nanomaterials Brief Report
An Al2O3 Gating Substrate for the Greater Performance of Field Effect Transistors Based on Two-Dimensional Materials Hang Yang 1 ID , Shiqiao Qin 2 ID and Xueao Zhang 1, * 1
2 3
*
ID
, Xiaoming Zheng 1,3 , Guang Wang 1 , Yuan Tan 1 , Gang Peng 1, *
College of Science, National University of Defense Technology, Changsha 410073, China;
[email protected] (H.Y.);
[email protected] (X.Z.);
[email protected] (G.W.);
[email protected] (Y.T.) College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, China;
[email protected] College of Physics and Electronics, Central South University, Changsha 410073, China Correspondence:
[email protected] (G.P.);
[email protected] (X.Z.); Tel.: +135-0748-0737 (G.P.); +138-7594-5032 (X.Z.)
Received: 14 August 2017; Accepted: 18 September 2017; Published: 22 September 2017
Abstract: We fabricated 70 nm Al2 O3 gated field effect transistors based on two-dimensional (2D) materials and characterized their optical and electrical properties. Studies show that the optical contrast of monolayer graphene on an Al2 O3 /Si substrate is superior to that on a traditional 300 nm SiO2 /Si substrate (2.4 times). Significantly, the transconductance of monolayer graphene transistors on the Al2 O3 /Si substrate shows an approximately 10-fold increase, due to a smaller dielectric thickness and a higher dielectric constant. Furthermore, this substrate is also suitable for other 2D materials, such as WS2 , and can enhance the transconductance remarkably by 61.3 times. These results demonstrate a new and ideal substrate for the fabrication of 2D materials-based electronic logic devices. Keywords: graphene; WS2 ; Al2 O3 gating substrate; field effect transistors
1. Introduction Two-dimensional (2D) materials, such as graphene and transition-metal dischalcogenides (TMDs), have attracted tremendous interest for possible applications in transistors [1–4], photodetectors [5,6], and touch panels [7,8] owing to their extraordinary properties. However, most efforts to date employ a 300 nm thick silicon dioxide (SiO2 ) substrate as the gate dielectric layer. This substrate is widely used is mainly because 2D materials can be readily visualized using an optical microscope due to optical interference [9–11]. Although they have led to many interesting scientific discoveries [12–14], applying 300 nm SiO2 substrates will greatly reduce the performance of the devices, especially the signal amplification capability, which is one of the most important parameters of contemporary integrated circuits [15,16]. The devices fabricated on SiO2 /Si substrates lack enough capability to regulate the Fermi surface of samples, thus requiring higher back-gate voltage [15]. Generally, the top-gate structure is adopted to enhance the gate capacitance of field effect transistors (FETs). However, its fabrication is challenging, as 2D materials lack dangling bonds [17,18]. Although many alternate approaches have been demonstrated, they inevitably result in the degradation of carrier mobility [19,20]. Previously, Liao et al. firstly reported that applying 72 nm Al2 O3 /Si substrates could improve the optical contrast and electrical properties of single-graphene FETs [21]. However, the Al2 O3 film they fabricated was not well insulated since they directly attached tape onto the Al2 O3 /Si substrate
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using mechanical exfoliation [6]. This conventional method may damage Al2 O3 films due to the strong adhesive force of the tape [21]. In our experiments, the deterministic transfer method was applied to transfer graphene onto an Al2 O3 /Si substrate to avoid this problem. In addition, the corrosion time of theNanomaterials HF solution which was 2017, 7,was 286 carefully controlled so that the wafer would be smoother,2 of 8 beneficial for forming a dense Al2 O3 film via atomic layer deposition (ALD) growth. Accordingly, adhesive force of the tape [21]. In our experiments, the deterministic transfer method was applied to our single-layer graphene FETs, compared to those of Liao’s work, achieved a greater performance. transfer graphene onto an Al2O3/Si substrate to avoid this problem. In addition, the corrosion time of Furthermore, systematically investigated FETs onwould an Al substrate, showing that 3 gating the we HF solution was carefully controlled soWS that2the wafer be2 O smoother, which was beneficial forsubstrate forming a dense Alsuitable 2O3 film via atomic layer (ALD) growth. Accordingly, our singlethis superior is also for other 2Ddeposition materials. layer graphene FETs, compared to those of Liao’s work, achieved a greater performance. Furthermore, we systematically 2. Experimental Details investigated WS2 FETs on an Al2O3 gating substrate, showing that this superior substrate is also suitable for other 2D materials.
