Significant Enhancement of MgZnO Metal-Semiconductor-Metal Photodetectors via Coupling with Pt Nanoparticle Surface Plasmons

Significant Enhancement of MgZnO Metal-Semiconductor-Metal Photodetectors via Coupling with Pt... We proposed and demonstrated MgZnO metal-semiconductor-metal (MSM) ultraviolet photodetectors (UV) assisted with surface plasmons (SPs) prepared by the radio frequency magnetron sputtering deposition method. After the decoration of their surface with Pt nanoparticles (NPs), the responsivity of all the electrode spacing (3, 5, and 8 μm) photodetectors were enhanced dramatically; to our surprise, comparing with them the responsivity of larger spacing sample, more SPs were gathered which are smaller than others in turn. A physical mechanism focused on SPs and depletion width is given to explain the above results. Keywords: MgZnO film, Ultraviolet photodetector, SPs, Electrode spacing Background photons can be enhanced [8–18]. In many recent ZnO is an attractive wide direct band gap (~ 3.37 eV) studies, Ag nanoparticles are considered to be a better oxide semiconductor featuring radiation hardness and material. But Ag could have been oxidized at the ZnO- environment friendliness. These characteristics make it Ag interface to form a layer of silver oxide (AgO) suitable for the fabrication of short-wavelength optoelec- eventually [19]. As a kind of novel and stability proper- tronic devices, such as UV photodetectors. However, ties metal in the world, the platinum (Pt) element has owing to the immaturity of p-type doping and other been an important candidate of the plasmonic material, related solar-blind technology, the performance of ZnO- whose SPs lies in the UV range. In addition, a metal- based UV photodetectors is still lower than expected. semiconductor-metal (MSM) structure has been prefer- For the fabrication of high-performance ZnO-based UV entially chosen for the MgZnO photodetectors, with the photodetectors, a common and effective method is im- advantages of a planar device structure, fast photo proving the material quality and optimizing the device response, and simplicity in fabrication process. However, technology, but this is usually a long-term process [1–7]. there has been rather limited systematic investigation of Recently, much attention has been paid to SPs for the combined effects of barrier height and depletion their fundamental scientific importance and promising width, even though it could promote the progress of practical applications. The SPs can be realized in coat- practical application and perfect fundamental physics. In ings on the surface of metal NPs by magnetron sputter- this work, the MgZnO UV photodetectors with different ing. The metal NPs on the surface can enhance the active layers and electrode spacings have been designed scattering of the incident photons and make more pho- and fabricated. tons reach the substrate, and thus, the absorption of the In this paper, we fabricated MgZnO MSM UV photo- detectors assisted with SPs prepared by the radio fre- * Correspondence: dayongjiangcust@126.com quency magnetron sputtering deposition method. Most School of Materials Science and Engineering, Changchun University of importantly, the responsivity of the photodetectors were Science and Technology, Changchun 130022, China enhanced by sputtering metal Pt NPs on the surface of Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Guo et al. Nanoscale Research Letters (2018) 13:168 Page 2 of 6 the device. In order to demonstrate SPs, then by com- paring with electrode spacing of 3, 5, and 8 μm the responsivity of larger spacing, more SPs are smaller than others in turn. In theory, more SPs, more photo- generated electron-hole pairs are then created and the photo current is accordingly increased. To our surprise, due to the responsivity of larger spacing sample, more SPs were gathered which are smaller than others, dem- onstrating that this method is a powerful complement for the improving performance of photodetectors. Methods/Experimental The MgZnO target was prepared by sintering mixture of 99.99% pure MgO and ZnO powders at 1000 °C for 10 h in air ambient then was placed on a zinc target. (The Fig. 2 The XRD spectra of the Mg Zn O film 0.24 0.76 two targets have connected closely by the high- temperature conductive tap. The diameter of Zn target is 7 cm.) Clearly, the MgZnO beam flow will be enclosed from 200 to 700 nm. The current-voltage (I-V) charac- by the Zn beam flow, reducing the losing of Zn atoms teristics of the MgZnO photodetectors are measured effectively [20]. The composition of the MgZnO film can under a 20 V bias using an Agilent 16442A Test Fixture. be controlled easily, even at high substrate temperature. The spectral response for the MgZnO photodetectors is The quartz substrates were successively cleaned recorded using a Zolix DR800-CUST. 30 min with acetone, ethanol, and deionized water, then blown dry with air before deposition. The MgZnO film Results and Discussion was grown on the quartz substrate first, with a total The XRD patterns of the MgZnO films in different sput- pressure of 3 Pa, a sputtering