Highly Sensitive Switchable Heterojunction Photodiode Based on

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Functional Inorganic Materials and Devices

Highly-sensitive switchable heterojunction photodiode based on epitaxial Bi2FeCrO6 multiferroic thin films Wei Huang, Joyprokash Chakrabartty, Catalin Harnagea, Dawit Gedamu, Ibrahima Ka, Mohamed Chaker, Federico Rosei, and Riad Nechache ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00459 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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ACS Applied Materials & Interfaces

Highly-sensitive switchable heterojunction photodiode based on epitaxial Bi2FeCrO6 multiferroic thin films Wei Huang,† Joyprokash Chakrabartty,† Catalin Harnagea,† Dawit Gedamu,‡ Ibrahima Ka,‡ Mohamed Chaker,† Federico Rosei,*,†,§ and Riad Nechache*,‡ †

INRS -Centre Énergie, Matériaux et Télécommunications, 1650, Boulevard Lionel-

Boulet, Varennes, Québec J3X 1S2, Canada. ‡

École de Technologie Supérieure, 1100 Rue Notre-Dame Ouest, Montréal, Québec H3C

1K3, Canada. §

Institute for Fundamental and Frontier Science, University of Electronic Science and

Technology of China, Chengdu 610054, People’s Republic of China.

•

Supporting Information

KEYWORDS: epitaxial thin films, perovskite multiferroics, polarization switching, heterojunction photodiode, photoresponse.

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ABSTRACT: Perovskite multiferroic oxides are promising materials for the realization of sensitive and switchable photodiodes, due to their favorable band gap (< 3.0 eV), high absorption coefficient and tunable internal ferroelectric polarization. A high speed switchable photodiode based on multiferroic Bi2FeCrO6 (BFCO)/SrRuO3 (SRO) layered heterojunction was fabricated by pulsed laser deposition. The heterojunction photodiode exhibits a large ideality factor (n = ~5.0) and a response time as fast as 68 ms, thanks to the effective charge carrier transport and collection at the BFCO/SRO interface. The diode can switch direction when the electric polarization is reversed by an external voltage. The time-resolved photoluminescence decay of the device measured at ~500 nm demonstrates an ultrafast charge transfer (lifetime = ~6.4 ns) in BFCO/SRO heteroepitaxial structures. The estimated responsivity value at 500 nm and zero bias is 0.38 mA/W which is so far the highest reported for any ferroelectric thin-film photodiode. Our work highlights the huge potential for using multiferroic oxides to fabricate highly-sensitive and switchable photodiodes.


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1. INTRODUCTION Since the discovery of the ferroelectric photovoltaic (FEPV) effect1,2, ferroelectric (FE) materials have been widely studied for applications in solar technologies, including photovoltaics (PV) and hydrogen generation.3-6 When illuminated by photons of energy above the band gap, a FE material generates excitons which subsequently split into an electron/hole pair by the internal intrinsic electric field of the FE and can then collected by electrodes. The phenomenon is promoted by the polarization-induced internal electric field in FE films, also called the depolarization electric field.7 This mechanism is different from charge separation that occurs in conventional p−n junction based PV cells, which is caused by a gradient in the electrochemical potential.8 In p–n junction based PV cells, the difference of the Fermi level of photogenerated carriers between two electrodes represents the photovoltage, which is generally smaller than the band gap and the total open-circuit voltage, usually lower than 1 eV and is determined by the difference in work functions of the two electrodes.9 In contrast, non-centrosymmetric FE materials are known to generate photovoltages which can exceed their band gap, typically in the range of 3–4 eV for FE oxides with ABO3 perovskite crystal structure.10,11 These noncentrosymmetric FE materials allow reversible directions for the photocurrent and photovoltage, simply by switching the FE polarization direction.12-16 An additional advantage of FE thin films in PV devices consists in the possibility of regulating barrier heights at the FE/electrodes interface by using its intrinsic spontaneous polarization, thereby allowing to tune the magnitude of the photovoltage in the structure.17 There are as yet no commercial technologies based on the FEPV effect, as a result of the low charge carrier mobility in the conduction band as well as the large band gap (typically larger than



