Ultraviolet Photodetector Based on a Beta-Gallium Oxide/Nickel Oxide/Beta-Gallium Oxide Heterojunction Structure

Abstract

In this paper, an n–p–n structure based on a β-Ga₂O₃/NiO/β-Ga₂O₃ junction was fabricated. The device based on the β-Ga₂O₃/NiO/β-Ga₂O₃ structure, as an ultraviolet (UV) photodetector, was compared with a p–n diode based on a NiO/β-Ga₂O₃ structure, where it showed rectification and 10 times greater responsivity and amplified the photocurrent. The reverse current increased in proportion to the 1.5 power of UV light intensity. The photocurrent amplification was related to the accumulation of holes in the NiO layer given by the heterobarrier for holes from the NiO layer to the β-Ga₂O₃ layer. Moreover, the device could respond to an optical pulse of less than a few microseconds.

1. Introduction

Gallium oxide (Ga₂O₃) has attracted attention recently as a next-generation power semiconductor material. Among the several poly-types of Ga₂O₃, β-Ga₂O₃ with a bandgap of 4.9 eV has the most thermodynamically stable crystal structure. Due to its optimal physical properties, several power devices and ultraviolet (UV) photodetectors have been studied [1,2,3,4,5]. However, it is difficult to obtain p-type conduction in β-Ga₂O₃. Therefore, devices based on β-Ga₂O₃ are currently restricted to Schottky and metal oxide semiconductor devices, such as Schottky diodes and field-effect transistors, respectively.

As a p-type metal oxide semiconductor, NiO, CuGaO₂, Cu₂O, and NiCo₂O₄, which have band gaps of 3.7 eV, 3.6 eV, 2.4 eV, and 2.1 eV, respectively, have been studied. NiO has the widest band gap among them. Kokubun et al. [6] proposed a NiO/β-Ga₂O₃ p–n heterojunction diode using the rare metal oxide NiO, which has p-type conduction and demonstrated good rectification of the diode. The same research group suggested that NiO was suitable for β-Ga₂O₃ in crystal orientation [7]. Since then, various diodes based on the NiO/β-Ga₂O₃ p–n heterojunction have been studied for application in power devices [8,9] or UV photodetectors [10,11,12,13].

UV light rays pose serious health problems for humans, such as damage to the skin, skin cancer, and white cataracts. UV light is classified as UV-A, -B, and -C, corresponding to wavelengths of 315–400 nm, 280–315 nm, and 100–280 nm, respectively. Because UV-A and -B cause light damage to the skin, it is important to be vigilant for UV light illumination. A UV-C photodetector is required for concerns of UV radiation through the holes in the ozone layer. UV-C light is also used for sterilization purposes. In order to protect against UV light, proper UV photodetectors are required. Therefore, many types of UV photodetectors have been studied [14,15,16,17,18,19,20]. There are several approaches, such as amorphous film, nano-rods, and combinations with other materials.

The wavelength of the photodetector relates to the bandgap of the active layer. If a photodetector for UV-B or -C is needed, a bandgap semiconductor wider than 3.9 eV should be used. Al_XGa_1-XN and Ga₂O₃ are typical candidates. Because a larger aluminum mole fraction of Al_XGa_1-XN is required for a wider bandgap, the difficulties inherent in growing crystals are increased. Meanwhile, β-Ga₂O₃ is a promising candidate for a UV-C photodetector because β-Ga₂O₃ is chemically stable, and commercial substrates are available. It is also expected that it can be used as a solar-blind UV photodetector.

Several UV photodetectors based on Ga₂O₃ have been studied. The resistance type [10] is based on the mechanism of the photoconductor. In the diode type, short-circuit current or reverse current [11,12,13] is used as the sensor signal. In the diodes based on the NiO/β-Ga₂O₃ p–n heterojunction, there are some difficult assignments relating to the hetero growth and interface trap due to the heterojunction. Although the author has studied the crystal orientation relationship between NiO and β-Ga₂O₃, there remain issues to be resolved.

In UV photodetectors, low dark current, responsivity, and response times are important factors. Several device structures have been studied to achieve these requirements. However, the amplification of photocurrent has not been reported in devices based on β-Ga₂O₃. A new device is required to obtain this amplification of the photocurrent. Therefore, in this study, the amplification of photocurrent by fabricating a β-Ga₂O₃/NiO/β-Ga₂O₃ structure is described. The device, based on a β-Ga₂O₃/NiO/β-Ga₂O₃ structure, has never been reported. The author studied the device as a UV photodetector. The photodetector exhibited 10 times greater responsivity and amplified the photocurrent. Moreover, a response with a high speed of the order of microseconds was demonstrated using pulse-driven UV-LED. The photodetection and amplification mechanism is discussed using a schematic band model.

