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Controlling the electronic properties of SWCNT FETs via modification of the substrate surface prior to atomic layer deposition of 10 nm thick Al2O3 film
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 455701 (http://iopscience.iop.org/0957-4484/24/45/455701) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 455701 (10pp)
doi:10.1088/0957-4484/24/45/455701
Controlling the electronic properties of SWCNT FETs via modification of the substrate surface prior to atomic layer deposition of 10 nm thick Al2O3 film Joonsung Kim1,4 , Jangyeol Yoon1,4 , Junhong Na2 , Seongmin Yee2 , Gyu Tae Kim2 and Jeong Sook Ha1,3 1
Department of Chemical and Biological Engineering, Korea University, Seoul 136-701, Korea School of Electrical Engineering, Korea University, Seoul 136-701, Korea 3 KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Korea 2
E-mail: [email protected]
Received 18 March 2013, in final form 4 June 2013 Published 18 October 2013 Online at stacks.iop.org/Nano/24/455701 Abstract We demonstrate the controllability of the electronic transport properties of single-walled carbon nanotube (SWCNT) field effect transistors (FETs) via the use of 10 nm thick atomic-layer-deposited aluminum oxide (Al2 O3 ) gate dielectric films, where the substrate surfaces were modified with differently functionalized self-assembled monolayers (SAMs) prior to their growth, namely SAMs with hydrophobic (−CH3 ) or hydrophilic (−OH) groups. Al2 O3 grown on a hydrophilic surface causes the SWCNT FETs to keep their intrinsic p-type transfer characteristics by alleviating the electron-doping effect originating from defects in the Al2 O3 film. However, the SAM with methyl groups increases the defect density of the Al2 O3 film, enhancing the n-type transfer characteristics and inducing ambipolar to n-type behavior in the SWCNT FETs. In this work, we find clues about the distribution of charged defects in the Al2 O3 film, which strongly influences the transfer characteristics of the SWCNT FETs, by measuring the thickness-dependent flat band voltages. S Online supplementary data available from stacks.iop.org/Nano/24/455701/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
threshold swings. In order to apply thin-film dielectric layers to various devices including flexible/stretchable ones, the growth of the dielectric layers should be carried out at temperatures low enough for the polymer substrate. Among the various growth techniques, atomic layer deposition (ALD) is known to be adequate for the growth of thin high-k dielectric layers at low temperatures with a thickness controllable at the nanometer scale [2, 3]. Aluminum oxide (Al2 O3 ) is known to have superior insulating properties with a much lower leakage current density and a higher dielectric constant than the conventionally used
As basic electronic devices, field effect transistors (FETs) show a very strong dependence of their electronic properties on the gate dielectric layer [1]. As a result, there has been extensive research on the preparation of very thin high-quality dielectric layers with very robust insulating properties. In particular, high-k dielectrics have shown superior device performance with low operation voltages and small sub4 These authors contributed equally to this work.
0957-4484/13/455701+10$33.00
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SiO2 film. Therefore, Al2 O3 films have frequently been used as dielectric layers in various nanomaterial-based FETs as well as in conventional thin-film FETs [4]. Single-walled carbon nanotubes (SWCNTs) have been actively investigated because of their superior electronic properties, mechanical strength, and flexibility. The pristine SWCNTs grown by chemical vapor deposition (CVD) are a mixture of metallic and semiconducting CNTs. Semiconducting SWCNT FETs with conventional SiO2 gate dielectrics show p-type transfer characteristics in ambient air, since the adsorbed O2 retracts electrons from the SWCNTs, producing excess holes as major charge carriers [5]. However, the application of an ALD-grown Al2 O3 dielectric layer to SWCNT FETs produces ambipolar transfer properties, which should be overcome [6]. This result has been attributed to the electrostatic doping (ESD) effect of the defect sites in the Al2 O3 film [7–9]. In particular, Al2 O3 films grown by ALD at low temperatures intensify such doping effects because of the higher defect density in the film. Therefore, there should be effort to understand and mitigate such undesired ESD effects of Al2 O3 films on the electronic transfer in SWCNT FETs. For the fabrication of devices using CVD-grown SWCNTs, thermal release tape is mostly used to transfer the SWCNTs from the SiO2 or quartz substrates to the device substrates, such as SiO2 . In this process, Au evaporation onto a CVD-grown SWCNT film makes an SWCNT-embedded Au film that is subsequently transferred onto the device substrate by thermal release tape. Then, Au is etched away to leave SWCNTs [10, 11]. However, transfer of SWCNTs onto Al2 O3 using this method is not easy, since Al2 O3 film has a low surface energy insufficient for adhesion of the sacrificial gold layer [12]. Therefore, solution casting of SWCNTs onto Al2 O3 has most frequently been used. Since the direct transfer of CVD-grown SWCNTs simplifies device fabrication, it is necessary to develop a method for the direct transfer of SWCNTs onto the Al2 O3 dielectric layer. In this work, we systematically investigated the effect of Al2 O3 dielectric film grown on differently functionalized growth substrates on the electronic transport properties of SWCNT FETs. Depending upon the hydrophilicity of the modified surface, the quality of the grown Al2 O3 film was changed, resulting in different electrical properties. Measurements of the film-thickness-dependent flat band voltages and simulation studies on the electric field distribution could explain the experimentally observed influence of the Al2 O3 dielectric layer. We also devised a transfer method for CVD-grown SWCNTs onto ALD-grown Al2 O3 substrate, the ‘enamel transfer method’.
chemical stoichiometry of the film was ascertained using the Auger profile data. The detailed growth process is similar to the one previously reported [1, 13]. 2.2. Surface treatment with SAMs For the surface modification prior to the ALD growth of the Al2 O3 film, SAMs with different functional groups were formed by dipping the substrate in the SAM solution for 12 h. On the Au surface, 2 mM of 11-mercapto-1-undecanol (MUO) and 1-decanethiol (DT) in ethanol were used. The SiO2 surface was treated with 1 mM of OTS SAM in ethanol or with UV-ozone (UVO) for 30 min. After treatment with the SAM solution or UVO, the samples were rinsed with toluene and deionized water. Measurement of the contact angle was performed after the SAM treatment. 2.3. Growth of SWCNTs Randomly networked SWCNTs were grown on the SiO2 surface using diluted ferritin as a catalyst via CVD at 925 ◦ C. The density of SWCNTs could be controlled by varying the concentration of the ferritin catalyst. In particular, the metallic percolation path could be suppressed by reducing the SWCNT density [14]. Methane (26 sccm, 99.995%) and H2 (60 sccm, 99.99%) gases were passed through a methanol/ethanol (1:1 volume ratio) bubbler and supplied as carbon sources. The average diameter of the grown SWCNTs was estimated to be 1.4 nm by analyzing the Raman radial breathing mode [15]. About 60% of the grown SWCNTs showed p-type semiconducting transfer properties in SWCNT FETs using SiO2 dielectric layers [14]. 2.4. Transfer of SWCNTs Randomly networked SWCNT films were transferred onto the Al2 O3 layer using the enamel transfer method, which consisted of four steps: (1) deposition of a 100 nm-thick Au layer onto as-grown SWCNT films by e-beam evaporation, (2) spin coating of an enamel layer consisting of nitrocellulose and RSF resin onto an SWCNT-embedded Au film and baking of the film at 65 ◦ C in a dry oven for 15 min, (3) peeling of the enamel/Au/SWCNT film from the donor substrate and transfer onto an Al2 O3 surface wetted with ethanol, and (4) etching of the enamel by acetone and subsequent etching of the Au by Au etchant, leaving SWCNT films on the Al2 O3 layer. The characteristics of the transferred SWCNTs were confirmed by Raman spectroscopy and SEM analysis. 2.5. Fabrication of SWCNT FETs
2. Experimental details
Both top- and local bottom-gate SWCNT FETs were fabricated (supporting figure S1 available at stacks.iop.org/ Nano/24/455701/mmedia). For the fabrication of a local bottom-gate device, a Au/Ti gate electrode pattern with a width of 50 µm was formed on a SiO2 surface by photolithography. In order to increase the yield of the lift-off process, LOR-2A (Microchem) was spin-coated prior to coating of the photoresist, which prevented deterioration of
2.1. Preparation of Al2 O3 thin film Al2 O3 films were grown using the ALD technique. The growth temperature was 100 ◦ C, and trimethylaluminum (TMA) and water vapor were used as precursor molecules for aluminum and oxygen. According to the SEM data, the growth rate was estimated to be 0.1 nm/cycle, and the 2
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Figure 1. (a) Auger depth profile taken from Al2 O3 film grown on a Au substrate. The background is a cross-sectional SEM image of the Al2 O3 . (b) Leakage current density versus film thickness of Al2 O3 (red circles) and Al2 O3 grown on the MUO-treated Au (black squares), which was measured at V = 2 V. (c) Leakage current density versus bias voltage taken from 10 nm-thick Al2 O3 films grown on bare, DT-treated, and MUO-treated Au surfaces. (d) Thickness-dependent breakdown voltage taken from Al2 O3 films grown on bare, DT-treated, and MUO-treated Au surfaces.
