Polarity modulation in compositionally tunable Bi₂O₂Se thin films

Abstract

Two-dimensional (2D) materials have emerged as one of most promising candidates to meet the demands of beyond-silicon technology. Among 2D semiconductors, Bi₂O₂Se (BOSe) stands out as a channel material for advanced electronic applications, due to its high electron mobility and the formation of a native high-k dielectric layer. However, the fabrication of p-type 2D BOSe transistors remains challenging. Here, we report an area-selective doping method at low temperatures (~600 K, compatible with back-end-of-line processes) of pulsed laser deposited BOSe thin films, enabling the modulation of their carrier polarity via the introduction of Zn²⁺ substitutional dopants. Taking advantage of this doping strategy, we demonstrate the fabrication of a 2D vertical p-n homojunction with an on/off ratio in photoresponse of ~10⁶ and planar transistors based on p-doped BOSe homojunctions. Our results help promoting the application of this material system towards the development of the next-generation electronics.

Bi₂O₂Se is a promising 2D semiconductor with high electron mobility and native high-k dielectric layers, but its p-type doping remains challenging. Here, the authors report a low-temperature substitutional doping method to fabricate 2D Bi₂O₂Se p-n junctions and p-type transistors

Introduction

Silicon has been the dominating semiconductor for microelectronics since the invention of integrated circuits (ICs) in the late 1950s. Its moderate bandgap (~1.1 eV), excellent chemical stability, and abundance in the lithosphere enable the complex procedure and further development in the semiconductor industry. Besides, the native silicon dioxide can serve as a capping layer to protect silicon from contamination and also as the gate dielectric in silicon-based field-effect transistors (FETs) owing to its dielectric properties and the excellent interfacial quality between Si and SiO₂. However, following Moore’s law, the scaling of silicon-based ICs at sub-10 nm technology nodes faces several challenges, such as interfacial imperfection¹, short-channel effect², mobility degradation³, and thickness fluctuation³. Thus, it is crucial to explore materials and develop high-mobility semiconductors. Possible material systems, including SiGe⁴, Ge⁵, III-V⁶ semiconductors, oxide semiconductors⁷, and two-dimensional (2D) materials⁸, have been investigated and shown their great potential to extend Moore’s law. However, the lack of a high-quality native oxide for these semiconductor materials impedes further development, making it hard to challenge silicon dominance. Though much effort has been made into the oxides, they are generally non-stoichiometric and highly defective at the oxide-semiconductor interface compared to Si/SiO₂.

In search of a feasible candidate, Bi₂O₂Se (BOSe), an air-stable quasi-2D semiconductor material with high electron mobility^9,10, has been discovered since 2017¹¹. In addition to its superior electronic characteristics, the native oxide layer, Bi₂SeO₅ (BSO), can be conformally formed through thermal oxidation¹², UV-assisted intercalative oxidation¹³, and oxygen plasma oxidation¹⁴ of the underlying BOSe semiconductor, ensuring an atomically sharp and chemically clean interface. Credited with the high dielectric constant (є=15 ~ 20), the insulating BSO can be directly adopted as an ideal gate dielectric for the BOSe/BSO self-assembled FETs, showing an excellent Hall mobility up to ~20000 cm²/V*s (Large Hall mobility up to 20,000 cm²/V*s of nanoflake BOSe at 1.9 K; Bilayer BOSe only shows the mobility approaching 450 cm²/V*s) and a sub-threshold swing of ~65 mV/dec¹¹ (For this case, SS ~ 65 mV/dec, the thickness of applied BOSe is up to 6.5 nm). Numerous BOSe based electronic devices have been demonstrated due to their excellent performance and abundant physical properties such as logic devices^15,16, optoelectronics^17–20, thermoelectric^21,22, memory^23,24, and IoT applications^25,26. However, integration for transistors is essential for developing further logic devices. In contrast to n-type semiconductors, superior and reliable p-type semiconductors with excellent transfer characteristics are still scarce²⁷. Though research has demonstrated the integration with two n-FETs for direct-coupled FET logic, combining one n-FET and one p-FET still possesses many advantages, such as low power consumption²⁸, better noise immunity²⁹, and fast switching speed³⁰. Leveraging this site-specified doping technology can meet the demands for complementary characteristics for well-functioning devices. On the other hand, a compatible process facilitates the monolithic 3D integration in the semiconductor industry³¹. With these understandings, a pathway to homogeneous BOSe integration should be developed for being adopted into practical applications.

