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Formation of high conductive nano-crystalline silicon embedded in amorphous
silicon-carbide films with large optical band gap
Yang Ji, Dan Shan, Mingqing Qian, Jun Xu, Wei Li, and Kunji Chen
Citation: AIP Advances 6, 105107 (2016);
View online: https://doi.org/10.1063/1.4965922
View Table of Contents: http://aip.scitation.org/toc/adv/6/10
Published by the American Institute of Physics
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AIP ADVANCES 6, 105107 (2016)
Formation of high conductive nano-crystalline silicon
embedded in amorphous silicon-carbide films
with large optical band gap
Yang Ji, Dan Shan, Mingqing Qian, Jun Xu,a Wei Li, and Kunji Chen
National Laboratory of Solid State Microstructures and School of Electronic Science
and Engineering and Collaborative Innovation Center of Advanced Microstructures,
Nanjing University, Nanjing 210093, China
(Received 21 August 2016; accepted 8 October 2016; published online 17 October 2016)
High conductive phosphorus-doped nano-crystalline Si embedded in SiliconCarbide (SiC) host matrix (nc-Si:SiC) films were obtained by thermally annealing doped amorphous Si-rich SiC materials. It was found that the room
conductivity is increased significantly accompanying with the increase of doping concentrations as well as the enhanced crystallizations. The conductivity can
be as high as 630 S/cm for samples with the optical band gap around 2.7 eV,
while the carrier mobility is about 17.9 cm2 / V·s. Temperature-dependent conductivity and mobility measurements were performed which suggested that the carrier transport process is strongly affected by both the grain boundaries and the
doping concentrations. © 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license
(http://creativecommons.org/licenses/by/4.0/). [http://dx.doi.org/10.1063/1.4965922]
I. INTRODUCTION
Recently, un-doped and doped nano-crystalline Si (nc-Si) embedded in amorphous SiC host
matrix (nc-Si:SiC) films have attracted much attention since they can be applied for many kinds of
optoelectronic devices, such as the next generation of solar cells and light emitting devices.1–6 It was
reported that the band offset both in conduction band and valence band for nc-Si:SiC system is the
lowest compared with other conventionally used dielectric materials (such as SiO2 and SiNx ), which is
helpful for the electron and hole transport and in turn to improve the device performance. For example,
it was reported that, by using p-type nc-Si:SiC films to get hetero-junction solar cells with n-type
crystalline-Si (c-Si), the spectral response in the visible light range can be enhanced compared with
that of conventional c-Si p-n junction solar cells.7 Moreover, an intense visible light emission from
nc-Si:SiC films deposited using advanced electron-cyclotron-resonance chemical vapor deposition
(ECRCVD) technique was also observed at room temperature under laser excitation.8 X. Xu et al.
used laser crystallization technique to get the p-i-n structures containing nc-Si:SiC films and they
observed the improved electroluminescence due to the enhanced radiative recombination probability.9
Usually, nc-Si:SiC films can be obtained by growing Si-rich SiC films or amorphous Si/SiC
multilayers with subsequently thermal annealing at high temperature.5–12 By controlling the Si/C
ratio or the post-annealing conditions, the optical band gap can be tunable which brings convenience
to the design of optoelectronic devices. However, the SiC matrix with a large band gap usually has
a poor conductive property and in turn impedes the carrier transport process which will deteriorate
the device performance.13,14 Lechner et al. fabricated the doped nc-Si films and they obtained the
maximum conductivity up to about 5 S/cm with the very small conductivity activation energy.15
From the view point of device application, it is highly desired to get nc-Si:SiC films with high
conductivity at the suitable optical band gap (>2.5 eV) so that they can act as the window layer
aThe corresponding Author: Jun Xu Email: [email protected] Tel:+8613585174001 Fax: 86-25-83595535
2158-3226/2016/6(10)/105107/9
6, 105107-1
© Author(s) 2016
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or the conductive layer in nano-crystalline SiC film-based optoelectronic devices without absorbing