Figure 1 illustrates the fabrication process of graphene- (or WS2 )-based FETs on Al2 O3 /Si Experimental substrates. 2. Firstly, an Al2Details O3 film was deposited on silicon (doped n++, conductivity: 0.01–0.02 Ω·cm) Figuretechnique 1 illustrates the fabrication process of graphene-((CH (or WS FETsdistilled on Al2O3/Si substrates. wafers via the ALD using tri-methyl-aluminum water as the source 3 2))-based 3 Al) and Firstly, an Al2O3 film was silicon (doped n++, conductivity: 0.01–0.02 Ω·cm) wafers via the (reaction temperature: 250 ◦ C).deposited Prior toonthe deposition of Al2 O3 , a native SiO2 layer was removed ALD technique using tri-methyl-aluminum ((CH3)3Al) and distilled water as the source (reaction with a 5% (mole ratio)250 hydrofluoric acid (HF) solution min).SiO After the Al2 O3 film was deposited, temperature: °C). Prior to the deposition of Al2O3,(40 a native 2 layer was removed with a 5% graphene (or WS ) was transferred onto that substrate via the deterministic transfer method [22] (mole ratio) hydrofluoric acid (HF) solution (40 min). After the Al 2 O 3 film was deposited, graphene 2 (or WS 2 ) was transferred onto that substrate via the deterministic transfer method [22] (see Figure (see Figure S1). At last, the source and drain contacts were patterned using e-beam lithography (EHT: S1). At last, the source and drain contacts were patterned using e-beam lithography (EHT: 10 kV, 10 kV, aperture size: 30 µm, beam current: 217.1 pA), and 10 nm Ti/50 nm Au were deposited using aperture size: 30 μm, beam current: pA), and 10 nm Ti/50 nm Au were deposited using e-beam −5 217.1 e-beam evaporation Pa, evaporation rate: 0.5 −5 Pa, evaporation evaporation(vacuum: (vacuum: 1 1 × 1010 rate: Ti: 0.5 Å/s;Ti: Au: 1.5Å/s; Å/s). Au: 1.5 Å/s).
Figure 1. The of process of fabricating graphene WS2)) field field effect (FETs) on Al2on O3/SiAl O /Si Figure 1. The process fabricating graphene (or(orWS effecttransistors transistors (FETs) 2 2 3 substrates. HF: hydrofluoric acid. substrates. HF: hydrofluoric acid.
The topography of the samples was characterized via atomic force microscopy (AFM, NT-MDT company, Moscow, scanning mode: Semi-contact, frequency: 1.01 (Hz), (AFM, scanningNT-MDT The topography of the Russia, samples was characterized via scanning atomic force microscopy electron microscopy (SEM, Raith company, Dortmund, Germany) and optical microscopy (Nikon company, Moscow, Russia, scanning mode: Semi-contact, scanning frequency: 1.01 (Hz), scanning company, Tokyo, Japan). The Raman and contrast spectra were recorded with Confocal Raman electron microscopy company, Dortmund, Germany) and532 optical microscopy Spectrometer(SEM, (WiTec Raith company, Ulm, Germany, exciting laser wavelength: nm, spot size: 2 μm). (Nikon company, Tokyo, Japan). and contrast spectra were recorded with Confocal The thickness of theThe Al2ORaman 3 film was obtained with GES-5 ellipsometer (Sopra Company, Annecy, Raman France) and calculated to be approximately nm. All characterizations were conducted ambient Spectrometer (WiTec company, Ulm, Germany,70exciting laser wavelength: 532 nm,inspot size: 2 µm). conditions and at room temperature (300 K). The electrical properties were measured with 4200-SCS The thickness of the Al2 O3 film was obtained with GES-5 ellipsometer (Sopra Company, Annecy, probe system (Keithley Company, Cleveland, OH, USA).
France) and calculated to be approximately 70 nm. All characterizations were conducted in ambient 3. Results and Discussion conditions and at room temperature (300 K). The electrical properties were measured with 4200-SCS probe system (Keithley Cleveland, USA). is uniform over a large area (50 μm 50 μm). Figure 2b As shown Company, in Figure 2a, the Al2O3 film OH, illustrates the height distribution of the local area, which mainly varies from 4 to 6 nm. The parameters of surface roughness are given in Table S1. Based on the measurements, the Si surface is 3. Results and Discussion extremely smooth after HF treatment. In addition, the average surface roughness of the ALD-grown
As shown Figure 2a, the Al2 O3 film is uniform over a large area (50 µm × 50 µm). Figure 2b Al2O3in film is 1.26 nm. To understand the dielectric properties of the Al 2O3 film, I-V characteristics wereThe firstly illustrates the height distribution of the local area, which mainly varies from 4 to 6 nm. parameters measured based on metal-insulator-semiconductor (MIS) devices with Al2O3 and SiO2 insulating of surface roughness are given in Table S1. Based on the measurements, the Si surface is extremely layers on silicon wafers (shown in Figure 2c). When bias voltage increased to 10 V, the tunneling smooth after HF treatment. In addition, the average surface roughness of the ALD-grown Al O film current of Al2O3 was only one tenth of that of SiO2. This indicates that the Al2O3 dielectric layer can 2 3 is 1.26 nm. To understand the dielectric properties of the Al2 O3 film, I-V characteristics were firstly measured based on metal-insulator-semiconductor (MIS) devices with Al2 O3 and SiO2 insulating layers on silicon wafers (shown in Figure 2c). When bias voltage increased to 10 V, the tunneling current of
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Al2 O3 was only one tenth of that of SiO2 . This indicates that the Al2 O3 dielectric layer can withstand Nanomaterials 2017, 7, 286 3 of 8 a higher gate voltage, resulting in greater modulation of the Fermi level of 2D materials. In general, withstand a higher gatedielectric voltage, resulting in greater modulation the Fermi level of 2D(F–N) materials. the I-V characteristic of the layer can be described viaofFowler–Nordheim tunneling In[21,23]: general, the I-V characteristic of the dielectric layer can be described via Fowler–Nordheim (F–N) behavior tunneling behavior [21,23]: 2 J = AEox exp(− B/Eox ) (1) J = AEox2 exp(− B Eox )
(1)
where J is current density, Eox is the electric field, and A and B are constants considering carrier is current density, is the electricApparently, field, and A and B are considering effectivewhere massJand barrier height,Eoxrespectively. based on constants Figure 2d, when thecarrier electric field effective mass and barrier height, respectively. Apparently, based on Figure 2d, when the electric is large, it is in good agreement with the theoretical model [24]. However, in the case of small electric field is large, it is in good agreement with the theoretical model [24]. However, in the case of small fields, due to the influence of electrical noise in the environment, the experimental curve exhibits electric fields, due to the influence of electrical noise in the environment, the experimental curve fluctuation [19]. exhibits fluctuation [19].
