- Email: [email protected]
Investigating electrical properties of controllable graphene nanoribbon field effect transistors
Physica B 583 (2020) 412022
Contents lists available at ScienceDirect
Physica B: Physics of Condensed Matter journal homepage: http://www.elsevier.com/locate/physb
Investigating electrical properties of controllable graphene nanoribbon field effect transistors Jianping Wang a, Quan Wang a, b, c, * a
Zhenjiang Key Laboratory of Advanced Sensing Materials and Devices, School of Mechanical Engineering, Jiangsu University, Zhenjiang, 212013, PR China State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Shanghai, 200050, PR China c State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene nanoribbon Focused ion beam Field effect transistor Mobility On/off current ratio
It is always an important issue to open the energy gap for graphene while retaining its high carrier mobility. Narrowing down large-area graphene into graphene nanoribbons (GNRs) is one of the ways to open the energy gap to achieve field-effect transistors (FET) suitable for logic circuits. But it has been a problem to obtain GNRs with controllable width and specific boundary structure. In this work, we proposed a simple high-precision preparation method for GNRs. In the process, GNRs with a width of 200 nm and smooth edges were prepared by a focused ion beam (FIB) etching process. Graphene nanoribbon field-effect transistors (GNR-FET) were fabricated by electron beam lithography (EBL). In this device, due to the proximity of the edge of the Heþ during the FIB etching process, the carbon atom structure of the GNRs edge was changed, resulting in variations in electron transport properties. The electrical performance test demonstrated that the on/off current ratio of the GNR-FET device was up to 103 at room temperature. The structural defects of the GNRs caused the device carrier mobility down to 371.6 cm2V 1s 1. The structural defects in the GNRs edge introduced by FIB can improve the on/off current ratio of the device and hence enhanced its electrical regulation performance. Our work provides a simple method of making controllable GNRs to fabricate field effect transistors.
1. Introduction
important method employed to open the graphene energy gap [8,9]. Other studies revealed that energy gap was affected by the localized edge states of the GNRs, and the range of gap was inversely proportional to the width [10,11]. The boundary conditions and width of GNRs are critical to the electrical properties of GNR-FET [12–14]. Due to the limitations of the boundary conditions and width of the GNRs, the preparation of GNRs faces many challenges. How to obtain GNRs with controlled width and specific boundary structure has always been a problem. At present, there are some different techniques to prepare GNRs. Kosynkin et al. [15] prepared GNRs by cleavage of carbon nanotubes. Multi-walled carbon nanotubes were suspended in concentrated sulfuric acid and then placed in 500 wt% KMnO4 solution at room temperature (22 � C) for 1 h, and finally transferred to 50 � C–70 � C for 1 h to obtain GNRs. This method yielded GNRs with widths of ~100 nm and sharp edges. Wang et al. [16] prepared GNRs by chemical derivation. The graphite was heated to 1000 � C for 60 s to a gas, and the exfoliated graphite obtained was dispersed in an organic solvent. After 30 min of
Since the exfoliation of graphene in 2004 [1], Graphene, as an atomically thin material, is of great interest for electronic devices because of its outstanding characteristics, e.g., high carrier mobility and good optical, mechanical, electrical and thermal properties [2–5]. However, the absence of an energy gap in graphene causes difficulty in achieving high on/off current ratio in its field effect transistor (FET) for logic circuit devices [6]. Therefore, how to reduce the off current and improve the on/off current ratio of the device is important to ensure the excellent mobility of the graphene. In 2007, Xia et al. [7] used bilayer graphene as the channel of the FET. It was found that when the vertical electric field was applied to the bilayer graphene, it opened the energy gap of graphene and increased the on/off current ratio of the FET. The test results exhibited that the on/off current ratio reached 200 at room temperature and 2000 at a low temperature of 20 K. In addition to this method, cutting a 2D graphene sheet into one dimensional graphene nanoribbons (GNRs) was another
* Corresponding author. School of Mechanical Engineering, Jiangsu University, Zhenjiang, 212013, PR China. E-mail address: [email protected] (Q. Wang). https://doi.org/10.1016/j.physb.2020.412022 Received 16 November 2019; Received in revised form 15 January 2020; Accepted 16 January 2020 Available online 18 January 2020 0921-4526/© 2020 Elsevier B.V. All rights reserved.