the Au/Ti electrode on the Al2 O3 film during the process by forming undercuts. Then, a 10 nm-thick Al2 O3 film was grown on the SAM-treated Au/Ti electrode. After transfer of the SWCNT film onto the Al2 O3 layer via the enamel transfer method, Au/Ti source and drain electrodes were patterned by photolithography and lift-off techniques. Finally, SWCNT channels were isolated via O2 RIE by using patterned photoresist (AZ5214E) as a mask layer. For the top-gate FET, Au/Ti source and drain electrodes were deposited on the SWCNT/SiO2 surface via e-beam evaporation, and the channels were isolated by O2 RIE. A 10 nm-thick Al2 O3 dielectric layer was grown on top of the SAM-treated SWCNT/SiO2 surface via ALD. Finally, the Au/Ti gate electrode was formed. Electrical measurements were performed using a B1500A semiconductor device analyzer.
(SEM) image. It is clearly shown that a 10 nm-thick stoichiometric Al2 O3 film with an Al/O ratio of 2/3 was grown. Figure 1(b) shows that the leakage current density of the grown Al2 O3 film decreased with increasing thickness, as expected. Furthermore, the addition of an MUO SAM prior to the growth of the Al2 O3 film improved the insulating properties, and the effect of the SAM was more noticeable with a thinner Al2 O3 film. This can be explained in terms of the effect of the carbon chains of the SAM as a barrier for the electron transfer between electrodes [16]. The changes in the leakage current density of the film with varying bias voltage for three differently prepared 10 nm-thick Al2 O3 films are shown in figure 1(c). In the absence of any SAM, the leakage current density continuously increased with increasing bias. With a DT SAM, the leakage current density remained at 10−8 A cm−2 up to 7 V and increased with increasing bias above 7 V. However, the leakage current density was stable up to 10 V when an MUO SAM was used, demonstrating the improvement in the breakdown voltage (Vb ) of the Al2 O3 film. This effect is tentatively attributed to electron capture by the hydroxyl groups or metal alkoxides located in the interface between the Al2 O3 and the MUO. Figure 1(d) shows the change in the Vb value of the Al2 O3 film with varying thickness. As expected, Vb increased with increasing thickness. The DT SAM did not influence
3. Results and discussion 3.1. Growth and characterization of Al2 O3 film on substrates modified with SAM Figure 1(a) shows the Auger depth profile of an ALD-grown 10 nm-thick Al2 O3 film on a Au substrate at 100 ◦ C and the corresponding cross-sectional scanning electron microscopy 3
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Vb , but a dramatic increase in Vb was observed with the MUO SAM: a merely 10 nm-thick Al2 O3 /MUO hybrid film had a Vb of over 20 V. For the 20 and 40 nm-thick Al2 O3 /MUO hybrid films, breakdown phenomena were not observed even under 40 V. When electrons are injected from an electrode into the unmodified Al2 O3 film, collisions can occur among electrons and atoms inside the film producing an impact ionization [17–19]. The impact ionization detaches the chemically bonded oxygen atoms from the lattices so that electrons can penetrate the dielectric layer easily, while the mobility of oxygen vacancies is not sufficiently high for them to move easily. Therefore, oxygen vacancies start to accumulate when a high voltage is applied between the electrodes, and the accumulated oxygen vacancies enhance the ion conductivity so that more electrons can be penetrated easily, leading to electrical breakdown of the dielectric film. However, hydroxyl (−OH) groups in the MUO SAM are reported to capture the electrons [20], and these make the film more hydrophilic. Thus, these hydroxyl groups form an oxygen-rich layer that can subsequently neutralize the effects of the oxygen detachment process. The existence of hydroxyl groups in the interface between the Al2 O3 and MUO SAM was confirmed by the hysteresis curve, which depended on the bias sweep direction (supporting figure S2 available at stacks.iop.org/Nano/24/ 455701/mmedia). The Al2 O3 and Al2 O3 /DT (supporting figure S2(a) available at stacks.iop.org/Nano/24/455701/ mmedia) films grown on Au surfaces showed symmetric hysteresis regardless of the bias sweep direction. However, the Al2 O3 film grown on an MUO SAM showed distinctively different hysteresis curves depending on the sweep direction (supporting figure S2(b) available at stacks.iop.org/Nano/24/ 455701/mmedia). To confirm the effects of the methyl and hydroxyl functional groups on the hysteresis phenomena, the leakage currents through Al2 O3 /DT and Al2 O3 /MUO films were compared (supporting figure S3 available at stacks.iop. org/Nano/24/455701/mmedia). A noticeable difference in the leakage current for different voltage sweeping directions was only observed for the Al2 O3 /MUO film, which confirms the existence of localized electron-trapping sites due to hydroxyl groups [21]. The initial state of the film growth is considered to play a critical role in the quality of the grown bulk film, as reported in a previous work by Kobayashi et al [22]. Accordingly, we expect that the chemical state of the substrate surface prior to ALD growth will strongly influence the growth of bulk Al2 O3 film. Next, in order to understand the effect of the hydrophilicity of the growth surface on the charged defect density of the ALD-grown Al2 O3 films, we measured the capacitance–voltage (C–V) curves of the grown Al2 O3 films (supporting figure S4 available at stacks.iop.org/Nano/ 24/455701/mmedia) and estimated the flat band voltage (VFB ), which is closely related to the charged defect density. To make the p-type SiO2 surface more hydrophilic, it was treated with UV-ozone (UVO) for 30 min, while it was treated with an OTS SAM to make it hydrophobic. After the modification, Al2 O3 films were grown at 100 ◦ C via ALD to investigate the effect of the surface properties
on the generation of charged defects inside the film. Then, the flat band voltage (VFB ) was measured for different film thicknesses [23–25]. Here, UVO and OTS mean the ALDgrown Al2 O3 films on UVO-treated and OTS-SAM-treated SiO2 substrates, respectively. The flat band voltage VFB can be expressed in terms of the fixed oxide charges (Qf ), distributed oxide charges (ρox ), and the work-function difference between the metal gate and the semiconductor (8MS ) for the Al2 O3 film, as shown in equation (1) [26]. The capacitance of the oxide film (Cox ) can be expressed as in equation (2). Z tox 1 x Qf − − ρox (x) dx (1) VFB = 8MS − Cox Cox t ox 0 A Cox = εr ε0 . (2) tox Here, tox is the thickness of the oxide film, A is the area of the electrodes, εr is the relative dielectric constant, and ε0 is the vacuum permittivity. Since the oxide charge distribution plays a strong influence on the electronic properties of MOSFETs, it is necessary to know the real profile of oxide charges for accurate characterization [27]. However, we assume here that ρox has a constant value (ρ) inside the same film regardless of the position for a simple estimation. Then equation (1) can be transformed into equation (3). VFB = 8MS −
1 Qf tox − ρt2 . Aεr ε0 2Aεr ε0 ox
(3)
To obtain the VFB values, circular Pt electrodes (diameter = 100 µm, thickness = 100 nm) were deposited on the Al2 O3 by using a metal shadow mask and the e-beam deposition method. The VFB values were estimated using a Mott–Schottky plot following the previous work by Mehta et al [25]. The Mott–Schottky plot which comes from equation (4) expresses the linear relation between C−2 and V [28]. 2 kT C−2 = 2 V − VFB − . (4) e A Neεr ε0 Here N, V, e, T, and k are the donor density, applied bias voltage, unit charge of an electron, temperature in Kelvin, and Boltzmann constant, respectively. When C−2 converges to zero, VFB can be estimated from the measurement of C−2 as a function of V . Therefore, the linear region of normalized C−2 values was fitted linearly to estimate VFB in figure 2(a). Following the work by Mehta et al, we only used the linear region of the plot and ignored the kT/e term because it is much smaller than the variation of the data. Next, the thickness-dependent VFB data from figure 2(a) were fitted with equation (3) (figure 2(b)). From the curve fitting, Qf and ρ values were estimated using the method of undetermined coefficients. The Qf values of the UVO and OTS samples were estimated to be −5.2 × 10−11 and −3.3 × 10−11 C, respectively, and the ρ values were +7.0 × 10−13 and +2.6 × 10−12 C nm−1 , respectively. It was found that Qf and ρ showed opposite charges. According to the previous work by Shin et al [9], this result supports our 4
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Figure 2. (a) Normalized C−2 versus bias for different thicknesses of Al2 O3 films grown on UVO (top) and OTS (bottom)treated SiO2 surfaces. The black, red, and blue plots are for 10, 20, and 30 nm-thick Al2 O3 films, respectively. Ten samples of each thickness were used for the measurement. (b) Thickness-dependent flat band voltage (VFB ) of Al2 O3 films grown on OTS (triangles) and UVO (circles) treated SiO2 . The curve fitting was carried out according to equation (3) in the text. (c) O 1s and Al 2p peaks of the XPS spectrum taken from 2 nm-thick Al2 O3 film grown on bare (blue), DT (red) and MUO (black) treated Au surfaces. (d) O(1s)/Al(2p) peak ratio normalized to that of the Al2 O3 film grown on bare Au. The insets show the contact angles of water measured on the surfaces before the ALD growth of the Al2 O3 film.
