High Oxygen Sensitivity of TiO2 Thin Films Deposited by ALD

The gas sensitivity and structural properties of TiO2 thin films deposited by plasma-enhanced atomic layer deposition (ALD) were examined in detail. The TiO2 thin films are deposited using Tetrakis(dimethylamido)titanium(IV) and oxygen plasma at 300 °C on SiO2 substrates followed by annealing at temperatures of 800 °C. Gas sensitivity under exposure to O2 within the temperature range from 30 °C to 700 °C was studied. The ALD-deposited TiO2 thin films demonstrated high responses to O2 in the dynamic range from 0.1 to 100 vol. % and low concentrations of H2, NO2. The ALD deposition allowed the enhancement of sensitivity of TiO2 thin films to gases. The greatest response of TiO2 thin films to O2 was observed at a temperature of 500 °C and was 41.5 arb. un. under exposure to 10 vol. % of O2. The responses of TiO2 thin films to 0.1 vol. % of H2 and 7 × 10–4 vol. % of NO2 at a temperature of 500 °C were 10.49 arb. un. and 10.79 arb. un., correspondingly. The resistance of the films increased due to the chemisorption of oxygen molecules on their surface that decreased the thickness of the conduction channel between the metal contacts. It was suggested that there are two types of adsorption centers on the TiO2 thin films surface: oxygen is chemisorbed in the form of O2– on the first one and O– on the second one.


Introduction
Oxygen is the most important gas for human life, and there is a widespread demand for O 2 sensors and for measuring the O 2 concentration in ambient environment.Oxygen detection at an over 1 vol.% level with a high accuracy is essential to control the reactive chemical concentration in the chemical industry and metallurgy [1] and to analyze exhaust gas composition of automobile engines [2,3].TiO 2 belongs to the large material class of metal oxide semiconductors [4][5][6][7][8][9][10].It is attractive for developing O 2 sensors due to its low cost and high chemical and thermal stability [11][12][13][14][15][16][17][18][19][20][21].Now, commercial O 2 sensors are based on bulk and thick-film TiO 2 structures [6].Such structures are not highly sensitive to O 2 .A well-known method for optimizing gas-sensitive properties of materials is the use of thin film structures [16].Film thickness d plays a key role in the gas sensitivity of thin film structures.It has been shown that the optimal thickness of TiO 2 films providing a high gas sensitivity should be comparable to the Debye length L D .The L D = 10-50 nm for TiO 2 at an electron concentration of n = 10 16 -10 18 cm −3 and a permittivity of ε 0 = 18.9.Atomic layer deposition (ALD) is a highly promising method for growing very thin TiO 2 films (d = 10-50 nm) with large homogeneity and reproducibility of structural and electrical properties.The ALD allows deposition of continuous films with high precision control over thickness and impurities levels [21,22].
Previously, the ALD method was used to produce thin films structures of various metal oxide semiconductors, mainly based on SnO 2 (see Table 1).At the same time, the gas-sensitive properties of such structures were studied under the exposure to low concentrations of toxic gases and H 2 .Detailed studies on the O 2 sensitivity of ALDdeposited metal oxide films have not been practically carried out.Therefore, the purpose of this work is to gain a deep insight into the gas-sensitive properties of the ALD-TiO 2 thin films under O 2 exposure and to explain them by proposing a theoretical model.[32] p-TiO 2 70 NO 10 RT 1.244 [33] P3HT/ZnO NWs -NH 3 5 RT 1.35 [34] In Table 1, c g is the gas concentration; T is the operating temperature; S is the response; CNT is the carbon nanotubes; QDs is the quantum dots; IGZO is the indium gallium zinc oxide; NSs is the nanospheres; NShs is the nanosheets; P3HT is the poly(3-hexylthiophene); NWs is the nanowires; and RT is the room temperature.

