CCTO Thin Films Research Articles

The room temperature signature of micro to nanocrystalline calcium copper titanate (CCTO) thin film by a modified sol-gel technique and tailoring its material property

This study aims to identify an alternative route for the low-temperature synthesis of calcium copper titanate (CCTO) thin films and to explore a new understanding of precursor composition strategy and its relationship to thin-film combustion synthesis using a modified sol-gel technique. Micro-sized to nano-sized CCTO thin films were effectively deposited onto various substrates using a rapid and modified sol-gel process at remarkably low temperatures (∼100 °C), with room temperature phase formation of CCTO observed via FTIR and Raman spectroscopy. Calcium nitrate, copper nitrate, and titanium (IV) butoxide served as precursor materials to create the sol for spin coating. The resulting CCTO thin films were polycrystalline, uniform, and compact. The surface morphology and optical properties of these films were analyzed using Atomic Force Microscopy (AFM), Field Emission Scanning Electron Microscopy with Energy Dispersive X-Ray Analysis (FESEM-EDAX), X-Ray Diffraction (XRD), and UV–VIS spectrophotometry. XRD confirmed various phases of CCTO on different substrates, with significant intensity variations in the peaks at different annealing temperatures. The XRD patterns on three different substrates-glass, SnO2 coated glass, and SiO2 coated Si—revealed different CCTO phases such as CCTO (301), CCTO (220), CCTO (400), and CCTO (422). EDAX analysis confirmed the presence of Ca, Cu, Ti, and O. The band gap varied from 2.5 to 3.8 eV depending on the annealing temperature. Capacitive vs. frequency response, dielectric loss, and I-V characteristics of the films were measured using MIM (Metal-Insulator-Metal) or SIM (Semiconductor-Insulator-Metal) structures.

High-performance piezoelectric energy harvesting in amorphous perovskite thin films deposited directly on a plastic substrate

Most reported thin-film piezoelectric energy harvesters have been based on cantilever-type crystalline ferroelectric oxide thin films deposited on rigid substrates, which utilize vibrational input sources. Herein, we introduce flexible amorphous thin-film energy harvesters based on perovskite CaCu3Ti4O12 (CCTO) thin films on a plastic substrate for highly competitive electromechanical energy harvesting. The room-temperature sputtering of CCTO thin films enable the use of plastic substrates to secure reliable flexibility, which has not been available thus far. Surprisingly, the resultant amorphous nature of the films results in an output voltage and power density of ~38.7 V and ~2.8 × 106 μW cm−3, respectively, which break the previously reported record for typical polycrystalline ferroelectric oxide thin-film cantilevers. The origin of this excellent electromechanical energy conversion is systematically explored as being related to the localized permanent dipoles of TiO6 octahedra and lowered dielectric constant in the amorphous state, depending on the stoichiometry and defect states. This is the leading example of a high-performance flexible piezoelectric energy harvester based on perovskite oxides not requiring a complex process for transferring films onto a plastic substrate.

Preparation and characterization of Mg-doped CaCu3Ti4O12 thin films

Mg-doped CaCu3−xMgxTi4O12 (x=0, 0.05, 0.1, 0.15, 0.2, at.%) thin films were prepared by a modified sol−gel method. A comparative study on the microstructure and electrical properties of Mg-doped CaCu3Ti4O12 (CCTO) thin films was carried out. The grain sizes of the Mg-doped CCTO thin films were smaller in comparison to the undoped CCTO films. Furthermore, compared to undoped CCTO films, Mg-doped CCTO thin films obtained higher dielectric constant as well as excellent frequency stability. Meanwhile, Mg doping could reduce the dielectric loss of CCTO thin films in the frequency range of 104−106 Hz. The results showed that the Mg-doped CCTO thin films had the better electrical characteristics compared with the undoped CCTO films. The nonlinear coefficient of Mg-doped CCTO thin films at x=0.15 and x=0.1 was improved to 7.4 and 6.0, respectively.

