Infrared Properties and Terahertz Wave Modulation of Graphene/MnZn Ferrite/p-Si Heterojunctions

Infrared Properties and Terahertz Wave Modulation of Graphene/MnZn Ferrite/p-Si Heterojunctions MnZn ferrite thin films were deposited on p-Si substrate and used as the dielectric layer in the graphene field effect transistor for infrared and terahertz device applications. The conditions for MnZn ferrite thin film deposition were optimized before device fabrication. The infrared properties and terahertz wave modulation were studied at different gate voltage. The resistive and magnetic MnZn ferrite thin films are highly transparent for THz wave, which make it possible to magnetically modulate the transmitted THz wave via the large magnetoresistance of graphene monolayer. Background impurity scattering and cavity effect [16]. Further, by Infrared (IR) and terahertz (THz) devices are highly im- using Bi-doped YIG (k ~12.0) as dielectric materials portant for many electronic systems such as radar [1], in the graphene/Bi:YIG (50 nm)/p-Si heterostructure, wireless communication [2], and security systems [3]. modulation depth of 15% and speed of 200 kHz were Thus it is critical to explore the materials [4–7] and achieved from 0.1 to 1.2 THz by applying gate structures [8–14] that can be used in the infrared and voltage [17]. terahertz range. Recently it is found that the transmis- According to previous studies, dielectric layer can sion of THz wave can be modulated with graphene field largely affect the performance of GFET that was effect transistor (GFET) by tuning the intraband transi- used for THz and infrared wave devices. By care- tions of graphene monolayer [8]. In their original GFET fully screening the dielectric materials, it is possible THz modulator, B. Sensale-Rodeiguez and coworkers to tune the performance of GFET. In prior studies, use 92 nm SiO as the gate dielectric material, which nonmagnetic high-k dielectric layers were used for achieved modulation depth of 15% and modulation terahertz and infrared GFET devices, where elec- speed of 18 Kb/s of THz wave [8]. D. Zhang and co- trical signal is extracted or applied. However, bi- workers investigated the optical THz modulation of gra- functional magnetic and dielectric layers have not phene/SiO (150 nm)/p-Si GFET, which can be tuned by been studied for GFET for terahertz and infrared gate voltage [15]. applications, which could be tuned by external Later, it was found that the THz wave modulation magnetic field. Here, we introduce 150 nm sput- of GFET could be improved by replacing the gate di- tered MnZn ferrite thin films as the dielectric ma- electric with high-k and dense Al O thin film, terials of GFET for THz and infrared applications. 2 3 which is grown by atomic layer deposition [16]. As a high-k [18] and magnetic materials, MnZn fer- Modulation depth of 22% and speed of 170 kHz was rite thin films could perform as an excellent dielec- achieved in the graphene/Al O (60 nm)/p-Si GFET tric layer and also introduce new functionalities in 2 3 by varying the gate voltage [16]. The improved the GFET THz and infrared devices. Response of modulation is attributed to the reduced Coulomb the graphene/MnZn ferrite/p-Si GFET to the infra- red illumination was observed by comparing the I-V curves with and without infrared illumination at * Correspondence: halong@uestc.edu.cn different gate bias. Meanwhile, electrical modulation State Key Laboratory of Electronic Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China © The Author(s). 2017 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. Zhang et al. Nanoscale Research Letters (2017) 12:482 Page 2 of 7 of THzwavewas achievedbythe GFET as the gate voltage was varied. Subtle change of transmitted THz wave was also observed as the external mag- netic field was varied. Methods Mn Zn Fe O thin films were prepared by RF mag- 1-x x 2 4 netron sputtering. The target material was produced by co-precipitation of Fe(NO ) ,Mn(NO ) ,and 4 3 4 3 Zn(NO ) solution, which is calcined at 950–1000 °C 4 2 for 2 h, then pressed into a 60-mm disc, and finally sintered at 1250 °C for 3.5 h. The films were depos- ited on (100) p-Si substrates at 200–300 °C under −4 base pressure of 4 × 10 Pa and oxygen concentra- tion of 0–25% (P /(P +P )). The film (150 nm) O2 O2 Ar Fig. 1 XRD patterns of samples on p-Si(100) substrate and sputtered was annealed in vacuum between 400 and 700 °C under different RF magnetron sputtering powers 100, 120, 140, 160, under pressure of 0.08 Pa–5.0 Pa for 1.5 h. and 180 W The crystal structures of Mn Zn Fe O thin films 1-x x 2 4 were characterized using Cu Kα X-ray diffraction (XRD, D/max 2400 X Series X-ray diffractometer, MnZn ferrite/p-Si heterostructures. Graphene was Tokyo, Japan) at 40 kV and 100 mA. The micro- fabricated by chemical vapor deposition (CVD) structures of the Mn Zn Fe O thin films were in- method in a tube furnace [19]. The transfer method 1-x x 2 4 vestigated using a scanning electron microscope of graphene monolayer was adapted from reference (SEM: JOEL JSM6490LV). The surface arithmetic [20]. To fabricate the GFET, the electrode of gate, average roughness (Ra) and root mean squared source, and drain was deposited by gold