Ambipolarity Regulation of Deep‐UV Photocurrent by Controlling Crystalline Phases in Ga2O3 Nanostructure for Switchable Logic Applications

Photoelectrochemical photocurrent switching (PEPS) effect can regulate the polarity of photocurrent, which has significant potential applications in areas such as logic gates, photosynapse, and artificial intelligence. In this work, it is reported for the first time that a pure Ga2O3 photoelectrochemical system exhibits ambipolar photocurrent behavior induced by deep ultraviolet, which is closely linked to the crystalline phase of Ga2O3 (α or β) and the surface states of oxygen vacancies. Spongy porous nanorod arrays (NRAs) of Ga2O3 designed here not only increase the contact area of Ga2O3 with the electrolyte but also can lower largely the reflection of light and improve light‐trapping capacity. For α phase Ga2O3, the photocurrent is in a forward direction under positive bias and shows a backward direction under negative bias in NaOH solution, exhibiting a distinct ambipolar photocurrent phenomenon, which can be attributed to more oxygen vacancy surface states and lower potential barrier at the semiconductor/electrolyte interface. Furtherly, the effect of the surface states on the ambipolar photocurrent behavior of α‐Ga2O3 NRAs is demonstrated by various treatment times of oxygen plasma, whose switching point moves from 0 V to −0.19 V with treatment for 30 min and continues to move in the negative direction with the increase of treatment time. Moreover, based on the ambipolar photocurrent behavior of α‐Ga2O3 NRAs, adjustable Boolean logic gates with voltage are prepared as the input source, offering a new path for the photoelectric device multifunctional integration needed in the Post‐Moore era, with a high accuracy manipulated by solar‐blind deep ultraviolet light.


Introduction
The development of new optoelectronic devices is an important topic due to classical semiconductor devices reaching the limit. [1][2][3] However, the polarity of photocurrent, as an important degree of freedom, has been neglected in the past. The direction of photocurrent (positive or negative) in photoelectric chemical devices can be regulated by changing the wavelength of the incident light or the applied bias potential, which is called the photoelectrochemical photocurrent switching (PEPS) phenomenon. [4][5][6][7][8] To be specific, the operation principle of photoelectrochemical (PEC) not only follows the typical carrier generation, separation, and migration processes that occur in conventional solid-state photodetectors but also involves a unique electrochemical procedure that includes the ion diffusion process at the semiconductor/electrolyte interface and the redox reaction in solution. [9][10][11][12] Therefore, by adjusting the chemical reaction in the working process, the carrier migration and transport process in the PEC device can be manipulated artificially, thus changing the photocurrent polarity. This is also the basis for the application of PEC devices in logic gates, photoelectric synapses, and other information devices. [13] According to previous reports, the main research on ambipolar PEC is to carry out molecular synthesis, [14] hybridization, [15][16][17] and inorganic nanoparticle modification [18,19] on different crystalline phases [20] TiO 2 material systems, and realize binary [21] and recently ternary [22] logic gate functions under different conditions. The ultimate realization of the application of ambipolar PEC devices not only requires devices to exhibit excellent ambipolar photocurrent behavior but also requires photoanode materials to have superior photochemical stability. However, the main structure of the PEPS effect reported in the past, namely the TiO 2 -based composite, still has a certain degree of imperfection. [23][24][25][26] Firstly, the material is usually conditioned by ultraviolet (UV) or visible light as a light signal whereas sunlight and fluorescent lamps contain light in these wavelengths which may cause interference. [26] Secondly, the surface adsorption layer of such materials contains metal Photoelectrochemical photocurrent switching (PEPS) effect can regulate the polarity of photocurrent, which has significant potential applications in areas such as logic gates, photosynapse, and artificial intelligence. In this work, it is reported for the first time that a pure Ga 2 O 3 photoelectrochemical system exhibits ambipolar photocurrent behavior induced by deep ultraviolet, which is closely linked to the crystalline phase of Ga 2 O 3 (α or β) and the surface states of oxygen vacancies. Spongy porous nanorod arrays (NRAs) of Ga 2 O 3 designed here not only increase the contact area of Ga 2 O 3 with the electrolyte but also can lower largely the reflection of light and improve light-trapping capacity. For α phase Ga 2 O 3 , the photocurrent is in a forward direction under positive bias and shows a backward direction under negative bias in NaOH solution, exhibiting a distinct ambipolar photocurrent phenomenon, which can be attributed to more oxygen vacancy surface states and lower potential barrier at the semiconductor/electrolyte interface. Furtherly, the effect of the surface states on the ambipolar photocurrent behavior of α-Ga 2 O 3 NRAs is demonstrated by various treatment times of oxygen plasma, whose switching point moves from 0 V to −0.19 V with treatment for 30 min and continues to move in the negative direction with the increase of treatment time. Moreover, based on the ambipolar photocurrent behavior of α-Ga 2 O 3 NRAs, adjustable Boolean logic gates with voltage are prepared as the input source, offering a new path for the photoelectric device multifunctional integration needed in the Post-Moore era, with a high accuracy manipulated by solar-blind deep ultraviolet light.
