A comparative study of graphene and graphite-based field effect transistor on flexible substrate

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1 Pramana J. Phys. (2018) 90:75 Indian Academy of Sciences A comparative study of graphene and graphite-based field effect transistor on flexible substrate KAPIL BHATT, CHEENU RANI, MONIKA VAID, ANKIT KAPOOR, PRAMOD KUMAR, SANDEEP KUMAR, SHILPI SHRIWASTAWA, SANDEEP SHARMA, RANDHIR SINGH and CCTRIPATHI University Institute of Engineering and Technology, Kurukshetra University, Kurukshetra , India Corresponding author. kapilbhattuiet@gmail.com MS received 23 August 2017; revised 21 November 2017; accepted 7 December 2017; published online 4 May 2018 Abstract. In the present era, there has been a great demand of cost-effective, biodegradable, flexible and wearable electronics which may open the gate to many applications like flexible displays, RFID tags, health monitoring devices, etc. Due to the versatile nature of plastic substrates, they have been extensively used in packaging, printing, etc. However, the fabrication of electronic devices requires specially prepared substrates with high quality surfaces, chemical compositions and solutions to the related fabrication issues along with its non-biodegradable nature. Therefore, in this report, a cost-effective, biodegradable cellulose paper as an alternative dielectric substrate material for the fabrication of flexible field effect transistor (FET) is presented. The graphite and liquid phase exfoliated graphene have been used as the material for the realisation of source, drain and channel on cellulose paper substrate for its comparative analysis. The mobility of fabricated FETs was calculated to be 83 cm 2 /V s (holes) and 33 cm 2 /Vs (electrons) for graphite FET and 100 cm 2 /V s (holes) and 52 cm 2 /V s (electrons) for graphene FET, respectively. The output characteristic of the device demonstrates the linear behaviour and a comprehensive increase in conductance as a function of gate voltages. The fabricated FETs may be used for strain sensing, health care monitoring devices, human motion detection, etc. Keywords. PACS No. Graphene; graphite; field effect transistor; mobility; transconductance; paper substrate Kk 1. Introduction A transistor is the basic building block of semiconductor industry. With advancements in technology, field effect transistor (FET) plays an important role in various electronics-based applications such as human motion detection and strain sensors [1], biological sensors [2], antennas [3] and in other biomedical applications [4]. In FET, the width of the conducting channel is the key in which its current-carrying capability varies with the application of an electric field. The applied voltages regulate the conductivity of FETs and in turn the output current. Searching for novel materials for the realisation of electronic devices, such as solar cells [5], tunable bandpass filters [6], supercapacitors [7], metal insulator metal structures [8], FETs [9], etc., graphene is being extensively investigated for various applications, i.e. energy harvesting [10], rectenna [11], biomedical-based applications, etc. Among these devices, there is a growing interest in application areas based on graphenebased FETs. Graphene has very interesting properties such as high mobility, better miniaturisation capability, one-atom thickness and 2D structure which make it one of the most suitable materials to use in FETbased applications [12]. Graphene was initially identified as the material that would replace silicon [13]. Due to the fragile nature, conventional silicon-based FETs are not suitable in flexible/wearable electronic applications. To deal with these kinds of issues, flexible electronics devices have been investigated [14]. For the fabrication of FETs, researchers worldwide are exploiting various flexible substrates, like PET [15], polyimide [16], PEN [17], etc. Moreover, polymerbased substrates used in flexible electronics are not completely biodegradable leading to serious environment contamination issues [18]. Therefore, there is a requirement to develop a low-cost, versatile and

