Giant Switchable Persistent Photoconductivity in Soft Chemistry Reduced SrTiO3

High tunability of photoconductivity is highly desired for applications in optical memories, sensors, and bioelectronics. Recently, room temperature persistent photoconductivity (PPC) in SrTiO3 (STO) has been revealed and has attracted great attention. However, reversible switching of the PPC in STO with a large on/off ratio remains challenging to date. Here, a giant switchable PPC in soft chemistry reduced STO is reported. An initial insulator‐to‐metal transition with on/off ratio up to 7 orders of magnitude is observed and about 5 orders of magnitude transition is found to be reversible. Via nuclear magnetic resonance measurements, it is uncovered that such unusual PPC is driven by the generation of excess carriers accompanied with a configuration evolution of the incorporated hydrogen from hydridic HO+ to protic Hi+ upon illumination. The work demonstrates giant switchable PPC transition in soft chemistry reduced perovskite oxides, providing a new platform for pursuing high performance sensors and nonvolatile optoelectronic memory devices.

ZnO. [46,47] In the case of the PPC in STO, the instability associated with substitutional hydrogen H O + under illumination was suggested to be account for this unique characteristic by density functional theory calculations, [48] but direct experimental evidence is still lacking. Moreover, reversible and effective switching of PPC in STO with larger on/off ratio remains to be explored.
In this work, we introduce hydride ions into STO single crystals by soft chemistry reduction method [49,50] and observe a giant tunable long living PPC effect triggered by illumination with an unprecedented conductivity enhancement up to 7 orders of magnitude. The 1 H NMR measurements reveal that the persistent resistive transition is driven by the configuration evolution of hydrogen from hydridic H O + to protic H i + upon illumination. Reversible switching of the conductance with 5 orders of magnitude is demonstrated via alternative thermal relaxation and illumination, which hold promising potential applications in sensors and nonvolatile optoelectronic memory devices. Figure 1 shows the schematic diagram of the whole experimental concept. A series of 5 × 5 × 0.5 mm 3 (001) STO single crystal samples were reduced to SrTiO 3−x H x via a soft chemistry reduction process by annealing in a silica tube that sealed with CaH 2 powder under ≈420-530 °C (as shown in Figure 1a). We found that the CaH 2 -annealed SrTiO 3−x H x single crystal has a reversible switching behavior of PPC under illumination (Low R s state, LRS) and thermal treatments (High R s state, HRS), as illustrated in Figure 1b. After being CaH 2 -annealed, the SrTiO 3−x H x single crystals were then exposed to a light of 405 nm and eventually obtained LRS. After a thermal treatment, the SrTiO 3−x H x single crystal could switch to the high R s state (HRS) and the resistance is out of the measurement range (>40 MΩ), which is the same as the original STO single crystals before introducing hydridic ions. More quantitative details of sheet resistance will be discussed in Figure 2. Figure 2a is the photoresponse of the as-annealed samples treated in ambient and vacuum conditions. The sheet resistance of CaH 2 -annealed SrTiO 3−x H x single crystal is 54 KΩ sq −1 in normal environment and barely changes in dark environment. After exposure to 405 nm illumination, the sheet resistance suddenly dropped to 0.04 KΩ sq −1 . After turning off the light, the increased conductivity sustains and shows little attenuation within the measurement timescale. Since oxygen vacancies can act as shallow donors and introduce n type charge carriers in STO, the formation of oxygen vacancies has been considered to be the cause of PPC effect in several oxide material systems such as Hf-In-Zn-O, [51] In 2 O 3 , [11] BaSnO 3 [52] etc. The oxygen in atmosphere may influence the PPC behavior, [18] and the generation and recombination of surface oxygen vacancies upon exposure to ultra violet (UV) light has been used for the manipulation of the photoconductivity in perovskite stannates. [53] As shown in Figure 2a, however, the same long living PPC behavior was observed in both ambient and vacuum conditions at room temperature in our experiments, excluding the formation of oxygen vacancies during illumination from the possible origins. Moreover, thermal treatments for STO single crystals in ultrahigh vacuum were also performed to intentionally create oxygen deficient samples. In the inset of Figure 2a, the vacuum-reduced STO samples with different levels of oxygen vacancy concentrations exhibit no such giant PPC effects. Therefore, the driving force for the giant PPC effects observed in our CaH 2 -reduced STO is most likely hydrogen incorporation instead of oxygen vacancies only. As shown in Figure S1 (Supporting Information), the mobility is about 1.2 cm −2 V −1 s −1 at room temperature, which is comparable with that of epitaxial LaAlO 3 /STO heterostructures (≈3-5 cm −2 V −1 s −1 ). [54][55][56][57] To characterize the PPC effect in CaH 2 -annealed STO, we extended the measurement timescale monitoring the resistance change as shown in Figure 2b. R o is the initial resistance for CaH 2 -annealed STO sample that has just been illuminated in a light with wavelength of 405 nm. After turning off the light, the resistance for the illuminated sample experienced a relatively quick increase in the first 2 h, followed by a flat and slow increase afterward. The normalized conductivity decay behavior can be described using a two-stage exponential function [16,18,19,52]