This work creates a p-type BOSe through a chemical doping process and integrates it with the pristine n-type BOSe. To obtain the p-type characteristics, Zn²⁺ is adopted as the dopant and substitutes the Bi³⁺ site, leading to a hole-dominating feature. Moreover, different device geometries, including planar and vertical stackings, are demonstrated to show the compatibility of n- and p-type BOSe. Inside the junctions, the epitaxial feature with an atomic sharp interface can be observed, beneficial for electrical properties³². For the planar homojunction, the electronic potential of n- and p-type BOSe can be resolved, and the current measurements of the PN homojunction show rectification behavior characteristics. Besides, transistors have been fabricated and show typical transfer characteristics. On the other hand, vertical BOSe-based PN junctions have the ability to integrate monolithically and homoepitaxially. The photoresponse in the visible light range (~600 nm) can reach a ~ 10⁶ on/off ratio, and the corresponding detectivity is 10¹³ Jones, invoking a vast potential for photodetectors. With these efforts, a comprehensive idea for obtaining a homogeneous integration in BOSe is delivered, paving the way for adopting this material system.

Results and discussion

Characterization of the doping features

First, the feasibility of the fabrication of p-type BOSe should be verified through the structural information. Using the pulsed laser doping process (Fig. 1), an elemental selecting platform has been established for identifying the doping influence. For the conventional ion implantation doping process, the energy used to accelerate the dopants is about a few keV to MeV. Though such high energy enables the dopants to penetrate the host materials (depth can reach about a few μm), the generated structural damage would lower the device performance. As for the proposed pulsed laser doping process, the laser wavelength used is 248 nm (~5 eV), which is a more moderate energy for preventing severe structural damage. However, such a relatively low energy limits the depth of the dopants. Though the depth is relatively lower than the depth by ion implantation, sustaining the crystalline quality of the host materials is better. Supplementary Figure. 1a, b shows the valance band maximum scans (X-ray photoelectron spectroscopy, XPS) and the schematic diagrams of the corresponding band structures for the doped BOSe samples. Based on the results, Zn ion is adopted as the dopant due to the significant lowering potential barrier between the Fermi-level and valance band maximum, more similar to a p-type semiconductor. Moreover, the chemical state of the Zn doped BOSe has been revealed through XPS, as shown in Supplementary Fig. 1c. With these efforts, the influence of the Zn dopants in BOSe can be revealed. To further realize the doping feature, a series of structural and elemental characterizations have been conducted. The unit cell of the obtained p-type BOSe is illustrated in Fig. 1b, with the Zn dopants substituting the Bi’s sites. To identify the distribution of Zn dopants in BOSe, cross-sectional high-angle annular dark-field-scanning transmission electron microscope (HAADF-STEM) images have been captured along with the depth profile for Zn and Bi, as shown in Fig. 1c. Due to that, the doped region (P) and the pristine BOSe share a same lattice without severe lattice distortion, the clear interface cannot be observed based on the crystal structure. However, the distribution can be revealed through the elemental depth profile. Based on the results, in the 20 nm-thick junction, the Zn signals can be detected for about 10 nm thick from the surface, while the Bi’s signals can be detected from the surface to the bottom. Based on the energy-dispersive spectroscopy (EDS) mapping of Fig. 1d, each element in the doped BOSe can be resolved (Fig. 1e–h). Zn and Bi ions occupy the same site, as concluded from the overlapping EDS signals, verifying the substitutional type. The accelerating energy for the pulsed laser doping process is about 5 eV (equal to ~482.4 kJ/mol). For a Bi₂O₂Se lattice, the strongest bonding is located at the Bi-O covalent bond, with about 102.5 KJ/mol bonding energy. Thus, the energy provided by the pulsed laser is sufficient to break the bonding and enable the doping process. A further theoretical calculation has been conducted to investigate the stability of the doped BOSe, which the substitution of Zn dopants to Bi site shows a stable condition and leads to a p-type modulation (see Supplementary Fig. 2). The replaced unbonded Bi atom should escape from the lattice. The driving force of Zn can be categorized into two categories. One is the kinetic energy provided by the pulsed laser. It helps the Zn dopants to penetrate the BOSe lattice, further achieving the doping process. The other one is diffusion. Due to that, the dopants bombard the BOSe from the surface, and the concentration of Zn is relatively higher than in the deeper regions. Thus, the concentration gradient is another factor that drives the Zn dopants. Meanwhile, the doped BOSe can still be oxidized into dielectric doped BSO layer (see Supplementary Fig. 3).