~3.3 eV, corresponding to the UV region of the solar spectrum), which typically results in limited power conversion efficiencies (PCE).18 Recently, several experimental approaches were developed to overcome these challenges; among these, band gap engineering of FE materials and band level alignments have led to significant breakthroughs in PV performance.15 Among FE materials, multiferroics exhibit more than one of the primary ferroic orders, namely ferromagnetism, ferroelasticity and ferroelectricity.19,20 These multiple functionalities make them promising candidates for applications in spintronics and optoelectronics,21-26 due to their switchable rectifying behavior, polarization-dependent photovoltage, efficient electron-hole pair separation at FE domain walls, and ultrafast carrier dynamics under above-bandgap femtosecond optical excitation.3,27-29 For example, BiFeO3 (BFO) is a typical multiferroic material, whose optical, electrical and ferroelectric properties have been intensively investigated in view of its use in PV devices and photodetectors. It possesses simultaneously FE and antiferromagnetic orders at room temperature with a large remnant polarization (~90 µC/cm2) and relatively narrow band gap (~2.67 eV).12,14,21 A heterojunction PV device using BFO as absorber was reported to yield a PCE of ~0.19% under AM 1.5 G illumination.16 A BFO-based highly-sensitive and fast-response photodetector with a responsivity of ~0.15 mA/W at 365 nm and a response speed of the order of tens nanoseconds was also reported.30 Recently, multiferroic Bi2FeCrO6 (BFCO) has been studied extensively for PV applications due to its small bandgap (tunable in the range ~1.4 − 2.1 eV), large light absorption coefficient (~2.0 × 105 cm-1) and effective electron-hole pair separation with large remnant polarization (~55 µC/cm2) which makes it an interesting material for


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applications in optoelectronics.31-33 Nechache et al.15 fabricated BFCO thin-film solar cells with an unprecedented PCE of ~3.3% for single-layer films under AM 1.5G illumination and a narrow band gap of ~1.5 eV, representing a 20 times higher PCE than that reported BFO-based PV devices. By controlling the Fe/Cr cationic order in the films, the band gap of BFCO, with FE polarizations ranging from 50 to 10 µC cm-2, can be tuned from 2.1 to 1.4 eV, respectively.15, 31,32 This approach led to the design of a multiabsorber system based on BFCO by stacking three layers with different bandgaps, leading to a record PCE of 8.1%. Multiferroic properties also offer opportunities for developing optoelectronic devices, including photodetectors, such as photodiodes, photomultipliers,30,34 etc. Among these applications, photodiodes are widely used in integrated circuits (for control and switching, or digital signal processing); consumer electronics devices; optical communications, and light regulation, due to its less complicated and low cost fabrication.35 It was previously demonstrated that photodiode devices on photoconductive mode exhibit a higher sheet resistance (typically of the order of ~ MΩ/□) than that of their external loads and lower photocurrent density (of the order of ~µA/cm2, usually under reverse bias condition).21,30,36,37 In addition, the multiferroic nature of BFCO allows to tune the photoelectric effect using magnetic fields or electric poling, endowing additional degrees of freedom for the next generation of optoelectronic devices. Here we report the fabrication of heteroepitaxial junction photodiodes based on epitaxial BFCO multiferroic thin films, sandwiched between a conducting perovskite SrRuO3 (SRO) bottom electrode and an indium tin oxide (ITO) transparent top electrode. In previous work,15 the FE polarization properties (or depolarization) were demonstrated