incident (or emitted) light too much.13,16
In our previous work, hydrogenated amorphous silicon carbide (a-SiC:H) films were prepared
in a plasma-enhanced chemical vapor deposition (PECVD) system. Nc-Si:SiC films were fabricated
after annealing at 900 ◦ C and 1000 ◦ C. It was found that for samples annealed at 1000 ◦ C, the average
size of nc-Si is 9 nm and the crystallinity is reached to 70 %. The optical band gap is enlarged from
1.9 eV to 2.2 eV with annealing temperature increasing, which can be attributed to the increase
of Si-C bonds. The conductivity reaches to 1.2×10-6 S/cm, which is 4 orders of magnitude higher
than that of as-deposited sample.14 However, the conductivity is still too low to apply in the related
devices such as thin film solar cells or light emitting devices. In the present work, nc-Si:SiC with
various phosphorus (P) doping concentrations are prepared by annealing P-doped amorphous Si-rich
SiC films. The micro-structures, optical and electronic properties are studied as a function of doping
concentrations. It is found that with increasing the P doping concentrations, the conductivity of
formed nc-Si:SiC is gradually increased and a very high conductive nc-Si:SiC film with conductivity
of 630 S/cm can be obtained. Furthermore, the temperature-dependent behaviors of conductivity and
mobility are studied, which reveal that the Fermi level indeed shifts to the conduction band with
doping concentration increasing and the transport mechanism changes with increasing the P doping
concentrations.
II. EXPERIMENT
Hydrogenated amorphous SiC films with various P doping concentrations were prepared on
quartz glass plates and p-Si substrates in a PECVD system using gas mixtures of pure SiH4 , CH4 ,
PH3 and H2 . The flow rate of SiH4 and CH4 was kept at 5 sccm and 1.5 sccm respectively. The flow
rate of PH3 (1 % diluted in H2 ) was changed from 0 sccm to 5 sccm for various doping concentrations.
Here, we define the doping gas ratio R as the gas ratio of [PH3 ] to [SiH4 ] which is changed from
0 to 1. During the deposition process, the radio-frequency power, chamber pressure and substrate
temperature were kept at 30 W, 10 mTorr and 250 ◦ C respectively. The deposition time was 40 min.
Fig. 1(a) and (b) are the XPS spectra around 283 eV and 100 eV of as-deposited samples without
doping, which is related to the C 1s and Si 2p signals, respectively. By integrating the C 1s peak
(∼283 eV) and the Si 2p peak (∼100 eV), the composition ratio of Si/C is about 20, which implies
that the as-deposited sample is Si-rich amorphous SiC film. It is also found that the Si 2p peak can be
decomposed into two sub-bands, which is assigned to Si-Si state (EBSi2p =99.6±0.1 eV) and Si-C state
(EBSi2p =100.4±0.1 eV), respectively.15 It is suggested from XPS spectra that the most of Si atoms
are bonded to neighboring Si and only small portion of Si are bonded to C atoms. It is noted that the
Si/C ratio is almost the same for un-doped and P-doped samples even after thermal annealing. XPS
spectra also reveal that the oxygen atom content is 2 %∼3 % in all the doped samples (not shown in
Fig. 1), which may be attributed to oxidation by air during both deposition and annealing process.
FIG. 1. (a) Si 2p and (b) C 1s spectra of as-deposited un-doped samples.
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The as-deposited samples were dehydrogenation at 450 ◦ C for 1 h in N2 ambient and annealing at
1000 ◦ C for 1 h in N2 atmosphere in sequence to form nc-Si:SiC films. It is found that the hydrogen is
completely effused out of the films after above thermal treatments. The changes of film microstructures
before and after annealing were characterized by Raman spectra using a Jobin Yvon Horiba HR800
spectrometer while the optical absorption was measured by a Shimadzu UV-3600 spectrophotometer
for samples deposited on quartz plates. To measure electronic properties (mobility, doping type, sheet
carrier concentration and sheet resistance) at room temperature and at higher temperature (from 300 K
to 410 K), a LakeShore 8400 series Hall effect measurement system was utilized by using Van der
Pauw configurations for samples on quartz substrates. The films deposited on p-Si substrates were
used to prepare samples for cross-section transmission electron microscopy (TEM) observations.
