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Abstract
Self-powered deep ultraviolet photodetectors (DUV PDs) are essential in environmental monitoring, flame detection, missile guidance, aerospace, and other fields. A heterojunction photodetector based on p-CuI/n-ZnGa2O4 has been fabricated by pulsed laser deposition combined with vacuum thermal evaporation. Under 260 nm DUV light irradiation, the photodetector exhibits apparent self-powered performance with a maximum responsivity and specific detectivity of 2.75 mA/W and 1.10 × 1011 Jones at 0 V. The photodetector exhibits high repeatability and stability under 260 nm periodic illumination. The response and recovery time are 205 ms and 133 ms, respectively. This work provides an effective strategy for fabricating high-performance self-powered DUV photodetectors.
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1. Introduction
Self-powered deep ultraviolet photodetectors (DUV PDs) are renowned for their exceptional photoresponse even without an additional bias, making them indispensable in a multitude of applications including, optical imaging, environmental monitoring, flame detection, missile guidance, and short-range communications [1–5]. Zinc gallate (ZnGa2O4), characterized by an ultra-wide bandgap (Eg) semiconductor material with a direct Eg of approximately 5.2 eV, stands out as one of the prime candidates for crafting DUV PDs. Its stellar photoelectric properties and high chemical stability, render it an ideal choice [6–11]. In recent times, the spotlight has increasingly turned towards ZnGa2O4, as it finds utility in diverse optoelectronic realms such as vacuum fluorescence displays, low-voltage field-emission displays, and DUV PDs [12–15].
ZnGa2O4-based DUV PDs have been achieved through metal-semiconductor-metal (MSM) and heterojunction structures. Zhang et al. reported MSM structure PD using the PLD technique, with a maximum responsivity of 7.02 × 10−6 A/W under 262 nm light irradiation at 10 V [16]. Horng et al. successfully prepared MSM Schottky DUV PDs on an Al2O3 substrate with a 230 nm DUV responsivity of 0.46 A/W at 10 V [17]. However, the ZnGa2O4-based MSM structure DUV PDs exhibit a much lower responsivity. Han et al. reported a heterojunction PD based on p-Si/n-ZnGa2O4 with a peak response at 242 nm at 0 V [18]. Han et al. prepared a 4H-SiC/ZnGa2O4 heterojunction with a maximum responsivity (R) of 115 mA/W at 244 nm and an external quantum efficiency (EQE) of 58.4% at 0 V [19]. Liu et al. reported self-powered amorphous ZnGa2O4/NiO heterojunction UV PD with high sensitivity and stability, achieving a high R of 48.19 mA/W at 0 V due to the type II band alignment [20]. However, the small Eg of Si results in a large dark current in the device [21–23]. In addition, the preparation process of 4H-SiC and p-NiO is very cumbersome and highly polluting, which will affect the development of ZnGa2O4-based PDs. Hence, to achieve high-performance ZnGa2O4-based DUV PDs, it is imperative to investigate wide-bandgap p-type semiconductor materials with enhanced mobility or carrier concentration [24,25].
CuI is a transparent semiconductor with an Eg of 3.1 eV. It possesses a weak internal electrostatic field, high bonding energy, and low interatomic bonding force, which are favorable for applying high photovoltaic efficiency devices [26–28]. Owing to the presence of intrinsic Cu vacancies in the crystal lattice, copper iodide (CuI) exhibits intrinsic p-type conductivity [29,30]. Furthermore, CuI boasts high hall mobility, exceptional transmittance, and excellent absorption capabilities in the UV region, making it an exceptional candidate for UV PDs [31–33]. Given that p-CuI and n-ZnGa2O4 possess a type II energy band alignment structure, photogenerated carriers can undergo easier and swifter separation and migration, facilitating the creation of self-powered DUV PDs based on energy level matching [34–36].
