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Resistive switching and optical properties of strontium ferrate titanate thin film prepared via chemical solution deposition

Resistive switching and optical properties of strontium ferrate titanate thin film prepared via... Journal of Advanced Ceramics 2021, 10(5): 1001–1010 ISSN 2226-4108 https://doi.org/10.1007/s40145-021-0483-0 CN 10-1154/TQ Research Article Resistive switching and optical properties of strontium ferrate titanate thin film prepared via chemical solution deposition Jun LI, Xingui TANG , Qiuxiang LIU, Yanping JIANG, Zhenxun TANG School of Physics and Optoelectric Engineering, Guangdong University of Technology, Guangzhou 510006, China Received: September 17, 2020; Revised: April 10, 2021; Accepted: April 12, 2021 © The Author(s) 2021. Abstract: The polycrystalline strontium ferrate titanate (SrFe Ti O , SFTO) thin films have been 0.1 0.9 3 successfully prepared by chemical solution method. By analyzing the current–voltage (I–V) characteristics, we discuss the conduction mechanism of SFTO. It is found that the number of oxygen vacancy defects is increased by Fe ion doping, making SFTO be with better resistive switching property. Fe ion doping can also enhance the absorption of strontium titanate to be exposed to visible light, which is associated with the change of energy band. The band gap width (2.84 eV) of SFTO films is figured out, which is less than that of pure strontium titanate. Due to more oxygen vacancy defects caused by Fe ion doping, the band gap width of strontium titanate was reduced slightly. The defect types of SFTO thin films can be determined by electron paramagnetic resonance spectroscopy. In addition, we analyzed the energy band and state density of SFTO by first-principles calculation based on density functional theory, and found that Fe ion doping can reduce the band gap width of strontium titanate with micro-regulation on the band structure. A chemical state of SFTO was analyzed by X-ray photo electron spectroscopy. At the same time, the structure and morphology of SFTO were characterized by X-ray diffraction and scanning electron microscope. This study deepens further understanding of the influence of Fe ion doping on the structure and properties of strontium ferrate titanate, which is expected to be a functional thin film material for memristor devices. Keywords: SrTiO ; thin films; resistive switching; oxygen vacancy; first principles of applications (energy harvesting, memory device, 1 Introduction oxygen sensor, catalysis, fuel cell cathodes, etc.) [1–5]. There are a number of researches by first-principles studies During the past few decades, a large number of experimental of defective perovskite to investigate electronic structure and theoretical studies on ABO perovskite compounds and performance [6–11]. As for SrTiO , it can be changed have been carried out, especially the typical material into a p-type material by substituting Fe to Ti ions [7]. SrTiO . One motive force to study SrTiO perovskite is 3 3 Some previous work revealed the interesting properties the opportunity to regulate and control its electronic such as ferroelectric [8], photochromic [9], oxygen and ionic defect structures, so as to gain a wide range sensitive properties [10,11], as well as Jahn–Teller distortion [12]. In addition, dielectric properties, defect state, and conduction mechanism have been studied in * Corresponding author. strontium ferrate titanate system [13–16]. E-mail: xgtang@gdut.edu.cn www.springer.com/journal/40145 1002 J Adv Ceram 2021, 10(5): 1001–1010 In recent years, strontium ferrate titanate has attracted stabilizer, respectively. First, strontium acetate was more and more attention in studying defect chemistry, dissolved in methanol and iron nitrate was dissolved in carrier transport properties, and ferroelectric [17,18]. 36% acetic acid. In order to dissolve the strontium Compared with the pure SrTiO , defect state like oxygen acetate and iron 9-hydrate nitrate completely, we put vacancies in strontium ferrate titanate sustains charge the beaker on the heating table and stirred it for 2 h to balance due to the diverse valence states between Fe be dissolved at a temperature of 50 ℃. Secondly, appropriate amount of butyl titanate was taken from and Ti ions. The formation of vacancies and defect state affects both electrical and magnetic behaviors [19]. As the measuring cylinder, and 2 drops (about 0.04 mL) of acetylacetone were added to stabilize it. Finally, we the previous report said, the potential barrier height is influenced as a result of oxygen vacancy concentration mixed the three solutions together and bottled the mixed – 2– solution after filtering. The concentration of the final and chemisorption of O and O at the grain boundary [20]. mixed solution was 0.2 mol/L. In order to ensure the Fe ion doping of 10% content improves the ferroelectric quality of the prepared thin film, we put the prepared of SrTiO exactly. Moreover, ferromagnetic properties in solution stand at room temperature for one day and perovskites depend on not only oxygen vacancies and observed that there was no precipitate before casting the defects but also the annealing atmosphere and temperature film. FTO-coated glass substrates were cleaned completely [21]. Afterward, the annealing atmosphere effects on before the preparation. We put the FTO-coated glass device characteristics have been investigated [22,23]. substrates into ultrasonic cleaner for 15 min and dried the Up to now, iron-substituted SrTiO system has been substrates on the heating platform. The above operation studied by multifarious experimental and theoretical needs to be repeated three times to ensure that the researches. Nevertheless, there is still not any thorough substrates are thoroughly cleaned. The solution was elucidation of the relation to the concentration of point dropped onto the FTO substrate and spun at a speed of defects. An intensive understanding of the energy band 4000 rpm for 30 s. Two high speed spins were performed regulation–property relation is essential. In this work, according to the thickness of the required film. After that, we presented the preparation and electric properties thin film samples were baked on a heating table at 400 ℃ including resistive switching characteristic, ferroelectric, for 30 min. Hydrofluoric acid with low concentration and optoelectronic of SrFe Ti O thin film deposited 0.1 0.9 3–σ will not corrode the FTO layer. Therefore, we used the on fluorine doped tin oxide (FTO) coated glass substrate hydrofluoric acid (0.1 mol/L) to wipe one corner on the via chemical solution deposition method. In this study, SFTO layer of the thin film samples––in order to plate it is the first time to utilize the first-priciples theoretical the electrode on the surface after annealing. At the end, calculation based on density functional theory (DFT) all the thin film samples (SFTO and STO) were annealed for investigating the electronic structure and state density at 650 ℃ for 15 min in the oxygen atmosphere by the of perovskite material SrFe Ti O with 10% Fe ions x 1–x 3–σ rapid thermal annealing furnace RTP-1000D4 facility. to elaborate the experimental results, which can give us