Figure 2. Characterization of 70 nm Al2O3 film prepared by atomic layer deposition (ALD). (a) Atomic
Figure 2. Characterization of 70 nm Al2 O3 film prepared by atomic layer deposition (ALD). (a) Atomic force microscopy (AFM) image and corresponding (b) height distribution of film surface (areas in force microscopy image and corresponding distribution of film surface andheight SiO2 films. (d) Flow–Nordheim (F–N) (areas fitting in blue blue dashed(AFM) box). (c) Tunneling currents of Al2O3(b) dashed curve box). of(c)the Tunneling currents of Al O and SiO films. (d) Flow–Nordheim (F–N) fitting curve metal-insulator-semiconductor 2 3 (MIS) device. 2 of the metal-insulator-semiconductor (MIS) device. Optical contrast is the difference in visual properties that enables us to distinguish an object from other objects and the background. Figure 3a,b shows the optical image of graphene on SiO2/Si and Optical contrast is the difference visualthe properties enables us to distinguish an object Al2O3/Si substrates, respectively. To in quantify contrast ofthat graphene on different substrates, the from other objects and the background. Figure 3a,b shows the optical image of graphene on color images are converted to gray-scale images. By calculation [25], the absolute value of contrastSiO2 /Si of graphene respectively. on the Al2O3/Si To substrate (−0.12) significantly than on thatdifferent on the SiOsubstrates, 2/Si and Al2intensity O3 /Si substrates, quantify theis contrast of higher graphene substrate (−0.05). Furthermore, from the contrast spectrum shown in Figure 3c, the absolute value the color images are converted to gray-scale images. By calculation [25], the absolute value ofofcontrast on the Al2O3/Si substrate in the 450~700 nm wavelength range is always higher than that intensitythe ofcontrast graphene on the Al2 O3 /Si substrate (−0.12) is significantly higher than that on the SiO2 /Si on the SiO2/Si substrate. The best contrast of graphene on the Al2O3/Si substrate could be obtained substrate (−0.05). Furthermore, from the contrast spectrum shown in Figure 3c, the absolute value of with 450 nm and 550 nm illuminations. As depicted in Figure 3d, the G peak and the 2D peak of the contrast on the the 450~700 nm wavelength always higher than that −1 for 2D graphene on Al the2 O Al32/Si O3/Sisubstrate substrate in experience a red-shift (8.3 cm−1 for range G peakisand 3.3 cm on the SiO /Si substrate. The best contrast of graphene on the Al O /Si substrate could be ⁄ obtained 2 The Raman shift could be simplified with the harmonic oscillator 2 3 , peak). model [26]: ∆ ∆ is550 the Raman shift, is the mechanical constant, and m is 3d, the effective mass. and Because thepeak of with 450where nm and nm illuminations. As depicted in Figure the G peak theof2D −1formed presence spotted islands on the Al2O3/Si substrate, a tensile stress is graphene on theofAl for G onto peakgraphene, and 3.3 which cm−1 for 2D 2 O3 /Si substrate experience a red-shift (8.3 cm leadsRaman to a decrease andbe subsequently red-shift of the Raman vibrationmodel peak of [26]: graphene on p β/m, peak). The shift in could simplifiedthe with the harmonic oscillator ∆k = the Al2O3/Si substrate [26].
where ∆k is the Raman shift, β is the mechanical constant, and m is the effective mass. Because of the presence of spotted islands on the Al2 O3 /Si substrate, a tensile stress is formed onto graphene, which leads to a decrease in β and subsequently the red-shift of the Raman vibration peak of graphene on the Al2 O3 /Si substrate [26].