to be at the basis for photogenerated charge separation. Reducing the BFCO bandgap with the degree of cationic Fe/Cr ordering results in weakening the material’s FE polarization and thus the driving force, which, in turn, increases the recombination rate of photogenerated carriers in the films. For fabricating photodiodes with fast photoresponse, it is necessary to ensure a low recombination rate of photogenerated carriers with a fast separation and an effective transport in heterostructures. Here, we used BFCO thin films with a relatively large bandgap (~2.5 eV) and a huge FE polarization of ~40 µC cm-2, as active layer to fabricate a photodiode. This device architecture (ITO/BFCO/SRO) effectively improves carrier collection and transport efficiency with a fast-transient response of the order of dozens of ms, leading to an ultrafast charge transfer (i.e., lifetime = 6.4 ns) and photoresponse of 0.38 mA/W at 500 nm in absence of voltage bias. 2. RESULTS AND DISCUSSION 2.1. Characterization of BFCO thin films. Figure 1a displays the x-ray diffraction (XRD) pattern acquired from the BFCO film. The (0 0 l) (l = 1, 2, 3) peaks confirm the caxis orientation of the film without any secondary phase. Stars and points correspond to the (00l) Kβ line of copper and tungsten contamination of the x-ray cathode tube, respectively. A more detailed analysis reveals that BFCO crystallizes in a pseudocubic phase with c-axis lattice constant of 3.967 Å caused by a compressive strain induced by the out-ofplane lattice mismatch with the SrTiO3 (STO) substrate (−1.6 %).38 The (0 0 l) reflections of the 15 nm-thick SRO layer overlap with those of the BFCO (0 0 l) reflections. As mentioned in previous work,15,31,32 the degree of cationic ordering of the BFCO phase is a crucial parameter for controlling its optical properties. The Fe/Cr cationic ordering in the BFCO phase was characterized by performing asymmetrical θ–2θ


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scans around (111) STO reflections. Schematic illustrations of the double perovskite BFCO crystal structure presenting Fe/Cr octahedral stacking along [001] and [111] cubic directions are displayed in Figure S1 in Supporting Information. The degree of Fe/Cr cationic ordering in the obtained films was estimated from the normalized intensity ratio of the superlattice peak to the main (111) single perovskite reflection of BFCO (cf. Figure S2 in Supporting Information). The ratio (R) of the degree of Fe/Cr cationic ordering is as low as ~0.1 %, revealing that the films are highly disordered. Figure 1b displays Φ-scans of the BFCO 110 and STO 110 planes for a BFCO/SRO/STO(100) heterostructure. Four sharp peaks are obtained for BFCO, indicating a good in-plane orientation of the films. These reflections are at the same positions than those of STO substrate, which is indication of a cube-on-cube epitaxial growth of BFCO with respect to the substrate with insignificant lattice tilt. XRD reciprocal space mapping (RSM) of BFCO thin films around the STO (204) reflection is illustrated in Figure 1c. The small condensed reciprocal area suggests high quality epitaxial growth of the BFCO thin film on the STO substrate. The presence of an extra spot indicates the existence of BFCO phase in the films, which we denote as disordered (d-BFCO) phase. The in-plane lattice parameter estimated from the positions of BFCO (204) reflection is ~3.925 Å, which suggests an in-plane compressive strain of 0.5%. Based on previous work,15,31,32 the BFCO films with low ratio of Fe/Cr cationic ordering (R) should exhibit a relatively wide band gap (Eg), compared to films with higher cationic ordering. For example, BFCO with R = 5.4% has a low Eg (~1.4 eV), and BFCO with R = 0.3% has a larger Eg (~2.1 eV).15 To verify this hypothesis, we performed



optical absorbance measurements at visible range (cf. Figure 1d). The inset of Figure 1d presents the Tauc plots of (αhv)2 versus hv. Using the linear extrapolation of the Tauc plots to zero, the calculated direct Eg of the disordered BFCO is derived as ~2.5 eV. This value is close to the previously reported Eg (~2.4 eV for the highly disordered BFCO phase with low R).15

2.2. Photodiode properties of devices. The typical J(E) characteristics of the devices were measured both in dark and under AM 1.5G illumination. The schematic layout of the measured devices is illustrated in Figure 2a. The power density of the incident light was varied from 50 to 100 mW/cm2. We found a substantial nonlinear dependence of the current density J as a function of the applied electric field E (Figure 2b). For a given E, the electrical conductivity rapidly increases with light intensity. This can be explained in terms of the excess of charge carriers generated by photoexcitation by photon energies above and close to Eg of BFCO. The electrons being excited from the valence band to the conduction band are transferred to the adjacent interfaces by the electrodes. Considering that the defects concentration in high quality BFCO single crystals is low, we conclude that the carriers generated from defects and SRO electrode do not affect the conductivity of the devices. The J(E) curves recorded at room temperature (~300 K) for BFCO based devices exhibit a diode-like behavior21 (cf. Figure 2b). The reverse saturation current density is about 0.22 µA/cm2 at −22.05 kV/cm measured under 1 sun illumination. For the p−n ideal diode, the forward current density follows an exponential relationship with applied voltage, given by21:


