Effect of oxygen mixing percentage on structural, optical and electrical properties of ZnTiO3 thin films grown by RF magnetron sputtering

Perovskites are important composites in the area of multidisciplinary applications. The enhancement in properties of the composite can be achieved by carefully choosing and tuning the properties of the thin film at the deposition. In this paper, ZnTiO3 thin films were deposited on quartz and N-Si substrates at various oxygen mixing percentages (OMP) from 0 to 100% using RF magnetron sputtering technique. The deposited thin films were annealed at 600 °C for 1h and confirmed the cubic perovskite structure of ZnTiO3. The roughness value of the thin film is (0.25–0.74 nm) < 1 nm and it is identified from the atomic force microscope. The optical properties of ZnTiO3 thin film indicated the highest refractive index of 2.46, at 633 nm with optical bandgap values of 3.57 eV, with a thickness of 145 nm and 25 OMP. Electrical characteristics of ZnTiO3 thin film measured from the conventional thermionic emission technique are: reverse saturation current I0 is 5.83 × 10–11 A, Barrier efficiency ϕB,eff is 0.87 eV, and Ideality factor η is 2.35.


Introduction
Perovskites are one of the crucial composites in the field of multidisciplinary applications. For several decades, metal oxide semiconductor (MOS) thin films have played a significant role in interdisciplinary research and in their applications. In the present scenario, researchers are working on MOS composites. Generally, metal oxide composites form ABX 3 perovskites. Here 'X' is anions such as O, F, I, N, or halogens. 'B' is transition metal elements such as Ti, Mg, Pb, Fe, Cu, Ta, Zr, Al, Cr, Mn. 'A' is metal cations such as Ca, Zn, Ag, Cs, K, Na, Cd, Pb, Ba, La [1]. Fuel cells, non-linear optics, memory devices, gas sensors, photocatalysis, solar energy conversion, water splitting, decomposition are some of the important applications of perovskite [2][3][4][5]. The metal oxides such as ZnO, TiO 2 , SnO 2 , MnO 2 , CuO, WO 3 are known as wideband semiconductors. ZnO and TiO2 thin films have extensive applications among these metal oxides due to their intrinsic properties, mixed oxides formations, and transition metal doping. Incorporation of ZnO with TiO 2 leads to Zn-Ti-O ternary oxides, which ensure the separation of electron-hole pairs efficiently [6]. Under the light, illumination contributes to optoelectronic device applications such as photovoltaic and dye-sensitized solar cells, light-emitting diodes, sensors, flat panel displays, and photodetectors. ZnO-TiO 2 composite exists in zinc Orthotitanate (Zn 2 TiO 4 in cubic spinel crystal structure), zinc titanate (ZnTiO 3 in either cubic spinel or hexagonal perovskite structure), and zinc poly titanate (Zn 2 Ti 3 O 8 in cubic spinel structure) [7]. The ideal cubic crystal structure of ZnTiO 3 is rare and often distorted with reduced symmetry [8].
However, to obtain good quality stoichiometric and crystalline thin films: dopants, chemical composition, substrate temperatures, annealing temperatures, minimum thickness, critical phase, and suitable oxygen atmosphere are very important. A small weight percentage (1 wt%) of SnO 2 has doped to ZnTiO 3 stoichiometric composition [9]. It improves the properties of ZnTiO 3 thin film because a minimal quantity of SnO 2 can enhance the intensity of the cubic nature of ZnTiO 3 thin film and increase the surface adatom mobility of the charges leads to the coalition of smaller grains [10,11]. Several researchers studied the effect of oxygen mean pressures and concluded the impact of OMP on the thin films structural, electrical, and optical properties. The phase composition, crystal structure, and optical behaviour of the metal oxide thin films can be controlled by adjusting the oxygen flow rate in the sputtering process [12][13][14][15][16].
This paper reports a systematic study on structural, optical, morphological, and electrical properties of ZnTiO3 thin films deposited at oxygen mean pressures (OMP) variant by Radio Frequency sputtering. A small concentration of SnO 2 was doped to the ZnTiO 3 sputtering target. The crystallinity and structure are determined with XRD. The thin film optical constants, thickness, and excitation coefficients of ZnTiO 3 thin films were measured using the Swanepoel envelope technique. Roughness and morphology were observed from AFM, the best and optimized conditions were used to determine I-V, R-V characteristics of ZnTiO 3 thin film deposited on N-Si substrate with Ag electrodes. As per the author's knowledge, very few literatures are available for I-V characteristics of ZnTiO 3 thin film.

