Perovskite Stannate Heterojunctions for Self‐Powered Ultraviolet Photodiodes Operated in Extreme Environments

High‐performance UV photodetectors call for sensitive and energy‐efficient signal detection in extreme environments. To satisfy the requirement of a UV detection without an external power consumption, self‐powered UV photodetectors must be realized by an optimal combination of heterostructure with maximum built‐in potential using novel wide‐bandgap materials. Here, self‐powered UV photodiodes are designed via the band engineering of a wide‐bandgap Sr(Sn,Ni)O3/BaSnO3 heterojunction for the first time. Based on the theoretical concept of acceptor doping by Ni substitution in SrSnO3, remarkably, this heterojunction with a conduction band offset of 0.94 eV shows strong nonlinear electrical characteristics with extremely low Idark (≈100 fA) owing to the spatial gradient of the potential barrier across the interfaces, outstanding photo‐to‐dark current ratio (>107 at 25 °C and > 104 at 300 °C), and high stability under various extreme conditions upon UV illumination even without external bias (V = 0 V). This study suggests a novel strategy that utilizes band engineering to maximize sensitivity and minimize energy consumption in harsh environments for UV imaging using the newly discovered wide‐bandgap semiconductors.


Introduction
[3][4][5][6][7] UV photodetectors are used in a DOI: 10.1002/aelm.202300530 Moreover, UV photodetectors used in the aforementioned applications are usually operated in harsh environments (such as high temperature, high vacuum, high humidity, and high radiation exposure); UV photodetectors should exhibit high stability even in extreme environments. [3,6]lthough Si-based UV photodetectors are technically mature and commercially available for sensing UV radiation, the narrow bandgap of Si fundamentally limits UV light absorption because of heat dissipation, which leads to a low quantum efficiency. [3,11]urthermore, the facile band-to-band excitation of intrinsic carriers in Si by thermal energy results in a high dark current and degrades the PDCR in Si-based UV photodetectors.This necessitates the use of a visible color filter to avoid signal noise from universal visible light. [3,11]12] However, highperformance UV photodetectors should possess high sensitivity, even in miniaturized device configurations, and low power consumption.Therefore, the development of novel materials and heterojunctions for realizing efficient photodetectors is urgently required to meet the demands of real-time applications.
[15][16][17] Owing to the high dispersion of the conduction band with the Sn 5s orbital characteristics and the low electron-phonon scattering rate, n-type BaSnO 3 (BSO) exhibits an unprecedentedly high electron mobility at room temperature (μ e ≈ 320 cm 2 V −1 s −1 at n 3D = 8 × 10 19 cm −3 ), which is the highest value among those reported for the wide-bandgap semiconductors at similar carrier concentrations. [14,18]The high μ e would be advantageous for enhancing the photoresponse and switching speed of the UV photodetector composed of stannate semiconductors.Moreover, these materials with perovskite structures show a high thermal stability even at high temperatures, [14,19] and alloying A-site alkaline-earth elements can facilitate the tuning of the bandgap of perovskite stannates (E g = 3.1-5.2eV), [15,20,21] thereby enabling the control of the band offset for efficient photoexcited carrier collection.
The use of the perovskite stannate heterostructure (e.g., ASnO 3 /AʹSnO 3 ) with high μ e and thermal stability would present a new strategy to achieve high-performance UV photodetectors that can be operated in extreme environments using bandgap engineering.In particular, the pn junction would maximize the built-in potential across the depletion region; the UV photodetector can be operated without an external power supply (i.e., self-powered UV photodetector). [8,22,23]However, efficient selfpowered and durable UV photodetectors based on perovskite stannate have not been realized owing to the difficulty in the acceptor doping of perovskite stannate thin films. [24]Therefore, search for an optimal combination of heterostructure to realize self-powered devices remains lacking. [25,26]ere, for the first time, we demonstrate self-powered UV photodiodes with excellent sensitivity and chemical robustness in extreme environments using a perovskite stannate pn heterojunction.Motivated by the theoretical concept that Ni substitution into the Sn site (Ni ′′ Sn ) lowers the Fermi level of SrSnO 3 toward the valence band maximum, we experimentally demonstrate that the p-Sr(Sn,Ni)O 3 (SSNO)/n-BSO heterojunction diode exhibits a type II staggered band alignment with a conduction band offset of 0.94 eV and shows strong nonlinear electrical characteristics with extremely low I dark (≈100 fA).Notably, a significant increase in the photocurrent (i.e., 100 fA → 1.8 μA) was observed upon UV illumination even without an external bias (V = 0 V), which exhibited excellent self-powered operation capability along with a short response time.Owing to the highly inert characteristics of perovskite stannates, the PDCR of the p-SSNO/n-BSO photodiodes show an outstanding value (>10 7 at 25 °C and >10 4 at 300 °C) and stability under extreme conditions (i.e., high temperature, vacuum, and immersion in water) without an external power supply.The enhanced mismatch of the Fermi level across the pn heterojunction photodiode results in a spatial gradient of the potential barrier across the interfaces, which is attributed to the outstanding PDCR in harsh environments.These results emphasize the importance of band engineering using robust oxide semiconductors for fabricating exceptional optoelectronic devices resistant to harsh environments.