J. Wang and Q. Wang
Physica B: Physics of Condensed Matter 583 (2020) 412022
sonication, the large particles were removed by centrifugation to obtain GNRs with a width of 10–50 nm. The resulting GNR-FET on/off current ratio was up to 107 at room temperature. Sun et al. [17] used a high-precision photoresist as a mask to pattern single-layer graphene by electron beam lithography (EBL). 50 nm wide mask strip was obtained by this method and then etched by an oxygen plasma etching technique to obtain GNRs of different widths. The resulting device on/off current ratio was approximately 47 at room temperature and 105 in a 5.4 K low temperature environment. Candini [18] presented a novel type of phototransistor device, made of bottom-up CVD-grown GNRs as the channel material and multilayer graphene as the electrodes. In this work, we report a simple method to fabricate ultra-narrow GNRs by the FIB processing system. It exhibits high controllability and accuracy. In contrast to other doping strategies, almost all elements can be introduced into the target materials by ion implantation and it does not introduce other impurity elements [19]. Heþ ion beam with 5 nm processing line width guarantees GNR etching accuracy. Owing to this way, GNRs with smooth edges and controllable width are achieved. Compared with the cracked multi-walled carbon nanotube method and the chemical synthesis method, this technique avoids contamination and residue of photoresist. We completed the device preparation with EBL and tested its electrical performance using a four-probe station. We studied the off-state current, carrier mobility, the on/off current ratio of the device, and analyzed the changes in electrical properties caused by structural damage of carbon atoms at the edge of the GNR. Our finding offers more freedom in construction of FET with controllable graphene nanoribbon as the channel material. 2. Experimental details 2.1. Characterization of single-layer graphene samples
Fig. 1. Raman spectra of the graphene. (a) graphene Raman spectra. (b) gra phene Raman Mapping.
In this experiment, a monolayer graphene grown by chemical vapor deposition (CVD) on Cu foils was selected and transferred to a Si sub strate as a raw material for preparing a GNR device. Raman spectroscopy was performed to identify the thin film. There are many peaks in the Raman spectrum of graphene. The main characteristic peaks are D peak (near 1350 cm 1), G peak (near 1580 cm 1) and 2D peak (near 2670 cm 1). Fig. 1(a) shows the Raman spectrum of graphene at 532 nm excitation wavelength. It is found that the position of the graphene characteristic peak is the same as that reported in the literature [20]. The D peak was not distinct, and the intensity of the 2D peak intensity was about twice of the G peak. This revealed that there were few defects in the sample [21,22]. Fig. 1(b) depicts a Raman mapping image of graphene. The area where the scale exceeds 1 was a monolayer gra phene. It can be seen that most of the sample surface is monolayer graphene except for a small amount of graphite.
the GNR after FIB etching. Fig. 4(a) shows the GNR patterned image under a 200x optical microscope. The surrounding wireframe was the FIB etching path. The purpose of the patterning was to separate the GNR from the surrounding graphene film for subsequent device construction. Fig. 4(b) shows the GNR image with a 1000x optical microscope. It was found no signs of miscut or fracture of GNR prepared by FIB etching. 2.3. Preparation of the electrode In this work, the source and drain electrodes (~50 nm thick) were patterned by EBL (Elionix ELS-F125G8). The electrodes materials was Ti/Au. Ohmic contacts between metal and graphene were verified from linear current-voltage response. Afterward, the photoresist (E-Beam Resist ARP6200.09) was spun onto the samples. Low spin rate of 2500 rpm was used to avoid any rolling or damage of graphene during coating. Then, EBL was employed to pattern the photoresist into the desired shape as etching mask for graphene after baking. Finally, the sample was developed and fixed. In this process, the substrate was dried in advance to increase the bonding ability of the photoresist. After spincoating the photoresist, the back of the substrate needed to be cleaned with acetone. The goal was to eliminate residual photoresist to improve exposure accuracy. In the end, an electrode was prepared for the sample by an evapo ration coating system. The electrode was a Ti/Au material of 10 nm Ti and 40 nm Au. The purpose of using Ti was to enhance the contact be tween the electrode and the substrate. The sample of GNR-FET device was obtained with channel width of 200 nm and length of 2 μm. These dimensions were controlled by EBL patterning. The morphology of the device by optical microscopy was shown in Fig. 5.