Fig. 1. The fabrication and the atomic-scale observation of the Zn-doped BOSe.

a The schematic diagram of the pulsed laser doping process, and the corresponding device structures, including planar and vertical junction. b The schematic diagram of the doped BOSe. c–h The cross-sectional high-angle annular dark-field-scanning trFansmission electron microscope (HAADF-STEM) image and depth profile of the interface between Zn-doped region and pristine BOSe, sharing a same crystal structure.

Structural characterization of the device

For a better demonstration and to facilitate the subsequent measurements, a bottom-gate transistor made of epitaxial BOSe/BSO/Nb:STO heterostructure is fabricated. The schematic diagram of the built BOSe/BSO/Nb:STO heterostructure is shown in Fig. 2a, in which the heterostructure is similar to a bottom-gate device structure. Next, to realize the structural information, X-ray diffraction and cross-sectional STEM images have been conducted. Figure 2b shows the out-of-plane L-scan of BOSe/BSO/Nb:STO heterostructure. The theta-2 theta scan presents the pristine phase of BOSe, BSO_, and STO substrate. Only (00 L) series signals of BOSe and BSO appear along with STO (00 L) signals. Meanwhile, the full width at half maximum (FWHM) of the BSO(002) and BOSe (004) peaks revealed by the rocking curve measurements are ~0.05°and ~0.13°, respectively, as shown in Fig. 2c, proving the superior crystal quality of the heterostructure. To further verify the epitaxial feature, the phi scans shown in Fig. 2d were used to determine the in-plane (IP) orientation. The four-fold symmetry along (001) orientation is observed, and four sets of peaks at 90° intervals are displayed. The peak alignment leads to the epitaxial relationship of the heterostructure as (002)BOSe//(002)BSO//(001)Nb:STO. Furthermore, the reciprocal space mapping (RSM), as shown in Fig. 2e, suggests a 0.6% out-of-plane tensile strain for the BOSe layer and a 2.2% out-of-plane tensile strain for the BSO layer. Such results show that the films were clamped on the STO substrate. Finally, the cross-sectional STEM images of the heterostructure with atomic-scale resolution is shown in Fig. 2f (The low-mag image is shown in Supplementary Fig. 4). Based on the result, the arrangement of each atom can be clearly observed, suggesting the optimized growth condition for each layer.

Fig. 2. Structural characterization of the device structure.

a The schematic diagram of the bottom-gate device structure, BOSe/BSO/Nb:STO. (BOSe: Bi₂O₂Se, BSO: Bi₂SeO₅, Nb:STO: Nb-doped SrTiO₃). b–e The out-of-plane L-scan, rocking curve, in-plane phi scan, and RSM (Reciprocal spacing mapping), and (f) cross-sectional STEM images of the BOSe/BSO/STO heterostructure.

