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Abstract
In recent years, ultraviolet (UV) photodetectors based on nanomaterials of wide-bandgap semiconductors have emerged as a hot topic for miniaturizing these devices and saving energy. Herein, for the first time, we report a micro p-n nanowire-heterojunction constructed from a NiO-coated Ni core-shell nanowire combined with a ZnO layer (NiO@ZnO) and a UV photodetector based on this micro p-n nanowire-heterojunction. The micro NiO@ZnO-nanowire-heterojunction shows good rectification effects with a rectification ratio of 6000 at a ± 2 V applied bias and a turn-on voltage of 0.5 V. The UV photodetector exhibits excellent performances of self-powered UV photodetection with a peak photoresponsivity of 17 mA/W under zero bias at the wavelength of 312 nm. The cutoff wavelength is located at 362 nm, and a dark current is ∼0.25 pA. Our findings provide an alternative approach to miniaturize UV detectors for daily carrying based on nanowire-heterojunction materials.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultraviolet (UV) photodetectors have attracted tremendous attentions in recent years in civil and military requirements, such as flame detection, missile warning, sewage monitoring, etc [1–4]. UV-enhanced silicon photodiodes are the most common devices for UV photodetection due to their well-established technology. However, the Si-based UV detectors need additional filters to filter low energy photons and their degradation issue is inevitable, leading to higher fabrication cost and complexity [5]. Wide-bandgap semiconductors, such as SiC, diamond, III-nitrides, ZnO, Ga2O3, and so on, emerge as better candidates for the UV photodetectors due to their room temperature operation capability and the intrinsic visible blindness [4–13]. In addition, the thermal conductivity of the wide-bandgap semiconductors is in general significantly higher than that of silicon, which makes them suitable for high-temperature and high-power applications of the UV detections. In spite of all their promising characteristics, there are still some impediments to exhibit the full potential performance for the UV photodetectors fabricated from the wide-bandgap semiconductors, such as large volume, large bias and energy consumption.
With the integration and miniaturization of electronic chips, photodetectors face the problems for difficulty to combine with electronic chips with a large scale. The existing commercial UV photodetectors are not conducive to daily carrying because of their large size and energy consumption. How to miniaturize the device and save energy is a hot topic in recent years. Nowadays the smart phones, wristbands, and other smart devices are widely used, and people are trying to make the UV photodetectors miniaturized, low energy consuming or without external energy, and portable [14–18]. One-dimensional materials such as nanowires, due to their high photoresponsivity and good independence, are believed to be potential for miniaturizing the photodetectors [18–25]. Our work tries to put forward a reliable solution to realize the detection of human skin carcinogenic band UV light in the regime of 290∼320 nm, since medical reports showed that UV light of 290∼320 nm is carcinogenic to human skin [26–28]. This kind of UV detectors is suitable for people’s daily life to monitor the UV light they received.
NiO is an intrinsic p-type wide-bandgap semiconductor due to the defect of Ni vacancy with a band gap of between 3.6∼4.1 eV. As a wide-bandgap semiconductor, NiO is a good candidate for the UV photodetectors. However, NiO exhibits insulator properties due to the complex electron entanglement, which hinders its applications [29]. On the other hand, ZnO is another candidate for the UV photodetectors with the suitable bandgap of 3.37 eV. Intrinsic ZnO is an n-type semiconductor due to the oxygen vacancies, however, it is difficult to realize stable p-doping in ZnO [30]. Moreover, most of the ZnO-based UV photodetectors are metal-semiconductor-metal contact structures, which resulting in a large amount of energy loss in use. Taking into account the alignments of the energy band structures between ZnO and NiO, an UV photodetector can be constructed using a ZnO/NiO nanowire-heterojunction with the intrinsic absorption in the UV wavelength. In addition, this UV detector is miniature by using the ZnO/NiO nanowires. To the best of our knowledge, there are no relevant reports in the literatures on this type UV detector.
In this work, a micro p-n heterostructure constructed from one-dimensional NiO-coated Ni core-shell nanowire combined with ZnO layer (NiO@ZnO) have been fabricated. This p-n nanowire-heterojunction shows typical rectifying properties with the turn-on voltage of 0.5 V and the rectifying ratio of above 103. The NiO@ZnO-nanowire-heterojunction also exhibits stable transport properties, which is very important for fabricating the devices. An UV detector based on this NiO@ZnO-nanowire-heterojunction exhibits excellent UV photodetections. The experimental results indicate that the nanowire-heterojunction UV detector has attractive potential applications in the portable UV photodetectors.