III. RESULTS AND DISCUSSION
Fig. 2(a) is the P 2p XPS spectra for 1000 ◦ C annealed SiC films with doping gas ratio R=0,
0.6 and 1, respectively. There are no detectable signals related to P atoms for un-doped samples as
shown in Fig. 2(a). However, a weak peak centered at 129.6 eV, which is attributed to the formation
P-Si bonds,17 can be observed for P-doped samples with R=0.6 and the peak becomes stronger for
samples with R=1. It is estimated that the P doping concentration is about 2.1 % and 2.3 % for
samples with R= 0.6 and 1, respectively. We also measured the depth-profile XPS spectra as shown
in Fig. 2(b), in which, each spectra was detected after Ar+ ion etching for various time (the etching
speed was 0.1∼0.2 nm/s), which corresponds to the maximum detected position of about 120 nm
from the surface. It is found that the P-related signals are not changed obviously with the depth,
indicating atomic distribution is quite uniform in our samples.
Fig. 3 is the Raman scattering spectra of as-deposited and 1000 ◦ C annealed samples with various
P doping concentrations. A broad peak at 473 cm-1 can be found in as-deposited films, which is related
to the transverse optical (TO) phonon mode of amorphous Si-Si phases. After annealing at 1000 ◦ C,
a narrow peak appears at 516 cm-1 , indicating the transformation from a-Si to nc-Si.18 It is found that
for annealed samples, Raman peak is almost unchanged with increasing the P doping concentrations,
while the Raman intensity is gradually enhanced which indicates the enhanced crystallization. In order
to estimate crystallinity of annealed films, three components are decomposed in Raman spectra (see
the inset in Fig. 3), which is a crystalline component peaked at 516 cm-1 , an amorphous component
peaked at 473 cm-1 , and an intermediate component peaked at 504 cm-1 corresponding to Si-Si bonds
at grain boundaries.19–22 The crystallinity (X c ) can be estimated based on the following formula:23
Xc = Ic /(Ic + 0.88Ia ),
(1)
where, I a and I c are integrated intensities of the amorphous component peaked at 473 cm-1 and the
crystalline component peaked at 504 cm-1 and 516 cm-1 respectively. The calculated results X c for the
FIG. 2. P 2p spectra of annealed samples (a) with different R values and (b)with different depths.
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FIG. 3. Raman spectra of samples with various P doping concentrations. The inset presents the deconvolution of the Raman
spectrum of annealed un-doped sample.
intrinsic samples is about 76 % after 1000 ◦ C annealing. With increasing the P doping concentrations,
X c goes up and can be reached as high as 91 % for samples with doping ratio R=1. It seems that
introduction P impurities are helpful for promoting the crystallization. The effect of P doping on the
structures of nc-Si films was also discussed previously.24 It was reported that with increasing the
P doping concentrations, the grain size of formed nc-Si is increased and the nc-Si film becomes more
ordered which may be related to the reduced strain in the films. However, the further work is need to
exactly understand the enhanced crystallinity due to the P doping.
The microstructures after 1000 ◦ C annealing are further studied by cross-sectional TEM observations. Fig. 4(a) shows the cross-section TEM image of 1000 ◦ C annealed sample. It is seen that the
film thickness is about 170 nm. Fig. 3(b) and (c) are the high resolution cross-section TEM images for
annealed samples with doping gas ratio R=0.6 and 1, respectively. The formation of Si nano-crystals
can be clearly identified in the figures and the lattice constant is 0.31 nm which corresponds to the Si
(111) orientation. The average size of nano-dots is 12 nm for sample with doping ratio R=0.6, which
is increased to 15 nm for sample with R=1. The TEM observations also suggest the crystallization is
enhanced with increasing the P doping concentrations. This is consistent with the Raman scattering
results.
