High-Performance NiO/TiO2/ZnO Photovoltaic UV Detector

The ultraviolet (UV) photodetector has found many applications, ranging from optical communication to environmental monitoring. There has been much research interest in the development of metal oxide-based UV photodetectors. In this work, a nano-interlayer was introduced in a metal oxide-based heterojunction UV photodetector to enhance the rectification characteristics and therefore the device performance. The device, which consists of nickel oxide (NiO) and zinc oxide (ZnO) sandwiching an ultrathin dielectric layer of titanium dioxide (TiO2), was prepared by radio frequency magnetron sputtering (RFMS). After annealing, the NiO/TiO2/ZnO UV photodetector exhibited a rectification ratio of 104 under UV irradiation of 365 nm at zero bias. The device also demonstrated a high responsivity of 291 A/W and a detectivity of 6.9 × 1011 Jones at +2 V bias. Such a device structure provides a promising future for metal oxide-based heterojunction UV photodetectors in a wide range of applications.


Introduction
The ultraviolet (UV) photoelectric detector has attracted much research interest, as it has found many important applications in optical communication, remote control, astronomical imaging, bioaerosol detection, fire detection, and environmental monitoring [1][2][3][4][5][6]. In recent years, there has been much research on the use of wide band gap metal oxide semiconductor materials, such as NiO, ZnO, TiO 2 , Ga 2 O 3 , SnO 2 , etc., to develop UV photodetectors. These materials offer good stability, rapid response, high sensitivity, and low production cost, which are essential criteria for the applications of UV photodetectors [7][8][9][10].
NiO is a p-type semiconductor material. It has a direct band gap of~3.6-4.3 eV [11], a high melting point of 1960 • C, and a relatively high exciton binding energy of 110 meV [12]. Furthermore, the environmental friendliness and abundance of NiO make it an attractive material in the fabrication of UV photodetectors for widespread applications [13][14][15]. ZnO is an n-type semiconductor material with a band gap energy of~3.3 eV [16,17]. It is inexpensive, non-toxic, and provides high optical transparency. Moreover, the material can be easily synthesized on an industrial scale [18,19].
There are several reports on the study of UV photodetectors based on NiO/ZnO heterojunction. For example, Rana [20] et al. fabricated an Ag-NWs/NiO film/ZnO film/FTO UV detector, and the responsivity of the device was 0.29 A/W. Zhang [21] and Shen [22] prepared Cr/Au/NiO nanofiber/ZnO nanofiber/Al and Ag/ZnO nanorod/NiO film/FTO UV detectors and the responsivities of these devices were 0.42 and 0.44 mA/W, respectively. However, these UV photodetectors based on NiO/ZnO heterojunction exhibited relatively poor photo-response and device performance. Jia [23] et al. reported a p-NiO/SiO 2 /n-ZnO ultraviolet detector, with the device showing high performance with responsivity and detectivity of 5.77 A/W and 1.51 × 10 11 Jones, respectively. To obtain a higher performance of the UV photovoltaic detector, TiO 2 is used instead of SiO 2 . Adding a dielectric layer is equivalent to adding a barrier in the heterojunction of the device, and different barriers can change the efficiency of the carrier transport [24]. The band gap of TiO 2 is smaller than that of SiO 2 ; meanwhile, TiO 2 has a larger dielectric constant than SiO 2 , which is easier to excite electrons and more conducive to charge storage.
In this work, an ultra-thin dielectric layer of TiO 2 was introduced between the NiO and ZnO layers to enhance the performance of the metal oxide-based heterojunction UV photodetector. After annealing, the NiO/TiO 2 /ZnO heterojunctions exhibited good rectification characteristics with a high rectification ratio of 10 4 . The use of the TiO 2 interlayer and its role was investigated. The photodetector demonstrated low dark current and excellent optical response. Under 365 nm UV illumination, the maximum responsivity and detectivity of the device was 291 A/W and 6.9 × 10 11 Jones, respectively.

Materials
Quartz substrate, sputtering targets (such as nickel oxide, zinc oxide target, and titanium dioxide), and ITO were purchased from ZhongNuo Advanced Material (Beijing) Technology Co., Ltd. (Beijing, China). The purity of all the target materials was 99.99%.