power of 120 W, at room tering time are shown in Fig. 2. Here is a diffraction temperature. Finally, the top Au finger electrodes were peak located at about 34.84°, which can be indexed to constructed through lithography and wet etching, which the (002) plane of MgZnO, and it means that MgZnO were 500 μm long and 5 μm wide with 3-, 5-, and 8-μm films crystals are typically fabricated along the c-axis. spacing, and the sum of finger pairs was 15 (Fig. 1 shows The intensities of the without Pt NPs and with sputter- the schematic of photodetector). ing Pt NP MgZnO peaks are nearly the same, which can The phase identification of the MgZnO film is charac- prove that the sputtering deposition Pt NPs deposited terized by the Rigaku Ultima VI X-ray diffractometer on the surface of MgZnO films and had no effect on the (XRD) with Cu Kα radiation (λ = 1.54184 Å) at 40 kV crystal quality of the films. Figure 3 illustrates the optical and 20 mA. A PerkinElmer Lambda 950 Spectrometer is absorption spectra of the without Pt NPs and with sput- used for the absorbance spectra in the wavelength range tering Pt NP MgZnO films [21, 22]; the result suggests that the enhancement of absorption occurs for the Fig. 1 The 3D schematic of Mg Zn O UV PDs with 0.24 0.76 MSM structure Fig. 3 UV-visible absorption spectra of the Mg Zn O film 0.24 0.76 Guo et al. Nanoscale Research Letters (2018) 13:168 Page 3 of 6 detector with as-deposited Pt NPs due to SP modes. Compared with the pristine MgZnO film, the absorption of the MgZnO films coating with Pt NPs is enhanced in the spectrum range. Simultaneously, the MgZnO films were characterized by energy-dispersive spectrometer (EDS), and the magnesium concentration is about 24% (the inset of Fig. 3). Plane-view SEM image of MgZnO surface with sputtering for 20 s, with Pt NPs, are shown in Fig. 4. The average diameter of the Pt NPs is about 6. 26 ± 0.50 nm. Figure 5 shows the responsivity of the MgZnO photo- detectors (with different electrode spacing) versus inci- dent light wavelength at 5 V bias. The responsivity enhancement tendencies were totally increased by dec- orating the Pt NPs. Notably, under the same conditions, Fig. 4 Plane-view SEM image of MgZnO surface with sputtering for all the photodetectors increase with decreasing electrode 20 s, with Pt NPs spacing (3, 5, and 8 μm). Therefore, the dominant com- ponent of the responsivity enhancement is the effect of the Pt NPs. The results indicate that the enhancement range of the responsivity can be controlled easily, which Fig. 5 The responsivity of the MgZnO photodetectors (with different electrode spacing) versus incident light wavelength at 5 V bias Guo et al. Nanoscale Research Letters (2018) 13:168 Page 4 of 6 mechanism for the plasmonic scattering effect has been described in the literature. Thus, the scattered light then acquires a certain angular spread in the MgZnO layer. As a result, the incident light will pass several times through the semiconductor, increasing the effective optical path length. More importantly, increasing the op- tical path length can enhance light absorption. The photoresponse spectra of the with Pt NPs were gradually higher than that of without Pt NP devices (Fig. 7a shows the schematic of SPs). (2) The depletion width (W) ex- plains why the responsivity of all MgZnO photodetectors increases with decreasing electrode spacing at the same bias. The depletion width can be described as [23] Fig. 6 The non-linear I-V characteristics for the MgZnO photodetectors 1=2 indicate that the classic Schottky metal-semiconductor contacts have W ¼½ 2ε εðÞ ψ þ V =qN ð1Þ 0 1 0 d been achieved where ɛ is the absolute dielectric constant, ɛ is the 0 1 differs from conventional methods such as change bias relative dielectric constant, ψ is the built-in potential, V voltage. To our surprise, due to the responsivity of larger is the bias voltage, q is the electron charge, and N is the spacing sample, more SPs were gathered which are donor concentration. As the electrode spacing increases, smaller than others. In theory, because more SPs appear, the area of the semiconductor thin film will increase, more photo-generated electron-hole pairs are then which refers to the effective resistance increases. ɛ , ɛ , 0 1 created and the photo current is accordingly increased. ψ , V, q, and N are invariants, thus resulting in widening 0 d The phenomenon is inconsistent with the theory. The as the electrode spacing increases, resulting in a de- non-linear I-V characteristics (shown in Fig. 6) for the crease in the voltage acting on the depletion region. One MgZnO photodetectors indicate that the classic can see only the bias effects of the depletion width; the Schottky metal-semiconductor contacts have been voltage applied on the depletion region is that it reduces achieved. It is also shown that the dark current is en- as the electrode spacing increases. Hence, any photo- larged with the decreasing electrode spacing at the same generated carriers in this region would be swept out by bias, which can be explained by the depletion width of the high electric field and drift to the metal