Ferroelectric ultraviolet photodetector material with ultrafast response speed

Domain: physics
Published: 2025-12-30
Editorial outcome: major_revision

Abstract
Ferroelectric films are promising to be used for high-performance photodetectors due to their intrinsic electric fields and high dielectric constants. However, the presence of high-density domains with varying polarization directions can severely degrade comprehensive performance. Here, we fabricate high-quality SrAl 11-δTiO 19 (SATO) ferroelectric films through a solid-state reaction. The SATO film possesses a magnetoplumbite-type structure with polarization along the c-axis and exhibit the possibility of single-domain ferroelectrics. Ferroelectric performance tests show that the remnant polarization of SATO film reaches 7.8 μC/cm2 and the polarization retention exceeds 500 hours. Optoelectronic performance measurements reveal that the SATO photodetector exhibits excellent performance with response wavelength of 330 nm, responsivity of 860 mA/W, detectivity of 1.63×1013 Jones, switching ratio of 1.9×104, and ultrafast rise/fall response speed of 6.8 ns/17.7 ns (i.e., nearly 10000 times faster than traditional photodetectors). The outstanding properties highlight SATO as an outstanding candidate for next-generation photodetectors.

Introduction
Photodetectors (PDs) are widely utilized in various fields, including information communications, environmental monitoring, space exploration, etc., owing to their capability to convert optical signals into electrical signals 1–6. The rapid development of real-time communication, high-precision detection, and deep ultraviolet (UV) information transmission requires PDs to possess ultra-fast response speed, high responsivity, high detectivity, and short response wavelength (<400 nm) 5. In traditional PDs, these four parameters are difficult to achieve optimally at the same time. For example, β-Ga 2 O 3 -based p–n junction PDs can obtain high responsivity (>1 A/W) and high detectivity (>1015 Jones), but the response speed is very slow (>1 s) 7. Si-based Schottky PDs have a high response speed of 30 ps and a high efficiency of more than 50%, but they can only detect visible and infrared light due to the small bandgap 8. AlN-based PDs (~6.2 eV) can achieve ms-level deep UV light signal detection, but the responsivity is very low (10−3 A/W) due to the very large electrical resistance 9. Built-in electric field is one of the key factors that tailor the performance of PDs since it affects significantly the separation efficiency of photogenerated carriers 3. In traditional p-n junction and Schottky PDs, heterointerfaces must be constructed to generate the built-in electric field, which increases the complexity of devices. Moreover, defects such as dislocations, vacancies, and impurities are easily involved in the heterointerfaces, thereby significantly degrading the performance of PDs 10.

Ferroelectric materials have recently been considered as one of the most ideal PD materials because they possess an intrinsic built-in electric field induced by the spontaneous ferroelectric polarization, which endows ferroelectric PDs with the advantages of simple structure and stable performance 11,12. Furthermore, ferroelectric materials can greatly reduce the noise level of PDs since they usually have high dielectric constants 13. For example, BaTiO 3 and LaFeO 3 based ferroelectric PDs have achieved the noise level of nA 14,15. However, current ferroelectric PDs are also facing limitations in their comprehensive performance. For example, although BiFeO 3 films achieve ns-level response time, their responsivity remains as low as 10−2 mA/W 15,16. BaTiO 3 /ZnO junction PDs exhibit a high responsivity of up to 100 mA/W but suffer from s-level response time 17. α-In 2 Se 3 PDs offer a balanced performance in terms of responsivity and response speed but have a low photoelectric conversion efficiency 18. As well known, most ferroelectric materials have a high density of ferroelectric domains with different polarization directions as shown in Supplementary Fig. S1, which will not only slow down the response speed of PDs due to the scattering and annihilation of photogenerated carriers at domain walls and the increase in carrier transport distance, but also decrease the responsivity and detectivity of PDs due to the decrease in photocurrent caused by the mutual cancellation of built-in electric fields 19. To improve the comprehensive performance of ferroelectric PDs, preparing single-domain ferroelectric thin films with large remnant polarization and a wide bandgap is a very promising approach, but it still remains unexplored.

AB 12 O 19 magnetoplumbite compounds have attracted intense interest due to their abundant physical properties. For example, SrFe 12 O 19 is widely used in microwave absorbers, magnetic recording media and sensors, high-frequency electromagnetic (EM) devices, etc., due to its superior magnetic parameters, high magnetic permeability, and low electrical conduction loss 20. BaFe 12 O 19 is the best choice for commercial low-cost permanent magnet materials due to its high coercivity and good chemical and thermal stability. It is widely used in motors, speakers, sensors, and other devices 21. In addition, PbFe 12 O 19 is a natural multiferroic material with broad application prospects in non-volatile memory, ferroelectric photovoltaics, and other fields 22,23. As a structurally similar derivative, SrAl 12 O 19 was predicted to have ferroelectricity but has not yet been experimentally proved 24. As shown in the schematic atomic model in Supplementary Fig. S2, the unit cell of SrAl 12 O 19 comprises alternating rock-salt (i.e., denoted by R) and spinel (i.e., denoted by S) blocks in the sequence of SRS*R* (* denotes 180° rotation around c axis). The misalignment of the positive and negative charge centers of the AlO 5 bipyramid in spinel layers makes SrAl 12 O 19 exhibit the ferroelectricity. Due to the strong c-axis anisotropy, SrAl 12 O 19 and related derivatives are good candidates for the preparation of single-domain ferroelectric thin films with polarization direction along the c-axis.

In this study, a new ferroelectric compound SrAl 11-δTiO 19 (SATO) derived from SrAl 12 O 19 was synthesized by the solid-state reaction of AlN and SrTiO 3 (STO). The atomic structure of SATO was determined by aberration-corrected transmission electron microscopy (TEM). Measurements of the ferroelectric and optoelectronic properties of SATO thin films were carried out. It was found that the SATO thin films had high remnant polarization (P r) and robust ferroelectric retention. The SATO PDs exhibited outstanding comprehensive performance with ultrafast response speed, high responsivity, high detectivity, low noise, and large switching ratio.

Results
Single-crystalline AlN thin films were grown on STO substrates by pulsed laser deposition. High-resolution X-ray diffraction (HRXRD) pattern in Supplementary Fig. S3a suggests that the AlN (0001) thin film and STO (111) substrate have a perfect expitaxial relationship. By annealing the as-prepared AlN thin films in air at 1500 °C for 5 h, SATO thin films were fabricated via the solid-state reaction of AlN and STO. The corresponding HRXRD pattern in Supplementary Fig. S3b suggests that the SATO has the same magnetoplumbite-type crystal structure as SrAl 12 O 19 and grows epitaxially on the STO substrate. To understand the stoichiometry and charge balance of SATO, electron energy-loss spectroscopy (EELS) analysis was carried out. Ti L 2,3 edges of SATO and STO are shown in Supplementary Fig. S4. It is clear that the Ti L 2,3 edges of SATO and STO have very similar fine structures with four peaks, suggesting that the Ti ions in SATO have the valence state of +4. To maintain charge neutrality, Al vacancies must exist in SATO. The chemical formula of SATO can be defined as SrAl 11-δTiO 19 , where δ represents a parameter related to Al vacancies. Energy dispersive X-ray spectroscopy (EDS) measurements were carried out, and the results are shown in Supplementary Fig. S5 and Supplementary Table S1. After background subtraction, normalization, and integration of the EDS spectrum, the average atomic ratios of Sr, Al, Ti and O in the SATO film are 1.00:10.71:1.02:19.02. The atomic ratio of Sr to Ti is close to 1, similar to that in the SrTiO 3 substrate. Since the structure of SATO is derived from SrAl 12 O 19 after one Ti atom replaces one Al atom, vacancies must exist in the other eleven sites of Al. Since the valence states of Ti and Al ions are +4 and +3, respectively, the chemical composition of SATO should be SrAl 10.67 TiO 19 to reach the complete electrical neutrality (δ=0.33 in SrAl 11-δTiO 19 ), which is consistent with the EDS measurements considering the accuracy of this technique.

To investigate the microstructure of the SATO thin film and mechanisms for the solid-state reaction of AlN and STO, TEM observations were carried out. Figures 1a and b display the cross-sectional bright-field TEM image and corresponding selected-area electron diffraction (SAED) pattern of the as-deposited AlN thin film. The film-substrate interface is flat and has no secondary phase. The bright/dark contrast within the AlN thin film signifies the columnar growth characteristics. The SAED pattern suggests that the AlN thin film grows epitaxially on the STO substrate with an orientation relationship of AlN (0006) [112̅0] // STO (111) [1̅10]. Figures 1c and d show the cross-sectional bright-field TEM image and corresponding SAED pattern of the SATO thin film. As one can see, both the film-substrate interface and film surface are flat. The SATO thin film has a similar thickness to the AlN thin film. The epitaxial relationship between SATO thin film and STO substrate is SATO (0001) [112̅0] // STO (111) [1̅10]. These results suggest that Sr and Ti atoms in the STO substrate diffused into the AlN thin film, then the solid reaction occurred in air and formed the SATO thin film. In addition, it is very clear that the SATO thin film exhibits a uniform contrast and has no visible ferroelectric domain walls, indicating that the as-prepared SATO thin film exhibits the possibility of single-domain ferroelectrics. SAED tilting series of SATO are shown in Supplementary Fig. S6, from which we confirmed that the SATO has a hexagonal magnetoplumbite crystal structure with lattice constant of a=b=5.74 Å, c=22.32 Å, α=β=90°, and γ=120°. The bright-field TEM images of the SATO thin film along the [101̅0] and [0001] zone axes are shown in Supplementary Fig. S7. There are no ferroelectric domain walls that can be observed in the SATO thin film, which indicates that the SATO ferroelectric film probably has a single-domain structure.

To reveal the atomic structure of SATO, high-angle annular dark-field (HAADF) and annular bright-field (ABF) imaging were carried out in a scanning transmission electron microscope (STEM). HAADF images can show heavy atomic columns since the intensity of atomic columns is approximately proportional to Z 1.7 (Z: atomic number) 25, while the ABF images can present all the atomic columns, including O 26. Figure 2 show the HAADF and ABF STEM images of SATO along the [112̅0], [11̅00], and [0001] zone axes. Atomic models are inserted into each image for a better understanding of the atomic structure of SATO. As shown in Fig. 2a and d, the SATO consists of alternating rock-salt and spinel blocks in the sequence of SRS*R* along the c-axis. Sr atoms occupy the center of O dodecahedrons in the rock-salt blocks. Al atoms occupy five distinct Wyckoff positions (Please refer to Fig. S2): 2a, 12k, 4f 1 in O octahedra, 4f 2 in O tetrahedra, and 2b in O bipyramids. All the HAADF and ABF images in Fig. 2 confirm that the SATO has a hexagonal magnetoplumbite crystal structure like SrAl 12 O 19 . To determine the atomic sites of Ti in SATO, atomic-resolution EDS elemental maps along the [112̅0] zone axis were obtained and shown in Fig. 3. As indicated by white arrows, the distribution of Ti atoms in SATO is ordered, and they partially substitute the Al atoms at the 4f 1 Wyckoff position. Simulated HAADF and ABF images of SATO along the [112̅0], [11̅00], and [0001] zone axes using the atomic model of hexagonal magnetoplumbite are shown in Supplementary Fig. S8. They are consistent well with the experimental images shown in Fig. 2, which proves again the hexagonal magnetoplumbite crystal structure of SATO.

Ferroelectric and piezoelectric properties of the SATO thin film were measured at room temperature, and the results are shown in Fig. 4. Figure 4a presents the surface morphology of the SATO thin film obtained by piezoresponse force microscopy (PFM). The SATO thin film displays a well-defined hexagonal plate-like structure with a surface roughness (RA) of ~2 nm, indicating high-quality film growth. Figure 4 elucidates the piezoelectric properties through PFM lithography and local switching spectroscopy (SS). The local ferroelectric switching behavior characterized by phase-voltage hysteresis and amplitude-voltage butterfly loops in Fig. 4c can preliminarily prove that SATO thin films may have ferroelectricity. To determine the initial domain state of the virgin SATO film and verify the reproducibility of the PFM measurements, we performed multiple measurements at four randomly selected positions on a 10×10 mm sample under nearly identical measurement conditions, as shown in Supplementary Fig. S9. The initial piezoelectric amplitude and phase images at all four positions display uniform contrast, which indicates that the virgin SATO film possibly possesses a single-domain nature. By applying a custom pattern with +10 V and −10 V DC voltages, the large-scale amplitude and phase images reveal distinct 180° contrast polarization signals between the center and edge regions, highlighting the robust piezoelectric response of the SATO thin film. The PFM measurements at all positions showed similar characteristics, indicating that the SATO film is uniform with a good reproducibility of ferroelectric properties. Remarkably, as one can see from the Supplementary Fig. S10, ferroelectric retention measurements demonstrate that the ferroelectric polarization in the SATO thin film can be maintained for over 500 h without significant degradation, showcasing an excellent long-term stability. To exclude the possibility that the ferroelectricity comes from the AlN thin film grown on the Nb:STO substrate, we performed PFM characterizations and showed the results in Supplementary Fig. S11. As can be seen, it is difficult to write domains in the AlN film, which demonstrates that the AlN thin film is not ferroelectric.

To unequivocally establish the intrinsic ferroelectric nature of the SATO thin film, the macroscopic polarization-electric field (P-E) hysteresis loops were measured through the Positive-up-negative-down (PUND) method under different voltages at 300 K and 1 kHz, as shown in Fig. 4d. The PUND mode is one of the most accurate methods for measuring the remanent polarization of ferroelectric materials. It can eliminate the influence of dielectric and leakage effects and reflect the intrinsic characteristics of ferroelectricity. Supplementary Fig. S12 shows the measurement parameters of the PUND mode. In the test, the write voltage and read voltage were both 10 V. It is clear that the remanent polarization gradually increases with the voltage. After the voltage is increased to 10 V and higher, the remanent polarization remains unchanged (~7.8 μC/cm2), and the polarization saturation is reached, as shown in Fig. 4d. The coercive field (E c) of the SATO thin film is up to 21 MV/m at 10 V voltage. These ferroelectric properties are nearly comparable to those of BiFeO 3 27, indicating that SATO is a promising ferroelectric material. Figure 4e shows the results of ferroelectric fatigue testing. After 106 cycles, the remanent polarization intensity did not attenuate significantly, which indicates that the SATO film has a strong fatigue stability. The atomic origin of the ferroelectricity of SATO has been experimentally identified as the displacement of Al atoms within the AlO 5 bipyramids, as depicted in Supplementary Fig. S13 and Fig. S14. As can be seen, the polarization of SATO is vertically upward, indicating that SATO is a uniaxial ferroelectric along the c-axis. Thus, the SATO thin film belongs to the first-type displacement-type ferroelectrics.

