【Member Papers】Continuous Tunable Energy-Band Tailoring Boosts Extending Sensing Waveband Based on (InₓGa₁₋ₓ)₂O₃ Solar-Blind Photodetectors

      The ultra-wide bandgap semiconductor gallium oxide (Ga₂O₃) shows great application potential for solar-blind UV photodetection, but the wide bandgap also results in low conductivity, causing electrical signals arising from PDs are always too weak to be transmitted in practical applications. Moreover, the bandgap of Ga₂O₃ astricts the detecting wavelength of PDs to be only about one-third of the solar-blind waveband, failing to achieve full range detection for the solar-blind UV light, which results in limitations in practical applications, such as the inability to accurately match the absorption peak wavelength of the given micro-organism or molecule in biochemical sensors. In addition, controllable bandgap tuning is very beneficial for the construction of heterojunctions. Currently, introducing metal atoms with similar structure to Ga atoms is the most commonly used bandgap tailoring method for Ga₂O₃. Both In and Ga belong to the III-group, sharing similar electronic structures and comparable atomic radii. Moreover, the optical bandgap of In­₂O₃ is about 3.7 eV, which is smaller than that of Ga₂O₃ (4.2-5.3 eV), so it is expected that the energy band tailoring of Ga₂O₃ can be achieved by constructing (In_xGa_1-x)₂O₃ alloys.

      In this work, we propose a novel method to prepare (In_xGa_1-x)₂O₃ alloy thin films by using pure metal reaction sources (metal Ga and metal In) in a plasma-enhanced chemical vapour deposition (PECVD) system and achieve continuous tunability of the bandgap by varying the In doping content.

      We prepared nine alloy films with different In content, and the XRD results of all the samples are shown in Figure 1(a), from which it can be observed that when the In content x is larger than 0.2, a phase segregation appears in the films, and films with smaller In content only show diffraction peaks of β-Ga₂O₃. By zooming in on the (222) plane diffraction peaks of In₂O₃ and (In_xGa_1-x)₂O₃ films with x ≥ 0.3 (Figure 1(b)), it is observed that the peaks show a tendency to shift to a smaller 2θ angle as the In content is increased, which implies an increase in the interplanar distance, and it is more visually illustrated by the lattice spacing measured from TEM image in Figure 1(e). In addition, from the XPS results, both In and Ga elements can be observed, and by fitting the O 1s spectra, it is found that both the oxygen vacancies and the surface adsorbed oxygen in the films show an increasing trend with the increase of the In content, as shown in Figure 1(c)(d).

      As shown in Figure 2, the bandgap of (In_xGa_1-x)₂O₃ thin films can be continuously adjusted in the range of 3.91-4.91 eV. The responsivity peaks of the alloy-based metal-semiconductor-metal (MSM) photodetectors lie between 254-295 nm and show a red-shift trend with the increase of In content, which proves that the doping of In atoms can play an effective role in modulating the bandgap as well as the detection band region of Ga₂O₃. As shown in Figure 3, the performance of some alloy photodetectors is more superior compared to Ga₂O₃ photodetectors, and the PDCR of (In_0.02Ga_0.98)₂O₃ device is as high as 10⁶, which can be attributed to the synergistic effect of multiple factors such as variations in bandgap, oxygen vacancies, lattice defects, and grain boundary barriers as depicted in Figure 4(a)(b). In order to explore the potential applications of (In_xGa_1-x)₂O₃ alloy photodetector devices, we further investigated their deep-ultraviolet imaging capabilities. The schematic of the imaging test system and the image sensing results of the three devices are shown in Figure 4(c)(d), respectively. The photocurrent of the Ga₂O₃ photodetector is too small to be transmitted in practical applications, which will result in a low image resolution. The alloy photodetector, on the other hand, produces a much larger photocurrent, resulting in a clearer image.

      This study achieves the continuous tunability of the bandgap of (In_xGa_1-x)₂O₃ alloy films and the broadening of the detection band region of the corresponding photodetectors through the introduction of In atoms into Ga₂O₃ for energy band tailoring, providing a new idea for high-performance full solar-blind UV detection.

Figure 1. (a) XRD images of Ga₂O₃, In₂O₃, and (In_xGa_1-x)₂O₃ thin films. (b) Zoom-in XRD images around the (222) diffraction peak of In₂O₃ and three (In_xGa_1-x)₂O₃ films. (c) XPS spectrum of Ga 3d and In 4d of (In_0.08Ga_0.92)₂O₃. (d) Calculated oxygen vacancies and chemisorbed oxygen concentrations in all samples. (e) HRTEM image of (In_0.08Ga_0.92)₂O₃ film, the inset is the corresponding SAED pattern.

Figure 2. (a) Tauc’s plot of In₂O₃, Ga₂O₃, (In_xGa_1-x)₂O₃ films. (b) Normalized response spectra of Ga₂O₃ and (In_xGa_1-x)₂O₃ PDs.

Figure 3. (a) I–V, (b) I–t curves, (c) PDCR, (d) R, D^*, EQE of Ga₂O₃ and (In_xGa_1-x)₂O₃ PDs. The wavelength of the light source was 254 nm, the light intensity was 420 μW/cm², and the illumination area was 0.25 mm².