X-ray diffraction, X-ray photoelectron spectroscopy enlightenment to explore and verify the effect of doping (XPS), and electron paramagnetic resonance spectroscopy in band gap regulation. were used to characterize the structure, chemical states, and defect center of the thin film samples. The current– voltage (I–V) characteristic of the Au/SFTO/FTO/Glass 2 Experimental device was measured with the two-probe method by the Keithley 2400 programmable electrometer under room The method we used in our work to prepare the temperature. Additionally, first-principles calculations based SrFe Ti O (SFTO) thin film on the FTO (the 0.1 0.9 3–σ on density functional theory were used to analyze the thickness is 330 nm and the resistance is 7 Ω) coated band structure and electronic states of the SFTO samples. glass substrate (purchased from the School Experimental Raw Materials Purchasing Platform, Guangdong University of Technology) was chemical solution deposition (CSD) 3 Results and discussion [24]. The raw materials we selected were strontium acetate (0.859 g), iron 9-hydrate nitrate (0.161 g), and 3. 1 Structure, morphology, and chemical states butyl titanate (1.225 mL). In addition, methanol, and 36% acetic acid and acetylacetone were selected as solvent and To prove the crystalline state of the strontium ferrate www.springer.com/journal/40145 J Adv Ceram 2021, 10(5): 1001–1010 1003 titanate thin film we prepared, X-ray diffraction was sample. Two peaks of Sr 3d locating at 132.10 eV (Sr 3d ) 5/2 applied to characterize the structure of the samples. As and 133.78 eV (Sr 3d ) are observed in Fig. 3(a), 3/2 we can seen from Fig. 1, the red and blue curves represent which indicates Sr ions with chemical state of 2 [27]. the XRD results of SFTO and pure strontium titanate Two peaks of Ti 2p and Ti 2p are located at 457.52 3/2 1/2 (STO was prepared with SFTO at the same time and same and 463.24 eV in Fig. 3(b), which indicates Ti ions conditions), respectively. It can be seen from the periodic with chemical state of 4 . The O 1s spectra are shown table of chemical elements that the ionic radius of iron in Fig. 3(c), and the binding energy of O 1s spectra possesses two peaks. The stronger peak appears at is smaller than that of titanium. According to the Bragg 2– diffraction equation (2dsinθ = nλ), the XRD diffraction 528.59 eV and it is closely related to O ions which is peaks of SFTO shift slightly to the direction of small associated with O element in SFTO lattice [4]. The angle. Peaks of 23.3° (100), 32.5° (110), 46.6° (200), lower peak at 530.54 eV is owing to intermediate and 58.2° (211) crystal planes of SFTO were observed oxidation state for O element. It may be related to the obviously which are corresponding to JCPDS No. 35-0734 chemical-adsorbed oxygen on the thin film surface, from the Joint Committee on Power Diffraction Standards which is associated with defects such as oxygen (JCPDS) database, indicating that SFTO thin film is vacancies. As shown in Fig. 3(d), Fe 2p XPS spectra crystallized in cubic structure. No second phase was and 2p . The positions of contain a doublet of Fe 2p 3/2 1/2 observed in the X-ray diffraction result. Therefore, Fe the peaks are located at 709.60 and 723.25 eV, respectively. ions successfully replace Ti ions in strontium titanate It indicates Fe element with chemical state of 3 [28]. matrix and the same perovskite structure of strontium Due to spin–orbit coupling, Fe 2p peak is stronger 3/2 2+ 6 than Fe 2p peak. Fe electronic configuration is 3d , ferrite as strontium titanate is formed [25]. The scanning 1/2 3+ 5 2+ electron microscopy (SEM) result of cross section is while Fe is 3d . It means that the Fe would have a 3+ longer life-time in the comparison to Fe , and therefore shown in Fig. 2(a). As can be seen from the results of the cross section image (Fig. 2(a)), the stratification is the selected full width at half maximum (FWHM) of Fe 2p peak is expected to be smaller than the Fe 2p peak. obvious. The top layer is the strontium ferrate titanate 1/2 3/2 thin film, while the middle layer is FTO conductive layer, the bottom layer is the glass, the thickness of SFTO film is about 120 nm, and the particles are relatively dense [26]. Figure 2(b) shows the morphology observed by the atomic force microscopy (AFM). From Fig. 2(b), we can see the slight sharp grain morphology with the typical surface roughness which is closely related to the annealing temperature (SFTO was treated at the high temperature of 650 ℃). In order to analyze the chemical composition, proportion, and chemical valence of SFTO thin film sample, XPS results are shown in Fig. 3. Narrow spectrum scanning was acquired from the XPS spectra for the Sr 3d, Ti 2p, Fig. 1 XRD patterns of the SFTO film on FTO/Glass substrates. Ti O thin film O 1s, and Fe 2p levels of SrFe 0.1 0.9 3–σ Fig. 2 (a) Cross section SEM image and (b) AFM image of the SFTO/FTO/Glass device. www.springer.com/journal/40145 1004 J Adv Ceram 2021, 10(5): 1001–1010 Fig. 3 XPS survey of the SrFe Ti O thin film and the fitted narrow sweeping results of (a) Sr 3d, (b) Ti 2p, (c) O 1s, and 0.1 0.9 3–σ (d) Fe 2p. In addition, the bimodal spectra of Fe 2p indicate Fe FTO/Glass device increases with the gradual increase + 3+ 4+ element with 4 valence. Therefore, both Fe and Fe of negative voltage at the beginning, and the device is in a low resistance state (LRS), as shown in path 1 in the ions exist in SFTO thin film [29]. It has been reported in Ref. [30] that the increase of charge at the Fe site Fig. 4(b). When the negative bias exceeds the threshold voltage of 2.8 V, the current rapidly decreases from leads to the shifting of Fe 2p spectrum toward higher –2 –4 1.7×10 to 6×10 mA, and the state of the device binding energy. Thus, it can be concluded that SrTiO 3+ 4+ switches from LRS to high resistance state (HRS). As with 10% Fe ion doping can increase Fe and Fe ions the negative bias decreases, the current approaches 0, effectively, resulting in the formation of oxygen vacancy as shown in path 2 in Fig. 4(b). When the scanning defects [30]. The process can be briefly written into the voltage is from 0 to 3 V, the current of the thin film defect reaction equations to deepen the understanding device gradually increases from 0. When the voltage of Fe ion doping effect. The reactions are as following exceeds the forward threshold voltage of 2.95 V, the [31]: current drops rapidly, and the device switches from 2Ο  2V 4 eΟ (1)  Ο 2 LRS to HRS, as shown in path 3 in Fig. 4(b). When the 4+ 3+ forward bias gradually decreases from 3 to 0 V, the 2Fe + 2e′ → 2Fe (2) current intensity also decreases to 0, returning to the 3+ 2+ 2Fe + 2e′ → 2Fe (3) original state. At this point, a complete cycle test is where e' is the free electron, Ο represents the oxidation completed. It can be concluded that the SFTO thin film of lattice oxygen, and V is the oxygen vacancy. device has the resistance switching behavior and the performance of memristors. Under the same test conditions, 3. 