To reveal the performance of ferroelectric SATO film as a photodetector, we carried out photoelectric response tests. Supplementary Fig. S15 shows the schematic diagram of SATO photodetector testing. Due to the existence of spontaneous polarization in the SATO thin film, a built-in electric field is generated in the opposite direction of the polarization. When high-energy light irradiates the ferroelectric thin film, photogenerated carriers are quickly separated due to the built-in electric field, thereby converting the received optical signal into an electrical signal for detection. To elucidate the optoelectronic properties of the SATO thin film, we conducted measurements using a comprehensive photoelectric testing system equipped with a monochromator, and showed the results in Fig. 5. Figure 5a shows the logarithmic I-V curves of the SATO thin film under different light wavelengths ranging from 250 to 470 nm. In the dark, the SATO thin film exhibits a low current density of less than 1 nA/cm², indicating a very low noise level when used as a PD material. Under illumination, the photocurrent of the SATO PDs significantly increases, demonstrating effective separation of photogenerated carriers within the thin film. The photocurrent reaches its maximum at a wavelength of 330 nm, suggesting that SATO is highly suitable for fabricating UVB (280–320 nm) PDs. In addition, Supplementary Fig. S16 shows that the SATO film has a large dielectric constant (>16), low dielectric loss (<0.1), and excellent frequency stability at room temperature. Figure 5b displays the I-V curves under different light intensities at 330 nm. The SATO thin film shows excellent detection capability for both weak light (down to 50 μW/cm²) and strong light (up to 500 μW/cm²), indicating its proficiency in detecting weak signals. Figure 5c presents the I–T curves of the SATO PDs at 330 nm under bias voltages of −2, −6, and −10 V. The photocurrent density increases with increasing bias voltage, reaching a maximum of approximately 10-4 A/cm² at −10 V. At a bias of −2 V, the SATO detector exhibits the largest difference between photocurrent and dark current, implying the best signal-to-noise ratio for information detection. Furthermore, long-term repeated on-off switching measurements shown in Supplementary Fig. S17 reveal that the photocurrent and dark current remain stable, and the SATO detector has excellent reproducibility and detection reliability.

To quantitatively evaluate the performance of the SATO PDs, we prepared three devices with different effective areas (Supplementary Fig. S18) to calculate important parameters such as responsivity (R), detectivity (D*), and on/off ratio (I on/I off) using the following formulas: R= I ph / PS, D*= R / (2qJ d) 1/2, I on/I off = I ph / I d, where I ph, P, S, q, J d, and I d represent the photocurrent, light intensity, effective device area, electronic charge, dark current density, and dark current, respectively 28. As depicted in Fig. 5d, the responsivity and detectivity of the SATO PDs increase with the wavelength from 250 nm to 320 nm. Beyond 320 nm, both parameters sharply decrease with the increasing wavelength. At 320 nm under a bias of −2 V, the responsivity and detectivity of the SATO film reach as high as 860 mA/W and 1.63×1013 Jones, respectively, which significantly outperform most currently known ferroelectric materials. Supplementary Fig. S19 shows the responsivity and detectivity of different devices at different laser power intensities under 330 nm illumination. The three devices exhibit the same feature. The responsivity and detectivity gradually decrease with the increase of laser intensity. Figure 5e shows the on/off ratios at different wavelengths under a bias of −2 V, with the highest ratio exceeding 104 at 320 nm. To reveal the response speed of the SATO PDs, a time-resolved photoresponse test was conducted, as shown in Fig. 5f. Generally, the response speed is the time taken for the photocurrent to transform from 10% to 90%. As can be seen, the rise time (τ r) and fall time (τ f) took only 6.8 ns and 17.7 ns respectively, which is nearly 10000 times faster than traditional photodetectors like BaTiO 3 29–31 and PbTiO 3 32. The photoelectric switching response and transient photoelectric response of SATO with different polarization directions were measured. As shown in Supplementary Fig. S20, the SATO photodetector has a strong and stable photoelectric response regardless of positive or negative polarization. More importantly, as shown in Supplementary Fig. S21, the SATO photodetectors with positive and negative polarizations have ultra-fast response speed. Their rise/fall times are 6.6 ns/17.7 ns and 8.7 ns/12.2 ns, respectively. Similar tests using other devices with different effective areas were carried out. The current intensity, effective area, laser intensity, responsivity, detectivity and response time of three SATO photodetector devices are listed in the Supplementary Table S2. All the SATO photodetector devices exhibit fast response speed. The ultrafast (ns-scale) response speed indicates that SATO-based optoelectronic devices can be applied to many fields, such as instant communication, which represents an important progress in the field of PD materials.

To compare the comprehensive performance of the SATO ferroelectric PDs with other ferroelectric PDs, Fig. 6 present the response times, responsivities, and detectivities of the ferroelectric PDs reported so far 15,17,18,29–47. It is clear that most ferroelectric PDs have response times ranging from seconds to milliseconds; only a few achieve the microsecond level. The responsivities and detectivities are generally less than 200 mA/W and 1012 Jones, respectively. Although some devices achieve response speeds of the nanosecond level, their responsivity and detectivity are far below 0.01 mA/W and 108 Jones, which do not meet the requirements for PD applications. As one can see, the SATO ferroelectric PDs in this work have not only ultrafast response speed (<10 ns), but also very high responsivity (>500 mA/W) and detectivity (>1013 Jones). Thus, the SATO ferroelectric PDs exhibit great potential in applications for real-time detection and the optoelectronic communication industries.

In summary, we have successfully developed a SATO ferroelectric PD thin film through a solid-state reaction of AlN thin film and STO substrate. The SATO thin film exhibits a magnetoplumbite-type uniaxial ferroelectric structure with a remanent polarization up to 7.8 μC/cm2 and robust ferroelectric retention exceeding 500 h. Optoelectronic performance measurements reveal that the SATO thin film is an outstanding ultraviolet photodetector material with a response wavelength of 330 nm, responsivity of 860 mA/W, detectivity of 1.63×1013 Jones, switching ratio of 1.9×104. More importantly, the SATO photodetector has an ultrafast response speed with a rise time of 6.8 ns and a fall time of 17.7 ns. The superior performance, driven by the displacement of Al within oxygen trigonal bipyramids, positions SATO as a promising candidate for advanced photodetection applications in many fields, such as real-time communication, space exploration, pharmaceuticals, environmental monitoring, and so on.

Methods
Material and microscopic observations
Epitaxial AlN thin films were grown on STO (111) substrates (10×10×0.5 mm) using the pulsed laser deposition (PLD, KrF excimer laser, 248 nm) method 48–50. Before growth, the STO substrates were annealed in air at 1000 °C for 10 h to obtain an atomically flat surface, and then transferred to a high vacuum chamber (<10−5 Pa) and baked at 800 °C for 1 h. For the growth of AlN thin films, the growth temperature of 850 °C, N 2 pressure of 10−3 Pa were adopted. The energy density of 3.5 J/cm2 and frequency of 5 Hz of the laser were used. The distance between the target and the substrate was set as 8 cm. After deposition, the AlN films were annealed at 1500 °C in air for 5 h. As a result, single-crystal epitaxial SATO thin films were formed by the solid-state reaction of AlN thin films and STO substrates.

TEM samples were prepared using the standard procedures of ion-milling, including cutting, mechanical grinding, dimpling, and Ar-ion thinning. To obtain electron-transparent and minimally damaged samples, the thinning voltage was progressively reduced from 5 keV to 0.1 keV, and the thinning angle was adjusted from 7° to 5°. Bright-field imaging and SAED patterns were recorded at 200 kV using a JEM-2100 (JEOL Co., Ltd) TEM. STEM HAADF, ABF images, and the atomic-scale elemental mapping were obtained using an aberration-corrected STEM (Themis 300, FEI) equipped with a probe corrector (CEOS, GmbH) and super-EDS. The acceleration voltage, convergence angle, and camera length were set to 300 kV, 21.4 mrad, and 115 mm, respectively. Collection semiangles for HAADF and ABF were 50-200 mrad and 11–22 mrad, respectively. HAADF and ABF images were simulated based on the multi-slice theory using the Dr. Probe software package 51,52. This approach allowed for accurate modeling of the complex interactions between the electron beam and the sample, providing insights into the structural details observed in the experimental TEM images. The simulation parameters were adopted according to the experimental conditions.

Ferroelectric and PD performance measurements
The electrodes were fabricated using a photolithography technique, followed by sputtering the Pt top electrode with a thickness of 50 nm and a size of 50×50 μm2 on the film 53. The polarization-electric field (P–E) hysteresis loop was measured on a ferroelectric tester (Precision Multiferroic, Radiant Tech, USA.; TF-Analyser 3000, aixACCT, Germany) at 1 kHz and room temperature (25 °C) employing the Remanent Hysteresis or Positive-up-negative-down (PUND) mode. Piezoresponse force microscopy (PFM) measurements were performed using a Cypher Asylum Research platform to characterize the local piezoelectric response. Conductive Pt-coated silicon cantilevers were used, and the sample was mounted on the PFM stage by conductive adhesive tape. Measurements were conducted in contact mode, with the cantilever in contact with the sample surface and a 10 V, 20 kHz AC voltage applied between the tip and the bottom electrode. The out-of-plane piezoresponse signal was recorded as a function of the applied voltage, and the PFM amplitude and phase images were obtained to visualize the ferroelectric domain structure.

