Fabrication and Ultraviolet Response Characteristics of All-Oxide Bi₂O₃/Ga₂O₃ Heterojunction

Abstract

Heterojunctions are commonly used in optoelectronic devices to improve device performance. However, interface defects and lattice mismatch often hinder carrier transport and reduce efficiency, emphasizing the need for further exploration of diverse heterojunction structures. In this study, a heterojunction device constructed from Bi₂O₃ and Ga₂O₃ is demonstrated. The microstructures and photoelectrical properties of Bi₂O₃ and Ga₂O₃ thin films were investigated. Bi₂O₃ and Ga₂O₃ thin films show a bandgap of 3.19 and 5.10 eV. The Bi₂O₃/Ga₂O₃ heterojunction-based device demonstrates rectification characteristics, with a rectification ratio of 2.72 × 10³ at ±4.5 V and an ON/OFF ratio of 1.07 × 10⁵ (4.5/−3.9 V). Additionally, we fabricated a sandwich-structured photodetector based on the Bi₂O₃/Ga₂O₃ heterojunction and investigated its ultraviolet photoresponse performance. The photodetector exhibits low dark current (0.34 pA @ 3.9 V) and fast response rise/fall time (<40/920 ms). This work offers important perspectives on the advancement of large-area, low-cost, and high-speed Bi₂O₃ film-based heterojunction photodetectors.

1. Introduction

The fundamental mechanisms of heterojunction devices mainly rely on the band structures and carrier transport properties of different semiconductor materials, which contributes to their widespread use in high-speed switching devices, solar cells, photodetectors, and more [1,2,3,4,5,6,7]. For heterojunction photodetectors, due to the differing energy band structures of the two materials, band bending and interfacial charge distribution are formed, leading to the generation of a built-in electric field. When suitable light illuminates the heterojunction photodetectors, photons are absorbed by the semiconductor, generating electron-hole pairs. Under the influence of a built-in electric field, the carriers transport in opposite directions, reach the electrode, and form a photocurrent. The inherent characteristics of heterojunctions enable heterojunction photodetectors to achieve high photoresponse performance, such as fast response, high gain, and even self-powering capabilities. Ultraviolet (UV) photodetectors as one important type of photodetectors have attracted tremendous attention due to the demand for missile tracking, scientific analysis, ozone hole monitoring, flame detecting, and so on [8,9,10,11]. Recently, wide-bandgap semiconductor materials have emerged as a promising choice and present rapidly development in UV detection [5,12]. Among the heterojunction UV photodetectors, type I (stradding) band alignments are usually constructed to improve the carrier separation efficiency [13].

Bi₂O₃, as a p-type metal oxide semiconductor material, possesses many intriguing properties such as wide bandgap, strong light absorption, non-toxic nature, and environmental stability, thereby exhibiting great potential for UV detection [14,15,16,17]. There are four main polymorphs of Bi₂O₃, namely α-, β-, γ-, and δ-, corresponding to monoclinic, tetragonal, body-centered cubic, and face-centered cubic, respectively [16,18,19,20]. Two-dimensional γ- Bi₂O₃ UV photodetector has been reported and has shown promising UV detection ability with a responsivity of 64.5 A/W and an ultrafast response speed of 0.29 ms (rise time)/0.87 ms (fall time) [21]. Praveen et al. reported a β-Bi₂O₃/0D SnO₂ quantum dot heterojunction UV detection device by combining an electrospinning technique and facile-one step hydrothermal process, which exhibits a responsivity of 62.5 μA/W and a detectivity of 4.5 × 10⁹ Jones [22]. However, the reported Bi₂O₃-based photodetectors are mainly based on nanoscale Bi₂O₃ material. Bi₂O₃ film-based photodetectors, especially heterojunction photodetectors, are rare, which is mainly attributed to Bi₂O₃’s poor conductivity. The conductivity of Bi₂O₃ films is very sensitive to both the deposition technique and deposition conditions [23]. Regarding the high performance mentioned above, it is necessary to investigate the heterojunction formed between the p-type material Bi₂O₃ and an n-type material.