Effects of Y Doping on Dielectric and Varistor Properties of CaCu3Ti4O12 Thin Films

Ca1−3x/2YxCu3Ti4O12 (x = 0, 0.05, 0.1, 0.15, and 0.2) (CCTO) thin films were prepared by a modified sol–gel method. The effects of Y doping on the microstructure and electrical properties of the CCTO thin films were investigated. According to x-ray diffraction analysis, the main phase of all Y-doped CCTO thin films is cubic Im $$ \bar{3} $$ . Meanwhile, some secondary phases were defected. Scanning electron microscopy analysis showed that Y doping can easily lead to a smaller grain size and larger porosity in CCTO thin films. Room-temperature dielectric analysis showed that the dielectric constant of different Y doping samples at 1 kHz was 4213, 4523, 4974, 4824, and 4777, and the dielectric losses were 0.053, 0.051, 0.062, 0.067, and 0.049, respectively. The study of Y-doped CCTO thin films showed that Y doping increased the dielectric constant of the films at low frequencies, but at the same time it increased their dielectric loss and decreased their dielectric frequency and temperature stability range. Y-doping had little effect on the nonlinearity of the films and did not significantly change their nonlinear coefficients.

Giant Dielectric Behaviour of Nb Modified CCTO Thin Film Prepared by Modified Sol–Gel Route

Modified sol–gel technique has been used to prepare the Nb doped CCTO ceramic thin film. X-ray diffraction of the prepared film confirms the cubic perovskite structure of the material. The micrographs obtained from the Scanning Electron Microscope reveal plenty of rods like microstructure within the scan area. This is being reported for the first time in the studied film. Dielectric constant is measured with the frequency range 100 Hz–1 MHz and it is observed that, with increase in frequency the dielectric constant decreases. Further, it is also observed that, the rate of decrease of er with frequencies increases with increasing temperature. The dielectric loss of the material decreases with increasing frequency and at low frequency region the dielectric loss increases with increase in temperature. The studied material shows relaxor behaviour (γ = 1.87) and is confirmed by peaks broadening at low frequency with increasing temperature and the peaks shift to low frequency region with decreasing temperature. The broad band spectrum of high dielectric constant is seen over the studied temperature range. This relaxation behaviour arises due to space charge polarization. The absorption and transmittance nature of the film is also presented and the calculated value of energy band gap is found to be 2.52 eV.

Enhanced dielectric properties and grain boundary potentials in sulfur-doped CaCu3Ti4O12 thin films

CaCu3Ti4O12 (CCTO) has been reported to possess a colossal dielectric constant owing to the intrinsic interfacial polarization via charge accumulations across the grain boundary. Herein, we explore the effects of unusual anion-doping on the dielectric properties of sputter-deposited CCTO thin films using an example of sulfur-doping. A post-annealing process of the films was utilized in a flowing H2S atmosphere for the sulfur-doping. The incorporation of sulfur into the perovskite structure was evidenced with the changes in chemical states, such as the reduced cations of Cu+ and Ti3+, the increased concentration of oxygen vacancies, and the formation of S-O bonds. The sulfurized CCTO thin films demonstrated an enhanced relative permittivity of ∼620 at 100 Hz, which is substantially better than that of the unsulfurized film. Direct measurement of the grain-boundary potential using Kelvin probe force microscopy suggests that the enhanced relative permittivity is associated with an increased Schottky barrier height.