2. Experimental sections
One-dimensional Ni nanowires were synthesized by hydrothermal reduction method using Ni chloride hexahydrate at 90 ℃ and under a magnetic field of 0.2 T [30–35]. Small amount of Ni nanowires were dissolved in ethanol solution and dispersed by ultrasonic method, and the dispersed solution was dripped on an Al2O3 substrate with a dropper. Single Ni nanowires were selected to etch electrode patterns at one end of Ni nanowires by UV lithography, and then the metal Pt was sputtered by a small sputtering instrument (KYKY, sbc-12) and treated with degumming. After treatment, a Ni nanowire with one end covered by a platinum electrode can be obtained. After that, it was placed in a tube furnace at 400 ℃ to obtain a NiO coated Ni (Ni@NiO) core-shell nanowire [36,37]. At the other end of the NiO nanowire, a ZnO layer was grown by pulsed laser deposition (ADNANOTEK CORP, PLD-18L) at 600 ℃ and under an oxygen pressure of 1×10−3 Pa, and the laser energy is 300 mJ and the frequency is 3 Hz. Finally, the sample was degummed, and the metal indium (In) was pressed on the ZnO, and the NiO@ZnO-nanowire-heterojunction device was fabricated, as shown in Fig. 2(e).
The morphology and microstructure of the NiO nanowires were characterized by scanning electron microscope (SEM, JSM-6700F/INCA-ENERGY). The crystal structures of the NiO nanowires and the ZnO layer were characterized by x-ray diffractometer instrument (XRD, HAOYUAN INSTRUMENT, DX-2700) with Cu Kα line as radiation source. The structure of the NiO nanowires was characterized by transmission electron microscope (TEM, JEOL-2010, Japan). The electrical characteristics of the NiO@ZnO-nanowire-heterojunction were measured by a semiconductor analyzer (Keithley, 4200-SCS), and the photoelectric properties of the NiO@ZnO-nanowire-heterojunction device were performed using an UV responsivity instrument (Zolix, dsr101uv-b).
3. Results and discussion
The morphology and composition of the Ni@NiO nanowires were characterized by SEM, and Figs. 1(a) and 1(b) show the SEM surface topography of a single Ni and Ni@NiO nanowires, respectively. It can be seen from Figs. 1(a) and (b) that there are some spiny substances on the surface of the Ni nanowire, and the spiny substances of the Ni nanowires change to the granular substances on the surface of the NiO nanowires by the thermal treatments. Because of the individual differences in nanowires, the average diameter of nanowires is 300 nm and the length of nanowires is 10∼50 µm. The thickness of the NiO layer is ∼ 55 nm, extracted from the thermogravimetric data [36]. Figure 1(c) shows the XRD characteristic curve of the Ni@NiO nanowire. It can be seen that there are two peaks from Ni in the center of the Ni@NiO nanowire, and the positions of the peaks respectively are 44.62° and 51.82°, which can respectively correspond to (111) and (200) of face-centered cubic Ni. The peaks of 37.39° and 43.39° correspond to the diffraction peaks of (111) and (200) planes of face-centered cubic NiO, respectively. Figure 1(d) is the TEM image of the Ni@NiO nanowire. It can be seen that a NiO (200) crystal plane structure is formed on the surface of the nanowire. From the above results, it can be confirmed that the nanowire is a structure with the core Ni coated by NiO. Figure 2(a) shows the XRD scan curve of the ZnO layers grown on an Al2O3 substrate. The characteristic peaks of (0002) and (0004) of the hexagonal close-packed ZnO are observed at 34.52° and 72.54°, respectively. The half-width of the peaks is 0.26°, indicating the high quality of the epitaxial ZnO layer.
Fig. 1. (a) The SEM morphology of the Ni nanowire. (b) The SEM morphology of the Ni@NiO nanowire. (c) XRD patterns of the Ni@NiO nanowire. (d) The TEM morphology of the Ni@NiO nanowire, and the inset showing the low resolution TEM morphology of the Ni@NiO nanowire.
