Static and Dynamic Performance of Complementary Inverters Based on Nanosheet α‑MoTe2 p‑Channel and MoS2 n‑Channel Transistors Atiye Pezeshki,†,# Seyed Hossein Hosseini Shokouh,†,# Pyo Jin Jeon,† Iman Shackery,‡ Jin Sung Kim,† Il-Kwon Oh,§ Seong Chan Jun,‡ Hyungjun Kim,§ and Seongil Im*,† †
Institute of Physics and Applied Physics, ‡School of Mechanical Engineering, and §School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, South Korea S Supporting Information *
ABSTRACT: Molybdenum ditelluride (α-MoTe2) is an emerging transition-metal dichalcogenide (TMD) semiconductor that has been attracting attention due to its favorable optical and electronic properties. Field-effect transistors (FETs) based on few-layer α-MoTe2 nanosheets have previously shown ambipolar behavior with strong p-type and weak n-type conduction. We have employed a direct imprinting technique following mechanical nanosheet exfoliation to fabricate high-performance complementary inverters using α-MoTe2 as the semiconductor for the p-channel FETs and MoS2 as the semiconductor for the n-channel FETs. To avoid ambipolar behavior and produce α-MoTe2 FETs with clean p-channel characteristics, we have employed the highworkfunction metal platinum for the source and drain contacts. As a result, our α-MoTe2 nanosheet p-channel FETs show hole mobilities up to 20 cm2/(V s), on/off ratios up to 105, and a subthreshold slope of 255 mV/decade. For our complementary inverters composed of few-layer α-MoTe2 p-channel FETs and MoS2 n-channel FETs we have obtained voltage gains as high as 33, noise margins as high as 0.38 VDD, a switching delay of 25 μs, and a static power consumption of a few nanowatts. KEYWORDS: MoTe2 nanosheet, MoS2 nanosheet, complementary inverter, switching speed, voltage gain
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that MoTe2 shows structural and electronic phase transition. The structural phase transition from the hexagonal (2H) phase to the monoclinic (distorted octahedral or 1T) phase is reversible at a high temperature.29 Monolayer α-MoTe2 exhibits a direct optical bandgap of 1.10 eV,27 while its bulk form is an indirect semiconductor with the band gap of 0.9−1.0 eV.30,31 For few-layer α-MoTe2 FETs with Ti source and drain contacts, ambipolar conduction with an electron mobility of 0.03 cm2/(V s) and a hole mobility of 0.3 cm2/(V s) has recently been reported,32 while MoTe2 nanosheet FETs with thermally annealed source/drain contacts have shown p-channel operation with notably enhanced hole mobilities of 10 to 30 cm2/(V s).33 Taking advantage of the ambipolar characteristics of the MoTe2 FETs with the Ti source and drain contacts,
ransition-metal dichalcogenides (TMDs) are a wellknown type of two-dimensional (2D) nanomaterials with the common formula MX2, where M is a transition metal element from group IV−VII (M = Mo, W, Nb, Re, and so on) while X is a chalcogen element (X = S, Se, Te). In general, M atoms are sandwiched between X atoms to form a single layer, and each layer can be stacked together via van der Waals forces,1 so that 2D TMDs can be easily cleaved using cellophane tape2,3 or similar techniques.4−8 Among TMD families that form ultrathin layers, MoS2 and WSe2 have been most extensively studied as model 2D materials for n- and ptype (or ambipolar) conduction,9,10 respectively, demonstrating their potentials in a variety of devices, such as field effect transistors (FETs),11−14 phototransistors,15−18 logic inverters,19−24 and p−n diodes.25,26 Recently, hexagonal α-MoTe2 (or MoTe2) has been attracting attention because of its favorable optical and electronic properties.3,27,28 Interestingly, it is also reported © XXXX American Chemical Society
Received: October 13, 2015 Accepted: December 3, 2015
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Figure 1. (a) Optical microscope (OM) image of our Mo-based complementary inverter comprising p-MoTe2 and n-MoS2 FETs with 50 nmthin ALD oxide dielectric and patterned Au gate. The direct imprinting technique was used to place both nanoflakes to be aligned on the same Au patterned gate. (b) Photographic image of inverter 1 fabricated on glass. (c) Raman spectra of 5L-thin MoTe2 flake on glass substrate. (d) Topological line profile of 28 nm-thick MoS2 flake on Al2O3/ patterned Au gate as obtained by AFM. (e) Schematic 3D view of our Mo-based complementary inverter.
