Photodetectors which can convert optical signals into electrical signals has received intensive attention due to their promising applications in many fields, including military warning, environmental monitoring, chemical and biological analysis, communication security, ^[1–3] With the development of nanometer technology and microelectronic processes, photodetectors with better performance have been fabricated in different structures.^[4–6] However, for the complexity of the process and high cost of materials, the photodetectors cannot be widely used. On the other hand, due to the absorption of deep ultraviolet (DUV) light by ozone sphere, photodetectors operating in a wavelength range of 200 nm–280 nm (so-called solar-blind region) do have the advantages of higher sensitivity and lower false alarm rate. Over the past decade, a large number of wide-bandgap semiconductors such as SiC, AlGaN, MgZnO, and β-Ga₂O₃ have been employed to fabricate solar-blind photodetectors.^[7–17] Moreover, photodetectors based on wide-bandgap semiconductors are expected to work in harsh conditions like astronautical exploration, to break through the limitations of the bulky power supply for the commercial DUV photodetector based on vacuum photo-multiplier tubes.

The β-Ga₂O₃ (band gap: ∼4.9 eV), as a next-generation wide-bandgap semiconductor, is considered as an ideal candidate for photoelectronic and electronic devices.^[18–22] Until now, various types of the β-Ga₂O₃ based solar-blind photodetectors have been investigated, including single crystal, thin film, nanostructure, metal-semiconductor-metal (MSM), heterojunction.^{[15,19,23–29]} Among them, heterojunction structure can efficiently enhance the photogenerated carrier transport by tuning the barrier height and realize carrier multiplication via impacting ionization process under relatively large bias, suggesting its promising application in high performance photodetectors. For instance, ZnO–Ga₂O₃ core–shell nanowire based solar-blind avalanche photodetector was reported by Zhao et al. The photodetector shows the highest reported responsivity of 1.3 × 10³ A/W with a rise time shorter than 20 and a decay time of . Moreover, for practical purposes, a heterojunction photodetector based on thin-film type is appealing due to its good adhesion and high reproducibility. To ensure good photodetector performance based on the heterojunctions, materials with smaller lattice mismatch are highly desired. So far, several single crystal substrates (Si, Nb:SrTiO₃, Ga:ZnO, SiC) are employed to form heterojunctions with Ga₂O₃ films.^[25,30–33] In this study, we deposit ZnO thin films and β-Ga₂O₃ thin films on sapphire substrates orderly to fabricate a vertically structured heterojunction. The photodetector based on the thin-film heterojunction exhibits excellent rectification characteristics and self-powered features.

The β-Ga₂O₃/ZnO heterojunctions were grown on (0001) c-plane sapphire substrates by laser molecular beam epitaxy (LMBE) with a KrF laser operating at a wavelength . The laser energy was about 400 mJ at a frequency of 2 Hz. During the deposition, the chamber was evacuated to a base pressure of 1 × 10⁻⁵ Pa. The distance between target and substrate was 50 mm, and the growth temperature was 750 °C. The ZnO layers were first deposited on Al₂O₃ substrates. The subsequent β-Ga₂O₃ thin films were grown on the surface of ZnO films under the same condition by introducing a shelter to cover part of the ZnO films. A schematic view of the β-Ga₂O₃/ZnO heterojunction device is shown in Fig. 1(a). The film structure was characterized by a Bruker D8 Advance x-ray diffractometer (XRD) using Cu ( Å) radiation. In order to measure the current–voltage (I–V) curves and photoresponse of β-Ga₂O₃/ZnO heterojunctions, Indium electrodes (0.1 mm × 0.1 mm) were fabricated on both the β-Ga₂O₃ layer and ZnO layer. The I–V characteristic curves of the β-Ga₂O₃/ZnO heterojunctions were measured by Keithley 2450. A low-pressure mercury lamp of 254/365 nm wavelength was employed for the photoresponse measurement.

Fig. 1.