The optical band gaps E opt of annealed samples with various P doping ratio are deduced from
Tauc formula:25
(αhv)1/2 = B(hv − Eopt ),
(2)
where, α is the absorption coefficient, hv is photon energy and B is a constant. As shown in Fig. 5(a),
the optical band gap is gradually increased from 2.2 eV to 2.8 eV for 1000 ◦ C annealed samples
with increasing the doping ratio R from 0 to 1. It is noticed that the optical band gap is enlarged after
annealing compared with that of as-deposited films, which can be clearly seen in the inset of Fig. 5(b)
that the sample becomes more transparent after annealing. It has been reported that optical band gap
is enlarged after annealing in B-doped nc-SiOx Ny films,13 which was explained as the contribution
both from the quantum confinement effects and the increased amount of SiOx composition due
to formation of Si nano-crystals. In our previous work, we also found that the existence of grain
boundary after annealing can influence the optical band gap obviously.26 Therefore, the increasing
optical band gap with doping ratio R can be ascribed to the increased influences of grain boundaries
due to enhanced crystallization as well as the increased amount of Si-C bonds due to the formation
of nc-Si phases though we cannot completely rule out of the effect of oxygen contamination during
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FIG. 4. Cross-section TEM image of 1000 ◦ C annealed sample with doping ratio R=0.6 (a) and high resolution TEM images
of 1000 ◦ C annealed samples with doping ratio R=0.6 (b) and R=1 (c), respectively.
the annealing process. However, it is argued that the oxygen content measured by XPS for all the
samples is 2 %∼3 % both for as-deposited and annealed samples as we mentioned before.
Fig. 5(b) shows the transmittance spectra of samples with R=0.22 before and after annealing.
Though existence of the strong interference effect due to the film thickness (∼170 nm), it can be seen
that the transmittance is improved in the visible light range, especially for the spectral region with
the wavelength less than 550 nm after annealing. It is also observed that the transmittance can be
FIG. 5. (a) Optical band gap of 1000 ◦ C annealed samples as a function of doping ratio R. (b) Transmittance spectra of
samples (R=0.22) before and after annealing with a characteristic thickness of ∼170 nm. Insets are the pictures of samples
(R=0.22) before and after annealing.
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further improved in the samples with the larger R values (not shown here) due to their higher band
gap as shown in Fig. 5(a). It is worth noting that the transmittance spectra shown in Fig. 5(b) are from
the samples with thickness of ∼170 nm, which is far more beyond the demand thickness (∼10 nm)
in actual thin film solar cells as a window layer.5 Based on the measured absorption coefficient, it
is estimated that the transmittance will exceed 80% in the whole visible spectral range for annealed
samples with a thickness of 10 nm.
Fig. 6 shows the room temperature conductivity σ and mobility µ as a function of P doping ratio
R for annealed samples. It is found that both the conductivity and mobility are gradually increased
with increasing the doping ratio R from 0 to 0.6. The conductivity can be as high as 630 S/cm with
the mobility of 17.9 cm2 /V·s, which is significantly higher than that of doped SiC and SiC powder
compact reported previously.14,27 Further increasing doping ratio R results in the slight reduction
of mobility but the conductivity still keeps its high level. The increasing of mobility with the doing
concentrations can be attributed to the improved crystallinity due to the enhanced crystallization
as discussed before which reduces scattering with the disordered amorphous structures. However,
further increasing P doping concentrations will cause the strong free-electron scattering and/or grain
boundary scattering which results in the slight decrease of mobility.
In order to further understand the carrier transport process of annealed samples, the temperaturedependent conductivity and mobility behaviors are studied. As shown in Fig. 7, when doping ratio
R is lower than 0.6, the relationship between dark conductivity σ and the temperature T can be well
described as the formula, which usually used to describe the transport process in micro-crystalline
and nano-crystalline Si-based materials:13,28
σ = σ0 exp(-Ea /kB T ),
(3)
where, σ0 , E a and k B are constant parameter, conductivity activation energy and Boltzmann constant,
respectively. The activation energy E a can be deduced through Arrhenius plots as given in Fig. 7.28,29
It is found that E a is obviously decreased with increasing the P doping concentrations, from 400
meV of un-doped sample to 12 meV of sample with R=0.22, which means the Fermi level is very
closed to the bottom of conduction band due to the P doping so that the conduction electron density
can be increased significantly (n ∝ exp(-E a /k B T )). For sample with R>0.3, dark conductivity is
nearly independent of temperature between 300 K and 410 K, which means that the Fermi level
is located within the conduction band.30 With further increasing the doping gas ratio R, the more
activated P atoms leads to the further increase of carrier concentrations to increase the conductivity.
