.spmi.2017.10.009
A. Coyopol et al. / Superlattices and Microstructures xxx (2017) 1e7
3
Table 2
SRO and SiO2 thickness and Si-excess in the SRO/SiO2 multilayer structures.
Sample
Sample Topology
Si-excess (at.%)
Total Thickness (nm)
As-deposited
Annealed
1A
2A
3A
1B
2B
3B
1C
2C
3C
~2.5 nm-SRO/~6 nm-SiO2
~5 nm-SRO/~6 nm-SiO2
~7.5 nm-SRO/~6 nm-SiO2
~2.5 nm-SRO/~6 nm-SiO2
~5 nm-SRO/~6 nm-SiO2
~7.5 nm-SRO/~6 nm-SiO2
~2.5 nm-SRO/~6 nm-SiO2
~5 nm-SRO/~6 nm-SiO2
~7.5 nm-SRO/~6 nm-SiO2
5.2
107 ± 2.0
135 ± 2.5
165 ± 2.4
99 ± 3.0
130 ± 1.9
170 ± 3.2
107 ± 3.0
140 ± 2.4
168 ± 2.6
105
127
150
103
120
152
102
124
154
10.2
14.3
±
±
±
±
±
±
±
±
±
1.8
2.0
2.2
3.5
2.8
3.2
2.6
1.8
2.2
For the design of SRO/SiO2 ML structures, SRO layers with the same atomic concentration that monolayers were used. We
assume that the atomic concentration in SRO monolayers should be the same as in SRO/SiO2 MLs. K. J. Kim et al. [16] found
that oxidation of SRO layers during the SRO/SiO2 growth is negligible, indicating that the atomic concentration in both cases is
similar.
Fig. 1a and b shows, respectively, the PL spectra of thermally annealed SRO monolayers and SRO/SiO2 MLs with a SROthickness of 5 nm with different Si-excesses. The PL emission intensity of all spectra was normalized to the total thicknesses of each sample. Both, monolayers and multilayers, emit an intense and broad emission band in the red-orange region
(1.45e2.1 eV). An additional broad PL band between 2.5 and 3.1 eV (blue-green) was also observed in these films, but with a PL
intensity two orders of magnitude lower than the red one (not shown). This blue-green band, which has been related to the
combination of oxygen defect centers [17], was analyzed in more detail elsewhere [15].
The PL of SRO monolayers strongly increases as Si-excess decreases from 10.2 to 5.2 at.%, as observed in Fig. 1a. Nevertheless, a stronger PL emission is obtained in the ML structures (SRO-5 nm) for any Si-excess as compared to SRO monolayers
(see Fig. 1b). It is noteworthy to mention that the effective thickness of the SRO intermediate layers in each multilayer is
approximately 50 nm (Multilayers 2A, 2B and 2C), which is lower than the thickness of the monolayers, demonstrating the
effect of the multiple SRO/SiO2 layers in the PL intensity.
In order to determine and analyze the contribution of different luminescent centers, the PL spectra were deconvoluted.
Three PL bands located at ~1.72 eV, ~1.6 eV and ~1.52 eV are obtained, as shown in Fig. 1a and b. As we can see, the intensity of
the different emission bands changes as function of Si-excess and SRO-thickness. To study such effect, a ratio of emission
intensities was calculated taking the maximum PL intensity of each peak (~1.72 eV, ~1.6 eV and ~1.52 eV) that form the PL
spectrum of each SRO/SiO2 ML (IPL ML) divided by the maximum PL intensity of SRO monolayers (IPL Mono) located at the
same energy. This emission ratio (IPL ML/IPLMono) is shown in Fig. 2 for different silicon excess and SRO thickness in each
multilayer.
As it can be observed, the intensity ratio increases as the SRO-thickness and Si-excess increase, mainly the emission band
located around ~1.72 eV. It is observed that the maximum PL intensity is obtained when the SRO-thickness in the MLs is about
5 nm for any silicon excess. Moreover, this PL intensity ratio shows that MLs exhibit an intensity close to 1.4, 11 and 14 times
stronger respect to the monolayers, as observed in Fig. 2a), b) and 2c), when the Si-excess within the SRO nanofilm is 5.2, 10.2
and 14.3 at.%, respectively. So, the most intense photoluminescence is achieved using SRO nanofilms with 5 nm thick and
14 at.% of Si-excess. Such effect is important for the design of electroluminescent devices, particularly for supplying low
voltages, since a large number of SieNCs are generated as Si-excess increases.