Results and Discussion
Where τ 1 and τ 2 are the two relaxation time constants for the charge carriers. In the fitting of the data with Equation (1), Adv. Electron. Mater. 2023, 9,2300068

(b)
Optical fiber Figure 1. Schematic diagrams of switchable persistent photoconductivity in soft chemistry reduced STO. a) hydrogen absorption into STO single crystal by annealing in a silica tube sealed with CaH 2 powder under a high vacuum of ≈10 −3 Pa and b) a reversible switching between high and low resistance states in reduced STO single crystal by thermal annealing and light illumination. τ 1 and τ 2 are ≈45 min and 1036 h, representing the initial fast decay and the subsequent slow decay, respectively. These relaxation times are larger than the time constants in most of the systems that exhibit PPC effect, [9,15,58,59] indicating the stability of PPC in CaH 2 -annealed STO at room temperature. The relaxation process of the photoconductance is dependent on the traps that inhibit the recombination. Here, the relatively fast decay component is attributed to the photogenerated effect caused by deep level traps [60] introduced in the annealing process, such as the hydrogen related energy levels and oxygen defect complexes. And the origin of longer decay component is most likely due to the large barrier caused by the repulsion between charged oxygen vacancies and interstitial hydron ions, [48] which will be discussed later.
To investigate the wavelength dependence of photoconductivity, we use monochromatic light of successively increasing photon energy to illuminate the as-annealed samples and monitor their resistance simultaneously. As depicted in Figure 2c, in the low energy range, only small changes were observed in the resistance values. Until exposure to the threshold energy at about 3 eV, which is slightly smaller than the bandgap energy, the resistance began to decrease markedly. At last, it came to a low resistance state after the sample was exposed to higher energy light illumination. The threshold energy can be explained by the electron excitation from H O + related in-gap levels to the conduction band, which has been proposed by theoretical calculations. [48] To further investigate the wavelength dependence of long lasting photogenerated carrier, free carrier absorption spectrum was also collected after each exposure to monochromatic light of successively increasing photon energy for 5 min. In Figure 2d, it shows a clear enhancement of the absorbance after illumination. A threshold photon energy about 3 eV was also obtained, which is consistent with the value observed from our transport measurements. In order to investigate the light intensity dependence of the resistance, we measured the resistance of a series of CaH 2 -annealed SrTiO 3−x H x at various light intensities under an illumination of 405 nm. And the resistance is nearly saturated at a light intensity of around 6 W m −2 (shown in Figure S2, Supporting Information).
Thermal treatments in ambient condition were performed to explore the switching behavior of the PPC effect in STO (Figure 3). For a typical material with PPC effects, an energy barrier between the metastable state and the initial state restrains the relaxation of delocalized charge carriers after the illumination. As shown in Figure 3a, a series of annealing processes under ambient dark condition with different maximum temperature has been performed on the same sample. At the end of each annealing cycle, the sample was exposed to light illumination of 405 nm wavelength at room temperature. Throughout the whole process, the resistance was monitored with a reading voltage of 0.5 V and I-V curves were recorded at room temperature before and after every illumination (Figure 3a-c). As shown in Figure 3a,c, at higher annealing temperature, the thermal treatment can drive the sample from metallic to  insulating state. An afterward light illumination restores the sample back to metallic state due to the PPC effect ( Figure 3b). Compared to the pristine CaH 2 -annealed sample, the thermal annealing in ambient condition can enhance the PPC transition and reach an on/off ration as large as 7 orders of magnitude in the sample annealed in 240 °C (Figure 3b), which is among the highest on/off values achieved in persistent photoconductors and phototransistors. [7,8,17,19] As shown in Figure 3d, reversible switching of the resistance states between high and low conductance with 5 orders of magnitude on/off ratio is clearly seen when the annealing temperature is 200 °C, which is about two orders of magnitude higher than the PPC effect previously reported in STO. [38] The resistance of CaH 2 -annealed SrTiO 3−x H x single crystal is 1.6 × 10 4 KΩ after a thermal treatment and then dropped to 0.245 KΩ upon exposing to 405 nm illumination, which can be calculated from Figure 3d. We now turn to discuss the mechanism for the observed PPC effects. Recent experiments indicated that oxygen vacancy and hydrogen are indispensable for the PPC effect observed in STO crystals. [39] Oxygen vacancy is generally considered to be doubly charged as shallow donors, while H is ubiquitous and can be incorporated into oxides with diverse configurations. A model regarding hydrogen configuration has been recently proposed, [48] in which the excitation of electron from hydrogen related levels in titanates upon illumination is suggested to induce the long lasting PPC effect, as the schematic shown in Figure 4a. [48,61] Considering the negatively charged hydride is incorporated in the substitutional site (H O + ), upon exposure to light higher than the activation energy of H + (≥2.9 eV), [38,48] one electron from the H O + related level is excited to the conduction band. This is consistent with the threshold energy (≈3 eV) in our experimental results as shown in Figure 2c. The proton left in the oxygen vacancy site is metastable and easily moves out to interstitial site, where it is stabilized by forming OH bond or trapped by V Sr . The excess carriers induce an increase in conductance which can sustain after the light illumination is turned off. At room temperature, the reset process is suppressed by the energy barrier induced by the metastable state, according to the hydrogen related model, as depicted in Figure 4b. [38] While at elevated temperature, the reset process can be enabled by thermal excitation. While this hydrogen related model can explain the observed phenomena, however, direct experimental evidence is still lacking up to date. 1 H NMR spectroscopy, which is effective to resolve the local structure of the hydrogen nuclei and distinguish hydride and hydroxide ions in solid materials, [45,62,63] Mis was used to provide direct experimental support of the model. The samples were grinded to powder and the high resolution 1 H NMR spectra were collected at magic-angle spinning (MAS) condition. In order to clarify the hydrogen ion evolution upon illumination, 1 H NMR measurements were carried out for the as-annealed and illuminated samples after the same soft  can be observed, which should be ascribed to H species in surface hydroxyl and residual Ca(OH) 2 [64] and are thus not intrinsic to the sample composition. Instead, we focus on those features that are clearly different from the reference signals. For the as-annealed sample, there is a broad peak centered at about −4 ppm, and this signal was also observed in CaH 2 -annealed BaTiO 3 sample and attributed to the hydrogen in the substitutional site with −1 valence state. [62,63,65] In the spectrum of the illuminated sample, this hydridic signal disappears and at the same time the weight of resonance centered at around 5.2 ppm is significantly enhanced compared to the as-annealed STO. The enhancement of this protic signal indicates that a great amount of structural hydrogen in the interstitial site with +1 valence state appears. [61,62,63] As such, the spectral weight shift in our 1 H NMR measurements provides direct evidence for the evolution of hydrogen valence state and configuration as depicted in Figure 4a