Fig. 2. (a) XRD scan of the ZnO layer grown on sapphire substrate. (b) AFM morphology of the ZnO layer surface. (c) The thickness of the ZnO layers ≈ 60 nm. The inset is the AFM image of the step of the ZnO layer. (d) The thickness of the Pt electrode ≈ 100 nm. The inset is the AFM image of the step of the Pt electrode. (e) The schematic structure of heterojunction device and the section model composed of nickel oxide coated nickel nanowires and zinc oxide. (f) SEM image of the NiO@ZnO-nanowire-heterojunction UV photodetector.
Figure 2(b) is an AFM image of ZnO with the roughness Ra of about 7.31 nm and a columnar bulge of 40∼60 nm on the surface. The surface morphology is similar to ZnO grown along c-axis which has wurtzite structure, consistent with the conclusion of the XRD results. The thicknesses of the ZnO layer and Pt electrode were measured by AFM, as shown in Figs. 2(c) and (d). From the height maps, we can obtain that the thicknesses of the ZnO layer and Pt electrode are about 60 and 100 nm, respectively.
The sketch of the NiO@ZnO-heterojunction structure with the metal electrodes is shown in Fig. 2(e). It can be seen that that Ni is connected to platinum electrode, and the ZnO layer is connected to the In electrode. NiO and ZnO form a p-n heterojunction. ZnO and Ni are separated as shown in Fig. 2(e). The SEM image of the NiO@ZnO-heterojunction structure with the metal electrodes is shown in Fig. 2(f), and the length of the nanowire is about 50 µm.
Figures 3(a) and (b) show the I-V characteristics of the Ni electrode on NiO and the In electrode on ZnO, respectively. This symmetrical structure of metal/semiconductor/metal which we specially constructed is in order to check whether the existence of Schottky barrier in our device. It shows that all the interfaces of the metal/semiconductor heterojunctions in the device exhibit a good ohmic contact. The possibility of any Schottky junction in this device can be excluded, which indicates that the rectifying effect of the device comes from the p-NiO/n-ZnO heterojunction as shown in Fig. 3(c). The bias voltage in Fig. 3(a) is relatively small, because the electrical properties of nanowires without the ZnO coating structures are extremely unstable. It may due to the resistance change at the NiO interface and the formation of conducting filament channels. Figure 3(c) is the I-V characteristic curve of the device under dark conditions, showing typical rectification properties. The rectifying ratio of forward and negative currents is 6000 at ± 2 V, and the turn-on voltage is about 0.5 V. This means that the device may operate in self-powered mode without any external bias. Contrast experiments under 365 nm illumination and dark conditions are shown in Fig. 3(d). The photocurrent is obviously larger than the dark current, which proves that the photocurrent of the device is sensitive to the detection of 365 nm. It can be seen from Fig. 3(d) that the enhancement of the photocurrent under the negative bias is larger than that under the positive voltage.
Fig. 3. (a) I-V characteristic curve of Ni/NiO/Ni. (b) I-V characteristic curve of In/ZnO/In. (c) I-V characteristic curves of the heterojunction structure in dark environment (black dots and line) and under the light (365 nm, 0.23 mW/mm2, blue dots and line). (d) The logarithmic form from Fig. (c).
Figure 4 (a) is the I-t characteristic curve of the UV photodector under the 0 V bias with 365 nm UV light on and off repeatedly, among them the dark current is 0.25 pA (0 V). It can be seen that the UV photodetector can work very well at the self-powered mode. For comparing, the I-V characteristic curves under visible light and dark conditions were also performed, however, the photocurrent and the dark current do not change significantly, indicating that the detector has high sensitivity to the UV light detection.
Fig. 4. (a) The I-t characteristic curve under the light with 365 nm wavelength and intensity of 0.23 mW/mm2 on-off repeatedly at 0 V bias. (b) The optical response spectrum characteristic curve of the NiO@ZnO-nanowire-heterojunction under 0 V bias. (c) The NiO@ZnO-heterojunction energy band model diagram. (d) Principle of detecting UV light at 0 V bias.