In this work, ZnGa2O4 and CuI thin films were prepared by PLD and vacuum thermal evaporation, respectively. A self-powered p-CuI/n-ZnGa2O4 DUV PD was constructed, and the optoelectronic properties were investigated. Under DUV (260 nm) light irradiation, the PDs showed excellent and obvious self-powered performance with a high responsivity of 2.75 mA/W and a specific detectivity of 1.10 × 1011 Jones at 0 V. The PDs exhibit high repeatability and stability under 260 nm periodic illumination, with response and recovery times of 205 ms and 133 ms, respectively. The current findings open up new avenues for the advancement of high-performance ZnGa2O4-based self-powered DUV photodetectors.
2. Experiments
2.1 Materials and device preparation
Commercial Ceramic targets of ZnGa2O4 (Kurt J. Lesker Co.), CuI powders (Alfa Aesar, 99.998%), and gold particles (99.999%) were used as source materials. Double-sided polished Al2O3 (10 mm × 10 mm) single crystal used as substrate. Clean all substrates in an ultrasonic cleaner using acetone as a solvent for 15 minutes, then blow dry quickly with an air pressure gun (N2).
The film of ZnGa2O4 was prepared on Al2O3 substrates by PLD technique with a growth rate of ∼1 Å/s (COMPexPro201 KrF excimer laser, 8 Hz, 300 mJ, background pressure: 1.0 × 10−7 Pa, substrate temperature: 500°C). Subsequently, the films were annealed for 60 min at 800°C in an oxygen atmosphere (vacuum below 500 Pa) using a vacuum tube furnace (SKGL-1200C) [16].
Vacuum thermal evaporation was used to prepare the CuI film (chamber pressure: 3.0 × 10−4 Pa, substrate temperature: 100°C, growth rate was ∼0.1 Å/s). 200 nm-thick CuI films were deposited on the top surface of ZnGa2O4 thin films. After that, 50 nm-thick ohmic contact Au electrodes (evaporation rate between 0.1 ∼ 0.5 Å/s) were prepared on the top surface of p-CuI and n-ZnGa2O4 by thermal evaporation, respectively. Finally, the p-CuI/n-ZnGa2O4 PD was fabricated; refer to Supplement 1 S1 for a detailed preparation flowchart. More detailed information can be found in the previous reports [28,37].
2.2 Characterization and measurements
The crystal structures of ZnGa2O4 and CuI thin films were analyzed by X-ray diffraction (GIXRD, glancing-angle: 0.1°, Bruker D8 ADVANCE). The optical spectra of ZnGa2O4 and CuI thin films were measured using the UV-visible spectrophotometer (UV-2600i, Shimadzu) at room temperature (RT). A field-emission scanning electron microscope examined the morphology of the ZnGa2O4 thin films, CuI thin film, and heterojunction device (FE-SEM, FEI Nano Nova 450). Current-voltage (I-V) curves were measured using a Keithley 2912B digital source meter. Photoresponse of the p-CuI/n-ZnGa2O4 PDs was performed using a 150 W Xe lamp with a grating monochromator (RF5301PC, SHIMADZU Corporation, 150 W). The carrier concentrations were measured by a Hall effect measurement system (Lake Shore Model 8404).
3. Results and discussion
Figure 1(a) shows the GIXRD patterns of the ZnGa2O4 thin films deposited on Al2O3 substrates by PLD; the diffraction peaks of the (220), (311), (222), and (400) are consistent with the cubic ZnGa2O4 (PDF#38-1240) without any detectable second phase, indicating the successful fabrication of ZnGa2O4 thin films. Figure 1(b) shows the GIXRD patterns of the CuI thin films, which were prepared on the top surface of ZnGa2O4 thin films through masks using vacuum thermal evaporation. Except for the diffraction peaks of ZnGa2O4 thin films, the other peaks match the face-centered cubic structure CuI (PDF#75-0831). A local enlargement of 2θ from 35° to 37°, corresponding to the (311) diffraction peak of ZnGa2O4, is shown right of Fig. 1(b). The results indicate that high crystalline quality ZnGa2O4 and CuI thin films can be successfully fabricated using PLD and vacuum thermal evaporation techniques, respectively [38,33].