2 I–V characteristics the resistive switching characteristics of SrTiO cannot Resistive switching behavior of Au/SFTO/FTO/Glass be found. We will discuss the reasons in the following device is shown in Fig. 4. Voltage sweeping (0 → –3 → sections. Figure 4(c) shows the I–V curve under semi- 0 → +3 → 0 V) is employed for I–V measurement. It logarithmic coordinates. The 100-cycle I–V curves are can be seen from Fig. 4(b) that the current of the Au/SFTO/ shown in Fig. S1 in the Electronic Supplementary Material www.springer.com/journal/40145 J Adv Ceram 2021, 10(5): 1001–1010 1005 (ESM). We can calculate the switch ratio from Fig. 4(c). applied bias, which is confirmed by the slopes of fitting Switch ratio (R /R ) at –2.8 V is about 30. We also curves of 1.05 and 1.07, respectively. However, conduction High Low tested the switch retention characteristics to ensure the mechanism switches to space-charge limited current (SCLC) resistive switching stability of the strontium ferrate mechanism in high voltage region, which is confirmed titanate. The cycle endurance results are shown in Figs. by the slopes of fitting curves of 1.93 and 2.13. In 5(a) and 5(b). Resistances of HRS and LRS are read at general, SCLC mechanism is mainly related to the presence –2.8 V and +2.8 V. Average ratio of HRS/LRS are about of defect centers. The current density of space-charge 30 and 10, respectively. From the results we can conclude can be explained as following [32]: limited current J SCLC that the resistive switching property of strontium ferrate 9V titanate has not only good stability but also higher J   (4) SCLC 8d switching ratio compared with the pure strontium titanate. where V is the bias voltage, d is the thickness of the thin In order to figure out the resistive switching mechanism film, ε is the dielectric constant of the thin film, and μ of Au/SFTO/FTO/Glass device, I–V characteristics under is the electron mobility. Under the application of bias negative and positive applied bias are replotted as logI voltage, the defect centers can capture the free carriers vs. logV and shown in Figs. 6(a) and 6(b), respectively. excited by the electric field. In order to understand the The slopes of fitting curve in LRS under negative and conduction mechanism of strontium ferrate titanate thin positive applied bias are 1.05 and 1.09, respectively, film devices more intuitively, a reasonable model is corresponding to the ohmic conduction mechanism. proposed to explain the role of oxygen vacancy in the For HRS, ohmic conduction mechanism is dominant in conduction process shown in Fig. 7. low voltage region under both negative and positive Fig. 4 Resistance properties of the SFTO/FTO/Glass device. (a) Measurement schematic diagram of the Au/SFTO/FTO/Glass device. (b) I–V characteristic curve in Cartesian coordinates at the bias voltage of ±3 V and I–V characteristics measuring structure is shown in the inset. The inset is the I–V characteristic of pure SrTiO in Cartesian coordinates. (c) I–V characteristic curve in logarithmic coordinates and the direction of measurement is shown in the order of arrows from 1 to 4. The inset is the I–V characteristic of pure SrTiO in logarithmic coordinates. Fig. 5 Resistance retention characteristics at: (a) –2.8 and (b) +2.8 V of the SFTO/FTO/Glass device. www.springer.com/journal/40145 1006 J Adv Ceram 2021, 10(5): 1001–1010 Fig. 6 Analysis of the conductance mechanism under (a) negative and (b) positive voltage bias fitted by the double logarithm. Fig. 7 Schematic diagram of the resistance switching phenomenon of Au/SFTO/FTO/Glass device based on the Schottky barrier. Oxygen vacancies are inevitable in the preparation of shown in Fig. 7(b). When the reverse bias voltage is thin films and the redistribution of electric charge on the applied, the barrier at the interface will increase, the surface of the devices is generated when the electrodes oxygen vacancy will release the captured electrons, and are plated to form the structure of the strontium ferrate the Fermi energy level of the film will decrease. However, titanate thin film devices. Therefore, before the device due to the increase of the barrier, only a small part of structure is formed without bias voltage, due to different the electrons on the surface of the gold electrode can work functions of the metal electrode and the strontium cross the barrier and enter the conduction band of the ferrate titanate thin film, the two different materials semiconductor thin film, thus forming an HRS which is contact at the interface and form the Schottky junction. shown in Fig. 7(c). When the direction of electric field is The electrons cannot freely cross the Schottky barrier, constantly changed, the strontium ferrate titanate thin and the Fermi level of the gold electrode is at the same film device can switch between high and low resistance level with the Fermi level of the strontium ferrate states. Additionally, the more oxygen vacancies the thin titanate thin film which is shown in Fig. 7(a). E is the film has, the higher ratio of high and low resistance states conduction band in the SFTO band structure, E is the the thin film would have. By doping Fe ions, the defect valence band, and E is the Fermi level. W represents concentration of strontium ferrate titanate thin film can be FS D the barrier width formed by the contact between gold changed to produce more oxygen vacancies which affect electrode and SFTO film. When the forward bias voltage the migration of the free electrons. Thus, by contrasting is added, the potential barrier at the interface will decrease, the I–V characteristics between the strontium ferrate the Fermi energy level of the strontium ferrate titanate titanate thin film and the pure strontium titanate thin film thin film will rise, and the electrons will fill into the (Figs. 4(b) and 4(c)) and the conduction mechanism oxygen vacancy and be captured by the oxygen vacancy. simulation diagram analysis (Fig. 7), we can draw the A large number of electrons tend to move toward the conclusion that strontium ferrate titanate thin film has direction of the gold electrode, forming an LRS, as relatively favorable resistive switching performance www.springer.com/journal/40145 J Adv Ceram 2021, 10(5): 1001–1010 1007 due to the increase of oxygen vacancies. obtained for SFTO thin film as shown in Fig. 8(b), being lower than the E of pure SrTiO (3.24 eV). The g 3 3. 3 Optical properties doping of Fe ions can lower the E of strontium titanate. Thus, SFTO thin films have better electric properties UV–Vis absorption spectra of SFTO thin film are shown which are expected for the application of photodetector in Fig. 8(a). We utilize UV–Vis spectrophotometer with faster response [33]. (ThermoFisher Evolution 220) to measure absorption spectrum of SFTO and STO. Before the measurement, we 3. 