2. Experimental

2.1. Device Structure and Fabrication

The devices were fabricated using purchased β-Ga₂O₃ substrates through a thin-film formation technique by the sol–gel method. Lift-off of the SiO₂ sacrificial layer, photolithography, and formation of metal electrodes were also performed.

Figure 1 shows the cross-sectional structure of the device and a photograph of its top view. The (001) β-Ga₂O₃ epitaxial substrates were used and purchased from Novel Crystal Technology Inc. The substrates were Sn-doped n-type substrates with a carrier density of 5 × 10¹⁸ cm⁻³ and had a Si-doped 8-µm epitaxial layer with a carrier density of 3 × 10¹⁶ cm⁻³. A 100 nm-thick film of the 10% Li-doped NiO was selectively formed on the β-Ga₂O₃ substrate. Afterward, a 100 nm-thick layer of the undoped β-Ga₂O₃ was selectively formed on the NiO layer. In both cases, the sol–gel method was used, and the SiO₂ sacrificial layer was removed using the lift-off process.

A SiO₂ thin film was formed on the β-Ga₂O₃ substrate through spin coating of a SiO₂ solution and annealing. The SiO₂ thin film was etched to make a hole for the selective formation of the NiO layer after patterning by photolithography, where the Li-doped NiO layer was formed using the sol–gel method. The 10% Li was added as a solution for the Ni. The resistivity of the 10% Li-doped NiO film was about 0.5 Ω·cm. The concentration and mobility of holes in the doped NiO were ~3 × 10²⁰ cm⁻³ and 0.05 cm²/Vs, respectively [6,21]. The absorption properties of the Li-doped NiO film are shown in our previous work [21].

Once again, a SiO₂ thin film was formed on the sample. The SiO₂ thin film was etched through the same lithographic process, and a smaller hole was formed. Then, an undoped β-Ga₂O₃ layer was formed on the NiO layer through the sol–gel method using a gallium isopropoxide solution [22]. The condition of the heat treatment was 750 °C 1 h. In our recent experiment, the resistivity of β-Ga₂O₃ formed on the MgO substrate, which has the same cubic structure and lattice constant as NiO, was ~150 Ω·cm. The absorption properties of the β-Ga₂O₃ film prepared by the sol–gel method are shown in our previous work [22]. For information purposes, the transmission spectra of the Li-doped NiO and β-Ga₂O₃ films formed on the sapphire substrate are shown in Supplementary Figure S1.

The SiO₂ layers were chemically etched using an HF solution to remove the NiO and β-Ga₂O₃ layers formed on the SiO₂ layer. The NiO and β-Ga₂O₃ layers on the β-Ga₂O₃ substrate were selectively retained, as shown in Figure 1.

Ohmic Ti/Al/Pt/Au electrodes were selectively formed on the top of the β-Ga₂O₃ layer and the bottom of the substrate through an annealing process at 500 ℃. The bottom electrode on the β-Ga₂O₃ substrate had a hole positioned underneath the top electrode for illumination of UV light. The diameter of the hole was around 1 mm. Finally, an Au electrode was selectively formed on the NiO layer. The diameter of the top electrode on the β-Ga₂O₃ layer was 0.5 mm, the diameter of the β-Ga₂O₃ layer was 0.7 mm, and the area of the NiO layer was approximately 1 mm².

The X-ray diffraction patterns (2θ–ω scan) of the samples were measured before all the metal electrodes were formed. The vertical line was on a logarithmic scale. Because the (001) β-Ga₂O₃ substrate was used, strong (001)-related reflection peaks were observed in the patterns, as shown in Figure 2. The peak from the NiO layer was a weak reflection of the (133) plane of NiO. The crystal orientation of the NiO thin film formed on the (001) β-Ga₂O₃ substrate was reported in a previous work. The (133)-oriented NiO layer was formed on the (001) β-Ga₂O₃ substrate. However, because the (133) plane was slightly inclined to the (001) plane of β-Ga₂O₃, the NiO (133)-related reflection peak was weakly observed. The reflection peaks of β-Ga₂O₃ were not observed except for the (001)-related diffraction. This suggests that the top β-Ga₂O₃ layer formed on the NiO layer is oriented to (001), the same as the (001) β-Ga₂O₃ substrate. The crystal orientation of the β-Ga₂O₃ thin film formed on NiO was studied using a MgO substrate, which has the same cubic structure and a similar lattice constant.