Fig. 1. GIXRD patterns of (a) ZnGa2O4 films were deposited on Al2O3 substrates using the PLD technique. (b) CuI thin films were prepared on the top surface of ZnGa2O4 thin films through masks using vacuum thermal evaporation.
Figure 2(a) shows the absorption spectra of ZnGa2O4 films on Al2O3 substrate at 200-900 nm. The absorption edge is located at 250-260 nm with a direct band gap characteristic of ZnGa2O4. The Eg can be obtained from Tauc plots in Eq. (1).
where α, h, ν, and A are the absorption coefficient, Planck constant, light frequency, and proportionality constant, respectively.Fig. 2. Absorption spectra of (a) ZnGa2O4 thin films, (b) CuI thin film, inset shows optical band gap values.
The Eg of the ZnGa2O4 thin film in this work was calculated to be ∼5.0 eV, which was close to the reported value (4.6-5.2 eV) [9], as shown in the inset of Fig. 2(a). The absorption spectra of the CuI thin film on the Al2O3 substrate at 370-800 nm with an absorption peak at 407 nm are displayed in Fig. 2(b). The calculated Eg of the CuI thin film was ∼2.99 eV, consistent with the literature reports [28] shown in the inset of Fig. 2(b).
Figure 3(a) and 3(b) show the surface and cross-sectional SEM images of the ZnGa2O4 films and the p-CuI/n-ZnGa2O4 heterojunction. A multilayer structure composed of CuI, ZnGa2O4, and Al2O3 substrate was observed, confirming that the thickness of the CuI layer was ∼ 200 nm and the ZnGa2O4 layer was ∼ 500 nm. The ZnGa2O4 and CuI thin films all have a dense and smooth surface, with fewer holes in them, and the interface is relatively clear and smooth without broken tiny grains. Therefore, the density of interface defects was low, which will help to reduce the leakage and short-circuit currents in the heterojunction device. Figure 3(c) shows the energy-dispersive X-ray spectroscopy (EDS) mappings of the p-CuI/n-ZnGa2O4 heterojunction cross-section. Cu, I, Zn, Ga, and O were uniformly distributed. The above results demonstrate that high crystalline quality ZnGa2O4 and CuI thin films can be successfully fabricated by PLD and thermal evaporation techniques. A three-dimensional photograph of the surface of a ZnGa2O4 film in a 5 × 5 µm2 region was displayed in Supplement 1 Fig. S2.
Fig. 3. (a) The surface and cross-sectional SEM images of the (a) ZnGa2O4 thin films and (b) p-CuI/n-ZnGa2O4 heterojunction. (c) EDS mapping of p-CuI/n-ZnGa2O4 heterojunction.
The p-CuI/n-ZnGa2O4 PDs were constructed after the 50-nm-thick Au electrodes were prepared by thermal evaporation, as shown in Fig. 4(a). The contact between Au and p-CuI as well as n-ZnGa2O4 was investigated. The linear relationship of the current-voltage (I-V) curves in Fig. 4(b) reflects the ohmic contacts at the Au/CuI and Au/ZnGa2O4 interfaces. Further, the optoelectronic properties of the PD were investigated. Figure 4(c) shows the I-V curves of the PD in the dark and under 260 nm UV light irradiation (6.5 µW/cm2). The current of the PD under 260 nm UV irradiation is greater than that in the dark, and the I-V curve shifts to the upper right concerning that in the dark, showing a significant photoresponse characteristic with a zero-bias photocurrent production [39]. The photocurrent was ∼2 × 10−10 A, the dark current was ∼2.1 × 10−12 A, and the current on/off ratio was ∼ 105 at 0 V. These results indicate that the p-CuI/n-ZnGa2O4 PD can be operated without voltage.
