Fluorination-enhanced photoconductive effect in a wide band gap Ca3Ti2O7-x F x thin films

In this work, Ca3Ti2O7-x F x thin films on (110) SrTiO3 substrates were prepared by two steps as deposited via pulsed laser deposition and fluorinated with polyvinylidene fluoride. Despite the unchanged crystal structure of the fluorinated films, the changed valence state can be used to confirm the incorporation of F−1 and the weakened chemical bond of Ca–O. Furthermore, we found that the photoelectric switch can be observed at a wide range of light wavelength from 405 nm to 808 nm. It is found that the photosensitivity of 4 × 104 (405 nm) in the fluorine has been increased by two orders of magnitude, which is most likely due to the deep energy levels in the reduced band gap of 2.3 eV. This work paves the way for the enhanced photoconductive devices via the anionic defect engineering.


Introduction
The 327 Ruddlesden-Popper (RP) layered A 3 B 2 O 7 compounds have been continuously investigated due to their special crystal structure and varying physical properties [1][2][3][4]. The inserted A-O layers in the 327 phase is different from the familiar perovskite ABO 3 , which can bring in two different layers defined as rock-salt and perovskite layers [5]. In such orthorhombic oxides, two most studied systems are the Ca 3 Ti 2 O 7 (CTO 7 ) and Ca 3 Mn 2 O 7 compounds. The competition between interlayer rumpling and rotations occurred in this layer structure can lead to ferroelectricity or even multiferroictiy [4,6]. Although the ferroelectricity in RP material was derived from first principles calculations, the relevant switching of polarization still has been rarely observed in the pure 327 compounds. Therefore, the previous studies have come back to the traditional tuning method of cation substitution. To overcome the larger energy barrier for switchable polarization, the cation doping at the A-sites is a very efficient way for hybrid improper ferroelectricity in the CTO 7 -based bulk [2,7,8]. Except for the above multiferroic behavior, the improved photocatalytic activity and photoluminescence have been formed via Rh and Eu substitution, respectively [9,10]. Furthermore, to the pure CTO 7 , it is worth pointing out that the stoichiometric bulk shows a direct gap of 3.94 eV while the Sr substitution for the reduced energy gap can be neglected [11]. Hence, we need to find out some else way to search the tuning effect on the electronic structure in the thin films with a wide band gap.
Despite the traditional cation doping, the anionic doping has been confirmed to be an effective alternative to enhance the physical properties of oxide films. The first one is oxygen vacancy, which is introduced during the deposition with high vacuum. A length of chemical bond and angle of the oxygen octahedron rotation can be changed by the oxygen vacancy effect [12]. However, the strain is often accompanied by this doping, causing that a competition mechanism can further result in an opposite tuning effect on the material character [13][14][15][16]. The Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. second one is fluorinated via the annealing after the deposition, from the thermal decomposition of polyvinylidene fluoride (PVDF) [17][18][19]. Monovalent F -1 ions substituting for O 2+ sites provide not only a certain concentration of electron donors but also a simulation as half oxygen vacancy [20]. Thus a wide range of varying in crystal lattice and resistance with the fluorine content can be realized in the fluorinated films [19,21]. Additionally, the distortion of Mn octahedral can induce a local magnetic order in the electron doped SrMnO 3 -based system [22,23]. Here we report the effective way to prepare three groups of samples including CTO 7-δ , CTO 7 and CTO 7-x F x films and compare the defect-induced photoconductive character of them. Our investigation begins with the definite crystal structure and verified fluorination in the CTO 7-x F x films. After the fluorinated treatment, the changed strain and chemical bonds can further lead to the subsequent photoelectric effect. Especially in fluorinated sample, there is a strong dependence between the photoelectric properties and the light of varied wavelength. Furthermore, we discuss that the origin of improved photoconductive behavior in the CTO 7-x F x film is the anionic doping and electronic reconstruction, via photosensitivity measurement and first-principles calculation.

