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Oxygen Pressure Influence on Properties of Nanocrystalline LiNbO3 Films Grown by Laser Ablation

Energy conversion devices draw much attention due to their effective usage of energy and resulting decrease in CO2 emissions, which slows down the global warming processes. Fabrication of energy conversion devices based on ferroelectric and piezoelectric lead-free films is complicated due to the difficulties associated with insufficient elaboration of growth methods. Most ferroelectric and piezoelectric materials (LiNbO3, BaTiO3, etc.) are multi-component oxides, which significantly complicates their integration with micro- and nanoelectronic technology. This paper reports the effect of the oxygen pressure on the properties of nanocrystalline lithium niobate (LiNbO3) films grown by pulsed laser deposition on SiO2/Si structures. We theoretically investigated the mechanisms of LiNbO3 dissociation at various oxygen pressures. The results of x-ray photoelectron spectroscopy study have shown that conditions for the formation of LiNbO3 films are created only at an oxygen pressure of 1 × 10−2 Torr. At low residual pressure (1 × 10−5 Torr), a lack of oxygen in the formed films leads to the formation of niobium oxide (Nb2O5) clusters. The presented theoretical and experimental results provide an enhanced understanding of the nanocrystalline LiNbO3 films growth with target parameters using pulsed laser deposition for the implementation of piezoelectric and photoelectric energy converters.


Introduction
Over the past few decades, the range of wireless wearable sensors and portable electronic devices has expanded significantly, and in most cases, their power supply is provided by electrochemical is carried out by calculating and analyzing the temperature dependencies of the change in Gibbs free energy considering deposition modes.

Thermodynamic Simulation
Laser ablation includes complex non-stationary processes: fast heating, overheating, and rapid nucleation. The description of the thermal mechanisms of laser ablation (surface evaporation, homogeneous boiling, and phase explosion) is a complex task and is accurately described in terms of non-equilibrium thermodynamics [36]. However, for preliminary theoretical estimates, we used equilibrium thermodynamics approaches based on the calculation of the Gibbs free energy temperature dependence.
It can be assumed that the ablated LiNbO 3 target can dissociate into individual components since the temperature at the interaction region of laser radiation with the target surface usually reaches several thousand degrees Celsius and significantly exceeds the LiNbO 3 melting temperature [37,38].
In order to study the processes related to the dissociation of LiNbO 3 , it is essential to determine possible dissociation reactions of lithium niobate by calculating and analyzing temperature dependences of change in Gibbs free energy (∆G) considering the nonlinear temperature dependences of the thermo-physical properties of materials [39]: where ∆H and ∆S -change of enthalpy [J/mol] and entropy [J/K] of a reaction, T -temperature [K]. The temperature dependences of the change in Gibbs free energy are calculated using the FactSage 6.2 software package for chemical reaction analysis (GTT-Technologies, Herzogenrath, Germany), which has a regularly updated electronic database of temperature dependences of the materials' thermophysical parameters. Calculating ∆G, we take into account not only the possibility of interaction between the components (for example, lithium and niobium oxides can interact with oxygen formed as a result of LiNbO 3 dissociation) but also the influence of background pressure in the growth chamber. Such calculations allow us to promptly evaluate the optimal window of partial oxygen pressures, as well as temperature.
In order to analyze the effect of oxygen pressure on the LiNbO 3 dissociation reactions, the following decomposition reactions of lithium niobate in a vacuum (1 × 10 −5 Torr) and oxygen atmosphere (1 × 10 −2 Torr) are identified: Stoichiometric coefficients in the equations of chemical reactions are taken into account, but omitted here, in order to simplify the perception of the results.

Experimental Methods
To synthesize LiNbO 3 films, we use the nanotechnological cluster complex NANOFAB NTK-9 (NT-MDT, Zelenograd, Russia), comprising the PLD module Pioneer 180 (Neocera Co., Beltsville, MD, USA). LiNbO 3 congruent target (Kurt J. Lasker, 99.9% purity) is ablated by excimer KrF laser (λ = 248 nm) (Coherent Inc., Santa Clara, CA, USA). Energy density on the target surface is maintained at 1.5 J/cm 2 . In all experiments, the target-substrate distance (100 mm), number of pulses (50,000), pulse repetition rate (10 Hz), and laser pulses energy on the target surface (150 mJ) are kept constant. Background oxygen pressure in the growth chamber varied from 1 × 10 −5 Torr to 1 × 10 −2 Torr. Films are obtained with a thickness of 45-90 nm at the heater temperature of 600 • C on SiO 2 (100 nm)/Si structures. The effect of the SiO 2 buffer layer thickness on the morphological parameters of LiNbO 3 films is presented in [40].
