Ga₂O₃ is an ultra-wide bandgap semiconductor, which has a direct bandgap of 4.9 eV. Also, it has the superior thermodynamic and chemical stability, making it suitable for the development of deep ultraviolet optoelectronic devices. Ga₂O₃ exhibits the superior photosensitive and resistive properties, which has a promising application in sensor-memory integrated optoelectronic devices. In this paper, the crystal structure, composition, and morphology of the Ga₂O₃ thin film grown via pulsed laser deposition (PLD) was discussed, and then an optoelectronic device with Pt/Ga₂O₃/NSTO/In structure was designed and constructed. The physical mechanism of the Ga₂O₃-based device was investigated. The device process was optimized to achieve both high sensitivity ultraviolet photo-response and stable resistive switching characteristics. This work can provide theoretical guidance and technical support for the development of novel Ga₂O₃-based multifunctional optoelectronic devices.

Ga₂O₃ thin films were grown on (100) Nd∶ SrTiO₃ (NSTO) substrates by the PLD method. The target material used was a pure Ga₂O₃ ceramic target prepared by a solid-state method. Before thin film deposition, the ceramic target was pre-ablated for 3 min to remove surface contaminants. The film deposition was performed at pulse laser energy of 300 mJ, O₂ partial pressure of 3.0 Pa, and substrate temperature of 650 ℃. The target rotation speed was 5 r/min, the substrate rotation speed was 10 r/min, the pulse laser frequency was 5 Hz, and the deposition time was 30 min. Based on the deposited Ga₂O₃ thin films, a device with metal/Ga₂O₃/electrode sandwich structure was further constructed. At room temperature, Pt metal dot electrodes were fabricated on the surface of Ga₂O₃ thin films by a DC magnetron sputtering method as a top electrode of the device, and then weld the metal indium (In) directly onto the back of the NSTO substrate as a bottom contact electrode. The above Pt metal electrode preparation process was run at DC source power of 100 W, working pressure of 1.5 Pa, Ar flow rate of 10 mL/min, substrate rotation of 10 r/min, and sputtering time of 60 s.

The structure of Ga₂O₃ thin films was characterized by a model D8 Discover type high-resolution four circle single crystal X-ray diffractometer. The elemental composition of Ga₂O₃ thin film material was analyzed by a model Escalab 250Xi X-ray photoelectron spectroscope. The surface and cross-sectional morphology of the films was characterized by a model Sigma 500 field emission scanning electron microscope. A semiconductor electrical performance testing system was constructed by a model Keithley 2635 digital source meter, probe station, and electromagnetic shielding dark box, to examine the I-V scanning loop, multi-level resistive state, retention, and fatigue resistance characteristics of the device. A 255 nm LED light source was driven by a digital signal generator to test the time-dependent UV photoresponse characteristics. A Omni-λ 3027 setup consisting of monochromator, 150 W xenon lamp light source, chopper, collimating light converter, and phase-locked amplifier, was utilized to measure the dark current, spectral response, UV-visible suppression ratio, and detectivity of the photodetector.

The deposited β-Ga₂O₃ film with small particles and a flat and dense surface exhibits (004) plane preferential orientation. The Pt/Ga₂O₃/NSTO/In device with both fast self-driven ultraviolet photoresponse and stable resistive switching characteristics was fabricated. Under illumination, the photocurrent of the device rapidly increases to its maximum value and then gradually decreases to a steady state possibly due to the reduction in the Schottky junction built-in electric field caused by the injection of photogenerated carriers into the barrier region. The self-driven photoresponse is attributed to the efficient separation of photo-generated carriers by a built-in electric field formed at the Pt/Ga₂O₃ interface. On the other side, the device exhibits a stable bipolar resistive switching behavior. Alternating -5 V and +3 V pulse voltages with a 10 ms pulse width results in a resistive switching ratio of nearly 10⁴. A rapid switching between multi-level resistance states occurs at different pulse voltages. The intrinsic oxygen vacancy defect in Ga₂O₃ thin film as the trap center dominates the capture/release process of charge carriers, thereby maintaining the resistance state. Changing the pulse voltage can control the amount of electron injection, thus affecting the height of the interface barrier, which can explain the multi-level resistive characteristics of the device. The resistance state shows a good retention, and there is no significant degradation within 10⁴ s, showing the nonvolatile characteristics. In addition, the device also exhibits stable and fast switching characteristics after multiple repeats, indicating potential applications in memory devices. The mechanism of resistive switching effect can be attributed to the interface type resistance mechanism. The variation of the Schottky barrier caused by external electrical injection and the carrier trapping/detrapping process caused by the oxygen vacancy defect trap center jointly determine the resistive switching effect.

The fabricated device had a fast UV photoresponse (τ_r/τ_d=60 ms/120 ms) with a peak responsivity of 13.4 mA/W and 1.49×10¹¹ Jones (λ=250 nm) at 0 V bias. According to the analysis, the Schottky contact formed at the Pt/Ga₂O₃ interface resulted in a fast self-driven photoresponse at 0 V bias. In addition, the fabricated device also exhibited a stable bipolar resistive switching behavior with a high/low resistance ratio of 10⁴. The fast switching between multiple resistance state was achieved with the superior anti-fatigue and retention characteristics at different pulse voltages. The external electric field and oxygen-vacancy trap centers was considered as the main origin of resistive switching effect in Ga₂O₃-based optoelectronic device.