4 Electron paramagnetic resonance spectroscopy use blank sample (which provided from the measurement of SFTO and STO system) for calibration. After that, we choose the measurement mode of reflectance. The wavelength range In order to determine the defect type of the thin film of is from 330 to 600 nm. In order to ensure the accuracy strontium titanate with 10% Fe iron doping, we studied of reflection spectra, we repeat the measurement three its electron paramagnetic resonance (EPR) spectra. EPR times for each sample. We also utilize the same UV–Vis is a magnetic resonance technique used to characterize the spectrophotometer to measure the transmittance. The test unpaired electrons or single electron states of materials. wavelength range keeps the same with the reflection The relative intensity of SFTO is three times larger measurement. Transmittance of SFTO and STO thin film than that of the pure SrTiO . In fact, there are intrinsic rises with the increase of test wavelength. It is noted defects (such as oxygen vacancies and line defect) in that the reflection and transmittance measurement on pure SrTiO crystal structure and the defect states would different thin film samples must be calibrated respectively. produce defect energy level in the band gap which can We repeat the transmittance measurement three times trap the electrons during the electron transition process. for each sample as well for attaining accuracy results. From the EPR spectra, g factor of the sample can also be At last, we use the reflection spectra and transmission calculated and used to determine whether there is an spectrum of SFTO and STO thin film samples to calculate electron defect center or a whole defect center by comparing the absorption spectrum respectively. From Fig. 8(a), It it with the g factor of the free electron. Some researchers 2– 3+ can be seen that SFTO thin film absorbs more UV light used EPR technology to analyze O ions and Ti ions on with a wavelength lower than 365 nm, and the UV the surface of polycrystalline TiO and SrTiO , respectively 2 3 response is relatively sensitive. According to Tauc’s [34,35]. Figure 9 shows the EPR results of SFTO thin law, the relationship between absorption coefficient (α) film sample. It is found that the EPR peak occurs at a and photon energy (hν) can be expressed as position of 3800 Gs magnetic field intensity. The g factor α = (1/d) × ln(1/T) (5) of strontium titanate SFTO thin film sample can be calculated by the following equation: hν = 1240/λ (6) g = 0.7144773 ×/B (7) where T and λ are the transmittance and wavelength, 0 respectively. The optical band gap (E ) can be derived where  is the frequency and B is the magnetic field. g g 0 by the tangent line of the known curve. E of 2.84 eV is factor of SFTO thin film sample is 1.9237, which is Fig. 8 Comparison of optical properties between Fe doped strontium titanate and pure strontium titanate. (a) UV–Vis absorption spectrum with the wavelength range from 330 to 600 nm. (b) The calculated band gap results are the points at which the tangent line intersects the horizontal axis according to the Tauc’s law. www.springer.com/journal/40145 1008 J Adv Ceram 2021, 10(5): 1001–1010 It is closely related to the Fe–O bond length. Fe–O bond length will change when Fe ions substitute to Ti ions, producing relative displacement of oxygen and related oxygen defects [36]. The experiments show that Jahn–Teller distortion decreases with the increase of Fe ion concentration. Further analysis on electronic state density, band structure, and absorption spectrum can be studied via first-principles calculation based on density functional theory for SFTO. In order to better illustrate the Ti O , band structure and electron state density of SrFe 0.1 0.9 3–σ SrTiO and SrFeO are also studied. First-principles 3–σ 3–σ Fig. 9 EPR spectroscopy of strontium titanate films with calculations are carried out via the Vienna ab initio iron content of 10%. The defect type can be determined simulation package (VASP) utilizing the projector by calculating g from the EPR spectroscopy. augmented waves (PAW) technique [37]. Before the calculation based upon the density functional theory less than that of free electron (2.0023), indicating that (DFT), the k-point grid size and the advolution are set there is an electron defect center in the thin film sample. up with respect to the cut of energy. We ensure that To some extent, it can be concluded that Fe ions have each structure (referring to SrFe Ti O , SrTiO , and 0.1 0.9 3–σ 3–σ successfully replaced the Ti ions in strontium titanate matrix. SrFeO ) has a appropriate number of k points for the 3–σ Fe ions can attract more electron density distribution integration of Brillouin-zone. The electronic energies are than Ti ions and lead to the redistribution of electron –5 converged to at least 10 eV/atom. As for the modeling of cloud density near the Fe-doped impurity, which is SFTO (10% Fe atoms on the Ti site), it is important to closely related to the formation of oxygen vacancies. build an appropriate crystalline structure which is shown in Fig. S1 in the ESM. Calculation results are 3. 5 First-principles calculations shown in Fig. 10. Among them, Figs. 10(a) and 10(b) show It has been reported that a phenomenon of Jahn–Teller the band structure and electron state density of SrTiO , 3–σ distortion in strontium ferrate titanate was found [17]. respectively. The top of the valance band and the bottom Fig. 10 Calculated band structures and density of electron states for (a, b) SrTiO , (c, d) SrFe Ti O , and (e, f) SrFeO . In 3 0.1 0.9 3–σ 3 all cases, the presentation of the valence band and conduction band is concluded. www.springer.com/journal/40145 J Adv Ceram 2021, 10(5): 1001–1010 1009 of conduction band are mainly made up of O 2p states was enhanced. By absorption spectrum calculation, the and Ti 3d states, respectively. Figures 10(e) and 10(f) of the sample was 2.84 eV, slightly band gap width E illustrate the band structure and electron state density smaller than the band gap width 3.24 eV of strontium of SrFeO . It is obviously seen from the comparison titanate. The experimental results were consistent with the 3– of the electron state density for spinning up and spinning theoretical calculation. down of the electron densities, where the effect of spin polarization is derived from the Fe 2p states, a little bit Acknowledgements from Fe 3d states. The results in Figs. 10(e) and 10(f) also indicate that there is a strong superposition of Fe This work was supported by the National Natural Science 3d orbital electron states and O 2p orbital electron states Foundation of China (Grant No. 11574057), the Guangdong near the Fermi level. In addition, the contribution of the Basic and Applied Basic Research Foundation (Grant No. main electron states near the Fermi level is derived 2021A1515012607), and the Science and Technology from the Eg states of the Fe ions and O 2p electron Program of Guangdong Province of China (Grant No. states. Figures 10(c) and 10(d) show the band structure 2017A010104022). and electron state density of SrFe Ti O . The top 0.1 0.9 3–σ of the valence band consists of O 2p orbital electron Electronic Supplementary Material states, while the bottom of virtual bands mainly originates from Ti 3d electron states [38]. It can be concluded that Supplementary material is available in the online version the band gap of strontium titanate with 10% Fe-doping of this article at https://doi.org/10.1007/s40145-021-0483-0. is smaller than that of the pure strontium titanate, which indicates that Fe ions can regulate the energy band of References strontium titanate and form the defect state effectively. 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Resistive switching and optical properties of strontium ferrate titanate thin film prepared via chemical solution deposition

