Plasma Enhanced Atomic Layer Deposition of Tantalum (V) Oxide

: The tantalum oxide thin ﬁlms are promising materials for various applications: as coatings in optical devices, as dielectric layers for micro and nanoelectronics, and for thin-ﬁlms solid-state lithium-ion batteries (SSLIBs). This article is dedicated to the Ta-O thin-ﬁlm system synthesis by the atomic layer deposition (ALD) which allows to deposit high quality ﬁlms and coatings with excellent uniformity and conformality. Tantalum (V) ethoxide (Ta(OEt) 5 ) and remote oxygen plasma were used as tantalum-containing reagent and oxidizing co-reagent, respectively. The inﬂuence of deposition parameters (reactor and evaporator temperature, pulse and purge times) on the growth rate were studied. The thickness of the ﬁlms were measured by spectroscopic ellipsometry, scanning electron microscopy and X-ray reﬂectometry. The temperature range of the ALD window was 250–300 ◦ C, the growth per cycle was about 0.05 nm/cycle. Different morphology of ﬁlms deposited on silicon and stainless steel was found. According to the X-ray diffraction data, the as-prepared ﬁlms were amorphous. But the heat treatment study shows crystallization at 800 ◦ C with the formation of the polycrystalline Ta 2 O 5 phase with a rhombic structural type (Pmm2). The results of the X-ray reﬂectometry show the Ta-O ﬁlms’ density is 7.98 g/cm 3 , which is close to the density of crystalline Ta 2 O 5 of the rhombic structure (8.18 g/cm 3 ). The obtained thin ﬁlms have a low roughness and high uniformity. The chemical composition of the surface and bulk of Ta-O coatings was studied by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy. Surface of the ﬁlms contain Ta 2 O 5 and some carbon contamination, but the bulk of the ﬁlms does not contain carbon and any precursor residues. Cyclic voltammetry (CVA) showed that there is no current increase for tantalum (V) oxide in a potential window of 3–4.2 V and has prospects of use as protective coatings for cathode materials of SSLIBs. using tantalum (V) ethoxide and remote oxygen plasma by different methods. The chemical composition at the surface and in-depth, structure (phase composition, thin ﬁlms density, roughness), the morphology of Ta-O thin ﬁlms on the silicon and stainless steel substrates were studied. The electrochemical performance is provided by cyclic voltammetry.


Introduction
Thin films of tantalum oxide are used for antireflection coatings for strongly curved glass lenses [1], as dielectric layers for micro and nanoelectronics [2]. They also have great prospects for solid-state lithium-ion batteries (SSLIB) [2] as a solid-state electrolyte and functional coatings for cathodes.
It is important to use low deposition temperatures to prevent structural and chemical changes in the surface of the coated material while obtaining films for previously mentioned purposes. The ability to cover 3D-oriented surfaces with precise thickness control is also important. All these tasks can be solved by the Atomic Layer Deposition (ALD) [3]. The essence of the method lies in the chemical interaction of reagent vapors with the substrate and subsequent purging with an inert gas. Due to its peculiarities, the method makes

