Journal of Non-Crystalline Solids 598 (2022) 121925
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Journal of Non-Crystalline Solids
journal homepage: www.elsevier.com/locate/jnoncrysol
Electronic structure of silicon oxynitride films grown by plasma-enhanced
chemical vapor deposition for memristor application
T.V. Perevalov a, *, V.A. Volodin a, b, G.N. Kamaev a, A.A. Gismatulin a, S.G. Cherkova a,
I.P. Prosvirin c, K.N. Astankova a, V.A. Gritsenko a, d
a
Rzhanov Institute of Semiconductor Physics SB RAS, 13 Lavrentiev aven., Novosibirsk 630090, Russia
Novosibirsk State University, 2 Pirogov str., Novosibirsk 630090, Russia
Boreskov Institute of Catalysis SB RAS 5 Lavrentiev aven., Novosibirsk 630090, Russia
d
Novosibirsk State Technical University, 20K. Marx aven., Novosibirsk 630090, Russia
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Silicon oxynitride
Chemical vapor deposition
Electronic structure
XPS
FTIR
Memristors
The electronic structure and optical properties of SiOxNy:H films enriched with silicon obtained by plasmaenhanced chemical deposition are studied. It is shown, that with the plasma generator power growth, the
content of silicon (amorphous silicon clusters) and oxygen decreases, whereas the nitrogen content increases.
Thus, the SiOxNy:H film composition can be effectively varied both by changing the gas flow ratio in the growth
chamber and by changing the plasma generator power. The electronic stricture of SiOxNy of various x and y
values is calculated from the first principles for the model structures, and the energy diagram, as well as the
bandgap dependence on the oxygen content, is obtained. It is found that p+-Si/SiOxNy:H/Ni structures, have the
properties of memristor bipolar type: they are reversibly switched between high and low resistance states. These
memristors are forming-free: the initial state has a close resistance to the low resistance state.
1. Introduction
Silicon oxide and silicon nitride enriched with silicon (SiOx<2 and
SiNx<4/3) are dielectrics compatible with the standard technological
processes of modern microelectronics and perspective for using them as
an active layer in a resistive random access memory (RRAM) [1–5].
RRAM elements or memristors are characterized by the reversible
resistive switching of the oxide layer in a metal-dielectric-metal struc­
ture between high resistance states (HRS) and low resistance states
(LRS). According to the generally accepted concept, the resistive
switching occurs during the current pulse action and the electro­
migration of oxygen vacancies in SiOx or nitrogen vacancies in SiNx,
with the conductive filament formation: Si rich nanowires with a
diameter of 1–5 nm [2,6]. It is not surprising that silicon oxynitride films
(SiOxNy) also could be used as a memristor active layer [7–12]. Despite
the prospects of using this dielectric for memristors, in particular, the
possibility of combining the advantages of SiOx and SiNx-based mem­
ristors, it has been studied significantly less than SiOx and SiNx.
Silicon oxynitride films can be obtained by various methods. SiOxNy
films obtained by magnetron deposition do not contain hydrogen, and it
is possible to vary the oxygen content in the films by changing the ox­
ygen flow in the reactor [8]. SiOxNy films obtained by the treatment in
the oxygen plasma of stoichiometric silicon nitride, deposited by
low-pressure chemical vapor deposition at 810 ◦ C, were studied in [9]. A
two-layer SiOxNy/Si3N4 structure was obtained, and the memristors
were fabricated. In these memristors, the resistive switching with a large
memory window (resistance ratio in ON- and OFF-states) was observed,
but the forming process is required for them. A two-layer structure based
on silicon oxynitrides was also used to fabricate a memristor and a
selector in one cell in the studies [10]. In [11] it was shown that Ag:
SiOxNy-based memristors are forming-free and belong to the type
conductive bridge random access memory with programmable metalli­
zation cells.
The memristors based on silicon oxynitride films grown by the
plasma-enhanced chemical vapor deposition (PECVD) method have
been obtained recently in [12]. It was shown that they are of stable
intermediate resistance states, in addition to HRS and LRS, and the
possibility of using SiOxNy-based memristor for neural networks was
demonstrated. The advantage of the PECVD method for the SiOxNy film
synthesis is low deposition temperature. The use of deposition processes
* Corresponding author.
E-mail address: timson@isp.nsc.ru (T.V. Perevalov).
https://doi.org/10.1016/j.jnoncrysol.2022.121925
Received 15 July 2022; Received in revised form 14 September 2022; Accepted 15 September 2022
Available online 13 October 2022
0022-3093/© 2022 Elsevier B.V. All rights reserved.