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2226-4108
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10.1007/s40145-021-0483-0
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Abstract

Journal of Advanced Ceramics 2021, 10(5): 1001–1010 ISSN 2226-4108 https://doi.org/10.1007/s40145-021-0483-0 CN 10-1154/TQ Research Article Resistive switching and optical properties of strontium ferrate titanate thin film prepared via chemical solution deposition Jun LI, Xingui TANG , Qiuxiang LIU, Yanping JIANG, Zhenxun TANG School of Physics and Optoelectric Engineering, Guangdong University of Technology, Guangzhou 510006, China Received: September 17, 2020; Revised: April 10, 2021; Accepted: April 12, 2021 © The Author(s) 2021. Abstract: The polycrystalline strontium ferrate titanate (SrFe Ti O , SFTO) thin films have been 0.1 0.9 3 successfully prepared by chemical solution method. By analyzing the current–voltage (I–V) characteristics, we discuss the conduction mechanism of SFTO. It is found that the number of oxygen vacancy defects is increased by Fe ion doping, making SFTO be with better resistive switching property. Fe ion doping can also enhance the absorption of strontium titanate to be exposed to visible light, which is associated with the change of energy band. The band gap width (2.84 eV) of SFTO films is figured out, which is less than that of pure strontium titanate. Due to more oxygen vacancy defects caused by Fe ion doping, the band gap width of strontium titanate was reduced slightly. The defect types of SFTO thin films can be determined by electron paramagnetic resonance spectroscopy. In addition, we analyzed the energy band and state density of SFTO by first-principles calculation based on density functional theory, and found that Fe ion doping can reduce the band gap width of strontium titanate with micro-regulation on the band structure. A chemical state of SFTO was analyzed by X-ray photo electron spectroscopy. At the same time, the structure and morphology of SFTO were characterized by X-ray diffraction and scanning electron microscope. This study deepens further understanding of the influence of Fe ion doping on the structure and properties of strontium ferrate titanate, which is expected to be a functional thin film material for memristor devices. Keywords: SrTiO ; thin films; resistive switching; oxygen vacancy; first principles of applications (energy harvesting, memory device, 1 Introduction oxygen sensor, catalysis, fuel cell cathodes, etc.) [1–5]. There are a number of researches by first-principles studies During the past few decades, a large number of experimental of defective perovskite to investigate electronic structure and theoretical studies on ABO perovskite compounds and performance [6–11]. As for SrTiO , it can be changed have been carried out, especially the typical material into a p-type material by substituting Fe to Ti ions [7]. SrTiO . One motive force to study SrTiO perovskite is 3 3 Some previous work revealed the interesting properties the opportunity to regulate and control its electronic such as ferroelectric [8], photochromic [9], oxygen and ionic defect structures, so as to gain a wide range sensitive properties [10,11], as well as Jahn–Teller distortion [12]. In addition, dielectric properties, defect state, and conduction mechanism have been studied in * Corresponding author. strontium ferrate titanate system [13–16]. E-mail: xgtang@gdut.edu.cn www.springer.com/journal/40145 1002 J Adv Ceram 2021, 10(5): 1001–1010 In recent years, strontium ferrate titanate has attracted stabilizer, respectively. First, strontium acetate was more and more attention in studying defect chemistry, dissolved in methanol and iron nitrate was dissolved in carrier transport properties, and ferroelectric [17,18]. 36% acetic acid. In order to dissolve the strontium Compared with the pure SrTiO , defect state like oxygen acetate and iron 9-hydrate nitrate completely, we put vacancies in strontium ferrate titanate sustains charge the beaker on the heating table and stirred it for 2 h to balance due to the diverse valence states between Fe be dissolved at a temperature of 50 ℃. Secondly, appropriate amount of butyl titanate was taken from and Ti ions. The formation of vacancies and defect state affects both electrical and magnetic behaviors [19]. As the measuring cylinder, and 2 drops (about 0.04 mL) of acetylacetone were added to stabilize it. Finally, we the previous report said, the potential barrier height is influenced as a result of oxygen vacancy concentration mixed the three solutions together and bottled the mixed – 2– solution after filtering. The concentration of the final and chemisorption of O and O at the grain boundary [20]. mixed solution was 0.2 mol/L. In order to ensure the Fe ion doping of 10% content improves the ferroelectric quality of the prepared thin film, we put the prepared of SrTiO exactly. Moreover, ferromagnetic properties in solution stand at room temperature for one day and perovskites depend on not only oxygen vacancies and observed that there was no precipitate before casting the defects but also the annealing atmosphere and temperature film. FTO-coated glass substrates were cleaned completely [21]. Afterward, the annealing atmosphere effects on before the preparation. We put the FTO-coated glass device characteristics have been investigated [22,23]. substrates into ultrasonic cleaner for 15 min and dried the Up to now, iron-substituted SrTiO system has been substrates on the heating platform. The above operation studied by multifarious experimental and theoretical needs to be repeated three times to ensure that the researches. Nevertheless, there is still not any thorough substrates are thoroughly cleaned. The solution was elucidation of the relation to the concentration of point dropped onto the FTO substrate and spun at a speed of defects. An intensive understanding of the energy band 4000 rpm for 30 s. Two high speed spins were performed regulation–property relation is essential. In this work, according to the thickness of the required film. After that, we presented the preparation and electric properties thin film samples were baked on a heating table at 400 ℃ including resistive switching characteristic, ferroelectric, for 30 min. Hydrofluoric acid with low concentration and optoelectronic of SrFe Ti O thin film deposited 0.1 0.9 3–σ will not corrode the FTO layer. Therefore, we used the on fluorine doped tin oxide (FTO) coated glass substrate hydrofluoric acid (0.1 mol/L) to wipe one corner on the via chemical solution deposition method. In this study, SFTO layer of the thin film samples––in order to plate it is the first time to utilize the first-priciples theoretical the electrode on the surface after annealing. At the end, calculation based on density functional theory (DFT) all the thin film samples (SFTO and STO) were annealed for investigating the electronic structure and state density at 650 ℃ for 15 min in the oxygen atmosphere by the of perovskite material SrFe Ti O with 10% Fe ions x 1–x 3–σ rapid thermal annealing furnace RTP-1000D4 facility. to elaborate the experimental results, which can give us X-ray diffraction, X-ray photoelectron spectroscopy enlightenment to explore and verify the effect of doping (XPS), and electron paramagnetic resonance spectroscopy in band gap regulation. were used to characterize the structure, chemical states, and defect center of the thin film samples. The current– voltage (I–V) characteristic of the Au/SFTO/FTO/Glass 2 Experimental device was measured with the two-probe method by the Keithley 2400 programmable electrometer under room The method we used in our work to prepare the temperature. Additionally, first-principles calculations based SrFe Ti O (SFTO) thin film on the FTO (the 0.1 0.9 3–σ on density functional theory were used to analyze the thickness is 330 nm and the resistance is 7 Ω) coated band structure and electronic states of the SFTO samples. glass substrate (purchased from the School Experimental Raw Materials Purchasing Platform, Guangdong University of Technology) was chemical solution deposition (CSD) 3 Results and discussion [24]. The raw materials we selected were strontium acetate (0.859 g), iron 9-hydrate nitrate (0.161 g), and 3. 