Al2 O3 film, the contact angle of water was measured to be 77◦ , 107◦ , and 32◦ for bare, DT-treated, and MUO-treated Au surfaces, respectively. These results imply that water molecules adsorbed on the hydrophilic surface supply more nucleation sites for the initial growth of the Al2 O3 film, resulting in a lower defect density of the film in the later stage of growth as compared to the film grown on a hydrophobic surface. This is consistent with the results from the flat band voltage measurements: the Al2 O3 grown on the hydrophilic surface had lower ρ but higher Qf values. Here, the absolute value of the charge density might not be accurate, since the calculation was made under the Mott–Schottky plot estimation and the assumption of a constant ρ value throughout the whole film. However, the result that the hydrophilicity of the substrate surface affects the quality of the whole film is consistent with previous work [23]. At the same time, we expect that the existence of the negative charges near the interface between the Al2 O3 film and the substrate surface should be considered in studying the interaction between the channel materials and the dielectric film in FETs. Moriyama et al reported [8] that the positive charges near the CNT channel help to change the band junction between the electrode and the channel to promote electron transport, while the negative charges help the hole transport. If this is
hypothesis: the oxygen-rich region observed in the initial stage of the film growth generates a negatively charged layer, but positive charges are generated in the bulk film because of the oxygen vacancies. These positive charges in the bulk film are well known, and they are usually used to explain the type conversion of CNT FETs [8]. However, the existence of negative charges near the interface between the Al2 O3 and the substrate has not yet been studied well. X-ray photoelectron spectra of 2 nm-thick Al2 O3 films grown on differently modified Au surfaces were obtained in order to investigate the chemical stoichiometry in the initial stage of the growth (figure 2(c)). The relative ratio between the O 1s peak and the Al 2p peak indicates an O/Al ratio higher than 1.5, which shows that the Al2 O3 film is more oxygen rich than the bulk Al2 O3 . The stoichiometries of the films grown on the DT-SAM-treated, bare, and MUO-SAM-treated Au substrates were AlO2.25 , AlO2.28 , and AlO2.31 , respectively. The much higher oxygen content of the film in the initial stage of growth is consistent with the previously reported results [9, 29]. Figure 2(d) shows that the oxygen ratio of the film increases with increasing hydrophilicity of the substrate surface: compared to the O/Al ratio for the film on bare Au, the O/Al ratio was ∼2% higher for the Al2 O3 film on the MUO-SAM-treated Au but was ∼2% lower for the Al2 O3 film on the DT-SAM-treated Au. Prior to the growth of the 5
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Figure 3. (a) SEM images of CNTs transferred onto the Al2 O3 surface once (left) and twice (right) via the enamel transfer method. (b) Distribution of current through the SWCNT channels after the one (black) and two (blue) transfers measured at a 1 V bias. The red bars are for sonicated SWCNTs after the first transfer. Here, Cr is used for the electrodes (thickness = 50 nm, channel length = 40 µm, channel width = 40 µm). The inset shows the number density of SWCNTs with the repetition of enamel transfer.
true, the high Qf and low ρ values of the Al2 O3 grown on the hydrophilic surface are expected to be advantageous for hole transport, while the low Qf and high ρ values of the Al2 O3 grown on the hydrophobic surface are expected to be favorable for electron transport. Therefore, we investigated the effect of the Al2 O3 dielectric film on the transfer characteristics of SWCNT FETs.
via the above-mentioned enamel transfer method. In the top-gate SWCNT FETs, SWCNTs were transferred onto the SiO2 surface using the thermal release tape method. Then, SAMs with different functional groups were formed on the SWCNT-covered SiO2 substrates. Finally, a 10 nm-thick Al2 O3 layer was grown at 100 ◦ C by ALD, and the Au/Ti top-gate electrode was deposited. In both devices, 10 µm wide SWCNT channels were formed through partial O2 reactive ion etching (RIE) to reduce the number and length of percolation paths of the metallic SWCNTs. Au/Ti (50/10 nm) electrodes were defined by photolithography. Here, a resist for lift-off (LOR, Micro Chem.) was used to increase the yield of the lift-off process. It is well known that oxygen molecules adsorbed on the SWCNT surface in ambient air induce p-type transfer properties in SWCNT FETs. In order to investigate the effect of the Al2 O3 gate dielectric layer on the electronic transfer characteristics of SWCNT FETs, we compared the transfer characteristics of FETs fabricated with different SAM layers beneath the Al2 O3 film. Figure 4(a) shows the transfer curves of the local bottom-gate SWCNT FETs with 10 nm-thick Al2 O3 , Al2 O3 /DT, and Al2 O3 /MUO gate dielectric layers. FETs with Al2 O3 dielectric layers showed typical ambipolar transfer curves, which can be explained in terms of the reduction of the Schottky barrier thickness by the defect sites in the high-k dielectric [32], where defects inside the Al2 O3 film had an ESD effect on the SWCNT FETs. The FETs also had ambipolar properties after the insertion of the DT SAM. However, when an MUO SAM was inserted prior to the growth of Al2 O3 film, the p-type characteristics were maintained. Here, we define the on and off currents as the currents measured at gate biases of Vg = −2 and +2 V, respectively. Figure 4(b) shows the distribution of the on/off current ratio for FETs with the three different dielectric layers. For the devices with Al2 O3 and Al2 O3 /DT dielectrics, the on/off ratio was about 1, indicating ambipolar transfer properties. However, half of the devices with the Al2 O3 /MUO dielectric showed on/off ratios of 10, and the other half exhibited ratios of 100, clearly demonstrating the dominant p-type transfer property, as shown in the transfer
3.2. Electronic properties of SWCNT FETs with Al2 O3 gate dielectric film Figure 3(a) shows SEM images of the SWCNTs transferred onto Al2 O3 films once and twice via the enamel transfer method (supporting figure S5 available at stacks.iop.org/ Nano/24/455701/mmedia). As can be clearly seen, the density of the SWCNTs increased with the number of transfer processes. Figure 3(b) shows the distribution of the current flowing through the SWCNT channel; both the length and the width of the channel were 40 µm, and Cr electrodes were used. The inset shows that the areal number density of SWCNTs increased from 2 to 5 CNTs µm−2 after the second transfer. Accordingly, the current increased with increasing number of transfers. After the first transfer, the surface was sonicated to test the adhesion of the transferred SWCNTs, and there was no noticeable decrease in the current, indicating the strong adhesion of the transferred SWCNTs to the Al2 O3 surface. Interestingly, the increase in the current with transfer time did not linearly correspond to an increase in the tube density. This is attributed to the fact that the number and length of metallic percolation paths increase with increasing areal tube density, which adds to the contribution from semiconducting tubes, resulting in a higher increase in current than that in areal tube density [14, 30, 31]. Next, we fabricated two different types of SWCNT FETs: local bottom-gate and top-gate FETs. The local bottom-gate FETs were fabricated as follows. On Au/Ti substrates as bottom-gate electrodes, SAMs with different functional groups were formed. Then, a 10 nm-thick Al2 O3 film was grown on the SAM layer at 100 ◦ C using the ALD technique. Finally, CVD-grown SWCNTs were transferred 6
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Figure 4. (a) Transfer curves measured at Vds = −1 V and (b) distribution of on/off current ratios, Ion/off , of local bottom-gate SWCNT FETs using Al2 O3 gate dielectric films grown on bare (red circles), DT-treated (blue triangles), and MUO-treated (black) Au surfaces.