T.V. Perevalov et al.
Journal of Non-Crystalline Solids 598 (2022) 121925
with a low thermal budget is important, since it allows obtaining
memristor structures at the end of all technological processes (the
so-called back-end-of-line processes) [13]. However, PECVD silicon
oxynitride contains a significant amount of hydrogen (Si–H bonds) and
is usually indicated as SiOxNy:H. The hydrogen content in such films is
uncontrolled, and it can significantly affect the electrophysical charac­
teristics of the films. Usually, when synthesizing silicon oxynitride by
the PECVD method, ammonia (NH3) and monosilane (SiH4) are used,
since it is an easily controlled process due to the proximity dissociation
energies of NH3 (3.6 eV) and SiH4 (3.1 eV). It is known that using ni­
trogen (N2), that has a dissociation energy of about 9.9 eV, instead of
NH3 makes it possible to reduce the hydrogen content in the films [14].
One of the key problems in the RRAM development today is the high
voltage of the memristor structure first switching from the initial resis­
tance states to the LRS (problem of forming). A possible solution of this
problem for memristors based on SiOx, SiNx and, most likely, SiOxNy:H is
using an active layer highly enriched with silicon. SiOxNy:H films
enriched with silicon can be synthesized by the PECVD method by
reducing the oxygen and nitrogen content in the plasma-forming gas
mixture.
Thus, the aim of this work is to study the electronic structure and
optical properties of SiOxNy:H films enriched with silicon of variable
compositions deposited by the PECVD method from a mixture of SiH4,
N2 and O2 and to ascertain whether the films obtained are suitable for
the use as the active medium of a forming-free RRAM cell.
spectroscopy and Fourier transformed infrared (FTIR) spectroscopy. The
Raman spectra were recorded using a T64000 spectrometer (Horiba
Jobin Yvon) at room temperature in the backscattering geometry; for
excitation, a solid-state laser with the wavelength λ = 514.5 nm was
used. The laser beam power reaching the sample was 1 mW, and the spot
diameter was 20 μm. The light with the 514.5 nm wavelength absorption
in the studied films is small, and used measurement mode did not lead to
the local heating of the samples during the measurement. Thus, all
Raman spectra for 200 nm thick films were measured at room temper­
ature. Thin films that have been grown for the manufacture of mem­
ristors have not been studied using Raman spectroscopy since the Raman
signal from them is very small. The spectral resolution was better than 2
cm− 1. The FT-801 Fourier spectrometer (SIMEKS, Russia) was used to
record FTIR absorption spectra. The spectral range was 650–4000 cm− 1,
and the spectral resolution was 4 cm− 1. A silicon substrate without a film
was used for a reference signal.
The optical properties of the SiOxNy:H films in the near-IR, visible
and UV ranges were studied using transmission and reflectance spec­
troscopy. To study the transmission spectra, the same films were grown
on transparent silica substrates. The SF-56 spectrophotometer (LOMOSpektr, Russia) with a special setup for the reflection study was used.
The spectral resolution was 2 nm, and the measurement range was from
1100 to 190 nm. To obtain the optical absorption dispersion α, the
developed computer program was used, which considers the interfer­
ence in the thin SiOxNy:H film.
The electronic structure of SiOxNy was calculated within the density
functional theory (DFT) in a periodic supercell model in the Quantum
ESPRESSO package [16]. Since, according to the XPS data, the studied
films are enriched with silicon, model structures with x = 0.33, 0.5 and
0.33 < y < 1.33 were created for SiOxNy electronic structures simulation
according to the following principles. Firstly, SiN0.33O1.5 and SiN0.5O1.25
structures were created as it was proposed in [17]: by replacing a pair
(four) of oxygen atoms with nitrogen and removing one (two) oxygen
atoms in the 18- (24-) atomic cell α-SiO2 (β-SiO2). The choice of the
nitrogen atom and the vacancy position were determined by the search
for the configuration with the lowest total energy of the structure.
Secondly, in the structures found, pairs of oxygen atoms were sequen­
tially removed from all possible positions and a structure with a minimal
energy was also selected. Thus, SiN0.33Oy structures with y = 1.33, 1.0,
0.67 and 0.33, as well as SiN0.5Oy structures with y = 1.25, 1.0, 0.75 and
0.5, were created. Structural relaxation calculations were carried out
with a local exchange-correlation functional PBEsol, and the total den­
sity of electronic states (TDOS) spectra were calculated with a hybrid
functional PBE0 with the Hartree-Fock exchange fraction of 0.16.