evaporation. roughness (RMS) have been measured by an atomic Thestructure of theGFETusing MnZn ferriteas forcemicroscope(AFM: VeecoMutimodeNano4). gate dielectric material is shown in Scheme 1. The The saturation induction was tested by an Iwatsu GFET was then characterized by a semiconductor BH analyzer (SY8232). The magnetic properties of parameter analyzer (Agilent 4155B) with a probe sta- the films were measured by a vibrating sample tion (SUMMIT 1100B-M). For IR characterization, magnetometer (VSM, MODEL: BHV-525). the I-V curves was measured under the IR illumin- After optimizing the growth conditions of ation (λ = 915 nm, P =1W),which wascompared Mn Zn Fe O thin films on p-Si, graphene mo- with that in the dark environment. Terahertz wave 1-x x 2 4 nolayers were then transferred from copper foil onto transmission was measured by a THz time domain the Mn Zn Fe O thin films to form graphene/ (TDS) system upon application of gate voltage and/ 1-x x 2 4 or external magnetic field. The external magnetic field is generated by a home-made copper coil. Results and Discussion Figure 1 shows the XRD patterns of the Mn Zn Fe O ferrites thin films on p-Si (100) 1-x x 2 4 Table 1 The roughness and grain size of MnZn ferrite thin film deposited at different RF power RF sputtering Ra RMS The length of The width of power (W) (nm) (nm) maximum maximum grains (nm) grains (nm) 90 53.70 64.96 250.19 40.84 100 65.24 81.35 156.36 21.30 120 80.26 93.91 230.11 24.48 130 74.56 88.51 347.03 38.31 Scheme 1 The GFET using 150 nm MnZn ferrite thin film as the gate dielectric material 150 75.07 90.62 533.74 34.34 Zhang et al. Nanoscale Research Letters (2017) 12:482 Page 3 of 7 substrates sputtered under RF powers of 100, 120, magnetic coercivity (Hc) are obtained. Figure 3c 140, 160, and 180 W, respectively. Spinel structure shows the Ms and Hc of the MnZn ferrite thin films of MnZn ferrite thin films was obtained under dif- annealed under the pressure of nitrogen gas up to ferent sputtering powers. The (311) diffraction peak 4 Pa. Below 3 Pa, the highest Ms and lowest Hc are is the strongest, indicating the best crystallinity at obtained at 0.5 Pa. Above 3 Pa, the Ms decrease deposition power of 160 W. Table 1 shows the sur- dramatically, which could be because of the reaction face arithmetic average roughness (Ra) and root between nitrogen gas and the thin film. Figure 3d mean squared roughness (RMS), and the length and shows Ms and Hc of the ferrite thin film as a func- width of maximum grains of the ferrite films on tion of the annealing temperature at nitrogen pres- the p-Si (100) substrates. As shown in Table 1, the sure of 1.5 Pa. The Ms (Hc)value of theMnZnthin surface roughness (Ra and RMS) of the MnZn fer- films reaches the maximum (minimum) value of rite thin films increases with the RF power. How- 330 kA/m (1600 A/m = 20 Oe) at 550 °C. The max- ever, very low RF power will affect the formation of imum Ms and the minimum Hc corresponding the MnZn ferrite thin films. The roughness of the best crystallinity of the MnZn thin films, which in MnZn ferrite thin films would affect the perform- consistent with the XRD data in Fig. 3a. At higher ance of the GFET IR and THz devices, which we temperature and gas pressure, the surface atoms of discuss later. the thin film were nitrided into impurities, which The SEM and AFM images of the MnZn ferrite deteriorate the magnetic properties of MnZn ferrite thin filmsonp-Sisubstratesare showninFig.2. thin film. As a result, the MnZn thin films were pre- The grains of MnZn ferrite thin films could be pared at annealing temperature of 550 °C and under clearly observed. After annealed, the grain size in- vacuum pressure below 3 Pa. creases as shown in Fig. 2b, d. Figure 3a shows the Graphene grown on the same copper foil was then XRD patterns of the MnZn ferrite thin films transferred onto MnZn ferrite thin films to make annealed at different temperatures. The (311) peak GFETs with structure shown in Scheme 1. Here, we of the MnZn ferrite thin film is the strongest when fabricated GFET with MnZn ferrite thin films sput- the film is annealed at 550 °C. The magnetic hyster- tered at 100 and 150 W and annealed in the optimal esis loops of these thin films were also measured by condition as discussed above. Figure 4a, b shows the VSM at room temperature and are shown in Fig. 3b, electrical current measured between drain and from which the saturation magnetization (Ms)and source as a function of applied gate voltage for the Fig. 2 SEM images of (a) as-deposited and (b) annealed MnZn ferrite thin film, (c) and (d) show the corresponding AFM images Zhang et al. Nanoscale Research Letters (2017) 12:482 Page 4 of 7 Fig. 3 Characterization of sputtered MnZn thin films. (a) XRD patterns and (b) hysteresis loops of MnZn thin films annealed at 350, 450, 550, 650, and 750 °C. Saturation magnetization (Ms) of the MnZn thin films when annealed under the pressure from 0.0 Pa to 4.5 Pa in (c) and temperature from 450 to 700 °C in (d) twoGFETs.Duringmeasurement,the applied voltage monolayer could suppress the transport of charge between source and drain is kept constant at 1 V. carriers, leading to higher resistance [22]. The current gradually increases as the