2 75 Page 2 of 6 Pramana J. Phys. (2018) 90:75 energy-efficient fabrication methods on biodegradable substrates for applications like touch sensors [19], memory devices [20], etc. Recently, cost-effective, large-scale production techniques such as screen printing and ink-jet printing methods [20], bar coating techniques [21], etc. have been reported. The fabrication process using screen printing and ink-jet printing techniques requires electrically conductive ink along with mask and other fabrication set-ups. To the best of our knowledge, a number of reports, separately on graphene and graphite-based FETs on paper substrate [22,23] are available, but no comparative study on graphene and graphite-based FETs is reported in literatures. In this report, we demonstrate a cost-effective, simple technique for the fabrication of FETs on cellulose paper which can be easily realised in any laboratory. Further, a comparative study of graphene and graphite-based FETs is presented and it is shown that graphene-based FETs show better mobility than graphite-based FETs. 2. Experimental section 2.1 Fabrication of FET A rectangle of 25 mm length and 5 mm width is drawn over both sides of the cellulose fibre paper (thickness more than 75 µm measured from digital micrometre) using graphite pencil 10B (having higher quantity of graphite content which makes it more conductive) (figure 1). The drawing is repeated 8 10 times to form a good conductive thin film. One side of the film is used as the gate, while the other side ends are used as the source and the drain. The region between the source and the drain acts as a channel with the cellulose paper acting as both the substrate as well as the dielectric material. For the fabrication of graphene FETs, graphene was prepared in-house using liquid phase exfoliation method [24] and was applied on the paper substrate using paint brush flat head size 2 (Ralson Art Taklon, USA) (figure 2). 2.2 Device characterisation The surface morphology of the conductive traces formed from graphite pencil and graphene layers were analysed using FESEM microscopy. The images using Hitachi SU8000 FESEM (Punjab University, Chandigarh) are recorded as shown in figure 3, showing porosity and orientation of the cellulose fibre paper, morphology of graphite and graphene on the paper. In addition, the Raman spectra recorded using RENISHAW Raman spectrometer (IISER, Mohali), exhibit simple structures characterised by two principal bands, G and 2D bands, as shown in figure Results and discussion The fabrication method of graphite FETs is shown in figure 1a and its schematic is shown in figure 1b. On cellulose paper (which acts as the dielectric) the source, drain, gate and channel are made of graphite (10B pencil) trace. Pencil trace was repeated till a thick layer of graphite is formed on the substrate. Figures 2a and 2b illustrate the fabrication method and schematic of graphene-based FET on the cellulose paper. The coating of graphene layer on the paper substrate has been done using the paint brush. It is apparent from the schematic Figure 1. (a) Fabrication method of the graphite FET with graphite pencil trace on cellulose paper and (b) schematic diagram of the graphite FET.

3 Pramana J. Phys. (2018) 90:75 Page 3 of 6 75 Figure 2. (a) Fabrication method of the FET with graphene by paint brush on cellulose paper and (b) schematic diagram of the graphene FET. Figure 3. (a) SEM image showing porosity and orientation of the cellulose fibre paper, (b) SEM image showing morphology of graphene on paper and (c) SEM image of the graphite film deposited on the paper. Figure 4. Raman spectra showing D and G peaks of (a) graphite and (b) graphene. of both the transistors that one side of the graphite and graphene film is used as the gate and two ends of the other side are used as the source and the drain with region between the source and the drain acting as the channel. Porosity and orientation of the cellulose paper were investigated by scanning electron microscopy as shown in figure 3a, which also ensures the proper deposition of graphene and graphite film on its surface as apparent from figures 3b and 3c. Figures 4a and 4b show the Raman spectra of graphite and graphene with D and G peaks. The intensity ratio of the D and G bands helps to estimate the defects of graphene. In figure 4b, the ratio of D to G peaks (ID /IG ) was found to be which clearly indicates the

4 75 Page 4 of 6 Pramana J. Phys. (2018) 90:75 Figure 5. Output characteristics of (a) graphite FET and (b) graphene FET. Figure 6. Transfer characteristics of (a) graphite FET and (b) graphene FET. synthesis of high-quality graphene. The spectrum shows that the D/G ratio in graphene is less due to which it has less defects and high electrical conductivity compared to graphite [24]. The performance of the FETs was measured by electrical characterisation at room temperature with a humidity level of 70%. The characteristics of the transistors were recorded using Keithley-2450 source meter. Figure 5 shows the output characteristics which demonstrate that I d (drain current) varies linearly with V ds for different gate voltages and the FETs show significant increase in the conductance induced by applying gate voltages. The transfer characteristics of graphite- and graphenebased FETs are shown in figures 6a and 6b. The fabricated graphite and graphene FETs display ambipolar nature for different gate voltages with positive Dirac shift near 0.6 V and 1 V, respectively. This Dirac shift is a function of the drain to source voltages [25] and depends on the fabrication conditions [26]. The mobility of the fabricated FETs was calculated by direct transconductance method using transconductance (g m ) value determined from transfer characteristics. The transconductance (g m ) of holes and electrons of the graphite FET was found to be µa/v and µa/v, and for graphene, it was measured to be 1.08 µa/v and µa/v, respectively. From direct transconductance method, the mobility (µ) can be calculated as µ = g m (w/l) C 0 V ds. (1) Here, C 0 is the specific capacitance which is measured to be 54 nf/cm 2 by LCR meter (by fabricating Cu paper dielectric Cu structure) of the paper dielectric material. For w/l ratio of 0.2 for both graphite and graphene FETs, the mobility of the fabricated FETs was calculated to be 83 cm 2 /V s (holes) and 33 cm 2 /V s (electrons) for graphite FET and 100 cm 2 /V s (holes) and 52 cm 2 /Vs (electrons) for graphene FET (table 1). The fabricated graphite- and graphene-based FETs on paper substrate find possible applications as strain sensors. Although strain sensors can be fabricated in many simple configurations [30,31], FET-type structures are more sensitive. The enhanced sensitivity of the sensor in [18] is reported to be due to microcontact reversible effect of the film (graphite/graphene) on the paper substrate and the dependence of conductance of the film on the contacts between the graphite/graphene nanosheets.