Optimized norm-conserving Vanderbilt pseudopotentials and the
plane-wave basis cutoff energy of 1088 eV were used. Searching the
structure was carried out with a 2 × 2 × 2 K-point grid, and TDOS
calculations were performed with a 12 × 12 × 12 grid. The method used
gives a close to the experimental bandgap value Eg = 8.0 eV for SiO2
[18].
To measure the current-voltage (I–V) characteristics, the metalinsulator-semiconductor (MIS) structures with 33 nm silicon oxy­
nitride were fabricated. The 200 nm thick Ni top electrodes with an area
of ~0.5 mm2 were deposited by magnetron sputtering in the Ar atmo­
sphere using a special mask. To improve the back contact, the Ni layer of
the same thickness was deposited on the back side of p+-Si. The I–V
characteristics of the MIS structures were measured using an Agilent
B2902A two-channel source-meter using connected micro positioners as
probe electrodes. The I–V measurements at different temperatures were
carried out in the LTS420E Linkam cell by using the Keythley 2400 and
temperature controller Linkam T95.
2. Materials and methods
SiOxNy:H films enriched with silicon were synthesized by PECVD
from a gas mixture of SiH4, N2 and O2 on the p-type Si (100) wafers
cleaned from natural oxide. The PECVD setup with a wide-aperture
inductive plasma source with frequency 13.56 MHz and a turbomo­
lecular pump was used. The residual pressure in the reactor was less than
10− 6 Torr. The substrate temperature was kept at 200 ◦ C. The SiH4 flow
supplied to the reaction zone (gas mixture of 10% SiH4 diluted with Ar)
was constant and kept to 10 sccm. The films were synthesized at two
nitrogen flow rate values 5 and 6 sccm. This nitrogen flow values range
is optimal as it was found earlier when studying the PECVD SiNx films,
since, at a lower flow, the films are not suitable for memristors, and, at a
higher one, the nitrogen content in plasma increases very slowly. Ni­
trogen gas was diluted with oxygen, so that the oxygen concentration in
the mixture is less than 1%. The low oxygen content in the gas mixture is
needed to obtain the films highly enriched with silicon since oxygen has
a very high chemical activity with silicon and, besides, the high oxida­
tion rate is expected due to the presence of Ar, as it was observed earlier
for the silicon oxidation [15]. The three films synthesized at the 5 sccm
N2 flow with plasma generator power 50, 100 and 150 W (further
indicated as 5–50, 5–100 and 5–150), and a film synthesized at the 6
sccm N2 flow with plasma generator power 100 W (denoted 6–100) were
obtained. For X-ray photoelectron spectroscopy (XPS) and optical
studies, silicon oxynitride films with a thickness of about 200 nm were
obtained.
The SiOxNy:H film electronic structure and composition were studied
with XPS. The photoelectron spectra were recorded using an SPECS
spectrometer with a PHOIBOS-150-MCD-9 analyzer and an FOCUS-500
monochromator (Al Kα radiation, hν = 1486.74 eV, 200 W). The binding
energy (BE) scale was pre-calibrated using the positions of the peaks of
Au4f7/2 (BE = 84.0 eV) and Cu 2p3/2 (BE = 932.67 eV) core levels. The
peaks binding energy (BE) was calibrated by the C 1s peak position
(284.8 eV) corresponding to the surface hydrocarbon-like deposits. The
survey and the narrow spectra were registered at the analyzer pass en­
ergy 20 eV. The atomic ratios of the elements were calculated from the
integral photoelectron line intensities (Si 2p, O 1s and N 1s), which were
corrected by the corresponding sensitivity factors based on Scofield
photoionization cross-sections.