ions or nanoparticles, which are easily oxidized or reduced (inorganic matter) and photodegraded (organic matter), and are unstable in an acid/alkaline environment. [27,28] Finally, due to the limitations of the TiO 2 material system itself, there is a small potential barrier at the solid/liquid interface and the ambipolar photocurrent behavior is inconspicuous. Therefore, the photoelectric anode of an ambipolar PEC device can still be optimized and improved by introducing new materials with robust features and suitable bandgaps. Ga 2 O 3 is an ultra-wide bandgap semiconductor, with an excellent chemical stability and a suitable conduction band (CB) energy level position, which is an ideal alternative material to TiO 2 for ambipolar photocurrent devices. [29,30] Due to the ultrawide bandgap of 4.9 eV, Ga 2 O 3 exhibits solar-blind photoelectric characteristics, meaning that it is photosensitive only to deep UV wavelengths less than 280 nm which doesn't exist on the surface of the earth. [31][32][33] As a result, the PEPS effect devices based on Ga 2 O 3 materials manipulated by solar-blind light, with the advantage of insensitivity to ambient light, will possess a high accuracy. Meanwhile, Ga 2 O 3 has a higher CB bottom and a lower valence band (VB) top than TiO 2 , which also means it has a larger potential barrier at the semiconductor/electrolyte solid/ liquid interface with a stronger redox capacity, contributing to the separation, and transport of the photogenerated carriers. Moreover, Ga 2 O 3 has excellent chemical stability, and the device can work stably in acidic or alkaline solutions. [34,35] It is well known that Ga 2 O 3 has a variety of isomerides, and different crystal structures have different surface states and surface active sites, which means that Ga 2 O 3 materials with different crystal phases will exhibit different ambipolar photocurrents in PEC devices. Because the surface states and surface active sites can directly affect the transport kinetics and chemical reaction process of photogenerated carriers. [36,37] In this paper, we systematically investigated PEPS effect in Ga 2 O 3 PEC system with different crystalline phases (α or β) and explains the reasons for the differences through oxygen plasma treatment experiments. After all these, we observed and reported the ambipolar photocurrent behavior of α-Ga 2 O 3 in NaOH solution for the first time and constructed XOR/OR logic gates.