Figure 5. (a) Transfer curves measured at Vds = −1 V for the global bottom-gate SWCNT FETs using 300 nm-thick bare (red circles) and OTS-treated SiO2 (blue triangles). (b) Transfer curves measured at Vds = −1 V and (c) distribution of on/off current ratios, Ion/off , of top-gate SWCNT FETs using Al2 O3 gate dielectric films grown on bare (red circles) and OTS-treated (blue triangles) SiO2 surfaces.
defined as the currents measured at Vg = +2 and −2 V, respectively, since the n-type transfer property is more prominent with the top gate. Figure 5(c) shows the distribution of the on/off current ratio for SWCNT FETs with Al2 O3 and Al2 O3 /OTS top-gate dielectric layers. Ambipolar but rather n-type properties with on/off ratios of 1–10 were observed for the Al2 O3 dielectric, but n-type properties with on/off current ratios of mostly ∼100 were observed for the Al2 O3 /OTS dielectric. Since the OTS SAM did not affect the electronic properties of the SWCNT FETs (figure 5(a)), such an enhancement of the n-type properties is attributed to the ESD effect produced by the Al2 O3 film grown on the OTS SAM. It is expected that the growth of an Al2 O3 film on a hydrophilic surface (SiO2 ) would produce a lower defect density than that on a hydrophobic surface (OTS-covered SiO2 ). Therefore, the electron-doping effect due to defects in the Al2 O3 film would be enhanced for the Al2 O3 /OTS layer, resulting in the type conversion of p-type SWCNT FETs, as was also confirmed in the C–V measurements (figure 2(b)). Such a doping effect was maintained for longer than 30 days without any external stimulation like heating. It is expected that the effect of the positive charges due to defects in the bulk Al2 O3 film on the SWCNT FETs
curve. These data suggest that an MUO SAM should alleviate the ESD effects produced by the defects in Al2 O3 dielectric film; the lower defect density (ρ) in the Al2 O3 film grown on the hydrophilic surface than in that grown on the hydrophobic surface had already been ascertained by measuring the flat band voltage (figure 2(b)). Prior to the electrical measurement of the top-gate SWCNT FETs using Al2 O3 gate dielectric layers, the effect of the OTS SAM on the electronic transfer characteristics of the bottom-gate SWCNT FETs using SiO2 gate dielectrics was investigated. In the transfer curves shown in figure 5(a), there is no noticeable difference between the curves obtained with and without the SAM, which is consistent with the previously reported result [13]. This suggests that there is negligible or no generation of charge traps due to organic chains. Therefore, we can exclude the possibility of an extra charge Qf generated by the OTS SAM. Figure 5(b) shows the transfer curves obtained from top-gate SWCNT FETs with Al2 O3 and Al2 O3 /OTS gate dielectrics. For a 10 nm-thick Al2 O3 gate dielectric, ambipolar transfer properties were observed. Application of an OTS SAM beneath the 10 nm-thick Al2 O3 film enhanced the n-type transfer property. Here, the on and off currents are 7
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Figure 6. Upper left corner: simulation of the charge distribution around an SWCNT in the top-gate FET using a 10 nm-thick Al2 O3 film with nSCD/pSCD = 4, where the Al2 O3 film consists of an 8 nm-thick positively charged bulk layer on top of a 2 nm-thick negatively charged layer. Here, a floating top gate is assumed. Others: change in the electric field, marked with arrows, around an SWCNT for different nSCD/pSCD ratios. The direction of the electric fields around the CNT is changed at an nSCD/pSCD ratio of ∼4.
will be opposite to that of the negative charges generated during the initial growth of the Al2 O3 film. In the local bottom-gate SWCNT FETs, it is reasonable that the p-type transfer characteristics were conserved when the Al2 O3 /MUO dielectric was used, since a very low density of positive charges was observed, while ambipolar transfer properties were observed with Al2 O3 and Al2 O3 /DT, whose positive charge densities are higher. In the top-gate SWCNT FETs, it is apparently strange that the n-type transfer property became dominant when an Al2 O3 layer was used, since negative charges were located directly on top of the SWCNTs and could inhibit the ESD due to positive charges in the bulk film. In order to understand such apparently contradictory phenomena, a numerical simulation on the distribution of the electric field around SWCNTs was carried out based on Poisson’s equation. In the simulation, we investigated the changes in the electric field distribution around the SWCNTs due to variation of the ratio between the negative space charge density (nSCD) and positive space charge density (pSCD). For the 10 nm-thick Al2 O3 film, the negatively charged region is considered to be a 2 nm-thick layer in contact with the top surface of the CNTs, while the positively charged region is an 8 nm-thick bulk layer on top of the negatively charged region (upper left corner of figure 6). The simulation results (figure 6) show that more electrons seem to reside in the CNT channel when the ratio of nSCD/pSCD is smaller than 4. On the other hand, more holes seem to be induced when the ratio is larger than 4. This means that even though there is a high density of negative charges in the shallow layer near the CNTs, positive charges in the bulk film would play a dominant role in the CNTs until the density of negative charges near the CNT is
four times higher than that of positive charges. Therefore, the reason why the Al2 O3 film with the OTS SAM has more positive charges and fewer negative charges, and thus has an advantage for electron transport compared to the Al2 O3 film grown on the UVO-treated SiO2 , is explained by these simulation data. As a result, in the case of a local bottom-gate FET, the negative charges remain a distance away from the channel. Thus, regardless the quantity of negative charges that can help in the hole transport, the channel is affected by the positive charges near the CNTs. Here, the number of positive charges in the Al2 O3 film grown on the hydrophilic MUO-covered surface is lower than that for the films grown on bare Au or hydrophobic DT-covered Au. In addition, in this device structure, the SWCNTs are exposed to ambient air and have adsorbed O2 molecules, which help to maintain the p-type characteristics of the SWCNT FETs. Therefore, the electron-doping effect is alleviated. In the case of the top-gate device, the Al2 O3 film grown on top of the OTS-covered SiO2 surface, which is hydrophobic, would have a higher number of positive defect sites in the bulk oxide than that in the film grown on the bare hydrophilic SiO2 surface. In addition, there are fewer negative charges located near the CNTs and on the substrate. Thus, in total, enhancement of the doping effect from the positive charges in the bulk oxide is observed, as these effects overcome the effects of the negative charges near the CNTs.
4. Conclusion We have shown the controllability of the electronic transport properties of SWCNT FETs via the use of ALD-grown 8
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10 nm-thick Al2 O3 gate dielectric film, where the growth substrate surface was modified by functionalized selfassembled monolayers prior to the growth at 100 ◦ C. It was observed that the quality of the Al2 O3 film, which depends strongly on the hydrophilicity of the growth surface, controlled the type of transfer in the SWCNT FETs. Al2 O3 grown on a hydrophilic surface alleviated the electron-doping effect, maintaining the p-type characteristics of the original SWCNT FETs. However, an SAM with methyl groups enhanced the positively charged defect density of the Al2 O3 film and enhanced the n-type transfer characteristics, leading to ambipolar and ultimately n-type nature of the SWCNT FETs. This work shows that the electronic transfer properties can be controlled by application of an organic/inorganic hybrid dielectric layer. Simulation studies on the distribution of the electric field near the SWCNTs and the measurement of flat band voltages were able to explain these transfer properties of SWCNT FETs using Al2 O3 dielectric film.
[9]
[10]
[11]
[12] [13]
Acknowledgments
[14]
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (Grant No. 2013R1A2A1A01016165 and 2012M3A7B4049863). We also thank KU-KIST Graduate School Program of Korea University, Korea. The authors would also like to thank Minho Choi at Shareshare, Inc., for programming of the FET data organizer and Minsung Kim at Kyungsung University for graphic designs.