The morphology of the obtained films is studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM) in semi-contact mode using a Nova Nanolab 600 scanning electron microscope (FEI. Co., Eindhoven, the Netherlands) and a Ntegra probe nanolaboratory (NT-MDT, Zelenograd, Russia), respectively. The crystal structure and elemental composition of the obtained LiNbO 3 films are studied by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) using Rigaku MiniFlex 600 (Rigaku Co., Tokyo, Japan) and Kratos Axis Ultra X-ray Photoelectron Spectroscopy (XPS) instrument (Kratos Analytical Ltd., Manchester, UK), respectively. XPS spectra were analyzed using the OPUS 7.0 software (Bruker Co., Billerica, MA, USA). The charge carriers concentration and mobility are determined by measuring the Hall electric moving force using an Ecopia HMS-3000 measurement system (Ecopia Co., Anyang, Republic of Korea). The spectral dependencies of the optical characteristics (refractive index n and absorption coefficient k) are studied on spectral ellipsometer M-2000X (J.A. Woollam Co., Lincoln, NE, USA) under the beam angle of 65 • in the wavelength range from 240 nm to 1000 nm with 10-nm pitch. The spot size is about 2 × 5 mm. Figure 1 shows the temperature dependences ∆G of LiNbO 3 dissociation reactions in vacuum and oxygen atmosphere. The temperature range is determined by the temperatures of the laser plume and the substrate (maximum and minimum temperatures, respectively), based on the data presented in the literature [37] and the theoretical estimation of the laser plume parameters according to [41].  Analysis of the dependences has shown that the most probable dissociation reaction is (4) both for vacuum (1 × 10 −5 Torr) and oxygen atmosphere (1 × 10 −2 Torr), which occurs at temperatures above 2113 K and 2533 K, respectively. Dissociation of LiNbO 3 into individual elements (7) is possible when the temperature increased to 5443 K (in a vacuum).

Theoretical Results
The ∆G value of the remaining reactions is positive in the entire temperature range at an oxygen pressure of 1 × 10 −2 Torr, hence the forward direction of the reaction is impossible in the temperature range from 773 K to 8773 K. Figure 2 shows temperature dependences ∆G of (4) and (7) at various oxygen pressures. As a result of the analysis of thermodynamic regularities, it is found that the LiNbO3 dissociation is a multi-stage process, depending on temperature and the value of oxygen pressure. At the first stage, lithium niobate dissociates into oxides with lower oxides (Li2O, Nb2O5). According to ΔG analysis of Li2O, Nb2O5 decomposition reactions show that the oxides decompose and completely dissociate into Li, Nb, and O2 at temperatures above 2050 K [42]. At pressure 1 × 10 −2 Torr, (7) becomes impossible, and LiNbO3 decompose into Li2O and Nb2O5 according to (4) [43]. With the subsequent propagation of laser plume toward the substrate, its temperature decreases, and the conditions for reverse reactions of Li and Nb interaction with O2 are created, as well as the formation of their oxides and lithium niobate. Figure 3 shows the dependence of the LiNbO3 film thickness on oxygen pressure measured by different methods. We applied three mutually independent methods to measure the thickness of LiNbO3 films: (1) the focused ion beam cut; (2) liquid etching [44]; (3) spectral ellipsometry [45]. In addition, with increasing oxygen pressure in the growth chamber, the character of the plasma interaction in the laser plume changes, which causes phase formation and mass transfer during PLD [46]. A decrease As a result of the analysis of thermodynamic regularities, it is found that the LiNbO 3 dissociation is a multi-stage process, depending on temperature and the value of oxygen pressure. At the first stage, lithium niobate dissociates into oxides with lower oxides (Li 2 O, Nb 2 O 5 ). According to ∆G analysis of Li 2 O, Nb 2 O 5 decomposition reactions show that the oxides decompose and completely dissociate into Li, Nb, and O 2 at temperatures above 2050 K [42]. At pressure 1 × 10 −2 Torr, (7) becomes impossible, and LiNbO 3 decompose into Li 2 O and Nb 2 O 5 according to (4) [43]. With the subsequent propagation of laser plume toward the substrate, its temperature decreases, and the conditions for reverse reactions of Li and Nb interaction with O 2 are created, as