For SRO monolayers, the main PL band in red-orange region (1.45e2.2 eV) tends to decrease as Si-excess increases from 5.2
to 14.3 at.% (Fig. 1b). It has been suggested that for a medium Si-excess (<9 at.%), oxygen-related defects, such as Si]O
(nonbridged oxygen passivation) and/or ultra-small oxidized SieNCs (diameters <2 nm) are the most optically active centers
that promote an intense emission in red region (~1.5e1.7 eV). Accordingly, SieNCs with diameters larger than 3 nm (Siexcess > 10 at.%) are not direct light-emitting centers [18,19]. Thus, a considerable Si-excess (>10 at.%) in SRO films is not
necessary to obtain a maximum PL emission. Therefore, in this work SieNCs around 3 nm in size are possibly synthesized and
confined in the ML structures by changing the SRO-thickness and Si-excess. In order to analyze the phase separation (SieSiO2)
and then the formation of SieNCs upon Si-excess and annealing temperature, FTIR and XPS (in depth profile) measurements
were done. It is well known that the most sensitive SieO vibration mode to structural and compositional changes within a
silica matrix is the SieO stretching mode. Fig. 3 shows the evolution of the FTIR spectra in the range of 800e1280 cm1 for
SRO monolayers and SRO/SiO2 MLs with a SRO-thickness of 5 nm, before and after thermal annealing.
The SieOeSi stretching band shifts towards low wavenumbers (1047e1027 cm1) as the silicon content increases (from
5.2 to 14.3 at.%) within as-deposited monolayers, as observed in Fig. 3a. An additional IR band of low intensity is observed
around 880 cm1. Such vibrational mode disappears when samples are thermally annealed (1100 C), indicating a rearrangement of the SiOx network due to a diffusion of Si atoms, which promotes Si-SiOx phase separation and the formation of
SieNCs in SRO layers [15,20,21]. The phase separation and SieNCs formation in monolayers is corroborated by the wellknown shift observed in the SieOeSi stretching band position (1047e1080 cm1) after thermal annealing [15,21].
Please cite this article in press as: A. Coyopol et al., Enhancement of the luminescence by the controlled growth of silicon
nanocrystals in SRO/SiO2 superlattices, Superlattices and Microstructures (2017), https://doi.org/10.1016/j.spmi.2017.10.009
A. Coyopol et al. / Superlattices and Microstructures xxx (2017) 1e7
2500
Si-excess (at.%)
5.2
10.2
14.3
500
0
1500
1000
1.6 eV
1.72 eV
1.6 eV
1500
1000
Si-excess (at.%)
5.2
10.2
14.3
2000
1.52 eV
PL intensity (arb. units)
2000
b)
1.52eV
a)
PL intensity (arb. units)
2500
1.72 eV
4
500
0
1.50
1.65
1.80
1.95
2.10
2.25
1.50
1.65
Energy (eV)
1.80
1.95
2.10
2.25
Energy (eV)
Fig. 1. PL spectra of a) SRO monolayers and b) SRO/SiO2 MLs (SRO-5 nm) with 5.2 at.% 10.2 at.%, and 14.3 at.% of Si-excess thermally annealed at 1100 C.
5.2 at.%
14
IPL ML / IPL Mono
6
4
2
0
2
3
4
5
6
7
10
8
6
4
2
0
8
c)
12
10
8
14.3 at.%
14
1.72 eV
1.60 eV
1.52 eV
12
10
16
b)
10.2 at.%
14
1.72 eV
1.60 eV
1.52 eV
12
IPL ML / IPL Mono
16
a)
IPL ML / IPL Mono
16
2
SRO-tickness in ML (nm)
3
4
5
6
7
8
8
6
4
1.72 eV
1.60 eV
1.52 eV
2
0
2
SRO-tickness in ML (nm)
3
4
5
6
7
8
SRO-tickness in ML (nm)
Fig. 2. The emission ratio (IPL ML/IPLMono) as a function of SRO-thickness and Si-excess: a) 5.2 at%, b) 10.2 at.% and c) 14.3 at.%.