The SiOxNy:H film structure was studied using both Raman scattering
3. Results and discussion
The XPS spectra of the studied films demonstrate the presence of
intense peaks from Si, N, C and O (Fig. 1(a)). The C 1s peak intensity is
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Journal of Non-Crystalline Solids 598 (2022) 121925
Fig. 1. XPS of the four silicon oxynitride films: (a) survey spectra, (b) Si 2p level, (c) N 1s level and (d) O 1s level with deconvolution (lines).
almost the same for all samples and is due to a random hydrocarbon-like
pollution on the film surface. As is shown below, according to the FTIR
spectroscopy data, there is no carbon in the film bulk. On the Si 2p level
photoelectron spectra there are three peaks that can be attributed to the
binding energies corresponding to Si 2p in silicon (99.4 eV), Si 2p in
silicon nitride (101.7 eV) and Si 2p in silicon oxide (103.2 eV) [19,20]
(Fig. 1(b)). However, the presented Si 2p spectra are not decomposed
correctly into the sum of three Gaussian functions, and, therefore, the
studied films are not a simple mixture of Si, SiO2 and Si3N4. The signal
from silicon (Si+0, 99.4 eV) is most significant for sample 5–50, which
indicates the greatest enrichment in silicon (and presence of silicon
clusters), compared to other samples. The relative intensity of the 101.7
eV signal in the XPS Si 2p, as well as the intensity of N 1s (397.5 eV),
increases in a sequence of samples 5–50, 5–100, 6–100 and 5–150 (Fig. 1
(c)). This indicates an increase in the Si–N bonds concentration and the
nitrogen content in the studied series, which is quite expected. The
atomic ratio y = [N]/[Si] for samples 5–50, 5–100, 6–100 and 5–150 is
0.09, 0.34, 0.43 and 0.51 respectively. The increase in the nitrogen
concentration in this sequence of samples is expected. The hydrogen
related states identification in the studied films is hardly possible due to
the lack of criteria for determining the spectral characteristics of Si–H
and N–H bonds against the background of a signal from silicon (in the Si
2p XPS) and nitrogen atoms (in the N 1s XPS) in many different charge
states.
For the correct identification of oxygen chemical state in the studied
samples, the deconvolution for O 1s spectra was carried out (Fig. 1(d)).
Taking into account the relatively high content of carbon in the nearsurface region, the main O 1s peak at 532.5 eV can be attributed to
Si–O bonds, while the peaks at 531.3 ± 0.2 eV and 533.5 ± 0.2 eV to
– O and C–O oxygen functional groups, respectively [21]. The
C–
parameter x = [O]/[Si] estimation gives the values of 0.87, 0.77, 0.78
and 0.69 for samples 5–50, 5–100, 6–100 and 5–150, respectively.
Although the absolute accuracy of the atomic ratio determination by the
XPS does not allow a reliable establishing of the second decimal place,
nevertheless, the method’s relative accuracy allows one to accurately
compare the data for different samples. So, one can say that the oxygen
concentration in samples 5–100 and 6–100 is almost the same. With an
increase in the plasma generator power, a monotonic decrease of the
oxygen content is observed. The decrease in the oxygen content with an
increase in the plasma generator power can be explained by the corre­
sponding increase in the nitrogen content and a very low oxygen con­
centration in the plasma-forming mixture. It is worth considering that
the obtained oxygen content is, probably, slightly overestimated due to
the oxidation of the samples surface in the air. Therefore, the conclu­
sions made regarding the synthesis parameters effect on the oxygen
content in PECVD silicon oxynitride films require an independent
confirmation by other methods. Nevertheless, the XPS results analysis
makes it possible to roughly identify the studied SiOxNy 5–50, 5–100,
6–100 and 5–150 films in their composition as SiO0.9N0.1, SiO0.8N0.3,
SiO0.8N0.4 and SiO0.7N0.5. The accuracy of the presented values in the
film composition is typical of the XPS method and is 5–10%.
The analysis of Raman spectra confirms the presence of excess Si
(Si–Si bonds) in the studied SiOxNy:H films (Fig. 2). Since the films are
almost transparent or semitransparent, the features from the silicon or
quartz substrate appear in their spectra. An intense peak at 520.5 cm− 1
from the long-wavelength optical phonons in Si and broad bands from
the two-phonon scattering (2TA and TA+LA) is observed (Fig. 2(a)). For
sample, 5–50 distinct bands from amorphous Si (a-Si) are visible. These
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Journal of Non-Crystalline Solids 598 (2022) 121925
Fig. 3. FTIR spectra of SiOxNy:H films synthesized under various conditions.
vibrations decreases [27]: in our case it is ~950 cm− 1. Thus, the IR
spectroscopy data show that almost all kinds of tetrahedra can be
contain in the films.