Experimental procedure
The ZnTiO 3 thin films were deposited on quartz and N-Si substrates using the RF magnetron sputtering method. The stoichiometric ZnTiO 3 with a small weight percentage (1 wt%) of SnO 2 produced a sputtering target using the conventional solid-state reaction method. Elementary powders such as Zinc Oxide (ZnO, * 30 nm, 99.8% pure, Zincite phase), Titanium Dioxide (TiO 2 , * 35 nm, 99.9% pure, Anatase phase), Tin Oxide (SnO 2 , * 80 nm, 99.9% pure, cassiterite phase) were mixed using a planetary ball mill (Fritsch GmbH, Germany) with prescribed stoichiometry. The mixed powders were dried to room temperature. The powders were uni-axially pressed to produces a ZnTiO 3 target of 60 mm diameter and sintered at 450°C for 4 h. Before deposition, the sputter chamber has been evacuated up to 1.0 9 10 -6 mbar base pressure. To obtain 3.0 9 10 -2 mbar pressure, the chamber is pumped with argon (Ar) and oxygen (O) gases. The sputtering power is fixed to 50 W. The ZnTiO 3 thin films were deposited at room temperature and annealed at 600°C. For the uniform deposition rate, and the same thickness, deposition time was varied. To vary oxygen mixing percentage, different percentages of oxygen (O) and Argon (Ar) gases were applied to the chamber (Oxygen-Argon percentages: 0:100, 25:75, 50:50, 75:25, 100:0%) [17]. The thickness of the films and deposition rate were optimized using UV-VIS Spectroscopy (Veeco-Dektak 6 M). The thin films' purity of phase and crystal structure was obtained using an X-ray diffractometer (Rigaku, TTRAX III 18 kW) with Cu-K a radiation (k = 1.5406 Å ). Atomic force microscope (Agilent, 5500 series) is employed for roughness and surface morphology analysis thin films. ZnTiO 3 (N-Si, Quartz) thin film was fabricated, and a top electrode (Ag) was deposited by thermal evaporation. UV-VIS-NIR Spectrophotometer (UV 3101PC, SHIMADZU) is employed for the spectral transmission characteristics in the 200-2500 nm wavelength range. To study the vibrational modes and FWHM of the thin film reviewed from Raman spectroscopy (LABRAM HR800, JOBIN YVON). I-V and R-V characteristics of the thin film were obtained from a 4-point probe station and Keithley (4200 SCS).

X-ray diffraction
The X-ray diffraction (XRD) patterns of the ZnTiO 3 thin films produced at room temperature under various oxygen atmosphere conditions (0-100%) are shown in Fig. 1a. It is seen that the thin film deposited at room temperature is purely amorphous, which indicates that no crystallization occurred. At 600°C, the ZnTiO 3 phase was found, and all peaks are confined to a cubic perovskite structure (ICDD: 00-039-0190). No secondary phases were found in the composite [18,19]. Main reflections are obtained at (2 2 0), (3 3 1), (4 2 2), (5 1 1), (4 4 0) planes. The plane (3 1 1) at 2h = 35.58°is the predominant peak. The highest intensity of peak at 2h = 51.75°is Si substrate, which indicates the orientation of ZnTiO 3 thin film grown on N-type Si substrate. Crystallite size (D) is calculated from Scherrer approximation, which is defined as where D is the average crystallite size, k is the X-ray wavelength, b is the width of the X-ray peak on the 2h axis, normally measured as the full width at half maximum (FWHM), h is the Bragg angle, and k is Scherrer constant. Since k is not known for the present material system, k = 0.9 was used, making all D calculations estimates. The average crystallite size estimated with Eq. (1) is increased from 3.5 to 6.2 nm, with an increase of OMP from 0 to 25. Crystallite size decreases from 6.2 to 4.2 nm with an increase of OMP from 25 to 100%, which is confined to the nanocrystalline nature of ZnTiO 3 . It can be correlated that the sputtered atoms react with oxygen molecules which generates redistribution of energy and heat on the substrate's surface. This process concurrently promotes sputtered species migration and crystallization. For ZnTiO 3 thin films initially, O 2 helps the crystalline growth up to 25 OMP, then the growth gradually decayed up to 100 OMP. The trends in lattice volume, D spacing, crystallite size, lattice strain, and lattice constant with respect to OMP were calculated and presented in Fig. 1b. The thin film deposited at 12.5-25% OMP is better for ZnTiO 3 thin film fabrication.