complementary-like inverters were reported in ref 32, but due to lack of proper electrical isolation of the FETs, these inverters showed a very small voltage gain of about 1.5, even at a relatively high supply voltage of 18 V. To our knowledge, complementary circuits using p-channel MoTe2 nanosheet FETs have not been reported so far. Complementary and complementary-like circuits fabricated using p-channel FETs based on other TMD nanosheet semiconductors, such as WSe2
or phosphorene, have recently been reported, but these reports focused mainly on the voltage transfer characteristics (VTC), while reports on the dynamic switching behavior of such circuits are still very rare (see Table S1).21,22,24,32,34−37 In the present study, we adopted a direct imprinting technique following mechanical exfoliation to fabricate highperformance heterogeneous but Mo-based complementary inverters which use α-MoTe2 as the semiconductor for the pB
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Figure 2. (a) Transfer characteristics (ID−VGS) of the MoTe2 (black curve) and MoS2 (blue curve) transistors at VD = −1 and 1 V, respectively. IG exhibits low gate leakage current. (b) Output characteristics (ID−VDS) of the p-MoTe2 and n-MoS2 nanoflake FETs show good linear behavior and symmetry. (c) Field effect mobility plots of MoTe2 (black curve) and MoS2 (blue curve) transistors at |VD| = 1 V. (d) Voltage transfer characteristics (VTC) curves of Mo-based complementary inverter (inverter 1 on glass) at different supply voltages of 1, 3, and 5 V, respectively, showing almost ideal transition voltages of 0.51, 1.5, and 2.55 V. The right axis values indicate the voltage gain (= −dVOUT/dVIN) of the inverters which is ∼22 at VDD = 5 V. (e) Near ideal noise margins, NML ≈ 0.35 VDD and NMH ≈ 0.39 VDD, were also obtained at VDD = 5 V.
1c and Figure S1c achieved using 532 nm excitation, in-plane E12g (∼383 cm−1) and out-of-plane A1g vibration modes in MoS2 are much higher than those of MoTe2 in wavenumber (E12g ≈ 228 cm−1) since the bond length between Mo and S atoms is shorter than that between Mo and Te (see Figure S1c for the Raman data of 2L-thin MoS 2 in Supporting Information). The thickness of the MoTe2 layer for the pchannel of inverter 1 is estimated to be 5L (∼3.5 nm) by its Raman spectra27,28 (see Supporting Information for detailed estimation method) and confirmed by AFM data (Figure S1a) while that of the n-channel MoS2 layer is estimated to be 40L (28 nm) according to AFM scan in Figure 1d. But, for our next complementary inverter on glass (inverter 3), we used 3L MoTe2 and 2L-thin MoS2. Figure 1e shows a schematic 3D view of our complementary inverter, where input (VIN), output (VOUT), and supply voltage (VDD) terminals are indicated. MoTe2 and MoS2 flakes are transferred by a direct imprinting method on 50 nm-thick atomic layer deposited (ALD) Al2O3 (below which a patterned back gate electrode, Au, is located).