The optoelectronic performance was measured using a Keithley 4200 semiconductor characterization system and a xenon (Xe) lamp equipped with a monochromator, providing a tunable light source with wavelengths ranging from 200 nm to 900 nm 54. For transient photoconductivity measurements, a sophisticated transient photoresponse system was employed. This system comprised a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (Continuum Electro-Optics, MINILITE II) with a pulse duration of 2 – 3 ns at 330 nm, a resistor, and an oscilloscope (Tektronix MSO/DPO5000). This setup enabled precise measurement of the sample’s response to light pulses, offering comprehensive insights into its optoelectronic properties.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript describes the optoelectronic performance of the SATO thin film, which shows a high ultraviolet\nphotodetection sensitivity with a responsivity of 860 mA/W, a detectivity of 1.63 × 1013 Jones, and a switching ratio of 104.\nThe SATO photodetector has an extremely ultrafast response speed with a rise time (τr) of 6.8 ns and a fall time (τf) of 17.7\nns. Major comments/corrections are as follows for further clarification and improvement.\n1) The manuscript highlights the potential of ferroelectric thin films for high-performance photodetectors, primarily due to their\nintrinsic electric fields and polarization-related effects. To further support this claim, it would be beneficial to include a\ndetailed analysis of the dielectric properties of the SATO thin film. Parameters such as dielectric constant, dielectric loss,\nand their frequency dependence would provide valuable insight into the material's capability for charge storage and\nscreening, which are critical for optimizing photodetector performance. Including these measurements would strengthen the\ndiscussion on the multifunctional role of ferroelectricity in optoelectronic applications.\n2) The statement \"AB₁₂O₁₉ magnetoplumbite compounds have garnered significant attention owing to their multifunctional\nphysical properties, including ferromagnetism, ferroelectricity, high dielectric constant, and wide bandgap. These\ncharacteristics make them promising candidates for applications in spintronics, microwave devices, and multifunctional\nelectronic materials.\" It is superficial in nature, without any strong evidence/reference.\n3) The polarization hysteresis data presented in Fig. 4f convincingly demonstrate the ferroelectric nature of the SATO thin\nfilm, with a notable remanent polarization (Pr) of 7.8 μC/cm² and a coercive field (Ec) of 21 MV/m at 300 K and 1 kHz. To\nfurther substantiate the intrinsic ferroelectric behavior and rule out potential non-ferroelectric contributions (e.g., from leakage\nor dielectric effects), it would be valuable to include Positive-Up-Negative-Down (PUND) measurements. Additionally, a\nfatigue analysis showing the stability of polarization over repeated electric field cycling would strengthen the case for the\nmaterial's potential in non-volatile memory or other ferroelectric device applications. Including these characterizations would\nsignificantly enhance the credibility and completeness of the ferroelectric performance assessment. More importantly, the\nauthors have NOT highlighted the primary cause of the development/origin of polarization; it must be illustrated in the\nrevised manuscript\n4) The HAADF and ABF STEM images of the SATO thin film along the [112 ̅ 0], [11 ̅ 00], and [0001] zone axes are clearly\npresented, and the overlaid atomic models aid significantly in visualizing the local atomic arrangement. Polarization\nmapping based on the HAADF-STEM data is recommended to enhance the structural analysis further. Quantitative mapping\nof cation displacements—especially the relative shifts between A-site and B-site cations or between cations and anions—\ncould provide direct insight into the local polarization vectors and their directional variation. This addition would confirm the\npresence of spontaneous polarization at the atomic scale and establish a stronger structure-property correlation for the\nobserved ferroelectric behavior in SATO.\n5) In Figure 5, (a) & (b), legends are not showing clearly, so increase the size; (b) all the graphs overlapped at different\npower intensities, so enlarge this figure; (c) caption does not mention power intensity; (e) Y-axis label doesn't show clearly,\nso increase the size, (f) not mentioned wavelength and power intensity, so also mention in the caption.\n6) The transient photoconductivity measurements and their results make this paper unique and might deserve publication in\nNature Communications. However, the authors have just put one graph, Fig. 5 (f), for the same. It requires its repeatability\n(long time ON/OFF), current intensity (absolute value), different laser intensity, device effective area (at least three sizes of\nthe device), etc., to calculate the responsivity and detectivity of the photodetector (PD) as claimed!! I would be happy to see\nthe detailed analysis of Fig. 5 (f), which supports and corroborates the claims. A table (supplementary) is needed for all the\ninputs with responsivity and detectivity calculations.", "reviewer_2": "This manuscript presented SrAl11TiO19 (SATO) thin films as a novel ferroelectric material for ultrafast ultraviolet\nphotodetectors. While the TEM structural analysis is generally well-executed, the ferroelectric characterization requires\nsubstantial reinforcement. Several additional issues warrant attention.\nMajor Concerns\n1. Inadequate Ferroelectric Characterization\nP-E Hysteresis Analysis: The P-E loops presented in Figure 4f lack essential electric field dependence data. Demonstrating\npolarization saturation at high fields is critical for confirming intrinsic ferroelectric switching behavior. The current data are\ninsufficient to distinguish between true ferroelectric switching and other polarization mechanisms.\nPFM Domain Analysis: While the PFM images successfully demonstrate voltage-induced domain writing (Figures 4b and\n4d), the study lacks PFM characterization of pristine, as-grown film regions. This baseline comparison is crucial for\ndetermining whether the observed ferroelectricity represents an intrinsic material property or results from field-induced\neffects.\n2. Stoichiometry and Charge Balance Issues\nThe reported SATO stoichiometry raises fundamental questions about charge neutrality. The manuscript must address how\nelectronic balance is maintained in this composition—specifically, the oxidation state of titanium and the potential role of\nintrinsic defects. This analysis is crucial for understanding both the insulating properties and ferroelectric behavior of SATO.\n3. Substrate Interaction Effects\nThe SATO films are grown on Nb-doped STO substrates and processed at 1500C. At these elevated temperatures, Nb\ndiffusion from the substrate into the SATO layer is highly probable. The authors must evaluate this possibility and discuss its\npotential impact on film conductivity, ferroelectric properties, and photoresponse characteristics.\n4. Polarization-Dependent Photoresponse\nA defining characteristic of ferroelectric photodetectors is their ability to exhibit polarization-switchable performance. The\nstudy lacks critical experiments demonstrating how photoresponse parameters (photocurrent magnitude, responsivity,\nresponse time) vary with deliberate polarization reversal in the SATO film. Such measurements would provide compelling\nevidence for ferroelectricity-enhanced photodetection.\nMinor Concerns\nDevice Visualization: Including a photograph of the fabricated SATO photodetector would help readers better understand\nthe device architecture and experimental configuration.\nDomain Structure Representation: The schematic diagram of ferroelectric domains in Supplementary Figure S1 does not\naccurately reflect typical domain patterns observed in ferroelectric thin films. Domain illustrations should be grounded in\nexperimental observations from either the literature or the authors' work to ensure scientific accuracy and educational value.\nAddressing these concerns—particularly the ferroelectric characterization gaps and stoichiometry questions—will\nsubstantially strengthen the manuscript's scientific rigor and enhance the credibility of claims regarding this novel\nferroelectric photodetector material.", "reviewer_3": "In the paper “Ferroelectric ultraviolet photodetector material with ultrafast response speed”, Yan et al present an outstanding\noptoelectronic performance of the single-domain ferroelectric SATO thin film. The ferroelectric properties have been\ndiscussed and they provide excellent optoelectronic performances for ultraviolet region, such as responsibility of 860mA/W,\nthe rise time of 6.8 ns and the fall time of 17.7 ns. I think the paper is well organized, and the presented experimental results\nare very interesting. However, several important points concerning the ferroelectric properties and the underlying\nmechanisms that enable such high optoelectronic performance remain unclear.\n1. The authors used the PFM and polarization hysteresis loops to show the ferroelectric behavior of SATO thin film. While\nthe authors state that the high dielectric constants of ferroelectric materials lead to lower noise level, it will be helpful if they\ncan show dielectric behavior of SATO thin film and compare with other ferroelectric materials.\n2. The authors state that the ferroelectric polarization induces intrinsic built-in electric field increasing optoelectronic\nproperties. I am wondering whether the ferroelectricity can be tailored in SATO thin film, that the effect of ferroelectric\npolarization can be confirmed. They could measure optoelectronic performances using the SATO thin films with different\nspontaneous polarization.\n3. It was demonstrated that the single-domain ferroelectric thin films can result the high optoelectronic performances. Is all\nsingle-domain ferroelectric thin films are possible to result the high optoelectronic performances? If not, what distinguishes\nSATO from other candidates? A brief comparison with other single-domain ferroelectric materials would help\ncontextualize the advantages of the SATO system.\n4. The SATO-based device demonstrates a nanosecond-scale response time, which is remarkable. What could be origin of\nthe ultrafast response speed compared to other conventional photodetectors?"}, "author_response": "Reply to referees’ questions and comments:\nManuscript ID: NCOMMS-25-34130-T\nTitle: Ferroelectric ultraviolet photodetector material with ultrafast response speed\nAuthors: Xuexi Yan, Tingting Yan, Lingli Li, Yi Cao, Xinwei Wang, Jinghui Wang, Ang Tao, Tingting\nYao, Yixiao Jiang, Weijin Hu, Xiaosheng Fang, Hengqiang Ye, Xiu-Liang Ma, Chunlin Chen\nAugust 2, 2025\nFirst of all, we would like to thank the reviewers for reviewing our manuscript very carefully and\nputting forth many constructive comments. We have revised our manuscript thoroughly based on these\ncomments. Changes, including minor ones not related to the reviewers’ comments, are indicated in red\nfonts in the revised main text and Supporting Information. Our responses to specific comments follow\nbelow:\nReply to referee 1 (R1):\nWe appreciate the general recognition by the referee that “This manuscript describes the\noptoelectronic performance of the SATO thin film, which shows a high ultraviolet photodetection\nsensitivity with a responsivity of 860 mA/W, a detectivity of 1.63 × 1013 Jones, and a switching ratio\nof 104. The SATO photodetector has an extremely ultrafast response speed with a rise time (τ) of 6.8\nr\nns and a fall time (τ) of 17.7 ns.”\nf\nIn the meanwhile, the referee also raises some specific questions and comments which are\nsummarized into six major aspects. We fully understand the referee’s concerns, and here we address\nall the questions and discuss all the comments one-by-one in the following.\nQuestion and comment (R1.1): The manuscript highlights the potential of ferroelectric thin films for\nhigh-performance photodetectors, primarily due to their intrinsic electric fields and polarization-\nrelated effects. To further support this claim, it would be beneficial to include a detailed analysis of the\ndielectric properties of the SATO thin film. Parameters such as dielectric constant, dielectric loss,\nand their frequency dependence would provide valuable insight into the material's capability for charge\nstorage and screening, which are critical for optimizing photodetector performance. Including these\nmeasurements would strengthen the discussion on the multifunctional role of ferroelectricity in\noptoelectronic applications.\nReply to Question and comment (R1.1):\nThank you very much for your valuable suggestion. According to your suggestions, we performed\nmeasurements of the dielectric constant and dielectric loss as a function of frequency in the SATO thin\nfilm, as shown in Supplementary Fig. S15. As one can see that the SATO thin film exhibits a large\ndielectric constant (>16) and low dielectric loss (<0.1) at room temperature. More importantly, the\ndielectric constant and dielectric loss decrease slowly with increasing frequency, which indicates that\nthe SATO film has good frequency stability. Thus, the large dielectric constant, low dielectric loss, and\nexcellent frequency stability all prove that SATO film has excellent charge storage capacity, which is\ncrucial for its excellent performance in optoelectronic applications.\nThe following sentences and Supplementary Fig. S15 were added in the revised manuscript and\nSupplementary Information:\nSupplementary Fig. S15 shows that the SATO film has a large dielectric constant (>16), low\ndielectric loss (<0.1), and excellent frequency stability at room temperature.\nSupplementary Fig. S15 | Dielectric constant and dielectric loss as a function of frequency in the\nSATO thin film. The SATO film exhibits a large dielectric constant, low dielectric loss, and excellent\nfrequency stability at room temperature.\nQuestion and comment (R1.2): The statement \"AB O magnetoplumbite compounds have garnered\n12 19\nsignificant attention owing to their multifunctional physical properties, including ferromagnetism,\nferroelectricity, high dielectric constant, and wide bandgap. These characteristics make them promising\ncandidates for applications in spintronics, microwave devices, and multifunctional electronic\nmaterials.\" It is superficial in nature, without any strong evidence/reference.\nReply to Question and comment (R1.2):\nThanks for your precious suggestions. We revised the introduction of the manuscript and added\napplication examples of AB O magnetoplumbite to demonstrate its potential as a functional material.\n12 19\nThe following sentences and references were added in the revised manuscript:\nAB O magnetoplumbite compounds have attracted intense interest due to the abundant physical\n12 19\nproperties. For example, SrFe O is widely used in microwave absorbers, magnetic recording media\n12 19\nand sensors, high-frequency electromagnetic (EM) devices, etc. due to its superior magnetic\nparameters, high magnetic permeability and low electrical conduction loss.20 BaFe O is the best\n12 19\nchoice for commercial low-cost permanent magnet materials due to its high coercivity and good\nchemical and thermal stability. It is widely used in motors, speakers, sensors and other devices.21 In\naddition, PbFe O is a natural multiferroic material with broad application prospects in non-volatile\n12 19\nmemory, ferroelectric photovoltaics and other fields.22,23 As a structurally similar derivative, SrAl O\n12 19\nwas predicted to has ferroelectricity but has not yet been experimentally proved.24\nReferences:\n20. Anantharamaiah, P. N., Shashanka, H. M., Saha, S., Haritha, K. & Ramana, C. V. Aluminum\nDoping and Nanostructuring Enabled Designing of Magnetically Recoverable Hexaferrite Catalysts.\nACS Omega 7, 6549–6559 (2022).\n21. Pullar, R. C. Hexagonal ferrites: A review of the synthesis, properties and applications of\nhexaferrite ceramics. Prog. Mater Sci. 57, 1191–1334 (2012).\n22. Tan, G. -L. & Li, W. Ferroelectricity and ferromagnetism of M‐type lead hexaferrite. J. Am. Ceram.\nSoc. 98, 1812–1817 (2015).\n23. Shen, S.-P. et al. Quantum electric-dipole liquid on a triangular lattice. Nat. Commun. 7, 10569\n(2016).\n24. Li J., Medina E. A., Stalick J. K., Sleight A. W. & Subramanian M. A. Structural studies of\nCaAl O , SrAl O , La Al O , and CaAl NiTiO with the hibonite structure;\n12 19 12 19 2/3+δ 12–δ 19 10 19\nindications of an unusual type of ferroelectricity. Z. Naturforsch. B 71, 475–484 (2016).