1 Structure, morphology, and chemical states butyl titanate (1.225 mL). In addition, methanol, and 36% acetic acid and acetylacetone were selected as solvent and To prove the crystalline state of the strontium ferrate www.springer.com/journal/40145 J Adv Ceram 2021, 10(5): 1001–1010 1003 titanate thin film we prepared, X-ray diffraction was sample. Two peaks of Sr 3d locating at 132.10 eV (Sr 3d ) 5/2 applied to characterize the structure of the samples. As and 133.78 eV (Sr 3d ) are observed in Fig. 3(a), 3/2 we can seen from Fig. 1, the red and blue curves represent which indicates Sr ions with chemical state of 2 [27]. the XRD results of SFTO and pure strontium titanate Two peaks of Ti 2p and Ti 2p are located at 457.52 3/2 1/2 (STO was prepared with SFTO at the same time and same and 463.24 eV in Fig. 3(b), which indicates Ti ions conditions), respectively. It can be seen from the periodic with chemical state of 4 . The O 1s spectra are shown table of chemical elements that the ionic radius of iron in Fig. 3(c), and the binding energy of O 1s spectra possesses two peaks. The stronger peak appears at is smaller than that of titanium. According to the Bragg 2– diffraction equation (2dsinθ = nλ), the XRD diffraction 528.59 eV and it is closely related to O ions which is peaks of SFTO shift slightly to the direction of small associated with O element in SFTO lattice [4]. The angle. Peaks of 23.3° (100), 32.5° (110), 46.6° (200), lower peak at 530.54 eV is owing to intermediate and 58.2° (211) crystal planes of SFTO were observed oxidation state for O element. It may be related to the obviously which are corresponding to JCPDS No. 35-0734 chemical-adsorbed oxygen on the thin film surface, from the Joint Committee on Power Diffraction Standards which is associated with defects such as oxygen (JCPDS) database, indicating that SFTO thin film is vacancies. As shown in Fig. 3(d), Fe 2p XPS spectra crystallized in cubic structure. No second phase was and 2p . The positions of contain a doublet of Fe 2p 3/2 1/2 observed in the X-ray diffraction result. Therefore, Fe the peaks are located at 709.60 and 723.25 eV, respectively. ions successfully replace Ti ions in strontium titanate It indicates Fe element with chemical state of 3 [28]. matrix and the same perovskite structure of strontium Due to spin–orbit coupling, Fe 2p peak is stronger 3/2 2+ 6 than Fe 2p peak. Fe electronic configuration is 3d , ferrite as strontium titanate is formed [25]. The scanning 1/2 3+ 5 2+ electron microscopy (SEM) result of cross section is while Fe is 3d . It means that the Fe would have a 3+ longer life-time in the comparison to Fe , and therefore shown in Fig. 2(a). As can be seen from the results of the cross section image (Fig. 2(a)), the stratification is the selected full width at half maximum (FWHM) of Fe 2p peak is expected to be smaller than the Fe 2p peak. obvious. The top layer is the strontium ferrate titanate 1/2 3/2 thin film, while the middle layer is FTO conductive layer, the bottom layer is the glass, the thickness of SFTO film is about 120 nm, and the particles are relatively dense [26]. Figure 2(b) shows the morphology observed by the atomic force microscopy (AFM). From Fig. 2(b), we can see the slight sharp grain morphology with the typical surface roughness which is closely related to the annealing temperature (SFTO was treated at the high temperature of 650 ℃). In order to analyze the chemical composition, proportion, and chemical valence of SFTO thin film sample, XPS results are shown in Fig. 3. Narrow spectrum scanning was acquired from the XPS spectra for the Sr 3d, Ti 2p, Fig. 1 XRD patterns of the SFTO film on FTO/Glass substrates. Ti O thin film O 1s, and Fe 2p levels of SrFe 0.1 0.9 3–σ Fig. 2 (a) Cross section SEM image and (b) AFM image of the SFTO/FTO/Glass device. www.springer.com/journal/40145 1004 J Adv Ceram 2021, 10(5): 1001–1010 Fig. 3 XPS survey of the SrFe Ti O thin film and the fitted narrow sweeping results of (a) Sr 3d, (b) Ti 2p, (c) O 1s, and 0.1 0.9 3–σ (d) Fe 2p. In addition, the bimodal spectra of Fe 2p indicate Fe FTO/Glass device increases with the gradual increase + 3+ 4+ element with 4 valence. Therefore, both Fe and Fe of negative voltage at the beginning, and the device is in a low resistance state (LRS), as shown in path 1 in the ions exist in SFTO thin film [29]. It has been reported in Ref. [30] that the increase of charge at the Fe site Fig. 4(b). When the negative bias exceeds the threshold voltage of 2.8 V, the current rapidly decreases from leads to the shifting of Fe 2p spectrum toward higher –2 –4 1.7×10 to 6×10 mA, and the state of the device binding energy. Thus, it can be concluded that SrTiO 3+ 4+ switches from LRS to high resistance state (HRS). As with 10% Fe ion doping can increase Fe and Fe ions the negative bias decreases, the current approaches 0, effectively, resulting in the formation of oxygen vacancy as shown in path 2 in Fig. 4(b). When the scanning defects [30]. The process can be briefly written into the voltage is from 0 to 3 V, the current of the thin film defect reaction equations to deepen the understanding device gradually increases from 0. When the voltage of Fe ion doping effect. The reactions are as following exceeds the forward threshold voltage of 2.95 V, the [31]: current drops rapidly, and the device switches from 2Ο  2V 4 eΟ (1)  Ο 2 LRS to HRS, as shown in path 3 in Fig. 4(b). When the 4+ 3+ forward bias gradually decreases from 3 to 0 V, the 2Fe + 2e′ → 2Fe (2) current intensity also decreases to 0, returning to the 3+ 2+ 2Fe + 2e′ → 2Fe (3) original state. At this point, a complete cycle test is where e' is the free electron, Ο represents the oxidation completed. It can be concluded that the SFTO thin film of lattice oxygen, and V is the oxygen vacancy. device has the resistance switching behavior and the performance of memristors. Under the same test conditions, 3. 