channel FETs and MoS2 as the semiconductor for the nchannel FETs. Our p-channel FET with nanosheet α-MoTe2 showed very high on-state current along with decent mobility of 12−20 cm2/(V s), since we used a high-workfunction metal, platinum (Pt), for the source and drain (S/D) contact. As a result, our Mo-based nanosheet complementary inverters on glass show good static and dynamic behavior. For the first inverter, which was operated with a supply voltage of 5 V, we obtained a voltage gain of 22, a noise margin of 0.35 VDD, a transition voltage of exactly half the supply voltage, and a switching delay of 25 μs. For the second inverter, which was operated with a supply voltage of 1 V, we measured a voltage gain of 33, a noise margin of 0.38 VDD, and a static power consumption of a few nanowatts. Optical microscopy images of mechanically exfoliated MoTe2 and MoS2 nanosheets are shown in Figure 1 a, where they play respectively as the p- and n-channel of FETs with a common bottom gate for a complementary inverter on glass (inverter 1). Figure 1b shows the photo image of inverter 1 fabricated on glass. The S/D electrodes for MoTe2 FET are Pt whose workfunction would be deeper than the Fermi level of p-MoTe2 (no Schottky barrier may form).38 As for the S/D electrode for n-channel MoS2 FET, Au/Ti was used when Ti was predeposited as a contact but a Au layer was then deposited as an overlay to prevent any oxidation of Ti. The common gate electrode was 50 nm-thick Au on glass substrate. According to the Raman spectroscopy data in Figure
RESULTS AND DISCUSSION Figure 2a shows the transfer curves of p- and n-FETs fabricated on glass for inverter 1, and the curves appear quite symmetric at the gate voltage (VGS) of 0 V along with the respective threshold voltages of +2.4 and −1.2 V. Both FETs display the high on-state current (drain current, ID) of more than ∼20 μA although p-FET with 5L MoTe2 shows 2 orders of magnitude C
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Figure 3. Output voltage dynamics (VOUT) achieved from inverter 1 at VDD of 5 V and under square wave input voltage pulses between 0 and 5 V at (a) 1 kHz and (b) 10 kHz, (c) 20 kHz, (d) 50 kHz. At shortest, 25 μs is observed for full 5 V scale dynamic switching.
Figure 4. Output voltage dynamics (VOUT) achieved at VDD of 2 V and under square wave input voltage pulses between 0 and 2 V at 1 kHz on (a) SiO2/p+-Si (inverter 2) and (b) glass substrate (inverter 1). Schematic 3D views of Mo-based complementary inverters are shown below corresponding plots. Although similarly thin MoTe2 and MoS2 were used for the two inverters, the complementary device on SiO2/p+-Si showed a long RC delay that might be caused by overlap capacitance between p+-Si and metal electrode patterns, which cover a large area.
behavior is apparent for both FETs; in fact, the good ohmic contact between Pt and p-channel MoTe2 was obtained even without any kind of thermal annealing, while its counterpart, the MoS2 channel, usually required postannealing for better contact with Au/Ti.39 The anneal-free ohmic contact between Pt and p-MoTe2 is quite surprising and we attribute this effect
higher off-state current (∼130 pA) than that of n-MoS2 FET. Subthreshold swing (SS) values of p- and n-FETs are estimated to be ∼255 and 221 mV/Dec, respectively, and the inferior SS value of p-FET originates from its high OFF current ID. The symmetric ID behavior was again confirmed in the output curves of p- and n-FETs in Figure 2b, for which excellent ohmic D
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Figure 5. (a) Transfer characteristics (ID−VGS) of the MoTe2 (black curve) and MoS2 (blue curve) transistors at VD = −1 and 1 V, respectively. IG exhibits gate leakage current. (b) Field effect mobility plots of MoTe2 (black curve) and MoS2 (blue curve) transistors at |VD| = 1 V. (c) Output characteristics of the p-MoTe2 and n-MoS2 nanoflake FETs show good linear behavior but appear not quite symmetric (17 μA vs 1.5 μA as maxim ID in the plot). (d) Voltage transfer characteristics (VTC) curves of inverter 3 at different supply voltages of 0.25, 0.5, and 1 V, showing almost ideal transition voltages. The right axis values indicate the voltage gain (= −dVOUT/dVIN) of the inverters which is as high as ∼33 at VDD = 1 V. (e) Near ideal noise margins, NML ≈ 0.41 VDD, NMH ≈ 0.38 VDD, were also obtained at VDD = 1 V. (f) Power consumption characteristics of our complementary inverter shows less than 4 nW as a peak at VDD of 1 V and subnanowatt power consumption of 0.4 nW at 250 mV.