\nQuestion and comment (R1.3): The polarization hysteresis data presented in Fig. 4f convincingly\ndemonstrate the ferroelectric nature of the SATO thin film, with a notable remanent polarization (Pr)\nof 7.8 μC/cm² and a coercive field (Ec) of 21 MV/m at 300 K and 1 kHz. To further substantiate the\nintrinsic ferroelectric behavior and rule out potential non-ferroelectric contributions (e.g., from leakage\nor dielectric effects), it would be valuable to include Positive-Up-Negative-Down (PUND)\nmeasurements. Additionally, a fatigue analysis showing the stability of polarization over repeated\nelectric field cycling would strengthen the case for the material's potential in non-volatile memory or\nother ferroelectric device applications. Including these characterizations would significantly enhance\nthe credibility and completeness of the ferroelectric performance assessment. More importantly, the\nauthors have NOT highlighted the primary cause of the development/origin of polarization; it must be\nillustrated in the revised manuscript.\nReply to Question and comment (R1.3):\nWe sincerely appreciate your constructive suggestions on our work. The PUND mode is one of the\nmost accurate methods for measuring the remanent polarization of ferroelectric materials. It can\neliminate the influence of dielectric and leakage effects and reflect the intrinsic characteristics of\nferroelectricity. Fig. 4f in the manuscript was measured through positive-up-negative-down (PUND)\nmethod at 300 K and 1 kHz. Supplementary Fig. S10 shows the measurement parameters of the PUND\nmode. In the test, the write voltage and read voltage were both 10 V. The delay time between P and U\nor N and D is fixed 0.5 ms. In addition, we added hysteresis loop measurements at different voltages\nthrough the PUND mode, as shown in Supplementary Fig. S11a. It can be found that as the voltage\nincreases, the remanent polarization gradually increases. When the voltage is higher than 10 V, the\nremanent polarization remains unchanged (~ 7.8 μC/cm2), which indicates that the polarization\nsaturation has been reached.\nTo explore the potential application of SATO films in non-volatile memory or other ferroelectric\nmicro-devices, ferroelectric fatigue testing was carried out, as shown in Supplementary Fig. S11b. The\nvoltages applied to the test were 10 V and 12 V, respectively. After 106 cycles, the remanent\npolarization intensity did not attenuate significantly, which indicates that the SATO film has a strong\nfatigue stability.\nThe origin for the ferroelectricity of SATO has been experimentally identified as the off-centre\ndisplacement of Al3+ in the AlO bipyramids, as depicted in Supplementary Fig. S12. The polarization\n5\nof SATO is along the c axis. Thus, the SATO thin film belongs to the first-type displacement-type\nferroelectrics.\nThe following sentences and Supplementary Fig. S10, Fig. S11, and Fig. S12 were added in the\nrevised manuscript and Supplementary Information:\nTo unequivocally establish the ferroelectric nature of the SATO thin film, macroscopic polarization\nhysteresis loops were measured through Positive-up-negative-down (PUND) method at 300 K and 1\nkHz, as shown in Fig. 4f. The PUND mode is one of the most accurate methods for measuring the\nremanent polarization of ferroelectric materials. It can eliminate the influence of dielectric and leakage\neffects and reflect the intrinsic characteristics of ferroelectricity. Supplementary Fig. S10 shows the\nmeasurement parameters of the PUND mode. In the test, the write voltage and read voltage were both\nHysteresis loops measured under different voltages are shown in Supplementary Fig. S11a. It is\nclear that the remanent polarization gradually increases with the voltage. After the voltage is increased\nto 10 V and higher, the remanent polarization remains unchanged (~ 7.8 μC/cm2) and the polarization\nsaturation is reached. Supplementary Fig. S11b shows the results of ferroelectric fatigue testing. After\n106 cycles, the remanent polarization intensity did not attenuate significantly, which indicates that the\nSATO film has a strong fatigue stability. The atomic origin for the ferroelectricity of SATO has been\nexperimentally identified as the displacement of Al atoms within the AlO bipyramids, as depicted in\nSupplementary Fig. S10 | Parameters of positive-up-negative-down (PUND) measurements of\nthe SATO films. The write and read voltages are both 10V.\nSupplementary Fig. S11 | Ferroelectric fatigue properties of the SATO thin film tested by PUND\nmeasurements. (a) Hysteresis loops of the SATO thin film under different voltages. (b) Fatigue\ncharacteristics of the SATO thin film under 10 V and 12 V. Inset are the PUND loops before and after\ncycling. After 106 cycles, the polarization intensity exhibits a strong fatigue stability without\nsignificant attenuation.\nSupplementary Fig. S12 | Atomic origin of the ferroelectricity of SATO. (a) and (b) HAADF\nimages showing the upward and downward polarities, respectively. The red and green dotted circles\nrepresent the O and Al atoms in the AlO bipyramid, respectively. The upward and downward\n5\ndisplacement of Al atoms inside the AlO bipyramids leads to the emergence of ferroelectricity.\nthe [112̅0], [11̅00], and [0001] zone axes are clearly presented, and the overlaid atomic models aid\nsignificantly in visualizing the local atomic arrangement. Polarization mapping based on the HAADF-\nSTEM data is recommended to enhance the structural analysis further. Quantitative mapping of cation\ndisplacements—especially the relative shifts between A-site and B-site cations or between cations and\nanions—could provide direct insight into the local polarization vectors and their directional variation.\nThis addition would confirm the presence of spontaneous polarization at the atomic scale and establish\na stronger structure-property correlation for the observed ferroelectric behavior in SATO.\nReply to Question and comment (R1.4):\nThanks for your valuable suggestion. We used the CalAtom software package\n(https://github.com/FangLinF/CalAtom) and self-developed MATLAB scripts to perform quantitative\nanalysis on the HAADF STEM images, including gaussian filtering of atomic intensity profile, atomic\ncoordinate position identification, element type classification, and calculation of the Al3+ offset in the\nAlO trigonal bipyramid. As shown in supplementary Fig. S13, the direction and length of the arrows\n5\nrepresent the direction of the ferroelectric polarization and displacement, respectively. The length of\nthe arrows has been magnified 25 times to visualize more clearly the displacement of the Al atoms.\nThe displacement of Al atoms in this image ranges from 7 to 15 pm.\nThe following sentences and Supplementary Fig. S13 were added in the revised Supplementary\nInformation.\nAs can be seen, the polarization of SATO is vertically upward, indicating that SATO is a uniaxial\nferroelectric along the c-axis. Thus, the SATO thin film belongs to the first-type displacement-type\nferroelectrics.\nSupplementary Fig. S13 | Analysis of the ferroelectric polarization in SATO. (a) HAADF STEM\nimage of SATO along [112̅0] axis zone. (b) Quantitative analysis of polarization displacement in the\nHAADF image. The direction and length of the green arrows represent the displacement of Al atoms\ninside the AlO bipyramids. The length of the arrows has been magnified by 25 times to visualize more\n5\nclearly the displacement of Al atoms. The displacement of Al atoms in this image ranges from 7 to 15\npm. Scale bar, 1nm.\nQuestion and comment (R1.5): In Fig. 5, (a) & (b), legends are not showing clearly, so increase the\nsize; (b) all the graphs overlapped at different power intensities, so enlarge this figure; (c) caption does\nnot mention power intensity; (e) Y-axis label doesn't show clearly, so increase the size, (f) not\nmentioned wavelength and power intensity, so also mention in the caption.\nReply to Question and comment (R1.5):\nThanks for your suggestion. Fig. 5 has been carefully adjusted and added to the revised manuscript.\nFig. 5 | Photoelectric properties the SATO films. (a) Logarithmic I-V curves of the SATO PDs under\ndifferent light wavelengths (250-470 nm). The SATO PDs have the strongest response to 330 nm UV\nlight. (b) I-V curves of the SATO PDs under different power density at 330 nm. (c) I-T curves under\n330 nm on-off illumination at -2, -6, and -10 V of the SATO PDs. (d) Responsivity, detectivity, and (e)\nOn/off ratio curves at different wavelengths. At 330 nm, the responsivity, detectivity, and on/off ratio\nof the SATO PDs at -2 V are as high as 860 mA/W, 1.63 × 1013 Jones, and 1.9 × 104, respectively. (f)\nTime-resolved transient photoresponse curves. The SATO PDs exhibits extremely fast response speed\nwith rise time (τ) of 6.8 ns and fall time (τ) of 17.7 ns.\nr f\nQuestion and comment (R1.6): The transient photoconductivity measurements and their results make\nthis paper unique and might deserve publication in Nature Communications. However, the authors\nhave just put one graph, Fig. 5 (f), for the same. It requires its repeatability (long time ON/OFF),\ncurrent intensity (absolute value), different laser intensity, device effective area (at least three sizes of\nthe device), etc., to calculate the responsivity and detectivity of the photodetector (PD) as claimed!! I\nwould be happy to see the detailed analysis of Fig. 5 (f), which supports and corroborates the claims.\nA table (supplementary) is needed for all the inputs with responsivity and detectivity calculations.\nReply to Question and comment (R1.5):\nThank you very much for your recognition and high evaluation on this study and thanks for your\nprecious suggestion. First, we added the long-term repeated on-off switching measurements of the\nSATO photodetector, as shown in Supplementary Fig. S16. The measurement was carried out under\n330 nm laser illumination with a power density of 50 μW/cm2. As one can see, the currents of the\nSATO photodetector remained almost unchanged during the test process of up to 2000s, which\nindicates that the SATO photodetector has an excellent reproducibility and detection reliability.\nPhotodetectors with groove widths ranging from 5 μm to 17 μm were fabricated by a direct-write\nlithography system, as shown in Supplementary Fig. S17. To evaluate the performance of the\nphotodetector, we selected three devices with groove widths of 5 μm, 8 μm, and 11 μm for systematic\nmeasurement. That is, the corresponding effective area of these devices are 2.5 × 10-6 cm2 (Device 1),\n4.0 × 10-6 cm2 (Device 2), and 5.5 × 10-6 cm2 (Device 3). Then, we measured the three devices at\ndifferent laser power densities and calculated the Responsivity (R) and detectivity (D*), as shown in\nSupplementary Fig. S18. It can be found that the devices with three effective areas all have excellent\nweak light detection capabilities as low as 50 μW/cm2.\nIn order to determine the effect of polarization direction on the photoelectric response of SATO thin\nfilms, we pre-polarized the AlN thin films using high voltage before testing. As shown in the\nSupplementary Fig. S19, the currents formed by the SATO photodetectors pre-polarized at 30V and -\n30V under 330 nm illumination are almost equal, which indicates that the effects of positive and\nnegative polarization on the performance of the photodetector are actually equivalent. In addition, we\nhave added time-resolved transient photoresponse measurements dependent on the polarization\ndirection, as shown in Supplementary Fig. S20. As one can see that the SATO photodetector has a\nstable photoresponse regardless of whether it is in the positive or negative polarization state, as shown\nin Supplementary Fig. S20a and d. More importantly, the SATO photodetectors with positive and\nnegative polarizations have ultra-fast response speed with rise and fall times of 6.6 ns/17.7 ns and\n8.7ns/12.2 ns, respectively (Supplementary Figs. S20), which is basically consistent with the response\nspeed of the unprepolarized SATO photodetector (Fig. 5f in the main text).\nFinally, we made statistics on the current intensity, effective area, laser intensity, responsivity,\ndetectivity and response time of SATO photodetector devices, as shown in the Supplementary Table\nS1. All the SATO photodetector devices we measured have extremely fast response speed, which can\ngreatly reduce the response delay of functional devices and improve the efficiency of information\nexchange and detection.\nThe following sentences and Supplementary Fig. S16 to Fig. S20 were added in the revised\nmanuscript and Supplementary Information:\nFurthermore, long-term repeated on-off switching measurements shown in supplementary Fig. S16\nreveal that the photocurrent and dark current remain stable, and the SATO detector has the excellent\nreproducibility and detection reliability.\nTo quantitatively evaluate the performance of the SATO PDs, we prepared three devices with\ndifferent effective areas (Supplementary Fig. S17)\nSupplementary Fig. S18 shows the responsivity and detectivity of different devices at different laser\npower intensity under 330 nm illumination. The three devices exhibit the same feature. The\nresponsivity and detectivity gradually decrease with the increase of laser intensity.\nThe photoelectric switching response and transient photoelectric response of SATO with different\npolarization directions were measured. As shown in Supplementary Fig. S19, the SATO photodetector\nhas a strong and stable photoelectric response regardless of positive or negative polarization. More\nimportantly, as shown in Supplementary Figs. S20, the SATO photodetectors with positive and\nnegative polarizations have ultra-fast response speed. Their rise/fall times are 6.6 ns/17.7 ns and\n8.7ns/12.2 ns, respectively. Similar tests using other devices with different effective area were carried\nout. The current intensity, effective area, laser intensity, responsivity, detectivity and response time of\nthree SATO photodetector devices are listed the Supplementary Table S1. All the SATO photodetector\ndevices exhibit extremely fast response speed.\nSupplementary Fig. S16 | Long-term stability of the SATO photodetector under 330 nm laser\nillumination with a power intensity of 50 μW/cm2. There is almost no significant change in the\ncurrent, indicating the excellent stability and reliability of the SATO photodetector.\nSupplementary Fig. S17 | Optical microscope images of the SATO photodetectors fabricated by\nphotolithography. The size of the Pt electrode is 50 μm × 50 μm. Scale bar, 500 μm in (a) and 50 μm\nin (b).\nSupplementary Fig. S18 | Responsivity and detectivity curves at different laser power density.\nAll the devices have excellent detection capabilities of weak light.\nSupplementary Fig. S19 | I-T curves under 330 nm on-off illumination at positive and negative\npolarizations of the SATO photodetector. The photocurrents under positive and negative\npolarizations are almost equal.\nSupplementary Fig. S20 | Time-resolved transient photoresponse curves of the SATO\nphotodetectors with positive and negative polarizations. The SATO photodetectors with positive\nand negative polarizations have ultra-fast response speed. Their rise/fall times are 6.6 ns/17.7 ns (a-c)\nand 8.7ns/12.2 ns (d-e), respectively.\nTable S1 | Statistical summary of the optoelectronic performance of SATO photodetectors.\nSincerely thank you very much for your constructive comments, suggestions and valuable questions.\nYour comments and important questions were very helpful in improving our manuscript.\nReply to referee 2 (R2):\nWe appreciate the general recognition by the referee that “This manuscript presented SrAl11TiO19\n(SATO) thin films as a novel ferroelectric material for ultrafast ultraviolet photodetectors. While the\nTEM structural analysis is generally well-executed, the ferroelectric characterization requires\nsubstantial reinforcement.” In the meanwhile, the referee also raises some specific questions and\ncomments. We fully understand the referee’s concerns, and address all the comments one-by-one in\nthe following.\nMajor Concerns\nQuestion and comment (R2.1): Inadequate Ferroelectric Characterization:\nP-E Hysteresis Analysis: The P-E loops presented in Figure 4f lack essential electric field dependence data.\nDemonstrating polarization saturation at high fields is critical for confirming intrinsic ferroelectric\nswitching behavior. The current data are insufficient to distinguish between true ferroelectric switching\nand other polarization mechanisms.\nPFM Domain Analysis: While the PFM images successfully demonstrate voltage-induced domain\nwriting (Figures 4b and 4d), the study lacks PFM characterization of pristine, as-grown film regions.\nThis baseline comparison is crucial for determining whether the observed ferroelectricity represents\nan intrinsic material property or results from field-induced effects.\nReply to Question and comment (R2.1):\nThank you very much for your precious suggestion. To reveal the intrinsic ferroelectricity of SATO\nthin films, we performed P-E loops measurements at different voltages through the Positive-up-\nnegative-down (PUND) mode, as shown in Supplementary Fig. S11a. It can be found that as the\nvoltage increases, the remanent polarization gradually increases. When the voltage is greater than 10\nV, the remanent polarization is stable (~ 7.8 μC/cm2), which indicates that polarization saturation has\nbeen reached. In addition, to reveal that SATO has stable intrinsic ferroelectricity, we carried out\nferroelectric fatigue measurements, as shown in Supplementary Fig. S11b. The voltages applied to the\ntest were 10 V and 12 V, respectively. After 106 cycles, the remanent polarization intensity did not\nattenuation significantly, which indicates that the SATO film has a strong fatigue stability.