Results and Discussion
Prior to the experiment, we first evaluated the feasibility of inducing p-type characteristics in SrSnO 3 via Ni doping by performing density functional theory (DFT) calculations.The substitution of Ni on a Sn site (Ni Sn ) was considered because of the similar ionic radii of Ni (r Ni 2+ = 0.70 Å) and Sn (r Sn 4+ = 0.72 Å). [27] Because the valency of Ni is lower than that of Sn, Ni Sn act as an acceptor that can accept up to two electrons from the host.The relative formation energies of Ni Sn are plotted in the neutral, −1, and −2 charge states (Figure 1a).The transition level (q/qʹ), at which the relative stability between two different charge states, q and qʹ, is equal, can be regarded as a thermodynamic defect level.Interestingly, (0/−1) and (−1/−2) are formed below the midgap, implying that Ni doping shifts the Fermi level of SrSnO 3 to a position below the midgap by accepting electrons from the host (Ni × Sn → Ni ′′ Sn + 2h + ) despite the deep defect levels.To confirm this possibility, we calculated the equilibrium Fermi level (E F ) as a function of the doping concentration, considering the charge neutrality condition. [28]E F in SSNO is near 0.6−0.9eV above the valence band maximum, when the compensation from other native defects is neglected (red line in Figure 1b).Considering the compensation because of the formation of oxygen vacancies (V ⋅⋅ O ), the E F increases to 1.2−1.6 eV above the valence band maximum (VBM, blue line in Figure 1b).The simulation results clearly demonstrate that the E F of SSNO can be located below the midgap, so that SSNO can exhibit p-type characteristics (Figure 1b).
][31] Therefore, based on the theoretical prediction, there should be a certain difference in E F between SSNO and BSO, which develops a built-in potential by Fermi level alignment at the heterointerface of SSNO and BSO.This gradient of the conduction band would block the random collection in the dark and simultaneously exert a quasi-electric force on the carriers, which accelerates the efficient separation and collection of photogenerated excess carriers under illumination.
We experimentally tested these theoretical predictions using an all-stannate epitaxial SSNO/BSO heterostructure as an active layer, grown by pulsed laser deposition on a 100 nm-thick epitaxial 5% La-doped BaSnO 3 (LBSO) bottom electrode (BE).The electrical conductivity of the 5% LBSO film was measured to be ≈6000 S cm −1 . [17]This degenerate semiconductor is suitable for transparent electrodes to collect photogenerated carriers with BSO/LBSO ohmic contact in this heterojunction device.(Figure S1, Supporting Information) The composition of Ni in SSNO was fixed at 10% to fully incorporate the Ni dopants into the SrSnO 3 lattice framework.The thickness of the heterostructure was controlled at ≈60 and 80 nm for the SSNO and BSO epitaxial layers, respectively.Nominally stoichiometric BSO and SrSnO 3 films were optimized by adjusting the laser plume dynamics for accurate cation stoichiometry, as previously described. [17,31]Thereafter, 50 nm-thick LBSO top electrodes (TEs) were deposited on the SSNO/BSO heterostructures.Top and bottom LBSO electrodes partially transmitted ultraviolet light from 93% to 15% as wavelength decreases from 370 to 250 nm, respectively (Figure S2, Supporting Information).
