Ultraviolet (UV) detection technology has emerged as a popular photodetection technology in recent years, following laser and infrared detection technologies^[1,2]. It has important applications in both military and civilian fields, including ozone hole monitoring, space communication, flame detection, and missile detection^[3–5]. As a n-type semiconductor with an ultrawide bandgap (4.9 eV), gallium oxide (Ga2O3) is a natural UV detection material^[6–8]. Moreover, it also has excellent stability with a very high breakdown field, high electron saturation rate, and strong radiation resistance, which enables it to become the preferred material for high-voltage-resistance electronic devices and deep UV photoelectronic detectors^[9–11]. In recent years, there have also been many reports on deep UV solar-blind detectors based on Ga2O3^[12,13]. Although Ga2O3 has excellent photoelectric properties and stability, due to its inherent electronic structure and large energy bandgap, it has disadvantages such as low responsivity, slow response speed, and large dark current^[14–16]. Additionally, the insufficient light absorption ability under low light intensity limits its photodetection ability in dim light environments^[17–19].

To optimize device performance, various strategies have been adopted. For example, in constructing a pn junction structure, the built-in electric field can significantly promote the carrier separation and transport characteristics in the interfacial depletion region^[20]. However, it is extremely difficult to achieve p-type doping of Ga2O3^[21,22]. In previous studies, efforts have also been made to construct pn heterojunctions by combining n-type Ga2O3 with p-type semiconductors, such as NiO, SiC, Si, and GaN^[7]. A two-dimensional (2D) Bi thin film is a p-type semiconductor^[23]. It has good stability in the air, and it possesses an exceptionally high carrier mobility of 220cm2V−1s−1 and a variable bandgap width spanning 0.075 eV (bulk) to 0.24 eV (single layer)^[24]. When the thickness exceeds four monolayers, a film with a (111) orientation and a stable structure will be preferentially synthesized^[25,26]. Additionally, as a 2D material featuring metallic topological surface states, Bi exhibits spin polarization, scattering-free characteristics, and highly efficient surface conduction properties, thereby enhancing the overall performance of the device^[27]. In this work, we fabricated a deep UV photodetector (PD) based on the 2D Bi/Ga2O3 heterojunction, which exhibits an extremely low dark current and showcases remarkable detectivity and responsivity under low-light conditions. This work provides a significant clue for the subsequent development of Ga2O3 heterojunction PDs.

300 nm thick Ga2O3 films were deposited on the (000l) oriented sapphire substrates by metal–organic chemical vapor deposition (MOCVD) at 750°C with a pressure of 25 Torr (1 Torr≈133.322 Pa). Subsequently, 2D Bi thin films, with a thickness of approximately 10 nm and an area of 2mm2, were deposited on the Ga2O3 film via the pulsed laser deposition (PLD) method with the assistance of a square-shaped mask. In the deposition chamber, the Bi target was positioned at a distance of 6 cm from the substrate, and the chamber pressure was set at approximately 4×10−5Pa. The temperature was set at 100°C. The energy of the laser pulse was 50 mJ, with a frequency of 6 Hz and a spot size of 1.75mm2 (1.42mm×1.23mm). Both the substrate holder and the target holder rotate counterclockwise at a uniform speed. After the film deposition is completed, the vacuum is maintained for 1 h to allow the chamber to return to room temperature. Then, a layer of dotted Ti/Au electrodes is deposited on the Ga2O3 and the Bi film through magnetron sputtering to serve as ohmic contacts. Finally, the device was bonded to a printed circuit board (PCB) with conductive adhesive and wire-bonded with gold wires for electrical connection. The device synthesis process is shown in Fig. 1. An atomic force microscope (AFM, Dimension 3100, VEECO), an X-ray diffractometer (XRD, Smartlab, Rigaku), a laser micro-Raman spectrometer (DXR, Thermo Fisher Scientific), a multifunctional X-ray photoelectron spectrometer (XPS, ESCALAB QXi, Thermo Fisher Scientific), and an optical microscope (BX51M, Olympus) are used to study the thickness, size, microstructure, and crystallinity of the Bi film. A spectrophotometer (UV-1900, Macy) is used to record the UV-visible absorption spectrum of the Ga2O3 film. The Keithley 4200 semiconductor characterization system is employed to investigate the current-voltage (I-V) characteristics.