Results and Discussion
Ga 2 O 3 is known to have multiple isomers with various band gaps, energy level positions, surface states, and surface active sites, which will lead to the different energy band bending amounts in the solid (Ga 2 O 3 )/liquid (electrolyte) interface. Therefore, Ga 2 O 3 with various crystal phases may exhibit different polarity of photocurrent in PEC devices theoretically, as is shown in Figure 1a with α and β phases. Based on this, we consider to adjust and control the ambipolar photocurrent behavior by different crystal phases of Ga 2 O 3 and prepare PEC photocurrent switching devices with a high accuracy manipulated by solar-blind deep ultraviolet light. To achieve the above experimental design and to prepare a better PEC logic device, we designed Ga 2 O 3 nanorod arrays (NRAs) with a spongy porous structure here, which can not only increase the contact area of α-Ga 2 O 3 with the electrolyte as the electrolyte can immerse in the inside of NRAs but also can lower largely the reflection of light and improve light-trapping capacity (Figure 1b). The top-view and cross-sectional Field-Emission Scanning Electron Microscope (FESEM) of as-grown α-Ga 2 O 3 NRAs are shown in Figure 1c(i),(ii), respectively. The nanorods have an average length of 1.72 µm and a diameter ranging from 100 to 300 nm, with a prism-like tip and a sponge-like porous structure inside. The prism-like tip of the NRAs can be attributed to the orthorhombic structure of GaOOH NRAs. And the porous structure like sponge forms inside the NRAs due to the dehydration process of GaOOH NRAs, as described by 2GaOOH ⇒ α-Ga 2 O 3 + H 2 O, as shown in the bright-field Transmission Electron Microscope (TEM) image in Figure 1c(iii). Porous nanostructure with a hollow interior is pretty evident in an enlarged view of Figure 1c(iv). The spacing between two adjacent lattice fringes is calculated to be 0.222 nm (Figure 1c(v)), which corresponds to the (006) plane of α-Ga 2 O 3 with the corundum structure. Through the transmittance of the FTO substrate, the absorption edge can be seen at about 300 nm and the band gap of FTO is calculated at about 3.72 eV (Figure 2f). This means that deep UV light with the wavelengths below 300 nm cannot pass through the FTO substrate to Ga 2 O 3 NRAs. Thus, we construct the PEC photoelectric performance test system with the working electrode of Ga 2 O 3 NRAs grown on FTO facing the LED lamp to absorb as much UVC light as possible, as shown in Figure 3a.  where (V OC ) max is the maximum open-circuit potential, V fb is the flat band potential, and V redox corresponds to the redox potential of the solution. [37,38] The Voc data value of the oxygen plasma treated α-Ga 2 O 3 is larger than that of untreated α-Ga 2 O 3 while less than that of β-Ga 2 O 3 , which gradually increases with the oxygen plasma treatment time. It means that these surface states brought by oxygen vacancies of α-Ga 2 O 3 gradually reduce www.advelectronicmat.de as the oxygen treatment time increases. As a result, the band bending increases gradually, which will prevent these photogenerated electrons from entering the solution. Therefore, the negative photocurrent phenomenon disappears gradually and the ambipolar switching potential moves in the negative direction. These results further prove that the surface states caused by oxygen vacancies are the key factors affecting the ambipolar photocurrent. Figure 4a illustrates the energy band diagram of α-Ga 2 O 3 and β-Ga 2 O 3 in NaOH electrolyte solution for explaining their difference in the PEC photoelectric phenomenon. The photogenerated electrons in the CB of Ga 2 O 3 induced by 255 nm illumination either enter the electrolyte solution to produce a negative photocurrent or through the FTO substrate reach the external circuit to produce a positive photocurrent. In particular, the amount of band bending of Ga 2 O 3 determines whether these photogenerated electrons can transport from the CB to the electrolyte for the reduction reaction, leading to the negative photocurrent. For α phase Ga 2 O 3 (Figure 4a(i)), the photogenerated electrons in the CB can either enter into the NaOH solution through the space charge region as well as pass the FTO substrate reaches the external circuit, while the photogenerated electrons of β phase Ga 2 O 3 (Figure 4a(ii)) can merely traverse the FTO substrate to reach the external circuit. It can be attributed to more surface states of oxygen vacancy ( Figure 2e) and lower potential barrier at the semiconductor/ electrolyte interface