Materials and Methods
TiO 2 thin films were fabricated by the plasma-enhanced ALD technique using FlexAL ALD equipment (Oxford Instruments, Abingdon, UK).Thermally oxidized silicon plates (SiO 2 /Si) were used as substrates.Tetrakis(dimethylamido)titanium(IV) (TDMAT) [(CH 3 ) 2 N] 4 Ti (99.999%) (Sigma-Aldrich, St. Louis, MO, USA) was used as the metal precursor with carrier gas of Ar (99.999%) at a flow rate of 200 cm 3 /min.Oxygen inductively coupled plasma (ICP) was used as an oxidizer.The discharge was excited in an oxygen atmosphere (99.999%) by a generator with a frequency of 13.56 MHz and a power of up to 300 W. The PEALD pulse durations were set at 0.8 s for TDMAT injection, 3 s for Ar purge, 3 s for exposure to plasma discharge, and 2 s for Ar purge.The growth rate at a temperature of 300 • C was 0.09 nm/cycle.The thickness of the deposited TiO 2 films was 30 nm.A SENTECH Senduro spectral ellipsometer was used to estimate the thickness and the growth rate of TiO 2 films at measurements in the wavelength range of 320-1800 nm.
The as-deposited TiO 2 thin films were annealed at a temperature of 800 • C in an Ar atmosphere at a pressure of 2 kPa for 30 min.The rates of heating from RT to 800 • C and cooling from 800 • C to RT were 4 • C/min.The heating and cooling of samples were in an Ar atmosphere at a pressure of 2 kPa.
X-ray diffraction (XRD) was performed to determine the phase composition of the thin films and the crystal lattice parameters.XRD spectra of films were measured in a 3 of 14 2θ scanning mode employing a CuK a radiation operated at 45 kV and 40 mA.The X-ray source wavelength was 1.5406 Å.The microrelief of the film surface was studied by a Bruker Dimension Icon atomic force microscope (AFM).Cross-sectional images of the annealed samples were examined by a Jeol JEM 2100 PLUS transmission electron microscope (TEM) at an accelerating voltage of 200 kV in a bright field (BF) mode.The elemental composition of the films was determined by the BF-TEM mode by means of a JEOL EX-24261M1G5T energy dispersive X-ray spectroscopy (EDX) analyzer at a beam current of 1 nA.
To investigate the gas-sensitive properties, Pt contacts were deposited on the TiO 2 film surface by means of vacuum deposition through a shadow mask.The plate with the film and contacts was divided into separate samples.The prepared samples were planar metal-semiconductor-metal (MSM) structures on SiO 2 /Si substrates (Figure 1).The interelectrode distance was kept at 1 mm.The thickness of the Pt contacts was about 330 nm.
Micromachines 2023, 14, 1875 3 of 15 X-ray diffraction (XRD) was performed to determine the phase composition of the thin films and the crystal lattice parameters.XRD spectra of films were measured in a 2θ scanning mode employing a CuKa radiation operated at 45 kV and 40 mA.The X-ray source wavelength was 1.5406 Å.The microrelief of the film surface was studied by a Bruker Dimension Icon atomic force microscope (AFM).Cross-sectional images of the annealed samples were examined by a Jeol JEM 2100 PLUS transmission electron microscope (TEM) at an accelerating voltage of 200 kV in a bright field (BF) mode.The elemental composition of the films was determined by the BF-TEM mode by means of a JEOL EX-24261M1G5T energy dispersive X-ray spectroscopy (EDX) analyzer at a beam current of 1 nA.
To investigate the gas-sensitive properties, Pt contacts were deposited on the TiO2 film surface by means of vacuum deposition through a shadow mask.The plate with the film and contacts was divided into separate samples.The prepared samples were planar metal-semiconductor-metal (MSM) structures on SiO2/Si substrates (Figure 1).The interelectrode distance was kept at 1 mm.The thickness of the Pt contacts was about 330 nm.The current-voltage (I-V) characteristics and time dependences of the sample's resistance under the exposure to various gases were measured by means of a Keithley 2636A source meter and a sealed chamber with a Nextron MPS-CHH micro-probe station.A ceramic-type heater, installed in the sealed chamber, was used to heat the samples from RT to 700 °C with a temperature accuracy control of ±0.1 °C.The measurements were carried out under dark conditions and in a flow of dry N2, or in a gas mixture of dry N2 + dry O2.The flow rate of gas mixtures through the measurement chamber was maintained at 500 cm 3 /min.A pure dry air or a gas mixture of pure dry air + target gas was pumped through the chamber to examine the selectivity of the samples studied.H2, CO, CO2, NO2, NO and CH4 were selected as target gases.The source of pure dry air was a special generator.The concentration of the target gas in the mixture was controlled by a gas mixture generator with a Bronkhorst gas mass flow controller.The relative error of the gas flow rate did not exceed 1.5%.The voltage U applied to the samples during the measurements of time dependences of the resistance was kept at 3 V.  204), (116) and (220) Bragg reflections of the tetragonal anatase TiO2 phase (ICDD 00-021-1272), respectively.These results confirm the polycrystalline nature of the material grown.The wide amorphous halo at 2θ ≈ 22° is due to the SiO2 layer.The parameters of the tetragonal crystalline lattice of the film are determined as a = 3.78 Å and c = 9.50 Å.

Structural Properties of the ALD-Deposited TiO2 Thin Films
Figure 3 depicts typical annealed ALD-TiO2 film surface morphology images taken by AFM.The surface roughness parameters of the TiO2 thin films are Ra = 1.329 nm, Rq = 1.605 nm and Rz = 12.67 nm, where Ra is the arithmetic mean of the absolute values of the deviations of the film surface profile; Rq is the mean square value of the deviations of the film surface profile and Rz is the arithmetic mean of the greatest height of the profile of The current-voltage (I-V) characteristics and time dependences of the sample's resistance under the exposure to various gases were measured by means of a Keithley 2636A source meter and a sealed chamber with a Nextron MPS-CHH micro-probe station.A ceramic-type heater, installed in the sealed chamber, was used to heat the samples from RT to 700 • C with a temperature accuracy control of ±0.1 • C. The measurements were carried out under dark conditions and in a flow of dry N 2 , or in a gas mixture of dry N 2 + dry O 2 .The flow rate of gas mixtures through the measurement chamber was maintained at 500 cm 3 /min.A pure dry air or a gas mixture of pure dry air + target gas was pumped through the chamber to examine the selectivity of the samples studied.H 2 , CO, CO 2 , NO 2 , NO and CH 4 were selected as target gases.The source of pure dry air was a special generator.The concentration of the target gas in the mixture was controlled by a gas mixture generator with a Bronkhorst gas mass flow controller.The relative error of the gas flow rate did not exceed 1.5%.The voltage U applied to the samples during the measurements of time dependences of the resistance was kept at 3 V.  116) and (220) Bragg reflections of the tetragonal anatase TiO 2 phase (ICDD 00-021-1272), respectively.These results confirm the polycrystalline nature of the material grown.The wide amorphous halo at 2θ ≈ 22 • is due to the SiO 2 layer.The parameters of the tetragonal crystalline lattice of the film are determined as a = 3.78 Å and c = 9.50 Å.