Device Fabrication
The quartz substrate was cleaned in a mixture of ammonia, hydrogen peroxide, and deionized water (1:1:2) at 80 • C for 1 h, followed by dipping in deionized water for 30 s and then blow-dried with nitrogen. The sputtering power used to deposit NiO, TiO 2 , and ZnO films was 200, 100, and 200 W, for a duration of 3000, 50, and 3000 s, respectively. The sample was annealed at 300 • C for 2 h after deposition of the films. This was then followed by lithography and the deposition of electrical contacts to the device.

Characterization Techniques
X-ray diffraction (XRD, Rigaku D/Max-23) and transmission electron microscopy (TEM, Tecnai G2 F30 Twin) were used to analyze the crystal structure of the deposited films. The thickness of the film was measured by atomic force microscopy (AFM) using Bruker Dimension ICON, and the thickness of the dielectric film in the device was optimized based on the device performance. A scanning electron microscope (SEM, Nova NanoSEM 450) was used to provide a cross-sectional analysis of the deposited films and thickness measurements of different layers in the device. Keysight B1500A was used for electrical characterization of the device. All measurements were conducted at room temperature. was 70°. Figure 1c shows the lattice spacings of NiO, with 0.205, 0.241, and 0.147 nm corresponding to its (012), (101), and (110) planes, respectively. The angle between the (101) and (110) planes of NiO was 83°. The lattice spacings of TiO2 were 0.321, 0.163, and 0.149 nm corresponding to the (110), (220), and (002) planes, respectively, as shown in Figure  1d. It belonged to the rutile-type indices of crystal face, indicating that the prepared TiO2 exhibited good crystal quality in the rutile-type phase. The angle between the (110) and (002) planes of TiO2 was 65°. All interplanar angles are consistent with those reported in the literature. The atomic spatial arrangements of the crystal planes for the different materials are shown as insets ⅱ, ⅲ, and ⅳ in Figure 1b-d.  Figure 2a shows the cross-sectional SEM image of the deposited films. The film thickness, which has an important influence on the device performance, can be deduced from the image. From top to bottom, the image reveals the deposited films of ZnO, TiO2, NiO, and ITO having a thickness of ~749.4, 4.3, 381.9, and 176.6 nm, respectively, on the quartz substrate. The cross-sectional image also shows that the different metal-oxide films exhibited longitudinally dense growth mode. Figure 2b shows the XRD patterns of the ZnO film before (ZnO-1#) and after (ZnO-2#) annealing at 300 °C for 2 h. Compared with the standard PDF cards, peaks related to the (002) and (103) crystal planes were observed before annealing of the ZnO film. After annealing, an increase in the number of crystal planes was evident. The full width at half maximum (FWHM) of the (002) crystal plane was significantly narrowed after annealing. This indicates that the annealing of the ZnO film has led to a significant reduction in the defect density and improvement in the crystal quality, which is favorable to the transport of charge carriers in the material. Figure 2c shows the XRD patterns of the NiO film before (NiO-#1) and after (NiO-#2) annealing at 300 °C for 2 h. It was found that the number of NiO diffraction patterns remained the same before and after annealing. There were five crystal planes, namely, (101), (012), (110), (113) and (202), related to the NiO film (similar to the standard PDF cards). However, the intensity of all diffraction patterns became stronger after annealing, and the FWHM of these  Figure 2a shows the cross-sectional SEM image of the deposited films. The film thickness, which has an important influence on the device performance, can be deduced from the image. From top to bottom, the image reveals the deposited films of ZnO, TiO 2 , NiO, and ITO having a thickness of~749.4, 4.3, 381.9, and 176.6 nm, respectively, on the quartz substrate. The cross-sectional image also shows that the different metal-oxide films exhibited longitudinally dense growth mode. Figure 2b shows the XRD patterns of the ZnO film before (ZnO-1#) and after (ZnO-2#) annealing at 300 • C for 2 h. Compared with the standard PDF cards, peaks related to the (002) and (103) crystal planes were observed before annealing of the ZnO film. After annealing, an increase in the number of crystal planes was evident. The full width at half maximum (FWHM) of the (002) crystal plane was significantly narrowed after annealing. This indicates that the annealing of the ZnO film has led to a significant reduction in the defect density and improvement in the crystal quality, which is favorable to the transport of charge carriers in the material. Figure 2c shows the XRD patterns of the NiO film before (NiO-#1) and after (NiO-#2) annealing at 300 • C for 2 h. It was found that the number of NiO diffraction patterns remained the same before and after annealing. There were five crystal planes, namely, (101), (012), (110), (113) and (202), related to the NiO film (similar to the standard PDF cards). However, the intensity of all diffraction patterns became stronger after annealing, and the FWHM of these patterns was significantly narrowed. The diffraction patterns in Figure 2b,c were consistent with the observed crystal planes in the TEM images in Figure 1b Figure 2g. The grain boundary became less obvious after annealing. Figure 2h shows an AFM image of the TiO 2 film with a step edge. The line profiles taken across the step edge are shown in Figure 2i. From the line profiles, the thickness of the TiO 2 film was measured to be~4.3 nm, which is consistent with the thickness measured in Figure 2a. patterns was significantly narrowed. The diffraction patterns in Figure 2b,c were consistent with the observed crystal planes in the TEM images in Figure 1b,c. Figure 2d-f show the SEM images on the surface morphology of the three metal oxide films. Large particles with obvious boundaries were observed in the ZnO film. NiO particles were relatively smaller than those of ZnO but exhibited tapered features. The TiO2 film has much finer and denser particles with good size distribution. An SEM image of the top view of the ZnO/TiO2/NiO films is shown in Figure 2g. The grain boundary became less obvious after annealing. Figure 2h shows an AFM image of the TiO2 film with a step edge. The line profiles taken across the step edge are shown in Figure 2i. From the line profiles, the thickness of the TiO2 film was measured to be ~4.3 nm, which is consistent with the thickness measured in Figure 2a.     Figure 3b,c show the I−V characteristics of three different devices, namely, Device 'A', 'B' and 'C'. Device 'A' (represented by black curves) consisted of the NiO/ZnO heterojunction and did not exhibit rectification behavior. Device 'B' (represented by red curves), which consisted of an ultra-thin TiO 2 dielectric layer between the NiO and ZnO films, exhibited slight rectification behavior. Device 'C' (represented by blue curves) has the same device structure as Device 'B' but has undergone annealing treatment. It demonstrated very good rectification characteristics with an excellent rectification ratio of 10 4 . The results showed that the rectification characteristic of the device was significantly improved by introducing a dielectric inter-layer and annealing treatment. Figure 3d shows the energy band structure of the metal oxide-based UV photodetector. Photogenerated charge carriers are generated under UV light irradiation and driven by an external electric field, causing electrons to flow from NiO to ZnO while holes flow in the opposite direction. The role of the TiO 2 is to block the migration of charge carriers, hence improving the rectification characteristic of the device. Figure 3e shows the I−V characteristics of Device 'C' under dark condition and Sensors 2023, 23, 2741 5 of 7 365 nm light with a power density of 1.479 mW/cm 2 . When the UV light was switched on or off, the device exhibited good stability and repeatability. When the UV light was turned on (off), the current of the device changed rapidly, indicating that the heterojunction device has a fast UV response. The device has a rise time and a fall time (defined as the time required for the photocurrent to increase from 10% to 90% and drop from 90% to 10%) of 163.04 and 282.19 ms, respectively, at 0 V bias, as shown in Figure 3f. Responsivity (R) and detectivity (D*) are important performance parameters for a photodetector. They are calculated using the followings formulae [25]:

Device Performance and Analysis
where I ph and I d are photocurrent and dark current, respectively, P is optical power density, A is effective irradiation area, and q is unit charge. The plots of responsivity and detectivity of the photodetector in this study are shown in Figure 3g. The maximum value of the responsivity and detectivity of the device was 291 A/W and 6.9 × 10 11 Jones, respectively, comparable with the NiO/ZnO-based heterojunction UV detector, as shown in Table 1.

Conclusions
In this work, a metal oxide-based heterojunction UV photodetector consisting of NiO/TiO2/ZnO was prepared via RFMS. The implementation of a TiO2 dielectric layer between the NiO and ZnO films and annealing treatment significantly enhanced the rectification characteristic of the device. This led to a reduction in the dark current and an improvement in the optical response of the device. The device demonstrated a rectification ratio of 10 4 , and a rapid response under 365 nm UV illumination, with a maximum re-

Conclusions
In this work, a metal oxide-based heterojunction UV photodetector consisting of NiO/TiO 2 /ZnO was prepared via RFMS. The implementation of a TiO 2 dielectric layer between the NiO and ZnO films and annealing treatment significantly enhanced the rectification characteristic of the device. This led to a reduction in the dark current and an improvement in the optical response of the device. The device demonstrated a rectification ratio of 10 4 , and a rapid response under 365 nm UV illumination, with a maximum responsivity and detectivity of 291 A/W and 6.9 × 10 11 Jones, respectively.