2.2. Preparation of GNRs Different from the traditional lithography method, the FIB (Helios 600i dual beam system focused ion beam) was used to etch graphene to prepare GNRs. High-precision Heþ ion beam etching technology ensures GNRs with narrow width and smooth edges. The preparation process of GNRs is shown in Fig. 2. The graphene on the Cu film was transferred onto the Si/SiO2 substrate and then the graphene was patterned by FIB. As shown in Fig. 2(b), the scanning electron microscope was cooperated with FIB to complete the positioning and focusing during processing. The main parameters of FIB processing are as follows: Heþ ion source, 2.7 � 10 10 pC/μm2 ion dose, 1.00 μs ion residence time, and 50.14 nm etching depth. Fig. 2(c) illustrates the GNR image. In the FIB etching process, in order to improve the etching precision, we made the scanning direction perpendicular to the width direction of the GNR. Fig. 3 is the SEM image of the etching process. Subsequently, we performed optical microscopy characterization of 2
J. Wang and Q. Wang
Physica B: Physics of Condensed Matter 583 (2020) 412022
2.4. Electrical characterization and measurements All the electrical measurements were carried out in a four-probe station. (KEYSIGHT B1500A). In order to improve the performance of the device, the device was annealed in an argon atmosphere at 400 � C for 2 h to form an ohmic contact between the electrode and the sub strate. In addition, the absorbates and organic residues on graphene flakes from the previous fabrication process were removed by using annealing [23]. 3. Results and discussion 3.1. Electrical performance test of GNR-FET Fig. 6 illustrates the structure of the back-gate GNR-FET with edge structure in this work. The thickness of electrodes is 50 nm, and the width of GNR is 200 nm and the length is 2 μm on the SiO2 dielectric with the thickness of 285 nm. All the measurements were performed at room temperature. 3.2. Output characteristics of GNR-FET Fig. 7 shows the output characteristics of the back-gate GNR-FET device. The source-drain voltage was selected from 0.5 V to 0.5 V, and the gate voltage was 0 V, 10 V, 20 V and 30 V. Fig. 7 reveals that the output characteristic curve of the device always has a good linear rela tionship under different gate voltages Vg. At the same time, the curve passes through the origin of the coordinate. This reveals that the device electrode has good ohmic contact with the channel material. In addition, the slope of the output characteristic curve does not change significantly as the gate voltage changes. Because the width of the GNR is relatively wide, it cannot open the transport gap in graphene by minimizing its width [24]. Therefore, the GNR still retains some of the characteristics of the raw graphene. This is a reason why the device does not have good gate voltage regulation.
Fig. 2. FIB etching graphene to prepare GNR. (a) single-layer graphene trans ferred to the substrate. (b) the processing of GNR etched with FIB (those red particles are carbon atoms or Heþ ions irradiated during etching). (c) GNR controllably fabricated on Si substrate.
3.3. Transfer characteristics of GNR-FET Carrier mobility and on/off current ratio are two important in dicators for evaluating the merits of FETs. The mobility definition
Fig. 3. SEM images of GNR etched by FIB. (a) The GNR patterned to separate from the surrounding graphene. (b) The first etching of the GNR with the width of 5 μm and the length of 5 μm. (c) The second etching of the GNR with the width of 2 μm and the length of 2 μm. (d) The third etching of the GNR with the width of 200 nm and the length of 2 μm. 3
J. Wang and Q. Wang
Physica B: Physics of Condensed Matter 583 (2020) 412022
Fig. 4. Optical micrograph of GNR after etching.
Fig. 5. Morphological characterization of the finished GNR-FET sample under optical microscope.
Fig. 7. Output electrical characteristics of the GNR-FET at ambient temperature.
Fig. 6. GNR-FET diagram with edge structure.
defines the time it takes for a carrier to pass a unit distance under a unit electric field. The field-effect mobility μ of the GNR can be derived from the graphene field-effect transistor transconductance gm [6,25]. The transconductance formula is � � dId W gm ¼ ¼ μCox Vd (3-1) L dVg
where L is the length of the graphene nanoribbon channel, W is the nanoribbon width, Vd, Vg are the source-drain voltage and the gate voltage, Id is the source-drain current, Cox is the back-gate capacitance per area. The back-gate capacitance can be calculated by equation (3-3) Cox ¼
Therefore, the carrier mobility of GNRs can be expressed as
μ¼
L 1 dId � � W Cox Vd dVg
εox D
¼
ε0 ε D
(3-2)
where ε0 ¼ 8.85 � 10 12 F m 1 is the vacuum dielectric constant, ε ¼ 3.9 is the relative dielectric constant of SiO2, and D is the thickness of the SiO2. Fig. 8 illustrates the transfer characteristics (source-drain current Isd
(3-2)
4
J. Wang and Q. Wang
Physica B: Physics of Condensed Matter 583 (2020) 412022
Fig. 9. Corresponding Isd Vg characteristics shown with 1 V Vds.
