The 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. It is achieved by carefully choosing and tuning the properties of the thin-film at the deposition. In this paper, ZnTiO3 (ZTO) thin-films were being deposited on quartz and N-Si substrates by RF magnetron sputtering. The thin-films were developed at room temperature, oxygen percentage levels varying from 0 to 100, and annealed at 600oC. The electrical, optical, morphological, and structural properties were analyzed as a function of oxygen mixing percentage (OMP). The crystallinity of the cubic structured ZTO thin-film is found to be high at 25 OMP, and it is gradually decreased with increased OMP. The surface morphology of the thin-film is observed, and roughness is measured from the atomic force microscope. Raman Spectroscopy investigated the phase formation and the vibrational modes of the thin-film with their spectral de-convolution. The ZTO thin-films optical properties were investigated using transmittance spectra. The ZTO thin-film indicated the highest refractive index of 2.46, at 633nm with optical bandgap values of 3.57 eV, with a thickness of 145nm and 25 OMP. The refractive index, thin-film thickness, and excitation coefficient were analyzed using the Swanepoel envelope technique. Electrical characteristics of ZTO thin-film are measured from the optimized conditions of the thin-film with conventional thermionic emission (TE) technique.

Introduction: For several decades, metal oxide semiconductor (MOS) thin-films play a significant role in the interdisciplinary research areas, and it's applications. In the present scenario, researchers are working on MOS composites. Generally, metal oxide composites form ABX3 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 essential perovskite applications [2][3] [4]. The metal oxides such as ZnO, TiO2, SnO2, MnO2, CuO, WO3 are known as wideband semiconductors [5]. Among these metal oxides, ZnO and TiO2 thinfilms have extensive applications of their intrinsic properties, mixed oxides formations, and transition metal doping. Incorporation of ZnO with TiO2 leads to Zn-Ti-O ternary oxides which ensure the separation of electron-hole pair efficiently [6].  [12]. 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 behavior of the metal oxide thin-films can be controlled by adjusting the oxygen flow rate in the sputtering process [13][14][15][16].
The average crystallite size calculated with the equation (1) was increased from 3.5 nm to 6.19 nm, with an increase of OMP from 0 to 25. Crystallite size decreases from 6.19 to 4.2 nm with an increase of OMP from 25 to 100, which is confined to the nanocrystalline nature of ZnTiO3. 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 ZTO thin-films initially, O2 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 represented in Fig.1(b). The thin film deposited at 12.5 % OMP to 25 % OMP is the better condition for ZnTiO3 thin film fabrication. Tmax is the transmittance maxima and Tmin is the transmittance minima at a specific wavelength λ, and ns is the substrate's refractive index. The thin-films thickness (d) can be calculated by the following equation, n n d (4) n1 and n2 are the refractive indices of two adjacent maxima or minima at wavelengths λ1 and λ2.

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The films' thicknesses were in the range of 160-177 nm is almost constant. It is calculated that the refractive index (n) of the thin-films was from 2.35 to 2.46. It follows the same trend as XRD and AFM. usually, the refractive index depends on the thin-films crystallinity, electronic structure, and oxygen deficiencies.

EDS:
The Energy dispersive spectrum technique confirmed the elemental distribution of the ZTO composite. The peaks in the spectrum are similar to X-ray diffraction peaks. The predominant peak in the spectrum is N-Si substrate, which represents the orientation of the thin-film on(1 0 0) N-Si substrate.

Raman Spectra:
Fig. 6 exhibits the Raman spectra of ZTO 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 ZTO thin-film at different OMP. Table II represents the Raman modes and full width at half maxima. The bands persist cubic phase of ZTO, displays two Where I0 is the reverse saturation current, V is the applied voltage, Rs is the series resistance, η is the ideality factor, q is the charge, k is the Boltzmann constant, and T is the absolute temperature.
The reverse saturation current Io is given by where A is the contact area of ZTO (∼0 765×10 −2 cm −2 ), A * is the effective Richardson constant of ZTO (∼ 37 A cm −2 K −2 ), and ϕB, eff is the effective barrier height at zero bias. It is described as Taking natural logarithm on both sides of Eq. (1), we obtain ln(I) = ln(I0) + For a low current region of the forward bias I-V characteristics, the forward bias current is in the order of I0, the effect of Rs is negligible due to the negligible value of IRs. Thus, the reverse saturation current value I0 can be calculated from the intercept of ln I versus V plot (shown in Fig. 8(a)) with the current(I) axis for V = 0, which gives I0 ∼ 5.833 × 10 −11 A. The value of I0 is then used in Eq. (7) to determine the value of ϕB, 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 (η) is computed from the slope of the linear region of the forward bias ln I versus V plot as = ( ln ( ) ) From Eq. (9), the ideality factor(η) is estimated as ∼ 2.35, which is much larger than unity. The high η values represent the interfacial thin oxide layer, a wide distribution of barrier height, and the bias voltage dependence of the barrier height[27] [28].
To determine the value of the device's series resistance, we have used the Ri = dV /dI vs V plot of the measured I-V data, as shown in Fig. 8(b). where Ri is the bias-dependent resistance. The series resistance is almost negligible at lower values of current. At higher values of current Rs 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 ZTO thin-film and hence applied bias V = IRs. The input resistance Ri ~2.3*10 9 is determined from Fig. 8(c).

Conclusions:
RF magnetron sputtering was used to deposit ZTO thin-films on quartz, 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 o C were crystallized in pure ZTO cubic phase without any secondary phases.
The roughness value is < 1nm is depicted in ultra-fine thin-films deposited by the sputtering technique. The thinfilms refractive index is high at 25 OMP is 2.46, whereas the optical band gap varies from 3.4 eV to 3.6 eV with varying OMP. The stoichiometry of the thin-film meets the elemental composition. The vibrational modes of the Raman spectra representing the cubic structure of ZTO. The electrical parameters of the thin-films are Reverse saturation current I0 is 5.83x10 -11 A, Barrier efficiency is ϕB, eff is 0.87 eV, and ideality factor η is 2.35. The thinfilm parameters are more suitable for optoelectronic, microwave dielectric, and gas sensing applications.