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Figure 3. (a,b) Optical image of graphene on the SiO2/Si and Al2O3/Si substrates. (c) The contrast and Figure 3. (a,b) Optical image of graphene on the SiO2 /Si and Al2 O3 /Si substrates. (c) The contrast and (d) Raman spectra of graphene on SiO2/Si and Al2O3/Si substrates. Raw data and processing methods (d) Raman spectra of graphene on SiO2 /Si and Al2 O3 /Si substrates. Raw data and processing methods are shown in Figures S2 and S3. are shown in Figures S2 and S3. Figure 3. (a,b) Optical image of graphene on the SiO2/Si and Al2O3/Si substrates. (c) The contrast and and Al2OFETs 3/Si substrates. Raw and processing (d) Raman of graphene on SiO Next, the spectra electrical properties of 2/Si graphene on the Al 2O3data /Si substrate weremethods studied in are in Figures and S3. nitrogen. As depicted inS2Figure 4a,graphene the drain-source increases linearly pace with the Next, theshown electrical properties of FETs oncurrent the Al2(I Ods3)/Si substrate wereinstudied in nitrogen. bias voltage, indicating good ohmic contact between the graphene and the electrode. The aspect ratio As depicted in Figure 4a, the drain-source current (Ids ) increases linearly in pace with the bias voltage, electrical properties of1.5, graphene the image. Al2O3/Si were studied in (L/W)Next, of thethe channel is approximately shownFETs inand theonthe SEM Forsubstrate better comparison among indicating good ohmic contact between the as graphene electrode. The aspect ratio (L/W) of the nitrogen. samples, As depicted in FigureIds4a, current which (Ids) increases linearly pace with the different normalized (=Ithe ds drain-source / ) was applied, considered the in influence of the channel is approximately 1.5, as shown in the SEM image. For better comparison among different bias voltage, indicating good ohmic contact between the of graphene and the electrode. The aspect ratio aspect ratio. Figure 4b shows the transfer characteristics our devices (Vg means back-gate voltage). samples, normalized Ids is (=I applied, which considered the better influence of the aspect ratio. ds × L/W) was (L/W) of the channel approximately 1.5, as shown in Al the2O SEM image. For comparison among It is obvious that the curve slope of the device on the 3/Si substrate is significantly higher than Figure 4b shows the transfer characteristics of our devices (V means back-gate voltage). It is obvious gwhich different normalized Ids (=Ids a greater / ) was applied, considered of the that on thesamples, SiO2/Si substrate, indicating gate regulation ability of the 70the nminfluence Al2O3 dielectric that the curve slope offor the device on theAlAl substrate significantly higher than that on the aspect ratio. Figure 4b shows theon transfer ofwhen ouris devices g means back-gate voltage). layer. In addition, graphene the 2characteristics O23O /Si3 /Si substrate, the gate(Vvoltage increases from −5 to It isV, obvious that the curveaslope of the 2O3/Si substrate is significantly SiO2 /Si substrate, indicating greater gate regulation ability of the 70 is nm Al2 O3 dielectric layer. 3.6 the current decreases from 190 todevice 28.3 A,onsothe theAl unit on/off ratio evaluated to higher be 0.78than V−1. In that on SiO2unit /Si substrate, indicating a greater gate regulation of the 70 nm Al 2O3 dielectric addition, forthe graphene on the Al /Si graphene substrate, when the voltage increases from −5 Vto−1.3.6 V, However, the on/off ratio on the SiO2gate /Si ability substrate reaches only 0.09 2 O3for − 1 layer. In addition, for graphene on the Al 2O3/Si substrate, when the gate voltage increases from −5 to Accordingly, the magnification capability was easily estimated to increase by 8.7 times. The the current decreases from 190 to 28.3 A, so the unit on/off ratio is evaluated to be 0.78 V . However, −1 3.6 V, the current decreases from 190 to 28.3 A, so the unit on/off ratio is evaluated to be 0.78 V − 1 minimum on the Alon 2O3/Si slightly higher thanonly that 0.09 on the /Si substrate,. the the unit on/offconductance ratio for graphene thesubstrate SiO2 /Siissubstrate reaches V SiO. 2Accordingly, −1 However, the unit on/off ratio for graphene on the SiO2/Si substrate reaches only 0.09 V . which maycapability be due to induced impurities in the to transfer process, leading to more carriers in grapheme magnification was easily estimated increase by 8.7 times. The minimum conductance Accordingly, the magnification capability was easily estimated to increase by 8.7 times. The [8]. For further discussion, some significant parameters of FETs are listed in Table 1. on the Al2 O3 /Si substrate on is slightly higher thanisthat on the SiOthan substrate, which may be due 2 /Sithat minimum conductance the Al 2 O 3 /Si substrate slightly higher on the SiO 2 /Si substrate, The normalized transconductance gm can be extracted from the following [27]: to induced impurities ininduced the transfer process, moreleading carriers grapheme For further which may be due to impurities in theleading transfer to process, toin more carriers in[8]. grapheme d I L discussion, significant of FETs are listed Table 1. ds [8]. Forsome further discussion,parameters some significant of in FETs are listed in Table 1. gparameters (2) m = dVg W from The normalized transconductance fromthe thefollowing following [27]: The normalized transconductancegmgmcan can be be extracted extracted [27]:
dI ds LL dI ggmm== dVds W dVgg W
(2)
Figure 4. (a) Output characteristics of graphene FETs on the Al2O3/Si substrate at different gate voltages (−5~10 V). The inset shows an SEM image of the device. (b) Transfer characteristics of graphene FETs on different substrates. Figure 4. (a) Output characteristics of graphene FETs on the Al2O3/Si substrate at different gate Figure 4. (a) Output characteristics of graphene FETs on the Al2 O3 /Si substrate at different gate voltages (−5~10 V). The inset shows an SEM image of the device. (b) Transfer characteristics of voltages (−5~10 V). The inset shows an SEM image of the device. (b) Transfer characteristics of graphene FETs on different substrates.
graphene FETs on different substrates.