Fig. 8. Transfer curves of GNR-FET at ambient temperature.
At the same time, the dielectric layer SiO2 does not form a strong in-plane bond with graphene, resulting in lower electron mobility of GNR [29–32]. In addition, Ahn et al. reported that a high dose ion-implantation process may cause non-recovered crystalline defects. The mobility degradation arising from the Coulomb scattering and/or neutral crystalline defects deteriorated the value of the on-state current, and the increased off-state current mostly arises from the gate-induced drain leakage [33]. The carrier mobility in GNRs is limited by edge scattering.
vs gate voltage Vg) for different bias (source-drain) voltages Vsd. The device displays an n-type semiconductor-like behavior. When the source and drain voltages remain constant, in the range of 30 V–30 V, the source-drain current decreases with the decrease in the gate voltage, and finally stabilizes at around 65 nA. The presence of leakage current is mainly because the 200 nm graphene nanoribbon channel is relatively wide and still retains some raw graphene characteristics. Fig. 9 illustrates the transfer characteristics of a GNR-FET when the source-drain voltage is constant at 1 V. The damage caused by ion col lisions can serve as metastable electron traps, and these electron traps will capture a large number of electrons, resulting in a threshold voltage shift toward the positive direction. The device is dominated by electrons as majority carriers in the 30 V–30 V gate voltage range. The number of carriers of the GNR is low in the range of Vg (gate voltage) < VT (threshold voltage), while the device is in a low conduction state. Conversely, the number of carriers of the GNR increases, and the device is in a high conduction state. The carrier mobility of the GNR-FET can be calculated using the transfer characteristic curve of Fig. 9 in combina tion with equations (3-1), (3-2), and (3-3). The device carrier mobility is 371.6 cm2V 1s 1, which is lower than those previously reported results about 103 cm2V 1s 1 [26]. At the same time, it can be seen from the transfer characteristic curve of Fig. 9. This leads to a high on/off current modulation up to ~103 for Vsd ¼ 1 V. This is consistent with the on/off ratio of GNRs devices pre pared by oxygen plasma etching [27]. The device carrier mobility can be induced by applying gate voltage, but it is still leakage current. This may be because the GNRs still retains the semi-metallic properties of some raw graphene. In addition, GNRs was prepared using the focused ion beam etching. The high-energy ion beam causes GNRs edge structure defects during the etching process, which cannot guarantee the linear edge of the GNRs. The carrier mobility in GNRs is limited by edge scattering [11].
4. Conclusions In conclusion, we presented the electrical properties of GNRs with edge structure defects. The fabrication process starts with the CVD monolayer graphene flakes and combines with FIB technology and EBL process. The electrical performance test was carried out with a fourprobe station. The electrical performance of the device showed that the on/off current ratio of the device reached 103 at room temperature. The carrier mobility reaches 371.6 cm2V 1s 1, and the off-state current is 65 nA. It may be concluded that the high-energy particle beam destroyed the carbon atom structure at the edge of the GNRs during the etching process, which affected the electrical properties of the GNR. The enhanced on/off ratio of GNR-FET is achieved. At the same time, it is found that the defect can cause the carrier mobility of the GNR-FET device to decrease. Our findings offer more freedom in construction of FET with controllable GNRs as the channel material. CRediT authorship contribution statement Jianping Wang: Conceptualization, Methodology, Data curation, Writing - original draft. Quan Wang: Supervision, Writing - review & editing. Acknowledgment
3.4. Analysis of the causes of low carrier mobility
This work is supported by the National Natural Science Foundation of China (No. 51675246 and 91750112) and Zhenjiang Science & Technology Program (No. GY2019017). The authors would like to thank Qianjin Wang from Nanjing University for his support on FIB and Hao Jia from Southern University of Science and technology for his support on EBL. Many thanks are also given to Guanglong Ding from Shenzhen University and Jiangxian Meng from HORIBA for the technical support on measurement.