Gallium oxide (Ga₂O₃) is regarded as a promising candidate for electronic applications due to its large bandgap (~4.9 eV), high critical breakdown field, and large Baliga figure of merit, which make it suitable for next-generation high-power electronics [24,25]. Achieving stable p-type doping in Ga₂O₃ remains a significant challenge due to the existence of shallow donor states [26]. It is reported that the CuMO₂/Ga₂O₃ heterojunction self-powered photodetector demonstrates a high photo/dark current ratio of 2.3 × 10⁴, a responsivity of 25 μA/W, and a detectivity of 0.9 × 10¹¹ Jones [27]. Wang reported that the Ga₂O₃/ZnO heterojunction exhibits a responsivity and detectivity of 2.69 mA/W and 1.56 × 10¹⁰ Jones, and an ultrafast response time of rise/decay time with 56.6/43.4 ms under the regulation of piezoelectricity [28]. Although Ga₂O₃-based heterojunction has been reported in quantities, it is a great challenge to commercialization due to the inherent defects of heterostructure, such as lattice mismatch, which would lead to heterojunction interface degradation and result in the failure of heterojunctions [29,30,31]. Moreover, high-performance Ga₂O₃-based UV photodetectors depend on expensive substrates and equipment, which present significant challenges for large-area fabrication [32]. Therefore, there is a need to explore a wider range of heterojunctions.

The properties of Bi₂O₃ and Ga₂O₃—UV absorption property, comparable bandgap, and conductive type—make them excellent candidates for UV detection and to form a p-n heterostructure. Moreover, Ga₂O₃ and Bi₂O₃ are likely to form type I band alignments because the electron affinities are 3.2 eV for Ga₂O₃ and 4.9 eV for Bi₂O₃ [13,22,33]. Further, the basic characteristics of heterojunctions and UV response characteristics of Bi₂O₃/Ga₂O₃ heterojunction remain unclear. Therefore, in this work, thin-film-based transparent heterojunction was realized using p-type Bi₂O₃ and n-type Ga₂O₃ preparation on a glass substrate, which exhibits potential to detect UV light. The microstructures and photoelectrical properties of Bi₂O₃ and Ga₂O₃ thin films were investigated. The film-based Bi₂O₃/Ga₂O₃ heterojunction exhibits a high rectification ratio and UV response behavior. The results were a breakthrough in development of large-area and low-cost Bi₂O₃ film-based heterojunction UV photodetectors.

2. Experimental Procedure

Bi₂O₃/Ga₂O₃ heterojunction was synthesized via the magnetron sputtering technique, and the sputtering parameters are listed in

Table 1. Sputtering process parameters of the Bi₂O₃/Ga₂O₃ heterojunction.

Name	Sputtering Pressure (Pa)	Argon Flow (sccm)	Sputtering Power (W)	Time (Min)
Bi2O3	0.6	40	60	20
Ga2O3	0.5	40	150	60

. First, ITO glass substrates (3 cm × 3 cm) were ultrasonically cleaned with acetone, methanol, and then deionized water, before being dried in compressed N₂. After that, the Bi₂O₃ layer was deposited by radio frequency (RF) magnetron sputtering in an Ar atmosphere with a power of 60 W and a pressure of 0.6 Pa. Then, the Ga₂O₃ layer was deposited on top of the Bi₂O₃ layer using the same system. The power and sputtering pressure used in the deposition of Ga₂O₃ were 150 W, 0.5 Pa, respectively. Finally, the patterned ITO top electrode was deposited on the surface of the Ga₂O₃ layer by direct current magnetron sputtering, utilizing a stainless steel mask for precise patterning. Before all sputtering, the chamber was pumped down to a base pressure of 4 × 10⁻⁴ Pa. During device fabrication, Bi₂O₃ and Ga₂O₃ thin films were simultaneously prepared on glass substrates for microstructural and photoelectrical characterization. In addition, to verify the successful preparation of the Ga₂O₃ film, the individual Ga₂O₃ film was annealed at 800 °C in air for 2 h. The Bi₂O₃/Ga₂O₃ heterojunction devices were synthesized at room temperature and not annealed.

The as-grown samples were characterized by field emission scanning electron microscopy (SEM; Sigma 300, ZEISS, Oberkochen, Germany), X-ray diffraction analyses (XRD; D8 ADVANCE A25, Bruker, Karlsruhe, Germany), a UV–Vis spectrophotometer (UV-2700, Shimadzu, Kyoto, Japan), and X-ray photoelectron spectroscopy (XPS; Escalab 250xi, Thermo Fisher Scientific, East Grinstead, UK). A photoelectric test system comprising an LED lamp, probe station, and Keithley 2602B was used to evaluated the photoelectric characteristics of the fabricated devices. The optical power of LED-emitted radiation was accurately calibrated with a power meter (PM 100D, Thorlabs GmbH., Newton, NJ, USA). Photoresponse testing of the fabricated photodetectors was conducted at room temperature.