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= exp ( ) − 1, where J0 is the reverse saturation current density, q is the electron charge, V is the applied voltage, n is a constant called the ideality factor, kB is the Boltzmann constant and T is absolute temperature. For our device, we found an ideality factor of n ~5.0 at 300 K and a forward bias field of 9.80 kV/cm, as displayed in the inset of Figure 2b. This ideality factor, which is much larger than the ideal value (≈ 1.0) in conventional semiconductor p−n junctions, has been observed in perovskite-based oxide p−n junctions, where charge trapping at defects in the bulk is considered important for transport properties.39 This value is close to the reported n (≈ 4.7 at 350 K and ≈ 6.3 at 300 K) for the switchable multiferroic BFO-based diode.21

2.3. J(E) characteristics with FE polarization switching. FE polarization is crucially important for modulating the internal depolarization electric field and obtaining optimal energy band alignments in devices to enhance their photoconductive performance.6,15,16,32 The ferroelectric character of BFCO films grown on SRO/STO at the grain level is demonstrated by piezoresponse force microscopy (PFM). A sketch describing the PFM measurement setup is illustrated in Figure 3a. From the surface topography of 100 nmthick BFCO films performed using an atomic force microscope (AFM, implemented with the same PFM instrument), we determined a root mean square roughness (R.M.S.) of ~8 nm, indicating a relatively smooth surface throughout the BFCO film. The FE domains of BFCO can be individually or partially switched by applying ±8 V DC voltage pulses. Applying +8 V bias induces a homogeneous state with upward (pointing out of the surface) out-of-plane polarization, while –8 V bias results in a homogeneous state with 9 ACS Paragon Plus Environment


downward polarization (towards the SRO bottom electrode) (cf. Figure 3b). The resulting contrast is a clear proof that ferroelectricity in BFCO films occurs and can be switched upon the application of an external voltage. This is further confirmed by the observation of hysteresis loops in the local in-field z-piezoelectric signal (as shown in Figure S3 in Supporting Information). A macroscopic FE hysteresis loop (Figure S4 in Supporting Information), measured at 2 kHz, and reveals both a FE and a leaky behavior of the BFCO films. The maximum polarization along the [100] direction of BFCO pseudo-cubic unit cell, exhibiting R value of 0.1%, measured at an applied electric field of 750 kV/cm is ~40 µC cm-2, close to the previously reported value.15 This is an indication of the possibility to design a switchable photodiode device by controlling the electric polarization in multiferroic BFCO. To investigate the effect of FE polarization on the photo-electric behavior in multiferroic BFCO films, we repeated the J(E) measurement after the BFCO film was poled by applying different high voltage pulses under 1 sun illumination (100 mW/cm2). Henceforth, we define the positively polarized state as the state when spontaneous polarization in the sample is downward (state achieved by positive voltage pulses on the top electrode), and the negatively polarized state as the one achieved by negative pulses. The magnitude of the voltage pulses applied was 15 V. The effect of polarization on the J(E) curves of the device is shown in Figure 3c. First, we observe a short-circuit photocurrent density (Jsc) enhancement of two orders of magnitude compared to the ground state (without poling) in Figure 3c. In the positive state Jsc ≈ +2 µA cm-2 while this value reaches −4 µA cm-2 after poling the film with a –15 V pulse. The latter corresponds to a hundred-fold improvement of photocurrent density. This indicates that


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the devices which undergo electric poling under illumination exhibit a PV behavior. Secondly, we find that the shape of the J(E) curves are drastically changed, becoming almost symmetrical with respect to the axis origin: ()| ≈ −(−)| , which suggests that the band alignments in the device are reversed at the two interfaces with polarization switching30.