5 Pramana J. Phys. (2018) 90:75 Page 5 of 6 75 Table 1. Comparison assessment with literatures. Substrate used Channel material S/D/G electrodes Dielectric material Mobility Reference Paper Graphite Ag Paper with rgo µh = cm 2 /Vs,µe = cm 2 /Vs [27] Paper Graphite Ag Ion gel µh = 106 cm 2 /Vs,µe = 59 cm 2 /Vs [28] Plastic Graphene Cr/Au Ion gel µh = 300 cm 2 /Vs,µe = 250 cm 2 /Vs [29] PET Semiconductor material Graphene h-bn µh = 45 cm 2 /Vs,µe = 24 cm 2 /Vs [15] Paper Graphite Graphite Paper µh = 191 cm 2 /Vs,µe = 167 cm 2 /Vs [18] Paper Graphite Graphite Paper µh = 83 cm 2 /Vs,µe = 33 cm 2 /V s Present work Graphene Graphene Paper µh = 100 cm 2 /Vs,µe = 52 cm 2 /Vs 4. Conclusion A cost-effective, biodegradable cellulose paper as the substrate as well as the dielectric material for the fabrication of flexible FET is demonstrated. A comparative study of graphite and liquid-phase exfoliated graphene used as the source, drain, gate and channel on cellulose paper substrate is presented. The mobility of the fabricated FETs was calculated to be 83 cm 2 /Vs (holes) and 33 cm 2 /V s (electrons) for graphite FET and 100 cm 2 /V s (holes) and 52 cm 2 /V s (electrons) for graphene FET. In our fabricated FETs, graphenebased FETs show better mobility than graphite-based FETs. The fabricated FETs may be used for applications such as strain sensing, health care monitoring devices, human motion detection, etc. In future, for other dielectric materials on paper substrates, the FETs may yield better results for higher current ranges. Acknowledgements Support from Research Laboratory (ECE), University Institute of Engineering and Technology, Kurukshetra is gratefully acknowledged by the author. Authors are also thankful to Dr Y Dwivedi, Physics Department, NIT Kurukshetra, Haryana for various technical discussions. References [1] T Yamada, Y Hayamazu and Y Yamamoto, Nat. Nanotechnol. 6, 296 (2011) [2] Y Ohno, K Maehashi and K Matsumoto, Proc. SPIE 8031, 903 (2011) [3] O Habibpour, J Vukusic and J Stake, IEEE Trans. Microw. Theory Tech. 61, 841 (2013) [4] L Xiang, Z Wang, Z Liu, S E Weigum and Q Yu, IEEE Sens. J. 16, 8359 (2016) [5] T Han, H Kim, S Kwon and T Lee, Mater. Sci. Eng. R Rep. 118, 1 (2017) [6] W Su and B Chen, Pramana J. Phys. 89, 37 (2017) [7] R Singh and C C Tripathi, Int. J. Electrochem. Sci. 11, 6336 (2016) [8] M Shaygan, Z Wang, M S Elsayed, M Otto, G Iannaccone, A H Ghareeb, G Fiori, R Negra and D Neumaier, Nanoscale 9, (2017) [9] S J Kim, K Choi, B Lee and B H Hong, Ann. Rev. Mater. Res. 45, 63 (2015) [10] K Bhatt, S Shriwastava, S Kumar, Sandeep and C C Tripathi, Terahertz spectroscopy a cutting edge technology edited by Jamal Uddin (Intech, Croatia, EU, 2017) Chapter 5, pp [11] K Bhatt and C C Tripathi, Indian J. Pure Appl. Phys. 53, 827 (2015)

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