electrodes. the metal-semiconductor junction. Thus, the amount of photo-generated carriers will To reveal the nature of the interesting phenomenon, increase, making the trend of the responsivity contrary two possible reasons are proposed as the cause which re- to the spacing increase (Fig. 7b shows the schematic of sults between the enhanced responsivity and dark depletion width). However, all the photodetectors in- current: (1) In order to get the ideal combinatorial tar- crease with decreasing electrode spacing (3, 5, and gets of MgZnO photodetectors, we use Pt NPs to modify 8 μm); under the same NP size and density, larger elec- the device again. The incident light of matching wave- trode spacing has more excited NPs; and then ability of length interacts with the metal NPs efficiently over near-field is coupled to the semiconductor is stronger. scattering cross sections much larger than its geomet- More photo-generated electron-hole pairs are then rical cross-sections through coupling with SPs. The created, and the photo current is accordingly increased Fig. 7 a The schematic of SPs. b The schematic of depletion width Guo et al. Nanoscale Research Letters (2018) 13:168 Page 5 of 6 in theory. It is worth noting that the responsivity of all Funding The study received funding from the following: the National Key Research the photodetectors increase with decreasing electrode and Development Program of China (Grant No. 2016YFB0303805), National spacing (3, 5, and 8 μm) and bias voltage are constant. Natural Science Foundation of China (Grant No. 61774023), Scientific and As mentioned above, the dominant factor focuses on de- Technological “13th Five-Year Plan” Project of Jilin Provincial Department of Education (Grant No. JJKH20170609KJ), Postdoctoral Advanced Programs of pletion width to explain this interesting phenomenon. Jilin Province (2014), Postdoctoral Fund of Changchun University of Science All the results reveal a practicable route to improve the and Technology, and Scientific and Technological Development Project of responsivity of SPs. Here, as compared with other Jilin Province, China (Grant No 20150311086YY). commonly used materials or previous photodetectors, Availability of Data and Materials lots of Zn atoms are losing during the growth process, The datasets generated during and/or analyzed during the current study are which is due to the higher vapor pressure of Mg com- available from the corresponding authors on reasonable request. pared with Zn. It will have many defects formation into Authors’ Contributions the films due to the deficiency of Zn atoms. The photo ZG and DJ conceived and designed the study. ZG performed the carriers will be compounded by the defects, and the experiments. ZG wrote the paper. DJ reviewed and edited the manuscript. All authors read and approved the manuscript. responsivity of the solar-blind photodetectors will be reduced largely. In addition, owing to the losing of Zn Authors’ Information atoms, the disorder and fluctuation of contents is Zexuan Guo is a Ph.D student in School of Materials Science and Engineering, Changchun University of Science and Technology. difficult to avoid, and the tow tail phenomenon of the Dayong Jiang is a professor in School of Materials Science and Engineering, absorption edge will be following. As a result, the UV- Changchun University of Science and Technology. visible rejection ratio will decay accompanying with the Nan Hu is a graduate student in School of Materials Science and Engineering, Changchun University of Science and Technology. reducing of the detectivity. Consequently, the controlling Xiaojiang Yang is a graduate student in School of Materials Science and of the stoichiometric ratio in the films may be a route to Engineering, Changchun University of Science and Technology. improve the performance of the MgZnO photodetectors. Wei Zhang is a graduate student in School of Materials Science and Engineering, Changchun University of Science and Technology. The SPs can be realized in coatings on the surface of Yuhan Duan is a Ph.D student in Research Center for Space Optical metal NPs by magnetron sputtering. The metal NPs on Engineering, Harbin Institute of Technology, Harbin 150001, China. the surface can enhance the scattering of the incident Shang Gao is a professor in School of Materials Science and Engineering, Changchun University of Science and Technology. photons and make more photons reach the substrate, Qingcheng Liang is a professor in School of Materials Science and and thus, the absorption of the photons can be en- Engineering, Changchun University of Science and Technology. hanced. In theory, more SPs, more photo-generated Tao Zheng is a professor in School of Materials Science and Engineering, Changchun University of Science and Technology. electron-hole pairs are then created and the photo Jingwen Lv is a professor in School of Materials Science and Engineering, current is accordingly increased. In order to demonstrate Changchun University of Science and Technology. SPs by comparing with electrode spacing of 3, 5, and Competing Interests 8 μm the responsivity of