Figure 3 illustrates the scanning electron microscope (SEM) image of the β-Ga₂O₃ surface and NiO layers formed on the (001) β-Ga₂O₃ substrate. The top β-Ga₂O₃ layer was selectively formed by a lift-off process employing SiO₂ on the NiO layer, as described above. The NiO layer comprised small crystal grains, which maintained the crystal orientation. The top β-Ga₂O₃ layer on NiO comprised smaller crystal grains.

The top electrode on the β-Ga₂O₃ layer, the Au electrode on the NiO layer, and the bottom electrode on the β-Ga₂O₃ substrate are represented by E, B, and C, respectively, as shown in Figure 1. The structure between B and C corresponds to a p–n junction based on NiO/β-Ga₂O₃, which has a composition similar to that reported in [6]. The structure between E and C corresponds to an n–p–n junction based on β-Ga₂O₃/NiO/β-Ga₂O₃, which is a novel structure. The structure between E and C was compared with that between B and C.

In this study, a special electrode structure was not fabricated on the device so that the UV light could easily pass through the top electrode. Therefore, when the device was illuminated in the direction of the top electrode, the UV light that reached the junction region was weak because the top electrode may shut out the UV light. On the other hand, in the case when the device was illuminated on the bottom side, the wavelength region corresponding to the fundamental absorption of the β-Ga₂O₃ substrate is somewhat absorbed before the UV light reaches the junction region and the NiO layer. The UV light with a wavelength corresponding to the fundamental absorption of the NiO can reach the junction region by the window effect of β-Ga₂O₃ with a wider bandgap.

2.2. Measurements

The current–voltage characteristics were measured using a source-measure unit (Keithly 6487). UV light was irradiated from 0.1% to 100% using a deuterium (D2) lamp through several neutral-density (ND) filters. The light power density of the deuterium lamp was 22 mW/cm² as a rough estimate using a standard photodiode. The distance between the lamp and the bottom of the substrate was around 150 mm. To obtain the responsivity spectrum, a Xenon arc lamp was used with a monochromator as the optical excitation source. The wavelength was varied from 200 to 450 nm with an increment of 5 nm. The photoresponse spectra were compared based on the photoresponse of a calibrated photodiode.

A UV-LED with a peak wavelength of 310 nm was used as an optical source to measure the transient response curves. A pulse voltage was applied to the UV-LED, and the optical output was monitored using a Si avalanche photodiode (APD). UV light was irradiated on the bottom side of the UV photodetector with a β-Ga₂O₃/NiO/β-Ga₂O₃ through a quartz lens. The current response pulse of the detector was amplified and monitored using a digital oscilloscope.

3. Results and Discussion

3.1. Current–Voltage Characteristics

Figure 4a shows the current–voltage characteristics between the B and C electrodes corresponding to a p–n junction based on NiO/β-Ga₂O₃ structure. The current increased exponentially when the B electrode on the p-type NiO layer was positively biased. Under the reverse bias condition, the current was maintained at nearly 0.1 nA in the dark. The rectification ratio of the diode, which was calculated from the current at 5 V and −10 V bias, was about 1.8 × 10⁷. This rectifying property has been reported in a previous study [6]. Under the UV light illumination, the reverse current increased, whereas the forward current slightly increased in the low bias region. The rectification ratio of the diode, which was calculated from the current at 5 V and −10 V bias, was about 2.7 × 10⁴. This behavior between the B and C electrodes for UV light illumination is similar to the photodetection of a conventional p–n junction.

Figure 4b illustrates the current–voltage characteristics between the E and C electrodes corresponding to an n–p–n junction based on β-Ga₂O₃/NiO/β-Ga₂O₃ structure. The forward current between E and C was lower than that between B and C, suggesting a higher series resistance between E and C than that between B and C. Under the reverse bias condition, the current was maintained at 0.1 nA in the dark, and the reverse current increased; furthermore, the reverse current increased with increasing bias voltage under UV light illumination. This behavior is different from the B and C electrodes of the p–n junction based on NiO/β-Ga₂O₃. The rectification ratio of the diode, which was calculated from the current at 5 V and −10 V bias, was about 2.8 × 10⁴ and 35 in the dark and under UV light, respectively.