(2)
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Table 1. Significant parameters of graphene FETs on different substrates. Nanomaterials 2017, 7,Parameters 286
5 of 8 Minimum Maximum Mobility Dirac Point Conductivity Trascondutance Substrate Table 1. Significant parameters of graphene FETs on different substrates. 2 −1 −1 Al2 O3 283 µS 3.6 V −26.1 µS 6500 cm V ·s SiO 237 µS 18.7 V 2.6 µS 6780 cm2 V−1 ·s−1 Parameters Minimum Maximum 2 Dirac Point Mobility Substrate Conductivity Trascondutance Al2O3 3.6 V 6500 cm2 V−1·s−1 S −26.1 μS The black curve in Figure 283 5a μillustrates the transconductance variation of graphene on SiO2 18.7 V 6780 cm2 V−1·s−1 237 μS 2.6 μS
the Al2 O3 /Si substrate. It can be seen that the maximum negative transconductance and maximum positive transconductance areFigure −26.15aµSillustrates (Vg = −3.1 and 19.4 µS (Vg variation = 2.9 V), respectively. Compared The black curve in theV)transconductance of graphene on the with Al the2Omaximum g of graphene on the SiO /Si substrate (2.6 µS), it can be concluded that the m It can be seen that the maximum 2 3/Si substrate. negative transconductance and maximum regulation ability of the Al2 Oare layer is about times ofV), SiO in accordance positive transconductance −26.1 μS (V g = −3.1 V) and10 19.4 μS (Vthat g = 2.9 respectively. 3 dielectric 2 , which isCompared the maximum gm of Accordingly, graphene on the 2/Si substrate (2.6 dielectric μS), it can constant be concluded that with with previous estimations. theSiO value of effective for Al 2 Othe 3 is 9.2, ability of the 2O3 dielectric layer isconstant about 10 (8~10) times that of SiO , which is in accordance whichregulation is consistent with the Al theoretical dielectric of the Al22O film grown by ALD [28]. 3 with previous estimations. Accordingly, the value of effective dielectric constant for Al 2O3 is 9.2, The changes of the Fermi level of graphene can be fitted with the theoretical model [29]: which is consistent with the theoretical dielectric constant (8~10) of the Al2O3 film grown by ALD q √ of graphene can [28]. The changes of the Fermi level be fitted with the theoretical model [29]:
EF = hv F πn/2πq = hv F πε 0 ε(Vg − VD )/qd/2πq (3) E F = hvF πn / 2πq = hvF πε 0ε (Vg − VD ) / qd / 2πq (3) where EF is the Fermi level, n is the induced charge amount, h is the Planck constant, v F is the Fermi the Fermi level, n is the induced amount, h isThe the amount Planck constant, the Fermi where EF iselementary vF is speed, q is the charge, and VD is charge the Dirac point. of charges induced by the applied gate voltages on different is shown in the of Figure 5b. induced Obviously, as the Dirac point. Theinset amount of charges by the speed, q is the elementary charge,substrates and VD is the applied gate voltages ondielectric different constant substratesincreases, is shown the in the inset of Figure Obviously, as the thickness decreases and the shift of the Fermi5b. level of graphene on the decreases and the dielectric constant shift of the Fermi to level of graphene on Al2 O3thickness /Si substrate is far greater than that on theincreases, SiO2 /Si the substrate. In order evaluate the mobility the Al 2O3/Si substrate is far greater than that on the SiO2/Si substrate. In order to evaluate the mobility of the devices, a device model was used [27]. The extracted carrier mobility of graphene FETs on of the devices, a device model was The extracted carrier mobility2 of−graphene FETs on the 2 Vused −1 ·s−[27]. 1 , which the Al2 O3 /Si substrate is 6500 cm is similar to 6780 cm V 1 ·s−1 of the FETs on the Al2O3/Si substrate is 6500 cm2 V−1·s−1, which is similar to 6780 cm2 V−1·s−1 of the FETs on the SiO2/Si SiO2 /Si substrate. The replacement of the substrate does not lead to the degradation of the transport substrate. The replacement of the substrate does not lead to the degradation of the transport performance performance of the Furthermore, devices. Furthermore, transfer characteristic few-layer graphene FETs on an of the devices. transfer characteristic of few-layer of graphene FETs on an Al 2O3/Si Al2 O3substrate /Si substrate was depicted in Figure S4, showing that Al O gating substrate is also suitable 2 3 was depicted in Figure S4, showing that Al2O3 gating substrate is also suitable for few-layer for few-layer graphene. graphene.
Figure gm–Vg curves of graphene FETs on different substrates. (b) EF–Vg curves of graphene FETs Figure 5. (a)5.g(a) m –V g curves of graphene FETs on different substrates. (b) EF –V g curves of graphene on different substrates. The inset shows the variation tendency of the induced charge against gate FETs on different substrates. The inset shows the variation tendency of the induced charge against voltage. gate voltage.