The reason of the device with a low electron mobility lies in that graphene samples were prepared using CVD. The advantage of CVD preparation is that it can continuously control the production of singlelayer graphene. However, the electron mobility of graphene prepared by this method is low [28]. This is due to a large number of point defects in the growth of graphene prepared by CVD, as well as residual chemical impurities during transfer. In addition to this, there are substrate in teractions and phonon effects as well as specific CVD scattering effects. 5
J. Wang and Q. Wang
Physica B: Physics of Condensed Matter 583 (2020) 412022
References
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6
Contents lists available at ScienceDirect
Physica B: Physics of Condensed Matter journal homepage: http://www.elsevier.com/locate/physb
Investigating electrical properties of controllable graphene nanoribbon field effect transistors Jianping Wang a, Quan Wang a, b, c, * a
Zhenjiang Key Laboratory of Advanced Sensing Materials and Devices, School of Mechanical Engineering, Jiangsu University, Zhenjiang, 212013, PR China State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Shanghai, 200050, PR China c State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene nanoribbon Focused ion beam Field effect transistor Mobility On/off current ratio
It is always an important issue to open the energy gap for graphene while retaining its high carrier mobility. Narrowing down large-area graphene into graphene nanoribbons (GNRs) is one of the ways to open the energy gap to achieve field-effect transistors (FET) suitable for logic circuits. But it has been a problem to obtain GNRs with controllable width and specific boundary structure. In this work, we proposed a simple high-precision preparation method for GNRs. In the process, GNRs with a width of 200 nm and smooth edges were prepared by a focused ion beam (FIB) etching process. Graphene nanoribbon field-effect transistors (GNR-FET) were fabricated by electron beam lithography (EBL). In this device, due to the proximity of the edge of the Heþ during the FIB etching process, the carbon atom structure of the GNRs edge was changed, resulting in variations in electron transport properties. The electrical performance test demonstrated that the on/off current ratio of the GNR-FET device was up to 103 at room temperature. The structural defects of the GNRs caused the device carrier mobility down to 371.6 cm2V 1s 1. The structural defects in the GNRs edge introduced by FIB can improve the on/off current ratio of the device and hence enhanced its electrical regulation performance. Our work provides a simple method of making controllable GNRs to fabricate field effect transistors.
1. Introduction
important method employed to open the graphene energy gap [8,9]. Other studies revealed that energy gap was affected by the localized edge states of the GNRs, and the range of gap was inversely proportional to the width [10,11]. The boundary conditions and width of GNRs are critical to the electrical properties of GNR-FET [12–14]. Due to the limitations of the boundary conditions and width of the GNRs, the preparation of GNRs faces many challenges. How to obtain GNRs with controlled width and specific boundary structure has always been a problem. At present, there are some different techniques to prepare GNRs. Kosynkin et al. [15] prepared GNRs by cleavage of carbon nanotubes. Multi-walled carbon nanotubes were suspended in concentrated sulfuric acid and then placed in 500 wt% KMnO4 solution at room temperature (22 � C) for 1 h, and finally transferred to 50 � C–70 � C for 1 h to obtain GNRs. This method yielded GNRs with widths of ~100 nm and sharp edges. Wang et al. [16] prepared GNRs by chemical derivation. The graphite was heated to 1000 � C for 60 s to a gas, and the exfoliated graphite obtained was dispersed in an organic solvent. After 30 min of
Since the exfoliation of graphene in 2004 [1], Graphene, as an atomically thin material, is of great interest for electronic devices because of its outstanding characteristics, e.g., high carrier mobility and good optical, mechanical, electrical and thermal properties [2–5]. However, the absence of an energy gap in graphene causes difficulty in achieving high on/off current ratio in its field effect transistor (FET) for logic circuit devices [6]. Therefore, how to reduce the off current and improve the on/off current ratio of the device is important to ensure the excellent mobility of the graphene. In 2007, Xia et al. [7] used bilayer graphene as the channel of the FET. It was found that when the vertical electric field was applied to the bilayer graphene, it opened the energy gap of graphene and increased the on/off current ratio of the FET. The test results exhibited that the on/off current ratio reached 200 at room temperature and 2000 at a low temperature of 20 K. In addition to this method, cutting a 2D graphene sheet into one dimensional graphene nanoribbons (GNRs) was another
* Corresponding author. School of Mechanical Engineering, Jiangsu University, Zhenjiang, 212013, PR China. E-mail address: [email protected] (Q. Wang). https://doi.org/10.1016/j.physb.2020.412022 Received 16 November 2019; Received in revised form 15 January 2020; Accepted 16 January 2020 Available online 18 January 2020 0921-4526/© 2020 Elsevier B.V. All rights reserved.