Materials and Methods
Ta(OEt) 5 (Sigma Aldrich, 99.98% trace metal basis) was used to obtain tantalum oxide films on a Picosun R-150 set up in the reactor temperature range from 225 to 350 • C. Ta(OEt)5 was stored in a stainless steel bottle (PicohotTM 200, Picosun Oy, Espoo, Finland) and heated during synthesis to 70, 90, 110, 160, and 190 • C. The pulse time was varied from 0.5 s to 2 s, and purge time was changed from 5 s to 15 s. At an inert nitrogen purge, the gas pressure of 8-12 hPa is combined with vacuum pumping. The number of cycles for the samples ranged from 50 to 1000. Remote oxygen plasma was used as a co-reagent (oxidizer). Plasma power was 3000 W, and the frequency was 1.9-3.2 MHz. The total plasma pulse time was 19.5 s. First, Ar purge was carried out for 0.5 s at a flow rate of 40 sccm, Ar and O 2 plasma purge was carried out for 14 s at a flow rate of 90 sccm. And the last part was Ar purge for 5 s at a flow rate flow 40 sccm. Thus, the number of cycles for the samples ranged from 50 to 1000.
Monocrystalline silicon substrates (surface orientation (100), 40 × 40 mm, TelecomSTV Co., Ltd., Zelenograd, Russia) and stainless-steel disks (316SS, 16 mm diameter, Tob New Energy Technology Co., Ltd., Xiamen, China) were used in experiments as substrates for the ALD. Before deposition, the stainless-steel substrates were cleaned in an ultrasonic bath filled with acetone and then deionized water for 5 min.
Spectroscopic ellipsometry (SE) [40,41] was used to determine the thickness of the resulting films and to calculate the growth per cycle (GPC). Ellipsometric measurements were performed with a spectroscopic ellipsometer Ellips-1891 SAG (CNT, Novosibirsk, Russia) at wavelengths in the range λ = 250-900 nm (photon energy E = 1.4-5.0 eV) and light incidence angle ϕ = 70 • relative to the normal to the surface, from an external medium (air). Spectral dependences of both ellipsometric angles ψ and ∆ were measured simultaneously. The probe spot diameter was 3 mm. The calculations were carried out using the procedure for finding the model (Supplementary file) coefficients and film thickness using the software supplied with the ellipsometer. The thickness measurement error was ±1 nm.
X-ray reflectometry (XRR) and X-ray diffraction (XRD) studies were performed on a Bruker D8 DISCOVER diffractometer (Bruker, Billerica, MA, USA), Cu-Kα = 1.5406 Å. Grazing incidence XRD (GIXRD) modes, that are surface sensitive, were used for XRD measurements using a 2θ range of 20-65 • in 0.1 • steps and 1 s exposure at each step. The angle of incidence of the primary X-ray beam was 0.7 • . XRR measurements were carried out over an angular range of 0.2-5 • (0.01 • step) using symmetric scattering geometry. The results were processed by the Rietveld method using the TOPAS software package (version 5, Bruker, Billerica, MA, USA) and LEPTOS (version 7.7, Bruker, Billerica, MA, USA) for XRD and XRR, respectively. The approximate thickness, density, and the number of coatings and instrumental parameters (radiation wavelength, goniometer radius, slit width, etc.) were set using the LEPTOS software. Based on these data, the theoretical curve was modeled and fitted to best match with experimental data using the simplex method by varying the characteristics of the samples (roughness, thickness, and density). The χ 2 value for the modeled curve was 8.080 × 10 −3 .
X-ray photoelectron spectra (XPS) were obtained on an Escalab 250Xi spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). To obtain information about inner layers of the deposited film the Ar+ sputtering (500 eV for 90 s) was performed. The samples were excited by the X-ray radiation Al-Kα (1486.7 eV) at a pressure of 7 × 10 −8 Pa.
The scanning electron micrographs of flat and cross-sections were obtained by a Supra 55 VP scanning electron microscope (SEM, Zeiss, Oberkochen, Germany) with a Gemini-I column and a field emission cathode. The spatial resolution was about 1.3 nm at an accelerating voltage of 3 kV. A total of 3 randomly selected positions on the sample surface were examined. An Everhart-Thornley and an InLens secondary electron detectors were used for SEM studies. Energy Dispersive X-ray (EDX) analysis was performed using an INCA X-Max system (Oxford Instruments, High Wycombe, UK) mounted on a Supra 55 VP SEM.
Stainless steel plates (SS) with deposited films were used for electrochemical studies. Lithium foil, polyolefin porous film 2325 (Celgard, Charlotte, NC, USA), and TC-E918 (Tinci, Guangzhou, China) were used as a counter electrode, a separator and an electrolyte, respectively. The composition TC-E918 was a 1M solution of LiPF 6 in a mixture of organic carbonates (ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, vinyl carbonate). The coin cells (CR2032) were assembled in OMNI-LAB (VAC) glove box under argon atmosphere. Cyclic voltammetry (CV) was performed using a PGSTAT302N + potentiostat (Autolab, Utrecht, The Netherlands) in the range 0.01-4.30 V with a scan rate of 0.5 mV/s.