\non the Nb-doped STO substrate, we performed PFM characterizations, as shown in Supplementary Fig.\nS9. Electrode domains cannot be written in the AlN film, which demonstrates that AlN is not a\nferroelectric material. On the other hand, low-magnification TEM bright-field images and\ncorresponding selected area electron diffraction (SAED) characterization indicate that the AlN film\nhas completely phase-changed with the STO substrate and transformed into a SATO single-phase film\nafter high-temperature annealing in air, as shown in Fig. 1 in the main text and Supplementary Fig. S6\nin Supplementary Information. Both performance testing and structural characterization can prove that\nthe ferroelectricity comes from the SATO film, not the initially grown AlN film.\nThe following sentences and Supplementary Fig. S11 and Fig. S9 were added in the revised\nmanuscript and Supplementary Information.\nHysteresis loops measured under different voltages are shown in Supplementary Fig. S11a. It is\nclear that the remanent polarization gradually increases with the voltage. After the voltage is increased\nto 10 V and higher, the remanent polarization remains unchanged (~ 7.8 μC/cm2) and the polarization\nsaturation is reached. Supplementary Fig. S11b shows the results of ferroelectric fatigue testing. After\n106 cycles, the remanent polarization intensity did not attenuate significantly, which indicates that the\nSATO film has a strong fatigue stability.\nTo exclude the possibility that the ferroelectricity comes from the AlN thin film grown on the\nNb:STO substrate, we performed PFM characterizations and showed the results in Supplementary Fig.\nS9. As can be seen, it is difficult to write domains in the AlN film, which demonstrates that the AlN\nthin film is not ferroelectric.\nmeasurements. (a) Hysteresis loops of the SATO thin film under different voltages. (b) Fatigue\ncharacteristics of the SATO thin film under 10 V and 12 V. Inset are the PUND loops before and after\ncycling. After 106 cycles, the polarization intensity exhibits a strong fatigue stability without\nsignificant attenuation.\nSupplementary Fig. S9 | PFM characterizations of the AlN thin film on the Nb:STO (111)\nsubstrate. (a-c) PFM topography, amplitude, and phase images plots of the AlN film. Domains cannot\nbe written in the AlN film, suggesting that the AlN film is not ferroelectric. Scale bar, 2 μm.\nQuestion and comment (R2.2): Stoichiometry and Charge Balance Issues:\nThe reported SATO stoichiometry raises fundamental questions about charge neutrality.\nThe\nmanuscript must address how electronic balance is maintained in this composition—specifically, the\noxidation state of titanium and the potential role of intrinsic defects. This analysis is crucial for\nunderstanding both the insulating properties and ferroelectric behavior of SATO.\nReply to Question and comment (R2.2):\nThanks for your valuable this comment. Your suggestions have led us to think deeply on the charge\nbalance issues in SATO. SATO can be understood as Ti atoms partially replacing the Al atoms in\nSrAl O magnetoplumbite. Therefore, determining the valence state of Ti is key to understanding the\n12 19\nstoichiometry and charge balance of SATO. Here, we characterized the Ti L fine structure using\n2,3\natomic-resolved electron energy-loss spectroscopy (EELS) with an energy resolution of 0.3 eV, as\nshown in Supplementary Fig. S5. It can be clearly found that the EELS spectra of SATO and STO have\nthe same fine structures with four peaks, indicating that the valence of Ti in SATO is +4. To maintain\n17\nthe charge neutrality of the SATO structure, Al vacancies must exist in SATO. Thank you again for\nyour insightful suggestions.\nThe sentences and Supplementary Fig. S5 were added in the revised manuscript and\nSupplementary Information.\nThis compound is derived from SrAl O after replacing one Al atom by Ti. To understand the\n12 19\nstoichiometry and charge balance of SATO, electron energy-loss spectroscopy (EELS) analysis was\ncarried out. Ti L edges of SATO and STO are shown in Supplementary Fig. S5. It is clear that the Ti\n2,3\nL edges of SATO and STO have very similar fine structures with four peaks, suggesting that the Ti\n2,3\nions in SATO have the valence state of +4. To maintain the charge neutrality of the SATO structure,\nAl vacancies must exist in SATO.\nSupplementary Fig. S5 | EELS spectra showing the Ti L edges in STO and SATO. The Ti L\n2,3 2,3\nedges of SATO and STO have very similar fine structures with four peaks, suggesting that the Ti ions\nin SATO have the valence state of +4.\nQuestion and comment (R2.3): Substrate Interaction Effects:\nThe SATO films are grown on Nb-doped STO substrates and processed at 1500C. At these elevated\ntemperatures, Nb diffusion from the substrate into the SATO layer is highly probable. The authors must\nevaluate this possibility and discuss its potential impact on film conductivity, ferroelectric properties,\nand photoresponse characteristics.\nSincerely thanks for your valuable comment. To determine whether there is diffusion of Nb from\nthe substrate to the SATO thin film or not, we used the EDS spectra obtained in the aberration-corrected\nSTEM to characterize the elements and content in the SATO film. Supplementary Fig. S4 shows the\nEDS spectra of the 0.7at.% Nb: STO substrate and the SATO thin film. It can be found that except for\nthe peaks of Sr, Ti, and O, there is also an obvious peak of Nb in the EDS spectrum of the substrate,\nas shown by the arrows in Supplementary Fig. S4a. In the SATO film, only the peaks of Sr, Al, Ti, and\nO are observed and no Nb peak can be detected, as shown in Supplementary Fig. S4b. This indicates\nthat the Nb element in the substrate does not diffuse into the film during sample preparation.\nSupplementary Fig. S4 | EDS spectrum recorded from the 0.7% Nb: STO (111) substrate and\nSATO thin film. There are only Sr, Al, Ti, and O elements in the SATO thin film and no Nb element\ndiffused from the substrate.\nQuestion and comment (R2.4): Polarization-Dependent Photoresponse:\nA defining characteristic of ferroelectric photodetectors is their ability to exhibit polarization-\nswitchable performance. The study lacks critical experiments demonstrating how photoresponse\nparameters (photocurrent magnitude, responsivity, response time) vary with deliberate polarization\nreversal in the SATO film. Such measurements would provide compelling evidence for ferroelectricity-\nenhanced photodetection.\nReply to Question and comment (R2.4):\nusing high voltage in different directions and measured the effect of different polarization directions\non the photoresponse. As shown in the Supplementary Fig. S19, the currents formed by the SATO\nphotodetectors pre-polarized at 30V and -30V under 330 nm illumination are almost the same, which\nindicates that the effects of positive and negative polarization on the performance of the photodetector\nare actually equivalent. In addition, we have added time-resolved transient photoresponse\nmeasurements dependent on the polarization direction, as shown in Supplementary Fig. S20. As one\ncan see that the SATO photodetector has a stable photoresponse regardless of whether it is in the\npositive or negative polarization state, as shown in Supplementary Fig. S20a and d. More importantly,\nthe SATO photodetectors with positive and negative polarizations have ultra-fast response speed with\nrise and fall times of 6.6 ns/17.7 ns and 8.7ns/12.2 ns, respectively (Supplementary Figs. S20b, c, e,\nand f), which is basically consistent with the response speed of the no prepolarized SATO\nphotodetector (Fig. 5f in the main text).\nSimilarly, we performed the same test on other devices with different effective area and statistically\nsummarized on the current intensity, effective area, laser intensity, responsivity, detectivity and\nresponse time of SATO photodetector devices, as shown in the Supplementary Table S1. All the SATO\nphotodetector devices we measured have extremely fast response speed, which can greatly reduce the\nresponse delay of functional devices and improve the efficiency of information exchange and detection.\nThe following sentences and Supplementary Fig. S19, Fig. S20, and Table S1 were added in the\nrevised manuscript and Supplementary Information.\nThe photoelectric switching response and transient photoelectric response of SATO with different\npolarization directions were measured. As shown in Supplementary Fig. S19, the SATO photodetector\nhas a strong and stable photoelectric response regardless of positive or negative polarization. More\nimportantly, as shown in Supplementary Figs. S20, the SATO photodetectors with positive and\nnegative polarizations have ultra-fast response speed. Their rise/fall times are 6.6 ns/17.7 ns and\n8.7ns/12.2 ns, respectively. Similar tests using other devices with different effective area were carried\nout. The current intensity, effective area, laser intensity, responsivity, detectivity and response time of\nthree SATO photodetector devices are listed the Supplementary Table S1. All the SATO photodetector\ndevices exhibit extremely fast response speed.\npolarizations of the SATO photodetector. The photocurrents under positive and negative\npolarizations are almost equal.\nSupplementary Fig. S20 | Time-resolved transient photoresponse curves of the SATO\nphotodetectors with positive and negative polarizations. The SATO photodetectors with positive\nand negative polarizations have ultra-fast response speed. Their rise/fall times are 6.6 ns/17.7 ns (a-c)\nand 8.7ns/12.2 ns (d-e), respectively.\nMinor Concerns\nQuestion and comment (R2.5): Device Visualization: Including a photograph of the fabricated SATO\nphotodetector would help readers better understand the device architecture and experimental\nconfiguration.\nReply to Question and comment (R2.5):\nSincerely thanks for your suggestion. We used a direct-write lithography system to fabricate\nphotodetectors with groove widths ranging from 5 μm to 17 μm, as shown in Supplementary Fig. S17.\nIn this work, we selected photodetectors with groove widths of 5 μm, 8 μm, and 11 μm to evaluate\ntheir performance.\nThe following Supplementary Fig. S17 was added in the revised Supplementary Information.\nphotolithography. The size of the Pt electrode is 50 μm × 50 μm. Scale bar, 500 μm in (a) and 50 μm\nin (b).\nQuestion and comment (R2.6): Domain Structure Representation: The schematic diagram of\nferroelectric domains in Supplementary Fig. S1 does not accurately reflect typical domain patterns\nobserved in ferroelectric thin films. Domain illustrations should be grounded in experimental\nobservations from either the literature or the authors' work to ensure scientific accuracy and\neducational value.\nReply to Question and comment (R2.6):\nSincerely thanks you very much for this comment. We have redrawn the schematic diagram of the\nferroelectric domain structure based on the experimental results, as shown in Supplementary Fig. S1.\nSupplementary Fig.S1 was revised in the revised Supplementary Information.\nferroelectric photodetectors. (a) Spontaneous polarization and (b) built-in electric field distribution\nin traditional ferroelectric materials. Most of ferroelectric materials have a high density of ferroelectric\ndomains with different polarization directions. (c-f) Several common failure mechanisms of\nferroelectric photodetectors include scattering, annihilation, capture of photogenerated carriers at\ndomain walls, and cancellation of built-in electric fields between different domains.\nReference\n1. Yang, S. Y. et al. Above-bandgap voltages from ferroelectric photovoltaic devices. Nat.\nNanotechnol. 5, 143–147 (2010).\nQuestion and comment (R2.7): Addressing these concerns—particularly the ferroelectric\ncharacterization gaps and stoichiometry questions—will substantially strengthen the manuscript's\nscientific rigor and enhance the credibility of claims regarding this novel ferroelectric photodetector\nmaterial.\nThank you very much for your valuable comments of our work. We fully understand the referee’s\nconcerns and address all the questions one by one based on your comments.\nWe sincerely thank the reviewers for their constructive comments, suggestions, and valuable\nquestions. Your comments and important questions were very helpful in improving our manuscript.\nWe appreciate the general recognition by the referee that “Yan et al present an outstanding\noptoelectronic performance of the single-domain ferroelectric SATO thin film. The ferroelectric\nproperties have been discussed and they provide excellent optoelectronic performances for ultraviolet\nregion, such as responsibility of 860mA/W, the rise time of 6.8 ns and the fall time of 17.7 ns. I think\nthe paper is well organized, and the presented experimental results are very interesting.”\nIn the meanwhile, the referee also raises some specific questions and comments which are\nsummarized into four major aspects. We fully understand the referee’s concerns, and here we address\nall the questions and discuss all the comments one-by-one in the following.\nQuestion and comment (R3.1): The authors used the PFM and polarization hysteresis loops to show\nthe ferroelectric behavior of SATO thin film. While the authors state that the high dielectric constants\nof ferroelectric materials lead to lower noise level, it will be helpful if they can show dielectric behavior\nof SATO thin film and compare with other ferroelectric materials.\nReply to Question and comment (R3.1):\nThank you very much for your valuable suggestion. According to your suggestions, we performed\nmeasurements of the dielectric constant and dielectric loss as a function of frequency in the SATO thin\nfilm, as shown in Supplementary Fig. S15. The SATO thin film exhibits a high dielectric constant (>16)\nand low dielectric (<0.1) at room temperature, which indicates that the SATO film is a material with\nstrong polarity. More importantly, the dielectric constant and dielectric loss decrease slowly with\nincreasing frequency, which indicates that the SATO film has a good frequency stability. Thus, the\nlarge dielectric constant, low dielectric loss, and excellent frequency stability all prove that SATO film\nhas excellent charge storage capacity, which is crucial for its excellent performance in optoelectronic\napplications.\nThe following sentences and Supplementary Fig. S15 were added in the revised manuscript and\nSupplementary Information.\nSupplementary Fig. S15 shows that the SATO film has a large dielectric constant (>16), low\ndielectric loss (<0.1), and excellent frequency stability at room temperature.\nSATO thin film. The SATO film exhibits a large dielectric constant, low dielectric loss, and excellent\nfrequency stability at room temperature.\nQuestion and comment (R3.2): The authors state that the ferroelectric polarization induces intrinsic\nbuilt-in electric field increasing optoelectronic properties. I am wondering whether the ferroelectricity\ncan be tailored in SATO thin film, that the effect of ferroelectric polarization can be confirmed. They\ncould measure optoelectronic performances using the SATO thin films with different spontaneous\npolarization.\nReply to Question and comment (R3.2):\nWe sincerely appreciate your constructive suggestions on our work. We prepolarized the SATO film\nusing high voltage in different directions and measured the effect of different polarization directions\non the photoresponse. As shown in the Supplementary Fig. S19, the currents formed by the SATO\nphotodetectors pre-polarized at 30V and -30V under 330 nm illumination are almost the same, which\nindicates that the effects of positive and negative polarization on the performance of the photodetector\nare actually equivalent. In addition, we have added time-resolved transient photoresponse\nmeasurements dependent on the polarization direction, as shown in Supplementary Fig. S20. As one\ncan see that the SATO photodetector has a stable photoresponse regardless of whether it is in the\npositive or negative polarization state, as shown in Supplementary Fig. S20a and d. More importantly,\n27\nthe SATO photodetectors with positive and negative polarizations have ultra-fast response speed with\nrise and fall times of 6.6 ns/17.7 ns and 8.7ns/12.2 ns, respectively (Supplementary Figs. S20b, c, e,\nand f), which is basically consistent with the response speed of the no prepolarized SATO\nphotodetector (Fig. 5f in the main text).\nSimilarly, we performed the same test on other devices with different effective area and statistically\nsummarized on the current intensity, effective area, laser intensity, responsivity, detectivity and\nresponse time of SATO photodetector devices. The results are shown in the Supplementary Table S1.