Structural Properties of the ALD-Deposited TiO 2 Thin Films
Figure 3 depicts typical annealed ALD-TiO 2 film surface morphology images taken by AFM.The surface roughness parameters of the TiO 2 thin films are R a = 1.329 nm, R q = 1.605 nm and R z = 12.67 nm, where R a is the arithmetic mean of the absolute values of the deviations of the film surface profile; R q is the mean square value of the deviations of the film surface profile and R z is the arithmetic mean of the greatest height of the profile of the film surface.R q is lower than the value reported in ref. [21] devoted to ALD-deposited TiO 2 thin films.The relatively high surface roughness should lead to an increase in the surface-to-volume ratio and the surface density of adsorption centers for gas molecules, and, as a result of this, to an increase in responses to gases [35].
Micromachines 2023, 14, 1875 4 of 15 the film surface.Rq is lower than the value reported in ref. [21] devoted to ALD-deposited TiO2 thin films.The relatively high surface roughness should lead to an increase in the surface-to-volume ratio and the surface density of adsorption centers for gas molecules, and, as a result of this, to an increase in responses to gases [35].the film surface.Rq is lower than the value reported in ref. [21] devoted to ALD-deposited TiO2 thin films.The relatively high surface roughness should lead to an increase in the surface-to-volume ratio and the surface density of adsorption centers for gas molecules, and, as a result of this, to an increase in responses to gases [35].the film surface.Rq is lower than the value reported in ref. [21] devoted to ALD-deposited TiO2 thin films.The relatively high surface roughness should lead to an increase in the surface-to-volume ratio and the surface density of adsorption centers for gas molecules, and, as a result of this, to an increase in responses to gases [35].The contents of Ti and O elements in the films are measured to be ~27 at.% and ~73 at.% (Figure 5a,b), respectively.The increased O content in the films may be associated with features of the ALD process.There is also a peak corresponding to C in the EDX spectrum caused by the technological operations before the measurements of the spectrum.
Micromachines 2023, 14, 1875 5 of 15 The contents of Ti and O elements in the films are measured to be ~27 at.% and ~73 at.% (Figure 5a,b), respectively.The increased O content in the films may be associated with features of the ALD process.There is also a peak corresponding to C in the EDX spectrum caused by the technological operations before the measurements of the spectrum.

Gas-Sensetive Properties of the ALD-Deposited TiO2 Thin Films
At the next stage, the gas-sensitive properties of the ALD synthesized TiO2 thin films were investigated in detail.The exposure to O2 led to a reversible increase in the resistance of TiO2 thin films.The following ratio was chosen as the response Sox of samples to O2: where Rox is the resistance of TiO2 thin film in a gas mixture of dry N2 + dry O2; RN is the resistance of TiO2 thin film in dry N2 atmosphere.The temperature dependencies of the responses under the exposure to 10 vol.% and 40 vol.% of O2 (Figure 6a) had a maximum at T = 500 °C.

Gas-Sensetive Properties of the ALD-Deposited TiO 2 Thin Films
At the next stage, the gas-sensitive properties of the ALD synthesized TiO 2 thin films were investigated in detail.The exposure to O 2 led to a reversible increase in the resistance of TiO 2 thin films.The following ratio was chosen as the response S ox of samples to O 2 : where R ox is the resistance of TiO 2 thin film in a gas mixture of dry N 2 + dry O 2 ; R N is the resistance of TiO 2 thin film in dry N 2 atmosphere.The temperature dependencies of the responses under the exposure to 10 vol.% and 40 vol.% of O 2 (Figure 6a) had a maximum at T = 500 • C.
Micromachines 2023, 14, 1875 5 of 15 The contents of Ti and O elements in the films are measured to be ~27 at.% and ~73 at.% (Figure 5a,b), respectively.The increased O content in the films may be associated with features of the ALD process.There is also a peak corresponding to C in the EDX spectrum caused by the technological operations before the measurements of the spectrum.