gate voltage Figure 4c, d shows the comparison of the I-V curves was negatively increased. The current changes very under dark environment and infrared illumination slowly when the gate voltage is positively biased. for GFETs using 100 and 150 W sputtered MnZn The asymmetric I-V characteristics of the two GFETs ferrite thin films, respectively. The infrared light is could be a result of the thermoionic emission and at wavelength of 915 nm and power of 1 W in a interband tunneling at the junctions between the window of ~1 cm . The applied voltage between gated and access regions [21]. The resistance of the source and drain is 1 V. The I-V curve of the GFET graphene on the 100 W sputtered MnZn ferrite thin under infrared illumination is analogous to that mea- film is much smaller than that on the 150 W sput- sured in the dark environment, however, with signifi- tered thin film at the same gate bias, as compared in cantly enhanced current. The enhancement is much Fig. 4a, b. The larger resistance in Fig. 4b could be a stronger for the GFET using 100 W sputtered MnZn result of larger roughness of the 150 W sputtered ferrite thin films as dielectric layer than that using MnZn ferrite thin films, as compared in Table 1. 150 W sputtered MnZn ferrite thin film. The en- The roughness induced corrugation of the graphene hancement is ~7.5 times at gate voltage of 10 V for Zhang et al. Nanoscale Research Letters (2017) 12:482 Page 5 of 7 Fig. 4 IR characterization. (a) and (b)I -V curves of the GFET with MnZn ferrite thin film sputtered at 100 and 150 W, respectively. (c)and (d) sd sg compare the I -V curves under IR illumination and no illumination. The voltage applied between source and drain is 1.0 V for all curves sd sg 100 W sputtered MnZn ferrite thin film, which is magnetoresistance of the graphene/MnZn ferrite/p- ~2.5 times for the 150 W sputtered MnZn ferrite Si hetrojunction is shown in Additional file 1: thin film. Namely, the surface roughness of MnZn Figure S1 in the supplemental information. How- ferrite thin films could also affect the infrared opto- ever, the modulation of terahertz wave is subtle electronic properties. (5%), which could be because of the uneven surface The GFET with 100 W sputtered MnZn ferrite of MnZn ferrite thin films and/or the small change thin films was then used to examine the modulation of terahertz modulation with resistance. Graphene properties of THz waves. Figure 5a shows the trans- could feel much stronger and uniform fringe field mittance of THz waves through the GFET upon ap- on extremely smooth MnZn ferrite thin film, which plication of different gate bias. The transmittance could have larger magnetoresistance of graphene was measured by a THz pulse using a THz-TDS and give larger modulation depth by external mag- system, and the transmittance in the frequency do- netic field. main was obtained by Fourier’s transformation using air as the baseline. When the gate voltage is varied from Conclusions 25 V to −25 V, the resistance between the source and Graphene/MnZn ferrite/p-Si heterostructure was fab- drain is decreased, as shown in Fig. 4a.The reduction of ricated for IR and THz device applications. The resistance results in the reduced transmittance of THz MnZn ferrite thin film was deposited on the p-Si by wave, as shown in Fig. 5a. Namely, the transmission of magnetron sputtering, which was annealed before THz wave could be modulated by applying different gate used for GFET fabrication. The MnZn ferrite thin voltage of the GFET. The transmitted THz wave was also films provide an alternative dielectric material for the measured when an external magnetic field was applied, GFET IR and THz devices. As a magnetic and high- which is shown in Fig. 5b. As external magnetic field resistive thin film, it can strengthen the magneto- increases, the intensity of transmitted THz wave de- resistance of graphene and modulation of transmitted creases, which saturate above 50 Oe. The change of THz without introducing additional insertion loss. transmitted intensity of THz wave under external The surface roughness of the MnZn ferrite thin film magnetic field could be due to the extremely large can largely affect the performance of the IR and THz magnetoresistance of graphene [23]. The underneath devices. Higher performance could be achieved by MnZn ferrite thin film provides strong fringe field making MnZn ferrite thin film smoother. Such work upon magnetization by external magnetic field. The is in progress. Zhang et al. Nanoscale Research Letters (2017) 12:482 Page 6 of 7 characterizations. JL participated in the MnZn thin film fabrication. QYW analyzed the data. All authors read and approved the final manuscript. Competing Interests The authors declare that they have no competing interests. Publisher’sNote Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Received: 19 May 2017 Accepted: 26 July 2017 References 1. Cooper KB, Dengler RJ, Llombart N, Thomas B, Chattopadhyay G, Siegel PH (2011) THz imaging radar for standoff personnel screening. IEEE Trans Terahertz Sci Technol 1(1):169–182 2. Federici J, Moeller L (2010) Review of terahertz and subterahertz wireless communications. J Appl Phys 107(11):111101 3. Kemp MC, Taday