\nAll the SATO photodetector devices we measured have extremely fast response speed, which can\ngreatly reduce the response delay of functional devices and greatly improve the efficiency of\ninformation exchange.\nThe following sentences and Supplementary Fig. S19, Fig. S20, and Table S1 were added in the\nrevised manuscript and Supplementary Information.\nThe photoelectric switching response and transient photoelectric response of SATO with different\npolarization directions were measured. As shown in Supplementary Fig. S19, the SATO photodetector\nhas a strong and stable photoelectric response regardless of positive or negative polarization. More\nimportantly, as shown in Supplementary Figs. S20, the SATO photodetectors with positive and\nnegative polarizations have ultra-fast response speed. Their rise/fall times are 6.6 ns/17.7 ns and\n8.7ns/12.2 ns, respectively. Similar tests using other devices with different effective area were carried\nout. The current intensity, effective area, laser intensity, responsivity, detectivity and response time of\nthree SATO photodetector devices are listed the Supplementary Table S1. All the SATO photodetector\ndevices exhibit extremely fast response speed.\npolarizations of the SATO photodetector. The photocurrents under positive and negative\npolarizations are almost equal.\nSupplementary Fig. S20 | Time-resolved transient photoresponse curves of the SATO\nphotodetectors with positive and negative polarizations. The SATO photodetectors with positive\nand negative polarizations have ultra-fast response speed. Their rise/fall times are 6.6 ns/17.7 ns (a-c)\nand 8.7ns/12.2 ns (d-e), respectively.\nQuestion and comment (R3.3): It was demonstrated that the single-domain ferroelectric thin films\ncan result the high optoelectronic performances. Is all single-domain ferroelectric thin films are\npossible to result the high optoelectronic performances? If not, what distinguishes SATO from other\ncandidates? A brief comparison with other single-domain ferroelectric materials would help\ncontextualize the advantages of the SATO system.\nReply to Question and comment (R3.3):\nWe appreciate the valuable comments. Generally speaking, we believe that single-domain\nferroelectric films can improve the performance of photodetectors, especially the response speed. On\none hand, lattice defects such as domain boundaries often generate defect levels, which act as traps for\nphotogenerated carriers, shortening their lifetime and reducing photoelectric conversion efficiency. On\nthe other hand, defects such as domain boundaries can sometimes cause the material to be conducting\nand increase leakage current, which increases the dark current of the detector and affects its detection\naccuracy and signal-to-noise ratio. Furthermore, defects such as domain boundaries can scatter\nphotogenerated carriers, hindering their efficient transport and reducing carrier mobility, leading to a\ndecrease in optoelectronic performance. However, not all single-domain ferroelectric thin films\nnecessarily exhibit a high optoelectronic performance. This is because optoelectronic performance is\ninfluenced by multiple factors, including the material’s band structure, bandgap, and defect density.\nEven with a single-domain structure, achieving high optoelectronic performance is difficult if the\n30\nmaterial is unfavorable for light absorption.\nWe believe that the main reasons for SATO's excellent performance are as follows: First, its\nferroelectric polarization is theoretically only along the c-axis, which makes the migration path of\nphotogenerated carriers unidirectional, resulting in higher photoelectric conversion efficiency and\nfaster response speed; on the other hand, SATO has a large band gap and only absorbs ultraviolet light,\nso the environmental noise is low and the signal-to-noise ratio is stronger; in addition, through a large\nnumber of SATO growth experiments, we found that there are almost no other defects such as\ndislocations, twins, stacking faults, etc. in its lattice, which shows that SATO is easy to obtain high-\nquality single crystals and the photoelectric performance is closer to the theoretical value.\nQuestion and comment (R3.4): The SATO-based device demonstrates a nanosecond-scale response\ntime, which is remarkable. What could be origin of the ultrafast response speed compared to other\nconventional photodetectors?\nReply to Question and comment (R3.4):\nThank you very much for your recognition and high evaluation on this study and thanks for your\nprecious suggestion. The ultrafast response speed of SATO, which is nearly 10,000 times faster than\nconventional ferroelectric photodetectors (e.g., BaTiO , PbTiO ), originates from the unique structural\n3 3\nand physical properties of the SATO thin film.\nFirst, SATO only has ferroelectric polarization along the c direction. This structure can greatly\nshorten the migration path of photogenerated carriers, which is crucial to improving the response speed.\nConventional ferroelectric materials (ie., BaTiO , PbTiO ) typically exhibit high-density ferroelectric\n3 3\ndomains with random polarization directions. Domain walls in these materials act as scattering centers\nfor photogenerated carriers, which will prolong carrier transport paths, cause carrier annihilation, and\nsignificantly slow down the response speed.\nSecond, SATO has a strong and stable built-in electric field, as shown in and Fig. 4e in main text\nand Fig. S11 in the Supplementary Information, which facilitates the efficient separation of electron-\nhole pairs and minimizes the time required for photocurrent generation and decay. In multi-domain\nferroelectrics, the built-in electric fields from domains with opposite polarizations partially cancel each\nother, weakening the net field and reducing the separation efficiency of carriers.\nboundaries and dislocations, as shown in Fig. 1c and Supplementary Fig. S7. Such high-quality growth\nreduces defect-induced carrier trapping, which is a major cause of slow response in conventional\nphotodetectors. Thus, carriers in SATO can transport without significant trapping, which will\naccelerate the response.\nThe following sentences and Supplementary Fig. S11 were added in the revised manuscript and\nSupplementary Information.\nHysteresis loops measured under different voltages are shown in Supplementary Fig. S11a. It is\nclear that the remanent polarization gradually increases with the voltage. After the voltage is increased\nto 10 V and higher, the remanent polarization remains unchanged (~ 7.8 μC/cm2) and the polarization\nsaturation is reached. Supplementary Fig. S11b shows the results of ferroelectric fatigue testing. After\n106 cycles, the remanent polarization intensity did not attenuate significantly, which indicates that the\nSATO film has a strong fatigue stability.\nSupplementary Fig. S11 | Ferroelectric fatigue properties of the SATO thin film tested by PUND\nmeasurements. (a) Hysteresis loops of the SATO thin film under different voltages. (b) Fatigue\ncharacteristics of the SATO thin film under 10 V and 12 V. Inset are the PUND loops before and after\ncycling. After 106 cycles, the polarization intensity exhibits a strong fatigue stability without\nsignificant attenuation.\nWe sincerely thank all the reviewers for the precious comments, suggestions and questions. Your\ncomments and important questions really help us improve this manuscript, and encourage us\n32\ncontinuing an in-depth and all-round research in this subject. We do hope that the detailed response\ngiven to the points made above goes some way towards addressing the comments and the revised\nmanuscript can fit your requirements.\n33"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have improved the manuscript in line with queries, comments, and concerns raised by reviewers; however, I am\nstill not convinced why these SATO thin films show an increase in coercive field even after reaching of maximum\npolarization. An ideal ferroelectric system shows nearly the same coercive field after polarization saturation. Polarization\ngraphs also show nearly square hysteresis without saturation (as can be seen PFM data saturation after 12 V); it requires\nsome explanation by the authors.", "reviewer_2": "The authors have made substantial improvements to the manuscript and addressed most of the concerns raised in the initial\nreview. The additional experimental data, particularly, polarization-dependent photoresponse characterization,\nsignificantly strengthen the work. However, two important issues require further attention:\nWhile the authors have provided PFM measurements of the AlN precursor film (Supplementary Fig. S9), this does not\naddress the original concern about the pristine SATO film state. Required additional data is PFM amplitude and phase\nimages of virgin SATO film regions (without any prior electrical poling).\nThe current response addresses whether the precursor AlN film is ferroelectric, but the fundamental\nquestion about the intrinsic domain state of the SATO film remains unanswered.\nThe authors propose Al vacancies as the charge compensation mechanism for Ti⁴⁺ substitution. However, there appears to\nbe an inconsistency between this explanation and the reported compositional analysis. The EDS analysis reports Sr:Al:Ti:O\n= 1:11:1:19, suggesting single Ti substitution. If the charge balance requires Al vacancies (as stated: \"Al vacancies must\nexist in SATO\"), the expected stoichiometry would differ from SrAl₁₁TiO₁₉. A more detailed explanation is needed for how the\nproposed defect chemistry (Al vacancies + Ti⁴⁺ substitution) reconciles with the experimental composition data", "reviewer_3": "We thank the authors for their detailed and comprehensive reply. They provided additional information to clarify their claims.\nThe paper is now much clearer and can be considered for publication."}, "author_response": "Reply to referees’ questions and comments:\nManuscript ID: NCOMMS-25-34130A\nTitle: Ferroelectric ultraviolet photodetector material with ultrafast response speed\nAuthors: Xuexi Yan, Tingting Yan, Lingli Li, Yi Cao, Xinwei Wang, Jinghui Wang, Ang Tao, Tingting\nYao, Yixiao Jiang, Weijin Hu, Xiaosheng Fang, Hengqiang Ye, Xiu-Liang Ma, Chunlin Chen\nSeptember 2, 2025\nWe would like to thank all reviewers for reviewing our manuscript very carefully and\nacknowledging our work. We have further revised our manuscript based on the comments and\nsuggestions from Reviewers, which is indicated in red font in the revised main text and Supplementary\ninformation.\nReply to referee 1 (R1):\nQuestion and comment (R1.1): The authors have improved the manuscript in line with queries,\ncomments, and concerns raised by reviewers; however, I am still not convinced why these SATO thin\nfilms show an increase in coercive field even after reaching of maximum polarization. An ideal\nferroelectric system shows nearly the same coercive field after polarization saturation. Polarization\ngraphs also show nearly square hysteresis without saturation (as can be seen PFM data saturation after\nReply to Question and comment (R1.1):\nWe sincerely appreciate your recognition of our work and your precious comments. For ideal\nferroelectric materials, the homogeneity of their microstructures, absence of defects, and uniformity in\nferroelectric domain switching barriers enable them to maintain a constant coercivity after their\nhysteresis loops reach saturation. However, defects in ferroelectric materials will induce a notable\nlateral broadening of the hysteresis loop as the applied electric field or frequency increases beyond the\nsaturation threshold, which in turn leads to a gradual increase in coercivity.1,2,3 This phenomenon has\nbeen observed in many practical systems and the related physical mechanism has been discussed and\nclarified.\nThe factors contributing to the increase in coercivity of ferroelectric materials include defect pinning,\nnon-uniform electric field distribution, and leakage current coupled with charge injection, among\nwhich defect pinning is the most prevalent mechanism. Specifically, intrinsic defects (e.g., vacancies)\nand extrinsic defects (e.g., heterointerfaces) within the material can act as \"pinning centers\" that anchor\nferroelectric domain walls, thereby impeding their dynamic switching behavior.4,5,6 At relatively low\napplied voltages, ferroelectric domains only need to overcome the constraint of weak pinning sites to\ncomplete switching. However, as the voltage increases to higher levels, additional energy input is\nrequired to surmount the stronger pinning effects imposed by strong pinning defects (e.g., deep-trap\ndefects), thereby manifesting as an increase in coercivity.\nAs illustrated in our experimental results (Supplementary Fig. S12), when the applied voltage was\nincreased from 10 V to 12 V, the P-E loop exhibited a significant lateral broadening, while the\nsaturation polarization remained nearly unchanged. This experimental observation is well-aligned with\nthe characteristic behaviors of the defect pinning mechanism.\nAl vacancies must exist in SATO films to achieve charge balance. SATO is derived from SrAl O\n12 19\ndue to partial substitution of Al atoms by Ti atoms. Therefore, determining the valence state of Ti is\nkey to understanding the stoichiometry and charge balance of SATO. We characterized the Ti L fine\n2,3\nstructure using atomic-resolved electron energy-loss spectroscopy (EELS) with an energy resolution\nof 0.3 eV, as shown in Supplementary Fig. S4. It is clear that the EELS spectra of SATO and STO\nexhibit the same fine structures with four peaks, indicating that the valence of Ti in SATO is +4. To\nmaintain the charge neutrality, Al vacancies must exist in SATO. We define the chemical formula of\nSATO as SrAl TiO , where δ represents a parameter related to Al vacancies. To more accurately\n11-δ 19\ndetermine the chemical composition in the SATO thin film, we measured the compositions of several\ndifferent regions in the SATO film, as shown in Supplementary Tabel S1. After background subtraction,\nnormalization, and integration of the EDS spectra, the average atomic ratios of Sr, Al, Ti and O in the\nSATO film is 1.00:10.71:1.02:19.02. The atomic ratio of Sr to Ti is close to 1, similar as that in the\nSrTiO substrate. Therefore, the δ associated with the Al vacancies in the present SATO film is\n3\napproximately 0.3, which also satisfies the principle of electrical neutrality. We believe that the Al\nvacancies in the SATO film may be the reason for the lateral broadening of the hysteresis loop as the\nelectric field continues to increase after reaching saturation.\nThe following sentences and Supplementary Fig. S9 were added in the revised Supplementary\nInformation:\nDefects in ferroelectric materials will induce a notable lateral broadening of the hysteresis loop as\nthe applied electric field or frequency increases beyond the saturation threshold, which in turn leads to\na gradual increase in coercivity,1,2,3 as shown in Fig. S12a. The factors contributing to the increase in\ncoercivity of ferroelectric materials are the intrinsic defects (e.g., vacancies) and extrinsic defects (e.g.,\nheterointerfaces), which can act as pinning centers that anchor ferroelectric domain walls, thereby\nimpeding their dynamic switching behavior.4,5,6 As illustrated in the experimental results, when the\napplied voltage was increased from 10 V to 12 V, the P-E loop exhibited a significant lateral broadening,\nwhile the saturation polarization remained nearly unchanged. This experimental observation is well-\naligned with the characteristic behaviors of the defect pinning mechanism.\nSupplementary Fig. S4 | EELS spectra showing the Ti L edges in STO and SATO. The Ti L\n2,3 2,3\nedges of SATO and STO have very similar fine structures with four peaks, suggesting that the Ti ions\nin SATO have the valence state of +4.\nSupplementary Table S1 | Quantitative statistics of EDS spectra in different areas of SATO films.\nReference\n1. Balashova, E. et al. Structural properties and dielectric hysteresis of molecular organic ferroelectric\ngrown from different solvents. Crystals 11, 1278 (2021).\n2. Hao, P. et al. Highly enhanced polarization switching speed in HfO ‐based ferroelectric thin films\n2\nvia a composition gradient strategy. Adv. Funct. Mater. 33, (2023).\n3. Shcherbakov A. et al. Evolution of coercive voltage in a ferroelectric memory cell based on\nHf Zr O during its lifetime. Phys. Rev. Appl. 22, 064083 (2024).\n0.5 0.5 2\n4. Zhang, D. et al. Superior polarization retention through engineered domain wall pinning. Nat.\nCommun. 11, 349 (2020).\n5. Gao, P. et al. Revealing the role of defects in ferroelectric switching with atomic resolution. Nat.\nCommun. 2, 591 (2011).\n6. Bencan, A. et al. Domain-wall pinning and defect ordering in BiFeO probed on the atomic and\n3\nnanoscale. Nat. Commun. 11, 1762 (2020).