Gas-Sensetive Properties of the ALD-Deposited TiO2 Thin Films
At the next stage, the gas-sensitive properties of the ALD synthesized TiO2 thin films were investigated in detail.The exposure to O2 led to a reversible increase in the resistance of TiO2 thin films.The following ratio was chosen as the response Sox of samples to O2: where Rox is the resistance of TiO2 thin film in a gas mixture of dry N2 + dry O2; RN is the resistance of TiO2 thin film in dry N2 atmosphere.The temperature dependencies of the responses under the exposure to 10 vol.% and 40 vol.% of O2 (Figure 6a) had a maximum at T = 500 °C.The samples practically did not react when exposed to O 2 and had a high resistance, making it impossible to reliably register the response to gases at T < 450 • C. The presence of a maximum on the temperature dependence of the response is due to the influence of temperature on the processes of dissociation, adsorption/desorption of O 2 molecules, and is specific to metal oxide semiconductors [36,37].The response t res and recovery t rec times were determined upon exposure to 10 vol.% of O 2 according to the method described in ref. [38] to estimate the operation speed of the films studied.The obtained values of t res and t rec can only be used to compare the operation speed of sensors under similar experimental conditions.We note that the t res and t rec decrease exponentially with the increase in T (see Figure 6b).It is also worth noting that t res and t rec were practically the same at T = 450-700 • C. The t res and t rec did not exceed 30 s in the range of T = 600-700 • C. The t res and t rec were 51.5 s and 52.9 s, respectively, at temperature of the maximum response to O 2 .
R ox and R N decreased by 4% and 36%, correspondingly, during a cyclic exposure to 10 vol.% of O 2 (five cycles) (illustrated in Figure 7a); as a result, S ox increased by 50%.On the other hand, the response of the films to O 2 decreased by 6-7 times during storing in a sealed box at RT after the experiments at high T (Figure 7b) mainly due to a significant increase in R N .The healing of oxygen vacancies in TiO 2 may be the reason for the increase in R N at high T and exposure to high O 2 concentrations [39].This process is inertial and, consequently, manifested during prolonged testing of samples.To further stabilize the gas-sensitive properties of the films, doping with metal additives should be applied [40].The samples practically did not react when exposed to O2 and had a high resistance, making it impossible to reliably register the response to gases at T < 450 °C.The presence of a maximum on the temperature dependence of the response is due to the influence of temperature on the processes of dissociation, adsorption/desorption of O2 molecules, and is specific to metal oxide semiconductors [36,37].The response tres and recovery trec times were determined upon exposure to 10 vol.% of O2 according to the method described in ref. [38] to estimate the operation speed of the films studied.The obtained values of tres and trec can only be used to compare the operation speed of sensors under similar experimental conditions.We note that the tres and trec decrease exponentially with the increase in T (see Figure 6b).It is also worth noting that tres and trec were practically the same at T = 450-700 °C.The tres and trec did not exceed 30 s in the range of T = 600-700 °C.The tres and trec were 51.5 s and 52.9 s, respectively, at temperature of the maximum response to O2.
Rox and RN decreased by 4% and 36%, correspondingly, during a cyclic exposure to 10 vol.% of O2 (five cycles) (illustrated in Figure 7a); as a result, Sox increased by 50%.On the other hand, the response of the films to O2 decreased by 6-7 times during storing in a sealed box at RT after the experiments at high T (Figure 7b) mainly due to a significant increase in RN.The healing of oxygen vacancies in TiO2 may be the reason for the increase in RN at high T and exposure to high O2 concentrations [39].This process is inertial and, consequently, manifested during prolonged testing of samples.To further stabilize the gas-sensitive properties of the films, doping with metal additives should be applied [40].The rise of resistance R of the TiO2 thin film under the exposure to O2 and the drop of resistance after this exposure were approximated by the following functions, respectively: where t is time; R st ox is the stationary resistance of TiO2 thin films in a gas mixture of dry N2 + dry O2; R st N is the stationary resistance of TiO2 thin films in dry N2 atmosphere; A and B are constants; τ1 and τ2 are time constants.τ1 ≈ 23 s and τ2 ≈ 25 s at T = 500 °C and The rise of resistance R of the TiO 2 thin film under the exposure to O 2 and the drop of resistance after this exposure were approximated by the following functions, respectively: where t is time; R st ox is the stationary resistance of TiO 2 thin films in a gas mixture of dry N 2 + dry O 2 ; R st N is the stationary resistance of TiO 2 thin films in dry N 2 atmosphere; A and B are constants; τ 1 and τ 2 are time constants.τ 1 ≈ 23 s and τ 2 ≈ 25 s at T = 500 • C and exposure to 10 vol.% of O 2 for new samples, τ 1 ≈ 13 s and τ 2 ≈ 23 s for samples after 4 weeks of storing.The time constants τ 1 and τ 2 are related to the relaxation times of adsorption and desorption of oxygen molecules on the semiconductor surface.
Figure 8a illustrates the time dependence of resistance of TiO 2 thin films at T = 500 • C and stepwise increase in the O 2 concentration c ox (Figure 8b).The dependences of the response of TiO 2 thin films on c ox at T = 500 • C in dynamic range from 0.1 vol.% to 100 vol.% of O 2 and 0.1 vol.% to 6 vol.% of O 2 are presented in Figure 8c,d, correspondingly.The samples demonstrate a wide dynamic range from 0.1 vol.% to 100 vol.% of O 2 , but their responses to c ox < 1 vol.% are low.Detailed research is needed to enhance oxygen sensitivity at these low oxygen concentration ranges.