Planar integration and investigation on the electronic potential

Next, a planar integration, doped BOSe/BOSe PN homojunction, is demonstrated. After the BOSe/STO sample fabrication, a p-type region can be defined through the combination of Digital Light Processing (DLP) Maskless Exposure System and the pulsed laser doping process. For the planar integration, we first deposited the pristine BOSe. After that, the photolithography is applied to create the window for doping, and then the pulsed laser doping process is employed. The operating temperature is at room temperature. Consequently, the photoresist is then removed from the acetone solution and undergoes the IPA clean process. For the metal contact, Au (work function ~5.1 eV)/Pd (work function ~5.2 eV) is used for the source and drain electrodes to form ohmic contacts. The thickness of Pd is ~10 nm, and it serves as the first contact due to the closest work function to BOSe. After that, the Au was deposited for ~30 nm, a metal with air stability. In this way, the n- and p- regions are horizontally joined to form a BOSe-based PN homojunction, as shown in Fig. 3a. After that, we applied the voltage bias (V_SD) and measured the direct current (I_SD) of the BOSe-based PN homojunction, as shown in Fig. 3b. Under reversed bias, the diffusion potential barrier height between the p- and n- regions becomes too high to flow a significant current through the junction until the breakdown voltage (BR) is achieved (3.8 V), showing a typical rectification behavior characteristic of the PN diode³³. (The measurement of the NN junction is shown in Supplementary Fig. 5),

Fig. 3. Rectification behavior and the electronic potential of the PN junction.

a The optical image of the planar PN homojunction with patterned Au electrodes. b The I_D-V_D (I_D: drain current, V_D: drain voltage) characteristic of the planar PN homojunction. c KPFM image of the planar PN homojunction and (d). The corresponding line scan across the interface. e The result of the STS (scanning tunneling microscopy) measurement for pristine and doped BOSe. ($\mathrm{\Delta }{E}_{VB}$: shifted valence band, $\mathrm{\Delta }{E}_{CB}$: shifted conduction band) f. The band alignment of the BOSe-based planar PN homojunction. (EF Fermi level, CL core level, VBM valence band maximum, CBM conduction band minimum, $\mathrm{\Delta }{E}_{CL}$: shifted core level).

To further investigate the electronic potential difference in this planar PN homojunction, Kelvin potential force microscopy (KPFM) was conducted. The surface potential and work function should be modified by the dopants, leading to a clear contrast in the KPFM image in Fig. 3c. There is no significant difference in the surface morphology and noticeable height difference, suggesting that the contrast is attributed from the electronic potential difference (see Supplementary Fig. 6). The only difference between these two is whether there is a doping process. Furthermore, the line scan (Fig. 3d) across the interface suggests a ~ 0.35 eV potential difference, enough to alter the carrier’s polarity of BOSe (bandgap ~0.8 eV). In this way, the relationship between the hole carrier’s concentration and the dose of dopants can be constructed. The estimation of the carrier’s concentration is based on the intrinsic carrier concentration and the potential difference (|E_F -E_Fi|):

$$Carrierconcentration=N_i\exp \left(\frac{∣E_F-E_{Fi}∣}{k_B\times T}\right)$$

Where N_i is the intrinsic carrier concentration, E_F, and E_Fi are the Fermi level and intrinsic Fermi level, k_B is the Boltzmann constant, and T is the temperature. Supplementary Fig. 6 shows the KPFM images of the homojunction with different doses. An increasing potential difference can be observed along with the dose, suggesting that the carrier concentration is controllable. Consequently, the relation between the calculated induced carrier concentration and the pulsed number is shown in Figs. S7, S8. This quantitative doping process offers a playground for better modifying the electronic potential of BOSe and defining the induced carrier concentration.

