High-Sensitive Ultraviolet Photodetectors Based on ZnO Nanorods/CdS Heterostructures
© The Author(s). 2017
Received: 11 September 2016
Accepted: 25 December 2016
Published: 13 January 2017
The ultraviolet (UV) photodetectors with ZnO nanorods (NRs)/CdS thin film heterostructures on glass substrates have been fabricated and characterized. It can be seen that the UV photoresponsivity of such a device became higher as the ZnO NR length was increased in the investigation. With an incident wavelength of 350 nm and 5 V applied bias, the responsivity of photodetectors based on ZnO NR/CdS heterostructures with the ZnO NR length at 500, 350, and 200 nm and traditional CdS film were at 12.86, 3.83, 0.91, and 0.75 A/W, respectively. The measurement results of the fabricated photodetectors based on ZnO nanorods (NRs)/CdS heterostructures have shown a significant high sensitivity in the range of UV light, which can be useful for the application of UV detection.
As we know, some pollution detection rely on ultraviolet (UV) spectroscopy, with detectors measuring the strength of absorption lines for such pollutants as ozone, nitrous oxides, and sulfur dioxide. High photoresponsivity and low noise levels are basically required for photodetectors in such applications [1, 2]. Because traditional silicon-based technology has some limitations in the UV photodetection. Recently, research and development in photodetectors have been mainly focused on nanostructures; a small absorption region with a high surface area to volume ratio not only contributes to a short transport time and thus higher response speed but also allows for better structural compatibility with scaling-down devices [3–6].
The bandgap energy of Si is 1.1 eV; costly high-pass optical filters and phosphors are needed to stop low-energy photons. Therefore, the most studied metal chalcogenide semiconductors, cadmium sulfide (CdS), exhibit a direct intermediate bandgap (∼2.5 eV) relatively low work function, large refraction index, and excellent thermal and chemical stability . The fascinating material makes CdS one of the most important electronic and optoelectronic materials in nonlinear optical devices, photodetectors, waveguides, sensors, energy harvesting devices, or photoelectrochemical cells [8–11]. Photodetectors based on ZnO nanostructures have received extensive attentions due to their higher sensitivity in the UV range . So far, a large variety of semiconductor nanostructures have been applied in photodetectors and some mechanisms involved have also been partly revealed [13–15]. Therefore, it will attract attention that the heterojunction photodetectors with ZnO nanostructures can be operated in the UV region. Therefore, the morphology and photoresponsivity properties of the ZnO nanowire on CdS thin film will be characterized because of their large surface area to volume ratio.
In this study, we synthesized the ZnO nanowire (NW)/CdS film heterostructure on glass substrates and then used such a structure to fabricate UV photodetectors. To our knowledge, CdS/ZnO NW heterostructures have not been thoroughly investigated yet for photodetectors, which was the motivation of this research. In this work, novel heterostructures made of CdS nanofilm and ZnO nanowires were prepared by a hydrothermal growth. Compared to pristine CdS nanobelts synthesized in the first stage of this work, CdS/ZnO NW heterostructures take obvious advantages over pure CdS nanofilm with respect to the ratio of photocurrent to dark current and responsivity.
The CdS film was prepared according to the chemical bath deposition (CBD) procedure using cadmium chloride, thiourea [SC(NH2)2], and ammonium chloride, with purities of over 99.9%. Typically, aqueous solutions of 0.06 M CdCl2, 0.02 M SC(NH2)2, and 0.03 M NH4Cl were mixed in a glass beaker under magnetic stirring. The beaker was maintained at a reaction temperature of 70 °C using water bath. Various morphology films of CdS were deposited for 20, 30, and 40 min on the glass by CBD route. The various length of ZnO nanorods with 200, 350, and 500 nm were prepared by a two-step process. The 50-nm-thick ZnO seed layers were first deposited on CdS film by radio frequency (rf) magnetron sputtering. Then, we employed the photoresists in protecting the electrode patterns by lithography technique. Secondly, the sample with photoresist-protected interdigitated electrodes was subsequently immersed in the zinc nitrate: HMTA = 0.05 M:0.1 M aqueous solution for 1.0, 1.5, and 2.0 h at 90 oC. The ZnO nanorod photodetectors were finished by removing the photoresists from the interdigitated electrode surface of devices. This synthesize was similar to our previous study . The active area of the photodetector device was 150 × 160 μm2. The fingers of the Ag interdigitated (IDT) contact patterns were 150 μm long and 10 μm wide with 10-μm spacing.
Results and Discussion
The comparison on photoluminescence (PL) spectra between the CdS film and the ZnO NRs/CdS heterostructure are shown in Fig. 5b. A stronger PL peak centered at 428 nm was observed for the sample with a CdS film. On the other hand, it was found that a PL peak centered at 382 nm was revealed for a ZnO NW/CdS heterostructure film. As a result, it seem to be concerned with the ZnO composition. Furthermore, the broader PL peak from 475 to 700 nm (yellow band) could be probably attributed to the defects such as vacancies and stacking faults on the surface and interface of the ZnO NW/CdS heterostructure . CdS heterostructure/CdS heterostructure is with lower emission intensity compared to the pristine CdS film, hence the recombination of the photogenerated carriers which was suppressed in such a heterostructure . In addition, the higher PL emission intensity for ZnO NR (500 nm)/CdS heterostructure are in line with the higher structure crystallinity.
Comparison of the photosensitivity reported in the literature and this work
In summary, we have investigated the ZnO NRs on the CdS film for the application of photodetection, which show higher response compared to the pure CdS film. As a result, it can be attributed to the high surface-to-volume ratios of ZnO nanostructure easily providing a carrier collection efficiency. With an incident wavelength of 350 nm and 5 V applied bias, we found that responsivity of ZnO nanowire length at 500, 350, and 200 nm and non-ZnO on CdS film were 12.86, 3.83, 0.91, and 0.75 A/W, respectively. The responsivity would become higher as ZnO nanorod length increased. It was found that the time constant for turn-ON transient was τON = 62.4 s and turn-OFF was τOFF = 44.9 s ZnO for NR (500 nm)/CdS heterostructure. The results exhibit significant improvement in the performance of photoinduced properties; this device will be useful in the field of UV detection.
This work was supported by Ministry of Science and Technology of Taiwan under contract nos. MOST 104-2221-E-150-041, MOST 105-2221-E-150-029, and MOST 105-2221-E-492-027.
KT and KH carried out the synthesis and characterization of the samples, analyzed the results, and wrote the first draft of the manuscript. YJ and TH participated in the design, preparation, and discussion of this study. LW and TT contributed ideas for the growth of the samples and revised the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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