Atomic Layer Deposition of Ta-O Thin Films
To determine the optimal conditions for the tantalum oxide growth, the effect of the Ta(OEt) 5 evaporator temperature, the time of pulsing, purging and the reactor temperature on the GPC were studied ( Table 2). There was a low growth per cycle (less than 0.01 nm/cycle) at the evaporator temperatures from 70 • C to 110 • C, indicating that the evaporator temperature is insufficient for the required amount of the reagent enters the reactor (Figure 1a). At 160 • C and 190 • C, the average growth per cycle increases up to 0.043-0.049 nm/cycle and begin to stabilize. The further increasing temperature is not technically possible because of equipment limitations. Thus, it is necessary to heat the evaporator to at least 190 • C to carry out the tantalum oxide synthesis. The dependences of the average growth per cycle from the Ta(OEt)5 pulse time were studied to determine the reactor filling time required to reach saturation. The experiments were carried out at a constant reactor temperature (300 °C) and purging times (after Ta(OEt)5 pulse in 5 s, after plasma treatment in 15 s). With an increase of the Ta(OEt)5 pulse duration from 0.5 to 1.0 s, the value of the average growth per cycle augments from 0.042 to 0.049 nm/cycle, and after subsequent pulse time increase GPC does not grow (Figure 1b). Further increases pulse up to 2 s resulted in a slight decrease in the average growth per cycle. Thus, the saturation of the substrate precursor vapors under synthesis conditions can be achieved with a Ta(OEt)5 pulse time of more than 1.0 s.
To determine the effect of purge duration two series of samples were prepared with 1.0 s and 1.5 s of reactor filling time of Ta(OEt)5 were chosen (Figure 1с). The purge duration for both series of samples was set to 5 s, 10 s, 15 s. For both series of samples, the average growth rate was almost the same. Thus, neither adsorption time nor desorption time doesn't influence on the growth rate. We can assume that all available functional The dependences of the average growth per cycle from the Ta(OEt) 5 pulse time were studied to determine the reactor filling time required to reach saturation. The experiments were carried out at a constant reactor temperature (300 • C) and purging times (after Ta(OEt) 5 pulse in 5 s, after plasma treatment in 15 s). With an increase of the Ta(OEt) 5 pulse duration from 0.5 to 1.0 s, the value of the average growth per cycle augments from 0.042 to 0.049 nm/cycle, and after subsequent pulse time increase GPC does not grow (Figure 1b). Further increases pulse up to 2 s resulted in a slight decrease in the average growth per cycle. Thus, the saturation of the substrate precursor vapors under synthesis conditions can be achieved with a Ta(OEt) 5 pulse time of more than 1.0 s.
To determine the effect of purge duration two series of samples were prepared with 1.0 s and 1.5 s of reactor filling time of Ta(OEt) 5 were chosen (Figure 1c). The purge duration for both series of samples was set to 5 s, 10 s, 15 s. For both series of samples, the average growth rate was almost the same. Thus, neither adsorption time nor desorption time doesn't influence on the growth rate. We can assume that all available functional groups react during adsorption carried out during 1 s. And 5 s is sufficient to eliminate all excess of Ta(OEt) 5 adsorbed during less or equal 1.5 s.
The average growth per cycle dependences on the reactor temperature were studied to determine the temperature window of the synthesis (Figure 1d). With an increase of the reactor temperature from 225 • C to 350 • C, the average growth per cycle value decreases from 0.055 to 0.047 nm/cycle. It was determined that the largest growth per cycle was at 225 • C (0.055 nm/cycle). The high average growth per cycle value is probably a consequence of the precursor condensation on the substrate surface due to the relatively low reactor temperature and the small difference between the temperatures in the Ta(OEt) 5 evaporator and the reactor. The average growth per cycle values decreases to 0.049-0.050 nm/cycle in the 250 • C. After which in the temperature range of 250-300 • C remains close to constant. The growth per cycle decreases with an increase in temperature, which may be caused by a decrease in the number of functional groups.
The interval in the temperature range 250-300 • C can be considered as a "synthesis window" for ALD processes using Ta(OEt) 5 and oxygen plasma because of the constant growth rate (0.049-0.050 nm/cycle). The linear dependence of the tantalum oxide layer thickness on the number of ALD cycles indicates the absence of nucleation growth effects and the possibility of the film thickness precise control (Figure 1d inset). The obtained growth per cycle values are less than GPC showed in [8] (0.075 nm/cycle), where Ta(OEt) 5 and oxygen plasma were used as reagents. However, they are very close to the growth per cycle values (0.049 nm/cycle) obtained with water as a co-reagent at a temperature of 300 • C ( Table 1). The slight difference between the experimental and published data is probably due to the design differences of the used setups, the thickness measurement techniques.