PF, Cole BE, Cluff JA, Fitzgerald AJ, Tribe WR (2003) Security applications of terahertz technology. In: Hwu RJ, Woodlard DL (eds) Terahertz for military and security applications. Spie-Int Soc Optical Engineering, Bellingham, pp 44–52 4. Xiong Y, Wen QY, Chen Z, Tian W, Wen TL, Jing YL et al (2014) Tuning the phase transitions of VO thin films on silicon substrates using ultrathin Al2O3 as buffer layers. J Phys D Appl Phys 47(45):455304 5. Zhang D, Wen T, Xiong Y, Qiu DH, Wen Q (2017) Effect of Al O buffer 2 3 layers on the properties of sputtered VO thin films. Nano-Micro Letters 9:29 6. Li SB, Zhang P, Chen H, Wang YF, Liu DT, Wu J et al (2017) Mesoporous PbI2 assisted growth of large perovskite grains for efficient perovskite solar cells based on ZnO nanorods. J Power Sources 342:990–997 7. Li SB, Zhang P, Wang YF, Sarvari H, Liu DT, Wu J et al (2017) Interface engineering of high efficiency perovskite solar cells based on ZnO nanorods using atomic layer deposition. Nano Res 10(3):1092–1103 8. Sensale-Rodriguez B, Yan RS, Kelly MM, Fang T, Tahy K, Hwang WS et al (2012) Broadband graphene terahertz modulators enabled by intraband transitions. Nat Commun 3:780 9. Wen TL, Zhang DN, Wen QY, Liao YL, Zhang C, Li JY et al (2016) Enhanced optical modulation depth of terahertz waves by self-assembled monolayer of Fig. 5 THz characterization. (a) The spectrum of THz transmittance plasmonic gold nanoparticles. Advanced Optical Materials 4(12):1974–1980 from 0.2 to 1.0 THz at different gate voltages from −25 to 25 V, and 10. Wen QY, Tian W, Mao Q, Chen Z, Liu WW, Yang QH et al (2014) Graphene (b) the frequency domain spectrum under different external magnetic based all-optical spatial terahertz modulator. Sci Rep 4:7409 11. Liu DT, Li SB, Zhang P, Wang YF, Zhang R, Sarvari H et al (2017) Efficient fieldfrom0.63to0.70THz planar heterojunction perovskite solar cells with Li-doped compact TiO2 layer. Nano Energy 31:462–468 12. Liu JK, Li QQ, Chen M, Ren MX, Zhang LH, Xiao L et al (2016) Dielectric-like Additional file behavior of graphene in Au plasmon resonator. Nanoscale Res Lett 11:541 13. Wang L, Zhai SQ, Wang FJ, Liu JQ, Liu SM, Zhuo N et al (2016) A polarization-dependent normal incident quantum cascade detector Additional file 1: Figure S1. The magnetoresistance of graphene/MnZn enhanced via metamaterial resonators. Nanoscale Res Lett 11:536 ferrite/p-Si heterojunctions at Vsg = −15 V and room temperature. 14. Zhang P, Li SB, Liu CH, Wei XB, Wu ZM, Jiang YD et al (2014) Near-infrared (DOCX 47 kb) optical absorption enhanced in black silicon via Ag nanoparticle-induced localized surface plasmon. Nanoscale Res Lett 9:519 15. Zhang DN, Sun DD, Wen QY, Wen TL, Kolodzey J, Zhang HW (2016) Tuning Acknowledgements the optical modulation of wideband terahertz waves by the gate voltage of None. graphene field effect transistors. Compos Pt B-Eng 89:54–59 16. Mao Q, Wen QY, Tian W, Wen TL, Chen Z, Yang QH et al (2014) High-speed and broadband terahertz wave modulators based on large-area graphene Funding field-effect transistors. Opt Lett 39(19):5649–5652 This work was financially supported by National Key Research Development 17. Zhang DN, Jin LC, Wen TL, Liao YL, Wen QY, Zhang HW et al (2017) Program (No. 2016YFA0300801), National Natural Science Foundation of Manufacturing and terahertz wave modulation properties of graphene/ China (Nos. 51401046, 61131005, 51572042), International Cooperation Y3Fe5O12/Si hybrid nanostructures. Compos Pt B-Eng 111:10–16 Projects (No.2015DFR50870), Sichuan Science and Technology Projects (Nos. 18. Ahmed MA, El-Khawas EH, Radwan FA (2001) Dependence of dielectric 2014GZ0091, 2015GZ0069, 2014GZ0003), Fundamental Research Funds for behaviour of Mn-Zn ferrite on sintering temperature. J Mater Sci 36(20): the Central Universities (ZYGX2016J045), and the startup fund from the 5031–5035 UESTC. 19. Li XS, Cai WW, An JH, Kim S, Nah J, Yang DX et al (2009) Large-area synthesis of high-quality and uniform graphene films on copper foils. Authors’ Contributions Science 324(5932):1312–1314 DNZ conceived the idea, made the devices, and did the characterization. 20. Li XS, Zhu YW, Cai WW, Borysiak M, Han BY, Chen D et al (2009) Transfer of MQW optimized and fabricated the MnZn ferrite thin films. TLW supervised large-area graphene films for high-performance transparent conductive the work and wrote the paper. YLL and LCJ participated in the infrared light electrodes. Nano Lett 9(12):4359–4363 Zhang et al. Nanoscale Research Letters (2017) 12:482 Page 7 of 7 21. Satou A, Tamamushi G, Sugawara K, Mitsushio J, Ryzhii V, Otsuji T (2016) A fitting model for asymmetric I-V characteristics of graphene FETs for extraction of intrinsic mobilities. IEEE Trans Electron Devices 63(8):3300–3306 22. Deng SK, Berry V (2016) Wrinkled, rippled and crumpled graphene: an overview of formation mechanism, electronic properties, and applications. Mater Today 19(4):197–212 23. Gopinadhan K, Shin YJ, Jalil R, Venkatesan T, Geim AK, Neto AHC et al (2015) Extremely large magnetoresistance in few-layer graphene/boron-nitride heterostructures. Nat Commun 6:8337 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Nanoscale Research Letters Springer Journals