[15]
[16]
[17]
References
[18]
[1] Park H H, Kang P S, Kim G T and Ha J S 2010 Effect of gate dielectrics on the device performance of SnO2 nanowire field effect transistors Appl. Phys. Lett. 96 102908 [2] Groner M D, Fabreguette F H, Elam J W and George S M 2004 Low-temperature Al2 O3 atomic layer deposition Chem. Mater. 16 639 [3] Kim S K, Lee S W, Hwang C S, Min Y S, Won J Y and Jeong J 2006 Low temperature (
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Controlling the electronic properties of SWCNT FETs via modification of the substrate surface prior to atomic layer deposition of 10 nm thick Al2O3 film
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 455701 (10pp)
doi:10.1088/0957-4484/24/45/455701
Controlling the electronic properties of SWCNT FETs via modification of the substrate surface prior to atomic layer deposition of 10 nm thick Al2O3 film Joonsung Kim1,4 , Jangyeol Yoon1,4 , Junhong Na2 , Seongmin Yee2 , Gyu Tae Kim2 and Jeong Sook Ha1,3 1
Department of Chemical and Biological Engineering, Korea University, Seoul 136-701, Korea School of Electrical Engineering, Korea University, Seoul 136-701, Korea 3 KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Korea 2
E-mail: [email protected]
Received 18 March 2013, in final form 4 June 2013 Published 18 October 2013 Online at stacks.iop.org/Nano/24/455701 Abstract We demonstrate the controllability of the electronic transport properties of single-walled carbon nanotube (SWCNT) field effect transistors (FETs) via the use of 10 nm thick atomic-layer-deposited aluminum oxide (Al2 O3 ) gate dielectric films, where the substrate surfaces were modified with differently functionalized self-assembled monolayers (SAMs) prior to their growth, namely SAMs with hydrophobic (−CH3 ) or hydrophilic (−OH) groups. Al2 O3 grown on a hydrophilic surface causes the SWCNT FETs to keep their intrinsic p-type transfer characteristics by alleviating the electron-doping effect originating from defects in the Al2 O3 film. However, the SAM with methyl groups increases the defect density of the Al2 O3 film, enhancing the n-type transfer characteristics and inducing ambipolar to n-type behavior in the SWCNT FETs. In this work, we find clues about the distribution of charged defects in the Al2 O3 film, which strongly influences the transfer characteristics of the SWCNT FETs, by measuring the thickness-dependent flat band voltages. S Online supplementary data available from stacks.iop.org/Nano/24/455701/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
threshold swings. In order to apply thin-film dielectric layers to various devices including flexible/stretchable ones, the growth of the dielectric layers should be carried out at temperatures low enough for the polymer substrate. Among the various growth techniques, atomic layer deposition (ALD) is known to be adequate for the growth of thin high-k dielectric layers at low temperatures with a thickness controllable at the nanometer scale [2, 3]. Aluminum oxide (Al2 O3 ) is known to have superior insulating properties with a much lower leakage current density and a higher dielectric constant than the conventionally used
As basic electronic devices, field effect transistors (FETs) show a very strong dependence of their electronic properties on the gate dielectric layer [1]. As a result, there has been extensive research on the preparation of very thin high-quality dielectric layers with very robust insulating properties. In particular, high-k dielectrics have shown superior device performance with low operation voltages and small sub4 These authors contributed equally to this work.
0957-4484/13/455701+10$33.00
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SiO2 film. Therefore, Al2 O3 films have frequently been used as dielectric layers in various nanomaterial-based FETs as well as in conventional thin-film FETs [4]. Single-walled carbon nanotubes (SWCNTs) have been actively investigated because of their superior electronic properties, mechanical strength, and flexibility. The pristine SWCNTs grown by chemical vapor deposition (CVD) are a mixture of metallic and semiconducting CNTs. Semiconducting SWCNT FETs with conventional SiO2 gate dielectrics show p-type transfer characteristics in ambient air, since the adsorbed O2 retracts electrons from the SWCNTs, producing excess holes as major charge carriers [5]. However, the application of an ALD-grown Al2 O3 dielectric layer to SWCNT FETs produces ambipolar transfer properties, which should be overcome [6]. This result has been attributed to the electrostatic doping (ESD) effect of the defect sites in the Al2 O3 film [7–9]. In particular, Al2 O3 films grown by ALD at low temperatures intensify such doping effects because of the higher defect density in the film. Therefore, there should be effort to understand and mitigate such undesired ESD effects of Al2 O3 films on the electronic transfer in SWCNT FETs. For the fabrication of devices using CVD-grown SWCNTs, thermal release tape is mostly used to transfer the SWCNTs from the SiO2 or quartz substrates to the device substrates, such as SiO2 . In this process, Au evaporation onto a CVD-grown SWCNT film makes an SWCNT-embedded Au film that is subsequently transferred onto the device substrate by thermal release tape. Then, Au is etched away to leave SWCNTs [10, 11]. However, transfer of SWCNTs onto Al2 O3 using this method is not easy, since Al2 O3 film has a low surface energy insufficient for adhesion of the sacrificial gold layer [12]. Therefore, solution casting of SWCNTs onto Al2 O3 has most frequently been used. Since the direct transfer of CVD-grown SWCNTs simplifies device fabrication, it is necessary to develop a method for the direct transfer of SWCNTs onto the Al2 O3 dielectric layer. In this work, we systematically investigated the effect of Al2 O3 dielectric film grown on differently functionalized growth substrates on the electronic transport properties of SWCNT FETs. Depending upon the hydrophilicity of the modified surface, the quality of the grown Al2 O3 film was changed, resulting in different electrical properties. Measurements of the film-thickness-dependent flat band voltages and simulation studies on the electric field distribution could explain the experimentally observed influence of the Al2 O3 dielectric layer. We also devised a transfer method for CVD-grown SWCNTs onto ALD-grown Al2 O3 substrate, the ‘enamel transfer method’.
chemical stoichiometry of the film was ascertained using the Auger profile data. The detailed growth process is similar to the one previously reported [1, 13]. 2.2. Surface treatment with SAMs For the surface modification prior to the ALD growth of the Al2 O3 film, SAMs with different functional groups were formed by dipping the substrate in the SAM solution for 12 h. On the Au surface, 2 mM of 11-mercapto-1-undecanol (MUO) and 1-decanethiol (DT) in ethanol were used. The SiO2 surface was treated with 1 mM of OTS SAM in ethanol or with UV-ozone (UVO) for 30 min. After treatment with the SAM solution or UVO, the samples were rinsed with toluene and deionized water. Measurement of the contact angle was performed after the SAM treatment. 2.3. Growth of SWCNTs Randomly networked SWCNTs were grown on the SiO2 surface using diluted ferritin as a catalyst via CVD at 925 ◦ C. The density of SWCNTs could be controlled by varying the concentration of the ferritin catalyst. In particular, the metallic percolation path could be suppressed by reducing the SWCNT density [14]. Methane (26 sccm, 99.995%) and H2 (60 sccm, 99.99%) gases were passed through a methanol/ethanol (1:1 volume ratio) bubbler and supplied as carbon sources. The average diameter of the grown SWCNTs was estimated to be 1.4 nm by analyzing the Raman radial breathing mode [15]. About 60% of the grown SWCNTs showed p-type semiconducting transfer properties in SWCNT FETs using SiO2 dielectric layers [14]. 2.4. Transfer of SWCNTs Randomly networked SWCNT films were transferred onto the Al2 O3 layer using the enamel transfer method, which consisted of four steps: (1) deposition of a 100 nm-thick Au layer onto as-grown SWCNT films by e-beam evaporation, (2) spin coating of an enamel layer consisting of nitrocellulose and RSF resin onto an SWCNT-embedded Au film and baking of the film at 65 ◦ C in a dry oven for 15 min, (3) peeling of the enamel/Au/SWCNT film from the donor substrate and transfer onto an Al2 O3 surface wetted with ethanol, and (4) etching of the enamel by acetone and subsequent etching of the Au by Au etchant, leaving SWCNT films on the Al2 O3 layer. The characteristics of the transferred SWCNTs were confirmed by Raman spectroscopy and SEM analysis. 2.5. Fabrication of SWCNT FETs
2. Experimental details
Both top- and local bottom-gate SWCNT FETs were fabricated (supporting figure S1 available at stacks.iop.org/ Nano/24/455701/mmedia). For the fabrication of a local bottom-gate device, a Au/Ti gate electrode pattern with a width of 50 µm was formed on a SiO2 surface by photolithography. In order to increase the yield of the lift-off process, LOR-2A (Microchem) was spin-coated prior to coating of the photoresist, which prevented deterioration of
2.1. Preparation of Al2 O3 thin film Al2 O3 films were grown using the ALD technique. The growth temperature was 100 ◦ C, and trimethylaluminum (TMA) and water vapor were used as precursor molecules for aluminum and oxygen. According to the SEM data, the growth rate was estimated to be 0.1 nm/cycle, and the 2
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Figure 1. (a) Auger depth profile taken from Al2 O3 film grown on a Au substrate. The background is a cross-sectional SEM image of the Al2 O3 . (b) Leakage current density versus film thickness of Al2 O3 (red circles) and Al2 O3 grown on the MUO-treated Au (black squares), which was measured at V = 2 V. (c) Leakage current density versus bias voltage taken from 10 nm-thick Al2 O3 films grown on bare, DT-treated, and MUO-treated Au surfaces. (d) Thickness-dependent breakdown voltage taken from Al2 O3 films grown on bare, DT-treated, and MUO-treated Au surfaces.