Morphological and electrical properties of RF sputtered calcium copper titanate thin films with the incorporation of intermediate layer

In this research work, an attempt has been undertaken to improve the structural, morphological and electrical properties of RF sputtered CCTO thin films with the incorporation of HfO2 as a buffer layer and Zn as an intermediate layer. RF sputtering technique was used to deposit all the films with the optimized sputtering parameters. XRD pattern of HfO2 buffered CCTO thin films have shown an increase in (220) peak intensity with the enhancement in buffer layer thickness from 2.5 nm to 20 nm. Field emission scanning electron microscope images depicted the formation of smooth surface morphology. In addition, improved electrical and interface properties are observed with the increase in buffer layer thickness. CCTO films, deposited on 20 nm of HfO2 buffer layer, have shown lower leakage current of 3.15 × 10−7 Amp. In addition, reduced Dit value of 5.3 × 1010 eV−1cm−2 has been observed for film with 20 nm buffer layers. Improved morphological and electrical properties in CCTO films are attributed to suppression of defect states due to the presence of HfO2 buffer layer. Furthermore, the effect of different position of Zn intermediate layer, sandwiched between two CCTO layers, was also investigated. The appearance of splitted (220) peak has confirmed the incorporation of Zn in CCTO lattice. The surface morphology was found to be strongly dependent upon the multilayer geometry. Improved dielectric constant of 1078 at 100 kHz frequency is observed for the film configuration of 40-10-60, in which the Zn layer is situated towards the middle position.

Influence of annealing temperature on morphological and photocatalytic activity of sputter-coated CaCu3Ti4O12 thin film under ultraviolet light irradiation

CaCu3Ti4O12 (CCTO) thin films were deposited by radio frequency (RF) magnetron sputtering and subsequently annealed at 300, 400, 500, or 600 °C. The XRD analysis showed that the amorphous CCTO thin film transformed to more crystallite structure as the annealing temperature increased to 600 °C. Generally, the average crystallite size, grain size and roughness of the thin films increased with increasing annealing temperature. Photocatalytic activities of these thin films were evaluated by the effectiveness of Rhodamine B (RhB–C28H31ClN2O3) removal from solution under ultraviolet (UV) light illumination. In particular, the CCTO thin film annealed at 600 °C exhibited the highest photocatalytic activity, where it removed 67.4% of RhB dye in 4 h, at a rate constant of 0.217 min−1. This observation was ascribed to its higher crystallinity and larger grain size of CCTO thin film annealed at 600 °C.

Effect of Ar:N2 flow rate on morphology, optical and electrical properties of CCTO thin films deposited by RF magnetron sputtering

Abstract Calcium copper titanate (CCTO) thin films were deposited on indium tin oxide (ITO) substrates using radio frequency (RF) magnetron sputtering, at selected Ar:N 2 flow rates (1:1, 1:2, 1:4, and 1:6 sccm) at ambient temperature. The effect of Ar:N 2 flow rate on the morphology, optical and electrical properties of the CCTO thin films were investigated using FESEM, XRD, AFM, Hall effect measurement, and UV–Vis spectroscopy. It was confirmed by XRD analysis that the thin films were produced is CCTO with cubic crystal structure. As the flow rate of Ar:N 2 increased up to 1:6 sccm, the thin film thickness reduced from 87 nm to 35 nm while the crystallite size of CCTO thin film decreased from 27 nm to 20 nm. Consequently, the surface roughness of thin film was halved from 8.74 nm to 4.02 nm. In addition, the CCTO thin films deposited at the highest Ar:N 2 flow rate studied, at 1:6 sccm; are having the highest sheet resistivity (13.27 Ω/sq) and the largest optical energy bandgap (3.68 eV). The results articulate that Ar:N 2 flow rate was one of the important process parameters in RF magnetron sputtering that could affect the morphology, electrical properties and optical properties of CCTO thin films.

Non-Ohmic behavior of copper-rich CCTO thin film prepared through magnetron sputtering method

CaCu3+xTi4O12+x (x = 0, 0.1, 0.2, 0.4, 0.8) thin films with obvious non-Ohmic behaviors were prepared through magnetron sputtering method. The second phase of CuO and TiO2 were detected in all Cu-rich ceramic targets, and CuO was detected in thin film sample when x = 0.8 which contributed to the increased grain size. The nonlinear I–V behaviors were explained by Schottky emission and Poole–Frenkel electrons transportation mechanism. The nonlinear coefficient increased with x, reached the maximum when x = 0.4 and then decreased, which is in accordance with the changing trend of trap barrier height, meaning that Poole–Frenkel model can describe the non-Ohmic behaviors of Cu non-stoichiometric samples better than Schottky emission model.