The dominant peaks associated with the presence of hydrogen in the
studied films are the peaks due to the stretching vibrations of Si–H
bonds (2080–2220 cm− 1) and N–H bonds (~3335 cm− 1). If, in the
latter case, the vibration frequency is almost independent on the nearest
environment of the nitrogen atom (the peak position is within
3325–3350 cm–1 [23,28]), then, in the case of Si–H bonds, this
dependence is significant. In the case of the Si–Si3H tetrahedron, the
frequency is ~2000 cm− 1, and in the case of Si–Si2H2 and Si–SiH3
tetrahedra, the frequency reaches 2100–2120 cm− 1 [29]. In the case of
Si–SiυNδHƞ tetrahedra the frequency depends on parameters υ, δ, and ƞ,
and for our samples this peak position varies from 2080 to 2220 cm− 1.
Such a dependence was observed earlier for SiNx:H films when the
parameter x varied over a wide range [30]. The shift of the Si-H peak
position is most obvious for the 5–150 sample, which, according to the
XPS and Raman data, does not contain amorphous silicon clusters.
It is known that C–O bond vibration frequency is ~1170 cm− 1, and
C–O–H and C–H bond vibration frequencies are in the range from
1320 to 1455 cm− 1 [31]. Thus, there are no peaks from the C–H bond
vibrations in the studied films. Therefore, the carbon concentration is
insignificant throughout the films thickness. The carbon concentration is
high only in the near-surface region of the studied films, which is clearly
demonstrated by the XPS data (Fig. 1). However, the depth of the XPS
analysis is only 3–4 nm, whereas the FTIR signal is recorded from the
entire thickness of 200 nm films.
After subtracting the background, the FTIR spectra were decomposed
into Gaussian curves [32]. According to the analysis of this decompo­
sition, an increase in the plasma discharge power under the fixed N2 flow
leads to the decrease of the Si–H bonds concentration and increase of
the N–H bonds concentration. Hydrogen formed during the monosilane
dissociation saturates the dangling bonds of nitrogen. The Absorption
(A) depends on the film thickness (d), H (CH) concentration of and the
cross-section of absorption (σ ) as: A = d⋅CH⋅σ. This method described in
[23] was used to analyze the hydrogen concentration; the corresponding
scattering cross sections were taken from [33]. Thus, it was obtained
that, for samples 5–50, 5–100, 6–100 and 5–150, the N–H concentration
is 0.2, 1.2, 1.3 and 1.2 × 1022 cm− 3, whereas the Si–H concentration is
1.8, 0.9, 1.0 and 0.5 × 1022 cm− 3, respectively.
In the transmission and reflection spectra of SiOxNy:H films depos­
ited on quartz substrates one can see the features associated with the
interference in the spectral range where the absorption is weak (Fig. 4
(a)). Note that the quartz (fused silica) substrate is transparent in all
range; the absorption edge for Si3N4 is 4.6 eV and absorption edge for aSi depends on the hydrogen content and varies from 1.5 to 2 eV [34]. For
Fig. 2. Raman spectra of four SiOxNy:H films (a) on the Si substrate with the
signal from Si and (b) on the quartz substrate with the signal from quartz.
are two broad modes with maxima of ~480 cm− 1 (contribution mainly
from TO modes) and ~150 cm− 1 (contribution mainly from TA modes)
[22]. Narrow peaks with positions below 150 cm− 1 are the result of
inelastic scattering on vibrational-rotational modes from molecules
contained in the air. For samples 5–100 and 6–100, some weak bands
from a-Si are visible, too. In Fig. 2(b), it also can be seen that there are no
features in the Raman spectra of samples 5–100 and 6–100, except for
the contribution from the quartz substrate. The largest contribution
from the local vibrations of the Si–Si bonds (a-Si peaks) is observed for
sample 5–50. One can see that there are no features associated with the
scattering by local Si-Si bond vibrations in the spectra of the 5–150
sample. It can be concluded that the amount of excess silicon in the
PECVD SiOxNy:H films increases with the decrease of plasma discharge
power. This is, apparently, because the SiH4 dissociation energy is lower
than that for N2 and O2.
The polar Si–N, Si–O, Si–H and N–H bonds, which almost do not
appear in the Raman spectra, appear in the FTIR spectra (Fig. 3). In all
spectra, the peak with a position of 850–860 cm− 1 dominates, corre­
sponding to stretching modes of Si–N bonds [23]. Stretching vibrations
of Si–O bonds also appear in all spectra and one can see some shoulders
with a position of 950–1050 cm− 1. It is known that this mode frequency
strongly depends on the nearest environment of silicon and oxygen
atoms, in particular, on the parameter x in SiOx films [24]. Silicon
oxynitride films are a substitutional solid solution type system consisting
of Si–OυNδSiƞ tetrahedra, where υ+δ+ƞ = 4 [25]. The frequency of local
deformation vibrations of Si–O bonds is ~1080 cm− 1 at υ = 4 [24,26]
and, with an increase in the parameters δ and ƞ, the frequency of these
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Journal of Non-Crystalline Solids 598 (2022) 121925
the same for all films. The presence of excess silicon (and silicon clus­
ters) has the greatest influence on the bandgap value of oxynitride films:
namely, the more excess silicon is present, the smaller the bandgap is. In
addition, as shown below within the ab initio simulation, the nitrogen
content also affects the oxynitride film bandgap, but in such a way that,
with an increase in the nitrogen content, the band gap increases.