to the high workfunction of our sputter-deposited Pt.38 In Figure 2c, the linear mobilities of p- and n-FETs are plotted, and their respective peak values appear at ∼19.1 and ∼18 cm2 V−1 s−1, respectively. On the basis of their individual FET properties, the voltage transfer characteristics (VTC) curves and voltage gain plots of the Mo-based complementary inverter were obtained as shown in Figure 2d, where an inverter circuit is also shown as inset (where PBG stands for patterned back gate). Maximum voltage gain of ∼22 at a VDD of 5 V and ideal positive transition voltages approaching VDD/2 are observed in Figure 2d while good noise margins (NML ≈ 0.35 VDD, NMH ≈ 0.39 VDD) close to ideal values are thus achieved (Figure 2e). Dynamic inverter switching behavior was then characterized as shown in Figure 3 panels a, b, c, and d, according to which
the time domain plots for dynamic inverter switching are displayed at 1, 10, 20, and 50 kHz, respectively. As a result, 25 μs RC delay is observed at 20 kHz from 5 V full scale square pulse input (VIN), while 50 kHz pulse reduced the output (VOUT) scale by half (∼2.5 V) although the delay appears decreased to ∼10 μs. Such RC delay is attributed to the source/ gate overlap capacitances in the p- and n-FETs of our complementary device on glass. Unlike the dynamic behavior of complementary inverter on glass (inverter 1), the other complementary device on SiO2/p+-Si (inverter 2) showed a long RC delay although it has similar thicknesses for p-channel MoTe2 and n-channel MoS2. Figure 4a and 4b display the dynamic inverter switching of the complementary device on SiO2/p+-Si and on glass (see each E
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≈ 0.35 VDD, NMH ≈ 0.39 VDD), positive transition voltage of 2.5 at 5 V VDD, and 25 μs switching delay were displayed from the first device (inverter 1) while under 1 V operation the higher gain of 33 and superior noise margin (NML ≈ 0.41 VDD, NMH ≈ 0.38 VDD) have been achieved along with a few nW of static power consumption from the next complementary inverter (inverter 3), for which we reduced n-channel MoS2 thickness to 2L. We now conclude that our complementary inverter with MoTe2 and MoS2 channels would be promising in view of future 2D nanoelectronics.
schematic 3D device view below the corresponding time domain plot for inverter 2 and 1). The switching behavior was achieved at 1 kHz in 2 V full scale for VIN. At this inverting frequency, no signature of delay was observed from inverter 1 yet, but 480 μs-long delay is observed from inverter 2. It is around 20 times longer than that (25 μs) of inverter 1. We attribute such long RC delay to the overlap capacitance between p+-Si and metal electrode patterns, which would cover the large area of Al2O3 and SiO2. The static VTC curves and FET transfer curves for inverter 2 are seen in Figure S2 along with its photo image. As the last approach of our device fabrication, we attempted to fabricate the third complementary device on glass (inverter 3), which has a 2L-thin MoS2 for n-channel FET and 3L-thin MoTe2 p-channel (see Raman data in Figure S1 b along with the inset AFM data). This approach was inspired to minimize the static power consumption of the complementary inverter because our first device (inverter 1) with 28 nm-thick MoS2 nchannel might have a quite high supply current (5 μA of peak IDD at 5 V VDD resulting in ∼25 μW; see Figure S3) although every other property was close to those of an ideal complementary inverter device. Figure 5a shows the transfer curves of p- and n-FETs fabricated on glass for our inverter 3 (see the inset OM image in Figure 5b), and the curves appear not quite symmetric at their crossing VGS near 0.5 V. The respective threshold voltages for p- and n-FET appear to be 0.2 and 0.5 V. The 3L-thin MoTe2 p-FET displays 10 times higher ON current ID over ca. −15 μA than that (∼1.4 μA) of n-FET with 2L MoS2 as shown in the output curves of Figure 5c as well. As a result, the mobility of n-FET appears as low as ∼5 cm2/(V s) in Figure 5b while that of p-FET is still quite high as ∼20 cm2/V s at −5 V of VGS. Despite such loss of ID symmetry, inverter 3 demonstrates a higher voltage gain of ∼33 along with noise margins that are quite ideal (NML ≈ 0.41 VDD, NMH ≈ 0.38 VDD) at 1 V of VDD in Figure 5d,e. To minimize the static peak power consumption, we used 2L-thin MoS2 n-channel and also utilized the low VDD values of 0.25, 0.5, and 1 V as shown in their VTC curves of Figure 5c, in which transition voltages appear to be 0.13, 0.18, and 0.5 V, respectively. (Respective voltage gain was 3.5, 9, and 33). Finally, we could obtain a minimum power consumption peak of 0.4 nW at 0.25 V of VDD and a few nW at 0.5−1 V. These results are a hundred times smaller than those (∼300 nW at 1 V VDD) from the first complementary inverter (inverter 1) with 28 nm-thick MoS2. Likewise, our hetero- and Mo-based complementary inverter with p-MoTe2 and n-MoS2 could demonstrate superior properties in dynamic switching, noise margin, voltage gain, and static power consumption, depending upon the controlled thickness of channel flakes.