Methods
The as-deposited CTO 7-δ thin films on atomically flat SrTiO 3 (STO, 110) substrates were deposited via pulsed laser deposition (PLD, Lambda Physik, 248 nm, 3 Hz, 2 J cm −2 ). For the stoichiometric films, the fabrication process was maintained at the temperature of 850°C and oxygen pressure of 0.1 mbar. Then the precursor films wrapped in Al foil were annealed with 0.1g of PVDF, under a flowing gas of argon for 24 h to introduce fluorine. As shown in figure 1(a), the PVDF pellets need to be placed on the foil rather than directly on the film sample, in order to fully fluorine dope rather than the charcoal-like adhere [21]. The thickness of three samples are kept at around 50 nm. The lattice constants and epitaxial characters of films were determined via an x-ray diffraction (XRD) θ-2θ scan and reciprocal space mapping (RSM), using the Panalytical Empyrean concept, respectively. The presence and valence state of Ca, Ti, O, and F in the CTO 7-x F x film were confirmed by x-ray photoemission spectroscopy (XPS) at PHI5000 VersaProbe. Three sets of films were chosen to measure for the photoconductive effect, including as-deposited CTO 7-δ , CTO 7 (annealed in an oxygen atmosphere of 500 mbar) and CTO 7-x F x films. Two point contacts are directly made on the surface of films by wire bonding. Electrical measurements were performed with a Keithley 6517B Source Meter at room temperature. The light illumination is supplied by the various semiconductor laser of 50 mW/cm 2 , with various wavelength of , 405 nm, 532nm, 655nm, and 808nm [24]. To investigate the underlying mechanism of fluorination-induced photoconductive effect, the generalized gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE) within the frame work of density functional theory (DFT), as implemented in the Vienna ab initio simulation package (VASP) was carried out [25][26][27][28]. Monkhorst-Pack k-point meshes of 5×3×5 was used for 2×1×2 F-doped CTO 7 supercell [29]. A plane-wave cutoff energy of 450 eV was used for the self-consistent calculations. In order to obtain optimized geometries structures of fluorinated system, both lattice constants and atomic positions are fully optimized until the Hellmann-Feynman less than 0.02 eV Å −1 . Figure 1(b) shows the XRD θ-2θ measurement, only the STO (110) and CTOF (010) reflections can be observed in spectrum. Without any additional peaks in the above spectrum, the influence on the epitaxial relationships and crystal structure of the films can be neglected after fluoride treatment. However, differ from the stoichiometric CTO 7 film fabricated on the STO substrate, the peaks of CTOF film appear at the higher 2θ angle, indicated a slightly decrement of the lattice constant along vertical alignment [3]. To estimate the three dimensional lattice constants, the symmetric and asymmetric RSM were performed and shown in figures 1(c)-(e). The isolated CTOF (040) spot at a Q z of 0.7393 confirms the out-of-plane (020) CTOF //(110) STO , which is consistent with the separated peaks in the θ-2θ scan. Meanwhile, a slight broadening along the Q z reveals that the lattice along b-axis is fully relaxed in the fluoride film ( figure 1(c)). For the asymmetric CTOF (240) around the STO (310) reflection as shown in figure 1(d), the lattice along a-axis is partially relaxed and stretched for a slight in-plane tension strain, same as that in the CTO 7 film [3]. Back to the lattice along vertical alignment, an out-ofplane contraction can be obtained from a higher Q z of CTOF (0 4 10) spots than that of STO (222) in figure 1(e). Hence, based on the above RSM images, we can obtain an orthorhombic CTO 7-x F x structure at room temperature with the lattice constants a=5.469 Å, b=5.411 Å, c=19.257 Å. It is found that lattice misfit compared with the bulk is only 0.85% along the a-axis and −0.82% along the c-axis [30]. And the epitaxial relation between the film and substrate is To further investigate the valence state and fluorination of CTO 7-x F x films, x-ray photoelectron spectroscopy (XPS) has been performed. After using Lorentzian-Gaussian fitting and Shirley background, the Ca 2p and Ti 2p core level spectra both include one pair of spin-orbit doublets in figures 2(a) and (b). Similar with that in the CTO 7 bulk, the peaks of Ti 2p1/2 and Ti 2p3/2 hold at 464.3 eV and 458.5 eV with no change on the binding energies, showing that the Ti-O bonds are stable after fluorination (discussed later) [31]. On the contrary, the binding energies of Ca2p 1/2 and Ca2p 3/2 peaks shifted to higher positions of 351.5 eV and 347.9 eV, both increased by 1.4 eV than that in the CTO 7 bulk [31]. The peaks appears at higher binding energy due to the ionic character of the bonds and the weakened chemical bond of Ca-O, which is also responsible for the compensation of F inclusion [32]. Moreover, the asymmetric O1s spectra can be resolved into three peaks as shown in figure 2(c). Two higher peaks can be also observed in the CTO 7-δ ceramic, corresponding to the absorbed water (533.5 eV) and oxygen species absorbed on the surface (531.9 eV) [8,33]. However, the third one is related to lattice oxygen in the CTO 7-x F x films (529.7 eV), which is higher than that in the ceramic (528.9 eV, marked as red dashes) [33]. This change confirms that fluorination leads to decrease the valence electron density of O 2− [18]. Furthermore, we can infer that the valence state of F is −1 from F 1s spectra in figure 2(d), causing the above reduced density [22,34]. Figures 3(a)-(d) show the resistance switch in CTO 7-x F x film between that in darkness and under illumination, with the light of the constant intensity and various wavelengths. Whatever the wavelength, the sample resistance increases with the light on and decreases with the light off, presented good photoelectric responses at room temperature. In addition, it is found that the fluoride films take a more recovery time to dark resistance than to light resistance, attributed to the photogenerated charges trapped in the deep energy levels    [35,36]. However, the other photoelectric characters of CTO 7-x F x film are dependent on the wavelength, such as stability and photosensitivity. Especially, the both resistance with an 808 nm laser start to fluctuate after four cycles, which illustrate that the photoelectric effect is gradually eliminated. The most stable photoelectric behavior appears from 532 nm to 655nm. For the photosensitivity with the different light, we selected a continuous period of on-off-on, from the whole resistance switch curves. For comparison, the photosensitivity is defined as P s =R dark /R light , where R dark and R light is the resistance with the light off and on respectively. In figure 3(e), the photosensitivity increases with the decrement in wavelength from 10 1 (808 nm) to 4×10 4 (405 nm). This is because the more photogenerated charges can be produced with the increasing in the photon energy, leading to the reduced resistance under the illumination. As shown in figure 3(f), the photosensitivity of CTO 7 , CTO 7-δ and CTO 7-x F x films were compared to show that the fluorination can enhance the photoelectric behavior at room temperature. Based on the photosensitivity of CTO 7-x F x film as the reference, it is estimated to be 2×10 3 and 4×10 2 in the CTO 7-δ and CTO 7 , respectively. The best photoconductive property can still be presented in the CTO 7-x F x film with a 405 nm light. In addition, it should be notice that a recovery time in the CTO 7 film differs from another two films. It recovers directly to dark resistance after the light remove, like as that to light resistance. By contrast, the slower recovery of resistance both can be observed in the CTO 7-δ and CTO 7-x F x films, owing to the trapped centers caused by oxygen vacancy and fluorinated defect.