After fabricating circular-area SSNO/BSO verticalheterojunction devices with a diameter of 500 μm (Figure 1d), we then studied the current−voltage (IV) characteristics of the SSNO/BSO heterostructures in the dark (Figure 1e).Our heterostructure device showed strong nonlinear IV rectification behavior: Forward currents (I forward = 1.2 μA at +1.0 V) were more than two orders of magnitude higher than reverse currents (I reverse = 5.0 nA at −1.0 V).Moreover, the I dark of the heterojunction devices is as low as ≈100 fA at zero bias; the energy barriers after Fermi level alignment between SSNO and BSO are likely to prevent the flow of charge carriers across the depletion region of the SSNO/BSO heterostructure under dark conditions. [8,22,23]The reverse current and forward current of our SSNO/BSO heterojunction are more than an order lower than those from previously reported stannate pn homojunctions (K-doped BaSnO 3 /La-doped BaSnO 3 ), [32] which is attributed to the energy barrier after alignment in our heterojunction.
To confirm the band alignment between the SSNO and BSO heterojunctions, we performed both ultraviolet photoemission spectroscopy (UPS) and secondary electron cut-off (SECO) measurements of the SSNO and BSO epitaxial films using synchrotron radiation (Figure 1f,g; Figure S3, Supporting Information).The relative positions of the valence band maximum (E VBM ) from the Fermi level and work functions (Φ) were estimated using the linear extrapolation of the UPS spectra and the relationship Φ = h -(E F -E cut-off ), [33] respectively.The valence band edges of SSNO and BSO were located 1.4 and 1.86 eV lower than their E F .Estimating the indirect bandgap (E g ) from the optical transmission spectra (E g,SSNO = 4.06 eV; E g,BSO = 3.14 eV) (Figure S4, Supporting Information), which is consistent with a previous report, [14,34] the E F of SSNO is located below the center of the bandgap, thereby validating the calculated results indicating the acceptor-like characteristic of the Ni dopant in SSNO.Furthermore, Φ values of p-SSNO and n-BSO were determined to be 5.73 and 5.29 eV, respectively.These estimated values indicate a valence band offset, conduction band offset, and ΔE F of 0.02, 0.94, 0.44 eV, respectively, confirming a type II band alignment between p-SSNO and n-BSO (Figure 1h).
When the p-SSNO and n-BSO with opposite doping types were brought into contact, the higher E F of n-BSO than the former led to electron transfer to p-SSNO until the heterojunction reached the equilibrated E F .Furthermore, the built-in potential near the interface after contact impeded further transfer of the electrons, which led to rectifying behavior in this widebandgap stannate pn diode heterojunction. [8,22,23]When the diode was operated under reverse voltage conditions, the potential barrier (i.e., conduction band offset + built-in potential) increased to block charge flow across the junctions; the drift of minority carriers resulted in low reverse currents.In particular, the as-designed p-SSNO/n-BSO heterojunction strengthens both the conduction band offset and the built-in potential, which hinder the movement of high-mobility electrons across the stannate interface.In contrast, the reduction of the potential barrier under forward voltage conditions promoted the majority carriers to transfer across the heterojunction, which led to large forward currents.
Benefitting from the good diode characteristics with high rectification and a low reverse current, we further explored the UV-induced photocurrent (I P ) characteristics of p-SSNO/n-BSO wide-bandgap heterojunction photodiodes (Figure 2a).When broadband UV light ( = 250-385 nm; P = 100 mW cm −2 ) was illuminated on the p-SSNO/n-BSO heterojunction diodes, the I P response was observed under all bias conditions across the heterojunction.Photons with energy higher than E g create excess carriers, which are separated by an electric field (Figure 2a). [16]nterestingly, a significant increase in the current (i.e., 100 fA → 1.8 μA) is observed under UV light illumination even without external bias (V = 0 V), which demonstrates the excellent capability for self-powered operation (Figure 2b).Moreover, obvious photovoltaic response is observed in the profile across the open-circuit voltage (V OC ) and short-circuit current (I SC ) in the enlarged view of the photoresponse near zero bias (Figure 2c).