larger spacing, more SPs are The authors declare that they have no competing interests. smaller than others in turn. Publisher’sNote Conclusions Springer Nature remains neutral with regard to jurisdictional claims in In order to get the ideal MgZnO photodetectors, we fabri- published maps and institutional affiliations. cated MgZnO MSM ultraviolet photodetectors with dif- Author details ferent electrode spacings (3, 5, and 8 μm). Then, we have 1 School of Materials Science and Engineering, Changchun University of a novel approach (we use Pt NPs to modify the device) to Science and Technology, Changchun 130022, China. Research Center for Space Optical Engineering, Harbin Institute of Technology, Harbin 150001, increase the performance of the devices. To our surprise, China. by comparing with them the responsivity of larger spacing sample, more SPs were gathered which are smaller than Received: 16 November 2017 Accepted: 9 May 2018 others in turn. We detailed the wider depletion width, to explain the optimize responsivity, and we propose that the References SPs of Pt NPs have enhanced the scattering of incident 1. Liang S, Sheng H, Liu Y, Huo Z, Lu Y, Shen H (2001) ZnO Schottky ultraviolet photodetectors. J Cryst Growth. 225(2):110. light, which is beneficial for further investigation in films 2. Chen HY, Yu PP, Teng F, Zheng LX, Hu K, Fang XS (2016) Ultrasensitive self- photodetectors. Further study is underway to develop powered solar-blind deep-ultraviolet photodetector based on all-solid-state high-quality MgZnO UV photodetectors. polyaniline/MgZnO bilayer. Small 12:5809–5816 3. Zhao B, Chen HY, Zheng LX, Su LX, Fang XS (2017) An ultrahigh −1 Abbreviations responsivity (9.7 mA W ) self-powered solar-blind photodetector based on AgO: Silver oxide; EDS: Energy-dispersive spectrometer; MSM: Metal- individual ZnO-Ga2O3 Heterostructures. Adv Funct Mater 27:1700264 semiconductor-metal; NPs: Nanoparticles; SPs: Surface plasmons; UV: Ultraviolet 4. Wei M, Yao K, Liu Y, Yang C, Zang X, Lin L (2017) Solar-blind UV detector based on graphene microcrystalline diamond heterojunctions. Small 13:1701328 A Acknowledgements 5. Nasiri N, Bo R, Tricoli A (2016) Tunable band-selective UV-photodetectors by We would like to thank the testers in Analytical and Testing Center, 3D self-assembly of heterogeneous nanoparticle networks. Adv Funct Mater University, for their help in the SEM observation. 26:7359–7366 Guo et al. Nanoscale Research Letters (2018) 13:168 Page 6 of 6 6. Chen HY, Liu H, Zhaeng ZM, Hu K, Fang XS (2016) Nanostructured photodetectors: from ultraviolet to terahertz. Adv Mater 28:403–433 7. Tak Lam K, Hsiao YJ, Ji LW, Fang TH, Hsiao KH, Chu TT (2017) High-sensitive ultraviolet photodetectors based on ZnO nanorods/CdS heterostructures. Nanoscale Res Lett 12:31 etc 8. Ding R, Xu C, Gu B, Shi Z, Wang H (2010) Effects of Mg incorporation on microstructure and optical properties of ZnO thin films prepared by sol-gel method. Mater Sci Technol 26:601–604 9. P Wang, ZJ Song, JF He, XL Guo, Y Wang, LH Qiu, LX Guo, High responsivity MgZnO ultraviolet thin-film phototransistor developed using radio frequency sputtering, IEEE-TED, vol. 63, 4, (2016)1600–1607 10. Li JY, Chang SP, Hsu MH, Chang SJ (2017) Effects of internal gain and illumination-induced stored charges in MgZnO metal-semiconductor-metal photodetectors. Materials 10(2, pp. 1):26–133 11. Lee CT, Lin HY, Tsen CY (2015) Nanomesh electrode on MgZnO-based metal-semiconductor-metal ultraviolet photodetectors. Sci Rep 5:1–10 12. Wang P, Zhen QH, Tang Q, Yang YT, Guo LX, Ding K, Huang F (2013) Steady-state characteristics and transient response of MgZnO-based metal- semiconductor -metal solar-blind ultraviolet photodetector with three types of electrode structures. Optics Express (OSA) 21(15):18387–18397 13. Cheng CW, Sie EJ, Liu B, Huan CHA, Sum TC, Sun HD et al (2010) Surface plasmon enhanced band edge luminescence of ZnO nano rods by capping Au nanoparticles. Appl Phys Lett 96:071–107 14. Lin Y, Xu CX, Li JT, Zhu GY, Xu XY, Dai J et al (2013) Localized surface plasmon resonance-enhanced two photon excited ultraviolet emission of Au-decorated ZnO nanorod arrays. Adv Opt Mater: [14] 1, 940–945 15. Mahanti M, Basak D (2012) Highly enhanced UV emission due to surface plasmon resonance in Ag–ZnO nanorods. Chem Phys Lett 542:110–116 16. Dai J, Xu C, Xu X, Guo JY, Li JT, Zhu GY et al (2013) Single ZnO microrod ultraviolet photodetector with high photocurrent gain. ACS Appl Mater Interf 5:9344–9348 17. Tong C, Yun JY (2013) Nanoplasmonic enhanced ZnO/Si heterojunction metal–semiconductor–metal photodetectors. J Electron Mater: 42(5),889 18. Lu JF, Li JT, Xu CX, Li Y, Dai J, Lin Y, Wang SF (2014) Direct resonant coupling of Al surface plasmon for ultraviolet photoluminescence ehancement of ZnO microrods. ACS Appl Mater Interfaces 6:18301–18305 19. Romanyuk A, Oelhafen P (2007) Formation and electronic structure of TiO - Ag interface. Sol Energy Mater Sol Cells 91:1051 20. Jiang DY, Tian CG, Yang G, Qin JM, Liang QC, Zhao JX, Hou JH, Gao S (2015) MgxZn1−xO solar-blind photodetectors fabricated by RF magnetron sputtering with combinatorial targets. Mater Res Bull 67:158 21. Wang LK, Ju ZG, Zhang JY, Zheng J, Shen DZ, Yao B, Zhao DX, Zhang ZZ, Li BH, Shan CX (2009) Single-crystalline cubic MgZnO films and their application in deep-ultraviolet optoelectronic devices. Appl Phys Lett 95(13):131113 22. Fonoberov VA, Balandin AA (2005) Polar optical phonons in wurtzite spheroidal quantum dots: theory and application to ZnO and ZnO/MgZnO nanostructures. J Phys Condens Matter 17(7):1085 23. D Neamen, Semiconductor physics and devices. University of New Mexico (McGraw-Hill, Inc., 2002) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Nanoscale Research Letters Springer Journals