Finally, the electronic structure of the synthesized planar homojunciton can be revealed through scanning tunneling spectroscopy (STS), as presented in Fig. 3e. A shifted electronic potential (~0.22 eV) for the doped BOSe compared to the pristine one can be seen. After doping, the closer Fermi level to the valance band maximum in the doped BOSe suggests a classic p-type semiconductor characteristic. Meanwhile, if the current falls below the background noise level of 1 pA, it is considered within the bandgap (0.8 eV). Thus, the band alignment of the synthesized BOSe-based planar PN homojunction can be depicted in Fig. 3f. The energy difference of the core level corresponds to the Fermi-level energy shift in the band gap, turning the n-type into p-type BOSe. Meanwhile, the polarity can be also adjusted by selecting a suitable element, invoking more possibility to develop this material system based on this approach.

Fabrication of the complimentary metal-oxide-semiconductor (CMOS) logic devices

The area-selectivity of the proposed doping process with micro-processing (see Supplementary Fig. 9) facilitates the fabrication of the transistors based on the BOSe/BSO/Nb:STO heterostructure, with the N- and P- MOSFET transfer and output characteristics shown in Fig. 4a–d (optimized thickness of BOSe is ~8 nm). The transfer curves of the N-FET also show typical n-type behaviors with an on/off ratio of ~ 4E10⁵ and a steep rise of drain current with the subthreshold slope of ~68 mV dec⁻¹, approaching the thermal limit of ~60 mV dec⁻¹ at 300 K³⁴. Also, the linear and saturation regions can be observed in the I_ds-V_ds along with the increasing gate voltages, suggesting a superior gate control by the native BSO to the pristine n-type BOSe channel. On the other hand, the transfer curves (Fig. 4c) of the P-FET at different V_ds show a noticeable rise of the drain current with on/off ratios for ~ 5E10³ and field-effect hole mobility for 34 cm²V⁻¹s⁻¹. Furthermore, the influence of pulsed numbers to transfer characteristics is shown in Figs. S10, S11 (calculation detail is shown in Supplementary Fig. 12). This characteristic provides an opportunity for tuning the electrical properties through compositional modulation. The chemical composition that makes BOSe transform from n to p-type is (Zn_0.02Bi_0.98)₂O₂Se. Furthermore, the relation between pulsed number and the composition is shown in Supplementary Fig. 13. The amount of Zn in the Zn-doped BOSe lattice is nearly proportional to the pulsed number. Based on the results, the calculated maximum field-effect mobility in the synthesized P-FETs is ~83 cm²V⁻¹s⁻¹ while maintaining a moderate threshold voltage of approximately -0.8 V, and the output characteristics also exhibit typical p-type behaviors with linear I_ds-V_ds (Fig. 4d). The comparison of the device characteristics (I_on/I_off & hole mobility) is shown in the benchmark in Fig. 4e (detail comparison is shown in Supplementary Table 1). The induced hole mobility in the BOSe-based P-FET device is higher than other P-FETs while maintaining a moderate I_on/I_off ratio, invoking a huge potential for better integration. Meanwhile, fatigue tests for both n- and p-FETs have been conducted, suggesting the robust electrical properties of the devices (see Supplementary Fig. 14). Such efforts show significant advantages for future monolithic integration, such as the lower power consumption³⁵ and better electrical control³⁶. Taking the advantage of polarity modulation, a CMOS inverter circuit can be demonstrated, as shown in Fig. 4f. To prevent the gate leakage, the device structure has been changed to top-gated structure with a HfO₂ layer further deposited on top, which operates between 0 V (logic state 0) and 1 V (logic state 1). With these efforts, a flexible modulation for the device characteristics can be achieved for a wider range of applications.

Fig. 4. Demonstration of the CMOS (complementary metal-oxide-semiconductor) logic devices.

a–d Transfer and output characteristics of the N- and P-FET. (L: channel length, W: channel width, V_G: gate voltage) (Solid lines denote the drain current and the dot lines denote the gate leakage current). e State-of-the-art for the field-effect-hole-mobility^48–57. (Selected materials are p-type semiconductors with relative high field-effect hole mobility). f Voltage transfer curves of the CMOS inverter. (V_DD: operational voltage, GND: ground, V_in: input voltage, V_out: output voltage).