Figure 2(a) shows the XRD pattern of Bi with a SiO2/Si substrate. Except for the diffraction peaks of the SiO2/Si(400) substrate and the instrument diffraction peaks, all the diffraction peaks (003) (006) of the Bi film match perfectly with the standard database (PDF 85-1330), which corresponds to the hexagonal crystal structure of Bi with cell parameters (a=b=4.55ű0.01Å, c=11.86ű0.01Å, α=β=90°, and γ=120°). The strongest peak (003) indicates that the preferred orientation of the Bi film is along the hexagonal (001) direction, which can be alternatively defined as the (111) direction in the rhombohedral system. As shown in Fig. 2(b), Raman spectra were acquired from three Bi (111) thin films deposited on SiO2/Si substrates, with thicknesses of 3, 10, and 30 nm, respectively. The thickness was determined from AFM results, as shown in Figs. 2(c)–2(e). Two Raman modes are present at 69.5 and 95.8cm−1, corresponding to the characteristic vibration modes of Eg and A1g of Bi, respectively. It is found that the positions of these Raman peaks do not rely on the film thickness. Similar phenomena have been previously observed in the research on 2D Bi films fabricated via mechanical exfoliation^[28], which implies that the Bi films are unstrained. To investigate the electrical properties of Bi films, a Ti/Au array electrode was deposited on a 10-nm-thick Bi film via the magnetron sputtering technology. The electrode pattern features a channel length of 30 µm and a width of 500 µm. The output characteristic curve, illustrated in Fig. 2(f), reveals ohmic contact at the interface between the Ti/Au electrode and the Bi films. The ohmic contact, devoid of rectifying behavior, is of utmost significance for high-performance electronic devices.

The schematic diagram of the 2D Bi/Ga2O3 heterojunction is shown in Fig. 3(a). As displayed in Fig. 3(b), the lower segment exhibits the XRD pattern of single-crystal β-Ga2O3 epitaxially grown on an Al2O3 substrate. In contrast, the upper segment presents the XRD pattern of the heterojunction films formed by depositing Bi onto Ga2O3. In addition to the Al2O3 (0006) diffraction peaks, the Ga2O3 film exhibits prominent diffraction peaks. These peaks, respectively, correspond to the (20¯1), (40¯2), and (60¯2) peaks in the monoclinic phase β-Ga2O3 (JCPDS Card No. 43–1012).

To further elucidate the elemental composition of the 2D Bi/Ga2O3 thin film, the heterostructure of 2D Bi/Ga2O3 was characterized by XPS measurements. The XPS results were calibrated using the standard peak position of C 1s at 284.8 eV. The full-range XPS spectrum of the 2D Bi/Ga2O3 heterojunction thin film is presented in Fig. 4(a). Evidently, distinct elemental spectral lines corresponding to Ga, O, and Bi are observed in the thin-film spectrum. Subsequently, we determined the elemental components and chemical valence states of the elements within the Bi/Ga2O3 heterojunction thin film. By comparing it with the standard XPS spectral libraries, we found that the core level spectral line of the Ga 3d orbital of the Ga element is centered at 19 eV, and 284.8 eV corresponds to the core-level of the calibrated C 1s, the spectral line near 530 eV corresponds to the core level of O 1s, and the energy spectrum near 165 eV corresponds to the core level spectral line of Bi 4f. XPS fitting results of the core level spectrum of Bi 4f are shown in Fig. 4(b). The fine structure spectra of the Ga 3d orbital are meticulously depicted in Fig. 4(c). The characteristic peak corresponding to the Ga element is precisely resolved into two distinct subpeaks. These two subpeaks are, respectively, assigned to Ga2O3 and metallic Ga. Figure 4(d) presents the fitting analysis results of the O 1s core-level spectra based on the XPS technology. The O 1s peak is actually composed of two types of closely related chemical bonds. First, in the binding energy (BE) range of 530.59–530.99 eV, it is the gallium–oxygen (Ga–O) bond derived from the lattice structure of Ga2O3. Second, in the binding energy range of 531.79–532.22 eV, it is the carbon–oxygen (C–O) bond contributed by subvalent oxide. As shown in Fig. 4(e), this is the valence band maximum (VBM) of Ga2O3 that we obtained from the XPS spectra.