in α-Ga 2 O 3 NRAs than β-Ga 2 O 3 , which provides electron transport channels that allow these photogenerated electrons can enter the electrolyte. Oxygen vacancy defects on the surface of Ga 2 O 3 cause Fermi level pinning, which results in the decrease or failure of the constructed solid/liquid interface junction. [38,39] Thus, the photogenerated electrons of α-Ga 2 O 3 can jump the space charge region at the interface and enter into the NaOH solution to capture the O 2 (e − + O 2 → O 2 *− ) with the applied bias potential tuning from -0.6 to 0 V. However, for β phase Ga 2 O 3 , there is less surface states of oxygen vacancy (Figure 3e), which results in a larger built-in electric field and a higher potential barrier between NRAs and solution. These photogenerated electrons merely shift to the FTO substrate and cannot jump the space charge region to the solution due to the high barrier within our regulated voltage range (−0.6 to 0.6 V), thus it was not observed the ambipolar photocurrents phenomenon in β-Ga 2 O 3 PEC system. For the α-Ga 2 O 3 , after the oxygen plasma treatment (Figure 4a(iii)), the surface state of α-Ga 2 O 3 is reduced allowing a gradual increase in the amount of energy band bending as The ambipolar photocurrent behavior of α-Ga 2 O 3 /FTO in NaOH electrolyte solution can be regulated and controlled by bias potential and wavelength of incident light (Figure 4b(i-iv)). For α-Ga 2 O 3 , the amount of band bending is −0.136 V, and the energy level position of the band is 0.14 V versus the reducing hydrogen electrode ( Figure S6, Supporting Information), so the reversing voltage position is at 0 V. With the applied bias potential is >0 V, the energy level position of the external circuit is higher than the amount of band bending of α-Ga 2 O 3 in NaOH solution (Figure 4b(i,ii)). For the illumination of 255 nm light, both α-Ga 2 O 3 NRAs and FTO can trigger a photoelectric effect and produce photogenerated electrons, and the vast majority of these electrons will pass the FTO substrate transfer to the external circuit to produce a positive photocurrent (Figure 4b(i)). While for the illumination of 310 nm light, merely FTO produces photogenerated electrons which are also transmitted to the external circuit, leading to a positive photocurrent (Figure 4b(ii)). However, with the applied bias potential is less than V switch , the energy level position of the external circuit is higher than the amount of band bending of α-Ga 2 O 3 in NaOH solution. In this case, the vast majority of photogenerated electrons will enter the electrolyte solution to reduce with O 2 , leading to the negative photocurrent in PEC devices. However, these photogenerated electrons from α-Ga 2 O 3 NRAs are the exclusive ones that can enter into the NaOH solution although both α-Ga 2 O 3 and FTO produce photogenerated electrons when 255 nm light illuminates, due to the CB energy level of FTO is lower than the reduction energy level position of O 2 (Figure 4b(iii)). Under 310 nm light illuminates, these photogenerated electrons from FTO neither enter the solution nor the external circuit and recombined with the hole in VB, so no photocurrent generates (Figure 4b(iv)).
Our result of ambipolar photocurrent behavior in Ga 2 O 3 materials provides a new path for the photoelectric device multifunctional integration needed in the Post-Moore era, with a high accuracy manipulated by deep UV light. Herein, to explore the potential application of the ambipolar photocurrent behavior of α-Ga 2 O 3 /FTO in NaOH electrolyte solution, we prepared adjustable Boolean logic gates with voltage as the input source ( Figure 5). Deep UV light sources are denoted here as inputs, 255 nm light is A and 310 nm light is B. If the corresponding light source is switched ON and illuminates photo-electrode α-Ga 2 O 3 NRAs, this input is 1, otherwise, it's 0. The photocurrent is recognized as the output and is considered a 1 if it's >−3.2 µA or <−3.5 µA, otherwise, it is considered a 0.
At −200 mV versus SCE, illumination with LED light (255 nm, 500 µW cm −2 ) results in negative photocurrent, which is consistent with electrons transferring into solution and then reducing with electron receptors in NaOH solution. Illumination with LED (310 nm, 500 µW cm −2 ) gives positive www.advelectronicmat.de photocurrent due to electron transfer to the external circuit structure. Simultaneous illumination with two LEDs yields zero net currents as negative and positive photocurrents compensate effectively to prepare an XOR logic device (Figure 5a).