Vertical junction for photoresponse

After verifying the feasibility of planar integration, attention is now paid to the other device’s geometry-vertical junction. The depth the dopants triggered by stable laser energy can reach is about 10 nm from the surface (depth profile is shown in Supplementary Fig. 15). Thus, a vertical PN junction with a 10 nm p-type region (upper)/10 nm n-type region (lower) can be created and serve as a photoelectronic device. Figure 5a illustrates the schematic diagram of the vertical PN junction under illumination in the visible light range. In principle, the built-in electric field inside the PN diode is beneficial for separating the photogenerated carriers compared to those single-layer structures³. To identify the characteristics of the vertical PN junction, the transport measurements (I_D-V_D) (Fig. 5b) was conducted and shows the rectification behavior characteristic of a typical PN diode. After that, the photoresponse of the vertical junction under blue, green, and red light was measured at room temperature, as shown in Fig. 5c. Among three wavelengths, the best photoresponse in this vertical junction is located at ~532 nm (green light) under zero bias. Moreover, Fig. 5d–f shows the time-resolved photocurrent response under red, green, and blue light illumination of various power intensities. It is evident that the photoresponse continuously increases with the light intensity, and a maximum on/off current ratio of ~10⁶ (green light, 532 nm) can be achieved under 10 mW illumination. Consequently, the corresponding photoresponsivity (R) and photodetectivity (D) can be calculated based on the I-t curves measurements. The R value can be calculated by the equation of R = I_ph/(PS), where I_ph, P, and S are the photocurrent, incident power density, and functional areas of the device, respectively. The increasing photoresponsivity compared to the pristine BOSe can be attributed to the enhanced charge separation in PN junction (see Supplementary Fig. 16). On the other hand, D can be expressed as D* = RS^1/2/(2eI_dark)^1/2, where I_dark represents the dark current³⁷. Figure 5g shows the relationship between detectivity and responsivity under green light illumination. The detectivity increases linearly with the responsivity and shows an excellent performance of ~1.1E13 Jones when the responsivity is 0.11 A/W. Such a high value can be attributed to the dark current (<1E-14 A). Compared to other systems, it is evident that the synthesized vertical BOSe homojunction shows a superior photodetectivity in the visible light, as shown in the state-of-the-art^38–47 photodetectors (Fig. 5h). Finally, the fatigue test under green light illumination with 100 cycles has been conducted to identify the stability of the photoresponse, as presented in Fig. 5i. There is no significant difference in the photoinduced current in every cycle, suggesting a robust behavior in long-term usage. Overall, the performance of this vertical BOSe-based homojunction is better than that of several advanced nanomaterials-based photodetectors, which are promising in low-noise detection and low-power consumption devices.

Fig. 5. Photoresponse of the synthesized vertical PN junction.

a The schematic diagram of the vertical BOSe-based PN homojunction. (ITO: Indium tin oxide, built-in E: built-in electric field, circles with “h⁺” and “e^-“labels: hole and electron carriers, blue and pink arrows: electric and hole current). b The transport measurement of the synthesized junction. c The photoresponse of the vertical junction under blue, green, and red-light illumination. d–f The time-resolved photocurrent response under red, green, and blue light illumination of various power intensities. g The relation between detectivity and responsivity under green light illumination. h The state-of-the-art^38–47 photodetectors in the range of visible light (Selected systems are the photodetectors with relative high photodetectivity in the range of visible light). i The fatigue test under green light illumination in 1000 s’ period.