Micromachines 2023, 14, 1875 7 of 15 exposure to 10 vol.% of O2 for new samples, τ1 ≈ 13 s and τ2 ≈ 23 s for samples after 4 weeks of storing.The time constants τ1 and τ2 are related to the relaxation times of adsorption and desorption of oxygen molecules on the semiconductor surface.Figure 8a illustrates the time dependence of resistance of TiO2 thin films at T = 500 °C and stepwise increase in the O2 concentration cox (Figure 8b).The dependences of the response of TiO2 thin films on cox at T = 500 °C in dynamic range from 0.1 vol.% to 100 vol.% of O2 and 0.1 vol.% to 6 vol.% of O2 are presented in Figure 8c,d, correspondingly.The samples demonstrate a wide dynamic range from 0.1 vol.% to 100 vol.% of O2, but their responses to cox < 1 vol.% are low.Detailed research is needed to enhance oxygen sensitivity at these low oxygen concentration ranges.The effect of applied voltage on the response of TiO2 thin films to O2 was evaluated.The I-V characteristics of the samples were measured in dry N2 atmosphere and in a dry gas mixture of N2 + 10 vol.% of O2 (shown in Figure 9a).The I-V characteristics were approximated by the power function I ~ U z , where I is the electric current; z is a power index.The z value was 2.62 ± 0.05 in the N2 atmosphere and 2.22 ± 0.05 in the gas mixture The effect of applied voltage on the response of TiO 2 thin films to O 2 was evaluated.The I-V characteristics of the samples were measured in dry N 2 atmosphere and in a dry gas mixture of N 2 + 10 vol.% of O 2 (shown in Figure 9a).The I-V characteristics were approximated by the power function I~U z , where I is the electric current; z is a power index.The z value was 2.62 ± 0.05 in the N 2 atmosphere and 2.22 ± 0.05 in the gas mixture of N 2 + 10 vol.% of O 2 .The nonlinearity of the I-V characteristics was probably caused by the manifestation of an energy barrier at the Pt/TiO 2 interface.The response of TiO 2 thin films to O 2 in the range of U = 0.2-1.5 V practically did not change with voltage (see Figure 9b).S ox increased according to the power law S ox ~Uk with the increase in U from 1.5 V to 5 V, where k is a power index.k was 0.62 ± 0.05 at c ox = 10 vol.% and at T = 500 • C. of N2 + 10 vol.% of O2.The nonlinearity of the I-V characteristics was probably caused by the manifestation of an energy barrier at the Pt/TiO2 interface.The response of TiO2 thin films to O2 in the range of U = 0.2-1.5 V practically did not change with voltage (see Figure 9b).Sox increased according to the power law Sox ~ U k with the increase in U from 1.5 V to 5 V, where k is a power index.k was 0.62 ± 0.05 at cox = 10 vol.% and at T = 500 °C.A promising application of O2 sensors based on the ALD-TiO2 thin films is the monitoring of the exhaust gases of the internal combustion engines.In order to achieve this, it is necessary to measure the change in O2 concentration in the range of 6-10 vol.% in the exhaust gas mixture [2,41].In addition to O2, exhaust gases contain relatively high concentrations of H2, NOx, CHx, CO and CO2 [2,41,42].To create a gas mixture corresponding to exhaust gas is difficult.But the sensitivity of the ALD-TiO2 thin films to these gases with concentrations close to those of exhaust gases was investigated at the temperature of the maximum response to O2.The ALD-TiO2 thin films demonstrated a relatively high response to H2, NO and NO2.The experimental results are exhibited in Figure 10.The responses to relatively high concentrations of CO, CH4 and CO2 were insignificant or absent.Exposure to H2 led to a drop in the resistance of the films.The ratio of resistances in the pure dry air and in the gas mixture of pure dry air + reducing gas (H2, CO and CH4) was chosen as response.The exposure to NO and NO2 led to an increase in the TiO2 thin film resistance.The response to these gases was determined as a ratio of the resistances in the gas mixture of pure dry air + NO (NO2) and in the pure dry air.It is worth noting that the responses to 0.1 vol.% of H2 and 7 × 10 −4 vol.% of NO2 were the same.This indicates the high sensitivity of the films to low NO2 concentrations.A promising application of O 2 sensors based on the ALD-TiO 2 thin films is the monitoring of the exhaust gases of the internal combustion engines.In order to achieve this, it is necessary to measure the change in O 2 concentration in the range of 6-10 vol.% in the exhaust gas mixture [2,41].In addition to O 2 , exhaust gases contain relatively high concentrations of H 2 , NO x , CH x , CO and CO 2 [2,41,42].To create a gas mixture corresponding to exhaust gas is difficult.But the sensitivity of the ALD-TiO 2 thin films to these gases with concentrations close to those of exhaust gases was investigated at the temperature of the maximum response to O 2 .The ALD-TiO 2 thin films demonstrated a relatively high response to H 2 , NO and NO 2 .The experimental results are exhibited in Figure 10.The responses to relatively high concentrations of CO, CH 4 and CO 2 were insignificant or absent.Exposure to H 2 led to a drop in the resistance of the films.The ratio of resistances in the pure dry air and in the gas mixture of pure dry air + reducing gas (H 2 , CO and CH 4 ) was chosen as response.The exposure to NO and NO 2 led to an increase in the TiO 2 thin film resistance.The response to these gases was determined as a ratio of the resistances in the gas mixture of pure dry air + NO (NO 2 ) and in the pure dry air.It is worth noting that the responses to 0.1 vol.% of H 2 and 7 × 10 −4 vol.% of NO 2 were the same.This indicates the high sensitivity of the films to low NO 2 concentrations.