In addition to the silicon oxynitride electronic structure investiga­
tion, the TDOS spectra for the model SiOxNy of various compositions
were calculated (Fig. 5). The spectra were combined along the O 2s band
maximum below the valence band top EV for SiO2 by 17.9 eV since it is
the deepest level in the calculation, and it is least sensitive to the atomic
environment. The spectra demonstrate an approximately symmetrical
shift of the EV edge and the conduction band bottom EC edge to the
bandgap. With a decrease in x value, the valence band becomes wider
mainly due to the EV shifting. The SiOxNy valence band top is formed
mainly by Si 3p atomic orbitals corresponding to the bonding σ orbitals
of Si–Si bonds. The EV shifting towards high energies with a decrease in
the parameter x is explained by the fact that the addition of oxygen to
SiOxNy is accompanied by an increase in the energy of the Si–Si bond
binding orbitals. The electron density near the EC consists mainly of Si 3p
orbitals with a Si 3s admixture. The shift of EC to the lower energy region
can be explained by a decrease in the anti-binding σ* orbitals energy of
Si–Si bonds.
The monotonous growth of the Eg value with an increase in param­
eter x in SiOxNy for a structure with y = 0.33 is more uniform and faster
than for a structure with y = 0.5 (Fig. 6(a)). This indicates that an in­
crease in the nitrogen content in highly silicon enriched SiOxNy leads to
an increase in the oxynitride bandgap. The calculated dependence Eg
predicts, for the parameter x = 0.77, the bandgap values of 3.4 eV and
4.0 eV for SiOxN0.33 and SiOxN0.5, respectively. The calculated values
only slightly exceed the values obtained from the optical spectra for
SiO0.8N0.3 (3.28 eV) and SiO0.8N0.4 (3.42 eV) films; although, for sample
5–150 (SiO0.7N0.5), on the contrary, the calculated Eg value 3.9 eV is
slightly underestimated. Taking into account that, in the calculation
model for SiOxN0.5, the excessive nitrogen content is compared to the
Fig. 4. Transmission and reflection spectra (a) and absorption coefficient of
four samples SiOxNy:H films grown on quartz substrates (b).
the studied oxynitride films, the transparency increases in the sequence
5–50, 5–100 and 6–100 is clearly observed. This is consistent with the
results of XPS, FTIR and Raman spectroscopy that the excess silicon
content in films decreases in this sequence.
To obtain the optical gap (bandgap) value of the studied films, the
optical absorption dispersion spectra α were calculated based on the
transmission and reflection spectra taking into account the interference
effects (Fig. 4b). Optical absorption is also observed for quanta with an
energy less than the bandgap value Eg due to the presence of density
states tails near the conduction band bottom and the valence band top
(Urbah tails [35]). Therefore, the Eg value was determined as the photon
energy at which α is 104 cm− 1 (the so-called E04 gap). Thus, for the
studied silicon films 5–50 (SiO0.9N0.1), 5–100 (SiO0.8N0.3) and 6–100
(SiO0.8N0.4), the corresponding bandgap values are 2.5 eV, 3.3 eV and
3.4 eV. For the 5–150 (SiO0.7N0.5) sample that does not contain amor­
phous silicon clusters, the E04 gap is 4.1 eV. Attention is drawn to the
strong optical absorption edge shift into the short-wavelength region of
the spectrum under increasing the plasma generator power at which the
film was synthesized, whereas an increase in the N2 flow from 5 to 6
sccm almost did not change the Eg value. So, both the composition and
electrophysical properties of PECVD silicon oxynitride films can be
effectively varied by changing the plasma generator power.
The bandgap value of the studied films is determined mainly by their
atomic composition. The carbon effect on the bandgap value is hardly
noticeable since it is present mainly in the near-surface region of the
films, whereas the studied films are rather thick. The hydrogen effect on
the bandgap should be greater, but since the hydrogen concentration in
the studied films is approximately the same, this effect should be almost
Fig. 5. The TDOS spectra of SiOxNy for two fixed y values and various x values.