well as the formation of their oxides and lithium niobate. As a result of the analysis of thermodynamic regularities, it is found that the LiNbO3 dissociation is a multi-stage process, depending on temperature and the value of oxygen pressure. At the first stage, lithium niobate dissociates into oxides with lower oxides (Li2O, Nb2O5). According to ΔG analysis of Li2O, Nb2O5 decomposition reactions show that the oxides decompose and completely dissociate into Li, Nb, and O2 at temperatures above 2050 K [42]. At pressure 1 × 10 −2 Torr, (7) becomes impossible, and LiNbO3 decompose into Li2O and Nb2O5 according to (4) [43]. With the subsequent propagation of laser plume toward the substrate, its temperature decreases, and the conditions for reverse reactions of Li and Nb interaction with O2 are created, as well as the formation of their oxides and lithium niobate. Figure 3 shows the dependence of the LiNbO3 film thickness on oxygen pressure measured by different methods. We applied three mutually independent methods to measure the thickness of LiNbO3 films: (1) the focused ion beam cut; (2) liquid etching [44]; (3) spectral ellipsometry [45]. In addition, with increasing oxygen pressure in the growth chamber, the character of the plasma interaction in the laser plume changes, which causes phase formation and mass transfer during PLD [46]. A decrease We applied three mutually independent methods to measure the thickness of LiNbO 3 films: (1) the focused ion beam cut; (2) liquid etching [44]; (3) spectral ellipsometry [45]. In addition, with increasing oxygen pressure in the growth chamber, the character of the plasma interaction in the laser plume changes, which causes phase formation and mass transfer during PLD [46]. A decrease in the film growth rate might be associated with decreasing of the ablated particles mean free path in the transit space under increasing background pressure in the growth chamber. The experimental results show that the thickness of LiNbO 3 films decreases from 80.7 ± 7.8 nm (film deposition rate 0.9 nm/min) to 48.57 ± 3.5 nm (film deposition rate 0.54 nm/min) with the increase of oxygen pressure from 1 × 10 −5 Torr to 1 × 10 −2 Torr (Figure 3).

Experimental Results
The films obtained at residual and oxygen pressures of 1 × 10 −5 Torr and 1 × 10 −2 Torr, respectively, are chosen for XPS studies since they are characterized by a change in the mechanism of LiNbO 3 dissociation ( Figure 2).
The results of XRD analysis show that all obtained films have the nanocrystalline structure with the predominance of crystallites oriented in the (012), (110), and (024) planes ( Figure 4). Figure 4 shows a comparison of the XRD spectra of the films deposited at an oxygen pressure of 1 × 10 −2 Torr and 1 × 10 −5 Torr.
Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 12 in the film growth rate might be associated with decreasing of the ablated particles mean free path in the transit space under increasing background pressure in the growth chamber. The experimental results show that the thickness of LiNbO3 films decreases from 80.7 ± 7.8 nm (film deposition rate 0.9 nm/min) to 48.57 ± 3.5 nm (film deposition rate 0.54 nm/min) with the increase of oxygen pressure from 1 × 10 −5 Torr to 1 × 10 −2 Torr (Figure 3). The films obtained at residual and oxygen pressures of 1 × 10 −5 Torr and 1 × 10 −2 Torr, respectively, are chosen for XPS studies since they are characterized by a change in the mechanism of LiNbO3 dissociation ( Figure 2).
The results of XRD analysis show that all obtained films have the nanocrystalline structure with the predominance of crystallites oriented in the (012), (110), and (024) planes ( Figure 4). Figure 4 shows a comparison of the XRD spectra of the films deposited at an oxygen pressure of 1 × 10 −2 Torr and 1 × 10 −5 Torr. We excluded angles higher than 60° since peaks in that region are attributed to the substrate. In the case of the higher pressure (1 × 10 −2 Torr), two distinct reflections corresponding to the (012) and (110) crystal planes in the range from 15° to 60°. Moreover, this film has a single-phase structure, identified as the ferroelectric structure of bulk material from the group (R3c) LiNbO3 [47]. In comparison, lower pressure (1 × 10 −5 Torr) sample shows peaks that correspond to lithium and niobium oxides while showing no presence of LiNbO3.