In this work, we reported the p-type modulation on BOSe and further identified its crystallinity and electronic structure. After verifying the fundamental characteristics of the p-type BOSe, different device geometries, including the planar and vertical integration with the pristine n-type BOSe, have been obtained for the subsequent applications. For the planar integration, the electronic potential difference can be resolved in the doped BOSe (p-type)/BOSe (n-type) homojunction, indicated by the I-V curve with a rectification behavior characteristic of a typical PN diode. Furthermore, the transistors can also be fabricated based on the planar homojunction with a sharing global bottom BSO gate dielectric. The relative high field-effect hole mobility and the double enhancement load switch suggest the potential for further CMOS applications. Finally, the vertical stacking of the n- and p-type BOSe presented an epitaxially monolithic integration. The sharp interface and the excellent crystallinity led to the superior photoresponse in the range of visible light with a high on/off ratio and photodetectivity. With these efforts, further exploration and intriguing applications based on this material system are expected to push the next-generation electronics.

Methods

Sample preparation

The epitaxial BOSe was fabricated via PLD with a commercial Bi₂O₂Se target, while the doping process was conducted after the thin film deposition. Commercial STO single crystals were used as the substrates. The STO and 0.5 wt% Nb-doped STO substrates with 99.99% purity were secured from Eternal Stars International Co., Ltd. On the other hand, the targets with 99.99% purity used in this work are made by Ultimate Materials Technology Co., Ltd. The cleaned substrate was immediately loaded into the processing chamber to minimize contamination. A KrF excimer laser (λ = 248 nm; Lambda Physik, Coherent agent) was operated at a 10-Hz repetition rate and energy density of 1 J/cm². The vacuum chamber was evacuated to a pressure of 1 × 10⁻⁶ Torr before deposition. The BOSe layer was grown on STO substrate at 405 °C under 100 mTorr of O₂ pressure. Lastly, the cooling process was conducted with a cooling rate of 0.3 °C/s. After the deposition of BOSe, the temperature will be maintained at the original 405 °C, and the oxygen pressure of the chamber will be increased from 100 mtorr to 100 torr, employing this thermal oxidation for about 1 h, and cooling down with 0.1 °C/sec. Such a process ensures the complete oxidation of BOSe into BSO dielectric layer.

For the doping process, the ZnSe target was used and placed in the PLD chamber. Before the doping process, the vacuum chamber was evacuated to a pressure of 1 ×10⁻⁶ Torr, and the dose was controlled by the pulsed number and laser energy (power density), which can be set up through the laser equipment.

X-ray diffraction in NSRRC

The crystal structure and phase identification were characterized by synchrotron-based X-ray diffraction techniques at the beamlines 13 A and 17 A in the National Synchrotron Radiation Research Center in Hsinchu, Taiwan. The incident beam was monochromated at 9.3 keV (ca. 1.333 Å) with a Si(111) double-crystal mirror and then focused by a toroidal focusing mirror to obtain a higher-intensity beam. Four sets of slits were used to gain the detection resolution, where two sets of slits were placed in front of the samples to set beam size and the other two were placed after the sample (or before the scintillation counter) to decrease background noise. These diffraction measurements were then plotted in reciprocal lattice units normalized to the STO substrate (1 r.l.u. = 2 π / aSTO).

Scanning probing microscopy

In this study, The Asylum Research Cypher S was used for AFM and KPFM measurements. The microscope is situated in air with a temperature-stabilized acoustic isolation chamber. The probe used was ASYELEC.02-R2 with a spring constant of 42 N/m, resonance frequency of around 300 kHz, and a tip coating is Ti/Ir. The radius of the tip is approximately 32 nm. The scan area was set to 20 µm × 20 µm. The resolution was configured at 512 points/lines, and the scan rate was 1 Hz. KPFM operated in amplitude-modulated (AM-KPFM) mode, which is by far the most common measurement mode. Measurements presented were carried out with lift-heights. The tip oscillates at a frequency of 313 kHz, and the AC voltage used during the measurements was 3 V. Copper Foil Tape (3 M 1182) was used to attach the samples to metal discs, which were used to ground the sample surface.

Transmission electron microscopy and analysis

TEM samples were prepared on a dual-beam FIB system (Helios G4 UX, FEI, USA). HAADF-STEM images and EDS mapping were acquired on a JEOL ARM300, which can record high-resolution STEM images with a spatial resolution of 63 pm. The microscope was equipped with a double spherical aberration (CS) corrector and an X-ray energy dispersive spectrometer (JED-2300 Series) with two 158 mm² Silicon Drift Detectors (SDD).