J. Wang and Q. Wang
Physica B: Physics of Condensed Matter 583 (2020) 412022
sonication, the large particles were removed by centrifugation to obtain GNRs with a width of 10–50 nm. The resulting GNR-FET on/off current ratio was up to 107 at room temperature. Sun et al. [17] used a high-precision photoresist as a mask to pattern single-layer graphene by electron beam lithography (EBL). 50 nm wide mask strip was obtained by this method and then etched by an oxygen plasma etching technique to obtain GNRs of different widths. The resulting device on/off current ratio was approximately 47 at room temperature and 105 in a 5.4 K low temperature environment. Candini [18] presented a novel type of phototransistor device, made of bottom-up CVD-grown GNRs as the channel material and multilayer graphene as the electrodes. In this work, we report a simple method to fabricate ultra-narrow GNRs by the FIB processing system. It exhibits high controllability and accuracy. In contrast to other doping strategies, almost all elements can be introduced into the target materials by ion implantation and it does not introduce other impurity elements [19]. Heþ ion beam with 5 nm processing line width guarantees GNR etching accuracy. Owing to this way, GNRs with smooth edges and controllable width are achieved. Compared with the cracked multi-walled carbon nanotube method and the chemical synthesis method, this technique avoids contamination and residue of photoresist. We completed the device preparation with EBL and tested its electrical performance using a four-probe station. We studied the off-state current, carrier mobility, the on/off current ratio of the device, and analyzed the changes in electrical properties caused by structural damage of carbon atoms at the edge of the GNR. Our finding offers more freedom in construction of FET with controllable graphene nanoribbon as the channel material. 2. Experimental details 2.1. Characterization of single-layer graphene samples
Fig. 1. Raman spectra of the graphene. (a) graphene Raman spectra. (b) gra phene Raman Mapping.
In this experiment, a monolayer graphene grown by chemical vapor deposition (CVD) on Cu foils was selected and transferred to a Si sub strate as a raw material for preparing a GNR device. Raman spectroscopy was performed to identify the thin film. There are many peaks in the Raman spectrum of graphene. The main characteristic peaks are D peak (near 1350 cm 1), G peak (near 1580 cm 1) and 2D peak (near 2670 cm 1). Fig. 1(a) shows the Raman spectrum of graphene at 532 nm excitation wavelength. It is found that the position of the graphene characteristic peak is the same as that reported in the literature [20]. The D peak was not distinct, and the intensity of the 2D peak intensity was about twice of the G peak. This revealed that there were few defects in the sample [21,22]. Fig. 1(b) depicts a Raman mapping image of graphene. The area where the scale exceeds 1 was a monolayer gra phene. It can be seen that most of the sample surface is monolayer graphene except for a small amount of graphite.
the GNR after FIB etching. Fig. 4(a) shows the GNR patterned image under a 200x optical microscope. The surrounding wireframe was the FIB etching path. The purpose of the patterning was to separate the GNR from the surrounding graphene film for subsequent device construction. Fig. 4(b) shows the GNR image with a 1000x optical microscope. It was found no signs of miscut or fracture of GNR prepared by FIB etching. 2.3. Preparation of the electrode In this work, the source and drain electrodes (~50 nm thick) were patterned by EBL (Elionix ELS-F125G8). The electrodes materials was Ti/Au. Ohmic contacts between metal and graphene were verified from linear current-voltage response. Afterward, the photoresist (E-Beam Resist ARP6200.09) was spun onto the samples. Low spin rate of 2500 rpm was used to avoid any rolling or damage of graphene during coating. Then, EBL was employed to pattern the photoresist into the desired shape as etching mask for graphene after baking. Finally, the sample was developed and fixed. In this process, the substrate was dried in advance to increase the bonding ability of the photoresist. After spincoating the photoresist, the back of the substrate needed to be cleaned with acetone. The goal was to eliminate residual photoresist to improve exposure accuracy. In the end, an electrode was prepared for the sample by an evapo ration coating system. The electrode was a Ti/Au material of 10 nm Ti and 40 nm Au. The purpose of using Ti was to enhance the contact be tween the electrode and the substrate. The sample of GNR-FET device was obtained with channel width of 200 nm and length of 2 μm. These dimensions were controlled by EBL patterning. The morphology of the device by optical microscopy was shown in Fig. 5.