Intensity (arb. units)
0.16
0.14
As-deposited:
5.2%
10.2%
14.3%
Annealed:
5.2%
10.2%
14.3%
a)
-1
-1
0.10
1045 cm
-1
1027 cm
0.08
-1
1080 cm
As-deposited:
5.2%
10.2%
14.3%
1080 cm
0.12
-1
1055 cm
Annealed
b)
0.18
0.16
0.14
Annealed
Annealed:
5.2%
10.2%
14.3%
0.12
0.10
0.08
0.06
0.06
As-deposited
0.04
0.02
Intensity (arb. units)
0.18
0.04
As-deposited
-1
880 cm
0.02
SRO (5 nm)
0.00
880
960
1040
1120
1200
-1
Wavenumber (cm )
1280
880
960
1040
1120
1200
0.00
1280
-1
Wavenumber (cm )
Fig. 3. FTIR spectra of a) SRO monolayers and b) SRO/SiO2 multilayers (SRO-5nm) before and after thermal annealing (1100 C).
Similar effect is observed in thermally annealed SRO/SiO2 MLs (SRO-5 nm), as observed in Fig. 3b. The shift in the SieOeSi
stretching band was also observed from 1055 to 1080 cm1 after the thermal annealing. This effect, again, indicates a phase
separation (Si-SiOx) and therefore the SieNCs formation within the SRO-bulk layers and possibly in the SRO-SiO2 interfaces.
Please cite this article in press as: A. Coyopol et al., Enhancement of the luminescence by the controlled growth of silicon
nanocrystals in SRO/SiO2 superlattices, Superlattices and Microstructures (2017), https://doi.org/10.1016/j.spmi.2017.10.009
A. Coyopol et al. / Superlattices and Microstructures xxx (2017) 1e7
5
By other hand, all SRO/SiO2 MLs exhibited narrower SieOeSi stretching bands respect to the SRO monolayers, as observed in
Fig. 3. This effect has been related with the increasing of oxygen atoms per interface silicon in the SRO matrix as well as to the
amount of stress exerted on the SieNCs by effect of SiOx matrix [22]. These structural parameters can be controlled varying
the Si-excess in the SRO films. In our case, the narrow stretching band observed in SRO/SiO2 MLs can be related to a higher
structural order in the SRO matrix due to a compressive stress in SieNCs. In ML structures, the compressive stress exerted on
the SieNCs is higher unlike monolayers, where the structural order of the SRO matrix is the lowest resulting in a broad IR
spectrum. Thus, it is likely that smaller nanocrystals (diameters <3 nm) must be confined in the SRO/SiO2 MLs due to the high
stress.
The different SRO/SiO2 interfaces in the multilayers were analyzed by XPS in-depth profile. Fig. 4a shows the composition
in-depth profile of the as-deposited MLs with 5 nm-thick SRO layer corresponding to 5.2 at.% and 10.2 at.% of Si-excess. As we
can see, different changes in atomic concentration (x) are observed corresponding to different SRO-SiO2 interfaces as the etch
time increases. A better contrast between SRO and SiO2 stoichiometry is observed when the Si-excess and SRO-thickness is
increased to 14.3 at.% and 7.5 nm, respectively (sample 3C, before and after annealing), as observed in Fig. 4b. A densification
in thickness is observed in this multilayer after thermal annealing and it is attributed to a microstructural reordering in SRO
layers. The difference in thickness observed in SRO/SiO2 MLs with 5.2 at.% and 10.2 at.% of Si-excess (Fig. 4a) can be attributed
to Si content in SRO layers. It is widely known that SRO films with high Si-excess (~10 at.%) show a higher densification after
the thermal annealing process.
The atomic concentrations (x) obtained in as-deposited MLs with SRO-5 nm (x ¼ 1.37, x ¼ 1.6) are near to that one of SRO
monolayers (x ¼ 1.3, x ¼ 1.5). A reduction of the atomic concentration (x) in the SRO/SiO2 layers near the Si-substrate
(x ¼ 1.3/x ¼ 1.16) of the multilayer with SRO-7.5 nm and 14.3 at.% is observed after thermal annealing, as shown in
Fig. 4b. This effect could be related with a diffusion of Si atoms between SRO-SiO2 interfaces as well as within SRO layers after
thermal treatment. Such thermal annealing promotes the crystallization of Si clusters and SieNCs formation [15,21].
SieNCs with sizes (and density) about 1.68 ± 0.2 nm (~5.2 1011 cm2) and 3.2 ± 0.4 nm (~1.38 1012 cm2) were
observed in SRO monolayers with Si-excess of 5.9 and 14.9 at.%, respectively [15]. Moreover, the HRTEM analysis showed a
broad size distribution of SieNCs, which depends on the Si-excess, crystallization temperature and phase separation in the SiSiOx interface [15].
