A Self-Powered Fast-Response Ultraviolet Detector of p–n Homojunction Assembled from Two ZnO-Based Nanowires
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Nowadays, fabrication of micro/nano-scale electronic devices with bottom-up approach is paid much research attention. Here, we provide a novel micro/nano-assembling method, which is accurate and efficient, especially suitable for the fabrication of micro/nano-scale electronic devices. Using this method, a self-powered ZnO/Sb-doped ZnO nanowire p–n homojunction ultraviolet detector (UVD) was fabricated, and the detailed photoelectric properties were tested. At a reverse bias of −0.1 V under UV light illumination, the photoresponse sensitivity of the UVD was 26.5 and the rise/decay time of the UVD was as short as 30 ms. The micro/nano-assembling method has wide potential applications in the fabrication of specific micro/nano-scale electronic devices.
KeywordsZinc oxide Micro/nano-assembling p–n homojunction Ultraviolet photodetector
Due to the wide band gap of 3.37 eV, ZnO has attracted much attention for its potential applications in short-wavelength optoelectronic devices. Compared with another wide-band gap semiconductor of GaN, ZnO exhibits a lot of advantages, such as low-temperature synthesis, rich material source, low cost, and environmental friendliness. Since the first ZnO nanowire (NW) ultraviolet detector (UVD) was fabricated in 2002 , a lot of researches related to ZnO UVD have been reported. Generally, there are three types of ZnO UVDs from the point of view of different working principles: photoconductive UVD, Schottky junction UVD, and p–n junction UVD. Photoconductive UVD has a simple structure and a high gain [2, 3, 4], but its rise time and decay time are too long [5, 6, 7, 8, 9, 10] due to the adsorption and desorption of oxygen molecules. Comparatively, Schottky junction UVD exhibits faster response speed [11, 12]. For example, Wang et al. reported a Schottky junction UVD by utilizing Schottky contact instead of Ohmic contact in device fabrication, and the reset time drastically reduced from ~417 to ~0.8 s .
ZnO p–n junction UVD, as a self-powered device, has attracted a great deal of research attention due to its fast response speed and high stability which are the unique advantages of p–n junction UVD [14, 15, 16]. However, the difficulties in fabrication of high-quality p-type ZnO hinder the applications of ZnO-based p–n junction UVD . Most of ZnO p–n homojunctions are fabricated by two steps: growth of n-type ZnO NW section and p-type ZnO NW section [18, 19, 20]. Formation of ZnO p–n homojunction is much difficult due to the p-type doping difficulty of ZnO. Nevertheless, it is relatively easier to synthesize n-type ZnO and p-type ZnO NWs separately. Intrinsic ZnO has n-type conductivity, and group V elements are often used to form p-type doping of ZnO, such as nitrogen , phosphorus , arsenic , and antimony (Sb) [23, 24, 25, 26]. Along with the development of micro/nano-assembling techniques, bottom-up fabrication of micro/nano-scale electronic devices comes true.
In this work, we demonstrate an efficient method to fabricate a self-powered ZnO p–n homojunction UVD, and high sensitivity and ultra-fast photoresponse speed were obtained. At a reverse bias of −0.1 V under UV light illumination, the ZnO p–n homojunction UVD has a sensitivity of 26.5, and the rise time τ r (10–90 %) and decay time τ d (90–10 %) of 30 ms. The size of UVD is very small and it can be integrated into a single chip to realize multifunction device. Our work presents an efficient and accurate way to build micro/nano-scale electronic devices with high performances.
2 Experimental Section
2.1 Synthesis of ZnO NWs and Sb-Doped ZnO (Sb–ZnO) NWs
Two NWs of ZnO and Sb–ZnO were synthesized via a simple chemical vapor deposition (CVD) method in a horizontal quartz tube furnace. A mixture of high-purity powders (weight ratio of ZnO:graphite = 1:1 for ZnO NWs; molar ratio of Zn:Sb2O3:graphite = 4:1:2 for Sb–ZnO NWs ) were used as the raw materials. For ZnO NWs, Au film with 3–5 nm thickness was firstly deposited on the Si substrate by a sputtering deposition method. In the growth process of ZnO, a mixed gas of 100 sccm Ar and 5 sccm O2 was used as the carrier gas and oxygen source, whereas 200 sccm Ar and 8 sccm O2 were used in the fabrication process of Sb–ZnO NWs. For ZnO NWs, the chamber was heated to 950 °C at a rate of 50 °C min−1 and lasted 40 min under 1 atm pressure, whereas it was heated to 930 °C and lasted 50 min for Sb–ZnO NWs. Then, the furnace was cooled down to room temperature naturally.