Evidence of ferroelectric behaviour in CaCu3Ti4O12 thin films deposited by RF-sputtering

The origin of abnormal ferroelectric and unusual piezoelectricity in the polycrystalline CaCu3Ti4O12 (CCTO) thin films deposited by RF-sputtering on Pt/Ti/SiO2/Si (100) substrates was explored. The CCTO thin films, deposited at room temperature followed by annealing at 600?C for 2 h in a conventional furnace, have a cubic structure with lattice parameter a = 7.379 ? 0.001 ? and without any secondary phases. No polarization loss up to 1010 switching cycles, with a switched polarization ?P of 30 ?C/cm2 measured at 400 kV/cm was evidenced. The piezoelectric coefficient investigated by piezoresponse force microscopy (PFM) was approximately 9.0 pm/V. This may be the very first example of exploring the origin of ferroelectric behaviour for a material that possesses space charge polarization with highly resistive grain boundaries in the polycrystalline state.

CCTO thin film synthesis by radio frequency magnetron sputtering

In the present work CCTO thin film has successfully deposited on FTO substrate in a non-reactive (argon) atmosphere with frequency (RF) magnetron sputtering at 300 W. This work aims to determine the structural, morphological and optical properties of thin film using X-ray diffraction (XRD), atomic force microscopy (AFM) and Ultraviolet–visible spectroscopy (UV-Vis), respectively. The result shows that the nature of CCTO thin film is polycrystalline with cubic structure. XRD peaks confirmed the presence of CCTO phase with the crystal planes of (022), (013), (033), (024), and (224). The average crystallite size was 32 nm as measured by Willamson-Hall method. The surface roughness (Ra) and root mean square (RMS) of CCTO thin film were 26 nm and 33 nm, respectively. The maximum optical transmittance of CCTO thin film was more than 80% in the visible region (550-1000 nm) while calculated optical energy bandgaps was 3.2 eV.

Effects of deposition time on properties of CaCu3Ti4O12 thin film deposited on ITO substrate by RF magnetron sputtering at ambient temperature

CaCu3Ti4O12 (CCTO) thin film was deposited on ITO substrate using radio frequency (RF) magnetron sputtering at various deposition times (1–4 h). The significant effects of deposition time on the structural, morphological, optical and electrical properties were systematically investigated using XRD, AFM, FESEM, UV–Vis, and Hall Effect measurements. XRD study showed that all sputtered films have cubic structure. Higher deposition times of up to 4 h enhanced the film's crystallinity, as demonstrated by the increase in peak intensity and incremental change in crystal size (from 19.5 to 23.6 nm). Roughness (Ra) and root mean square (RMS) values were found to increase from 1.99 to 5.15 nm and 2.66–6.68 nm, respectively, when deposition times were increased. The surface morphology of CCTO thin films were found to be smooth, compact, and densely island-shaped in structure. The film thickness increased from 270 to 330 nm with increase in deposition time. Increase in deposition time also affected the optical transmittance values which declined to 70% at the visible range and the optical energy band gap which decreased from 3.76 to 3.29 eV. At 4 h deposition time, the films showed lower resistivity (of 8.22 Ω/sq), and higher Hall mobility (78.43 cm2/Vs) and carrier concentration (9.84 × 1015 cm−2). The conclusion drawn from this study revealed that by controlling the deposition time, better properties of CCTO thin films can be achieved.