When excess electronic carriers are excited by photons, whose energy is higher than the E g of the semiconductor, photogenerated electron-hole pairs can be separated by the internal potential in pn diode heterojunctions.In particular, despite the unfavorable hole collection between LBSO/SSNO contact, steeper bending at the edge of the conduction band can accelerate the collection of more electrons with low effective mass in the conduction band of perovskite stannates than holes in the valence band. [16,17,21]The accumulated electrons mostly contribute to the formation of a forward V OC .When the device was short-circuited, the photogenerated carriers generated a current flow (i.e., I SC ).Both V OC and I SC increase with the light power intensity (P L = 1.6-100 mW cm −2 ), as shown in Figure 2c.The photovoltaic behavior enables the self-powered operation of the stannate heterojunction photodiode.Indeed, we experimentally confirmed that the Ni addition in SSO increases the open circuit voltage and zero-bias photocurrent in stannate heterojunctions (Figure S5, Supporting Information): Ni doping in SSO strengthens the built-in potential of stannate pn heterojunctions, and the considerable spatial gradient of the conduction band edge can result in a quasi-electric force on the electrons, which promotes the efficient separation of photogenerated excess carriers and facilitates electron drifting toward the external circuit even without the external electrical bias. [8,12,23]s the P L increased, the I P and V OC of the heterojunction photodetector increased owing to an increase in the number of photogenerated electrons.Under illumination, the light-intensitydependent I P is related to the P L of UV illumination by the following relationship [16,35,36] where A is a scaling constant.For this heterojunction photodiode, the data points between log(I P ) and log(P L ) were linearly dependent.The linear interpolation yields  ≈ 0.77 in the entire range of P L (Figure 2e);  < 1 indicates the complex processes of electron-hole generation, trapping, and recombination at the SSNO/BSO interface. [16,35,36]V OC also increased with log (P L ) because of the increase in the number of accumulated electrons owing to the increased P L .
The zero-bias transient I P -t photoresponse of these heterojunction devices was monitored by the consecutive illumination of UV light with different intensities (P L = 1.6-100 mW cm −2 ) (Figure 2d).These self-powered UV photodiodes demonstrate excellent performance with an ultralow dark current (<10 −12 A) and a superhigh on/off ratio of I P (up to 10 7 ), exhibiting an outstanding signal-to-noise ratio and device stability over multiple operations.
To quantify the P L -normalized performance of the UV photodiode based on our heterojunction, the wavelength-dependent spectral responsivity (R * ()) and detectivity (D * ()) of the device were calculated using the following equations. [37]* () = I P − I dark P L (2) where A d and e are the effective device area receiving light and the unit charge, respectively.The maximum values of R * and D * are observed to be ≈11 mA W −1 and ≈10 12 Jones, respectively, (Figure 2f), at  = 310 nm (i.e., E ph,max = 4 eV), and excess photocarriers for I P are mainly generated at the depletion layer of the heterojunction because E g,BSO < E ph,max < E g,SSO .Additionally, the low effective mass of the conduction band facilitates the transfer of photoexcited electrons across the heterojunction, than hole transfer. [16,17,21] then measured a single cycle of photoresponse to characterize the switching speed of the p-SSNO/n-BSO UV heterojunction photodiodes by detecting the photocurrent during illumination using a UV laser pulse shot ( = 248 nm; pulse width = 20 ns).I P reaches the maximum value within 2 μs, which is considered a rise time (the time to reach 90% of the maximum value, Figure 2g), whereas I P exhibits a longer decay of ≈120 μs as a fall time (the time to reach 10% of the maximum value) because of the slow recombination of photocarriers (Figure 2h).The obtained response time of ≈120 μs is comparable to that of previously reported fast-response UV detectors, [3,5] suggesting that the band-engineered p-SSNO/n-BSO heterojunction has potential for high-speed photodetector applications.Therefore, the ultralow I dark and significant photoresponse of the p-SSNO/n-BSO photodetector provided excellent self-powered UV detection.
Due to the very high thermal stability of perovskite stannates, the as-developed heterojunction photodiodes can operate efficiently with superior stability and repeatability, even at a high working temperature of 300 °C (Figure 3a).In self-powered modes of repeated current−time (I P -t) photoresponses at different temperatures, the photocurrent instantaneously increases up to four orders of magnitude (i.e., I P ≈ 5.6 μA and I P /I dark ≈ 3.75 × 10 4 at 300 °C) with the response to the alternate illumination of UV light (P L = 100 mW cm −2 ) for 60 s (Figure 3b); the I P -t curves of the p-SSNO/n-BSO photodiodes show outstanding reproducibility and stability at high temperatures without an external power supply (Figure S3, Supporting Information).Notably, although an extremely low I dark (≈100 fA) was maintained below 150 °C, I dark began to increase at T ≥ 150 °C and reached 100 pA at T = 300 °C.