Infrared Properties and Terahertz Wave Modulation of Graphene/MnZn Ferrite/p-Si Heterojunctions

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Abstract

MnZn ferrite thin films were deposited on p-Si substrate and used as the dielectric layer in the graphene field effect transistor for infrared and terahertz device applications. The conditions for MnZn ferrite thin film deposition were optimized before device fabrication. The infrared properties and terahertz wave modulation were studied at different gate voltage. The resistive and magnetic MnZn ferrite thin films are highly transparent for THz wave, which make it possible to magnetically modulate the transmitted THz wave via the large magnetoresistance of graphene monolayer. Background impurity scattering and cavity effect [16]. Further, by Infrared (IR) and terahertz (THz) devices are highly im- using Bi-doped YIG (k ~12.0) as dielectric materials portant for many electronic systems such as radar [1], in the graphene/Bi:YIG (50 nm)/p-Si heterostructure, wireless communication [2], and security systems [3]. modulation depth of 15% and speed of 200 kHz were Thus it is critical to explore the materials [4–7] and achieved from 0.1 to 1.2 THz by applying gate structures [8–14] that can be used in the infrared and voltage [17]. terahertz range. Recently it is found that the transmis- According to previous studies, dielectric layer can sion of THz wave can be modulated with graphene field largely affect the performance of GFET that was effect transistor (GFET) by tuning the intraband transi- used for THz and infrared wave devices. By care- tions of graphene monolayer [8]. In their original GFET fully screening the dielectric materials, it is possible THz modulator, B. Sensale-Rodeiguez and coworkers to tune the performance of GFET. In prior studies, use 92 nm SiO as the gate dielectric material, which nonmagnetic high-k dielectric layers were used for achieved modulation depth of 15% and modulation terahertz and infrared GFET devices, where elec- speed of 18 Kb/s of THz wave [8]. D. Zhang and co- trical signal is extracted or applied. However, bi- workers investigated the optical THz modulation of gra- functional magnetic and dielectric layers have not phene/SiO (150 nm)/p-Si GFET, which can be tuned by been studied for GFET for terahertz and infrared gate voltage [15]. applications, which could be tuned by external Later, it was found that the THz wave modulation magnetic field. Here, we introduce 150 nm sput- of GFET could be improved by replacing the gate di- tered MnZn ferrite thin films as the dielectric ma- electric with high-k and dense Al O thin film, terials of GFET for THz and infrared applications. 2 3 which is grown by atomic layer deposition [16]. As a high-k [18] and magnetic materials, MnZn fer- Modulation depth of 22% and speed of 170 kHz was rite thin films could perform as an excellent dielec- achieved in the graphene/Al O (60 nm)/p-Si GFET tric layer and also introduce new functionalities in 2 3 by varying the gate voltage [16]. The improved the GFET THz and infrared devices. Response of modulation is attributed to the reduced Coulomb the graphene/MnZn ferrite/p-Si GFET to the infra- red illumination was observed by comparing the I-V curves with and without infrared illumination at * Correspondence: halong@uestc.edu.cn different gate bias. Meanwhile, electrical modulation State Key Laboratory of Electronic Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China © The Author(s). 2017 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. Zhang et al. Nanoscale Research Letters (2017) 12:482 Page 2 of 7 of THzwavewas achievedbythe GFET as the gate voltage was varied. Subtle change of transmitted THz wave was also observed as the external mag- netic field was varied. Methods Mn Zn Fe O thin films were prepared by RF mag- 1-x x 2 4 netron sputtering. The target material was produced by co-precipitation of Fe(NO ) ,Mn(NO ) ,and 4 3 4 3 Zn(NO ) solution, which is calcined at 950–1000 °C 4 2 for 2 h, then pressed into a 60-mm disc, and finally sintered at 1250 °C for 3.5 h. The films were depos- ited on (100) p-Si substrates at 200–300 °C under −4 base pressure of 4 × 10 Pa and oxygen concentra- tion of 0–25% (P /(P +P )). The film (150 nm) O2 O2 Ar Fig. 1 XRD patterns of samples on p-Si(100) substrate and sputtered was annealed in vacuum between 400 and 700 °C under different RF magnetron sputtering powers 100, 120, 140, 160, under pressure of 0.08 Pa–5.0 Pa for 1.5 h. and 180 W The crystal structures of Mn Zn Fe O thin films 1-x x 2 4 were characterized using Cu Kα X-ray diffraction (XRD, D/max 2400 X Series X-ray diffractometer, MnZn ferrite/p-Si heterostructures. Graphene was Tokyo, Japan) at 40 kV and 100 mA. The micro- fabricated by chemical vapor deposition (CVD) structures of the Mn Zn Fe O thin films were in- method in a tube furnace [19]. The transfer method 1-x x 2 4 vestigated using a scanning electron microscope of graphene monolayer was adapted from reference (SEM: JOEL JSM6490LV). The surface arithmetic [20]. To fabricate the GFET, the electrode of gate, average roughness (Ra) and root mean squared source, and drain was deposited by gold evaporation. roughness (RMS) have been measured by an atomic