Recently, we experimentally demonstrated that the phosphorus dopants can occupy the inner sites of Si
nc-Si substitutionally, which generate the impurity-related level in the gap to provide the conduction
electrons.31 In fact, with increasing the P doping concentrations, the measured carrier concentration
FIG. 6. Room temperature conductivity and mobility as a function of P doping ratio of 1000 ◦ C annealed samples.
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FIG. 7. Temperature-dependent dark conductivity of 1000 ◦ C annealed samples with various P doping ratio.
increases from 1.3×1014 cm-3 to 2.6×1020 cm-3 . On the other hand, thermal treatment can promote
crystallization in our samples which leads to the increase of carrier mobility (i.e. form 2.3 cm2 /V·s
for un-doped one to 17.9 cm2 /V·s for P-doped one with R=0.6) as we mentioned before. Since both
of the mobility and carrier concentration are increased, the conductivity is consequently increased
obviously as given in Fig. 6.
Fig. 8 shows the temperature-dependent mobility for samples with various P doping concentrations. It is found that the mobility µ increases with the temperature ranged from 300 K to 410 K for
intrinsic sample. At the present stage, the increase in mobility with temperature is not fully understood.
The similar behaviors were previously observed in microcrystalline Si films. It was explained in term
of the effect of grain boundaries since they can trap the carriers in the ionization defects states to form
the potential barrier. At high temperature, the carriers gain much more energy to pass through the
potential barrier which results in the increased mobility.26 Another possibility is the scattering process
FIG. 8. Temperature-dependent mobility of 1000 ◦ C annealed samples with various P doping ratio.
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via the ionized impurity centers which also induce the increased mobility with temperature. However, for samples with high P doping concentrations, they exhibit different temperature-dependent
behaviors. The mobility is slightly decreased with the measured temperature. It is suggested that the
impurity ionization carrier concentration is so high for samples with high doping concentrations that
carriers can saturate the trapping states and reduce the potential barrier height.26,32 Consequently,
grain boundary barriers are not serious obstacles for the carrier transport for heavily doped samples.
The carrier transport process is then dominantly controlled by phonon scattering mechanism and the
mobility decreases with increasing the temperature. As discussed before, for samples with P doping
ratio R higher than 0.22, they are degenerate semiconductors with carrier concentration of 1020 cm-3 ,
it is reasonable that the carrier transport characteristics in those samples are similar with metal-like
materials.33,34
IV. CONCLUSIONS
In summary, P-doped nc-Si:SiC films with high conductivity can be achieved by annealing
P-doped amorphous Si-rich SiC layer at 1000 ◦ C. It is found that the crystallinity of annealed samples
can be enhanced with increasing the P doping concentrations which is responsible for the improved
electronic properties. The obtained film exhibits the best conductivity of 630 S/cm with the optical
band gap of 2.7 eV while the measured Hall mobility is about 17.9 cm2 /V·s. Moreover, both the
temperature-dependent conductivity and mobility were studied. It is shown that with increasing the
doping ratio, the activation energy is gradually decreased which indicates the up-shift of Femi level
to the conduction band. For samples with low doping ratio, the mobility is increased with temperature
implying the influences of grain boundaries and ionized impurities. However, for samples with high
doping ratio, they show the metal-like behaviors that the mobility is decreased with temperature,
which suggests that the photon scattering mechanism dominates the carrier process in the heavilydoped films. Our results demonstrate that the present high conductive nanocrystalline films with large
optical band gap can act as the good window materials potentially used in Si-based optoelectronic
devices.
ACKNOWLEDGMENTS
This work was supported by 973 Project (2013CB632101), NSFC (No. 11274155), 333 Project of
Jiangsu Province (BRA2015284), PAPD and the Research Innovation Program for College Graduates
of Jiangsu Province (SJLX15 0019).
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