Significant Enhancement of MgZnO Metal-Semiconductor-Metal Photodetectors via Coupling with Pt Nanoparticle Surface Plasmons

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Abstract

We proposed and demonstrated MgZnO metal-semiconductor-metal (MSM) ultraviolet photodetectors (UV) assisted with surface plasmons (SPs) prepared by the radio frequency magnetron sputtering deposition method. After the decoration of their surface with Pt nanoparticles (NPs), the responsivity of all the electrode spacing (3, 5, and 8 μm) photodetectors were enhanced dramatically; to our surprise, comparing with them the responsivity of larger spacing sample, more SPs were gathered which are smaller than others in turn. A physical mechanism focused on SPs and depletion width is given to explain the above results. Keywords: MgZnO film, Ultraviolet photodetector, SPs, Electrode spacing Background photons can be enhanced [8–18]. In many recent ZnO is an attractive wide direct band gap (~ 3.37 eV) studies, Ag nanoparticles are considered to be a better oxide semiconductor featuring radiation hardness and material. But Ag could have been oxidized at the ZnO- environment friendliness. These characteristics make it Ag interface to form a layer of silver oxide (AgO) suitable for the fabrication of short-wavelength optoelec- eventually [19]. As a kind of novel and stability proper- tronic devices, such as UV photodetectors. However, ties metal in the world, the platinum (Pt) element has owing to the immaturity of p-type doping and other been an important candidate of the plasmonic material, related solar-blind technology, the performance of ZnO- whose SPs lies in the UV range. In addition, a metal- based UV photodetectors is still lower than expected. semiconductor-metal (MSM) structure has been prefer- For the fabrication of high-performance ZnO-based UV entially chosen for the MgZnO photodetectors, with the photodetectors, a common and effective method is im- advantages of a planar device structure, fast photo proving the material quality and optimizing the device response, and simplicity in fabrication process. However, technology, but this is usually a long-term process [1–7]. there has been rather limited systematic investigation of Recently, much attention has been paid to SPs for the combined effects of barrier height and depletion their fundamental scientific importance and promising width, even though it could promote the progress of practical applications. The SPs can be realized in coat- practical application and perfect fundamental physics. In ings on the surface of metal NPs by magnetron sputter- this work, the MgZnO UV photodetectors with different ing. The metal NPs on the surface can enhance the active layers and electrode spacings have been designed scattering of the incident photons and make more pho- and fabricated. tons reach the substrate, and thus, the absorption of the In this paper, we fabricated MgZnO MSM UV photo- detectors assisted with SPs prepared by the radio fre- * Correspondence: dayongjiangcust@126.com quency magnetron sputtering deposition method. Most School of Materials Science and Engineering, Changchun University of importantly, the responsivity of the photodetectors were Science and Technology, Changchun 130022, China enhanced by sputtering metal Pt NPs on the surface of Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Guo et al. Nanoscale Research Letters (2018) 13:168 Page 2 of 6 the device. In order to demonstrate SPs, then by com- paring with electrode spacing of 3, 5, and 8 μm the responsivity of larger spacing, more SPs are smaller than others in turn. In theory, more SPs, more photo- generated electron-hole pairs are then created and the photo current is accordingly increased. To our surprise, due to the responsivity of larger spacing sample, more SPs were gathered which are smaller than others, dem- onstrating that this method is a powerful complement for the improving performance of photodetectors. Methods/Experimental The MgZnO target was prepared by sintering mixture of 99.99% pure MgO and ZnO powders at 1000 °C for 10 h in air ambient then was placed on a zinc target. (The Fig. 2 The XRD spectra of the Mg Zn O film 0.24 0.76 two targets have connected closely by the high- temperature conductive tap. The diameter of Zn target is 7 cm.) Clearly, the MgZnO beam flow will be enclosed from 200 to 700 nm. The current-voltage (I-V) charac- by the Zn beam flow, reducing the losing of Zn atoms teristics