High-temperature IV characteristics (T ≥ 150 °C) were measured to confirm the photovoltaic effect of the self-powered photodiodes both under the dark and UV illumination conditions (Figure 3c,d).In the dark, both reverse and forward I dark gradually increase with T owing to the enhanced emission of electrons over the energy barrier, which simultaneously lowers V OC because of the reduced photovoltaic effect at high T values.Because I dark was determined by the thermionic emission over the interfacial energy barrier, the barrier height ( b ) could be estimated by plotting ln (I dark, rev /T 2 ) versus e/k B T using the following relationship. [38] Using the linear fitting of the data points,  b can be estimated as ≈0.88 eV (Figure 3e), which is similar to the measured conduction band offset at the interface between SSNO and BSO (≈0.94 eV).The increase in the conduction band offset by a suitable heterostructure results in a low-level dark current and a high PDCR of our photodiodes under UV illumination.
To benchmark the performance of the p-SSNO/n-BSO photodiodes in terms of sensitivity at high temperatures, their PDCR ((I P -I dark )/I dark )) was compared with those of the previously reported self-powered UV detectors using wide-bandgap semiconductors operated at high temperatures (Figure 3f, Table S1, Supporting Information for the reference).Excellent PDCR values were observed from 25 (PDCR (25 °C) = 2.22 × 10 7 ) to 300 °C (PDCR (300 °C) = 3.75 × 10 4 ) for our heterojunction UV diodes, exceeding those of previously reported self-powered UV photodetectors using wide-bandgap materials.The remarkable PDCR at high T can be attributed to the low I dark and high I p /I dark at 0 V bias owing to band engineering, as well as the excellent electron mobility and thermal stability of perovskite stannate.The enhanced mismatch of the Fermi level across the heterojunction by the addition of a Ni dopant in SSO, in conjunction with the conduction band offset between BSO and SSO, leads to a considerable spatial bending of the conduction band edge and electron barriers across the heterojunction; this potential barrier across the interfaces results in superhigh I p /I dark values even at high temperatures, leading to an outstanding PDCR.
In addition to the high-temperature stability, the p-SSNO/n-BSO photodiodes were evaluated to ensure stable operation in other extreme environments (i.e., air, vacuum, and immersed in water).Figure 3g shows the repeated I P -t photoresponses of p-SSNO/n-BSO photodiodes under consecutive 10 s-on and 20 soff conditions of UV illumination in the self-powered mode (at 0 V).In air (black line in Figure 3g), an I p /I dark value of <10 6 (i.e., I dark = 100 fA; I P = 1.8 μA) was observed without any degradation under repeated on/off operation.Notably, a higher I P of ≈4 μA and I p /I dark of the order of 10 6 can be reliably measured dur-ing repeated on/off operation in vacuum (red line in Figure 3g) compared to air.The increase in I P under vacuum conditions is attributed to the formation of a surface conducting layer at the side wall of p-SSNO/n-BSO photodiodes owing to UV-triggered surface oxygen removal. [16]This photodiode can be operated successfully even in liquid water (blue line in Figure 3g).Although I dark was increased to ≈0.1 μA owing to the water-induced surface ionic current, a rapid enhancement of the I P signal (≈2 μA) was observed under UV illumination.Therefore, the chemical stability of bulk perovskite stannates enables the successful operation of the as-developed heterojunction UV photodiode without an external power source under a wide range of thermodynamic conditions (i.e., temperature, pressure, and environment).

Conclusions
In summary, self-powered UV photodiodes operating in an extreme environment were achieved using an all-perovskite stannate pn heterostructure for the first time.The band-engineered p-SSNO/n-BSO epitaxial heterojunction diode exhibits strong nonlinear current−voltage characteristics owing to type II band alignment, exhibiting extremely low I dark (≈100 fA) in the reverse bias direction.Because of the increased potential variation across the depletion region of this designed junction, photogenerated electrons could be efficiently separated using the internal potential.Consequently, photocurrents dramatically increased upon UV illumination, even without an external electrical bias.Because of the exceptional stability of the stannate semiconductor, our pn stannate photodiodes demonstrated an outstanding PDCR (>10 7 at 25 °C) even at high temperatures (>10 4 at 300 °C) and under other extreme conditions without an external power supply.
Our study provides a fundamental strategy for tailoring the performance of optoelectronic devices using the band engineering of new oxide semiconductors.The proposed design rule for selfpowered heterojunction devices can be applied to the development of new types of highly durable UV photodetectors with lower power consumption.