To understand the mechanism of FE polarization-dependent

charge separation and transfer in our ITO/BFCO/SRO heterostructures, we investigated the energy band structure for each component material and alignments in accordance with previously reported values.15,32 From ultraviolet photoemission spectrometry (UPS) measurements (cf. Figure S5 in Supporting Information), the work functions (ϕ) of ITO and SRO are estimated to be 4.8 eV and 5.2 eV, respectively,15 while the electron affinity (χ) and ϕ for BFCO, are determined to be around −3.8 eV and −5.0 eV. In Figure 3d, these values are used to design the energy band structure with a conduction band of ~3.8 eV and a valence band of ~6.3 eV below the vacuum level for disordered BFCO. For our ITO/BFCO/SRO heteroepitaxial structure without external voltages applied, at the SRO/BFCO interface under thermal equilibrium, electrons pass from the conduction band of BFCO to SRO (since ϕSRO > ϕBFCO) until the Fermi levels equalize, thus forming a depletion region in BFCO with an upward band bending (cf. top panel of Figure 3d). This is a Schottky-type contact and is highly resistive (with a barrier height of 1.4 eV). In contrast, the contact region formed at the interface between BFCO and ITO, has a relatively low resistance (with a barrier height of 1.0 eV) inducing a downward band bending since ϕBFCO > ϕITO (cf. top panel of Figure 3d). The middle and bottom panel of Figure 3d show the modification of the energy band alignments in the presence of the FE polarization (P) for the two possible orientations. A positive voltage applied to the SRO



bottom electrode results in an electric field with opposite direction to the bottom barrier field and in the same direction as the top one. When the BFCO films are poled, the energy band diagram is modified as shown in the middle panel of Figure 3d for “upward” polarization, while the modified energy band alignment for “downward” polarization is illustrated in the bottom panel of Figure 3d. The origin of this change is attributed to the modulation of the energy band induced by polarization40: The positively charged holes and negatively charged electrons will move under the electric depolarization field, thus accumulate near the ITO and the SRO interfaces, respectively, when polarization is “downward” (and in opposite directions for “upward” polarization). These charge accumulations near interfaces further contribute to the shift of energy levels, resulting in a reduction or an increase of the barrier heights.

2.4. Photoresponse and charge carrier dynamics. Figure 4a shows the reliable and reversible switching of the photocurrent through the device biased at 0.3 V under chopped 1 sun illumination. The rise (as measured from 10% to 90%) and decay (as measured from 90% to 10%) times of the current for a single pulse were estimated to be ~90 and 68 ms, as shown in the top and bottom panels in Figure 4b, respectively. These values are close to the reported typical fast-transient responsive ZnO NWs/p−Si heterojunction photodiode.37 As mentioned before, when illuminated by energetic photons ≥ 2.5 eV, excitons (electron-hole pairs) are generated. Under the internal depolarization electric field in BFCO, at the interfacial region of BFCO/SRO, the excitons split into electrons and holes, the electrons at the conduction band of the BFCO is then transferred to SRO. Due to the low electrical resistivity of SRO (~2.0 × 10-4 Ω cm


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at 300 K),41 the total series resistance of the heterojunction structure (BFCO/SRO) is reduced. This effectively enhances the electron–hole separation and improves carrier transport in the heterojunction, leading to a faster response of the device. When the light is turned off, the excess electrons from the SRO transfer into the BFCO where they recombine with the holes, which is an inherently fast process.42 The On/Off ratio (≈ Ilight on

/ Ilight

off)

in our devices is relatively low (~7), which is more than two orders of

magnitude below the value reported for typical ZnO film-based UV detector (over 1000),43 a fact that we attribute to the high dark current of the device (e.g. ~0.1 µA/cm2). The transient times extracted from the electric measurements, however, do not represent the charge dynamics in the device, since the current transient depends on the measuring circuit (particularly on the bandwidth of the current preamplifier). Therefore, to gain insight and further understand the mechanism of charge transfer and carrier recombination in BFCO/SRO heteroepitaxial structure, we performed photoluminescence (PL) measurements. Under an excitation wavelength of 383 nm, the films exhibit a broad PL emission in the visible range with a maximum at ~500 nm (cf. Figure 4c). Charge separation and transfer process (cf. Inset of Figure 4c) in the BFCO/SRO junction were quantitatively determined using time resolved PL (TRPL) measurements at ~500 nm wavelength, as shown in Figure 4d. The charge transfer process from BFCO to SRO is ultrafast, with a lifetime of ~6.4 ns, implying an effective charge separation and efficient charge transfer with a low carrier recombination rate. In fact, a value within ~10 ns indicates that the charge carrier dynamics in our heteroepitaxial structure is faster than the typical values reported for FE oxide based photodetectors.30 Finally, we characterized the spectral response of our photodiode devices, by