At positive potentials, illumination with 255 nm light gives positive photocurrent phenomenon, as well as the 310 nm one. At +200 mV, photocurrent output under the influence of two light inputs (255 nm and 310 nm) follows OR logic giving positive output if at least one of the inputs is positive [ (Figure 5b]. Figure 5c demonstrates the logic system in which characteristics can be changed by way of the photocurrent regarded as programming output and the corresponding truth table is presented. In the case of lighting sources and bias potential as inputs, the OR/XOR logic is implemented based on ambipolar photocurrent behavior. This behavior is quite unique in photochemical logic systems, which can prepare not only photochemical logic devices but also photochemical synapses and artificial intelligence devices, etc. [40,41]

Conclusions
In this work, the α phase and β phase of porous Ga 2 O 3 vertically aligned NRAs are grown on FTO substrates through a simple and low-cost method of hydrothermal and post-annealing treatments. Meanwhile, the Ga 2 O 3 nanostructure was used as photoanode to construct novel photochemical devices. In NaOH solution, α-Ga 2 O 3 NRAs PEC photoanode exhibits an ambipolar photocurrent with a switching point at 0 V while β-Ga 2 O 3 device shows constants positive photocurrents, which can be attributed to more surface states of oxygen vacancy and lower potential barrier at the semiconductor/electrolyte interface in α-Ga 2 O 3 than β-Ga 2 O 3 . After that, α-Ga 2 O 3 was treated with oxygen plasma by different time lengths and combined with OCPT curves, the influence of surface states on ambipolar photocurrent behavior was explained in detail. More importantly, the ambipolar photocurrent behavior of α-Ga 2 O 3 /FTO in NaOH electrolyte solution can be regulated and controlled by bias potential and wavelength of incident light, which can Light sources are inputs. The choice between XOR and OR function is determined by programming input of potential bias. At +0.2 V OR logic is realized, at −0.2 V is XOR logic. The corresponding truth table is presented.
www.advelectronicmat.de provide a new path for the photoelectric device multifunctional integration needed in the Post-Moore era. Herein, we modulated the polarity and magnitude of the photocurrent of α-Ga 2 O 3 /FTO using 0.2 V/−0.2 V and 255 nm/310 nm as the bias potential and light source respectively, achieving adjustable Boolean logic gates (OR/XOR).

Experimental Section
The fluorine-doped tin oxide layer on the surface of the commercial FTO substrate (Shanghai Daheng Optics and Fine Mechanics Co., Ltd) was used as the seed crystal layer for the growth of NRAs. FTO was reclined inside a container of polytetrafluoroethylene (PTFE) under the optimum growth condition of a 0.2 g/40 mL Ga(NO 3 ) 3 aqueous and heated at 150 °C for 12 h in an oven. The FTO substrates have been treated with acetone, anhydrous ethanol, and deionized water in turn before use. The α-Ga 2 O 3 and β-Ga 2 O 3 NRAs were obtained by annealing GaOOH NRAs at 450 and 700 °C, respectively in atmospheric environment for 4 h. The LSV curves of FTO at different temperatures of annealing in NaOH solutions ( Figure S7, Supporting Information) to prove annealing at different temperatures has little effect on FTO. Plasma-enhanced chemical vapor deposition was employed to treat the α-Ga 2 O 3 NRAs in this work, oxygen as the oxidizing gas, and the plasma processing power is 50 W. The treating temperature was kept at 25 °C, while the oxygen flow rate was kept at 100 sccm. [42] The morphologies were characterized on a FESEM and a TEM. The crystalline structure was examined on an XRD and UV-Vis absorption spectra. The surface compositions were checked on an X-ray photoelectron spectroscopy (XPS) and the Raman spectra were measured on a high-resolution spectrometer equipped with a microscope. XPS (Thermo Fisher-K-Alpha) was used to analyzed the chemical composition of the prepared Ga 2 O 3 . The XPS instrument kept the base pressure better than 3.5 × 10 −7 mTorr when used to acquire core-level XPS spectra from the sample. The spectrometer used Al Kα excitation source (hν = 1486.6 eV), and the electron emission angle was 90°. The size of the analyzed area was a circle of 500 µm in diameter. Samples were sputter-cleaned with an Ar + ion beam, where the Ar + incidence angle was about 50° with the Ar + energy of 1000 eV. The scanning speed is 1 eV per step, and the passing energy is 100 eV. Charge neutralizer was used. The binding energy was calibrated by the work function method. FTIR spectra were measured on a Nicolet 5700 FTIR spectrometer. Several LEDs with wavelengths of 255, 310, 365, 390, and 405 nm were used to illustrate the photoelectrode that controls light ON and OFF.
The Ga 2 O 3 NRAs grown on FTO substrate were used as the working electrode, platinum foil (10 × 10 mm 2 ) as counter electrode, and saturated calomel electrode as the reference electrode. All LED lights with a power of 500 µW were used as the illumination source. A 0.5 m NaOH aqueous solution was used as the electrolyte. The active area of Ga 2 O 3 NRAs/FTO photoanode was fixed at 1 cm 2 . The Mott-Schottky plots and measurements were carried out at 1.0 kHz in dark.

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