\nReply to referee 2 (R2):\nWe appreciate the general recognition by the referee that “The authors have made substantial\nimprovements to the manuscript and addressed most of the concerns raised in the initial review. The\nadditional experimental data, particularly, polarization-dependent photoresponse characterization,\nsignificantly strengthen the work”. We sincerely thank the referee’s comments and suggestions, which\nare very helpful for improving the manuscript.\nQuestion and comment (R2.1): While the authors have provided PFM measurements of the AlN\nprecursor film (Supplementary Fig. S9), this does not address the original concern about the pristine\nSATO film state. Required additional data is PFM amplitude and phase images of virgin SATO film\nregions (without any prior electrical poling).\nThe current response addresses whether the precursor AlN film is ferroelectric, but the fundamental\nquestion about the intrinsic domain state of the SATO film remains unanswered.\nReply to Question and comment (R2.1):\nWe sincerely appreciate your valuable suggestion and apologize for any misunderstanding regarding\nyour original concern. We conducted PFM measurements on the SATO film in its initial state without\nany prior electrical poling, as shown in the Supplementary Fig. S9. Both the amplitude and phase\nimages of the virgin SATO film exhibit uniform contrast, indicating that there are no ferroelectric\ndomain walls in the observed region.\nThe following sentences and Supplementary Fig. S9 were added in the revised manuscript and\nSupplementary Information:\nSupplementary Fig. S9 shows the PFM measurements of SATO film in its initial state without any\nprior electrical poling. Both the amplitude and phase images of the virgin SATO film exhibit uniform\ncontrast, indicating that there are no ferroelectric domain walls in the observed region.\nelectrical poling. (a-c) PFM topography, amplitude, and phase images of the SATO thin film. Both\nthe amplitude and phase images of the virgin SATO film exhibit uniform contrast, indicating that\nthere are no ferroelectric domain walls in the observed region. Scale bar, 2 μm.\nQuestion and comment (R2.2): The authors propose Al vacancies as the charge compensation\nmechanism for Ti4+ substitution. However, there appears to be an inconsistency between this\nexplanation and the reported compositional analysis. The EDS analysis reports Sr:Al:Ti:O = 1:11:1:19,\nsuggesting single Ti substitution. If the charge balance requires Al vacancies (as stated: \"Al vacancies\nmust exist in SATO\"), the expected stoichiometry would differ from SrAl TiO A more detailed\n11 19.\nexplanation is needed for how the proposed defect chemistry (Al vacancies + Ti4+ substitution)\nreconciles with the experimental composition data.\nReply to Question and comment (R2.2):\nWe sincerely appreciate your valuable comments. According to the EELS spectra as shown in\nSupplementary Figure S5, the chemical valence state of Ti in the SATO thin film is +4. To achieve the\nelectrical neutrality, Al vacancies must exist in the SATO thin film. Therefore, it is more reasonable to\ndefine the chemical formula of SATO as SrAl TiO , where δ represents a parameter related to Al\n11-δ 19\nvacancies. To determine more accurately the chemical composition in the SATO thin film, we\nperformed EDS measurements in several different regions, as shown in Supplementary Tabel S1. After\nbackground subtraction, normalization, and integration of the EDS spectrum, the average atomic ratios\nof Sr, Al, Ti and O in the SATO film is 1.00:10.71:1.02:19.02. The atomic ratio of Sr to Ti is close to 1,\nsimilar as that in the SrTiO substrate. Since the structure of SATO is derived from SrAl O after\n3 12 19\none Ti atom replacing one Al atom, vacancies must be existed in the other eleven sites of Al. Since the\nvalence states of Ti and Al ions are +4 and +3 respectively, the chemical composition of SATO should\n6\nbe SrAl TiO to reach the complete electrical neutrality (δ=0.33 in SrAl TiO ), which is\n10.67 19 11-δ 19\nconsistent with the EDS measurements considering the accuracy of this technique.\nThe following sentences and Supplementary Table S1 were added in the revised manuscript and\nSupplementary Information:\nTo maintain the charge neutrality, Al vacancies must exist in SATO. The chemical formula of SATO\ncan be defined as SrAl TiO , where δ represents a parameter related to Al vacancies. Energy\n11-δ 19\ndispersive X-ray spectroscopy (EDS) measurements were carried out, and the results are shown in in\nSupplementary Fig. S5 and Supplementary Table S1. After background subtraction, normalization, and\nintegration of the EDS spectrum, the average atomic ratios of Sr, Al, Ti and O in the SATO film is\n1.00:10.71:1.02:19.02. The atomic ratio of Sr to Ti is close to 1, similar as that in the SrTiO substrate.\n12 19\nvacancies must be existed in the other eleven sites of Al. Since the valence states of Ti and Al ions are\n+4 and +3 respectively, the chemical composition of SATO should be SrAl TiO to reach the\n10.67 19\ncomplete electrical neutrality (δ=0.33 in SrAl TiO ), which is consistent with the EDS\n11-δ 19\nmeasurements considering the accuracy of this technique.\nSupplementary Table S1 | Quantitative statistics of EDS spectra in different areas of SATO films.\nWe appreciate the general recognition by the referee that “We thank the authors for their detailed\nand comprehensive reply. They provided additional information to clarify their claims. The paper is\nnow much clearer and can be considered for publication.”\nWe sincerely thank all reviewers for their valuable comments, suggestions, and questions. Your\ncomments have greatly helped improve this manuscript and encouraged us to pursue in-depth,\ncomprehensive research on this topic. We hope our detailed responses to the above points address your\nconcerns, and that the revised manuscript meets your requirements.\n8"}, {"round": 3, "reviewer_comments": {"reviewer_1": "The paper may be accepted for publication", "reviewer_2": "The authors have made substantial improvements to address most of the concerns raised in the initial review, and their\nefforts are acknowledged. However, the newly added Fig. S9 PFM data introduces a serious contradiction that\nfundamentally challenges the manuscript's central claims.\nThe PFM measurements in Fig. S9 demonstrate that the as-prepared SATO sample exhibits characteristics consistent with a\nparaelectric phase. While the amplitude image is presented in arbitrary units (a.u.), preventing direct quantitative comparison\nwith the results obtained under applied field, the phase image reveals distinctly different values compared to those observed\nafter electrical poling (Fig. 4b, 4d). If the as-prepared sample were truly in a ferroelectric phase, the phase values should\nshow consistency between the virgin state and post-poling measurements, at least within domains of the same polarization\norientation.\nThe collective PFM results strongly suggest the possibility of a field-induced ferroelectric phase transition rather than\nintrinsic ferroelectricity. However, this interpretation directly contradicts the TEM structural analysis, which concludes that the\nas-prepared sample possesses a ferroelectric phase based on observed atomic displacements and structural\ncharacteristics.\nThese conflicting interpretations cannot be reconciled without further clarification and additional experimental evidence.\nTherefore, the manuscript cannot be considered for publication until this critical contradiction is satisfactorily resolved."}, "author_response": "Reply to referees’ questions and comments:\nManuscript ID: NCOMMS-25-34130B\nTitle: Ferroelectric ultraviolet photodetector material with ultrafast response speed\nAuthors: Xuexi Yan, Tingting Yan, Lingli Li, Yi Cao, Xinwei Wang, Jinghui Wang, Ang Tao, Tingting\nYao, Yixiao Jiang, Weijin Hu, Xiaosheng Fang, Hengqiang Ye, Xiu-Liang Ma, Chunlin Chen\nSeptember 17, 2025\nWe would like to thank all reviewers for reviewing our manuscript very carefully and\nacknowledging our work. We fully understand the referee’s concerns and here we address all the\nquestions and discuss all the comments.\nReply to referee 1 (R1):\nThank you very much for your recognition that “The paper may be accepted for publication.”\nReply to referee 2 (R2):\nWe appreciate the general recognition by the referee that “The authors have made substantial\nimprovements to address most of the concerns raised in the initial review, and their efforts are\nacknowledged”.\nQuestion and comment (R2.1): The PFM measurements in Fig. S9 demonstrate that the as-prepared\nSATO sample exhibits characteristics consistent with a paraelectric phase. While the amplitude image\nis presented in arbitrary units (a.u.), preventing direct quantitative comparison with the results obtained\nunder applied field, the phase image reveals distinctly different values compared to those observed\nafter electrical poling (Fig. 4b, 4d). If the as-prepared sample were truly in a ferroelectric phase, the\nphase values should show consistency between the virgin state and post-poling measurements, at least\nwithin domains of the same polarization orientation.\ntransition rather than intrinsic ferroelectricity. However, this interpretation directly contradicts the\nTEM structural analysis, which concludes that the as-prepared sample possesses a ferroelectric phase\nbased on observed atomic displacements and structural characteristics.\nThese conflicting interpretations cannot be reconciled without further clarification and additional\nexperimental evidence. Therefore, the manuscript cannot be considered for publication until this\ncritical contradiction is satisfactorily resolved.\nReply to Question and comment (R2.1):\nThank you very much for your valuable comment. First, we sincerely apologize for the\nmisunderstanding caused by the Fig. S9 in the former revision. Regarding to the ferroelectricity of the\nSATO film, our considerations are as follows:\nHAADF STEM image in Fig. S13 shows clearly atomic displacements, which suggests that the as-\nprepared SATO film possesses a ferroelectric phase. TEM bright-field image in Fig. S7 shows that the\nSATO film has a uniform contrast. No ferroelectric domain walls can be observed. These results\nsuggest that the SATO ferroelectric film has a single-domain structure in the observed region.\nFurthermore, the PFM phase image of the SATO film exhibits a uniform contrast, indicating that the\nobserved region has the same ferroelectric polarization direction, which is consistent with the TEM\nobservations. Thus, the ferroelectricity of the SATO film should be intrinsic, rather than a voltage-\ndriven ferroelectric phase transition. Although the PFM phase image of a paraelectric material will\nalso show a uniform contrast, the reason is different. The PFM phase image of the SATO film exhibits\na uniform contrast because the whole observed region locates within a single ferroelectric domain of\nthe same polarization direction.\nRegarding the inconsistency between the phase values in Figure 4 and Figure S9 you mentioned, we\ncarefully considered this question based on the working principle of PFM. For PFM testing, the essence\nof the phase image is that during the cantilever’s vibration, its energy is dissipated or modulated by\nthe sample, causing the vibration to not fully synchronize with the drive signal. The PFM device detects\nthe phase lag of the forced vibration of the cantilever and converts the spatial distribution of the phase\ndifference into a visual contrast image. The baseline of the PFM phase image is determined by the\ncantilever’s initial phase. However, the initial phase of the cantilever is not a fixed value. The driving\nvoltage, sample position, and probe type will affect the initial phase of the cantilever. Therefore, it is\n2\ndifficult to make a direct quantitative comparison of the phase values obtained from two independent\nimages. Thus, the phase values in Figure 4 and Figure S9 are different since they were obtained from\ntwo different measurements in which the initial phases of the cantilever were different. We cannot\ncompare them directly.\nTo directly and quantitatively compare the phase changes of the SATO film under three conditions:\nno voltage, forward voltage, and reverse voltage, we re-performed a PFM test on the SATO film and\nshowed the results in Figure S9 in the new revision. As one can see in Fig. S9c, the phase values in the\npolarization direction. The phase values in the 0 V region and the -10V region are also identical,\nindicating that the initial direction of the ferroelectric polarization is perpendicular to the film surface\nand outward. Upon applying a +10V voltage, a 180° phase shift occurs, signifying a change in\nferroelectric polarization direction from being perpendicular to the film surface and outward to an\ninward orientation. These results are consistent with the TEM structural analyses.\nThe following sentences and Supplementary Fig. S9 were added in the revised manuscript and\nSupplementary Information:\nSupplementary Fig. S9 presents the PFM measurements of the SATO film under three conditions:\nno voltage, forward voltage, and reverse voltage. The amplitude and phase images of the virgin SATO\nfilm display uniform contrast, indicating the absence of ferroelectric domain walls in the observed\nregion. The contrast between the 0 V region and the -10 V region remains consistent, suggesting that\nthe initial ferroelectric polarization direction of the as-grown SATO film is perpendicular to its surface\nand directed outward. Upon applying a +10 V voltage, a 180° phase shift occurs, signifying a change\nin ferroelectric polarization direction from being perpendicular to the film surface and outward to an\ninward orientation.\nSupplementary Fig. S9 | PFM characterizations of the SATO thin films. (a-c) PFM topography,\n3\namplitude, and phase images of the SATO thin film. Both the amplitude and phase images of the SATO\nfilm exhibit a uniform contrast under 0 V and -10 V, indicating that the SATO is a single-domain\nferroelectric film with the polarization along out-of-plane direction. Scale bar, 2 μm.\nWe hope the new experimental results and responses can address your concerns, and that the revised\nmanuscript meets your requirements. Thank you very much for your valuable comments and precious\nsuggestions, which have greatly helped improve this manuscript.\n4"}, {"round": 4, "reviewer_comments": {"reviewer_2": "The manuscript reports highly intriguing and potentially impactful experimental results regarding ultrafast ferroelectric\nphotodetectors with exceptional performance characteristics. The main finding of nanosecond-scale response times\ncombined with high responsivity and detectivity appears robust and could attract broad interest within the optoelectronics\ncommunity. However, the interpretation of these results relies on several supporting datasets, which unfortunately exhibit\nsignificant inconsistencies and lack reproducibility.\nIn particular, the additional PFM data provided in response to the review (Fig. S9) do not reconcile with the results in Fig. 4\nand raise new concerns about reproducibility. In Fig. 4, PFM measurements demonstrate consistent piezoelectric amplitude\nregardless of polarization direction upon ±10 V application, supporting the claimed single-domain behavior. However, in Fig.\nS9, despite nearly identical measurement conditions, the region subjected to –10 V application exhibits a distinctly larger\nresponse than the region subjected to +10 V. Furthermore, although the application of –10 V from the unpoled state does not\ninduce any change in the polarization direction, an increase in the piezoelectric response is observed, which is in clear\ncontradiction with the single-domain nature claimed on the basis of the TEM observations.\nThe observed photodetection properties themselves appear genuine and noteworthy. However, the proposed ferroelectric\nmechanism relies on materials characterization that contains contradictory evidence and lacks internal consistency. Without\na clear and consistent experimental foundation, the mechanistic explanation for the exceptional photodetector performance\nremains insufficiently supported. While the authors have made efforts to address previous concerns, the newly provided data\nhave inadvertently highlighted fundamental issues with experimental reproducibility. In the current form, the manuscript does\nnot meet the standard for Nature Communications."}, "author_response": "Reply to referees’ questions and comments:\nManuscript ID: NCOMMS-25-34130C\nTitle: Ferroelectric ultraviolet photodetector material with ultrafast response speed\nAuthors: Xuexi Yan, Tingting Yan, Lingli Li, Yi Cao, Xinwei Wang, Jinghui Wang, Ang Tao, Tingting\nYao, Yixiao Jiang, Weijin Hu, Xiaosheng Fang, Hengqiang Ye, Xiu-Liang Ma, Chunlin Chen\nNovember 8, 2025\nWe are grateful to #2 reviewer for reviewing our manuscript carefully and recognizing our efforts.\nWe systematically discussed the reviewer’s concerns and conducted repeated measurements to confirm\nthe reviewer’s concerns about the reproducibility of the PFM results. We hope our response will\nreassure the reviewer and address their concerns.