Figure 4(b) shows the spectral responsivity curve under zero bias voltage. The spectral responsivity under the light of 312 nm reaches a maximum of 17 mA/W, and the external quantum efficiency is about 0.068. The cutoff wavelength is located at 362 nm, and the ultraviolet/visible contrast ratio is about 7. However, the spectral responsivity remains a certain value in the region where the wavelength is larger than 400 nm, which may come from the narrowing of the band gap of ZnO caused by the oxygen vacancies on the surface of ZnO. When the spectral responsivity is calculated, the area is chosen as the irradiated area of NiO and ZnO. If only the contact area of NiO and ZnO heterojunction is considered, the response will be much larger than the above result. Through analysis, we obtain a p-NiO/n-ZnO abrupt heterojunction structure, whose energy band diagram is shown in Fig. 4(c). We can see that the “spikes” in the energy band come from the junction of the heterojunction. The potential difference in the space charge region (VD) is 0.5 V. It can be seen from the literatures that the energy bands of intrinsic NiO and intrinsic ZnO are 3.71 and 3.37 eV, and the electron affinity potential χ1 and χ2 of NiO and ZnO are 1.41 and 1.31 eV, respectively [38–42]. The schematic diagram of the energy band of the self-powered UV photodetector under the illumination is shown in Fig. 4(d). At zero bias voltage, the electron-hole pairs are formed at the p-n junction due to the action of the built-in electric field. When the light is irradiated, the electron-hole pairs in the space charge region are separated to form photogenerated carriers, and a directional moving current is formed under the action of the p-n junction barrier. The photogenerated electrons in NiO flow to ZnO, and the photogenerated holes in ZnO flow toward NiO.
The repeatability and stability of the NiO@ZnO-nanowire-heterojunction self-powered UV detector were measured under a light intensity of 0.23 mW/mm2 and a voltage of −5 V. The photocurrent response of the UV detector under the light of 365 nm is shown in Fig. 5(a). When the UV light is turned on, the current rapidly increases and there is no obvious degradation in repeated measurements. Compared with the single NiO coated Ni nanowires without the ZnO coating layer, our UV photodetector exhibits the tolerance to the long-term applied voltage. Because of the individual difference of nanowires, the breakdown voltage of the device is very different. The general breakdown voltage is 20 V, and some breakdown voltage of the individual device can reach 100 V or more. The dark current recovery time at −5 V voltage is analyzed experimentally in Fig. 5(b). Fitting the curve and using the formula:
Fig. 5. (a) The I-t characteristic curve under the light with 365 nm wavelength and intensity of 0.23 mW/mm2 on-off repeatedly at −5 V voltage. (b) The decay time under the illumination condition of the same fig. (a). (c) The I-t characteristic curve under the light with 365 nm wavelength and intensity of 0.23 mW/mm2 on-off repeatedly at different voltage. (d) The I-t characteristic curve under the light with 365 nm wavelength with different intensities of light on-off repeatedly at −5 V voltage.
Where y0, A1, A2 are constant, and t1 and t2 are the decreasing time. The recovery time is divided into two processes, and one is a fast process and the other is a diffuse one. Because the fast process is caused by the decrease of carrier concentration at the interface of the NiO/ZnO heterojunction, the combination of electron-hole pairs under the action of an internal electric field. The diffusion process may be caused by the adsorption and desorption of the oxygen molecules on the surface of nanocrystalline materials. NiO is an intrinsic p-type semiconductor, and there are hole carriers on the surface. The negatively charged oxygen molecules are combined to form oxygen to adsorb on the surface of NiO. Because of the oxygen vacancies, the surface of ZnO directly adsorbs the oxygen molecules in the air, forming charged oxygen molecules attached to the ZnO surface. The decay time of the fast process is 2.93 s and that of the diffuse process is 51.48 s. The rise time at different voltages and different light intensities changes are shown in Figs. 5(c) and 5(d), respectively. The possible reason for the analysis is that the electron-hole pairs in the built-in electric field are separated under light. Some photogenerated carriers recombine with oxygen on the surface and do not enter the external circuit in time [43–45].
4. Conclusions
The p-NiO/n-ZnO nanowire-heterojunction and the UV detector based on this nanowire-heterojunction have been successfully fabricated for the first time. The NiO@ZnO-nanowire-heterojunction shows the excellent rectification effect with the rectification ratio of 6000 at ± 2 V and the turn-on voltage of 0.5 V. The UV photodetector exhibits the optical responsivity about 17 mA/W without the applied voltage at the wavelength of 312 nm. The photodetectors are excellent in stability, and the fall time is about 2.9 s. Our findings indicate that the nanowire-heterojunction UV detector have attractive potential applications in miniaturization of UV photodetectors for daily carrying.
Funding
National Natural Science Foundation of China (NSFC) (11574365); National Science Foundation for Distinguished Young Scholars of China (61425021).
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