2 I–V characteristics the resistive switching characteristics of SrTiO cannot Resistive switching behavior of Au/SFTO/FTO/Glass be found. We will discuss the reasons in the following device is shown in Fig. 4. Voltage sweeping (0 → –3 → sections. Figure 4(c) shows the I–V curve under semi- 0 → +3 → 0 V) is employed for I–V measurement. It logarithmic coordinates. The 100-cycle I–V curves are can be seen from Fig. 4(b) that the current of the Au/SFTO/ shown in Fig. S1 in the Electronic Supplementary Material www.springer.com/journal/40145 J Adv Ceram 2021, 10(5): 1001–1010 1005 (ESM). We can calculate the switch ratio from Fig. 4(c). applied bias, which is confirmed by the slopes of fitting Switch ratio (R /R ) at –2.8 V is about 30. We also curves of 1.05 and 1.07, respectively. However, conduction High Low tested the switch retention characteristics to ensure the mechanism switches to space-charge limited current (SCLC) resistive switching stability of the strontium ferrate mechanism in high voltage region, which is confirmed titanate. The cycle endurance results are shown in Figs. by the slopes of fitting curves of 1.93 and 2.13. In 5(a) and 5(b). Resistances of HRS and LRS are read at general, SCLC mechanism is mainly related to the presence –2.8 V and +2.8 V. Average ratio of HRS/LRS are about of defect centers. The current density of space-charge 30 and 10, respectively. From the results we can conclude can be explained as following [32]: limited current J SCLC that the resistive switching property of strontium ferrate 9V titanate has not only good stability but also higher J   (4) SCLC 8d switching ratio compared with the pure strontium titanate. where V is the bias voltage, d is the thickness of the thin In order to figure out the resistive switching mechanism film, ε is the dielectric constant of the thin film, and μ of Au/SFTO/FTO/Glass device, I–V characteristics under is the electron mobility. Under the application of bias negative and positive applied bias are replotted as logI voltage, the defect centers can capture the free carriers vs. logV and shown in Figs. 6(a) and 6(b), respectively. excited by the electric field. In order to understand the The slopes of fitting curve in LRS under negative and conduction mechanism of strontium ferrate titanate thin positive applied bias are 1.05 and 1.09, respectively, film devices more intuitively, a reasonable model is corresponding to the ohmic conduction mechanism. proposed to explain the role of oxygen vacancy in the For HRS, ohmic conduction mechanism is dominant in conduction process shown in Fig. 7. low voltage region under both negative and positive Fig. 4 Resistance properties of the SFTO/FTO/Glass device. (a) Measurement schematic diagram of the Au/SFTO/FTO/Glass device. (b) I–V characteristic curve in Cartesian coordinates at the bias voltage of ±3 V and I–V characteristics measuring structure is shown in the inset. The inset is the I–V characteristic of pure SrTiO in Cartesian coordinates. (c) I–V characteristic curve in logarithmic coordinates and the direction of measurement is shown in the order of arrows from 1 to 4. The inset is the I–V characteristic of pure SrTiO in logarithmic coordinates. Fig. 5 Resistance retention characteristics at: (a) –2.8 and (b) +2.8 V of the SFTO/FTO/Glass device. www.springer.com/journal/40145 1006 J Adv Ceram 2021, 10(5): 1001–1010 Fig. 6 Analysis of the conductance mechanism under (a) negative and (b) positive voltage bias fitted by the double logarithm. Fig. 7 Schematic diagram of the resistance switching phenomenon of Au/SFTO/FTO/Glass device based on the Schottky barrier. Oxygen vacancies are inevitable in the preparation of shown in Fig. 7(b). When the reverse bias voltage is thin films and the redistribution of electric charge on the applied, the barrier at the interface will increase, the surface of the devices is generated when the electrodes oxygen vacancy will release the captured electrons, and are plated to form the structure of the strontium ferrate the Fermi energy level of the film will decrease. However, titanate thin film devices. Therefore, before the device due to the increase of the barrier, only a small part of structure is formed without bias voltage, due to different the electrons on the surface of the gold electrode can work functions of the metal electrode and the strontium cross the barrier and enter the conduction band of the ferrate titanate thin film, the two different materials semiconductor thin film, thus forming an HRS which is contact at the interface and form the Schottky junction. shown in Fig. 7(c). When the direction of electric field is The electrons cannot freely cross the Schottky barrier, constantly changed, the strontium ferrate titanate thin and the Fermi level of the gold electrode is at the same film device can switch between high and low resistance level with the Fermi level of the strontium ferrate states. Additionally, the more oxygen vacancies the thin titanate thin film which is shown in Fig. 7(a). E is the film has, the higher ratio of high and low resistance states conduction band in the SFTO band structure, E is the the thin film would have. By doping Fe ions, the defect valence band, and E is the Fermi level. W represents concentration of strontium ferrate titanate thin film can be FS D the barrier width formed by the contact between gold changed to produce more oxygen vacancies which affect electrode and SFTO film. When the forward bias voltage the migration of the free electrons. Thus, by contrasting is added, the potential barrier at the interface will decrease, the I–V characteristics between the strontium ferrate the Fermi energy level of the strontium ferrate titanate titanate thin film and the pure strontium titanate thin film thin film will rise, and the electrons will fill into the (Figs. 4(b) and 4(c)) and the conduction mechanism oxygen vacancy and be captured by the oxygen vacancy. simulation diagram analysis (Fig. 7), we can draw the A large number of electrons tend to move toward the conclusion that strontium ferrate titanate thin film has direction of the gold electrode, forming an LRS, as relatively favorable resistive switching performance www.springer.com/journal/40145 J Adv Ceram 2021, 10(5): 1001–1010 1007 due to the increase of oxygen vacancies. obtained for SFTO thin film as shown in Fig. 8(b), being lower than the E of pure SrTiO (3.24 eV). The g 3 3. 3 Optical properties doping of Fe ions can lower the E of strontium titanate. Thus, SFTO thin films have better electric properties UV–Vis absorption spectra of SFTO thin film are shown which are expected for the application of photodetector in Fig. 8(a). We utilize UV–Vis spectrophotometer with faster response [33]. (ThermoFisher Evolution 220) to measure absorption spectrum of SFTO and STO. Before the measurement, we 3. 