measuring the responsivity in the 350 to 850 nm spectral range. As shown in Figure 5, a clear response is achieved for wavelengths between 400 and 560 nm, with a drop at 500 nm. The maximum responsivity of 0.38 mA/W is located around 500 nm (corresponding to a photon energy of ~2.50 eV), which matches the strong absorption of the disordered BFCO film (having an Eg ≈ 2.50 eV). A zoom around 700 nm of the same plot in a semilog scale (inset of Figure 5), suggests that the sample has also a weak response of ~10-2 mA/W within a wide window of the spectrum (wavelength from 600 to 800 nm). Such photoresponse evidences a sub-band-gap feature, similar to that observed in multiferroic BFO film-based photodetectors.30 Oxygen vacancies, which are often present in perovskite films, could be the origin of such behavior, and are presumed to act as donor impurities in BFCO films.15 This is also confirmed by the presence of a peak around ~700 nm found in PL spectra (cf. Figure 4c), which is attributed to the oxygen vacancies in the films.


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3. CONCLUSIONS AND PERSPECTIVES We reported the fabrication and characterization of a switchable heterojunction photodiode based on epitaxial BFCO multiferroic thin films. To this end, we focused on a large band gap (~2.5 eV) BFCO thin film, corresponding to a low degree of Fe/Cr cationic ordering (~0.1%) and switchable FE polarization properties of BFCO films. The device exhibits a substantial photocurrent density (~2.4 µA cm-2) at 0.3 V and a fast transient response, attributed to the effective charge carrier separation and collection at the interfacial region between BFCO and the SRO electrode. The TRPL decay measured at 500 nm wavelength demonstrated an ultrafast charge transfer (lifetime < 10 ns) in a BFCO/SRO heteroepitaxial junction. The responsivity spectrum shows a peak sensitivity of 0.38 mA/W at 500 nm at zero bias. These results could improve our understanding of charge conduction mechanisms in FE systems and advance the design of switchable devices combining FE, electronic, and optical functionalities. Further studies on FE photoconductivity and dielectric polarization will lead to the development of highlysensitive switchable optoelectronic devices based on inorganic perovskite multiferroics.



4. EXPERIMENTAL SECTION 100 nm-thick BFCO films were grown epitaxially on (100)-oriented single-crystalline STO substrates by pulsed laser deposition (PLD). A 15 nm-thick SRO buffer layer was grown on the substrate and serves as bottom electrode. Stoichiometric BFCO and SRO targets were ablated using a KrF excimer laser (wavelength of 248 nm and pulse duration of 25 ns) with an energy density of about 1.8 J/cm2 at a repetition rate of 10 Hz. The deposition temperature and oxygen partial pressure were 580 °C and 6 mTorr, respectively. To complete the device fabrication, 100 nm-thick ITO top electrodes were deposited on the BFCO films by PLD using the same parameters through a shadow mask with circular apertures (~0.5−1 mm in diameter). To study the optical properties of BFCO thin films, we investigated the optical absorption of BFCO films directly grown on transparent (100)-oriented STO substrates. The crystal structure, growth orientation, and lattice parameters of the films were examined using XRD (Panalytical X’pert pro diffractometer using Cu Kα radiation, Westborough, MA) by performing θ-2θ scan, Φ-scan, and RSM measurements, respectively. AFM (Bruker Enviroscope, Santa Barbara, CA) imaging was used to characterize the surface morphology and roughness of the grown film. The nanoscale FE properties of the BFCO films were studied using PFM implemented with the same AFM instrument. The macroscopic FE properties of BFCO films were assessed using a thinfilm analyzer system (TFA 2000) at a frequency of 2 kHz. The optical absorbance of BFCO thin films in the visible range was determined using a Cary 5000 UV–Vis-NIR spectrophotometer (Varian, Palo Alto, CA) with a scan speed of 600 nm/min. The work function and electron affinity of BFCO films were determined by ultraviolet