Atomic force microscope
The microstructural properties of the thin film are made with an atomic force microscope. The surface morphology, holographic roughness (internal), and typical 3-D representation of ZnTiO 3 thin films deposited with different OMP's is shown in Fig. 2a. The average roughness to be an increase from 0.25 to 0.74 nm with the increase of OMP from 0 to 25% and roughness decreases from 0.74 to 0.48 nm with the rise of OMP from 25 to 100%. AFM micrographs follow the same trend observed from XRD. But the films deposited in 100% oxygen and 0% oxygen atmosphere exhibited a uniform and homogeneous but low dense microstructure regarding surface topology and thickness. The grain growth enhancement may be optimized at 25% OMP for ZnTiO 3 thin film to improve the films crystallization in oxygen and argon mixed atmosphere. The roughness value is \ 1 nm is depicted in ultra-fine thin films deposited by the sputtering technique. The small roughness peaks can act as nanostructured absorption sites for sensing applications [20]. Figure 2b represents the RMS roughness of the deposited thin films. The optical constants were determined using the Swanepoel envelope technique [21,22]. The refractive index computed from the following equation:

Optical properties (UV-visible spectroscopy)
where T max is the transmittance maxima and T min is the transmittance minima at a specific wavelength k, and n s is the substrate's refractive index. The following equation can calculate the thin films thickness (d), n 1 and n 2 are the refractive indices of two adjacent maxima or minima at wavelengths k 1 and k 2 .
The films' thicknesses were in the range of 160-177 nm is almost constant. The refractive index (n) of the thin films was found to be 2.35-2.46. It follows the same trend as XRD and AFM. Usually, the refractive index depends on the thin films The optical bandgap energy (E g ) of the thin films are acquired from the extrapolated linear portion of (aht) m versus (ht) curve, where ht is the photon energy, a is the absorption coefficient. The measure of crystalline order b related to the bandgap energy is (aht) m = b (ht -E g ). The bandgap energy (E g ) is calculated by considering an allowed direct (m = 2) transition of electron between the highest occupied state of the valence band and the lowest unoccupied state of the conduction band. The thin films absorption edges at different OMP are shown in Fig. 3b. It is observed that the optical bandgap energy values are in the range of 3.4-3.6 eV. The bandgap variations might be due to reduced oxygen vacancies, variations in crystallinity, and improved grain size [23][24][25]. Figure 3c represents the refractive index (n f ), absorption coefficient (a), Excitation coefficient (k), and optical energy bandgap (E g ) of the ZnTiO 3 films concerning different OMP's.

Energy dispersive spectra
The Energy dispersive spectrum technique confirmed the elemental distribution of the ZnTiO 3 composite. Figure 4a shows the typical microstructure, elemental mapping, and Fig. 4b depicts the energy-dispersive spectra of ZnTiO 3 composite deposited on the N-si substrate at 25 OMP conditions. The experimental volume fraction of ZnTiO 3 composition is in agreement with theoretical volume fractions, confirming the ZnTiO 3 composites stoichiometry. The peaks in the spectrum are similar to X-ray diffraction peaks. The predominant peak in the spectrum is N-Si substrate, representing the thin film's orientation on (1 0 0) N-Si substrate.

Raman spectra
Figure 5a-f exhibit the Raman spectra of ZnTiO 3 composite recorded in the wavenumber range from 50 to 1000 cm -1 and their spectral de-convolution, the spectrum was fitted with Gaussian function using origin pro software. Seven to eight active Raman modes were identified for deposited ZnTiO 3 thin film at different OMP. Table 2 represents the Raman modes and full width at half maxima. The bands persist cubic phase of ZnTiO 3 , which displays two large and broad bands at 310.46, 432.60 cm -1 , and all the spectral features are of the first order. The result can be explained based on the order-disorder model of the central Ti ion. According to group theory ZnTiO 3 has ten Raman active modes 5 A g ? 5 E g . The bands at 102, 153, 433, 583 cm -1 are the E g modes and 310, 489, 795 cm -1 are related to Ag modes of the ZnTiO 3 Raman spectra [26,27].  Figure 6a shows the I-V characteristics of ZnTiO 3 thin film on N-type Silicon substrate which was deposited at 25 oxygen mixing percentage. The Ag electrodes were deposited on a thin film with thermal evaporation sputtering unit. The characteristics showed that the thin film has a non-linear and symmetrical response for both forward and reverse bias. The high resistivity of the film is also observed. In general, chemically deposited films have high resistance. It is due to low donor defect density and a large number of chemisorbed oxygen species. A large number of oxygen molecules are chemisorbed at the grain boundaries and on the surface of the thin film. The surface resistance of the thin film measured from the four-probe station is R S = 5.6 9 10 9 X.