3. Results and Discussion

Figure 1a shows the schematic illustration of the Bi₂O₃/Ga₂O₃ heterojunction photodetector, where the ITO electrodes are located on the top and bottom planes of the Bi₂O₃/Ga₂O₃ composite films. As shown in Figure 1b, the SEM image presents a typical sandwich structure, which reveals that the fabricated heterojunction is consistent with our design, and the thickness of bottom ITO electrode, Bi₂O₃ layer, Ga₂O₃ layer, and top ITO electrode are 110, 340, 140, and 190 nm, respectively. Further, it clearly shows that the films are dense and uniform. The XRD spectra of the as-synthesized samples, Bi₂O₃, Ga₂O₃, and 800 °C annealed Ga₂O₃ films, are displayed in Figure 1c,d. The XRD patterns of Bi₂O₃ film showed diffraction peaks that matched the PDF#71-2274, suggesting the formation of α phase Bi₂O₃. However, the relatively broad full width at half maximum (FWHM) of the diffraction peaks indicates that the crystallization quality of the Bi₂O₃ thin film is deficient. This phenomenon suggests that the thin film likely contains a substantial number of polycrystalline structures, which may account for the observed broadening of the FWHM. It can be observed that the as-deposited Ga₂O₃ film with 800 °C annealing shows diffraction peaks at 2θ values of 30°, 31.7°, 37.4°, and 64.7°, corresponding to the crystal plane indices of β-Ga₂O₃ (PDF # 43-1012) at (400), (002), (401), and (512), respectively. The as-deposited Ga₂O₃ film without annealing does not exhibit significant peaks, indicating that the Ga₂O₃ thin film is amorphous. This suggests that the amorphous Ga₂O₃ transforms into β-Ga₂O₃ upon annealing. Previous research has proven that Ga₂O₃ material synthesized by the RF magnetron sputtering method at RT demonstrates an amorphous structure [34,35].

Optical transmittance measurements, including transmittance ratio and optical bandgap, are carried out to evaluate the optical parameters of Bi₂O₃ and Ga₂O₃ films. The transmittance of the Bi₂O₃ and Ga₂O₃ films is investigated by a UV–Vis spectrophotometer, as shown in Figure 2a,b. The Bi₂O₃ film shows a transmittance value less than 60% in the wavelength range from 200 to 800 nm. Obviously, the Ga₂O₃ film has an average transmittance of over 80% and shows great absorption at wavelengths less than 280 nm. Commonly, for direct bandgap semiconductor material, the optical bandgap (E_g) can be estimated using the Tauc plots [36]. The estimated bandgap (E_g) and absorption coefficient (α) satisfy the following equation:

(α hν)² = B (hν − E_g)

(1)

where hν is the energy of the incident photons, and B is the absorption edge width parameter. The (αhν)² versus hν Tauc plot of Bi₂O₃ and Ga₂O₃ films is presented in Figure 2c,d. Utilizing the Tauc method, the optical bandgap is determined through extrapolation of the linear portion of the plot, with the intercept on the photon energy axis serving as the E_g. The values of the Ga₂O₃ and Bi₂O₃ films were estimated to be 5.10 and 3.19 eV, respectively.

XPS spectra of the samples with the fitted peaks based on Gaussian fitting analysis are shown in Figure 3, which are used to explore the bonding states of Bi₂O₃ and Ga₂O₃ films. Figure 3a depicts the XPS survey spectra of Bi₂O₃, showing the central energy level positions of Bi 4p, Bi 4d, Bi 4f, Bi 5d, and O 1s. The C 1s peak in the spectra originates from the hydrocarbons in the air. Figure 3b shows high-resolution spectrum of Bi 4f; the banding energies of Bi 4f_7/2 and Bi 4f_5/2 are located at 158.23 and 163.52 eV, respectively, which correspond to the Bi³⁺ in α-Bi₂O₃ [19,37]. The banding energies of O 1s are located at 529.03 and 530.47 eV, attributable to Bi-O and Bi-O-Bi, respectively, as shown in Figure 3c [18,19]. Figure 3d depicts the XPS survey spectra of Ga₂O₃. Figure 3e presents the high-resolution spectrum of Ga 2p; it is found that both Ga 2p_3/2 and 2p_1/2 exhibit two peaks, which may be due to the coexistence of multiple crystal phases in the sample. The Ga 3d high-resolution spectrum shown in Figure 3f reveals two fitted peaks, corresponding to the binding energies of Ga³⁺ and Ga⁺. The Ga³⁺ peak is associated with Ga³⁺ ions in Ga₂O₃, while the Ga⁺ peak indicates the presence of gallium in a sub-stoichiometric ratio, suggesting an oxygen deficiency in the material that leads to gallium adopting a lower oxidation state, and the weak peak on the left is attributed to the O 2s signal [38]. Figure 3g presents the O 1s high-resolution spectrum of the Ga₂O₃ thin film, consisting of two fitted peaks. The peak at 530.2 eV (O_I) corresponds to the O-Ga bond, while the peak at 531.6 eV (O_II) is attributed to the surface-oxygen defects [38,39,40]. The intensity ratio of O_II/(O_II + O_I) is 35.0%, indicating a significant number of oxygen vacancies in the Ga₂O₃ thin film.