In order to demonstrate evidence of SieNCs and determine the average crystal sizes, the SRO/SiO2 MLs were analyzed by
HRTEM. Fig. 5 shows the cross-view HRTEM images of the SRO/SiO2 ML with 5 nm SRO-thick for different silicon excess. As we
can see in Fig. 5a, the different SRO/SiO2 interfaces can be clearly observed in the ML with 5.2 at.% of Si-excess. Clear zones are
related to the SRO nano-layers. The total thickness is about 123 nm, which is very close to the optically measured (127 nm).
70
SRO/SiO2 - ML (SRO = 5 nm)
SiOx
a)
60
O
SRO
Atomic concentration (at.%)
50
x=1.6
x=1.37
Si
40
5.2 at%,
30
0
100
70
SiOx
60
200
10.2 at%
300
400
Etch time (s)
500
600
SRO/SiO2 - ML (SRO = 7.5 nm)
Si-excess: 14.3 at%
700
b)
O
SRO
50
x=1.3
x=1.16
7-13 at.%
Si
40
As-deposited,
30
0
100
200
Thermally annealed
300
400
500
600
700
Etch time (s)
Fig. 4. XPS in-depth profile of a) as-deposited multilayer (SRO-5 nm) with 5.2 at.%, 10.2 at.% of Si-excess and b) multilayer using SRO-7.5 nm with 14.3 at.%- before
and after thermal annealing.
Please cite this article in press as: A. Coyopol et al., Enhancement of the luminescence by the controlled growth of silicon
nanocrystals in SRO/SiO2 superlattices, Superlattices and Microstructures (2017), https://doi.org/10.1016/j.spmi.2017.10.009
6
A. Coyopol et al. / Superlattices and Microstructures xxx (2017) 1e7
Fig. 5. HRTEM images of thermally annealed multilayers (SRO-5nm) corresponding to a) 5.2 at.% b) 10.2 at.% c) 14.3 at.%.
The SieNC size is not fixed to the SRO thickness, but rather depends on the Si-excess within them, as observed in Fig. 5b
and c. Nevertheless, a better homogeneity in the average crystal size (<3 nm) was found in ML structures as compared to
monolayers. The SieNC size is about 1.7 ± 0.1, 2.1 ± 0.08 and 2.8 ± 0.09 nm with a density of 1.72 1011 cm2, 5.14 1011 cm2,
3.44 1012 cm2 for MLs with Si-excess about 5.2 at.%, 10.2 at.% and 14.3 at.%, respectively.
The effect of nanocrystal size reduction in SRO/SiO2 MLs with respect to SRO monolayers is shown in Fig. 6a. In both, SRO/
SiO2 MLs and SRO monolayers, an increase in the average nanocrystal size is observed as the Si-excess increases. However, in
the ML structures, the SieNCs tend to be confined due to the stress exerted by the SiOx matrix, so that the average nanocrystal
size decreases in the MLs compared to the monolayers with the same Si-excess.
The increase in the SieNC size according to the Si-excess is expected due to high Si content leads an enhanced in the
agglomeration of Si clusters after thermal annealing. Recent studies about SRO films confined by two SiO2 layers have shown
that Si atoms in excess from the SRO films mainly diffuse toward the center of the SRO layer, enhancing the Si atom aggregation and thus the SieNCs formation [10]. Nevertheless, as observed in the XPS in depth profile from this work, the Siexcess in the SRO nano-layers from SRO/SiO2 MLs reduces as compared to SRO monolayers. This effect is related with the Si
diffusion from the SRO films towards the SiO2 layer making them silicon rich instead of stoichiometric films. This Si-diffusion
produces three important effects: (i) the SieNCs formation near the edge of the SRO-SiO2 interfaces, as observed by HRTEM
micrographs (Fig. 5 bec), (ii) the reduction of the SieNC size as compared to SRO monolayers and (iii) and as a consequence,
the increased SieNCs density and improved PL intensity.
Regarding the presence of SieNCs and their PL properties, Wolking et al. suggested that light emission in red-orange
region originates from Si]O bonds, where oxidation can stabilize the energy position of the PL peak (~1.5e1.7 eV) from
small SieNCs (<3 nm) [23]. The model of Si]O bonds has been also used to explain the PL emission in connection to SieNCs
in a SiO2 matrix [24]. It was suggested later that the light-emitting centers can be stabilized not only at SieNCs surface but also
in a disordered network [25], which could be promoted from the phase separation of Si and SiO2. The emission of the Si]O
defect is around 1.5e2.1 eV and it strongly affect the PL intensity when the nanocrystal size is below 3 nm [23].