Morphology of Films on Si
Top view and cross-sectional images of Ta-O films on a Si substrate, obtained using scanning electron microscopy, are shown in Figure 2. The resulting tantalum oxide films were smooth and homogeneous, without visible defects and inclusions. The average thickness sample of Ta O_6 is about 46.3 nm. The smoothness of the film indicates the absence of nucleation effects during synthesis in the ALD "window". The average growth per cycle dependences on the reactor temperature were studied to determine the temperature window of the synthesis (Figure 1d). With an increase of the reactor temperature from 225 °C to 350 °C, the average growth per cycle value decreases from 0.055 to 0.047 nm/cycle. It was determined that the largest growth per cycle was at 225 °C (0.055 nm/cycle). The high average growth per cycle value is probably a consequence of the precursor condensation on the substrate surface due to the relatively low reactor temperature and the small difference between the temperatures in the Ta(OEt)5 evaporator and the reactor. The average growth per cycle values decreases to 0.049-0.050 nm/cycle in the 250 °C. After which in the temperature range of 250-300 °C remains close to constant. The growth per cycle decreases with an increase in temperature, which may be caused by a decrease in the number of functional groups.
The interval in the temperature range 250-300 °C can be considered as a "synthesis window" for ALD processes using Ta(OEt)5 and oxygen plasma because of the constant growth rate (0.049-0.050 nm/cycle). The linear dependence of the tantalum oxide layer thickness on the number of ALD cycles indicates the absence of nucleation growth effects and the possibility of the film thickness precise control (Figure 1d inset). The obtained growth per cycle values are less than GPC showed in [8] (0.075 nm/cycle), where Ta(OEt)5 and oxygen plasma were used as reagents. However, they are very close to the growth per cycle values (0.049 nm/cycle) obtained with water as a co-reagent at a temperature of 300 °C ( Table 1). The slight difference between the experimental and published data is probably due to the design differences of the used setups, the thickness measurement techniques.

Morphology of Films on Si
Top view and cross-sectional images of Ta-O films on a Si substrate, obtained using scanning electron microscopy, are shown in Figure 2. The resulting tantalum oxide films were smooth and homogeneous, without visible defects and inclusions. The average thickness sample of Ta O_6 is about 46.3 nm. The smoothness of the film indicates the absence of nucleation effects during synthesis in the ALD "window".

Chemical Composition of the Films
The EDX analysis was carried out to determine the chemical composition. It showed that the atomic concentration ratio of Ta:O in the film (with a thickness of 46.3 nm) is approximately 1:2.7. The observed ratio indicates the formation of tantalum (V) oxide during the synthesis. It can be concluded that there is no reduction of tantalum during the synthesis since Ta(OEt)5 was used for the deposition. The slight excess of the oxygen atomic concentration over ratios 2:5 may be due to a natural oxide layer on the silicon substrate surface.
A decrease in the energy of exciting radiation during the EDX makes it possible to reduce the depth of the analyzed region. Considering the small thickness of the analyzed