In order to study the chemical bonds of the grown films, XPS analysis has been used. Figure 5c shows the XPS survey spectra of LiNbO3 films. The obtained spectra show lines corresponding to Li 1s, Nb 4s, Nb 3d, Nb 3p, and O 1s bonds [48]. Figure 5 and 6 show the high-resolution XPS spectra of Li 1s and Nb 4s as well as Nb 3d lines for the grown LiNbO3 films. In the range of binding energies from 50 to 62 eV and from 202 to 214 eV, respectively, Li 1s, Nb 4s, and Nb 3d 3/2, Nb 3d 5/2 peaks are identified. To define various states of lithium and niobium atoms the peaks were decomposed by Gaussian functions [49][50][51]. XPS peaks from Nb 3d are decomposed into 3d 3/2 and 3d 5/ The analysis of XPS spectra in the range from 50 eV to 62 eV shows that under residual pressure of 1 × 10 −5 Torr (Figure 5b), the spectrum had only one Li 1s peak (55.2 eV), which attributes to Li2CO3. The Nb 4s peak corresponding to the LiNbO3 phase was absent. The peaks of Nb 3d 3/2 and Nb 3d 5/2 (Figure 6b) had maximum energy of 207.055 eV and 209.869 eV, respectively, which corresponds to Nb2O5 chemical bonds. The peaks corresponding to NbO, NbO2, and LiNbO3 are not detected.
The position of the peaks changed when oxygen is added during the film deposition (pressure 1 We excluded angles higher than 60 • since peaks in that region are attributed to the substrate. In the case of the higher pressure (1 × 10 −2 Torr), two distinct reflections corresponding to the (012) and (110) crystal planes in the range from 15 • to 60 • . Moreover, this film has a single-phase structure, identified as the ferroelectric structure of bulk material from the group (R3c) LiNbO 3 [47]. In comparison, lower pressure (1 × 10 −5 Torr) sample shows peaks that correspond to lithium and niobium oxides while showing no presence of LiNbO 3 .
In order to study the chemical bonds of the grown films, XPS analysis has been used. Figure 5c shows the XPS survey spectra of LiNbO 3 films. The obtained spectra show lines corresponding to Li 1s, Nb 4s, Nb 3d, Nb 3p, and O 1s bonds [48]. Figures 5 and 6 show the high-resolution XPS spectra of Li 1s and Nb 4s as well as Nb 3d lines for the grown LiNbO 3 films. In the range of binding energies from 50 to 62 eV and from 202 to 214 eV, respectively, Li 1s, Nb 4s, and Nb 3d 3/2, Nb 3d 5/2 peaks are identified. To define various states of lithium and niobium atoms the peaks were decomposed by Gaussian functions [49][50][51]. XPS peaks from Nb 3d are decomposed into 3d 3/2 and 3d 5/ The analysis of XPS spectra in the range from 50 eV to 62 eV shows that under residual pressure of 1 × 10 −5 Torr (Figure 5b), the spectrum had only one Li 1s peak (55.2 eV), which attributes to Li 2 CO 3 .
The Nb 4s peak corresponding to the LiNbO 3 phase was absent. The peaks of Nb 3d 3/2 and Nb 3d 5/2 ( Figure 6b) had maximum energy of 207.055 eV and 209.869 eV, respectively, which corresponds to Nb 2 O 5 chemical bonds. The peaks corresponding to NbO, NbO 2 , and LiNbO 3 are not detected.
The position of the peaks changed when oxygen is added during the film deposition (pressure 1 × 10 −2 Torr). At this pressure, films show XPS peaks at 206.89 eV and 209.635 eV corresponding to LiNbO 3 bonds while no peaks attributed to other types of bonds are detected. In the range of binding energies from 50 to 62 eV, one can see two peaks Li 1s (54.8 eV) and Nb 4s (60.2 eV), which corresponds to LiNbO 3 bonds (Figure 5a) [49][50][51]. Similarly, the peaks of Nb 3d 3/2 and Nb 3d 5/2 ( Figure 6a) had a maximum of 206.89 eV and 209.635 eV, which corresponds to the binding energy to LiNbO 3 bonds. Based on the results of XPS analysis, we can conclude that the films grown at 1 × 10 −5 Torr form a mixture of niobium oxide Nb2O5 and Li2CO3, which is associated with a lack of oxygen in the deposited film. This fact is confirmed by the results of SEM and AFM studies (Figure 7 and 8). The Based on the results of XPS analysis, we can conclude that the films grown at 1 × 10 −5 Torr form a mixture of niobium oxide Nb2O5 and Li2CO3, which is associated with a lack of oxygen in the deposited film. This fact is confirmed by the results of SEM and AFM studies (Figure 7 and 8). The decomposition of the multi-component material into individual atoms takes place during the ablation of LiNbO3 in the laser plume (7), and part of the oxygen atoms are pumped out by the Based on the results of XPS analysis, we can conclude that the films grown at 1 × 10 −5 Torr form a mixture of niobium oxide Nb 2 O 5 and Li 2 CO 3 , which is associated with a lack of oxygen in the deposited film. This fact is confirmed by the results of SEM and AFM studies (Figures 7 and 8). The decomposition of the multi-component material into individual atoms takes place during the ablation of LiNbO 3 in the laser plume (7), and part of the oxygen atoms are pumped out by the pumping system. The lack of oxygen in the deposited film is compensated for, when oxygen is added to the chamber (pressure 1 × 10 −2 Torr), which leads to the formation of a single-phase structure of LiNbO 3 . Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 12 pumping system. The lack of oxygen in the deposited film is compensated for, when oxygen is added to the chamber (pressure 1 × 10 −2 Torr), which leads to the formation of a single-phase structure of LiNbO3.