Impact of post-deposition annealing on RF sputtered calcium copper titanate thin film for memory application

Calcium copper titanate (CCTO) sputtering target was synthesized by solid state method. Structural analysis of target has confirmed the formation of CCTO phase. CCTO thin films were deposited by RF magnetron sputtering technique on p-Si (100) substrates with the working pressure and RF power of 5 × 10–3 mbar and 105 W, respectively. Post-deposition annealing was performed at different temperatures ranging from 650 °C to 950 °C. Evolution of CCTO polycrystalline peaks has been observed in the 950 °C annealed film. The formation of CCTO phase has also been confirmed by FTIR spectroscopy studies. The surface morphology of the annealed thin film was found to be modulated with the annealing temperature. Larger microstructures have been found for the films annealed at high temperature. Bandgap of films were calculated from UV–vis spectral analysis. Electrical properties of the annealed thin films were studied by fabricating Al/CCTO/Si MOS structures. The interface trap density (Dit) was calculated as 7.9 × 1010 eV−1 cm−2 for the films annealed at 950 °C. Improved bipolar resistive switching behavior has been observed for the films annealed at 950 °C.

Improvement in Dielectric Properties of CaCu3Ti4O12 Thin Film Over Pt(111)/Ti/SiO2/Si Substrate by Spin Coating Method

High quality calcium copper titanium oxide (CaCu3Ti4O12, CCTO) thin film has been deposited on Pt(111)/Ti/SiO2/Si substrate by sol–gel method using the spin coating technique. The structural analysis shows that CCTO thin film belonging to the cubic crystal structure with Im3 space group. The morphological result show that the film surface are smooth, fully covered, homogeneous, crack free with existence of many pinholes. No peaks of any impurities other than CCTO are detected in the energy dispersive analysis by X-ray spectra indicating the high purity of resultant product. The absorption band of Fourier transform infrared spectra is observed in the range of 380–700 cm−1 which arises from the mixed vibrations of CuO4 and TiO6 groups prevailing in the cubic CCTO. The luminescence spectrum of CCTO thin film has a weak green and strong red emission centered at 536 and 786 nm, respectively. The effective dielectric constant of CCTO dielectric layer in Si/SiO2/Ti/Pt/CCTO/Pt thin film capacitor is found to decrease with increase in frequency. It is found that capacitor reached a high dielectric constant (er) of 1067 and a capacitance (C) of 1.192 µF at 100 kHz due to the improved oxygen vacancies of trapped electrons and also from the effect of electrode-film interface. We compare our dielectric constant to previously reported results and it is recommended to CCTO use of an attractive material for the production of miniaturized capacitor in microelectronic applications.

Non-Ohmic properties of MgTiO3 doped CaCu3Ti4O12 thin films deposited by magnetron sputtering method

CaCu3Ti4O12-xMgTiO3 (x = 0, 0.05, 0.1 and 0.2) thin films were prepared through magnetron sputtering method and sintered at 850 °C for 1 h. Second phase of CuO and TiO2 were detected in MgTiO3 doped ceramic targets and CaTiO3 phase was observed when x = 0.2. SEM results showed that grain size increased with MgTiO3 doping, which is corresponding to the decrease of threshold voltage. Remarkable non-Ohmic behaviors of all samples were observed at different temperatures. Nonlinear coefficient increased from 2.803 (x = 0) to 3.799 (x = 0.2) and Schottky barrier height increased firstly then decreased as MgTiO3 doping. Double Schottky barrier model was used to explain the non-Ohmic characteristic of CCTO thin films.

Transformation of sputtered calcium copper titanate thin film into nanorods by sequential annealing

Rapid synthesis of long calcium copper titanate (CCTO) nanorods was carried out by sequential annealing. CCTO thin films have been deposited on p-Si substrate by RF sputtering technique and afterwards, the samples were thermally treated using a preheated furnace by varying the annealing temperature from 850°C to 1100°C. CCTO nanorods of 12µm lengths and 400–600nm diameters were synthesized at 1100°C. Based on the FESEM observations, a plausible growth mechanism has been proposed to explain the formation of nanorods. The (220) XRD peak of the CCTO film became prominent for the annealing temperature of 950°C. The presence of nanoscale crystals in amorphous matrix has been observed by HRTEM studies. The elemental mapping of CCTO nanorod has shown a spatial variation of elements throughout the nanorod. The oxide and interface charge density was found to be increased with the rise in annealing temperature.