We also systematically studied the electrical properties of few-layer WS2 on the Al2O3/Si
We also systematically studied the of electrical properties of few-layer WSand the Al2 O3 /Si substrate substrate (Figure 6a). The number layers was determined by Raman 2 on Photoluminescence (Figure 6a). (see The number layers was by Raman andstates Photoluminescence spectra spectra Figure S5). Aofsingle layer wasdetermined not used because the surface of single-layer TMDs are easily affected by the external environment in the process of device fabrication, thus losing the (see Figure S5). A single layer was not used because the surface states of single-layer TMDs are [30]. As Figure 6b shows, nonlinearly against the losing changethe in bias easilyintrinsic affectedproperty by the external environment in the the current processvaries of device fabrication, thus intrinsic voltage from to 1 V. is due to current the formation the Schottkyagainst barrier between WS2in and metal property [30]. As −1 Figure 6bThis shows, the variesofnonlinearly the change bias voltage contact, which was widely reported in previous studies [30–32]. However, from the inset of Figure from −1 to 1 V. This is due to the formation of the Schottky barrier between WS2 and metal contact,
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which was widely reported in previous studies [30–32]. However, from the inset of Figure 6b, it can be seen is maintained fairly wellfairly under condition of small bias bias voltage. Hence, 6b,that it canthe be linearity seen that the linearity is maintained wellthe under the condition of small voltage. the bias voltage is maintained at 0.1 V inatthe test. test. Hence, the bias voltage is maintained 0.1 following V in the following
(a) Three-dimensional schematicview viewof ofthe the few-layer few-layer WS (b)(b) Output characteristics FigureFigure 6. (a)6.Three-dimensional schematic WS2 2FETs. FETs. Output characteristics of few-layer WS2 on Al2O3/Si substrates in large bias and (inset) small bias voltage. (c) Transfer of few-layer WS2 on Al2 O3 /Si substrates in large bias and (inset) small bias voltage. (c) Transfer characteristics of few-layer WS2 FETs on different substrates. (d) gm–Vg curves of few-layer WS2 FETs characteristics of few-layer WS2 FETs on different substrates. (d) gm –V g curves of few-layer WS2 FETs on different substrates. on different substrates.
Figure 6c illustrates the transfer characteristics of few-layer WS2 on different substrates, and both of them6cdistinctly exhibit n-type behavior conduction [31]. WhenWS the2 on gatedifferent voltage changes fromand −10both Figure illustrates the transfer characteristics of few-layer substrates, to 10 V, devices on Al 2 O 3 /Si substrates turn from the off state (2.8 pA) to the on state (2.5 μA). of them distinctly exhibit n-type behavior conduction [31]. When the gate voltage changes from −10 to 5 V−1, which is far greater than that on the SiO2/Si Therefore, on/off ratio is turn as highly 10off 10 V, devices onthe Al2unit O3 /Si substrates fromasthe state (2.8 pA) to the on state (2.5 µA). Therefore, 3 −1 substrate (1.5 × 10 V ). As depicted in Figure 6d, the maximum transconductance (red rectangle) of the unit on/off ratio is as highly as 105 V−1 , which is far greater than that on the SiO2 /Si substrate few-layer WS2 can reach about 0.92 μS (Vg = 2.3 V), and the corresponding carrier mobility is (1.5 × 103 V−1 ). As depicted in2 Figure 6d, the maximum transconductance (red rectangle) of few-layer calculated to be 239 cm ·V−1·s−1. However, as the gate voltage continues to increase, the WS2 can reach about 0.92 µS (V = 2.3 V), and the corresponding carrier mobility calculated g transconductance starts to decline, indicating that the carrier mobility has reached isthe maximumto be 2 ·V−1 ·s−1 . However, as the gate voltage continues to increase, the transconductance starts to 239 cm value. Compared with the maximum transconductance of few-layer WS2 on the Si/SiO2 substrate (1.5 −2 μS), the gate decline, thatcontrol the carrier has reached the maximum Compared with × 10indicating abilitymobility was significantly improved (61.3 times).value. Furthermore, the Al 2O3 the maximum transconductance of few-layer WS on the Si/SiO substrate substrate is a better alternative for other 2D2 materials (such2as WS2). (1.5 × 10−2 µS), the gate control ability was significantly improved (61.3 times). Furthermore, the Al2 O3 substrate is a better alternative 4. Conclusions for other 2D materials (such as WS2 ). In summary, Al2O3/Si substrates are superior for the visualization of graphene and fabrication
4. Conclusions of graphene transistors. Compared with SiO2/Si substrates, Al2O3/Si substrates can enhance the optical contrast of graphene by up to 2.4 times. Furthermore, using the Al2O3 film as the gate In summary, Al2 O3 /Si substrates are superior for the visualization of graphene and fabrication dielectric, the transconductance of graphene FETs exhibited an approximately 10-fold increase. of graphene transistors. Compared with SiO2 /Si substrates, Al2 O3 /Si substrates can enhance the Significantly, this substrate is also more suitable for other 2D materials, such as WS2, and can opticalremarkably contrast ofenhance graphene by up to 2.4 times. Furthermore, using the Al2 O3 film as the gate dielectric, the transconductance by 61.3 times. the transconductance of graphene FETs exhibited an approximately 10-fold increase. Significantly, Supplementary The following are available online atsuch http://www.mdpi.com/2079-4991/7/10/286/s1, this substrate is alsoMaterials: more suitable for other 2D materials, as WS2 , and can remarkably enhance Figure S1: Steps ofby deterministic the transconductance 61.3 times.transfer method. Figure S2: Reflection spectra of graphene on different
substrates. Figure S3: Raman scanning image of graphene on different substrates. Figure S4: Transfer characteristic and Raman spectrumareofavailable few-layer graphene on an Al2O3/Si substrate. Figure S5: Supplementary Materials: The following online at http://www.mdpi.com/2079-4991/7/10/286/s1, Photoluminescence and Raman spectra of few-layer WS 2 on Al 2 O 3 /Si substrate. Table S1: Average roughness Figure S1: Steps of deterministic transfer method. Figure S2: Reflection spectra of graphene on different substrates. Al2O3). meanscanning square (Sq) and coefficient of kurtosis (Ska) ofsubstrates. three different substrates (Si, SiO2, characteristic Figure(Sa), S3: root Raman image of graphene on different Figure S4: Transfer and
Raman spectrum of few-layer graphene on an Al2 O3 /Si substrate. Figure S5: Photoluminescence and Raman spectra of few-layer WS2 on Al2 O3 /Si substrate. Table S1: Average roughness (Sa), root mean square (Sq) and coefficient of kurtosis (Ska) of three different substrates (Si, SiO2 , Al2 O3 ).