The phenomenon of light emission in SRO monolayers can be attributed to the presence and complete activation of Si]O
defects especially in SRO films with 5.9 at.% where the average SieNC size is about 1.68 ± 0.2 nm, as reported in a previous
work [15]. For SRO monolayers with a 14.36 at.%, a broad SieNC size distribution between 1 nm and 6 nm was observed by
3.4
3000
a)
3.2
Si-NC Size (nm)
PL Intensity (arb. units)
2500
3.0
Monolayers
ML: SRO-5nm
2.8
b)
2000
2.6
1500
2.4
2.2
1000
2.0
500
1.8
1.6
ML: SRO-5nm
ML: SRO-7.5nm
ML: SRO-2.5nm
Monolayer
4
6
8
10
12
Si-excess (at.%)
14
0
4
6
8
10
12
Si-excess (at.%)
14
Fig. 6. a) Si nanocrystal size and b) PL intensity in SRO monolayers and SRO/SiO2 MLs as a function of Si-excess.
Please cite this article in press as: A. Coyopol et al., Enhancement of the luminescence by the controlled growth of silicon
nanocrystals in SRO/SiO2 superlattices, Superlattices and Microstructures (2017), https://doi.org/10.1016/j.spmi.2017.10.009
A. Coyopol et al. / Superlattices and Microstructures xxx (2017) 1e7
7
HRTEM [15]. Therefore, we suggest that PL intensity decreases due to the increased SieNCs size, larger to the Bohr radius
conducting to possible annihilation of the radiative centers (Si]O).
In SRO/SiO2 MLs, the PL intensity tends to increase as SRO-thickness layer changes from 2 to 5 nm, however it tends to
decrease when SRO-thickness is 7.5 nm (see Fig. 2). SieNCs with diameters below 3 nm are necessary to obtain the activation
of the Si]O luminescence centers. Therefore, SRO/SiO2 MLs with SRO-5 nm where SieNC sizes of about 1.7 ± 0.1, 2.1 ± 0.08
and 2.8 ± 0.09 nm were obtained with Si-excess of 5.2 10.2 and 14.9 at.%, respectively, are suitable for the complete activation
of Si]O defects. The PL intensity for multilayers with SRO-thickness of 5 nm remains almost at the same order as the Siexcess increases, as shown in Fig. 6b. However, the PL intensity of SRO/SiO2 MLs with SRO-2.5 nm and SRO-7.5 nm and
SRO monolayers decreases as the Si-excess increases.
It was observed that the SieNCs density in SRO/SiO2 MLs is higher than SRO monolayers produced by a better control of the
NCs formation in SRO nanolayers confined between SiO2 layers. Moreover, SieNCs with sizes below 3 nm and a large number
of Si]O defects are created in SRO/SiO2 MLs with SRO-thickness of 5 nm. These effects explain the enhanced PL intensity of
multilayers making them suitable for the development of silicon-based light sources.
4. Conclusions
The PL properties of SRO monolayers as well as SRO/SiO2 ML structures obtained by sputtering were studied. An
improvement of the PL intensity is observed in the ML structures when the SRO-thickness is 5 nm, and for any Si-excess as
compared to monolayers. SRO/SiO2 MLs allow up to 14-fold PL enhancement as compared to SRO monolayers. A silicon
diffusion from SRO nano-layers towards the SiO2 ones reduces the Si content within the SRO allowing the SieNC size
reduction (thus increasing the SieNC density) as compared to SRO monolayers. SRO/SiO2 MLs with SRO thickness of 5 nm
present a higher density of SieNCs smaller than 3 nm as compared to the monolayers. Therefore, the intense emission
observed in SRO/SiO2 MLs with high Si-excess can be attributed to an increase of SieNCs density (<3 nm) as well as formation
and activation of Si]O defects. These results are of great importance for the development of electroluminescent devices.
Acknowledgements
Authors want to thank Oscar Solís, Cesar Leyva and Luis Gerardo Silva from CIMAV for the FIB preparation, TEM and XPS
nchez
measurements, respectively. F MoraleseMorales want to thank to CONACYT for the postdoctoral grant. A MoraleseSa
acknowledges the support received from CONACYTeCB #180992 and CONACYT-Scientific Infrastructure #269359.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
G.R. Lin, C.J. Lin, H.C. Kuo, Appl. Phys. Lett. 91 (2007), 093122.