of the MgZnO photodetectors are measured effectively [20]. The composition of the MgZnO film can under a 20 V bias using an Agilent 16442A Test Fixture. be controlled easily, even at high substrate temperature. The spectral response for the MgZnO photodetectors is The quartz substrates were successively cleaned recorded using a Zolix DR800-CUST. 30 min with acetone, ethanol, and deionized water, then blown dry with air before deposition. The MgZnO film Results and Discussion was grown on the quartz substrate first, with a total The XRD patterns of the MgZnO films in different sput- pressure of 3 Pa, a sputtering power of 120 W, at room tering time are shown in Fig. 2. Here is a diffraction temperature. Finally, the top Au finger electrodes were peak located at about 34.84°, which can be indexed to constructed through lithography and wet etching, which the (002) plane of MgZnO, and it means that MgZnO were 500 μm long and 5 μm wide with 3-, 5-, and 8-μm films crystals are typically fabricated along the c-axis. spacing, and the sum of finger pairs was 15 (Fig. 1 shows The intensities of the without Pt NPs and with sputter- the schematic of photodetector). ing Pt NP MgZnO peaks are nearly the same, which can The phase identification of the MgZnO film is charac- prove that the sputtering deposition Pt NPs deposited terized by the Rigaku Ultima VI X-ray diffractometer on the surface of MgZnO films and had no effect on the (XRD) with Cu Kα radiation (λ = 1.54184 Å) at 40 kV crystal quality of the films. Figure 3 illustrates the optical and 20 mA. A PerkinElmer Lambda 950 Spectrometer is absorption spectra of the without Pt NPs and with sput- used for the absorbance spectra in the wavelength range tering Pt NP MgZnO films [21, 22]; the result suggests that the enhancement of absorption occurs for the Fig. 1 The 3D schematic of Mg Zn O UV PDs with 0.24 0.76 MSM structure Fig. 3 UV-visible absorption spectra of the Mg Zn O film 0.24 0.76 Guo et al. Nanoscale Research Letters (2018) 13:168 Page 3 of 6 detector with as-deposited Pt NPs due to SP modes. Compared with the pristine MgZnO film, the absorption of the MgZnO films coating with Pt NPs is enhanced in the spectrum range. Simultaneously, the MgZnO films were characterized by energy-dispersive spectrometer (EDS), and the magnesium concentration is about 24% (the inset of Fig. 3). Plane-view SEM image of MgZnO surface with sputtering for 20 s, with Pt NPs, are shown in Fig. 4. The average diameter of the Pt NPs is about 6. 26 ± 0.50 nm. Figure 5 shows the responsivity of the MgZnO photo- detectors (with different electrode spacing) versus inci- dent light wavelength at 5 V bias. The responsivity enhancement tendencies were totally increased by dec- orating the Pt NPs. Notably, under the same conditions, Fig. 4 Plane-view SEM image of MgZnO surface with sputtering for all the photodetectors increase with decreasing electrode 20 s, with Pt NPs spacing (3, 5, and 8 μm). Therefore, the dominant com- ponent of the responsivity enhancement is the effect of the Pt NPs. The results indicate that the enhancement range of the responsivity can be controlled easily, which Fig. 5 The responsivity of the MgZnO photodetectors (with different electrode spacing) versus incident light wavelength at 5 V bias Guo et al. Nanoscale Research Letters (2018) 13:168 Page 4 of 6 mechanism for the plasmonic scattering effect has been described in the literature. Thus, the scattered light then acquires a certain angular spread in the MgZnO layer. As a result, the incident light will pass several times through the semiconductor, increasing the effective optical path length. More importantly, increasing the op- tical path length can enhance light absorption. The photoresponse spectra of the with Pt NPs were gradually higher than that of without Pt NP devices (Fig. 7a shows the schematic of SPs). (2) The depletion width (W) ex- plains why the responsivity of all MgZnO photodetectors increases with decreasing electrode spacing at the same bias. The depletion width can be described as [23] Fig. 6 The non-linear I-V characteristics for the MgZnO photodetectors 1=2 indicate that the classic Schottky metal-semiconductor contacts have W ¼½ 2ε εðÞ ψ þ V =qN ð1Þ 0 1 0 d been achieved where ɛ is the absolute dielectric constant, ɛ is the 0 1 differs from conventional methods such as change bias relative dielectric constant, ψ is the built-in potential, V voltage. To our surprise, due to the responsivity of larger is the bias voltage, q is the electron charge, and N is the spacing sample, more SPs were gathered which are donor concentration. As the electrode spacing increases, smaller than others. In theory, because more SPs appear, the area of the semiconductor thin film will increase, more photo-generated electron-hole pairs are then which refers to the effective resistance increases. ɛ , ɛ , 0 1 created and the photo current is accordingly increased. ψ , V, q, and N are invariants, thus resulting in widening 0 d The phenomenon is inconsistent with the theory. The as the electrode spacing increases, resulting in a de- non-linear I-V characteristics (shown