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Acknowledgments: The authors acknowledge financial support from the National Natural Science Foundation of China (Nos. 11574395, 61675234), the Open Foundation of State Key Laboratory of High Performance Computing (No. 201301-02), the Advanced Research Foundation of the National University of Defense Technology (No. zk16-03-40), and the research project of National University of Defense Technology (No. JC15-02-01). Author Contributions: Hang Yang and Gang Peng conceived and designed the experiments; Hang Yang, Xiaoming Zheng and Yuan Tan performed the experiments; Shiqiao Qin, Guang Wang and Xueao Zhang provided valuable suggestions; Hang Yang wrote the paper. All authors read and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2. 3.
4. 5. 6. 7.
8. 9.
10.
11. 12.
13.
14. 15.
16. 17. 18.
Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-layer MoS2 transistors. Nat. Nanotechnol. 2011, 6, 147–150. [CrossRef] [PubMed] Liao, L.; Lin, Y.C.; Bao, M.; Cheng, R.; Bai, J.; Liu, Y.; Qu, Y.; Wang, K.L.; Huang, Y.; Duan, X. High speed graphene transistors with a self-aligned nanowire gate. Nature 2010, 467, 305–308. [CrossRef] [PubMed] Seyler, K.L.; Schaibley, J.R.; Gong, P.; Rivera, P.; Jones, A.M.; Wu, S.; Yan, J.; Mandrus, D.G.; Yao, W.; Xu, X. Electrical control of second-harmonic generation in a WSe2 monolayer transistor. Nat. Nanotechnol. 2015, 10, 407–411. [CrossRef] [PubMed] Li, Y.; Zhang, H.; Yan, D.W.; Yin, H.-F.; Cheng, X.L. Secondary plasmon resonance in graphene nanostructures. Front. Phys. 2015, 10, 102–108. [CrossRef] Mueller, T.; Xia, F.; Avouris, P. Graphene photodetectors for high-speed optical communications. Nat. Photonics 2010, 4, 297–301. [CrossRef] Hang, Y.; Li, Q.; Luo, W.; He, Y.; Zhang, X.; Peng, G. Photo-Electrical Properties of Trilayer MoSe2 Nanoflakes. Nano 2016, 11, 1650082. [CrossRef] Kim, K.S.; Zhao, Y.; Jang, H.; Lee, S.Y.; Kim, J.M.; Kim, K.S.; Ahn, J.H.; Kim, P.; Choi, J.Y.; Hong, B.H. Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature 2009, 457, 706. [CrossRef] [PubMed] Kim, Y.; Kwon, Y.J.; Kang, E.L.; Oh, Y.; Um, M.-K.; Seong, D.G.; Lee, J.U. Flexible Textile-Based Organic Transistors Using Graphene/Ag Nanoparticle Electrode. Nanomaterials 2016, 6, 147. [CrossRef] [PubMed] Jung, I.; Pelton, M.; Piner, R.; Dikin, D.A.; Stankovich, S.; Watcharotone, S.; Hausner, M.; Ruoff, R.S. Simple Approach for High-Contrast Optical Imaging and Characterization of Graphene-Based Sheets. Nano Lett. 2007, 7, 3569–3575. [CrossRef] Skulason, H.S.; Gaskell, P.E.; Szkopek, T. Optical reflection and transmission properties of exfoliated graphite from a graphene monolayer to several hundred graphene layers. Nanotechnology 2010, 21, 295709. [CrossRef] [PubMed] Li, H.; Lu, G.; Yin, Z.; He, Q.; Li, H.; Zhang, Q.; Zhang, H. Optical Identification of Single-and Few-Layer MoS2 Sheets. Small 2012, 8, 682–686. [CrossRef] [PubMed] Chen, W.; Qin, S.; Zhang, X.A.; Zhang, S.; Fang, J.; Wang, G.; Wang, C.; Wang, L.; Chang, S. Current induced doping in graphene-based transistor with asymmetrical contact barriers. Appl. Phys. Lett. 2014, 104, 183. [CrossRef] Chen, W.; Qin, S.; Zhang, X.A.; Zhang, S.; Fang, J.; Wang, G.; Wang, C.; Wang, L.; Chang, S. Current self-amplification effect of graphene-based transistor in high-field transport. Carbon 2014, 77, 1090–1094. [CrossRef] Yin, Z.; Li, H.; Li, H.; Jiang, L.; Shi, Y.; Sun, Y.; Lu, G.; Zhang, Q.; Chen, X.; Zhang, H. Single-Layer MoS2 Phototransistors. ACS Nano 2012, 6, 74–80. [CrossRef] [PubMed] Lyu, H.; Lu, Q.; Huang, Y.; Ma, T.; Zhang, J.; Wu, X.; Yu, Z.; Ren, W.; Cheng, H.; Wu, H. Graphene Distributed Amplifiers: Generating Desirable Gain for Graphene Field-Effect Transistors. Sci. Rep. 2015, 5, 17649. [CrossRef] [PubMed] Guerriero, E.; Polloni, L.; Rizzi, L.G.; Massimiliano, B. Graphene audio voltage amplifier. Small 2012, 8, 357–361. [CrossRef] Lee, H.S.; Min, S.W.; Park, M.K.; Lee, Y.T.; Jeon, P.J.; Kim, J.H.; Ryu, S.; Im, S. MoS2 nanosheets for top-gate nonvolatile memory transistor channel. Small 2012, 8, 3111–3115. [CrossRef] [PubMed] Liao, L.; Bai, J.; Cheng, R.; Lin, Y.; Jiang, S.; Huang, Y.; Duan, X. Top-Gated Graphene Nanoribbon Transistors with Ultra-Thin High-k Dielectrics. Nano Lett. 2010, 10, 1917–1921. [CrossRef] [PubMed]
Nanomaterials 2017, 7, 286
19.