the Au/Ti electrode on the Al2 O3 film during the process by forming undercuts. Then, a 10 nm-thick Al2 O3 film was grown on the SAM-treated Au/Ti electrode. After transfer of the SWCNT film onto the Al2 O3 layer via the enamel transfer method, Au/Ti source and drain electrodes were patterned by photolithography and lift-off techniques. Finally, SWCNT channels were isolated via O2 RIE by using patterned photoresist (AZ5214E) as a mask layer. For the top-gate FET, Au/Ti source and drain electrodes were deposited on the SWCNT/SiO2 surface via e-beam evaporation, and the channels were isolated by O2 RIE. A 10 nm-thick Al2 O3 dielectric layer was grown on top of the SAM-treated SWCNT/SiO2 surface via ALD. Finally, the Au/Ti gate electrode was formed. Electrical measurements were performed using a B1500A semiconductor device analyzer.
(SEM) image. It is clearly shown that a 10 nm-thick stoichiometric Al2 O3 film with an Al/O ratio of 2/3 was grown. Figure 1(b) shows that the leakage current density of the grown Al2 O3 film decreased with increasing thickness, as expected. Furthermore, the addition of an MUO SAM prior to the growth of the Al2 O3 film improved the insulating properties, and the effect of the SAM was more noticeable with a thinner Al2 O3 film. This can be explained in terms of the effect of the carbon chains of the SAM as a barrier for the electron transfer between electrodes [16]. The changes in the leakage current density of the film with varying bias voltage for three differently prepared 10 nm-thick Al2 O3 films are shown in figure 1(c). In the absence of any SAM, the leakage current density continuously increased with increasing bias. With a DT SAM, the leakage current density remained at 10−8 A cm−2 up to 7 V and increased with increasing bias above 7 V. However, the leakage current density was stable up to 10 V when an MUO SAM was used, demonstrating the improvement in the breakdown voltage (Vb ) of the Al2 O3 film. This effect is tentatively attributed to electron capture by the hydroxyl groups or metal alkoxides located in the interface between the Al2 O3 and the MUO. Figure 1(d) shows the change in the Vb value of the Al2 O3 film with varying thickness. As expected, Vb increased with increasing thickness. The DT SAM did not influence
3. Results and discussion 3.1. Growth and characterization of Al2 O3 film on substrates modified with SAM Figure 1(a) shows the Auger depth profile of an ALD-grown 10 nm-thick Al2 O3 film on a Au substrate at 100 ◦ C and the corresponding cross-sectional scanning electron microscopy 3
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Vb , but a dramatic increase in Vb was observed with the MUO SAM: a merely 10 nm-thick Al2 O3 /MUO hybrid film had a Vb of over 20 V. For the 20 and 40 nm-thick Al2 O3 /MUO hybrid films, breakdown phenomena were not observed even under 40 V. When electrons are injected from an electrode into the unmodified Al2 O3 film, collisions can occur among electrons and atoms inside the film producing an impact ionization [17–19]. The impact ionization detaches the chemically bonded oxygen atoms from the lattices so that electrons can penetrate the dielectric layer easily, while the mobility of oxygen vacancies is not sufficiently high for them to move easily. Therefore, oxygen vacancies start to accumulate when a high voltage is applied between the electrodes, and the accumulated oxygen vacancies enhance the ion conductivity so that more electrons can be penetrated easily, leading to electrical breakdown of the dielectric film. However, hydroxyl (−OH) groups in the MUO SAM are reported to capture the electrons [20], and these make the film more hydrophilic. Thus, these hydroxyl groups form an oxygen-rich layer that can subsequently neutralize the effects of the oxygen detachment process. The existence of hydroxyl groups in the interface between the Al2 O3 and MUO SAM was confirmed by the hysteresis curve, which depended on the bias sweep direction (supporting figure S2 available at stacks.iop.org/Nano/24/ 455701/mmedia). The Al2 O3 and Al2 O3 /DT (supporting figure S2(a) available at stacks.iop.org/Nano/24/455701/ mmedia) films grown on Au surfaces showed symmetric hysteresis regardless of the bias sweep direction. However, the Al2 O3 film grown on an MUO SAM showed distinctively different hysteresis curves depending on the sweep direction (supporting figure S2(b) available at stacks.iop.org/Nano/24/ 455701/mmedia). To confirm the effects of the methyl and hydroxyl functional groups on the hysteresis phenomena, the leakage currents through Al2 O3 /DT and Al2 O3 /MUO films were compared (supporting figure S3 available at stacks.iop. org/Nano/24/455701/mmedia). A noticeable difference in the leakage current for different voltage sweeping directions was only observed for the Al2 O3 /MUO film, which confirms the existence of localized electron-trapping sites due to hydroxyl groups [21]. The initial state of the film growth is considered to play a critical role in the quality of the grown bulk film, as reported in a previous work by Kobayashi et al [22]. Accordingly, we expect that the chemical state of the substrate surface prior to ALD growth will strongly influence the growth of bulk Al2 O3 film. Next, in order to understand the effect of the hydrophilicity of the growth surface on the charged defect density of the ALD-grown Al2 O3 films, we measured the capacitance–voltage (C–V) curves of the grown Al2 O3 films (supporting figure S4 available at stacks.iop.org/Nano/ 24/455701/mmedia) and estimated the flat band voltage (VFB ), which is closely related to the charged defect density. To make the p-type SiO2 surface more hydrophilic, it was treated with UV-ozone (UVO) for 30 min, while it was treated with an OTS SAM to make it hydrophobic. After the modification, Al2 O3 films were grown at 100 ◦ C via ALD to investigate the effect of the surface properties
on the generation of charged defects inside the film. Then, the flat band voltage (VFB ) was measured for different film thicknesses [23–25]. Here, UVO and OTS mean the ALDgrown Al2 O3 films on UVO-treated and OTS-SAM-treated SiO2 substrates, respectively. The flat band voltage VFB can be expressed in terms of the fixed oxide charges (Qf ), distributed oxide charges (ρox ), and the work-function difference between the metal gate and the semiconductor (8MS ) for the Al2 O3 film, as shown in equation (1) [26]. The capacitance of the oxide film (Cox ) can be expressed as in equation (2). Z tox 1 x Qf − − ρox (x) dx (1) VFB = 8MS − Cox Cox t ox 0 A Cox = εr ε0 . (2) tox Here, tox is the thickness of the oxide film, A is the area of the electrodes, εr is the relative dielectric constant, and ε0 is the vacuum permittivity. Since the oxide charge distribution plays a strong influence on the electronic properties of MOSFETs, it is necessary to know the real profile of oxide charges for accurate characterization [27]. However, we assume here that ρox has a constant value (ρ) inside the same film regardless of the position for a simple estimation. Then equation (1) can be transformed into equation (3). VFB = 8MS −
1 Qf tox − ρt2 . Aεr ε0 2Aεr ε0 ox
(3)
To obtain the VFB values, circular Pt electrodes (diameter = 100 µm, thickness = 100 nm) were deposited on the Al2 O3 by using a metal shadow mask and the e-beam deposition method. The VFB values were estimated using a Mott–Schottky plot following the previous work by Mehta et al [25]. The Mott–Schottky plot which comes from equation (4) expresses the linear relation between C−2 and V [28]. 2 kT C−2 = 2 V − VFB − . (4) e A Neεr ε0 Here N, V, e, T, and k are the donor density, applied bias voltage, unit charge of an electron, temperature in Kelvin, and Boltzmann constant, respectively. When C−2 converges to zero, VFB can be estimated from the measurement of C−2 as a function of V . Therefore, the linear region of normalized C−2 values was fitted linearly to estimate VFB in figure 2(a). Following the work by Mehta et al, we only used the linear region of the plot and ignored the kT/e term because it is much smaller than the variation of the data. Next, the thickness-dependent VFB data from figure 2(a) were fitted with equation (3) (figure 2(b)). From the curve fitting, Qf and ρ values were estimated using the method of undetermined coefficients. The Qf values of the UVO and OTS samples were estimated to be −5.2 × 10−11 and −3.3 × 10−11 C, respectively, and the ρ values were +7.0 × 10−13 and +2.6 × 10−12 C nm−1 , respectively. It was found that Qf and ρ showed opposite charges. According to the previous work by Shin et al [9], this result supports our 4
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Figure 2. (a) Normalized C−2 versus bias for different thicknesses of Al2 O3 films grown on UVO (top) and OTS (bottom)treated SiO2 surfaces. The black, red, and blue plots are for 10, 20, and 30 nm-thick Al2 O3 films, respectively. Ten samples of each thickness were used for the measurement. (b) Thickness-dependent flat band voltage (VFB ) of Al2 O3 films grown on OTS (triangles) and UVO (circles) treated SiO2 . The curve fitting was carried out according to equation (3) in the text. (c) O 1s and Al 2p peaks of the XPS spectrum taken from 2 nm-thick Al2 O3 film grown on bare (blue), DT (red) and MUO (black) treated Au surfaces. (d) O(1s)/Al(2p) peak ratio normalized to that of the Al2 O3 film grown on bare Au. The insets show the contact angles of water measured on the surfaces before the ALD growth of the Al2 O3 film.