Chemical Composition of the Films
The EDX analysis was carried out to determine the chemical composition. It showed that the atomic concentration ratio of Ta:O in the film (with a thickness of 46.3 nm) is approximately 1:2.7. The observed ratio indicates the formation of tantalum (V) oxide during the synthesis. It can be concluded that there is no reduction of tantalum during the synthesis since Ta(OEt) 5 was used for the deposition. The slight excess of the oxygen atomic concentration over ratios 2:5 may be due to a natural oxide layer on the silicon substrate surface. A decrease in the energy of exciting radiation during the EDX makes it possible to reduce the depth of the analyzed region. Considering the small thickness of the analyzed coatings (3.9-46.3 nm), the minimum accelerating voltage of 3 kV was chosen to study the samples.
A clear signal from tantalum is observed even on the X-ray spectra with a coating thickness of 6 nm ( Figure 3). As the film thickness increases, the main peak of tantalum (M-series) shifts to the low-energy side due to the decrease in the effect of the silicon substrate. The intensity of the peaks in the spectral regions of 1.335 and 1.96 keV also increases. The intensity of the maxima corresponding to silicon atoms decreases. A sample of tantalum oxide (Ta-O_11), obtained at the reactor temperature of 300 • C during 500 cycles, was investigated by the XPS method for a detailed study of the chemical composition. Both the external surface and the coating volume were studied after etching the surface layer with argon ions. Tantalum, oxygen, and carbon were found on the sample surface (Table 3). After the surface layer etching, carbon peaks in the spectrum are no longer observed, indicating that there is no carbon in the coating. Presumably, the sample surface is contaminated with organic and inorganic carbon compounds deposited from the air. Based on the C1s level peak position and shape, carbon on the sample surface is predominantly in the C-C, C-H state (Figure 4). In addition, there is a small amount of C-OH. There are no compounds with C=O bonds, COOH, and carbonates on the surface. The O1s spectra of the surface exhibit an intense peak corresponding to Ta 2 O 5 and a small shoulder corresponding to compounds with C-O bonds. This peak shifts toward the higher energies during etching. According to the fact that no carbon was found in the etched sample and it cannot contain oxygen-containing phases C-O and C=O, the shift is caused by the reduction of Ta   According to the XPS spectra of the Ta4f level, Ta2O5 is located on the sample surface. The Ta to O 2:5 ratio also corresponds to this compound and coincides with the data obtained by the EDX. The peaks shift to the lower energies after etching, their maxima correspond to the TaO compound, and the Ta to O ratio becomes 1:1. According to the XRD data, the film volume consists of Ta2O5. Judging by that, it is likely that TaO was a result of the Ta 5+ reduction upon bombardment with the argon ions.

Crystal Structure
The XRD of the sample showed that the resulting films in ALD "window" are amorphous, Figure 5. In the range from 20 to 40° 2θ, a diffuse halo is noticeable. It signifies the crystallization onset of the tantalum (V) oxide phase. The fact that there are no metallic tantalum peaks indicates its absence in the film composition. It correlates with the data obtained by the XPS, in the spectra, the maxima corresponding to tantalum in the zerooxidation state were not found. After heat treatment (HT) at 800 °C for 15 min, intense peaks appeared in the diffractogram at 23, 29, 37.5, 45, 47, 51, and 56°, corresponding to the crystalline phase of Ta2O5 (orthorhombic, Pmm2). According to the ellipsometry data, the films' thickness before and after HT is not changed within the measurement accuracy (Supplementary file, Figure S3) thus we can assume that remains stable and cover substrate even after calcination at high temperature. According to the XPS spectra of the Ta4f level, Ta 2 O 5 is located on the sample surface. The Ta to O 2:5 ratio also corresponds to this compound and coincides with the data obtained by the EDX. The peaks shift to the lower energies after etching, their maxima correspond to the TaO compound, and the Ta to O ratio becomes 1:1. According to the XRD data, the film volume consists of Ta 2 O 5 . Judging by that, it is likely that TaO was a result of the Ta 5+ reduction upon bombardment with the argon ions.

Crystal Structure
The XRD of the sample showed that the resulting films in ALD "window" are amorphous, Figure 5. In the range from 20 to 40 • 2θ, a diffuse halo is noticeable. It signifies the crystallization onset of the tantalum (V) oxide phase. The fact that there are no metallic tantalum peaks indicates its absence in the film composition. It correlates with the data obtained by the XPS, in the spectra, the maxima corresponding to tantalum in the zerooxidation state were not found. After heat treatment (HT) at 800 • C for 15 min, intense peaks appeared in the diffractogram at 23, 29, 37.5, 45, 47, 51, and 56 • , corresponding to the crystalline phase of Ta 2 O 5 (orthorhombic, Pmm2). According to the ellipsometry data, the films' thickness before and after HT is not changed within the measurement accuracy (Supplementary file, Figure S3) thus we can assume that remains stable and cover substrate even after calcination at high temperature.
The X-ray reflectometry of the sample before HT ( Figure 6) showed that the films have a uniform density equal to 7.98 g/cm 3 , close to 7.96 g/cm 3 [42], and close to the crystal Ta 2 O 5 density of 8.18 g/cm 3 [43]. The lower density is due to the amorphous state of the tantalum (V) oxide. According to the XRR data, the films have low roughness, which is also confirmed by the SEM photography. The film thickness was 46.8 nm, close to the data obtained by spectral ellipsometry (46.3 nm).