2.2. Preparation of GNRs Different from the traditional lithography method, the FIB (Helios 600i dual beam system focused ion beam) was used to etch graphene to prepare GNRs. High-precision Heþ ion beam etching technology ensures GNRs with narrow width and smooth edges. The preparation process of GNRs is shown in Fig. 2. The graphene on the Cu film was transferred onto the Si/SiO2 substrate and then the graphene was patterned by FIB. As shown in Fig. 2(b), the scanning electron microscope was cooperated with FIB to complete the positioning and focusing during processing. The main parameters of FIB processing are as follows: Heþ ion source, 2.7 � 10 10 pC/μm2 ion dose, 1.00 μs ion residence time, and 50.14 nm etching depth. Fig. 2(c) illustrates the GNR image. In the FIB etching process, in order to improve the etching precision, we made the scanning direction perpendicular to the width direction of the GNR. Fig. 3 is the SEM image of the etching process. Subsequently, we performed optical microscopy characterization of 2
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Physica B: Physics of Condensed Matter 583 (2020) 412022
2.4. Electrical characterization and measurements All the electrical measurements were carried out in a four-probe station. (KEYSIGHT B1500A). In order to improve the performance of the device, the device was annealed in an argon atmosphere at 400 � C for 2 h to form an ohmic contact between the electrode and the sub strate. In addition, the absorbates and organic residues on graphene flakes from the previous fabrication process were removed by using annealing [23]. 3. Results and discussion 3.1. Electrical performance test of GNR-FET Fig. 6 illustrates the structure of the back-gate GNR-FET with edge structure in this work. The thickness of electrodes is 50 nm, and the width of GNR is 200 nm and the length is 2 μm on the SiO2 dielectric with the thickness of 285 nm. All the measurements were performed at room temperature. 3.2. Output characteristics of GNR-FET Fig. 7 shows the output characteristics of the back-gate GNR-FET device. The source-drain voltage was selected from 0.5 V to 0.5 V, and the gate voltage was 0 V, 10 V, 20 V and 30 V. Fig. 7 reveals that the output characteristic curve of the device always has a good linear rela tionship under different gate voltages Vg. At the same time, the curve passes through the origin of the coordinate. This reveals that the device electrode has good ohmic contact with the channel material. In addition, the slope of the output characteristic curve does not change significantly as the gate voltage changes. Because the width of the GNR is relatively wide, it cannot open the transport gap in graphene by minimizing its width [24]. Therefore, the GNR still retains some of the characteristics of the raw graphene. This is a reason why the device does not have good gate voltage regulation.
Fig. 2. FIB etching graphene to prepare GNR. (a) single-layer graphene trans ferred to the substrate. (b) the processing of GNR etched with FIB (those red particles are carbon atoms or Heþ ions irradiated during etching). (c) GNR controllably fabricated on Si substrate.
3.3. Transfer characteristics of GNR-FET Carrier mobility and on/off current ratio are two important in dicators for evaluating the merits of FETs. The mobility definition
Fig. 3. SEM images of GNR etched by FIB. (a) The GNR patterned to separate from the surrounding graphene. (b) The first etching of the GNR with the width of 5 μm and the length of 5 μm. (c) The second etching of the GNR with the width of 2 μm and the length of 2 μm. (d) The third etching of the GNR with the width of 200 nm and the length of 2 μm. 3
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Physica B: Physics of Condensed Matter 583 (2020) 412022
Fig. 4. Optical micrograph of GNR after etching.
Fig. 5. Morphological characterization of the finished GNR-FET sample under optical microscope.
Fig. 7. Output electrical characteristics of the GNR-FET at ambient temperature.
Fig. 6. GNR-FET diagram with edge structure.
defines the time it takes for a carrier to pass a unit distance under a unit electric field. The field-effect mobility μ of the GNR can be derived from the graphene field-effect transistor transconductance gm [6,25]. The transconductance formula is � � dId W gm ¼ ¼ μCox Vd (3-1) L dVg
where L is the length of the graphene nanoribbon channel, W is the nanoribbon width, Vd, Vg are the source-drain voltage and the gate voltage, Id is the source-drain current, Cox is the back-gate capacitance per area. The back-gate capacitance can be calculated by equation (3-3) Cox ¼
Therefore, the carrier mobility of GNRs can be expressed as
μ¼
L 1 dId � � W Cox Vd dVg
εox D
¼
ε0 ε D
(3-2)
where ε0 ¼ 8.85 � 10 12 F m 1 is the vacuum dielectric constant, ε ¼ 3.9 is the relative dielectric constant of SiO2, and D is the thickness of the SiO2. Fig. 8 illustrates the transfer characteristics (source-drain current Isd
(3-2)
4
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Physica B: Physics of Condensed Matter 583 (2020) 412022
Fig. 9. Corresponding Isd Vg characteristics shown with 1 V Vds.