Al2 O3 film, the contact angle of water was measured to be 77◦ , 107◦ , and 32◦ for bare, DT-treated, and MUO-treated Au surfaces, respectively. These results imply that water molecules adsorbed on the hydrophilic surface supply more nucleation sites for the initial growth of the Al2 O3 film, resulting in a lower defect density of the film in the later stage of growth as compared to the film grown on a hydrophobic surface. This is consistent with the results from the flat band voltage measurements: the Al2 O3 grown on the hydrophilic surface had lower ρ but higher Qf values. Here, the absolute value of the charge density might not be accurate, since the calculation was made under the Mott–Schottky plot estimation and the assumption of a constant ρ value throughout the whole film. However, the result that the hydrophilicity of the substrate surface affects the quality of the whole film is consistent with previous work [23]. At the same time, we expect that the existence of the negative charges near the interface between the Al2 O3 film and the substrate surface should be considered in studying the interaction between the channel materials and the dielectric film in FETs. Moriyama et al reported [8] that the positive charges near the CNT channel help to change the band junction between the electrode and the channel to promote electron transport, while the negative charges help the hole transport. If this is
hypothesis: the oxygen-rich region observed in the initial stage of the film growth generates a negatively charged layer, but positive charges are generated in the bulk film because of the oxygen vacancies. These positive charges in the bulk film are well known, and they are usually used to explain the type conversion of CNT FETs [8]. However, the existence of negative charges near the interface between the Al2 O3 and the substrate has not yet been studied well. X-ray photoelectron spectra of 2 nm-thick Al2 O3 films grown on differently modified Au surfaces were obtained in order to investigate the chemical stoichiometry in the initial stage of the growth (figure 2(c)). The relative ratio between the O 1s peak and the Al 2p peak indicates an O/Al ratio higher than 1.5, which shows that the Al2 O3 film is more oxygen rich than the bulk Al2 O3 . The stoichiometries of the films grown on the DT-SAM-treated, bare, and MUO-SAM-treated Au substrates were AlO2.25 , AlO2.28 , and AlO2.31 , respectively. The much higher oxygen content of the film in the initial stage of growth is consistent with the previously reported results [9, 29]. Figure 2(d) shows that the oxygen ratio of the film increases with increasing hydrophilicity of the substrate surface: compared to the O/Al ratio for the film on bare Au, the O/Al ratio was ∼2% higher for the Al2 O3 film on the MUO-SAM-treated Au but was ∼2% lower for the Al2 O3 film on the DT-SAM-treated Au. Prior to the growth of the 5
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Figure 3. (a) SEM images of CNTs transferred onto the Al2 O3 surface once (left) and twice (right) via the enamel transfer method. (b) Distribution of current through the SWCNT channels after the one (black) and two (blue) transfers measured at a 1 V bias. The red bars are for sonicated SWCNTs after the first transfer. Here, Cr is used for the electrodes (thickness = 50 nm, channel length = 40 µm, channel width = 40 µm). The inset shows the number density of SWCNTs with the repetition of enamel transfer.
true, the high Qf and low ρ values of the Al2 O3 grown on the hydrophilic surface are expected to be advantageous for hole transport, while the low Qf and high ρ values of the Al2 O3 grown on the hydrophobic surface are expected to be favorable for electron transport. Therefore, we investigated the effect of the Al2 O3 dielectric film on the transfer characteristics of SWCNT FETs.
via the above-mentioned enamel transfer method. In the top-gate SWCNT FETs, SWCNTs were transferred onto the SiO2 surface using the thermal release tape method. Then, SAMs with different functional groups were formed on the SWCNT-covered SiO2 substrates. Finally, a 10 nm-thick Al2 O3 layer was grown at 100 ◦ C by ALD, and the Au/Ti top-gate electrode was deposited. In both devices, 10 µm wide SWCNT channels were formed through partial O2 reactive ion etching (RIE) to reduce the number and length of percolation paths of the metallic SWCNTs. Au/Ti (50/10 nm) electrodes were defined by photolithography. Here, a resist for lift-off (LOR, Micro Chem.) was used to increase the yield of the lift-off process. It is well known that oxygen molecules adsorbed on the SWCNT surface in ambient air induce p-type transfer properties in SWCNT FETs. In order to investigate the effect of the Al2 O3 gate dielectric layer on the electronic transfer characteristics of SWCNT FETs, we compared the transfer characteristics of FETs fabricated with different SAM layers beneath the Al2 O3 film. Figure 4(a) shows the transfer curves of the local bottom-gate SWCNT FETs with 10 nm-thick Al2 O3 , Al2 O3 /DT, and Al2 O3 /MUO gate dielectric layers. FETs with Al2 O3 dielectric layers showed typical ambipolar transfer curves, which can be explained in terms of the reduction of the Schottky barrier thickness by the defect sites in the high-k dielectric [32], where defects inside the Al2 O3 film had an ESD effect on the SWCNT FETs. The FETs also had ambipolar properties after the insertion of the DT SAM. However, when an MUO SAM was inserted prior to the growth of Al2 O3 film, the p-type characteristics were maintained. Here, we define the on and off currents as the currents measured at gate biases of Vg = −2 and +2 V, respectively. Figure 4(b) shows the distribution of the on/off current ratio for FETs with the three different dielectric layers. For the devices with Al2 O3 and Al2 O3 /DT dielectrics, the on/off ratio was about 1, indicating ambipolar transfer properties. However, half of the devices with the Al2 O3 /MUO dielectric showed on/off ratios of 10, and the other half exhibited ratios of 100, clearly demonstrating the dominant p-type transfer property, as shown in the transfer
3.2. Electronic properties of SWCNT FETs with Al2 O3 gate dielectric film Figure 3(a) shows SEM images of the SWCNTs transferred onto Al2 O3 films once and twice via the enamel transfer method (supporting figure S5 available at stacks.iop.org/ Nano/24/455701/mmedia). As can be clearly seen, the density of the SWCNTs increased with the number of transfer processes. Figure 3(b) shows the distribution of the current flowing through the SWCNT channel; both the length and the width of the channel were 40 µm, and Cr electrodes were used. The inset shows that the areal number density of SWCNTs increased from 2 to 5 CNTs µm−2 after the second transfer. Accordingly, the current increased with increasing number of transfers. After the first transfer, the surface was sonicated to test the adhesion of the transferred SWCNTs, and there was no noticeable decrease in the current, indicating the strong adhesion of the transferred SWCNTs to the Al2 O3 surface. Interestingly, the increase in the current with transfer time did not linearly correspond to an increase in the tube density. This is attributed to the fact that the number and length of metallic percolation paths increase with increasing areal tube density, which adds to the contribution from semiconducting tubes, resulting in a higher increase in current than that in areal tube density [14, 30, 31]. Next, we fabricated two different types of SWCNT FETs: local bottom-gate and top-gate FETs. The local bottom-gate FETs were fabricated as follows. On Au/Ti substrates as bottom-gate electrodes, SAMs with different functional groups were formed. Then, a 10 nm-thick Al2 O3 film was grown on the SAM layer at 100 ◦ C using the ALD technique. Finally, CVD-grown SWCNTs were transferred 6
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Figure 4. (a) Transfer curves measured at Vds = −1 V and (b) distribution of on/off current ratios, Ion/off , of local bottom-gate SWCNT FETs using Al2 O3 gate dielectric films grown on bare (red circles), DT-treated (blue triangles), and MUO-treated (black) Au surfaces.