I-V and R-V characteristics
Chemisorption is the process of trapping conduction electrons from the negatively charged oxygen species (O 2-, O -). Due to this chemisorbed species, the resistivity of the oxide surface is high. For reducing gas molecules, the negatively charged chemisorbed species act as reaction centers. When reduction gases come in contact with the oxide surface, the trapped electrons release because of the reaction between gas molecules and the oxygen species. The electrons return to the conduction band, so the resistance is decreased. After the removal of the gas, the electrons are again trapped, and the resistance increases. Thus such high resistive films containing an enhanced density of chemisorbed species are particularly suitable for resistive mode gas sensor applications [28].
From the conventional thermionic emission (TE) theory, the forward I-V characteristics of the ZnTiO 3 thin film deposited on N-type Si substrate with Ag contacts can be delineated by where I 0 is the reverse saturation current, V is the applied voltage, R s is the series resistance g is the ideality factor, q is the charge, K is the Boltzmann constant, and T is the absolute temperature. The reverse saturation current I 0 is given by where A is the contact area of ZnTiO 3 (* 0.76 9 10 -2 cm -2 ), A * is the effective Richardson constant of ZnTiO 3 (* 37 A cm -2 K -2 ), and / B, eff is the effective barrier height at zero bias. It is described as Taking natural logarithm on both sides of Eq. (5), we obtain For a low current region of the forward bias I-V characteristics, the forward bias current is in the order of I 0 . The effect of R s is negligible due to the negligible value of IRs. Thus, the reverse saturation current value I 0 can be calculated from the intercept of lnI versus V plot (shown in Fig. 6b) with the current (I) axis for V = 0, which gives I 0 * 5.83 9 10 -11 A. The value of I 0 is then used in Eq. (7) to determine the value of / B, eff as 0.87 eV. This value may be deviated from the ideal value because of high surface states, generation-recombination, image force lowering effect in the depletion region, and the barrier inhomogeneities at the junction. Now, the ideality factor (g) is computed from the slope of the linear region of the forward bias ln I versus V plot as From Eq. (9), the ideality factor (g) is estimated as * 2.35, much larger than unity. The high g values represent the interfacial thin oxide layer, a wide   [29,30].
To determine the value of the device's series resistance, we have used the R i = dV/dI versus V plot of the measured I-V data, as shown in Fig. 7a. where R i is the bias-dependent resistance. The series resistance is almost negligible at lower values of current. At higher values of current R s shows a significant effect so that it exhibits nonlinear characteristics. In the high current region, the voltage drop IRs is much larger than the voltage appearing across the ZnTiO 3 thin film and hence applied bias V = IRs. The input resistance R i * 2.3 9 10 9 X is determined from Fig. 7b. Electrical parameters of ZnTiO 3 thin film measured from thermionic emission (TE) theory is shown in Table 3.

Conclusions
The RF magnetron sputtering was used to deposit ZnTiO 3 thin films on quartz and N-Si substrates. The influences of the OMP on structural, optical, morphological, and electrical properties were studied systematically. The deposited thin films annealed at 600°C were crystallized in pure ZnTiO 3 phase without any secondary phases. As per AFM results, the roughness value of the deposited thin film is \ 1 nm and hence it can be said that the films are ultra fine. As per the optical spectroscopy, it can be said that the thin films refractive index is high at 25 OMP is 2.46. The optical bandgap varies from 3.4 to 3.6 eV with varying OMP. The stoichiometry of the thin film meets the elemental composition. Eight to nine vibrational modes are observed from the Raman spectra, representing the cubic structure of ZnTiO 3 . The electrical parameters of the thin films are reverse saturation current I 0 is 5.8 9 10 -11 A, Barrier efficiency is / B, eff is 0.87 eV, and ideality factor g is 2.35. Hence it can be concluded that the estimated thin film parameters are more suitable for optoelectronic, dielectric, and gas sensing applications. Surface resistance (R S ) 5.59 9 10 9 X 2 Reverse leakage current (I 0 ) * 5.83 9 10 -11 A 3 Barrier efficiency (/ B, eff ) * 0.87 eV 4 Ideality factor (g) * 2.35 5 Input resistance R i * 2.3 9 10 9 X