Fig. 8. Transfer curves of GNR-FET at ambient temperature.
At the same time, the dielectric layer SiO2 does not form a strong in-plane bond with graphene, resulting in lower electron mobility of GNR [29–32]. In addition, Ahn et al. reported that a high dose ion-implantation process may cause non-recovered crystalline defects. The mobility degradation arising from the Coulomb scattering and/or neutral crystalline defects deteriorated the value of the on-state current, and the increased off-state current mostly arises from the gate-induced drain leakage [33]. The carrier mobility in GNRs is limited by edge scattering.
vs gate voltage Vg) for different bias (source-drain) voltages Vsd. The device displays an n-type semiconductor-like behavior. When the source and drain voltages remain constant, in the range of 30 V–30 V, the source-drain current decreases with the decrease in the gate voltage, and finally stabilizes at around 65 nA. The presence of leakage current is mainly because the 200 nm graphene nanoribbon channel is relatively wide and still retains some raw graphene characteristics. Fig. 9 illustrates the transfer characteristics of a GNR-FET when the source-drain voltage is constant at 1 V. The damage caused by ion col lisions can serve as metastable electron traps, and these electron traps will capture a large number of electrons, resulting in a threshold voltage shift toward the positive direction. The device is dominated by electrons as majority carriers in the 30 V–30 V gate voltage range. The number of carriers of the GNR is low in the range of Vg (gate voltage) < VT (threshold voltage), while the device is in a low conduction state. Conversely, the number of carriers of the GNR increases, and the device is in a high conduction state. The carrier mobility of the GNR-FET can be calculated using the transfer characteristic curve of Fig. 9 in combina tion with equations (3-1), (3-2), and (3-3). The device carrier mobility is 371.6 cm2V 1s 1, which is lower than those previously reported results about 103 cm2V 1s 1 [26]. At the same time, it can be seen from the transfer characteristic curve of Fig. 9. This leads to a high on/off current modulation up to ~103 for Vsd ¼ 1 V. This is consistent with the on/off ratio of GNRs devices pre pared by oxygen plasma etching [27]. The device carrier mobility can be induced by applying gate voltage, but it is still leakage current. This may be because the GNRs still retains the semi-metallic properties of some raw graphene. In addition, GNRs was prepared using the focused ion beam etching. The high-energy ion beam causes GNRs edge structure defects during the etching process, which cannot guarantee the linear edge of the GNRs. The carrier mobility in GNRs is limited by edge scattering [11].
4. Conclusions In conclusion, we presented the electrical properties of GNRs with edge structure defects. The fabrication process starts with the CVD monolayer graphene flakes and combines with FIB technology and EBL process. The electrical performance test was carried out with a fourprobe station. The electrical performance of the device showed that the on/off current ratio of the device reached 103 at room temperature. The carrier mobility reaches 371.6 cm2V 1s 1, and the off-state current is 65 nA. It may be concluded that the high-energy particle beam destroyed the carbon atom structure at the edge of the GNRs during the etching process, which affected the electrical properties of the GNR. The enhanced on/off ratio of GNR-FET is achieved. At the same time, it is found that the defect can cause the carrier mobility of the GNR-FET device to decrease. Our findings offer more freedom in construction of FET with controllable GNRs as the channel material. CRediT authorship contribution statement Jianping Wang: Conceptualization, Methodology, Data curation, Writing - original draft. Quan Wang: Supervision, Writing - review & editing. Acknowledgment
3.4. Analysis of the causes of low carrier mobility
This work is supported by the National Natural Science Foundation of China (No. 51675246 and 91750112) and Zhenjiang Science & Technology Program (No. GY2019017). The authors would like to thank Qianjin Wang from Nanjing University for his support on FIB and Hao Jia from Southern University of Science and technology for his support on EBL. Many thanks are also given to Guanglong Ding from Shenzhen University and Jiangxian Meng from HORIBA for the technical support on measurement.
The reason of the device with a low electron mobility lies in that graphene samples were prepared using CVD. The advantage of CVD preparation is that it can continuously control the production of singlelayer graphene. However, the electron mobility of graphene prepared by this method is low [28]. This is due to a large number of point defects in the growth of graphene prepared by CVD, as well as residual chemical impurities during transfer. In addition to this, there are substrate in teractions and phonon effects as well as specific CVD scattering effects. 5
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Physica B: Physics of Condensed Matter 583 (2020) 412022
References
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