Figure 5. (a) Transfer curves measured at Vds = −1 V for the global bottom-gate SWCNT FETs using 300 nm-thick bare (red circles) and OTS-treated SiO2 (blue triangles). (b) Transfer curves measured at Vds = −1 V and (c) distribution of on/off current ratios, Ion/off , of top-gate SWCNT FETs using Al2 O3 gate dielectric films grown on bare (red circles) and OTS-treated (blue triangles) SiO2 surfaces.
defined as the currents measured at Vg = +2 and −2 V, respectively, since the n-type transfer property is more prominent with the top gate. Figure 5(c) shows the distribution of the on/off current ratio for SWCNT FETs with Al2 O3 and Al2 O3 /OTS top-gate dielectric layers. Ambipolar but rather n-type properties with on/off ratios of 1–10 were observed for the Al2 O3 dielectric, but n-type properties with on/off current ratios of mostly ∼100 were observed for the Al2 O3 /OTS dielectric. Since the OTS SAM did not affect the electronic properties of the SWCNT FETs (figure 5(a)), such an enhancement of the n-type properties is attributed to the ESD effect produced by the Al2 O3 film grown on the OTS SAM. It is expected that the growth of an Al2 O3 film on a hydrophilic surface (SiO2 ) would produce a lower defect density than that on a hydrophobic surface (OTS-covered SiO2 ). Therefore, the electron-doping effect due to defects in the Al2 O3 film would be enhanced for the Al2 O3 /OTS layer, resulting in the type conversion of p-type SWCNT FETs, as was also confirmed in the C–V measurements (figure 2(b)). Such a doping effect was maintained for longer than 30 days without any external stimulation like heating. It is expected that the effect of the positive charges due to defects in the bulk Al2 O3 film on the SWCNT FETs
curve. These data suggest that an MUO SAM should alleviate the ESD effects produced by the defects in Al2 O3 dielectric film; the lower defect density (ρ) in the Al2 O3 film grown on the hydrophilic surface than in that grown on the hydrophobic surface had already been ascertained by measuring the flat band voltage (figure 2(b)). Prior to the electrical measurement of the top-gate SWCNT FETs using Al2 O3 gate dielectric layers, the effect of the OTS SAM on the electronic transfer characteristics of the bottom-gate SWCNT FETs using SiO2 gate dielectrics was investigated. In the transfer curves shown in figure 5(a), there is no noticeable difference between the curves obtained with and without the SAM, which is consistent with the previously reported result [13]. This suggests that there is negligible or no generation of charge traps due to organic chains. Therefore, we can exclude the possibility of an extra charge Qf generated by the OTS SAM. Figure 5(b) shows the transfer curves obtained from top-gate SWCNT FETs with Al2 O3 and Al2 O3 /OTS gate dielectrics. For a 10 nm-thick Al2 O3 gate dielectric, ambipolar transfer properties were observed. Application of an OTS SAM beneath the 10 nm-thick Al2 O3 film enhanced the n-type transfer property. Here, the on and off currents are 7
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Figure 6. Upper left corner: simulation of the charge distribution around an SWCNT in the top-gate FET using a 10 nm-thick Al2 O3 film with nSCD/pSCD = 4, where the Al2 O3 film consists of an 8 nm-thick positively charged bulk layer on top of a 2 nm-thick negatively charged layer. Here, a floating top gate is assumed. Others: change in the electric field, marked with arrows, around an SWCNT for different nSCD/pSCD ratios. The direction of the electric fields around the CNT is changed at an nSCD/pSCD ratio of ∼4.
will be opposite to that of the negative charges generated during the initial growth of the Al2 O3 film. In the local bottom-gate SWCNT FETs, it is reasonable that the p-type transfer characteristics were conserved when the Al2 O3 /MUO dielectric was used, since a very low density of positive charges was observed, while ambipolar transfer properties were observed with Al2 O3 and Al2 O3 /DT, whose positive charge densities are higher. In the top-gate SWCNT FETs, it is apparently strange that the n-type transfer property became dominant when an Al2 O3 layer was used, since negative charges were located directly on top of the SWCNTs and could inhibit the ESD due to positive charges in the bulk film. In order to understand such apparently contradictory phenomena, a numerical simulation on the distribution of the electric field around SWCNTs was carried out based on Poisson’s equation. In the simulation, we investigated the changes in the electric field distribution around the SWCNTs due to variation of the ratio between the negative space charge density (nSCD) and positive space charge density (pSCD). For the 10 nm-thick Al2 O3 film, the negatively charged region is considered to be a 2 nm-thick layer in contact with the top surface of the CNTs, while the positively charged region is an 8 nm-thick bulk layer on top of the negatively charged region (upper left corner of figure 6). The simulation results (figure 6) show that more electrons seem to reside in the CNT channel when the ratio of nSCD/pSCD is smaller than 4. On the other hand, more holes seem to be induced when the ratio is larger than 4. This means that even though there is a high density of negative charges in the shallow layer near the CNTs, positive charges in the bulk film would play a dominant role in the CNTs until the density of negative charges near the CNT is
four times higher than that of positive charges. Therefore, the reason why the Al2 O3 film with the OTS SAM has more positive charges and fewer negative charges, and thus has an advantage for electron transport compared to the Al2 O3 film grown on the UVO-treated SiO2 , is explained by these simulation data. As a result, in the case of a local bottom-gate FET, the negative charges remain a distance away from the channel. Thus, regardless the quantity of negative charges that can help in the hole transport, the channel is affected by the positive charges near the CNTs. Here, the number of positive charges in the Al2 O3 film grown on the hydrophilic MUO-covered surface is lower than that for the films grown on bare Au or hydrophobic DT-covered Au. In addition, in this device structure, the SWCNTs are exposed to ambient air and have adsorbed O2 molecules, which help to maintain the p-type characteristics of the SWCNT FETs. Therefore, the electron-doping effect is alleviated. In the case of the top-gate device, the Al2 O3 film grown on top of the OTS-covered SiO2 surface, which is hydrophobic, would have a higher number of positive defect sites in the bulk oxide than that in the film grown on the bare hydrophilic SiO2 surface. In addition, there are fewer negative charges located near the CNTs and on the substrate. Thus, in total, enhancement of the doping effect from the positive charges in the bulk oxide is observed, as these effects overcome the effects of the negative charges near the CNTs.
4. Conclusion We have shown the controllability of the electronic transport properties of SWCNT FETs via the use of ALD-grown 8
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10 nm-thick Al2 O3 gate dielectric film, where the growth substrate surface was modified by functionalized selfassembled monolayers prior to the growth at 100 ◦ C. It was observed that the quality of the Al2 O3 film, which depends strongly on the hydrophilicity of the growth surface, controlled the type of transfer in the SWCNT FETs. Al2 O3 grown on a hydrophilic surface alleviated the electron-doping effect, maintaining the p-type characteristics of the original SWCNT FETs. However, an SAM with methyl groups enhanced the positively charged defect density of the Al2 O3 film and enhanced the n-type transfer characteristics, leading to ambipolar and ultimately n-type nature of the SWCNT FETs. This work shows that the electronic transfer properties can be controlled by application of an organic/inorganic hybrid dielectric layer. Simulation studies on the distribution of the electric field near the SWCNTs and the measurement of flat band voltages were able to explain these transfer properties of SWCNT FETs using Al2 O3 dielectric film.
[9]
[10]
[11]
[12] [13]
Acknowledgments
[14]
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (Grant No. 2013R1A2A1A01016165 and 2012M3A7B4049863). We also thank KU-KIST Graduate School Program of Korea University, Korea. The authors would also like to thank Minho Choi at Shareshare, Inc., for programming of the FET data organizer and Minsung Kim at Kyungsung University for graphic designs.
[15]
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