METHODS For the fabrication of Mo-based complementary inverters on glass (inverter 1 and 3) and 285 nm-thick SiO2/p+-Si substrates (inverter 2), p-channel MoTe2 and n-channel MoS2 FETs with Au-patterned back gate were initially fabricated on the same substrate before coupling. First of all, the substrates were ultrasonically cleaned in acetone, methyl alcohol, and deionized water. Then 50 nm-thin Au was deposited and patterned as back gate using a DC magnetron sputtering system, followed by 50 nm-thin Al2O3 atomic layer deposition (ALD) for a high-k gate dielectric. As the next stage, we transferred MoTe2 and MoS2 nanoflakes (mechanically exfoliated from bulk crystals which are commercially available: α-MoTe2 from HQ Graphene and MoS2 from SPI Supplies) onto ALD Al2O3 dielectric by the direct imprinting technique,4,5 so that the channel flakes could be precisely aligned on a Au patterned back-gate electrode beneath Al2O3. The S/D electrodes were patterned by using a conventional photolithography process.40 Finally, 50 nm-thick Ti and Au (Au/Ti) were sequentially deposited as ohmic source/drain (S/D) contacts for MoS2 FET while100 nm-thick Pt was deposited as S/D for MoTe2 (see the Figure 1a and inset of Figure 5b). For the lift-off process, acetone and lift-off layer (LOL) remover were used. To finalize the formation of the complementary inverter, we connected the Au/Ti drain electrode of MoS2 to the Pt electrode of MoTe2 by Al wire bonding (Figure 1e). Electrical Measurements. All device characterizations were performed in the dark at room temperature using a semiconductor parameter analyzer (HP4155C, Agilent Technologies). For dynamic measurements, a function generator (AFG 310, Tektronix) and digital storage oscilloscope (TDS2014B, Tektronix) were used.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b06419. AFM and Raman spectra of MoS2 and MoTe2 to obtain thickness information, photographic and schematic 3D view of inverter 2, transfer curves, mobility plots, VTC curves, output voltage dynamics of inverter 2, Power consumption of inverter 1, and table regarding properties of published complementary and complementary-like inverters based on nanosheet transistors (PDF)
CONCLUSION In summary, we have fabricated high performance Mo-based complementary inverters on both SiO2/p+-Si and glass substrates, taking α-MoTe2 as a p-type channel for a FET and MoS2 as n-channel for the other FET. Our p-channel FET with nanosheet α-MoTe2 and Pt S/D showed very high onstate current along with excellent ohmic contact behavior, which was possible even without thermal annealing since Pt has a high-workfunction. As a result, our Mo-based complementary device on glass demonstrated high device performances in switching dynamics and electrostatic behavior; under 5 V operation a high voltage gain of ∼22, good noise margin (NML
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Author Contributions #
These authors (A.P. and S.H.H.S.) contributed equally to this work. Notes
The authors declare no competing financial interest. F
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