SEM of Tantalum Oxide Coated Steel
The surface of coatings deposited on the steel substrates differs from the sample deposited on silicon. The densely spaced particles ranging in size from several tens to one hundred nanometers (Figure 7) are presented on the surface. The average thickness of Ta-O_6 coating on the stainless steel is about 61.4 nm. The difference in the morphology of coatings on silicon and steel may be due to different surface roughness and functional groups of a different nature. The average growth per cycle is 0.0614 nm/cycle, that more GPC on the silicon surface (0.0463 nm/cycle). tantalum peaks indicates its absence in the film composition. It correlates with the data obtained by the XPS, in the spectra, the maxima corresponding to tantalum in the zerooxidation state were not found. After heat treatment (HT) at 800 °C for 15 min, intense peaks appeared in the diffractogram at 23, 29, 37.5, 45, 47, 51, and 56°, corresponding to the crystalline phase of Ta2O5 (orthorhombic, Pmm2). According to the ellipsometry data, the films' thickness before and after HT is not changed within the measurement accuracy (Supplementary file, Figure S3) thus we can assume that remains stable and cover substrate even after calcination at high temperature.  The X-ray reflectometry of the sample before HT ( Figure 6) showed that the films have a uniform density equal to 7.98 g/cm 3 , close to 7.96 g/cm 3 [42], and close to the crystal Ta2O5 density of 8.18 g/cm 3 [43]. The lower density is due to the amorphous state of the tantalum (V) oxide. According to the XRR data, the films have low roughness, which is also confirmed by the SEM photography. The film thickness was 46.8 nm, close to the data obtained by spectral ellipsometry (46.3 nm).

SEM of Tantalum Oxide Coated Steel
The surface of coatings deposited on the steel substrates differs from the sample deposited on silicon. The densely spaced particles ranging in size from several tens to one hundred nanometers (Figure 7) are presented on the surface. The average thickness of Ta-O_6 coating on the stainless steel is about 61.4 nm. The difference in the morphology of coatings on silicon and steel may be due to different surface roughness and functional groups of a different nature. The average growth per cycle is 0.0614 nm/cycle, that more GPC on the silicon surface (0.0463 nm/cycle). posited on silicon. The densely spaced particles ranging in size from several tens to on hundred nanometers (Figure 7) are presented on the surface. The average thickness of Ta O_6 coating on the stainless steel is about 61.4 nm. The difference in the morphology o coatings on silicon and steel may be due to different surface roughness and functiona groups of a different nature. The average growth per cycle is 0.0614 nm/cycle, that mor GPC on the silicon surface (0.0463 nm/cycle).

Electrochemical Studies
Cyclic voltammetry (CV) was carried out for samples of amorphous tantalum (V) oxide on steel substrates (TaO_9). The results of 5 cycles of CV are shown in Figure 8.

Electrochemical Studies
Cyclic voltammetry (CV) was carried out for samples of amorphous tantalum (V) oxide on steel substrates (TaO_9). The results of 5 cycles of CV are shown in Figure 8. During CV scans of nanofilms, a pronounced current increase appeared in the 3.0-0.1 V range. It was caused by conventional reactions and lithiation of Ta-O nanofilms [44], and a partial reduction in organic solvents (ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate) and functional additives (vinylene carbonate) of electrolytes with the formation of a gel-like solid electrolyte interface (SEI) film on the active layer [45]. However, there is no current amplification in the cathode potential range (4.3-3.0 V) (inset in Figure 8). Thus, the investigated coatings do not affect the electrochemical capacity in the region of cathodic potentials and do not interact with the electrolyte. In the works, the stabilization of Ni-rich cathodes is shown both when doping with tantalum [46] and when applying tantalum-containing oxide coatings [47]. Thus, they can be used as protective coatings for cathode materials [2].