Figure 5a,b illustrate the dependence of reverse current–voltage characteristics up to 100 V of the p–n junction based on the NiO/β-Ga₂O₃ structure and the device based on the β-Ga₂O₃/NiO/β-Ga₂O₃structure on the relative intensity of UV light illumination, respectively. The intensity of UV light illumination was varied using a D₂ lamp and several ND filters. The dependence of the current–voltage characteristics of B–C on the relative intensity of UV light illumination is also shown in Figure 5a. With an increase in the intensity of UV light illumination, the reverse current was increased, maintaining the saturation property. The UV light on–off ratio calculated from the current at 100 V was about 628.

Figure 5b also shows the dependence of the current–voltage characteristics of E–C on the relative intensity of UV light illumination. The characteristics of E–C were similar to the characteristics of B–C at an intensity of UV light illumination lower than 1%. However, with the increasing intensity of UV light illumination, the current did not maintain the saturation property and increased with an increase in the bias voltage. It was observed that the electrical resistance between E and C decreased with an increase in the relative intensity of UV light illumination. The reverse current of E–C at 100 V increased more than 10 times compared to the current of B–C under the condition of 100% UV light illumination. The UV light on–off ratio calculated from the current at 100 V was about 10130.

Figure 6 shows the relationship between the reverse current of the device biased at 100 V and the relative intensity of UV light illumination. Note that logarithmic scales were employed on both the horizontal and vertical axes. The relationship for B–C, which corresponds to the p–n junction based on the NiO/β-Ga₂O₃ structure, had a linear inclination of 0.84. In contrast, the relationship for E–C, which corresponds to the n–p–n device based on the β-Ga₂O₃/NiO/β-Ga₂O₃ structure, had a larger inclination of 1.5. The reverse current increases in proportion to the 1.5 power of UV light intensity. This means that there is photocurrent amplification in the device with the innovation of the β-Ga₂O₃/NiO/β-Ga₂O₃ structure. This is the first demonstration of photocurrent amplification in a UV photodetector based on a β-Ga₂O₃ or a β-Ga₂O₃/NiO junction.

3.2. Photo Responsivity

Figure 7a shows the responsivity spectrum of B–C, which corresponds to the p–n junction based on the NiO/β-Ga₂O₃ structure. The highest sensitivity was observed at 285 nm. With increasing reverse bias voltage from 10 to 100 V, the responsivity was increased at the same wavelength as the maximum responsivity.

Figure 7b shows the responsivity spectrum of E–C, which corresponds to the n–p–n device based on the β-Ga₂O₃/NiO/β-Ga₂O₃ structure. When the device was biased at 10 V, the responsivity spectrum, whose peak was at about 290 nm, was broad, and the maximum responsivity was 290 nm, similar to that of the p–n diode shown in Figure 7a. It should be noted that the scale of the vertical line in Figure 7b is about 10 times larger than that in Figure 7a. With an increase in bias voltage, the wavelength of the maximum responsivity moved to 275 nm, and the maximum responsivity increased up to 10 mA/W when the bias was 100 V. The maximum responsivity was about 10 times greater than the responsivity of B–C. When the maximum responsivity was at about 290 nm under the bias condition of 10 V, it is suggested that the generated carriers in the NiO layer contributed to the photocurrent. When the maximum responsivity was suddenly increased at ~275 nm under the bias voltage >40 V, the generated charge carriers near the interface between NiO and β-Ga₂O₃ may have contributed to the photocurrent. The generated holes flow to the NiO layer and accumulate in the NiO layer. They contribute to the amplification of the electron flow from the top β-Ga₂O₃ layer to the NiO layer.

In this study, the obtained responsivity was compared with that of several UV photodetectors based on Ga₂O₃.

Table 1. Comparison of photoresponsivity of UV photodetectors based on Ga₂O₃ structure.

UV Photodetectors
Structure	Photoresponsivity	References
Transparent amorphous Ga2O3	2.66 A/W 5.78 A/W	[14] [15]
ITO/β-Ga2O3	1720.2 A/W	[16]
ITO/NiO/β-Ga2O3	27.43 A/W	[10]
β-Ga2O3/NiO	415 mA/W	[11]
β-Ga2O3/NiO with Pt nanoparticles	4.27 mA/W	[12]
β-Ga2O3/NiO	57 μA/W	[13]
β-Ga2O3NW/CH3NH3PbI3/NiO NW	254 mA/W	[17]
β-Ga2O3/NiO/β-Ga2O3	10 mA/W	This work

shows the comparison of structural and photo responsivity. The highest photoresponsivity of 1720.2 A/W was achieved in the ITO/β-Ga₂O₃ structure [16]. Furthermore, in the transparent amorphous Ga₂O₃ structure, a high responsivity of 2.66 A/W and 5.78 A/W was obtained [15,16]. This suggests that a transparent electrode structure effectively enhances photo responsivity.