Zero energy corresponds to the valence band top position EV for SiO2; the
spectra are combined by the O 2s level peak (dash vertical line) and have the
Gaussian broadening with σ = 0.2 eV.
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Journal of Non-Crystalline Solids 598 (2022) 121925
Fig. 6. (a) The Eg value (a), and EV with the Ec (b) position relative to the
vacuum level dependences on the parameter x for SiOxN0.33 and SiOxN0.5.
SiO0.77N0.43 film, as well as the possible oxygen content overestimation
in the studied film, one can say that the agreement of theoretical and
experimental values is surprisingly good. This indicates the correctness
of the simple theoretical model used. Since the studied oxynitride films
contain a high hydrogen concentration, it can be expected that hydrogen
almost does not affect the dielectric bandgap value.
The calculated TDOS spectra, the known bandgap value for a-Si 1.6
eV [34], as well as the EV position relative to the vacuum level for a-Si
and SiO2 [36], allows as to construct an energy diagram of SiOxNy for
different oxygen compositions (Fig. 6(b)). In turn, this diagram allows
estimating the potential barriers for electrons (Фe) and holes (Фh) at the
a-Si/SiOxNy boundary for different values of parameter x. As one can
see, there is a monotonous and almost linear increase in the Фe and Фh
values with an increasing of parameter x. In addition, Фe is greater than
Фh for any parameter “x” value, as it takes place for the Si/Si3N4
boundary [37]. The difference between the Фe and Фh values increases
with the decrease of parameter x. The calculated energy diagram allows
estimating the Фe and Фh values for the studied oxynitride films with x
≈ 0.77: for samples 5–100 and 6–100, the corresponding Фe values are
1.2 eV and 1.6 eV, whereas the Фh values are 0.7 eV and 0.9 eV.
To find out the possibility using of the PECVD SiOxNy:H films
enriched with silicon as an active RRAM cell layer, the I–V characteristic
of the structures p+-Si/SiOxNy:H(33 nm)/Ni were measured. The struc­
ture with SiOxNy:H 5–50 has no memristor effect. The structures with
dielectric layer 5–100, 5–150 and 6–100 have typical bipolar memristor
I-V characteristics: they can switch reversibly between the HRS (ON) and
LRS (OFF). Since structures 5–100 and 6–100 have almost identical I-V
curves, here are only I–V characteristics for the two structures with
SiOxNy:H 6–100 and 5–150 (Fig. 7). As one can see, for the memristor
based on sample 5–150, the resistance before a certain switch almost
coincides with the LRS, whereas, for the memristor based on sample
6–100, it is even less than the LRS. So, the initial state can be considered
as the ON-state. Such memristors belong to the forming-free class.
Actually, the forming occurs as a procedure for the memristor switching
to the operating mode, but this does not require a pre-breakdown
voltage. Apparently, some conductive channels were formed at the
synthesis stage.
The plasma generator power increase from 100 to 150 W leads to the
Fig. 7. (a) I-V characteristic of a p+-Si/SiOxNy:H/Ni structure with dielectric
layer 6–100. The voltage sweep direction is shown with arrows.
increase of the memory window, i.e., the ratio of currents in the LRS and
HRS at 0.5 V from about 10 to 100. In addition, the LRS for the 5–150
memristor is lower than for the 6–100 sample, which indicates a
potentially lower power consumption of the device. However, although
an increase in the plasma power during the SiOxNy:H synthesis has a
positive effect on the memory window value and the resistance in the
LRS, the switching voltage increases, and it is undesirable. Most likely,
the obtained memristors are not optimal and there are certain synthesis
modes in which the memristor characteristics will be better. The search
for such modes is beyond the scope of the current studies. A detailed
study of the obtained memristor properties, namely, their cycling and
information storage time, as well as the charge transport mechanism and
trap nature, are the object of further investigations.
To identify which RRAM type the obtained memristors belong (to
valence change memory (VCM) RRAM or conductive-bridge RRAM), to
the current temperature dependences in the LRS state were measured
(Fig. 8). One can see, that in the LRS with the increasing temperature,
the current increases exponentially, and this growth is the same for
memristor structures 5–150 and 6–100. This suggests, firstly, that the
filament has a semiconductor-type conductivity and the trap nature
responsible for the conductivity for both studied dielectric is the same.