(a) (b) It is established that the diameter of clusters on the surface of the film obtained at a residual pressure of 1 × 10 −5 Torr is 92 ± 7.4 nm. With increasing oxygen pressure from 1 × 10 −5 Torr to 1 × 10 −2 Torr, the average roughness of the obtained films decreasing from 4.75 nm to 4.58 nm. Teardrop-shaped structures on the surface of the obtained films were identified as Nb2O5 [52,53]. Figure 9 shows the dependences of concentration and charge carrier mobility of LiNbO3 films as Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 12 pumping system. The lack of oxygen in the deposited film is compensated for, when oxygen is added to the chamber (pressure 1 × 10 −2 Torr), which leads to the formation of a single-phase structure of LiNbO3.
(a) (b) It is established that the diameter of clusters on the surface of the film obtained at a residual pressure of 1 × 10 −5 Torr is 92 ± 7.4 nm. With increasing oxygen pressure from 1 × 10 −5 Torr to 1 × 10 −2 Torr, the average roughness of the obtained films decreasing from 4.75 nm to 4.58 nm. Teardrop-shaped structures on the surface of the obtained films were identified as Nb2O5 [52,53]. Figure 9 shows the dependences of concentration and charge carrier mobility of LiNbO3 films as It is established that the diameter of clusters on the surface of the film obtained at a residual pressure of 1 × 10 −5 Torr is 92 ± 7.4 nm. With increasing oxygen pressure from 1 × 10 −5 Torr to 1 × 10 −2 Torr, the average roughness of the obtained films decreasing from 4.75 nm to 4.58 nm. Teardrop-shaped structures on the surface of the obtained films were identified as Nb 2 O 5 [52,53]. Figure 9 shows the dependences of concentration and charge carrier mobility of LiNbO 3 films as a function of oxygen pressure. Increasing oxygen pressure from 1 × 10 −5 Torr to 1 × 10 −2 Torr results in decreasing of charge carrier concentration in the range from 1.4 × 10 15 cm −3 to 9.7 × 10 11 cm −3 . In contrast, the mobility of charge carriers increased from 4.7 cm 2 /(V•s) to 16 cm 2 /(V•s). It is assumed that the electron mobility can change with changing the stoichiometry of LiNbO3: congruent LiNbO3 (Li to Nb ratio of about 94%) has lower electron mobility than a perfectly stoichiometric crystal (Li to Nb ratio is 1) [54]. This effect can be associated with changes in the phase composition of LiNbO3 films, as well as a decrease in the content of metallic Li and the defectiveness of the films, which is confirmed by the results of XPS, SEM, and AFM studies (Figures 5-8). Figure 10 shows typical spectral dependences of the optical constants of LiNbO3 films on wavelength. The Tautz-Lorentz model [55] (which applies both to dielectrics and semiconductors) is used for modeling the optical characteristics of the films. Obtained optical characteristics satisfy the Kramers-Kronig relations [56], and the film thicknesses are similar to the data obtained by the focused ion beam cut and liquid etching. It was found that the measurement results do not depend on the orientation of the samples, which indicates the isotropic nature of the optical characteristics of the obtained films. In the visible wavelength range, the refractive index decreases from 2.63 (at 350 nm) to 1.95 (at 800 nm). The absorption coefficient does not exceed 0.01. There is a slight decrease in the refractive index and a sharp increase in the absorption coefficient to 0.97 in the near-ultraviolet region of the spectrum. Increasing oxygen pressure from 1 × 10 −5 Torr to 1 × 10 −2 Torr results in decreasing of charge carrier concentration in the range from 1.4 × 10 15 cm −3 to 9.7 × 10 11 cm −3 . In contrast, the mobility of charge carriers increased from 4.7 cm 2 /(V·s) to 16 cm 2 /(V·s). It is assumed that the electron mobility can change with changing the stoichiometry of LiNbO 3 : congruent LiNbO 3 (Li to Nb ratio of about 94%) has lower electron mobility than a perfectly stoichiometric crystal (Li to Nb ratio is 1) [54]. This effect can be associated with changes in the phase composition of LiNbO 3 films, as well as a decrease in the content of metallic Li and the defectiveness