Several studies have investigated the UV photodetectors with a NiO/β-Ga₂O₃ structure [11,12,13]. Although the responsivity was low at the beginning, it was enhanced, and a higher responsivity of 27.43 A/W was achieved for the device based on an ITO/NiO/β-Ga₂O₃ structure [11]. The photodetector that used both nanowire and CH₃NH₃PbI₃ achieved a higher responsivity of 254 mA/W [17]. The responsivity of 10 mA/W obtained in this study is lower than other detectors reported in the literature [10,11]. However, the presented detector demonstrated the amplification of photocurrent for the first time. This is a remarkable point.

3.3. Transient Response

The transient responses of the device to an optical pulse were measured using a UV-LED whose peak wavelength of light emission was 310 nm. The UV light can reach the junction region by passing through the β-Ga₂O₃ substrate without absorption by the window effect, even under the UV light illumination from the substrate’s bottom. The UV-LED was operated by a pulse generator. Figure 8a shows the waveform of the photoemission from the UV illumination measured by using an avalanche photodiode module and a digital oscilloscope. The optical pulse width and repetition period were 2 and 15 µs, respectively.

Figure 8b shows the photoresponse curves of E–C and B–C electrodes reversely biased at 10 V. The E–C and B–C correspond to the device based on the β-Ga₂O₃/NiO/β-Ga₂O₃ and the NiO/β-Ga₂O₃ (B–C) structures, respectively. The current with a road resistance of 50 kΩ was amplified and recorded in the digital oscilloscope. Clear sensing responses to the UV light pulses of 2 µs in width were observed for both E–C and B–C. The response level of E–C was higher than that of B–C. It was demonstrated that the device was able to respond to the optical pulse for shorter than a few microseconds. This response time was rather quick compared to the results of milliseconds reported in [5]. When the device’s properties and the circuit system to detect current pulses are improved, the device is expected to respond at a higher speed.

4. Discussion

As described by the characteristics shown in Figure 4b, Figure 5b, Figure 6 and Figure 7b, the presented device based on the β-Ga₂O₃/NiO/β-Ga₂O₃ structure has the function of amplification contrary to the p–n diode based on the NiO/β-Ga₂O₃ structure. It is certain that the addition of the β-Ga₂O₃ top layer caused the amplification of the photocurrent. To study the amplification mechanism of the detector with a β-Ga₂O₃/NiO/β-Ga₂O₃ structure, we conducted several experiments and used a schematic band model.

Normalized photoresponsivities of several diodes based on β-Ga₂O₃ were compared using the UV photodetector with a β-Ga₂O₃/NiO/β-Ga₂O₃ structure. The photoresponsivity of the β-Ga₂O₃ Schottky diode with a thin Au electrode is shown in Figure 9a, with the structure shown in (a1). The highest responsivity was obtained at 225 nm, which corresponds to the fundamental absorption of β-Ga₂O₃.

The normalized photoresponsivities of the diode based on a NiO/β-Ga₂O₃ hetero p–n diode are shown in Figure 9b,c, with the structures shown in (b1) and (c1). In the case of (b), UV light was illuminated on the diode from the surface of the thin Au electrode on the NiO layer. The UV light could reach the NiO layer through the thin Au electrode. The highest responsivity was obtained at 335 nm, which is similar to that obtained in the case where UV light was illuminated on the E–C detector from the surface side of the top electrode, as described above. In the case of (c), UV light was illuminated on the NiO/β-Ga₂O₃ hetero p–n diode from the bottom side of the β-Ga₂O₃ substrate, and the highest responsivity was obtained at 285 nm.