The Mechanism of the Sensory Effect
The ALD-deposited TiO2 thin films annealed at Tann = 800 °C in Ar for 30 min are corresponding to the anatase phase.They are homogeneous and relatively smooth, without features of microrelief on the film surface which could affect the transport of charge carriers through the film.Therefore, the increase in the resistance of such TiO2 films under exposure to oxygen is due to the chemisorption of O2 molecules on their

The Mechanism of the Sensory Effect
The ALD-deposited TiO 2 thin films annealed at T ann = 800 • C in Ar for 30 min are corresponding to the anatase phase.They are homogeneous and relatively smooth, without features of microrelief on the film surface which could affect the transport of charge carriers through the film.Therefore, the increase in the resistance of such TiO 2 films under exposure to oxygen is due to the chemisorption of O 2 molecules on their surface.During chemisorption, oxygen captures electrons from the TiO 2 conduction band and forms a region depleted by charge carriers in the near-surface part of the semiconductor film, with a width W.There are no charge carriers in this region, and the electric current between the contacts flows through a layer of thickness (d − W), which is called a conduction channel.The negative charge on the surface of the n-type semiconductor film leads to the formation of the upward bending of energy band eV s , where vs. is the surface potential; e is the electron charge.It is shown that eV s ~Ni 2 [43,44], where N i is the surface density of chemisorbed oxygen ions.The relationship between W and eV s has the following form: where L D = [(εε 0 kT/(e 2 n)] 0.5 ; k is the Boltzmann constant; ε is the electric constant.The chemisorption of oxygen on the surface of TiO 2 thin films leads to an increase in W and eV s , as well as to a decrease in the thickness of the conduction channel that leads to an increase in the resistance of the film.At n ≈ 10 18 cm −3 , L D for the anatase TiO 2 phase increases linearly from 8.0 nm to 9.3 nm and does not exceed the film thickness.The TiO 2 film resistance in gas mixture N 2 + O 2 is given by where ρ N is the resistivity of the TiO 2 film in the dry N 2 atmosphere; l is the distance between the electrodes; b is the width of the TiO 2 film.The intrinsic surface states can be neglected for ionic semiconductors [43].Thus, in the N 2 atmosphere, eV s and W = 0, and R N = ρ N l/(bd).The expression for response to oxygen is High S ox takes place when the region depleted by charge carriers extends almost the entire thickness of the film, but there is a very thin conduction channel.The dependences of W and eV s on T and c ox (Figure 11a in the range of c ox = 0.1-2 vol.%, where l 1 and m 1 are the power indexes.l 1 = 0.90 ± 0.03 and m 1 = 0.45 ± 0.01.The equality m 1 = l 1 /2 follows from Expression (4).There are weaker power dependencies of eV s ~cox l 2 and W~c ox m 2 in the range of c ox = 4-100 vol.%, where l 2 and m 2 are the power indexes.At the same time, l 2 = 0.028 ± 0.006 and m 2 = 0.014 ± 0.003.We assume that the manifestation of two linear areas on the dependencies is due to the presence of two types of adsorption centers for oxygen molecules.The possibility of this was shown for SnO 2 and Ga 2 O 3 thin films [45,46].As a result, oxygen is chemisorbed in the form of O 2− on the first type of adsorption centers and in the form of O − on the second one.Chemisorption on the centers of the first type prevails in the range of c ox = 0.1-2 vol.%.Twice as many electrons are captured during the chemisorption of one oxygen molecule on this type of centers, which causes a sharp increase in W and eV s .Centers of the second type prevail in the range of c ox = 4-100 vol.% and the dependencies of W and eV s on c ox are much weaker.In this case, eV s ~(N i1 + N i2 ) 2 , where N i1 is the surface density of chemisorbed O 2− ions; N i2 is the surface density of chemisorbed O − ions.It is worth noting that it is possible to approximate the experimental dependence of the response on the O 2 concentration (see Figure 8c) by means of Expressions ( 4) and (6) in the case when eV s (c ox ) = a(c ox ) × c ox l 1,2 , where a is a function of c ox .We believe that this dependence of eV s (c ox ) is due to the manifestation of the dependence of the surface density of adsorption centers for oxygen molecules on c ox .
presence of two types of adsorption centers for oxygen molecules.The possibility of this was shown for SnO2 and Ga2O3 thin films [45,46].As a result, oxygen is chemisorbed in the form of O 2− on the first type of adsorption centers and in the form of O − on the second one.Chemisorption on the centers of the first type prevails in the range of cox = 0.1-2 vol.%.Twice as many electrons are captured during the chemisorption of one oxygen molecule on this type of centers, which causes a sharp increase in W and eVs.Centers of the second type prevail in the range of cox = 4-100 vol.% and the dependencies of W and eVs on cox are much weaker.In this case, eVs ~ (Ni1 + Ni2) 2 , where Ni1 is the surface density of chemisorbed O 2− ions; Ni2 is the surface density of chemisorbed O − ions.It is worth noting that it is possible to approximate the experimental dependence of the response on the O2 concentration (see Figure 8c) by means of Expressions ( 4) and ( 6) in the case when eVs(cox) = a(cox) × cox l1,2 , where a is a function of cox.We believe that this dependence of eVs(cox) is due to the manifestation of the dependence of the surface density of adsorption centers for oxygen molecules on cox.The effect of applied voltage on the response of metal oxide to gases has been poorly studied in the literature so far.For SnO2 films, it is shown that negatively charged ions of chemisorbed oxygen diffuse over the surface, participating in the transport of the electric current [47].The diffusion time of adsorbed oxygen over the surface is less than its lifetime on the surface at high electric fields.Negatively charged ions accumulate near the anode and form a high-resistance region at higher electric fields.An additional increase in the resistance contributes to an increase in response at higher U.
Within the framework of the proposed mechanism of the sensory effect, the sensitivity of the films to reducing gases is due to the interaction of their molecules with pre- The effect of applied voltage on the response of metal oxide to gases has been poorly studied in the literature so far.For SnO 2 films, it is shown that negatively charged ions of chemisorbed oxygen diffuse over the surface, participating in the transport of the electric current [47].The diffusion time of adsorbed oxygen over the surface is less than its lifetime on the surface at high electric fields.Negatively charged ions accumulate near the anode and form a high-resistance region at higher electric fields.An additional increase in the resistance contributes to an increase in response at higher U.
Within the framework of the proposed mechanism of the sensory effect, the sensitivity of the films to reducing gases is due to the interaction of their molecules with previously chemisorbed oxygen, as a result of which W and eV s , as well as the resistance of TiO 2 films, decrease.The mechanism of sensitivity of TiO 2 films to reducing gases was previously described in detail by our group in ref. [48].Oxidizing gases interact with the surface of TiO 2 films like oxygen.
The proposed sensory effect does not take into account the contribution of changes in the potential barrier at the Pt/TiO 2 interface under the exposure to gases.The Schottky barrier between a semiconductor and catalytically active metals such as Pt, Pd, Ru, Ir, and Ag, including Pt/TiO 2 structures, are known to exhibit sensitivity to H 2 and other gases [38,49,50].According to the corresponding sensory effect's mechanism, gas molecules (H 2 for example) undergo a dissociative adsorption on the catalytically active metal surface.Then, H atoms diffuse through the metal layer to the metal-semiconductor interface.A dipole layer of H atoms is formed at this interface, which leads to a decrease in the height of a potential barrier for electrons at the metal-semiconductor interface and an increase in the current.The diffusion of H atoms in Pt is characterized by lower diffusion activation energies and higher diffusion coefficients than those for the diffusion of O atoms in Pt [51][52][53][54][55][56].It can be estimated that the diffusion times of H atoms through a 330 nm thick Pt layer are less than 0.055 s.In contrast, the diffusion time of O atoms through the Pt contact layer is about 10 8 s.Thus, changes in the potential barrier at the Pt/TiO 2 interface under the exposure to O 2 should not be taken into account.However, this effect may be the main one under the exposure to other gases, such as H 2 and CO.
The gas-sensitive characteristics of TiO 2 films synthesized by different deposition methods under the exposure to O 2 are compared in Table 2, where NRA is nanorod array; NPs is the nanoparticles; NFs are the nanoflakelets; NTs is the nanotubes; RFMS is the radio frequency magnetron sputtering; DCMS is the direct current magnetron sputtering; IBSD is the ion beam sputtering deposition; GLAD + EBE is the glancing angle deposition with electron beam evaporation; AVO is the acid vapor oxidation; USP is the ultrasonic spray pyrolysis; TD + HM is the thermal decomposition assisted hydrothermal method; PEO is the plasma electrolytic oxidation; AO is the anodic oxidation; UV is the exposure to ultraviolet radiation.The gas-sensitive properties of ALD-deposited undoped TiO 2 thin films studied in this work are comparable or superior to the results reported for undoped TiO 2 thin films grown by other methods.High S ox requires heating of the structures to T > 300 • C. T is reduced to RT by using of the ultrathin films or nanostructures, exposure to UV.However, at the same time, t res and t rec significantly increase.On the other hand, an increase in S ox is achieved by doping of TiO 2 with Nb, Pd and Cr, introducing ZrO 2 , MoO 3 , forming of multilayer structures, and modifying the surface of films with nanoparticles.