To investigate the device’s photoresponse, a 254 nm UV light was vertically irradiated onto the 2D Bi/Ga2O3 heterojunction PD. The effective junction area (1.27×10−4cm2) was determined by the overlapping region. As shown in Fig. 5(a), the I-V curves of the device were obtained in dark and illumination under different light intensities. In a dark environment, it shows rectification characteristics with a dark current of 2.43×10−14A at −10V and 3.33×10−13A at 10 V, respectively. The photocurrent significantly increases with light intensity. At 10 V bias, from 0.1 to 100μW/cm2, the current surged from 1.97×10−10 to 5.83×10−8A, due to the increase in photoexcited electron–hole pairs. At 0.1μW/cm2, the photocurrent is 3 orders of magnitude higher than the dark current, indicating a large signal-to-noise ratio (SNR), which is important for practical applications. As shown in Fig. 5(b), in order to evaluate the photoresponse, the responsivity and detectivity were calculated by the following equations, respectively: R=Ip−IdPλS,(1)D*=RS2qId,(2)where Ip is the photocurrent, Id is the dark current, Pλ is the incident light-power density, S is the effective irradiation area, and q is the electron charge. At 10 V bias and 0.1μW/cm2, Rmax is 200 mA/W and Dmax* is 8.58×1011Jones, and the responsivity is much higher than that of the simple β-Ga2O3 PD and other heterojunction PDs such as MoS2/β-Ga2O3 and graphene/Ga2O3. The specific D* also has more advantages compared to traditional heterojunction detectors^[29–31]. Furthermore, the external quantum efficiency (EQE) and the photo-to-dark current ratio (PDCR) are also two crucial figures-of-merit (FOMs) for evaluating the performance of PDs, which can be extracted by the following equations, respectively: EQE=Rhcqλ×100%,(3)PDCR=Ip−IdId,(4)where R, h, λ, and c represent the responsivity, the Planck constant, the excitation wavelength, and the speed of light, respectively. The variations of the device’s EQE and PDCR with respect to light intensity are shown in Fig. 5(c). The maximum EQE is calculated to be 96.61% under an illumination of 0.1μW/cm2. The decrease of EQE at higher illumination power could be understood by the declined increment proportion of photoexcited electron−hole pairs^[32]. As depicted in Fig. 5(c), with the augmentation of light intensity, the PDCR surges from 593.81 to 1.54×105. This can be ascribed to the fact that upon an increase in light intensity, a greater number of photons are absorbed, inducing a copious generation of excited carriers. This gives rise to a notable upsurge in carrier density. The elevated carrier density, in turn, fortifies the built-in electric field. Concurrently, the likelihood of carrier impacting ionization escalates, while the probability of recombination is suppressed. Consequently, the collection efficiency of the photogenerated carriers is markedly enhanced. Figure 5(d) shows the temporal correlation of the photocurrent under varying bias voltages. The photocurrent exhibits recurrent on–off states in response to the incident UV light. In this research, the objective of which is to conduct an in-depth investigation into the photoresponse of the device functioning in the self-powered mode, we have meticulously measured the time response of the photocurrent under a light illumination intensity of 100μW/cm2 at discrete bias voltages, namely, 0, −5, and −10V. At a bias voltage of 0 V, the measured value of the photocurrent was approximately 2.5×10−10A. Notably, as the bias voltage was gradually increased, the photocurrent rised accordingly. Based on the data calculations from the figures above, the rise and fall times of the PD are accurately quantified. At a bias potential of 0 V, the rise time is ascertained to be 0.341 s, and the fall time is determined as 1.376 s. At a bias of −5V, the rise time measures 0.694 s, and the fall time is 0.3285 s. At a bias of −10V, the rise time is 0.872 s, and the fall time is 0.368 s. As the bias voltage increases, the rise time exhibits an elongation tendency. This phenomenon can be ascribed to the fact that under higher bias conditions, the augmented drift velocity of carriers promotes more extensive recombination of photogenerated carriers.

To further elucidate the working mechanism of the 2D Bi/β-Ga2O3 PD, the energy band diagram of 2D Bi and β-Ga2O3 is plotted in Fig. 6. According to previous research, β-Ga2O3 has a bandgap of 4.93 eV and an electron affinity of 4.0 eV^[33], while 2D Bi has a bandgap of 0.24 eV and an electron affinity of 4.4 eV^[34]. The Bi/Ga2O3 exhibits a type-I heterojunction. When in contact, electrons flow from Ga2O3 to 2D Bi, causing the energy bands to bend, which forms a built-in electric field from Ga2O3 to 2D Bi [Fig. 6(a)]. Under UV illumination [Fig. 6(b)], the interface’s built-in electric field enables rapid separation and migration of photogenerated electron–hole pairs towards the electrodes, achieving a fast response and a large photocurrent. Type-I heterojunctions can significantly reduce the dark current^[35], which is crucial for improving the detector’s sensitivity and SNR. Their unique energy band structure confines photogenerated carriers in the narrow-bandgap layer as a “well,”^[36,37] which suppresses the recombination of electron–hole pairs and enhances carrier collection efficiency, thus facilitating ultrafast and sensitive photodetection^[38]. Table 1 summarizes the main parameters of Bi/Ga2O3 heterojunctions and PD devices based on other material systems with similar configurations. Notably, the Bi/β-Ga2O3 heterojunction PD reported in this work significantly outperforms some existing Ga2O3-based heterojunction devices in terms of responsivity (200 mA/W) and detectivity (8.58×1011Jones). For example, the responsivities of the MoS2/Ga2O3 detector (2.05 mA/W)^[31], the GaN/Sn:Ga2O3 device (3.05 mA/W)^[39], and the MXenes/β-Ga2O3 detector (12.2 mA/W)^[41] are 1 to 2 orders of magnitude lower than those of this work. The outstanding performance of Bi/Ga2O3 PDs mainly results from the type-I band alignment and the high mobility of 2D Bi thin films.

In conclusion, we have successfully deposited large-area, uniform, and high-quality Bi thin films using the PLD technique and fabricated a high-performance PD based on the Bi/Ga2O3 heterojunction. This PD exhibits a very low dark current and relatively high responsivity. Notably, the maximum responsivity of the device is 200 mA/W, and its EQE can reach 96.61%. Moreover, under a light intensity of 0.1μW/cm2, the detectivity can still reach 8.58×1011Jones, and its response speed of around 0.3 s is also superior to that of PDs based solely on Ga2O3 thin films. This work also shows that combining the Bi thin film with semimetallic semiconductor properties with Ga2O3 can optimize photodetection performance, providing a theoretical basis for the combination of narrow-bandgap materials and ultrawide-bandgap materials in the future.