2.2 Device Fabrication and Characterization
The Au/Ti electrodes with 10 μm finger spacing were fabricated by optical lithography and lift-off process on oxidized Si substrate (300-nm SiO2). The thicknesses of Au and Ti layers are 50 and 10 nm, respectively. The micro/nano-assembling process was performed in a dual-beam scanning electron microscope (SEM)/focused ion beam (FIB) microscope (FEI Quanta 3D FEG) equipped with nanomanipulator (Oxford Instruments OmniProbe 100) and gas injection system (GIS), and therefore the whole operating process can be monitored in real time. Firstly, one ZnO NW with suitable width and length was selected, and the tip of the nanomanipulator was handled carefully to make the tip contact with one side of the ZnO NW lightly. Secondly, the tip of the nanomanipulator and the ZnO NW were welded together by direct-written Pt deposition. Thirdly, the other side of the ZnO NW was cut off by the FIB. Finally, the ZnO NW was extracted and transferred it to anywhere we want.
The ZnO NW and another Sb–ZnO NW were transferred to the electrodes on chip. It is necessary to make one side of Sb–ZnO NW contact with the ZnO NW transferred before. The other sides of Sb–ZnO NW and intrinsic ZnO NW were welded on the Au electrodes by direct-written Pt deposition. Care was taken to avoid any contamination on the NWs’ surface on the chip.
The morphology, crystalline structure, and element distribution of the as-synthesized samples are characterized by SEM (FEI Nova Nano-SEM 450) and high-resolution transmission electron microscope (HRTEM, FEI Titan G2 60-300). The I–V characteristics and photoresponse of the devices were measured using an Agilent B2901A with the time resolution of 20 μs, a portable UV lamp (λ = 365 nm, 0.3 mW cm−2), and a function generator. All the electrical properties and photoelectric properties were tested at room temperature under atmospheric condition.
3 Results and Discussion
The fabrication processes of ZnO/Sb–ZnO p–n homojunction are schematically shown in Fig. 2a, b. The Ohmic ZnO/Pt contact and Sb–ZnO/Pt contact were made carefully by the same processes described above. The inset of Fig. 2d shows the SEM image of the device, where the ZnO NW and Sb–ZnO NW are connected. The ZnO and Sb–ZnO NWs have the diameters of ~500 nm and the lengths of ~20 μm. The I–V curve (Fig. 2d) of ZnO/Sb–ZnO p–n homojunction displays significant rectification characteristics under dark condition. The rectification ratio of the ZnO p–n homojunction diode is about 3.3 × 102 at ±1 V, and the turn-on voltage is ~1 V.
The ZnO/Sb–ZnO p–n junction could be used as a UVD with fast response performance and high stability, or as a nano-scale photovoltaic cell. The micro/nano-assembling method has been proved to be an efficient and accurate way to build high-performance micro/nano-scale p–n junction UVD. The open-circuit voltage was improved using n-type ZnO and p-type ZnO with higher doping density, and it is much easier to control the doping density when the two kinds of NWs are synthesized separately. Moreover, with the help of the micro/nano-assembling method, we can freely combine semiconductor nanomaterials with different band gaps to build p–n junction photodetectors, which can be responsive to light of different wavelength ranges.
In conclusion, we fabricated a ZnO/Sb–ZnO p–n junction UVD by a micro/nano-assembling method. The self-powered UVD exhibits high sensitivity and stability, and the rise time and decay time reach 30 ms. This micro/nano-assembling method provides an efficient and accurate way to fabricate micro/nano-scale electronic devices with potential applications.
This work was supported by the National Natural Science Foundation of China (11374110, 51371085, 11304106). Y.H.G would like to thank Prof. Zhong Lin Wang for the support of experimental facilities in WNLO of HUST.
- 1.H. Kind, H.Q. Yan, B. Messer, M. Law, P.D. Yang, Nanowire ultraviolet photodetectors and optical switches. Adv. Mater. 14(2), 158–160 (2002). doi: 10.1002/1521-4095(20020116)14:2<158:Aid-Adma158>3.0.Co;2-W CrossRefGoogle Scholar
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