20. 21. 22.
23.
24. 25. 26. 27.
28.
29. 30. 31. 32.
8 of 8
Alaboson, J.M.P.; Wang, Q.H.; Emery, J.D.; Lipson, A.L.; Bedzyk, M.J.; Elam, J.W.; Pellin, M.J.; Hersam, M.C. Seeding atomic layer deposition of high-k dielectrics on epitaxial graphene with organic self-assembled monolayers. ACS Nano 2011, 5, 5223–5232. [CrossRef] [PubMed] Xuan, Y.; Wu, Y.Q.; Shen, T.; Qi, M.; Capano, M.A.; Cooper, J.A.; Ye, P.D. Atomic-layer-deposited nanostructures for graphene-based nanoelectronics. Appl. Phys. Lett. 2008, 92, 013101. [CrossRef] Liao, L.; Bai, J.; Qu, Y.; Huang, Y.; Duan, X. Single-layer graphene on Al2 O3 /Si substrate: Better contrast and higher performance of graphene transistors. Nanotechnology 2010, 21, 015705. [CrossRef] [PubMed] Dean, C.R.; Young, A.F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K.L.; et al. Boron nitride substrates for high-quality graphene electronics. Nat. Nanotechnol. 2010, 5, 722–726. [CrossRef] [PubMed] Liao, L.; Bai, J.; Qu, Y.; Lin, Y.C.; Li, Y.; Huang, Y.; Duan, X. High-κ oxide nanoribbons as gate dielectrics for high mobility top-gated graphene transistors. Proc. Natl. Acad. Sci. USA 2010, 107, 6711–6715. [CrossRef] [PubMed] Fallahazad, B.; Lee, K.; Lian, G.; Kim, S.; Corbet, C.M.; Ferrer, D.A.; Colombo, L.; Tutuc, E. Scaling of Al2 O3 dielectric for graphene field-effect transistors. Appl. Phys. Lett. 2012, 100, 093112. [CrossRef] Ni, Z.H.; Wang, H.M.; Kasim, J.; Fan, H.M.; Yu, T.; Wu, Y.H.; Feng, Y.P.; Shen, Z.X. Graphene thickness determination using reflection and contrast spectroscopy. Nano Lett. 2007, 7, 2758–2763. [CrossRef] [PubMed] Zheng, X.; Chen, W.; Wang, G.; Yu, Y.; Qin, S.; Fang, J.; Wang, F.; Zhang, X. The Raman redshift of graphene impacted by gold nanoparticles. AIP Adv. 2015, 5, 1530–1534. [CrossRef] Giubileo, F.; di Bartolomeo, A.; Martucciello, N.; Romeo, F.; Iemmo, L.; Romano, P.; Passacantando, M. Contact Resistance and Channel Conductance of Graphene Field-Effect Transistors under Low-Energy Electron Irradiation. Nanomaterials 2016, 6, 206. [CrossRef] [PubMed] Groner, M.D.; Elam, J.W.; Fabreguette, F.H.; George, S.M. Electrical characterization of thin Al2 O3 films grown by atomic layer deposition on silicon and various metal substrates. Thin Solid Films 2002, 413, 186–197. [CrossRef] Rao, C.N.; Sood, A.K.; Subrahmanyam, K.S.; Govindaraj, A. Graphene: The new two-dimensional nanomaterial. Angew. Chem. Int. Ed. 2009, 48, 7752–7777. [CrossRef] [PubMed] Huo, N.; Yang, S.; Wei, Z.; Li, S.; Xia, J.; Li, J. Photoresponsive and Gas Sensing Field-Effect Transistors based on Multilayer WS2 Nanoflakes. Sci. Rep. 2014, 4, 5209. [CrossRef] [PubMed] Cui, Y.; Xin, R.; Yu, Z.; Pan, Y.; Ong, Z.; Wei, X.; Wang, J.; Nan, H.; Ni, Z.; Wu, Y. High-Performance Monolayer WS2 Field-Effect Transistors on High-κ Dielectrics. Adv. Mater. 2015, 27, 5230–5234. [CrossRef] [PubMed] Ying, C.; Sun, H.; Peng, W. 2D Transition Metal Dichalcogenides and Graphene-Based Ternary Composites for Photocatalytic Hydrogen Evolution and Pollutants Degradation. Nanomaterials 2017, 7, 62. © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
