Comment on “Metal Semiconductor FieldEffect Transistor with MoS2/Conducting NiOx van der Waals Schottky Interface for Intrinsic High Mobility and Photoswitching Speed” Zhenyu Yang, Jingli Wang, and Lei Liao* Department of Physics and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University, Wuhan 430072, China n a recent paper, Lee et al.1 reported that the MoS2-based metal semiconductor field-effect transistors (MESFETs) with a few-layer and ∼10 layer MoS2 have very large mobility of 500−1200 cm2/(V s) at room temperature, by using the NiOx Schottky electrode, which makes van der Waals interface with MoS2. The mobility (μ) is calculated with the following equation,1 μ = Lgm/(eN3dtW),where gm is the transconductance, e is an electronic charge, N3d is the carrier density (cm−3) in the three-dimensional space, and t, W, and L are the thickness, width, and length of the channel, respectively. Because Lee et al. measured the carrier density (N2d) in the two-dimensional space as 4.03 × 1010 cm−2 at 300 K by using Hall measurements, so the μ of these MoS2 MESFETs is very large.1 However, we consider this value of N2d to be not accurate due to the following three reasons. First, the value of N2d (at Vg = 0 V) measured by other groups is 1012 cm−2 rather than 1010 cm−2 orders of magnitude, regardless whether at room temperature or at low temperature.2−5 In the experiment performed by Lee et al.,1 if the N2d of the MoS2 is around 1012 cm−2, the corresponding mobility of MoS2 should be 20−50 cm2/(V s). Second, there is a typical equation between the current I and carrier concentration N3d, as follows:
12.5 cm2/(V s) in the paper by Lee et al.1 This result is contradictory, so there is a mistake in the value of N2d. Moreover, according to the paper of Ye et al.,6 if the value of N2d is 4.03 × 1010 cm−2, the mobility of the device (Lch = 2 μm) can also be calculated to be 3000 cm2/(V s) by using eq 3 when the value of the gate voltage is 0 V, but the mobility of this device is only 28 cm2/(V s) in the paper of Ye et al.6 The two results are contradictory again, so it proved again that the value of N2d should be wrong. According to the paper of Zou et al.,7 although the current density is about 20 μA/μm and it is almost 100 times greater than the current density (about 0.2 μA/μm) of Lee et al.1 when Vds is 1 V and Vg is 0 V, we can find that the μ of the device is only 63.7 cm2/(V s). It is much smaller than the μ of Lee et al.1 It is an abnormal phenomenon that the current density is not big enough, while the μ is so big at room temperature in the paper of Lee et al.1 Third, we can calculate that the N2d at the MOSFET, according to the following formula3
I
I = N3deSν
N2d =
Vds L
(2)
where Vds is the voltage between source and drain and L is the channel length. Using eq 1 and 2, we can obtain μ μ=
IL eN2dWVds
(3)
where W is the channel width. In the experiment of Lee et al.,1 they made a four-layer MoS2 MISFET with a 50 nm Al2O3 top gate insulator (in ref 1; W/L = 10 μm/5 μm). We can see that the current (Ids) is about 5 × 10−5 A when the Vbg is 0 V. So if the value of N2d is 4.03 × 1010 cm−2, we can calculate the μ by using eq 3, and then the μ is 3872.33 cm2/(V s). However, the mobility of the device is only © XXXX American Chemical Society
e
(4)
where Cox = ε0εr/dox, Vbg,th is the value of the threshold voltage and is close to its pinch-off voltage estimated from the linear Id−Vg curves. According to the work of Li et al.,8 they have made a back-gate field-effect transistor with 100 nm channel lengths on a 90 nm SiO2, and we can probably get a threshold voltage of −20 V with Vds = 1 V at 300 K from their graph. Using formula 4, we can calculate that the N2d is equal to 4.79 × 1012 cm−2 when Vbg is 0 V. The N2d of the carrier concentration is also more than the 1010 cm−2 orders of magnitude.1 In summary, the N 2d values at 300 K using Hall measurements may be inaccurate in this paper.1 We believe that the value of Ids and gm are as important as the μ in the fieldeffect transistor. Different methods need to be used to make sure that the μ is right when the μ is abnormally large. The true electronic performance of MoS2 is very important for future research and application.
(1)
where S is the cross-sectional area of channel material and ν is the carrier drift velocity. The ν can be obtained from the electric field (E) and the mobility (μ), so ν can be given by
ν=μ
Cox(Vbg − Vbg,th)
Received: November 9, 2015
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DOI: 10.1021/acsnano.5b07083 ACS Nano XXXX, XXX, XXX−XXX
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AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
REFERENCES (1) Lee, H. S.; Baik, S. S.; Lee, K.; Min, S. W.; Jeon, P. J.; Kim, J. S.; Choi, K.; Choi, H. J.; Kim, J. H.; Im, S. Metal Semiconductor FieldEffect Transistor with MoS2/Conducting NiOx van der Waals Schottky Interface for Intrinsic High Mobility and Photoswitching Speed. ACS Nano 2015, 9, 8312−8320. (2) Neal, A. T.; Liu, H.; Gu, J. J.; Ye, P. D. Magneto-Transport in MoS2: Phase Coherence, Spin-Orbit Scattering, and the Hall Factor. ACS Nano 2013, 7, 7077−7082. (3) Radisavljevic, B.; Kis, A. Mobility Engineering and a MetalInsulator Transition in Monolayer MoS2. Nat. Mater. 2013, 12, 815− 820. (4) Cui, X.; Lee, G. H.; Kim, Y. D.; Arefe, G.; Huang, P. Y.; Lee, C. H.; Chenet, D. A.; Zhang, X.; Wang, L.; Ye, F.; et al. Multi-terminal Transport Measurements of MoS 2 using a van der Waals Heterostructure Device Platform. Nat. Nanotechnol. 2015, 10, 534− 540. (5) Baugher, B. W. H.; Churchill, H. O. H.; Yang, Y. F.; JarilloHerrero, P. Intrinsic Electronic Transport Properties of High-quality Monolayer and Bilayer MoS2. Nano Lett. 2013, 13, 4212−4216. (6) Liu, H.; Neal, A. T.; Ye, P. D. Channel Length Scaling of MoS2 MOSFETs. ACS Nano 2012, 6, 8563−8569. (7) Zou, X. M.; Wang, J. L.; Chiu, C. H.; Wu, Y.; Xiao, X. H.; Jiang, C. Z.; Wu, W. W.; Mai, L. Q.; Chen, T. S.; Li, J. C.; et al. Interface Engineering for High-performance Top-gated MoS2 Field-effect Transistors. Adv. Mater. 2014, 26, 6255−6261. (8) Li, X. F.; Yang, L. M.; Si, M. W.; Li, S. C.; Huang, M. Q.; Ye, P. D.; Wu, Y. Q. Performance Potential and Limit of MoS2 Transistors. Adv. Mater. 2015, 27, 1547−1552.
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DOI: 10.1021/acsnano.5b07083 ACS Nano XXXX, XXX, XXX−XXX