To confirm the contact type between Bi₂O₃ and the ITO electrode, as well as between Ga₂O₃ and the ITO electrode, I–V testing was conducted, and the results are shown in Figure 4a,b. The I–V characteristics of both Bi₂O₃–ITO and Ga₂O₃–ITO exhibit a linear behavior, suggesting the formation of a reliable ohmic contact between the semiconductor functional materials and the ITO electrode. Figure 1a shows the I–V test schematic diagram of the Bi₂O₃/Ga₂O₃ heterojunction, where the positive and negative poles are connected to the bottom and top ITO electrodes, respectively. Under dark conditions, the I–V characteristic plot of Bi₂O₃/Ga₂O₃ heterojunction exhibits a typical rectifying behavior, as shown in Figure 4c, suggesting that a p-n junction is formed in the contact interface of Bi₂O₃ and Ga₂O₃ layers. The Bi₂O₃/Ga₂O₃ heterojunction shows a high rectification ratio of 2.72 × 10³ under the bias of +4.5/−4.5 V, and an extremely low dark current (0.34 pA @ − 3.9 V), with a highest ON/OFF of 1.07 × 10⁷ (4.5/−3.9 V).

To investigate the UV response performance of Bi₂O₃/Ga₂O₃ heterojunction photodetectors, we characterized their I–V characteristics under UV illumination at wavelengths of 255, 310, and 365 nm with a light intensity of 56 mW/cm², as displayed in Figure 5a. During the experiments, the UV light was incident from the Ga₂O₃ side to ensure accurate experimental results. The results show that the Bi₂O₃/Ga₂O₃ heterojunction photodetector responds to 255, 310, and 365 nm UV light. To better reflect the device’s response capability to UV light, we calculated the photo-to-dark-current ratio (PDCR), responsivity (R), and detectivity (D*) of the device. The PDCR, R, and D* can be expressed by the following equations [34]:

$$\mathit{PDCR}={\displaystyle \frac{{\mathit{I}}_{\mathrm{photo}}-{\mathit{I}}_{\mathrm{dark}}}{{\mathit{I}}_{\mathrm{dark}}}}$$

(2)

$$\mathit{R}={\displaystyle \frac{{\mathit{I}}_{\mathrm{photo}}-{\mathit{I}}_{\mathrm{dark}}}{{\mathrm{P}}_{\mathit{\lambda }}\mathit{S}}}$$

(3)

$${\mathit{D}}^{*}=\mathit{R}\sqrt{{\displaystyle \frac{\mathit{S}}{2\mathit{q}{\mathit{I}}_{\mathrm{dark}}}}}$$

(4)