Experimental Section
Heterojunction Photo-Diode Fabrication using Epitaxial Stannate Thin Films: All-stannate Sr(Sn,Ni)O 3 (SSNO, 60 nm)/BaSnO 3 (BSO, 80 nm) hetero-epitaxial films as an active layer were grown on a 100-nm-thick epitaxial 5% La-doped BaSnO 3 (LBSO) bottom electrode (BE)/(001)-oriented SrTiO 3 single-crystal substrates by pulsed laser deposition (PLD) in a chamber with base pressure of 10 −6 Torr.Then, 50 nm-thick LBSO top electrodes (TE) were deposited on the SSNO/BSO heterostructures.Rotating SSNO, BSO, LBSO targets synthesized by a solid-state sintering were irradiated by a KrF excimer laser ( = 248 nm, Coherent Compex Pro 102F) at a repetition rate of 5 Hz and a fluence of ≈1.1 Jcm −2 .The Ni composition in SSNO was fixed at 10% to fully incorporate Ni dopants in SrSnO 3 lattice framework.The nominally stoichiometric BaSnO 3 and SrSnO 3 films were optimized by adjusting laser plume dynamics for accurate cation stoichiometry, as described previously.The growth temperature was fixed to be 850 and 700 °C by the growth optimization of BSO (LBSO) and SSNO thin films, respectively.The electrical conductivity of 5% LBSO films are measured to be ≈6000 S cm −1 ; this degenerate semiconductor is sufficient for transparent electrodes to collect photo-generated carriers in this heterojunction devices.The thickness of each film was determined using X-ray reflectometry measurement.
To fabricate vertical photodetectors with TE-LBSO and BE-LBSO, circular area of SSNO/BSO heterostructures with 500 μm diameter (blue/green color in Figure 1d) and that of TE-LBSO with 420 μm diameter (yellow region in Figure 1d) were processed using photolithography (AZ 5214 and MA6 mask aligner) and subsequent etching with 37% HCl solution for about 80 seconds.This process enabled to define active layer of heterojunction devices and contact BE-LBSO (yellow area in Figure 1d).
Photocurrent Measurement: The photocurrent measurement was performed in a chamber probe station equipped with an electrical measurement system, temperature controller and a precisely controllable gas flow system.While two-terminal heterojunction photodetectors were uniformly illuminated using a broadband ultraviolet mirror module ( = 250-385 nm) of mercury xenon light source (Asasi Spectra, Max-350, 300 W), the photocurrent was measured between TE-LBSO and BE-LBSO using source meter unit of semiconductor parameter analyzer (Keysight, B1500A).Zero-bias transient I P -t response of this heterojunction devices was monitored by consecutive illumination of ultraviolet light with different intensity (P L = 1.6-100 mW cm −2 ) at different temperature between 25 and 300 °C.The wavelength-dependent spectral responsivity (R * ()) and detectivity (D * ()) of the device was measured by using a wavelength-resolved monochromator system (Asasi Spectra, CMS-100).
Characterization using X-Ray Diffraction, UV-vis-NIR Spectrophotometry and X-Ray Photoemission Spectroscopy: Symmetrical −2 X-ray diffraction scan was conducted using an in-house HRXRD (Bruker D8 Discover) with Cu K1 radiation ( = 0.15 406 nm).To obtain band gap of SSNO and BSO films, optical transmittance was conducted on SSNO and BSO films grown on MgO substrates at wavelengths ranging from 280 to 2500 nm using a UV-vis-NIR spectrophotometer (Perkin Elmer Lambda 750S).The photon energy dependence of optical transition strengths could be expressed by the Tauc equation, which was given by (h) 1/2 = A(h -E g ) for indirect transition, where  is the measured optical absorption coefficient, h is the photon energy, E g is the transition energy (or band-gap), and A is a proportionality constant.The photoemission spectroscopy (PES) measurements were performed to obtain both the positions of valence band maximum (E VBM ) and the onset of the secondary electron cutoff region using 4D PES beam line (PLS-II) of Pohang Light Source-II (PLS-II, Pohang, Republic of Korea) with 95 eV X-ray photon energy in an ultra-high vacuum chamber (3 × 10 −10 Torr) at 300 K.An external electrical bias of −5 V was applied to the sample to clearly analyze the low-kinetic-energy region.Before the measurement, ion sputtering was performed for 10 min under an Ar pressure of 4 × 10 −8 Torr at an anode voltage of 2.5 keV in the preparation chamber.The positions of valence band maximum (E VBM ) from Fermi level and work functions (Φ) were estimated from linear extrapolation of UPS spectra and from the relationship (Φ = h -(E F -E cut-off )), respectively.