Thus, the p+-Si/SiOxNy:H/Ni memristors based on PECVD SiOxNy:H
films enriched with silicon are bipolar, forming-free and belong to the
VCM type of RRAM.
It is interesting to note that memristors based on PECVD SiOx:H and
SiNx:H qualitatively have the same properties as memristors based on
PECVD SiOxNy:H [3,5]. The memory window value for the studied
5–150 memristor is slightly larger than for a SiOx:H memristor, but less
than that of a SiNx:H memristor. At the same time, the LRS for
6
T.V. Perevalov et al.
Journal of Non-Crystalline Solids 598 (2022) 121925
CRediT authorship contribution statement
T.V. Perevalov: Conceptualization, Investigation, Writing – review
& editing, Visualization. V.A. Volodin: Conceptualization, Investiga­
tion, Writing – original draft, Project administration. G.N. Kamaev:
Resources. A.A. Gismatulin: Investigation. S.G. Cherkova: Investiga­
tion. I.P. Prosvirin: Formal analysis. K.N. Astankova: Formal analysis.
V.A. Gritsenko: Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data availability
Fig. 8. I-V characteristic of a SiOxNy:H film in the LRS at different
temperatures.
Data will be made available on request.
memristors based on SiOx:H and SiNx:H are characterized by the resis­
tance lower than the LRS for SiOxNy:H based memristors. Anyway, an
unambiguous quantitative comparison of the different memristor
properties is hardly possible due to the strong, but insufficiently
well-studied influence of the film composition (parameters x and y) on
the memristors properties based on these films.
Acknowledgments
The work was supported by the Russian Science Foundation, project
No. 22–19–00369. The electrophysical measurements and measure­
ments of Raman spectra are done on the equipment of the Center of
Collective Usage “VTAN” NSU supported by the Ministry of Education
and Science of Russia by agreement #075–12–2021–697. The ab initio
simulations were carried out at the NSU Supercomputer Center.
4. Conclusions
In this paper, the electronic structure and optical properties of silicon
oxynitride films enriched with silicon of four different compositions
obtained by PECVD was studied. The films were synthesized by varying
both the N2 + O2 flow and plasma discharge power. It was found that the
plasma discharge power significantly affects the film content: with its
growth, the content of silicon and oxygen decreases, while the nitrogen
content increases. The analysis of Raman spectra predicts the presence
of amorphous Si in the studied films. According to the XPS data, the
relative content of elements in samples 5–50 (SiO0.9N0.1), 5–100
(SiO0.8N0.3), 6–100 (SiO0.8N0.4) and 5–150 (SiO0.7N0.5) was determined.
The corresponding bandgap values for these films are 2.5 eV, 3.3 eV, 3.4
eV and 4.1 eV. According to the FTIR spectroscopy data, the studied
films contain a lot of hydrogen: the N–H (Si–H) bonds concentration
for the listed sequence of samples is 0.2 (1.8) × 1022, 1.2 (0.9) × 1022,
1.3 (1.0) × 1022 cm− 3, and 1.2 (0.5) × 1022 cm− 3. It is concluded that the
PECVD SiOxNy:H film composition and electrophysical properties can be
effectively varied by the plasma generator power. The electronic struc­
ture of SiOxNy of various x and y values was calculated within the DFT
for simple model structures. The correctness of the used model is
confirmed by the agreement of the calculated bandgap values with the
experimental ones. Based on the calculations, the SiOxNy energy dia­
gram is constructed. So, the Фe and Фh values for the a-Si/SiOxNy
interface for films under study were estimated.
It is established that the p+-Si/SiO0.8N0.3:H/Ni, p+-Si/SiO0.8N0.4/Ni
and p+-Si/SiO0.7N0.5/Ni structures have the I-V characteristic typical of
the memristor with bipolar switching. The measured current tempera­
ture dependence in the LRS state clearly shows that memristors belong
to the valence change RRAM. The memristors are forming-free since the
initial resistive state has a close resistance to the LRS, such that they are
initially in the ON state and the device functionalization procedure is not
required. An increase in the plasma power during the SiOxNy:H synthesis
leads to increasing the memory window value and decreasing the con­
ductivity in the LRS. The obtained memristors properties are qualita­
tively close to those for memristors based on PECVD SiOx:H and SiNx:H.
Thus, silicon oxynitride films enriched with silicon synthesized by
PECVD from a mixture of SiH4, N2 and O2 are appropriate for using them
as an active medium of a forming-free RRAM cell.
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