Thestructure of theGFETusing MnZn ferriteas forcemicroscope(AFM: VeecoMutimodeNano4). gate dielectric material is shown in Scheme 1. The The saturation induction was tested by an Iwatsu GFET was then characterized by a semiconductor BH analyzer (SY8232). The magnetic properties of parameter analyzer (Agilent 4155B) with a probe sta- the films were measured by a vibrating sample tion (SUMMIT 1100B-M). For IR characterization, magnetometer (VSM, MODEL: BHV-525). the I-V curves was measured under the IR illumin- After optimizing the growth conditions of ation (λ = 915 nm, P =1W),which wascompared Mn Zn Fe O thin films on p-Si, graphene mo- with that in the dark environment. Terahertz wave 1-x x 2 4 nolayers were then transferred from copper foil onto transmission was measured by a THz time domain the Mn Zn Fe O thin films to form graphene/ (TDS) system upon application of gate voltage and/ 1-x x 2 4 or external magnetic field. The external magnetic field is generated by a home-made copper coil. Results and Discussion Figure 1 shows the XRD patterns of the Mn Zn Fe O ferrites thin films on p-Si (100) 1-x x 2 4 Table 1 The roughness and grain size of MnZn ferrite thin film deposited at different RF power RF sputtering Ra RMS The length of The width of power (W) (nm) (nm) maximum maximum grains (nm) grains (nm) 90 53.70 64.96 250.19 40.84 100 65.24 81.35 156.36 21.30 120 80.26 93.91 230.11 24.48 130 74.56 88.51 347.03 38.31 Scheme 1 The GFET using 150 nm MnZn ferrite thin film as the gate dielectric material 150 75.07 90.62 533.74 34.34 Zhang et al. Nanoscale Research Letters (2017) 12:482 Page 3 of 7 substrates sputtered under RF powers of 100, 120, magnetic coercivity (Hc) are obtained. Figure 3c 140, 160, and 180 W, respectively. Spinel structure shows the Ms and Hc of the MnZn ferrite thin films of MnZn ferrite thin films was obtained under dif- annealed under the pressure of nitrogen gas up to ferent sputtering powers. The (311) diffraction peak 4 Pa. Below 3 Pa, the highest Ms and lowest Hc are is the strongest, indicating the best crystallinity at obtained at 0.5 Pa. Above 3 Pa, the Ms decrease deposition power of 160 W. Table 1 shows the sur- dramatically, which could be because of the reaction face arithmetic average roughness (Ra) and root between nitrogen gas and the thin film. Figure 3d mean squared roughness (RMS), and the length and shows Ms and Hc of the ferrite thin film as a func- width of maximum grains of the ferrite films on tion of the annealing temperature at nitrogen pres- the p-Si (100) substrates. As shown in Table 1, the sure of 1.5 Pa. The Ms (Hc)value of theMnZnthin surface roughness (Ra and RMS) of the MnZn fer- films reaches the maximum (minimum) value of rite thin films increases with the RF power. How- 330 kA/m (1600 A/m = 20 Oe) at 550 °C. The max- ever, very low RF power will affect the formation of imum Ms and the minimum Hc corresponding the MnZn ferrite thin films. The roughness of the best crystallinity of the MnZn thin films, which in MnZn ferrite thin films would affect the perform- consistent with the XRD data in Fig. 3a. At higher ance of the GFET IR and THz devices, which we temperature and gas pressure, the surface atoms of discuss later. the thin film were nitrided into impurities, which The SEM and AFM images of the MnZn ferrite deteriorate the magnetic properties of MnZn ferrite thin filmsonp-Sisubstratesare showninFig.2. thin film. As a result, the MnZn thin films were pre- The grains of MnZn ferrite thin films could be pared at annealing temperature of 550 °C and under clearly observed. After annealed, the grain size in- vacuum pressure below 3 Pa. creases as shown in Fig. 2b, d. Figure 3a shows the Graphene grown on the same copper foil was then XRD patterns of the MnZn ferrite thin films transferred onto MnZn ferrite thin films to make annealed at different temperatures. The (311) peak GFETs with structure shown in Scheme 1. Here, we of the MnZn ferrite thin film is the strongest when fabricated GFET with MnZn ferrite thin films sput- the film is annealed at 550 °C. The magnetic hyster- tered at 100 and 150 W and annealed in the optimal esis loops of these thin films were also measured by condition as discussed above. Figure 4a, b shows the VSM at room temperature and are shown in Fig. 3b, electrical current measured between drain and from which the saturation magnetization (Ms)and source as a function of applied gate voltage for the Fig. 2 SEM images of (a) as-deposited and (b) annealed MnZn ferrite thin film, (c) and (d) show the corresponding AFM images Zhang et al. Nanoscale Research Letters (2017) 12:482 Page 4 of 7 Fig. 3 Characterization of sputtered MnZn thin films. (a) XRD patterns and (b) hysteresis loops of MnZn thin films annealed at 350, 450, 550, 650, and 750 °C. Saturation magnetization (Ms) of the MnZn thin films when annealed under the pressure from 0.0 Pa to 4.5 Pa in (c) and temperature from 450 to 700 °C in (d) twoGFETs.Duringmeasurement,the applied voltage monolayer could suppress the transport of charge between source and drain is kept constant at 1 V. carriers, leading to higher resistance [22]. The current gradually increases as the gate voltage Figure 4c, d shows the comparison of the