where I_photo is the photocurrent, I_dark is the dark current, P_λ is the light power density, S is the effective illuminated area of 1.96 × 10⁻³ cm², and q is the electron charge. Compared with 255 nm, the fabricated photodetector exhibits a weaker UV response to 310 and 365 nm because only the Bi₂O₃ layer absorbs photons of the corresponding wavelengths. Clearly, under a bias voltage of − 3.9 V, the fabricated heterojunction photodetector reaches a maximum photo-to-dark current ratio value of 2.11 × 10⁴ @ 255 nm, 3.80 × 10³ @ 310 nm, and 1.66 × 10³ @ 365 nm. After calculation, it can be found that the 255 nm/310 nm rejection ratio (R_{255 nm}/R_{310 nm}) of the fabricated photodetector (5.14 @ −4.5 V) is an order of magnitude larger than R_{255 nm}/R_{365 nm} (11.2 @ −4.5 V). Figure 5b presents the I–V characteristic curves of the Bi₂O₃/Ga₂O₃ heterojunction photodetector under 255 nm UV illumination at with various light densities, including 5, 13, and 56 mW/cm². Under a bias voltage ranging from −3.9 to −4.5 V, the photocurrent increases with increasing light intensity. However, the current under forward bias remains almost unchanged. As Figure 5c shows, responsivity increases with the increase in bias voltage due to enhanced separation and collection efficiency of photogenerated carriers. In contrast, detectivity decreases as the bias voltage increases, owing to the rapid rise in dark current once the bias voltage exceeds the current zero voltage, as displayed in Figure 5d. Under 255 nm UV illumination at 5, 13, and 56 mW/cm², the fabricated photodetector shows the highest R and D* values of 3.38 × 10⁻⁵, 6.34 × 10⁻⁵, and 6.47 × 10⁻⁵ A/W (@−4.5 V), and 5.51 × 10⁹, 1.07 × 10¹⁰, and 1.05 × 10¹⁰ Jones (@−3.9 V), as listed in

Table 2. Photoresponse performance of the Bi₂O₃/Ga₂O₃ heterojunction photodetector.

Light Intensity (mW/cm2)	PDCR @−3.9 V	R @−4.5 V (A/W)	D* @−3.9 V (Jones)	τr/τf (ms)
56 @ 310 nm	3.80 × 103	6.58 × 10−6	9.93 × 108
56 @ 365 nm	1.66 × 103	3.02 × 10−6	4.33 × 108
56 @ 255 nm	2.11 × 104	3.38 × 10−5	5.51 × 109
13 @ 255 nm	9.54 × 103	6.34 × 10−5	1.07 × 1010	<40/920
5 @ 255 nm	3.58 × 103	6.47 × 10−5	1.05 × 1010

.

It should be emphasized that we have observed a fascinating phenomenon in the Bi₂O₃/Ga₂O₃ heterojunction. The minimum current (current zero) of the Bi₂O₃/Ga₂O₃ heterojunction photodetector devices does not occur at zero bias but at a negative bias, and it shifts towards a more negative bias as the illuminated UV wavelength increases or the light intensity decreases. Similar phenomena have been reported in Ga₂O₃-based UV detectors and NiO_x/PEDOT:PSS heterojunction solar cells [41,42,43]. The literature has reported that a dipole layer at the interface may result in a notable vacuum level shift, thereby altering the voltage at the current zero for the I–V curves [44]. The formation of the dipole layer at the interface typically results from either the p-n heterojunction or the presence of defect states. The XRD and XPS analysis results reveal that the Bi₂O₃ and Ga₂O₃ films contain a large number of oxygen vacancy defects. Therefore, in the Bi₂O₃/Ga₂O₃ heterojunction, the shift of the I–V curve is mainly due to the two factors mentioned above. Under UV light illumination, electrons in the valence band of Bi₂O₃ films are excited to the conduction band, resulting in a change in carrier concentration. This, in turn, causes the redistribution of charges at the heterojunction interface, leading to a change in the dipole layer. Additionally, defects in the heterojunction interface may be excited by absorbed photon energy, further affecting the characteristics of the dipole layer. Therefore, compared with UV illumination conditions, the device shows a higher shift of current zero under dark conditions.

Response time is a key parameter of photodetectors, reflecting how quickly they can respond to changes in optical signals. As shown in Figure 5, the response time can be determined from the current–time (I–T) characteristic curves, including response and recovery processes, i.e., rise time (τ_r, 10–90%) and fall time (τ_f, 90–10%). The Bi₂O₃/Ga₂O₃ heterojunction photodetector device has a fast response speed, with the rise and fall time of <40 ms and 920 ms. It is important to note that the time resolution of the photoresponsive testing system is 40 ms, indicating that the rise time of the device is faster than 40 ms. Additionally, the device exhibits a current spike at the moment of switching the 255 nm UV on and off, which may be due to the light-induced pyroelectric effect. The literature reports that wurtzite ZnO exhibits pyroelectric potential under light illumination due to its non-centrosymmetric crystal structure [45,46,47]. In our results, the sharp rising edge in the Bi₂O₃/Ga₂O₃ heterojunction photodetector is attributed to the non-centrosymmetric crystal structure of the Bi₂O₃ film.