of the films, which is confirmed by the results of XPS, SEM, and AFM studies (Figures 5-8). Figure 10 shows typical spectral dependences of the optical constants of LiNbO 3 films on wavelength. The Tautz-Lorentz model [55] (which applies both to dielectrics and semiconductors) is used for modeling the optical characteristics of the films. Increasing oxygen pressure from 1 × 10 −5 Torr to 1 × 10 −2 Torr results in decreasing of charge carrier concentration in the range from 1.4 × 10 15 cm −3 to 9.7 × 10 11 cm −3 . In contrast, the mobility of charge carriers increased from 4.7 cm 2 /(V•s) to 16 cm 2 /(V•s). It is assumed that the electron mobility can change with changing the stoichiometry of LiNbO3: congruent LiNbO3 (Li to Nb ratio of about 94%) has lower electron mobility than a perfectly stoichiometric crystal (Li to Nb ratio is 1) [54]. This effect can be associated with changes in the phase composition of LiNbO3 films, as well as a decrease in the content of metallic Li and the defectiveness of the films, which is confirmed by the results of XPS, SEM, and AFM studies (Figures 5-8). Figure 10 shows typical spectral dependences of the optical constants of LiNbO3 films on wavelength. The Tautz-Lorentz model [55] (which applies both to dielectrics and semiconductors) is used for modeling the optical characteristics of the films. Obtained optical characteristics satisfy the Kramers-Kronig relations [56], and the film thicknesses are similar to the data obtained by the focused ion beam cut and liquid etching. It was found that the measurement results do not depend on the orientation of the samples, which indicates the isotropic nature of the optical characteristics of the obtained films. In the visible wavelength range, the refractive index decreases from 2.63 (at 350 nm) to 1.95 (at 800 nm). The absorption coefficient does not exceed 0.01. There is a slight decrease in the refractive index and a sharp increase in the absorption coefficient to 0.97 in the near-ultraviolet region of the spectrum. Obtained optical characteristics satisfy the Kramers-Kronig relations [56], and the film thicknesses are similar to the data obtained by the focused ion beam cut and liquid etching. It was found that the measurement results do not depend on the orientation of the samples, which indicates the isotropic nature of the optical characteristics of the obtained films. In the visible wavelength range, the refractive index decreases from 2.63 (at 350 nm) to 1.95 (at 800 nm). The absorption coefficient does not exceed 0.01. There is a slight decrease in the refractive index and a sharp increase in the absorption coefficient to 0.97 in the near-ultraviolet region of the spectrum.

Conclusions
Studies of the properties of LiNbO 3 films grown by the PLD show that increasing oxygen pressure in the growth chamber has a significant effect on target dissociation mechanism, structure, composition, and properties of the deposited films. The results obtained by the theoretical assessment of thermodynamic processes show good agreement with the experimental data in the considered window of partial oxygen pressures and temperatures.
Analysis of XPS spectra shows that the formation of LiNbO 3 films is possible at an oxygen pressure of 1 × 10 −2 Torr. The films grown at residual pressure 1 × 10 −5 Torr do not contain sufficient oxygen to form LiNbO 3 , which leads to the formation of Nb 2 O 5 clusters on the films' surfaces.
It was discovered that the structure of the films becomes more fine-grained, and the mobility of charge carriers increases from 4 cm 2 /V·s to 16 cm 2 /V·s with the increase of oxygen pressure from 1 × 10 −5 Torr to 1 × 10 −2 Torr. The refractive index of the obtained films ranges from 1.95 to 2.05 depending on the wavelength (60-800 nm), and the absorption index does not exceed 0.01.
The study shows the possibility of fabrication of LiNbO 3 films with target properties by PLD. The obtained theoretical and experimental results make it possible to get LiNbO 3 films that can be used for the fabrication of promising lead-free energy converters for "green" energy devices.