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photoelectron spectroscopy (UPS) equipped with x-ray photoelectron spectroscopy using an ESCA Escalab 220i XL spectrometer with a monochromated Al Kα X-ray source (1486.6 eV). PL spectra were acquired using a 383 nm laser as an excitation source. The TRPL decay transients measured at 500 nm demonstrated the sensitivity of the device by the same laser excitation source. For device characterization, the current density (J) – electric field (E) characteristics were recorded using a source meter (Keithley 2400) while an AAA class Sun simulator equipped with 1.5 AM filter was used to illuminate the devices. Finally, the spectral responsivity was measured using a monochromator combined with an optical chopper, a lock-in amplifier, and a 500W Xe lamp. All the measurements were carried out at room temperature (RT, ~300 K).



ASSOCIATED CONTENT Supporting Information The characterization of BFCO thin films such as crystal structure, ferroelectric properties and UPS analyses are supplied as Supporting Information.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ORCID Wei Huang: 0000-0003-1170-1665


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ACKNOWLEDGMENTS We acknowledge financial support from the Canada Foundation for Innovation which funded our facilities for materials deposition and characterization as well as device fabrication and testing. F.R. and R.N. acknowledge NSERC for individual Discovery grants and for a joint Strategic grant in partnership with Hexoskin. F.R. is also grateful to the Canada Research Chairs program for partial salary support. F.R. acknowledges the government of China for a Chiang Jiang short term scholar award and Sichuan province for a 1000 talent short term award.

Author contributions W.H. and R. N. have developed both the concept of the deposition of the material and its integration into devices. Materials and devices characterization was done mostly by W.H., J.C. and C. H. performed AFM and PFM measurements. D.G and I.K. carried out PL and Responsivity measurements. Interpretations of the results and data analysis were discussed between all co-authors. The first draft of the manuscript was written by W.H. and corrected by D.G., I.K., M.C., F.R and R.N.. All authors have approved the final version of the manuscript before its submission. Competing financial interests: The authors declare no competing financial interests.



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Figure 1. (a) XRD θ–2θ scan of a BFCO thin film grown on STO (001) substrate. The stars correspond to (00l) Kβ line while the points indicate tungsten contamination of the x-ray tube cathode. (b) Φ-scan profiles around the (110) plane of BFCO thin film and STO (100) substrate. (c) RSM measurement of the BFCO/STO heterostructure around the (204) reflection of STO showing the spot mainly related to the disordered (d-) BFCO phase in the film. (d) Optical absorbance spectrum of transparent BFCO/STO. The inset shows the corresponding direct optical transitions of BFCO thin film at visible range.


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Figure 2. (a) Layout of measured devices with ITO/BFCO/SRO heterostructures and the representation of FeO6/CrO6 arrangements of the double-perovskite BFCO crystal. (b) Illumination intensity dependence of J(E) device characteristics. The inset shows the same curves as semi-log-plots.



Figure 3. (a) Sketch of the setup for PFM measurements. (b) Topography and out-ofplane PFM images after applied ±8 V pulses. (c) J(E) curves for devices at Virgin state and under ±15 V electric poling under illumination (100 mW/cm2). (d) Schematic of simplified energy band alignments for heteroepitaxial structures for an ideal metal– semiconductor interface without polarization (top of panel: virgin state), and positively/negatively poled at ±15 V (middle and bottom of panel, respectively).


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Figure 4. (a) Reversible switching of electrical current at 0.3 V biasing voltage under illumination for photodiode devices. (b) The rise (top of panel) and decay (bottom of panel) of the current in time for a single pulse in (a). (c) PL spectra of the heteroepitaxial structures under an excitation of 383 nm; Inset shows the schematic illustration of charge transfer at interfacial region of BFCO/SRO. (d) Time-resolved PL (TRPL) decay measured at 500 nm wavelength for the heteroepitaxial structures. The inset shows a magnified region of the TRPL spectra between 2–15 ns.



Figure 5. Photoresponsivity spectra of the device at 500 nm wavelength at zero bias. The inset shows a semi-log curve of the measured spectrum.


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