Figure 9d,e show the normalized photoresponsivities of the detector with NiO/β-Ga₂O₃ and β-Ga₂O₃/NiO/β-Ga₂O₃ structures. These responsivity spectra were obtained from the properties of the B–C and E–C electrodes biased at −100 V, respectively. The highest responsivities were obtained at 285 and 275 nm, respectively, and the wavelength of 285 nm is the same as that in Figure 9c. Because UV light was irradiated on the diode from the bottom of the 0.5-mm-thick Ga₂O₃ substrate, most of the UV light with shorter wavelengths corresponding to the fundamental absorption of β-Ga₂O₃ became weak when the light reached the region near the NiO/Ga₂O₃ junction. Though the UV light with shorter wavelengths was weak, the photoresponse at 275 nm, in other words, the photoresponse for higher-energy UV light with wavelengths shorter than 285 nm, increased quickly in the E–C detector. This indicates that the generated carriers near the interface between NiO and β-Ga₂O₃ layers_, which has a larger bandgap than NiO, contributed to the photocurrent. NiGa₂O₄ was synthesized at the interface between NiO and β-Ga₂O₃ under high-temperature conditions. The NiGa₂O₄ film has a bandgap value between that of NiO and β-Ga₂O₃. The holes generated in the β-Ga₂O₃ layer or Ga₂O₃ substrate may contribute to the photoresponse because the Ga₂O₃ layer on the NiO layer acts as a barrier for holes to flow toward the surface.

To elucidate the overview, a schematic band diagram of the device based on the β-Ga₂O₃/NiO/β-Ga₂O₃ structure is shown in Figure 10. In the p–n heterojunction comprising p-type NiO and n-type β-Ga₂O₃, the energy barrier for the flow from NiO to β-Ga₂O₃ for the holes in the NiO layer is greater than the energy barrier for the flow from β-Ga₂O₃ to NiO for the electrons in the β-Ga₂O₃ layer. The difference in the energy barrier was reported in a previous study [6]. The band offsets of $\mathsf{\Delta }$Ec and $\mathsf{\Delta }$Ev in the heterojunction between NiO and β-Ga₂O₃ were 2.2 and 3.4 eV, respectively.

Recombination centers exist at the interface between the NiO layer and the β-Ga₂O₃ substrate because of the lattice mismatch. The current in the NiO/β-Ga₂O₃ heterojunction diode is largely attributed to interface recombination. Furthermore, in the device based on the β-Ga₂O₃/NiO/β-Ga₂O₃ structure, the electrons in β-Ga₂O₃ may recombine with the holes in the NiO layer through the interface between the β-Ga₂O₃ and NiO layers.

UV light with a wavelength >260 nm reaches the NiO layer without attenuation, even when the UV light is illuminated from the bottom of the β-Ga₂O₃ substrate. When the device was illuminated with UV light and electron–hole pairs were generated on the NiO layer, the holes generated by the absorption of UV light in the NiO layer, including the holes that flowed to the NiO layer from the depletion layer of the p–n junction of B–C or B–E, are expected to accumulate in the NiO layer. The accumulated holes act as positive charges and positively bias the p–n heterojunction of B–E. Thus, the barrier height for electrons in the β-Ga₂O₃ layer mentioned above is decreased, which increases the electron flow from the β-Ga₂O₃ layer to the NiO layer. It is also expected that the accumulated holes decrease the electrical resistance of the NiO layer. This is an anticipated mechanism to amplify the photocurrent. In the p–n diode based on the NiO/β-Ga₂O₃ structure, the holes generated by the absorption of UV light in the depletion layer of the p–n junction of B–C reversely biased may flow to the NiO layer but will not accumulate in the NiO layer. Therefore, amplification of the photocurrent does not occur in the p–n diode structure.

5. Conclusions

The n–p–n structure was constructed by fabricating both the NiO layer formed on the β-Ga₂O₃ substrate and the β-Ga₂O₃ layer formed on the NiO layer. The device based on the β-Ga₂O₃/NiO/β-Ga₂O₃ (n–p–n) structure, as a UV photodetector, was compared with the p–n diode based on the NiO/β-Ga₂O₃ structure. The device based on the β-Ga₂O₃/NiO/β-Ga₂O₃ structure showed a 10 times greater responsivity and amplified the photocurrent. This amplification of the photocurrent is the first in a UV photodetector based on NiO and β-Ga₂O₃. The photocurrent of the device increased in proportion to the 1.5 power of relative UV light intensity. The photocurrent amplification is related to the accumulation of holes in the NiO layer provided by the heterobarrier for holes from the NiO layer to the β-Ga₂O₃ layer. Moreover, the device could respond to an optical pulse of less than a few microseconds.

The amplification of photocurrent is an important achievement. However, the photoresponsivity of the detector based on the β-Ga₂O₃/NiO/β-Ga₂O₃ structure is not high at this stage. In future studies, a structure that enables UV light to reach the junction region should be developed to improve the responsivity of the detector.