Table 3. Comparison of the critical parameters for the Bi₂O₃- and Ga₂O₃-based UV photodetector.

Structure	Fabricate Method	Wavelength (nm)	Responsivity (A/W)	Detectivity (Jones)	Rise/Fall Time (ms)	Refs
Bi2O3/SnO2  (nanofibers/ quantum dots)	Electrospinning and hydrothermal	UV	4.41 × 10−2	3.18 × 1012	100/- @ 10–90%	[22]
NiO/Bi2O3/TiO2  (film)	Spin-coating	254	1.45 × 10−1	5.50 × 1010	20.8/21.1 @ 10–90%	[48]
Bi2O3  (2D flakes)	CVD	365	64.5	1.32 × 1013	0.29/0.87 @ 10–90%	[21]
Bi2O3  (nanosheets)	hydrothermal	365	-	-	33/47 @ 0–1/e	[16]
CuSCN/Ga2O3  (film)	Spin-coating and MOCVD	270	5.5 × 10−3	3.81 × 1011	3.1 × 10−3/0.7 × 10−3 @ 10–90%	[49]
Ga2O3 (film)	HVPE	230 or 254	30.6	6.95 × 1015	18/1.3 × 10−3 @ biexponential	[50]
Ga2O3  (bulk crystal)	Czochralski	210	20.4	9.60 × 1015	17/27 @ biexponential	[51]
Ga2O3 (film)	ALD	254	5.836	7.72 × 1014	9.3 × 103/5.2 × 103	[52]
Bi2O3/Ga2O3 (film)	RF magnetron sputtering	255	6.47 × 10−5	1.05 × 1010	<40/920 @ 10–90%	This work

compares the critical parameters of Bi₂O₃- or Ga₂O₃-based UV photodetectors. The Bi₂O₃/Ga₂O₃ heterojunction photodetector, although not showing significant advantages in responsivity and detectivity, clearly exhibits a certain superiority in response speed. Notably, Bi₂O₃-based photodetectors with excellent performance are predominantly constructed from quasi-one-dimensional materials, which poses great challenges for large-area controllable deposition. Similarly, Ga₂O₃-based photodetectors rely on complex or costly fabrication techniques, resulting in high expenses for large-area preparation. Therefore, UV photodetectors based on Bi₂O₃ heterojunction thin films hold significant potential for rapid response and large-area applications.

Figure 6a presents the XPS valence band spectra of Bi₂O₃ and Ga₂O₃ films, in which the valence band maxima (VBM) positions were determined by extrapolating the valence band edge to the baseline. The calculated energy separations between the Fermi level and VBM are 1.01 eV for Bi₂O₃ and 2.80 eV for Ga₂O₃, respectively. Figure 6b shows energy band diagrams that explain the UV detection performance. The bandgap of Ga₂O₃ (5.10 eV) is larger than the energy of 255 nm UV light, whereas Bi₂O₃ has the opposite relationship. Therefore, with 255 nm UV light illumination on the device, the Bi₂O₃ in the bottom layer preferentially absorbs the light and generates photogenerated carriers. Furthermore, amorphous Ga₂O₃ contains a large number of defect states and impurity levels within its bandgap. These intermediate states provide lower energy levels, allowing electrons from the valence band to be excited into these defect states by absorbing 255 nm photons. Subsequently, the electrons can be further excited into the conduction band through thermal excitation or multiphoton processes. Under ultraviolet irradiation, photogenerated electron–hole pairs primarily form in the space-charge region. With reverse bias, the conduction band of Bi₂O₃ shifts upward, and that of Ga₂O₃ shifts downward, reducing band alignment mismatch. This enhances carrier movement and charge collection at the electrodes. Increasing light power density generates more carriers, while a forward bias reduces the space-charge region width, making carriers less sensitive to changes in light intensity.

4. Conclusions

In summary, all-oxide heterojunction and its UV response performance based on magnetron-sputtering-grown Bi₂O₃ and Ga₂O₃ thin films were reported. The structural characterization reveals α phase Bi₂O₃ and Ga₂O₃ shows polycrystalline structures. Furthermore, the Bi₂O₃/Ga₂O₃ heterojunction shows the highest ON/OFF ratio of 1.07 × 10⁷, and the photodetector shows a high PDCR of 2.11 × 10⁴ and a fast response speed of <40 ms rise time and 920 ms fall time. It is shown that Bi₂O₃/Ga₂O₃ heterojunction has potential for application in UV detection.