G.R. Lin, Y.H. Pai, C.T. Lin, C.C. Chen, Appl. Phys. Lett. 96 (2010), 263514.
C.H. Cheng, Y.C. Lien, C.L. Wu, G.R. Lin, Opt. Express 21 (1) (2013) 391e403.
N. Koshida, Device Applications of Silicon Nanocrystals and Nanostructures, Springer, New York, 2009.
A. Anopchenko, A. Marconi, E. Moser, S. Prezioso, M. Wang, L. Pavesi, G. Pucker, P. Bellutti, J. Appl. Phys. 106 (2009), 033104.
€nen, S. Novikov, L. Khriachtchev, J. Appl. Phys. 112 (2012), 094316.
T. Nikitin, R. Velagapudi, J. Sainio, J. Lahtinen, M. R€
asa
L. Jin, D. Li, D. Yang, D. Que, Appl. Phys. A 113 (2013) 121.
P.G. Han, Z.Y. Ma, Z.B. Wang, X. Zhang, Nanotechnology 19 (2008), 325708.
K.J. Kim, D.W. Moon, S.-H. Hong, S.-H. Choi, M.-S. Yang, J.-H. Jhe, J.H. Shin, Thin solid films 478 (2005) 21e24.
J.H. Yoon, Appl. Surf. Sci. 344 (2015) 213e216.
Y.H. Lin, C.L. Wu, Y.H. Pai, G.R. Lin, Opt. Express 19 (7) (2011) 6563e6570.
C.L. Wu, Y.H. Lin, G.R. Lin, E.E.E.J. Sel, Top. Quantum Electron. 18 (6) (2012) 1643e1649.
n-Salazar, I.E. Zaldívar-Huerta, M. Aceves-Mijares, J. Appl. Phys. 119 (2016), 215101.
J. Alarco
€ sele, Solid State Phenom. 94 (2003) 95e104.
M. Zacharias, L.X. Yi, J. Heitmann, R. Scholz, M. Reiche, U. Go
nez, A. Morales-Sa
nchez, J. Lumin. 176 (2016) 40e46.
A. Coyopol, M.A. Cardona, T. Diaz-Becerril, L. Licea-Jime
K.J. Kim, D.W. Moona, S.-H. Hong, S.-H. Choi, M.-S. Yang, J.-H. Jhe, J.H. Shin, Thin Solid Films 478 (2005) 21.
G.R. Lin, C.J. Lin, C.K. Lin, L.J. Chou, Y.L. Chueh, J. Appl. Phys. 97 (2005), 094306.
€s€
T. Nikitin, R. Velagapudi, J. Sainio, J. Lahtinen, M. Ra
anen, S. Novikov, L. Khriachtche, J. Appl. Phys. 112 (2012), 094316.
L. Khriachtchev, S. Ossicini, F. Iacona, F. Gourbilleau, Int. J. Photoenergy (2012) 21.
L.X. Yi, J. Heitmann, R. Scholz, M. Zacharias, Appl. Phys. Lett. 81 (2002) 428.
pez, M. Pacio, J.A. Luna-Lo
pez, J. Carrillo-Lo
pez, J. Nanomater (2012) 7.
A. Coyopol, G. García-Salgado, T. Díaz-Becerril, H. Ju
arez, E. Rosendo, R. Lo
G. Zatryb, A. Podhorodecki, J. Misiewicz, J. Cardin, F. Gourbilleau, Nanoscale Res. Lett. 8 (2013) 1e7.
M.V. Wolkin, J. Jorne, P.M. Fauchet, G. Allan, C. Delerue, Phys. Rev. Lett. 82 (1999) 197e200.
L. Khriachtchev, S. Novikov, J. Lahtinen, J. Appl. Phys. 92 (2002) 5856e5862.
L. Khriachtchev, M. Rasanen, S. Novikov, L. Pavesi, Appl. Phys. Lett. 85 (2004) 1511e1513.
Please cite this article in press as: A. Coyopol et al., Enhancement of the luminescence by the controlled growth of silicon
nanocrystals in SRO/SiO2 superlattices, Superlattices and Microstructures (2017), https://doi.org/10.1016/j.spmi.2017.10.009
Документ
Категория
Без категории
Просмотров
1
Размер файла
748 Кб
Теги
spmi, 2017, 009
1/--страниц
Пожаловаться на содержимое документа