in Fig. 6) for the crease in the voltage acting on the depletion region. One MgZnO photodetectors indicate that the classic can see only the bias effects of the depletion width; the Schottky metal-semiconductor contacts have been voltage applied on the depletion region is that it reduces achieved. It is also shown that the dark current is en- as the electrode spacing increases. Hence, any photo- larged with the decreasing electrode spacing at the same generated carriers in this region would be swept out by bias, which can be explained by the depletion width of the high electric field and drift to the metal electrodes. the metal-semiconductor junction. Thus, the amount of photo-generated carriers will To reveal the nature of the interesting phenomenon, increase, making the trend of the responsivity contrary two possible reasons are proposed as the cause which re- to the spacing increase (Fig. 7b shows the schematic of sults between the enhanced responsivity and dark depletion width). However, all the photodetectors in- current: (1) In order to get the ideal combinatorial tar- crease with decreasing electrode spacing (3, 5, and gets of MgZnO photodetectors, we use Pt NPs to modify 8 μm); under the same NP size and density, larger elec- the device again. The incident light of matching wave- trode spacing has more excited NPs; and then ability of length interacts with the metal NPs efficiently over near-field is coupled to the semiconductor is stronger. scattering cross sections much larger than its geomet- More photo-generated electron-hole pairs are then rical cross-sections through coupling with SPs. The created, and the photo current is accordingly increased Fig. 7 a The schematic of SPs. b The schematic of depletion width Guo et al. Nanoscale Research Letters (2018) 13:168 Page 5 of 6 in theory. It is worth noting that the responsivity of all Funding The study received funding from the following: the National Key Research the photodetectors increase with decreasing electrode and Development Program of China (Grant No. 2016YFB0303805), National spacing (3, 5, and 8 μm) and bias voltage are constant. Natural Science Foundation of China (Grant No. 61774023), Scientific and As mentioned above, the dominant factor focuses on de- Technological “13th Five-Year Plan” Project of Jilin Provincial Department of Education (Grant No. JJKH20170609KJ), Postdoctoral Advanced Programs of pletion width to explain this interesting phenomenon. Jilin Province (2014), Postdoctoral Fund of Changchun University of Science All the results reveal a practicable route to improve the and Technology, and Scientific and Technological Development Project of responsivity of SPs. Here, as compared with other Jilin Province, China (Grant No 20150311086YY). commonly used materials or previous photodetectors, Availability of Data and Materials lots of Zn atoms are losing during the growth process, The datasets generated during and/or analyzed during the current study are which is due to the higher vapor pressure of Mg com- available from the corresponding authors on reasonable request. pared with Zn. It will have many defects formation into Authors’ Contributions the films due to the deficiency of Zn atoms. The photo ZG and DJ conceived and designed the study. ZG performed the carriers will be compounded by the defects, and the experiments. ZG wrote the paper. DJ reviewed and edited the manuscript. All authors read and approved the manuscript. responsivity of the solar-blind photodetectors will be reduced largely. In addition, owing to the losing of Zn Authors’ Information atoms, the disorder and fluctuation of contents is Zexuan Guo is a Ph.D student in School of Materials Science and Engineering, Changchun University of Science and Technology. difficult to avoid, and the tow tail phenomenon of the Dayong Jiang is a professor in School of Materials Science and Engineering, absorption edge will be following. As a result, the UV- Changchun University of Science and Technology. visible rejection ratio will decay accompanying with the Nan Hu is a graduate student in School of Materials Science and Engineering, Changchun University of Science and Technology. reducing of the detectivity. Consequently, the controlling Xiaojiang Yang is a graduate student in School of Materials Science and of the stoichiometric ratio in the films may be a route to Engineering, Changchun University of Science and Technology. improve the performance of the MgZnO photodetectors. Wei Zhang is a graduate student in School of Materials Science and Engineering, Changchun University of Science and Technology. The SPs can be realized in coatings on the surface of Yuhan Duan is a Ph.D student in Research Center for Space Optical metal NPs by magnetron sputtering. The metal NPs on Engineering, Harbin Institute of Technology, Harbin 150001, China. the surface can enhance the scattering of the incident Shang Gao is a professor in School of Materials Science and Engineering, Changchun University of Science and Technology. photons and make more photons reach the substrate, Qingcheng Liang is a professor in School of Materials Science