Fig. 4. (a) Schematic illustration of the Au/p-CuI/n-ZnGa2O4/Au PD. (b) I-V curves of Au electrode on CuI and ZnGa2O4. (c) I-V characteristics of the p-CuI/n-ZnGa2O4 PD under the dark and 260 nm UV light irradiation (6.5 µW/cm2). (d) Responsivity spectra (red curve) and inset show the specific detectability (blue curve) of the PD at 0 V. (e) Time-dependent photoresponse of the p-CuI/n-ZnGa2O4 DUV PD at 0 V under 260 nm light illumination. (f) Response and recovery time of the PD at 0 V under 260 nm light illumination.
Responsivity (R) and specific detectivity (D*) are important parameters to quantify the response and detection ability of a PD, which can calculate Eqs. (2) and (3):
where Iph and Idark are the photocurrent and dark current generated by the PD, Plight is the power of the incident illumination, q is the electron charge (1.602 × 10−19 C), and S is the effective area of the film between the electrodes. The R and D* of the p-CuI/n-ZnGa2O4 PD in the wavelength range from 250 nm to 380 nm are shown in Fig. 4(d). The PD exhibits strong response and detectivity in the 255-260 nm wavelength range, indicating that the p-CuI/n-ZnGa2O4 detector can be used as a DUV PD. The maximum R and D* was 2.75 mA/W and 1.10 × 1011 Jones under 260 nm illumination with a light power density of 6.5 µW/cm2 at 0 V.The extra-quantum efficiency (EQE) of the PD is obtained from Eq. (4):
where c is the speed of light, λ is the wavelength of the light signal. The EQE of the p-CuI/n-ZnGa2O4 DUV PD at different wavelengths is shown in Supplement 1, Fig. S3, with a maximum value of 5.1%.To further investigate the stability and repeatability of the p-CuI/n-ZnGa2O4 DUV PD, the current-time (I-t) curves of the PD were measured at 0 V by cyclically irradiating the PD with 260 nm light (6.5 µW/cm2) and a switching time of 20 s. As shown in Fig. 4(e), DUV PD exhibits prominent and stable photoresponse and recovery characteristics, indicating that PD has high repeatability and stability. The light and dark current changes of p-CuI/n-ZnGa2O4 DUV PD in a response recovery period were characterized, as shown in Fig. 4(f). The response time (the time required for the photocurrent to increase from 10% to 90%) and the recovery time (the time required for the photocurrent to decrease from 90% to 10%) are 205 ms and 133 ms, respectively. The above results indicated that the built-in electric field at the p-CuI/n-ZnGa2O4 interface could achieve a relatively fast separation of hole and electron pairs. The p-CuI/n-ZnGa2O4 DUV PD has a faster response recovery time compared to the reported ZnGa2O4 or CuI-based PDs [16,17,34,40]. Refer to Supplement 1 Table 1 for a detailed parameter comparison.
The photoresponse characteristics of different light power densities were also investigated. Figure 5(a) shows the I-V characteristics of the p-CuI/n-ZnGa2O4 DUV PD under 260 nm irradiation at different soft power densities. The Iph increases rapidly with increasing light power density at 0 V. It also increases with increasing negative bias at the same light power density. The higher the optical power, the more photogenerated carriers are produced, increasing the potential of the built-in electric field, which is in the same direction as that of the reverse bias, and a larger forward bias is required to overcome the increased built-in electric field. Thus, the current at the 0 V point shifts downward, resulting in an overall downward shift in the photocurrent curve. When the optical power is enhanced, the photocurrent decreases. Furthermore, the defects in PD were explored by calculating the ideal factor (n). The n of the p-CuI/n-ZnGa2O4 DUV PD was obtained by Eq. (5):
Where q/kT = 39.57 and dV/dlnI = 1/slope [41]. The n of the PD was calculated to be 2.67, as shown in Fig. 5(b). When n exceeds 1, indicating the presence of some defects in the PD, which is consistent with fewer holes in ZnGa2O4 and CuI thin films as shown in Fig. 3. The presence of defects not only results in lower responsivity (2.75 mA/W) and specific detectivity (1.10 × 1011 Jones), but also leads to instability at high current conditions and generates a lot of heat during long-term operation. Therefore, it is necessary to continue improving the film's quality or to reduce the contact resistance between the film and the electrode by adding an intermediate layer to minimize defects.Fig. 5. (a) Photocurrent under 260 nm UV irradiation with different optical power densities. (b) The ideal factor of the p-CuI/n-ZnGa2O4 DUV PD.