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	Fig. 1. (a) Schematic diagram of constructed β-Ga2O3/ZnO heterojunction, (b) XRD patterns of c-plane Al2O3 substrate, ZnO/Al2O3, and β-Ga2O3/ZnO/Al2O3 samples, respectively.

Figure 1(b) shows the XRD patterns of the (0001) sapphire substrate, the ZnO layer, and the β-Ga₂O₃/ZnO heterojunction, respectively. The sapphire (0006) diffraction peak is utilized to calibrate and normalize the patterns. As shown in Fig. 1(b), except for the substrate diffraction peaks, the observed peaks can be indexed to the (002) lattice plane of ZnO (JCPDS Card no. 36-1451). In addition, it is clear that three peaks are observed around 18.8°, 38.2°, and 58.9° corresponding to the ( 01), ( 02), and ( 03) lattice planes of β-Ga₂O₃ (JCPDS Card no. 43-1021) in the pattern of heterojunction. The results indicate that the β-Ga₂O₃ thin films are grown in the ( 01) orientation.

Figure 2(a) shows the I–V characteristic curves of the β-Ga₂O₃/ZnO heterojunction in the dark and under the illumination of 254-nm and 365-nm light, respectively. Here, the forward bias is set as the current flowing from the ZnO thin film into the β-Ga₂O₃ thin film. The contact between the In–ZnO and the In–β-Ga₂O₃ is confirmed as Ohmic contact by the linear I–V curves. Both the ZnO film and β-Ga₂O₃ film are about 100-nm thick. The distance between In electrodes is about 1 cm. The carrier concentration of the as-grown ZnO layer is ∼5 × 10¹⁶ cm⁻³. On the other hand, the β-Ga₂O₃ layer is highly electrically resistive (sheet resistance over ). Thus no measurable conductivity was observed by the Hall measurement technique. The I–V curve of the β-Ga₂O₃/ZnO heterojunction presents an obvious rectifying property. The current increases sharply, presenting an on-state under an about 2-V positive bias and an off state under a reverse bias. The asymmetric ratio, I(4 V)/I(−4 V), is over 10⁵ as shown in the inset. Comparing with most other reports on Ga₂O₃-based heterojunctions, the asymmetry of I–V relation is very obvious in β-Ga₂O₃/ZnO heterojunction, indicating an excellent rectification characteristic. When the heterojunction is illuminated under 254-nm light, the photocurrent presents a pronounced increase from to at 5 V and to at −7 V. In contrast, the heterojunction is not sensitive to 365-nm light, indicating a solar-blind characteristic. The spectral responsivity ( ) is an important parameter to determine the sensitivity of the photodetector. A rejection ratio of value for 254 nm/365 nm is approximately 600 at −7-V bias.

Fig. 2.

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	Fig. 2. (a) I–V characteristic curves of the β-Ga2O3/ZnO heterojunction in the dark, under 254-nm and 365-nm illuminations, with inset showing I–V curve of heterojunction in the dark in log scale; (b) I–V characteristic curve of the In–ZnO–In structure; (c) I–V characteristic curve of In–β-Ga2O3–In structure.

The time-dependent photoresponse measurements are performed to evaluate the response speed of the photodetector by periodically turning the mercury lamp ON and OFF under different bias voltages (−7 V, −5 V, 0 V, and 5 V). The heterojunction exhibits robustness and good reproducibility performance, as the fluctuations in response curve are nearly identical and repeatable as shown in Fig. 3. The quantitative analysis of the current rise process and decay process are fitted by (involving the fitting of the time-dependent photoresponse curve) an exponential relaxation equation of the following type: where I₀ is the steady-state current, A is constant, and τ is the relaxation time constant. The rise time constant and decay time constant are marked in the figure. At zero-bias, the dark current is approximately 1.89 nA. Under the illumination of 254-nm wavelength light with an intensity of , the current increases rapidly to a stable value of 3.12 nA, revealing the self-powered characteristic of the heterojunction. When the UV light turns off, the current decreases instantaneously down to the initial dark value. The rise time and the decay time at zero bias are approximately 0.035 s and 0.032 s, respectively. In contrast, the response times for applied bias of −5 V, −7 V, and 5 V are slower than that for zero bias, specifically, is 0.065 s and is 0.033 s at −5 V; is 19.58 s and is 18.20 s at 5 V; is 13.74 s and is 15.01 s at −7 V.