Effects of deposition temperatures and substrates on microstructure and optical properties of sputtered CCTO thin film

Calcium copper titanium oxide (CaCu3Ti4O12, CCTO) films of 400nm thickness were deposited on ITO and FTO substrates by radio-frequency (RF) magnetron sputtering at different deposition temperatures (100, 150, 200, and 250°C). The influences of deposition temperatures and substrates on the structural, morphological, and optical properties of CCTO films were systematically investigated by XRD, AFM and UV–visible spectrophotometer. XRD study showed that all the CCTO films have a cubic structure. As the deposition temperature increased to 250°C, the film crystallinity was improved, the crystallite sizes became larger and the films deposited on FTO substrates exhibited better crystalline quality. AFM studies revealed that the surface roughness of the films was increased with increasing deposition temperatures for both substrates. The transmittance spectra of the films were more than 50% transparent in the visible region (500–900nm). It was also observed that the optical transmittance and energy band gaps reduced with increased deposition temperatures for both substrates.

Non-ohmic properties of CaCu3Ti4O12 thin films deposited By RF-sputtering

Calcium copper titanate (CaCu3Ti4O12, CCTO), thin films with polycrystalline nature have been deposited by RF sputtering on Pt/Ti/SiO2/Si (100) substrates at a room temperature followed by annealing at 600 °C for 2 h in a conventional furnace. The CCTO thin film present a cubic structure with lattice parameter a = 7.379 ± 0.001 A free of secondary phases. Dielectric spectroscopy was employed to examine the polycrystalline behaviour of CCTO material and the mechanisms responsible for the barrier-layer capacitances associated with Schottky-type barriers and the non-Ohmic properties. The film presents an electric breakdown field (Eb = 203 V cm−1) and then nonlinear coefficient (α = 6), which is even lower than that of the ZnO and SnO2 based varistors The observed electrical features of CCTO thin films are highly dependent on the [CaO12], [CaO4], [CuO11], [CuO11Vo x] and [TiO5· $$V_{O}^{ \bullet }$$ ] clusters.

Effects of annealing temperature on the structural, morphology, optical properties and resistivity of sputtered CCTO thin film

CaCu3Ti4O12 (CCTO) thin films with a thickness of 200 nm were deposited on ITO substrates by RF magnetron sputtering using a pure CCTO target. After the deposition, thin films were annealed at 400, 450, 500 and 550 °C, respectively, for 1 h. The effects of annealing temperature on the structural, surface morphology, optical properties and resistivity of (CCTO) thin films were investigated. The X-ray diffractometer results show that the thin films are polycrystalline in nature and are assigned to body-centered cubic perovskite configuration with a space group of Im-3. The intensity of the peaks and crystallinity gradually increased with the increase in annealing temperature. Microstructural investigation through FESEM showed that the grain size increased with increase in annealing temperature from 32 to 85 nm. The root mean square and roughness (Ra) were also enhanced with higher annealing temperatures, from 3.8 to 6.2 nm and from 4.7 to 7.7 nm, respectively, as confirmed by AFM. Increase in annealing temperature also affected the optical transmittance values which decreased to almost 60% at the visible range (550–850), as well as the optical energy band gap which decreased from 3.86 to 3.39 eV. The relevance between resistance behaviors and film microstructure is discussed. Therefore, it can be concluded that the desirable crystallinity, surface roughness, energy band gap and resistivity for 200 nm thick CCTO thin films deposited by RF magnetron sputtering can be achieved through the annealing process.