Conclusion
In summary, we report the observation of switchable giant PPC effect with extremely long retention time in soft chemistry reduced STO. By light illumination and thermal annealing, reversible metal-insulator transition with an on/off ratio of the corresponding resistive states up to 5 orders of magnitude was achieved. The possibility that the long lasting PPC driven by oxygen vacancy is excluded. The 1 H NMR measurements directly reveal that the evolution of hydrogen configuration upon illumination is responsible for the PPC effect. The room temperature switchable giant PPC effect in STO is of great interest among applications of sensors and nonvolatile optoelectronic devices.

Experimental Section
Sample Fabrication: Verneuil-grown STO bulk single crystals were from MTI Corp. Double side polished samples were sealed in an evacuated fused silica tube, embedded with 0.3 g of CaH 2 powder and annealed at moderate high temperatures in the range of ≈420-530 °C for 2 h. After the reduction reaction in hydride, residual Ca compounds (CaH 2 and CaO) at the surface of samples were removed by washing with isopropanol, and samples were naturally dried at nitrogen-flowing environment.
Photoelectric Characterizations: UV-visible spectrometry (Shimadzu UV3600) with Halogen lamp source was used to collect the optical absorption spectra. A light emitting diode (wavelength 405 nm) was used for illumination source. In order to investigate the wavelength dependence of PPC effect, excitation light for fluorescence spectrometer (PerkinElmer, LS55) was used to provide monochromatic illumination. Infrared absorption spectra were measured using a FTIR spectrometer (Thermo Fisher Scientific, Is50). High temperature electrical measurements were performed in a customed heating cavity with electrical feedthrough connected to multi-Source-Meter (Keithley, 2600), the sample could be illuminated by light through a quartz window.
Transport Measurements of SrTiO 3−x H x Sample: A four-probe van der Pauw method was used for transport measurements with ultrasonically wire-bonded aluminum wires as electrodes, which can ensure to create ohmic contact with the 5 × 5 × 0.5 mm 3 STO single crystals (schematic diagram as shown in Figure 1b. Temperature-dependent sheet resistance (R s ) and Hall effects of the CaH 2 -annealed SrTiO 3−x H x sample was measured in a Quantum Design physical property measurement system (PPMS).
NMR Spectroscopy: The as-annealed and illuminated single crystal samples were grinded to powder and packed into 3.2 mm ZrO 2 NMR rotors inside a N 2 -filled glove box. 1 H MAS NMR spectra were recorded at a Larmor frequency of 400.0 MHz, on a 9.4 T Bruker Avance III spectrometer. 1 H chemical shifts were referenced to trimethylsilane (TMS) at 0.0 ppm.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.