To verify that this DUV PD can work properly, the image detection test under 260 nm light irradiation was conducted. During the image detection, the corresponding images were generated on the mask with “L” and “H” patterns under 260 nm illumination, respectively; the imaging schematic diagram is shown in Supplement 1, Fig. S4. The PD will move horizontally and vertically to collect each pixel's current and send the corresponding current to the computer for counting (the pixel array is 5 × 5). The image based on the value of the currents is shown in Fig. 6. Both image profiles of “L” and “H” can be observed to indicate that p-CuI/n-ZnGa2O4 DUV PD can be used for image detection [42,43].
A classical type-II aligned energy band diagram of the CuI/ZnGa2O4 p-n heterojunction interface is shown in S5. To explore the carrier migration and separation mechanism of the self-powered p-CuI/n-ZnGa2O4 DUV PD, the energy band diagram based on Anderson model was constructed, reference Supplement 1 S6 for energy band modeling in dark conditions, 260 nm illumination is shown in Fig. 7. The electron affinity (χ) of n-ZnGa2O4 and p-CuI are 2.31 eV and 2.1 eV [44,45], respectively. The Eg of n-ZnGa2O4 and p-CuI are calculated to be ∼5.0 eV and ∼2.99 eV from Fig. 2. The Hall effect test results indicate that CuI thin film is a p-type semiconductor with a hole concentration of ∼1018 cm−3, the ZnGa2O4 thin film is an n-type semiconductor with an electron concentration of ∼1015 cm−3 [46,47].
Fig. 7. Schematic of the energy band diagram based on the Anderson model of p-CuI/n-ZnGa2O4 DUV PD under 260 nm irradiation at 0 V.
In the case of self-powered PDs, the built-in electric field separates the photogenerated charge carriers without applying driving voltage [48,49]. When the surface of the PD is irradiated by UV light, the incident light passes through the CuI layer with minimal absorption. It is absorbed by the ZnGa2O4 layer, generating photogenerated charge carriers within the PD. The internal electric field separates electron-hole pairs, driving the photogenerated holes through the CuI layer to the Au electrode. In contrast, the electrons are guided through the ZnGa2O4 layer to the Au electrode. Meanwhile, the photogenerated electrons are prevented from entering the CuI layer to form a self-composite due to a significant conduction band shift (ΔEC = 0.21 eV) and valence band shift (ΔEV = 2.22 eV). The electrons and holes, driven by their built-in electric fields, move towards the electrodes at both ends, so the photocurrent generated without applying an external voltage proves the self-powered performance during the period.
4. Conclusion
In conclusion, the p-CuI/n-ZnGa2O4 photodetectors were successfully fabricated, which exhibited apparent self-powered performance under 260 nm (DUV, 6.5 µmW/cm2) light irradiation. The maximum responsivity and specific detectivity of the p-CuI/n-ZnGa2O4 photodetector were 2.75 mA/W and 1.10 × 1011 Jones with an on/off ratio of 105 at 0 V. The photodetector exhibits high repeatability and stability under 260 nm periodic illumination, with response and recovery times of 205 ms and 133 ms, respectively. This work provides insight into the fabrication of high-performance self-powered DUV photodetectors and opens up further possibilities for practical applications of p-CuI/n-ZnGa2O4 DUV photodetectors.
Funding
National Natural Science Foundation of China (No. 12274189, No. 62075092); Young Taishan Scholars Program of Shandong Province of China (NO. tsqn202306239); Natural Science Foundation of Shandong Province (ZR2021MF121, ZR2022MA045, ZR2022QF052); Fundamental Research Program of Shanxi Province, China (Grant No. 202103021223185).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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