Conclusions
As a result, the optimal conditions for the production of tantalum (V) oxide nanofilms by the ALD were determined. The temperature effect of the container's evaporator with the reagent Ta(OEt)5, the time of its pulsing, purging, and the reactor temperature on the growth per cycle were studied. The results showed that to achieve the ALD growth of the substrate surface with Ta(OEt)5 vapors, an evaporator temperature above 190 °C and an pulse time of at least 1 s are required. A 5 s purge is sufficient to remove excess reagent and reaction products. The ALD temperature window is 250-300 °C, with an average growth per cycle at 0.043-0.050 nm/cycle. During CV scans of nanofilms, a pronounced current increase appeared in the 3.0-0.1 V range. It was caused by conventional reactions and lithiation of Ta-O nanofilms [44], and a partial reduction in organic solvents (ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate) and functional additives (vinylene carbonate) of electrolytes with the formation of a gel-like solid electrolyte interface (SEI) film on the active layer [45]. However, there is no current amplification in the cathode potential range (4.3-3.0 V) (inset in Figure 8). Thus, the investigated coatings do not affect the electrochemical capacity in the region of cathodic potentials and do not interact with the electrolyte. In the works, the stabilization of Ni-rich cathodes is shown both when doping with tantalum [46] and when applying tantalum-containing oxide coatings [47]. Thus, they can be used as protective coatings for cathode materials [2].

Conclusions
As a result, the optimal conditions for the production of tantalum (V) oxide nanofilms by the ALD were determined. The temperature effect of the container's evaporator with the reagent Ta(OEt) 5 , the time of its pulsing, purging, and the reactor temperature on the growth per cycle were studied. The results showed that to achieve the ALD growth of the substrate surface with Ta(OEt) 5 vapors, an evaporator temperature above 190 • C and an pulse time of at least 1 s are required. A 5 s purge is sufficient to remove excess reagent and reaction products. The ALD temperature window is 250-300 • C, with an average growth per cycle at 0.043-0.050 nm/cycle.
The study of the obtained nanofilms by XPS and EDX showed that the films contain no unreacted carbon-containing residues of the Ta(OEt) 5 precursor, and the stoichiometric ratio of tantalum and oxygen is close to Ta 2 O 5 . The nanofilms deposited on Si substrates have homogeneous morphology, low roughness, and the density (7.98 g/cm 3 ) are close to bulk tantalum (V) oxide (8.18 g/cm 3 ). The nanofilms obtained at 300 • C with a thickness of 46-47 nm are X-ray amorphous. However, they crystallize upon annealing at 800 • C with the formation of orthorhombic Ta 2 O 5 .
The average growth per cycle of nanofilms on stainless steel is 0.0614 nm/cycle, which is more than GPC on the silicon surface (0.0463 nm/cycle). This nanofilms are not homogenous and consist of densely spaced particles ranging from several tens to one hundred nanometers.
The obtained cyclic voltammograms analysis of the nanofilms deposited on steel substrates showed that the coatings in the cathode potential range (4.3-3.0 V) do not interact with the electrolyte and do not contribute to the electrochemical capacity.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/coatings11101206/s1, Figure S1: The measured and calculated spectra of ellipsometric angles ψ (a) and ∆ (b) at an angle of incidence ϕ = 70 • for a Ta-O layer with a thickness of d = 46.3 nm, Figure S2: According to the ellipsometric angles, the real ε1 and imaginary ε2 parts of the complex dielectric function for the ALD Ta-O sample with the layer thickness d = 46 nm were calculated by Cauchy model. Comparison with the data from for Ta-O deposited layer (d = 48 nm) by evaporation of tantalum in atomic oxygen plasma. In both cases, there was a Si substrate, Figure S3. Ellipsometric angles ψ (a) and ∆ (b) for the TaO sample before and after annealing (800 • C) and the corresponding dependences of the real part of the dielectric function ε1 (c). Data Availability Statement: Data available on request due to restrictions on privacy. The data presented in this study are available on request from the corresponding author.