Results and discussion
Back to the fluoride films, the first principles calculations were performed to uncover the underlying mechanism of enhanced photoelectric character. According to the above physical properties, two different positions of doped F ion (F1 and F2) were marked as black and red in the inset of figure 1(b). Specifically, the F1 position locates at the rock-salt layer while the F2 position locates at the perovskite layer. As shown in figure 4(a), the target system doped at F1 represents two distinct changes in the total density of states (TDOS). The first one is the decreasing in the band gap, from 2.45 eV (pure CTO 7 ) to 2.3 eV (fluorine) [8]. Both calculated band gap are lower than the bulk value of ∼3.94eV [11], because the GGA method were confirmed to underestimate the value of band gap [1]. Secondly, the distribution of TDOS appear at the Femi level (E F , marked as magenta dashes), showing that this doping system possesses a certain conductivity induced by electron filling. Deep into the partial DOS in figure 4(b), the contribution near E F is mainly from Ti d-bands, not from the incorporation of F. The additional electron donors created by monovalent F cause that the E F moves up to the occupied conduction band, which is the origin of a certain conductivity. Besides, the hybridization between O 2p and Ti 3d explain that the fluorination can weak the chemical bond of Ca-O rather than that of Ti-O. The similar changes can be observed in the second system with the doped position of F2 (figures 4(c) and (d)). Hence, with the incorporation of doping fluorion, the multiple effect of the reduced band gap and deep energy levels have an influence on the above photoelectric character.

Conclusion
In sum, we fabricated the CTO 7-δ thin films on the STO (110) substrates via PLD, then post-annealed in the oxygen and argon atmosphere for CTO 7 and CTO 7-x F x films, respectively. For the crystal character, the CTO 7-x F x film owns an orthorhombic structure with a slight tension along a-axis and compression along c-axis. From the changed valence states, we found that the F-doping can replace the lattice oxygen led to not only the decreased O 2− ions but also the weakened Ca-O bonds. Meanwhile, the photosensitivity of fluorinated film shows a dependent relation with the wavelength, and has a better photoelectric behavior than that in another two films. The first principles calculations suggest that the induced fluoride can induce the equivalent oxygen vacancy, resulted in the upper Femi level and reduced bang gap.