Conclusions
The structural and gas-sensitive properties under the exposure to O 2 within the temperature range from 30 • C to 700 • C of TiO 2 thin films deposited by atomic layer deposition on SiO 2 /Si substrates were studied.The structure of the films annealed at 800 • C in an Ar for 30 min corresponded to the anatase phase.They are homogeneous and relatively smooth.The ALD-TiO 2 thin films demonstrated high responses to O 2 in the dynamic range from 0.1 to 100 vol.% and to low concentrations of H 2 , NO 2 .The greatest response-41.5 arb.un.-was observed at a temperature of 500 • C under exposure

Figure 1 .
Figure 1.Microscopic photo of the sample based on TiO2 thin film.

Figure 2
Figure 2 illustrates a typical XRD spectrum of the annealed ALD-TiO2 thin film.Several peaks appear at 2θ = 25.3°,36.9°,37.8°, 38.2°, 48.0°, 54.0°, 55.1°, 62.7°, 68.8° and 70.3°.These peaks are associated with (101), (103), (004), (112), (200), (105), (211), (204), (116) and (220) Bragg reflections of the tetragonal anatase TiO2 phase (ICDD 00-021-1272), respectively.These results confirm the polycrystalline nature of the material grown.The wide amorphous halo at 2θ ≈ 22° is due to the SiO2 layer.The parameters of the tetragonal crystalline lattice of the film are determined as a = 3.78 Å and c = 9.50 Å.Figure3depicts typical annealed ALD-TiO2 film surface morphology images taken by AFM.The surface roughness parameters of the TiO2 thin films are Ra = 1.329 nm, Rq = 1.605 nm and Rz = 12.67 nm, where Ra is the arithmetic mean of the absolute values of the deviations of the film surface profile; Rq is the mean square value of the deviations of the film surface profile and Rz is the arithmetic mean of the greatest height of the profile of

Figure 1 .
Figure 1.Microscopic photo of the sample based on TiO 2 thin film.

Figure 2
Figure 2 illustrates a typical XRD spectrum of the annealed ALD-TiO 2 thin film.Several peaks appear at 2θ = 25.3 • , 36.9 • , 37.8 • , 38.2 • , 48.0 • , 54.0 • , 55.1 • , 62.7 • , 68.8 • and 70.3 • .These peaks are associated with (101), (103), (004), (112), (200), (105), (211), (204), (116) and (220) Bragg reflections of the tetragonal anatase TiO 2 phase (ICDD 00-021-1272), respectively.These results confirm the polycrystalline nature of the material grown.The wide amorphous halo at 2θ ≈ 22 • is due to the SiO 2 layer.The parameters of the tetragonal crystalline lattice of the film are determined as a = 3.78 Å and c = 9.50 Å.Figure3depicts typical annealed ALD-TiO 2 film surface morphology images taken by AFM.The surface roughness parameters of the TiO 2 thin films are R a = 1.329 nm, R q = 1.605 nm and R z = 12.67 nm, where R a is the arithmetic mean of the absolute values of the deviations of the film surface profile; R q is the mean square value of the deviations of the film surface profile and R z is the arithmetic mean of the greatest height of the profile of the film surface.R q is lower than the value reported in ref.[21] devoted to ALD-deposited TiO 2 thin films.The relatively high surface roughness should lead to an increase in the

Figure 4
Figure 4 illustrates a BF-TEM cross-sectional image of annealed ALD-TiO2 thin film on a substrate in the high-resolution mode.The interplane distance D corresponding to the (101) reflection of the anatase TiO2 phase is 0.353 nm, determined by fast Fourier transformation (FFT).The D values for the same plane determined by FFT and by analysis of the XRD pattern (0.352 nm) are the same.TiO2 thin films have a nanocrystalline structure with amorphous inclusions according to the TEM study.