I-V curves was negatively increased. The current changes very under dark environment and infrared illumination slowly when the gate voltage is positively biased. for GFETs using 100 and 150 W sputtered MnZn The asymmetric I-V characteristics of the two GFETs ferrite thin films, respectively. The infrared light is could be a result of the thermoionic emission and at wavelength of 915 nm and power of 1 W in a interband tunneling at the junctions between the window of ~1 cm . The applied voltage between gated and access regions [21]. The resistance of the source and drain is 1 V. The I-V curve of the GFET graphene on the 100 W sputtered MnZn ferrite thin under infrared illumination is analogous to that mea- film is much smaller than that on the 150 W sput- sured in the dark environment, however, with signifi- tered thin film at the same gate bias, as compared in cantly enhanced current. The enhancement is much Fig. 4a, b. The larger resistance in Fig. 4b could be a stronger for the GFET using 100 W sputtered MnZn result of larger roughness of the 150 W sputtered ferrite thin films as dielectric layer than that using MnZn ferrite thin films, as compared in Table 1. 150 W sputtered MnZn ferrite thin film. The en- The roughness induced corrugation of the graphene hancement is ~7.5 times at gate voltage of 10 V for Zhang et al. Nanoscale Research Letters (2017) 12:482 Page 5 of 7 Fig. 4 IR characterization. (a) and (b)I -V curves of the GFET with MnZn ferrite thin film sputtered at 100 and 150 W, respectively. (c)and (d) sd sg compare the I -V curves under IR illumination and no illumination. The voltage applied between source and drain is 1.0 V for all curves sd sg 100 W sputtered MnZn ferrite thin film, which is magnetoresistance of the graphene/MnZn ferrite/p- ~2.5 times for the 150 W sputtered MnZn ferrite Si hetrojunction is shown in Additional file 1: thin film. Namely, the surface roughness of MnZn Figure S1 in the supplemental information. How- ferrite thin films could also affect the infrared opto- ever, the modulation of terahertz wave is subtle electronic properties. (5%), which could be because of the uneven surface The GFET with 100 W sputtered MnZn ferrite of MnZn ferrite thin films and/or the small change thin films was then used to examine the modulation of terahertz modulation with resistance. Graphene properties of THz waves. Figure 5a shows the trans- could feel much stronger and uniform fringe field mittance of THz waves through the GFET upon ap- on extremely smooth MnZn ferrite thin film, which plication of different gate bias. The transmittance could have larger magnetoresistance of graphene was measured by a THz pulse using a THz-TDS and give larger modulation depth by external mag- system, and the transmittance in the frequency do- netic field. main was obtained by Fourier’s transformation using air as the baseline. When the gate voltage is varied from Conclusions 25 V to −25 V, the resistance between the source and Graphene/MnZn ferrite/p-Si heterostructure was fab- drain is decreased, as shown in Fig. 4a.The reduction of ricated for IR and THz device applications. The resistance results in the reduced transmittance of THz MnZn ferrite thin film was deposited on the p-Si by wave, as shown in Fig. 5a. Namely, the transmission of magnetron sputtering, which was annealed before THz wave could be modulated by applying different gate used for GFET fabrication. The MnZn ferrite thin voltage of the GFET. The transmitted THz wave was also films provide an alternative dielectric material for the measured when an external magnetic field was applied, GFET IR and THz devices. As a magnetic and high- which is shown in Fig. 5b. As external magnetic field resistive thin film, it can strengthen the magneto- increases, the intensity of transmitted THz wave de- resistance of graphene and modulation of transmitted creases, which saturate above 50 Oe. The change of THz without introducing additional insertion loss. transmitted intensity of THz wave under external The surface roughness of the MnZn ferrite thin film magnetic field could be due to the extremely large can largely affect the performance of the IR and THz magnetoresistance of graphene [23]. The underneath devices. Higher performance could be achieved by MnZn ferrite thin film provides strong fringe field making MnZn ferrite thin film smoother. Such work upon magnetization by external magnetic field. The is in progress. Zhang et al. Nanoscale Research Letters (2017) 12:482 Page 6 of 7 characterizations. JL participated in the MnZn thin film fabrication. QYW analyzed the data. All authors read and approved the final manuscript. Competing Interests The authors declare that they have no competing interests. Publisher’sNote Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Received: 19 May 2017 Accepted: 26 July 2017 References 1. Cooper KB, Dengler RJ, Llombart N, Thomas B, Chattopadhyay G, Siegel PH (2011) THz imaging radar for standoff personnel screening. IEEE Trans Terahertz Sci Technol 1(1):169–182 2. Federici J, Moeller L (2010) Review of terahertz and subterahertz wireless communications. J Appl Phys 107(11):111101 3. Kemp MC, Taday PF, Cole BE, Cluff JA, Fitzgerald AJ, Tribe WR (2003) Security