4 Electron paramagnetic resonance spectroscopy use blank sample (which provided from the measurement of SFTO and STO system) for calibration. After that, we choose the measurement mode of reflectance. The wavelength range In order to determine the defect type of the thin film of is from 330 to 600 nm. In order to ensure the accuracy strontium titanate with 10% Fe iron doping, we studied of reflection spectra, we repeat the measurement three its electron paramagnetic resonance (EPR) spectra. EPR times for each sample. We also utilize the same UV–Vis is a magnetic resonance technique used to characterize the spectrophotometer to measure the transmittance. The test unpaired electrons or single electron states of materials. wavelength range keeps the same with the reflection The relative intensity of SFTO is three times larger measurement. Transmittance of SFTO and STO thin film than that of the pure SrTiO . In fact, there are intrinsic rises with the increase of test wavelength. It is noted defects (such as oxygen vacancies and line defect) in that the reflection and transmittance measurement on pure SrTiO crystal structure and the defect states would different thin film samples must be calibrated respectively. produce defect energy level in the band gap which can We repeat the transmittance measurement three times trap the electrons during the electron transition process. for each sample as well for attaining accuracy results. From the EPR spectra, g factor of the sample can also be At last, we use the reflection spectra and transmission calculated and used to determine whether there is an spectrum of SFTO and STO thin film samples to calculate electron defect center or a whole defect center by comparing the absorption spectrum respectively. From Fig. 8(a), It it with the g factor of the free electron. Some researchers 2– 3+ can be seen that SFTO thin film absorbs more UV light used EPR technology to analyze O ions and Ti ions on with a wavelength lower than 365 nm, and the UV the surface of polycrystalline TiO and SrTiO , respectively 2 3 response is relatively sensitive. According to Tauc’s [34,35]. Figure 9 shows the EPR results of SFTO thin law, the relationship between absorption coefficient (α) film sample. It is found that the EPR peak occurs at a and photon energy (hν) can be expressed as position of 3800 Gs magnetic field intensity. The g factor α = (1/d) × ln(1/T) (5) of strontium titanate SFTO thin film sample can be calculated by the following equation: hν = 1240/λ (6) g = 0.7144773 ×/B (7) where T and λ are the transmittance and wavelength, 0 respectively. The optical band gap (E ) can be derived where  is the frequency and B is the magnetic field. g g 0 by the tangent line of the known curve. E of 2.84 eV is factor of SFTO thin film sample is 1.9237, which is Fig. 8 Comparison of optical properties between Fe doped strontium titanate and pure strontium titanate. (a) UV–Vis absorption spectrum with the wavelength range from 330 to 600 nm. (b) The calculated band gap results are the points at which the tangent line intersects the horizontal axis according to the Tauc’s law. www.springer.com/journal/40145 1008 J Adv Ceram 2021, 10(5): 1001–1010 It is closely related to the Fe–O bond length. Fe–O bond length will change when Fe ions substitute to Ti ions, producing relative displacement of oxygen and related oxygen defects [36]. The experiments show that Jahn–Teller distortion decreases with the increase of Fe ion concentration. Further analysis on electronic state density, band structure, and absorption spectrum can be studied via first-principles calculation based on density functional theory for SFTO. In order to better illustrate the Ti O , band structure and electron state density of SrFe 0.1 0.9 3–σ SrTiO and SrFeO are also studied. First-principles 3–σ 3–σ Fig. 9 EPR spectroscopy of strontium titanate films with calculations are carried out via the Vienna ab initio iron content of 10%. The defect type can be determined simulation package (VASP) utilizing the projector by calculating g from the EPR spectroscopy. augmented waves (PAW) technique [37]. Before the calculation based upon the density functional theory less than that of free electron (2.0023), indicating that (DFT), the k-point grid size and the advolution are set there is an electron defect center in the thin film sample. up with respect to the cut of energy. We ensure that To some extent, it can be concluded that Fe ions have each structure (referring to SrFe Ti O , SrTiO , and 0.1 0.9 3–σ 3–σ successfully replaced the Ti ions in strontium titanate matrix. SrFeO ) has a appropriate number of k points for the 3–σ Fe ions can attract more electron density distribution integration of Brillouin-zone. The electronic energies are than Ti ions and lead to the redistribution of electron –5 converged to at least 10 eV/atom. As for the modeling of cloud density near the Fe-doped impurity, which is SFTO (10% Fe atoms on the Ti site), it is important to closely related to the formation of oxygen vacancies. build an appropriate crystalline structure which is shown in Fig. S1 in the ESM. Calculation results are 3. 5 First-principles calculations shown in Fig. 10. Among them, Figs. 10(a) and 10(b) show It has been reported that a phenomenon of Jahn–Teller the band structure and electron state density of SrTiO , 3–σ distortion in strontium ferrate titanate was found [17]. respectively. The top of the valance band and the bottom Fig. 10 Calculated band structures and density of electron states for (a, b) SrTiO , (c, d) SrFe Ti O , and (e, f) SrFeO . In 3 0.1 0.9 3–σ 3 all cases, the presentation of the valence band and conduction band is concluded. www.springer.com/journal/40145 J Adv Ceram 2021, 10(5): 1001–1010 1009 of conduction band are mainly made up of O 2p states was enhanced. By absorption spectrum calculation, the and Ti 3d states, respectively. Figures 10(e) and 10(f) of the sample was 2.84 eV, slightly band gap width E illustrate the band structure and electron state density smaller than the band gap width 3.24 eV of strontium of SrFeO . It is obviously seen from the comparison titanate. The experimental results were consistent with the 3– of the electron state density for spinning up and spinning theoretical calculation. down of the electron densities, where the effect of spin polarization is derived from the Fe 2p states, a little bit Acknowledgements from Fe 3d states. The results in Figs. 10(e) and 10(f) also indicate that there is a strong superposition of Fe This work was supported by the National Natural Science 3d orbital electron states and O 2p orbital electron states Foundation of China (Grant No. 11574057), the Guangdong near the Fermi level. In addition, the contribution of the Basic and Applied Basic Research Foundation (Grant No. main electron states near the Fermi level is derived 2021A1515012607), and the Science and Technology from the Eg states of the Fe ions and O 2p electron Program of Guangdong Province of China (Grant No. states. Figures 10(c) and 10(d) show the band structure 2017A010104022). and electron state density of SrFe Ti O . The top 0.1 0.9 3–σ of the valence band consists of O 2p orbital electron Electronic Supplementary Material states, while the bottom of virtual bands mainly originates from Ti 3d electron states [38]. It can be concluded that Supplementary material is available in the online version the band gap of strontium titanate with 10% Fe-doping of this article at https://doi.org/10.1007/s40145-021-0483-0. is smaller than that of the pure strontium titanate, which indicates that Fe ions can regulate the energy band of References strontium titanate and form the defect state effectively. 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Journal

Journal of Advanced CeramicsSpringer Journals

Published: Oct 1, 2021

Keywords: SrTiO3; thin films; resistive switching; oxygen vacancy; first principles

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