Figure 4 .
Figure 4. BF-TEM cross-sectional image of annealed ALD-TiO2 thin film on SiO2/Si substrate, the insertion is the diffraction pattern.

Figure 4
Figure 4 illustrates a BF-TEM cross-sectional image of annealed ALD-TiO2 thin film on a substrate in the high-resolution mode.The interplane distance D corresponding to the (101) reflection of the anatase TiO2 phase is 0.353 nm, determined by fast Fourier transformation (FFT).The D values for the same plane determined by FFT and by analysis of the XRD pattern (0.352 nm) are the same.TiO2 thin films have a nanocrystalline structure with amorphous inclusions according to the TEM study.

Figure 4 .
Figure 4. BF-TEM cross-sectional image of annealed ALD-TiO2 thin film on SiO2/Si substrate, the insertion is the diffraction pattern.

Figure 4
Figure 4 illustrates a BF-TEM cross-sectional image of annealed ALD-TiO 2 thin film on a substrate in the high-resolution mode.The interplane distance D corresponding to the (101) reflection of the anatase TiO 2 phase is 0.353 nm, determined by fast Fourier transformation (FFT).The D values for the same plane determined by FFT and by analysis of the XRD pattern (0.352 nm) are the same.TiO 2 thin films have a nanocrystalline structure with amorphous inclusions according to the TEM study.

Figure 4
Figure 4 illustrates a BF-TEM cross-sectional image of annealed ALD-TiO2 thin film on a substrate in the high-resolution mode.The interplane distance D corresponding to the (101) reflection of the anatase TiO2 phase is 0.353 nm, determined by fast Fourier transformation (FFT).The D values for the same plane determined by FFT and by analysis of the XRD pattern (0.352 nm) are the same.TiO2 thin films have a nanocrystalline structure with amorphous inclusions according to the TEM study.

Figure 4 .
Figure 4. BF-TEM cross-sectional image of annealed ALD-TiO2 thin film on SiO2/Si substrate, the insertion is the diffraction pattern.

Figure 4 .
Figure 4. BF-TEM cross-sectional image of annealed ALD-TiO 2 thin film on SiO 2 /Si substrate, the insertion is the diffraction pattern.

Figure 6 .
Figure 6.(a) Temperature dependences of response to 10 vol.% and 40 vol.% of O 2 ; (b) Temperature dependences of response and recovery times upon exposure to 10 vol.% of O 2 .

Figure 7 .
Figure 7. Time dependence of resistance upon cyclic exposure to 10 vol.% of O2 at T = 500 °C for new sample (a) and after 4 weeks of storing (b).

Figure 7 .
Figure 7. Time dependence of resistance upon cyclic exposure to 10 vol.% of O 2 at T = 500 • C for new sample (a) and after 4 weeks of storing (b).

Figure 8 .
Figure 8.(a) Time dependence of resistance under stepwise increase in the O2 concentration at T = 500 °C; (b) Time profile of O2 concentration changes; (c) dependence of response on O2 concentration in dynamic range from 0.1 vol.% to 100 vol.% of O2 at T = 500 °C; (d) dependence of response on O2 concentration in dynamic range from 0.1 vol.% to 6 vol.% of O2 at T = 500 °C.

Figure 8 .
Figure 8.(a) Time dependence of resistance under stepwise increase in the O 2 concentration at T = 500 • C; (b) Time profile of O 2 concentration changes; (c) dependence of response on O 2 concentration in dynamic range from 0.1 vol.% to 100 vol.% of O 2 at T = 500 • C; (d) dependence of response on O 2 concentration in dynamic range from 0.1 vol.% to 6 vol.% of O 2 at T = 500 • C.

Figure 9 .
Figure 9. (a) I-V characteristics in N2 atmosphere and gas mixture of N2 + 10 vol. of O2 at T = 500 °C; (b) dependencies of the responses to 10 vol.% of O2 on applied voltage at T = 500 °C.

Figure 9 .
Figure 9. (a) I-V characteristics in N 2 atmosphere and gas mixture of N 2 + 10 vol. of O 2 at T = 500 • C; (b) dependencies of the responses to 10 vol.% of O 2 on applied voltage at T = 500 • C.
,b) are estimated by means of experimental S ox and calculated L D values.Pure TiO 2 thin films do not demonstrate reliably recorded sensitivity to O 2 at T < 450 • C.An increase in T stimulates dissociative adsorption of O 2 molecules.At the same time, high W and eV s are observed in the range of T = 500-650 • C, indicating a high surface density of chemisorbed O − ions.A further increase in T leads to a predominance of O − desorption, which leads to a sharp decrease in W and eV s , as well as the response of films.There are two linear areas on the dependencies of W and eV s on the O 2 concentration in double logarithmic coordinates (Figure 11b).eV s ~cox l 1 and W~c ox m 1

Figure 11 .
Figure 11.(a) Dependences of space-charge region width and bending of energy bands on temperature at cox = 10 vol.% of O2; (b) dependences of space-charge region width and bending of energy bands on oxygen concentration at T = 500 °C.

Figure 11 .
Figure 11.(a) Dependences of space-charge region width and bending of energy bands on temperature at c ox = 10 vol.% of O 2 ; (b) dependences of space-charge region width and bending of energy bands on oxygen concentration at T = 500 • C.

Table 1 .
Comparison of gas-sensitive characteristics of ALD-deposited metal oxide thin films.

Table 2 .
Comparison of sensitivity to O 2 for TiO 2 thin films deposited by different methods.