applications of terahertz technology. In: Hwu RJ, Woodlard DL (eds) Terahertz for military and security applications. Spie-Int Soc Optical Engineering, Bellingham, pp 44–52 4. Xiong Y, Wen QY, Chen Z, Tian W, Wen TL, Jing YL et al (2014) Tuning the phase transitions of VO thin films on silicon substrates using ultrathin Al2O3 as buffer layers. J Phys D Appl Phys 47(45):455304 5. Zhang D, Wen T, Xiong Y, Qiu DH, Wen Q (2017) Effect of Al O buffer 2 3 layers on the properties of sputtered VO thin films. Nano-Micro Letters 9:29 6. Li SB, Zhang P, Chen H, Wang YF, Liu DT, Wu J et al (2017) Mesoporous PbI2 assisted growth of large perovskite grains for efficient perovskite solar cells based on ZnO nanorods. J Power Sources 342:990–997 7. Li SB, Zhang P, Wang YF, Sarvari H, Liu DT, Wu J et al (2017) Interface engineering of high efficiency perovskite solar cells based on ZnO nanorods using atomic layer deposition. Nano Res 10(3):1092–1103 8. Sensale-Rodriguez B, Yan RS, Kelly MM, Fang T, Tahy K, Hwang WS et al (2012) Broadband graphene terahertz modulators enabled by intraband transitions. Nat Commun 3:780 9. Wen TL, Zhang DN, Wen QY, Liao YL, Zhang C, Li JY et al (2016) Enhanced optical modulation depth of terahertz waves by self-assembled monolayer of Fig. 5 THz characterization. (a) The spectrum of THz transmittance plasmonic gold nanoparticles. Advanced Optical Materials 4(12):1974–1980 from 0.2 to 1.0 THz at different gate voltages from −25 to 25 V, and 10. Wen QY, Tian W, Mao Q, Chen Z, Liu WW, Yang QH et al (2014) Graphene (b) the frequency domain spectrum under different external magnetic based all-optical spatial terahertz modulator. Sci Rep 4:7409 11. Liu DT, Li SB, Zhang P, Wang YF, Zhang R, Sarvari H et al (2017) Efficient fieldfrom0.63to0.70THz planar heterojunction perovskite solar cells with Li-doped compact TiO2 layer. Nano Energy 31:462–468 12. Liu JK, Li QQ, Chen M, Ren MX, Zhang LH, Xiao L et al (2016) Dielectric-like Additional file behavior of graphene in Au plasmon resonator. Nanoscale Res Lett 11:541 13. Wang L, Zhai SQ, Wang FJ, Liu JQ, Liu SM, Zhuo N et al (2016) A polarization-dependent normal incident quantum cascade detector Additional file 1: Figure S1. The magnetoresistance of graphene/MnZn enhanced via metamaterial resonators. Nanoscale Res Lett 11:536 ferrite/p-Si heterojunctions at Vsg = −15 V and room temperature. 14. Zhang P, Li SB, Liu CH, Wei XB, Wu ZM, Jiang YD et al (2014) Near-infrared (DOCX 47 kb) optical absorption enhanced in black silicon via Ag nanoparticle-induced localized surface plasmon. Nanoscale Res Lett 9:519 15. Zhang DN, Sun DD, Wen QY, Wen TL, Kolodzey J, Zhang HW (2016) Tuning Acknowledgements the optical modulation of wideband terahertz waves by the gate voltage of None. graphene field effect transistors. Compos Pt B-Eng 89:54–59 16. Mao Q, Wen QY, Tian W, Wen TL, Chen Z, Yang QH et al (2014) High-speed and broadband terahertz wave modulators based on large-area graphene Funding field-effect transistors. Opt Lett 39(19):5649–5652 This work was financially supported by National Key Research Development 17. Zhang DN, Jin LC, Wen TL, Liao YL, Wen QY, Zhang HW et al (2017) Program (No. 2016YFA0300801), National Natural Science Foundation of Manufacturing and terahertz wave modulation properties of graphene/ China (Nos. 51401046, 61131005, 51572042), International Cooperation Y3Fe5O12/Si hybrid nanostructures. Compos Pt B-Eng 111:10–16 Projects (No.2015DFR50870), Sichuan Science and Technology Projects (Nos. 18. Ahmed MA, El-Khawas EH, Radwan FA (2001) Dependence of dielectric 2014GZ0091, 2015GZ0069, 2014GZ0003), Fundamental Research Funds for behaviour of Mn-Zn ferrite on sintering temperature. J Mater Sci 36(20): the Central Universities (ZYGX2016J045), and the startup fund from the 5031–5035 UESTC. 19. Li XS, Cai WW, An JH, Kim S, Nah J, Yang DX et al (2009) Large-area synthesis of high-quality and uniform graphene films on copper foils. Authors’ Contributions Science 324(5932):1312–1314 DNZ conceived the idea, made the devices, and did the characterization. 20. Li XS, Zhu YW, Cai WW, Borysiak M, Han BY, Chen D et al (2009) Transfer of MQW optimized and fabricated the MnZn ferrite thin films. TLW supervised large-area graphene films for high-performance transparent conductive the work and wrote the paper. YLL and LCJ participated in the infrared light electrodes. Nano Lett 9(12):4359–4363 Zhang et al. Nanoscale Research Letters (2017) 12:482 Page 7 of 7 21. Satou A, Tamamushi G, Sugawara K, Mitsushio J, Ryzhii V, Otsuji T (2016) A fitting model for asymmetric I-V characteristics of graphene FETs for extraction of intrinsic mobilities. IEEE Trans Electron Devices 63(8):3300–3306 22. Deng SK, Berry V (2016) Wrinkled, rippled and crumpled graphene: an overview of formation mechanism, electronic properties, and applications. Mater Today 19(4):197–212 23. Gopinadhan K, Shin YJ, Jalil R, Venkatesan T, Geim AK, Neto AHC et al (2015) Extremely large magnetoresistance in few-layer graphene/boron-nitride heterostructures. Nat Commun 6:8337

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Nanoscale Research LettersSpringer Journals

Published: Aug 8, 2017

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