\nReply to referee 2 (R2):\nWe appreciate the general recognition by the referee that “The manuscript reports highly intriguing\nand potentially impactful experimental results regarding ultrafast ferroelectric photodetectors with\nexceptional performance characteristics. The main finding of nanosecond-scale response times\ncombined with high responsivity and detectivity appears robust and could attract broad interest within\nthe optoelectronics community.”\nQuestion and comment (R2.1): The manuscript reports highly intriguing and potentially impactful\nexperimental results regarding ultrafast ferroelectric photodetectors with exceptional performance\ncharacteristics. The main finding of nanosecond-scale response times combined with high responsivity\nand detectivity appears robust and could attract broad interest within the optoelectronics community.\nHowever, the interpretation of these results relies on several supporting datasets, which unfortunately\nexhibit significant inconsistencies and lack reproducibility.\nIn particular, the additional PFM data provided in response to the review (Fig. S9) do not reconcile\nwith the results in Fig. 4 and raise new concerns about reproducibility. In Fig. 4, PFM measurements\ndemonstrate consistent piezoelectric amplitude regardless of polarization direction upon ±10 V\napplication, supporting the claimed single-domain behavior. However, in Fig. S9, despite nearly\n1\nidentical measurement conditions, the region subjected to –10 V application exhibits a distinctly larger\nresponse than the region subjected to +10 V. Furthermore, although the application of –10 V from the\nunpoled state does not induce any change in the polarization direction, an increase in the piezoelectric\nresponse is observed, which is in clear contradiction with the single-domain nature claimed on the\nbasis of the TEM observations.\nThe observed photodetection properties themselves appear genuine and noteworthy. However,\nthe proposed ferroelectric mechanism relies on materials characterization that contains contradictory\nevidence and lacks internal consistency. Without a clear and consistent experimental foundation, the\nmechanistic explanation for the exceptional photodetector performance remains insufficiently\nsupported. While the authors have made efforts to address previous concerns, the newly provided data\nhave inadvertently highlighted fundamental issues with experimental reproducibility. In the current\nform, the manuscript does not meet the standard for Nature Communications.\nReply to Question and comment (R2.1):\nThank you very much for your comments. However, we cannot agree with your comments on PFM\nmeasurements and experimental reproducibility. Let’s return to the original comment in the 1st-round\nreview: to prove that the as-grown SATO ferroelectric thin film exhibits a single-domain characteristics.\nFirst, to determine the initial ferroelectric domain state of the as-grown thin film, it is necessary to\navoid any voltage interference since the polarization direction of ferroelectric domains are easily\naffected by applied voltage. Therefore, we believe that your conclusion about “In Fig. 4, PFM\nmeasurements demonstrate consistent piezoelectric amplitude regardless of polarization direction\nupon ±10 V application, supporting the claimed single-domain behavior” is not rigorous, because a\npositive or negative voltage was applied to the observation area in Figure 4. Amplitude images of PFM\nare not sensitive to the polarization direction, the single-domain nature can be supported by phase\nimages, instead of amplitude images. Second, surface charge injection and surface potential changes\nwill significantly affect the amplitude. Extensive studies have reported that the surface charge injection\nand potential changes are common and almost unavoidable during PFM measurements (Nat. Mater.\n2022, 21, 909-909; Nat. Commun. 2016, 7, 11502; Nat. Commun. 2017, 8, 15217; J. Semicon. 2019,\n40:9, 092002), which can clarify your concern on “Furthermore, although the application of –10 V\nfrom the unpoled state does not induce any change in the polarization direction, an increase in the\npiezoelectric response is observed”, which can explain your concern about “However, in Fig. S9, despite nearly identical\nmeasurement conditions, the region subjected to –10 V application exhibits a distinctly larger response\nthan the region subjected to +10 V.” Phase images in PFM measurements are highly sensitive to\npolarization direction. Therefore, if we want to determine the initial ferroelectric domain state of the\nas-grown thin film, we should focus more on identifying phase information (Adv. Funct. Mater. 2025,\n35, 2423225; Nat. Mater. 2022, 21, 909-909; Nat. Commun. 2017, 8, 15217). Amplitude images are\ntypically used to identify the locations of domain walls since amplitude value often abruptly changes\nat domain walls. The amplitude images in this study are uniform, suggesting that there are no domain\nwalls inside the measured regions, which is also consistent with the single-domain nature.\nIn summary, we believe there are two aspects to provide direct evidence for accurately proving the\ninitial domain state and polarization direction of SATO thin film: i) obtaining a PFM phase image\nwithout applied voltage, which can determine whether the film is a single-domain film; ii)\napplying positive and negative voltages polarization writing and comparing the phase images\nwith and without applied voltages, which can determine the polarization direction. Considering\nyour concerns about the reproducibility of our PFM measurements, we take this very seriously and\nhave immediately repeated the PFM results (shown in Supplementary Figure S9) and we are confident\nthat the PFM measurements of the SATO thin films have good reproducibility. As shown in Figure S9,\nwe measured four different positions on a 10 × 10 × 0.5 mm sample under nearly identical\nmeasurement conditions. We found that the initial amplitude and phase of all the positions had uniform\ncontrast, which directly indicated that the as-grown SATO film uniform and has an intrinsic single-\ndomain structure. Subsequently, we wrote polarization patterns at the same positions and found that\nthe regions with applied -10 V had similar phase contrast to the regions without applied voltage. This\nindicates that the initial direction of the ferroelectric polarization is perpendicular to the film surface\nand outward, which is consistent with our STEM HAADF image shown in Supplementary Figure S14.\nUpon applying a +10V voltage, a 180° phase shift occurs, signifying a change in\nferroelectric polarization direction from being perpendicular to the film surface and outward to an\ninward orientation. Therefore, based on the initial amplitude and phase images of the PFM, we can\nprove that the as-grown SATO film exhibits the single-domain nature. The polarization patterns written with\npositive and negative voltages prove that SATO possesses the intrinsic ferroelectricity.\nThe following sentences and Supplementary Fig. S9 of the new version were added in the revised\nmanuscript and Supplementary Information:\nTo determine the initial domain state of the virgin SATO film and verify the reproducibility of the\nPFM measurements, we performed multiple measurements at four randomly selected positions on a\n10 × 10 mm sample under nearly identical measurement conditions, as shown in Supplementary Fig.\nS9. The initial piezoelectric amplitude and phase images at all four positions display uniform contrast,\nwhich indicates that the virgin SATO film exhibits the single-domain nature. The phase contrast\nbetween the 0 V region and the -10 V region remains almost the same, suggesting that the initial\nferroelectric polarization direction of the as-grown SATO film is perpendicular to its surface and\ndirected outward. Furthermore, a +10 V writing voltage induces a 180° phase changes, suggesting the\nflip of the polarization direction from upward to downward. The PFM measurements at all positions\nshowed similar characteristics, indicating that the SATO film is uniform with a good reproducibility\nof ferroelectric properties.\nThank you again for your careful review. We sincerely hope the new experimental results and\nresponses can address your concerns and meet your requirements. Thank you very much for your\nvaluable comments, which have greatly helped improve this manuscript.\namplitude, and phase images over 10 × 10 μm2 at four different positions. The inset in a shows the\nmeasurement positions. Both the initial amplitude and phase images of the SATO film exhibit a\nuniform contrast under 0 V and -10 V, indicating that the SATO is a single-domain ferroelectric film\nwith the polarization along out-of-plane direction. Scale bar, 2 μm.\n5"}, {"round": 5, "reviewer_comments": {"reviewer_2": "The reviewer appreciates the additional PFM data (Fig. S9).\nThese PFM images demonstrate that the PFM measurements for the pristine regions exhibit a certain degree of\nreproducibility, which strengthens the basis for discussion.\nBased on the revised manuscript, I would like to summarize once again the evidence supporting the ferroelectricity of the\nSATO thin films:\nObservation of the displacement of Al atoms by HAADF\nClear polarization switching behavior in the PUND measurements\nPhotoresponse characteristics dependent on polarization direction\nThese results support the conclusion that SATO exhibits ferroelectricity.\nOn the other hand, the evidence for the single-domain nature of SATO requires more careful evaluation.\nThe authors clearly acknowledge that surface charge injection and changes in surface potential significantly influence the\nPFM amplitude. Indeed, the results in Fig. S9 show that the SATO film is strongly affected by surface charge injection—an\neffect much larger than the intrinsic piezoelectric response. Therefore, the possibility of charge-injection-induced contrast\ncannot be fully excluded, especially because an AC electric field is applied even in the pristine regions during PFM\nmeasurements.\nThus, while PFM results may serve as auxiliary information, they should not be treated as conclusive evidence. The\ndescription of a single-domain state should be revised to a more modest expression, such as “indicating the possibility of a\nsingle domain.”\nFurthermore, the presence of surface charge injection also affects the interpretation of the PFM data in Fig. 4. The\npolarization direction can be meaningfully discussed based on phase only when the piezoelectric response dominates over\nthe surface-charge-related contribution. However, because the SATO film is strongly affected by charge injection, the\nphase contrast in Fig. 4 should be removed.\nI strongly recommend that the primary evidence for ferroelectricity should rely on PUND measurements and structural\nanalysis.\nIf these revisions are made, the logical consistency of the paper will be greatly improved, and I believe it will reach a level\nworthy of publication in Nature Communications."}, "author_response": "Reply to referees’ questions and comments:\nManuscript ID: NCOMMS-25-34130D\nTitle: Ferroelectric ultraviolet photodetector material with ultrafast response speed\nAuthors: Xuexi Yan, Tingting Yan, Lingli Li, Yi Cao, Xinwei Wang, Jinghui Wang, Ang Tao, Tingting\nYao, Yixiao Jiang, Weijin Hu, Xiaosheng Fang, Hengqiang Ye, Xiu-Liang Ma, Chunlin Chen\nNovember 27, 2025\nWe are grateful to #2 reviewer for reviewing our manuscript carefully and recognizing our efforts.\nWe have fully incorporated the reviewer’s suggestions and revised the manuscript to improve its\nlogical consistency.\nReply to referee 2 (R2):\nWe appreciate the recognition by the referee that “The reviewer appreciates the additional PFM data\n(Fig. S9). These PFM images demonstrate that the PFM measurements for the pristine regions exhibit\na certain degree of reproducibility, which strengthens the basis for discussion.”\nQuestion and comment (R2.1):\nThe reviewer appreciates the additional PFM data (Fig. S9).\nThese PFM images demonstrate that the PFM measurements for the pristine regions exhibit a certain\ndegree of reproducibility, which strengthens the basis for discussion.\nBased on the revised manuscript, I would like to summarize once again the evidence supporting the\nferroelectricity of the SATO thin films:\nObservation of the displacement of Al atoms by HAADF\nClear polarization switching behavior in the PUND measurements\nPhotoresponse characteristics dependent on polarization direction\nThese results support the conclusion that SATO exhibits ferroelectricity.\nOn the other hand, the evidence for the single-domain nature of SATO requires more careful evaluation.\nThe authors clearly acknowledge that surface charge injection and changes in surface potential\nsignificantly influence the PFM amplitude. Indeed, the results in Fig. S9 show that the SATO film is\nstrongly affected by surface charge injection—an effect much larger than the intrinsic piezoelectric\nresponse. Therefore, the possibility of charge-injection-induced contrast cannot be fully excluded,\nespecially because an AC electric field is applied even in the pristine regions during PFM\nmeasurements.\nThus, while PFM results may serve as auxiliary information, they should not be treated as conclusive\nevidence. The description of a single-domain state should be revised to a more modest expression,\nsuch as “indicating the possibility of a single domain.”\nFurthermore, the presence of surface charge injection also affects the interpretation of the PFM data in\nFig. 4. The polarization direction can be meaningfully discussed based on phase only when the\npiezoelectric response dominates over the surface-charge-related contribution. However, because the\nSATO film is strongly affected by charge injection, the phase contrast in Fig. 4 should be removed.\nI strongly recommend that the primary evidence for ferroelectricity should rely on PUND\nmeasurements and structural analysis.\nIf these revisions are made, the logical consistency of the paper will be greatly improved, and I believe\nit will reach a level worthy of publication in Nature Communications.\nReply to Question and comment (R2.1):\nThank you very much for your valuable comments and suggestions. According to your precious\nsuggestions, the description of a single-domain state was revised to a more modest expression. We\nrevised the description of \"single-domain state\" to \"possibility of a single domain\" in the whole\nmanuscript, which is indeed the most comprehensive and prudent expression.\nWe agree that the primary evidence for ferroelectricity should rely on PUND measurements and\nstructural analysis. Thus, we rearranged the figures and removed the phase contrast in Fig. 4, and added\nthe ferroelectric fatigue properties tested by PUND measurements in Figs. 4d,e which were previously\nshown in Supplementary Figures. The phase contrast in Fig. 4 was moved to Supplementary Fig. S10\nto demonstrate that the SATO film exhibits excellent retention after poling.\nThe revised sentences and Fig. 4 are as following.\n“…exhibits the possibility of single-domain ferroelectrics”\n“…indicates that the SATO ferroelectric film probably has a single-domain structure”\n“…the virgin SATO film possibly possesses single-domain nature”.\nimage of the SATO thin film. The RA is about 2 nm. (b) PFM amplitude image polarized by ± 10 V\nbias voltages. The voltage of the probe tip (V ) was 2 V. (c) The local PFM amplitude (red) and phase\ntip\n(blue) hysteresis loops. (d) The polarization-electric field (P-E) hysteresis loops of the SATO thin film\nunder different voltages at 300 K, 1kHz. The SATO thin film has a remanent polarization of 7.8 μC/cm2\nand exhibits typical intrinsic ferroelectric characteristics. (e) Fatigue characteristics of the SATO thin\nfilm under 10 V and 12 V. Insets are the PUND loops before and after cycling. After 106 cycles, the\npolarization intensity exhibits a strong fatigue stability without significant attenuation. Scale bar, 2 μm.\nSincerely thank you again for your careful review and precious suggestions. We hope the responses\ncan address your concerns and meet your requirements. Thank you very much for your valuable\ncomments and suggestions, which have greatly helped improve this manuscript.\n3"}, {"round": 6, "reviewer_comments": {"reviewer_1": "The paper may be accepted for publication", "reviewer_2": "This paper has undergone multiple rounds of peer review, during which the authors made sincere and appropriate revisions,\nresulting in a significant improvement in its logical structure. All previously raised concerns have been addressed, and the\nmain conclusions of this study are supported by sufficient experimental evidence.\nNow, I'm confident that this paper is sufficiently worthy of publication in Nature Communications. Therefore, I strongly\nrecommend acceptance of this manuscript."}, "author_response": ""}]

Decision letter