and and thus, the absorption of the photons can be en- Engineering, Changchun University of Science and Technology. hanced. In theory, more SPs, more photo-generated Tao Zheng is a professor in School of Materials Science and Engineering, Changchun University of Science and Technology. electron-hole pairs are then created and the photo Jingwen Lv is a professor in School of Materials Science and Engineering, current is accordingly increased. In order to demonstrate Changchun University of Science and Technology. SPs by comparing with electrode spacing of 3, 5, and Competing Interests 8 μm the responsivity of larger spacing, more SPs are The authors declare that they have no competing interests. smaller than others in turn. Publisher’sNote Conclusions Springer Nature remains neutral with regard to jurisdictional claims in In order to get the ideal MgZnO photodetectors, we fabri- published maps and institutional affiliations. cated MgZnO MSM ultraviolet photodetectors with dif- Author details ferent electrode spacings (3, 5, and 8 μm). Then, we have 1 School of Materials Science and Engineering, Changchun University of a novel approach (we use Pt NPs to modify the device) to Science and Technology, Changchun 130022, China. Research Center for Space Optical Engineering, Harbin Institute of Technology, Harbin 150001, increase the performance of the devices. To our surprise, China. by comparing with them the responsivity of larger spacing sample, more SPs were gathered which are smaller than Received: 16 November 2017 Accepted: 9 May 2018 others in turn. We detailed the wider depletion width, to explain the optimize responsivity, and we propose that the References SPs of Pt NPs have enhanced the scattering of incident 1. Liang S, Sheng H, Liu Y, Huo Z, Lu Y, Shen H (2001) ZnO Schottky ultraviolet photodetectors. J Cryst Growth. 225(2):110. light, which is beneficial for further investigation in films 2. Chen HY, Yu PP, Teng F, Zheng LX, Hu K, Fang XS (2016) Ultrasensitive self- photodetectors. Further study is underway to develop powered solar-blind deep-ultraviolet photodetector based on all-solid-state high-quality MgZnO UV photodetectors. polyaniline/MgZnO bilayer. Small 12:5809–5816 3. Zhao B, Chen HY, Zheng LX, Su LX, Fang XS (2017) An ultrahigh −1 Abbreviations responsivity (9.7 mA W ) self-powered solar-blind photodetector based on AgO: Silver oxide; EDS: Energy-dispersive spectrometer; MSM: Metal- individual ZnO-Ga2O3 Heterostructures. Adv Funct Mater 27:1700264 semiconductor-metal; NPs: Nanoparticles; SPs: Surface plasmons; UV: Ultraviolet 4. Wei M, Yao K, Liu Y, Yang C, Zang X, Lin L (2017) Solar-blind UV detector based on graphene microcrystalline diamond heterojunctions. Small 13:1701328 A Acknowledgements 5. Nasiri N, Bo R, Tricoli A (2016) Tunable band-selective UV-photodetectors by We would like to thank the testers in Analytical and Testing Center, 3D self-assembly of heterogeneous nanoparticle networks. Adv Funct Mater University, for their help in the SEM observation. 26:7359–7366 Guo et al. Nanoscale Research Letters (2018) 13:168 Page 6 of 6 6. Chen HY, Liu H, Zhaeng ZM, Hu K, Fang XS (2016) Nanostructured photodetectors: from ultraviolet to terahertz. Adv Mater 28:403–433 7. Tak Lam K, Hsiao YJ, Ji LW, Fang TH, Hsiao KH, Chu TT (2017) High-sensitive ultraviolet photodetectors based on ZnO nanorods/CdS heterostructures. Nanoscale Res Lett 12:31 etc 8. Ding R, Xu C, Gu B, Shi Z, Wang H (2010) Effects of Mg incorporation on microstructure and optical properties of ZnO thin films prepared by sol-gel method. Mater Sci Technol 26:601–604 9. P Wang, ZJ Song, JF He, XL Guo, Y Wang, LH Qiu, LX Guo, High responsivity MgZnO ultraviolet thin-film phototransistor developed using radio frequency sputtering, IEEE-TED, vol. 63, 4, (2016)1600–1607 10. Li JY, Chang SP, Hsu MH, Chang SJ (2017) Effects of internal gain and illumination-induced stored charges in MgZnO metal-semiconductor-metal photodetectors. Materials 10(2, pp. 1):26–133 11. Lee CT, Lin HY, Tsen CY (2015) Nanomesh electrode on MgZnO-based metal-semiconductor-metal ultraviolet photodetectors. Sci Rep 5:1–10 12. Wang P, Zhen QH, Tang Q, Yang YT, Guo LX, Ding K, Huang F (2013) Steady-state characteristics and transient response of MgZnO-based metal- semiconductor -metal solar-blind ultraviolet photodetector with three types of electrode structures. Optics Express (OSA) 21(15):18387–18397 13. Cheng CW, Sie EJ, Liu B, Huan CHA, Sum TC, Sun HD et al (2010) Surface plasmon enhanced band edge luminescence of ZnO nano rods by capping Au nanoparticles. Appl Phys Lett 96:071–107 14. Lin Y, Xu CX, Li JT, Zhu GY, Xu XY, Dai J et al (2013) Localized surface plasmon resonance-enhanced two photon excited ultraviolet emission of Au-decorated ZnO nanorod arrays. Adv Opt Mater: [14] 1, 940–945 15. Mahanti M, Basak D (2012) Highly enhanced UV emission due to surface plasmon resonance in Ag–ZnO nanorods. Chem Phys Lett 542:110–116 16. 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Journal

Nanoscale Research LettersSpringer Journals

Published: Jun 5, 2018

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