Density Functional Theory Calculations: DFT calculations were carried out using Vienna Ab initio Simulation Package (VASP) code. [39]The cutoff energy of 500 eV was used for plane-wave basis set.For exchangecorrelation energy, a HSE06 hybrid functional was employed, [40] and the mixing parameter of Fock-exchange was set to 0.32 to reproduce the experimental band gap of SrSnO 3 .Every atomic coordination was relaxed until atomic forces became less than 0.05 eV Å −1 .For the Brillouin zone sampling, a (1/4 1/4 1/4) special k point was sampled for supercell calculations.To determine the V ′′ O concentration relating to the compensation in Figure 1b, the formation energy of V ′′ O following the procedure in previous literature is evaluated. [28,41]It is accounted for the oxygen-rich condition for the oxygen chemical potential, and the growth temperature was set to 700 °C.The ionized dopant concentration of Ni Sn was calculated by considering statistics of multicharge centers in semiconductors. [42]

Figure 1 .
Figure 1.Perovskite stannate heterojunction with nonlinear IV rectification behavior.a) Theoretical prediction of the charge transition level of the substitutional Ni dopant in the SrSnO 3 lattice framework, which shows the possibility of using Ni ′′ Sn as an acceptor.(Inset: crystal structure of substitutional Ni dopants in Sn sites).b) Estimated Fermi level of SSNO as a function of the Ni dopant concentration ([Ni Sn ]).c) Wide-angle range of −2 X-ray diffraction scan of all-stannate epitaxial TE-LBSO/SSNO/BSO/BE-LBSO heterostructure on STO substrates.d) Schematics (left) and plane-view (right) of SSNO/BSO vertical-heterojunction devices with a circular area.e) IV characteristics of SSNO/BSO heterostructures in the dark with linear (black) and log scales (blue), exhibiting strong nonlinear IV rectifying behavior.f) UV photoemission spectra (left) and secondary electron cut-off (right) of BSO (green) and SSNO (navy) for the valence band edge and work function, respectively.g) Tauc plots of BSO (green, top) and SSNO (navy, bottom) estimated using UV-vis optical transmittance spectra to obtain the indirect bandgap.h) Band alignment of the SSNO/BSO junction estimated using parameters from f and g before contact.

Figure 2 .
Figure 2. Performance of the self-powered SSNO/BSO wide-bandgap photodiode.a) Carrier dynamics illustrated by the band diagram of SSNO/BSO photodiodes under UV illumination.b) IV curves of the heterojunction photodiodes in the dark (black) and under UV light illumination (blue).c) IV curves as a function of the light power intensity (P L = 1.6-100 mW cm −2 ).d) Zero-bias transient I P -t photoresponse monitored by the consecutive illumination of UV light with different light intensities.e) Zero-bias photocurrent dependence of the light power intensity. denotes the slope of log (I P )-log (P L ) plot.f) wavelength-dependent spectral responsivity (R * ()) and detectivity.Single cycle of photoresponse to characterize the switching speed of the p-SSNO/n-BSO UV heterojunction photodiodes g) for rise time and h) for fall time.

Figure 3 .
Figure 3. Self-powered stannate UV photodiodes operated in extreme environments.a) Schematic of SSNO/BSO photodiodes operated at high temperatures.b) Self-powered modes of repeated I P -t photoresponses at different temperatures (25 °C ≤ T ≤ 300 °C) with the response to the alternate illumination of UV light (P L = 100 mW cm −2 ).High-temperature IV characteristics of photodiodes c) under UV illumination and d) in the dark.e) Plot of ln (I dark, rev /T 2 ) versus e/k B T at reverse bias to estimate barrier heights ( b ).f) Performance comparison of our p-SSNO/n-BSO photodiodes in terms of sensitivity at high temperatures (i.e., PDCR = (I P -I dark )/I dark )) with previously reported self-powered UV detectors using wide-bandgap semiconductors operated at high temperatures.g) Demonstration of p-SSNO/n-BSO photodiode operation in other extreme environments (i.e., air, vacuum, and immersed in water).Repeated I P -t photoresponses of p-SSNO/n-BSO photodiodes in the self-powered mode in extreme environments.