Fig. 3.

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	Fig. 3. Plots of time-dependent photoresponse of the β-Ga2O3/ZnO heterojunction under illumination of 254-nm wavelength light at different switching on/off bias voltages: (a) −7 V, (b) −5 V, (c) 0 V, and (d) 5-V bias.

The rectifying I–V and photoelectric characteristic of the heterojunction can be explained by the energy band diagram shown in Fig. 4. The photocurrent generation and transportation mechanism are considered by using the Anderson model. The band gap of β-Ga₂O₃ and ZnO are taken to be 4.9 eV and 3.37 eV, and the electron affinities are taken to be 4 eV and 4.35 eV respectively.^[34,35] Fermi levels are raised up after β-Ga₂O₃ has contacted ZnO, and the electrons will flow from β-Ga₂O₃ to ZnO. The accumulation region will be formed in the ZnO layer, while the depletion region will be formed in the β-Ga₂O₃ layer. When the heterojunction is in thermal equilibrium, Fermi levels of β-Ga₂O₃ and ZnO are uniform. Herein, a built-in electric field near the interface would be built to balance electron drift, called the space charge region. When a reverse bias is applied, the electric field direction is consistent with the built-in electric field. As the field in the space charge region widens, the barrier of the conduction band between β-Ga₂O₃ and ZnO increases, and the electron energy will diminish and be insufficient to pass through the barrier. When a forward bias is applied, the space charge region narrows and the barrier of the conduction band between β-Ga₂O₃ and ZnO lowers, leading to the facilitation of electron transfers. Under 254-nm DUV light illumination, the electrons are excited to jump from valence band to conduction band and the photogenerated electron-hole pairs are separated by the built-in electric field. The carrier concentration in the space charge region increases, resulting in the increase of the photocurrent. The applied bias can conduce to separating more photogenerated electron-hole pairs, which leads to an increase for photogenerated carrier concentration. However, due to the presence of oxygen vacancies in β-Ga₂O₃ and ZnO thin films, the photogenerated electrons will be captured by oxygen vacancies. Therefore, the slow response can be attributed to the extremely slow carrier trapping and releasing process under applied voltage. At zero bias voltage, the original built-in electric field intensity is insufficient to offset the increasing photocarrier drift. The number of carriers in the space charge region is a small fraction due to its narrow width. Hence, the photocurrent at zero bias voltage is small. However, the carriers released from the oxygen vacancy traps make almost no contribution to the current under zero bias, thus resulting in a fast response.

Fig. 4.

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	Fig. 4. Schematic energy band diagrams of β-Ga2O3/ZnO heterojunction. (0) Before contact. After contact under dark conditions at (a) 0 V bias, (b) reverse bias, and (c) positive bias; under 254-nm illumination at (d) 0 V bias, (e) reverse bias, and (f) positive bias.

The β-Ga₂O₃/ZnO heterojunctions are fabricated on sapphire substrates by laser molecular beam epitaxy. The obtained heterojunction presents an excellent rectifying characteristic and solar-blind photoelectric property. At 0-V bias, the photodetector exhibits high response speed and high stability, revealing self-powered characteristics. The observed self-powered performance can be related to the built-in electric field driven photogenerated electron-hole pairs’ separation in the depletion. An enhanced responsivity is obtained when the applied bias increases, owing to the increased electric field promoting carrier separation. The film-type heterojunction has a low cost and extensive application prospect.