X-ray photoelectron spectroscopy

The localized SR-PES technique provides a powerful method to obtain direct information about a band structure with varied polarization; soft X-rays (photon energy 400 eV) were used at the SPEM end station located at beamline 09 A1 of Taiwan Light Source in NSRRC. The soft X-ray beam was focused by the Fresnel zone plate, and the order-sorting aperture at the focal plane was about 100–200 nm in diameter. All measurements were undertaken near 300 K. The energy resolution was estimated to be better than 100 meV. Based on the SPEM images, the focused beam was movable to a specific location to record high-resolution PES of a microscopic area.

Scanning tunneling microscopy

The sample was subjected to heating at 200 °C for 20 min in the preparation chamber to remove adsorbed gases from the sample surface. After degassing, the sample was allowed to cool to near room temperature before being transferred to the chamber of STM for measurement. The type used in this study was an Omicron RT UHV STM. Tungsten tips, etched in NaOH solution, were employed as the probes. STS measurements were conducted under ultrahigh vacuum conditions with a base pressure of 6E-10 torr at room temperature to minimize environmental noise with the initial bias voltage was set to 2 V.

Macroscopic electrical characterization

The capacitance features of the Bi₂SeO₅ dielectric layer were investigated under100 mV AC voltage, 10 kHz AC frequency, and the voltage was measured from 1 V to 3 V, measured by a commercial instrument for ferroelectric properties (TFAnalyzer3000, aixACCT Systems). Meanwhile, the current measurements, photoresponse, and transfer characteristics were measured by a Keysight B1500A semiconductor analyzer to obtain the performance of the synthesized FET device and PN junction.

Theoretical calculation

Density Functional Theory (DFT) calculations of the band structure information for tetragonal Zn-doped Bi₂O₂Se can be effectively conducted using the QuantumATK package, designed for atomic-scale simulations. The electron basis set is constructed using the linear combination of atomic orbitals (LCAO) method, providing an efficient approach for geometry optimization and electronic property evaluations with balanced computational cost and accuracy. Moreover, applying the exchange-correlation function based on the generalized gradient approximation (GGA), specifically through the Perdew-Burke-Ernzer (PBE) scheme, ensures that our simulated band structure energies can meet with agreement to experimental findings beautifully. In terms of the complexities of ionic cores, norm-conserving (NC) pseudopotentials sourced from PseudoDojo (PDj) have been utilized, avoiding the explicit DFT calculations of core electrons, thus enhancing overall computational efficiency. In addition, the periodic boundary conditions (PBCs) have been applied in all three dimensions to represent the bulk system accurately.

Author contributions

Y. J. W. designed the experiments, synthesized the films, conducted the structural characterization, measurements of transfer characteristics, and current measurements. J. W. Z., J. C., and H. W. carried out the TEM, STEM, and EDS analysis. S. W. and Y. J. W. performed the SPM (AFM and KPFM) analysis. C. Y. L. conducted the STM measurements. J. T. H., C. Y. S., L. S. H., and I. S. C. helps the device fabrication. Y. C. C.(Yuan-Chih Chang) conducted the theoretical calculation. Z. Y., R. H., C. L. L., P. W. C., Y. L. C., Y. C. C.(Yi-cheng Chen), C. H. Y., and Y. H. C. supervised the research. All the authors contributed to the discussion and manuscript preparation and read the final manuscript.

Peer review

Peer review information

Nature Communications thanks Fei Yan, who co-reviewed with Amit Kumar Shringi; Hongtao Liu and the other, anonymous, reviewer for their contribution to the peer review of this work. A peer review file is available.

Data availability

Relevant data supporting the key findings of this study are available within the article and the Supplementary Information file. All raw data generated during the current study are available from the corresponding authors upon request.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-025-58198-3.