SUPPLEMENTARY INFORMATION doi: 1.138/nnano.21.279 Supplementary Material for Single-layer MoS 2 transistors B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis Device fabrication Our device fabrication begins with scotch-tape based micromechanical cleavage of commercially available, naturally occurring crystals of molybdenite (SPI supplies) using the method previously developed for graphene fabrication. 1 The scotch tape with ultrathin crystals is pressed against the surface of a substrate composed of degenerately doped Si with 27nm of SiO 2. The substrate is imaged using an optical microscope (Olympus BX51M) equipped with a color camera. Single layers of MoS 2 are located with respect to fiduciary markers. Monolayers can be easily identified by their optical contrast. We have previously established the correlation between the optical contrast and thickness as measured by AFM for a number of dichalcogenide materials, including MoS 2. 2 With this method, we can produce cca 1-3 single layers per area of 1cm 2. After identifying interesting monolayers we fabricate the first set of contacts using e- beam lithography and the bilayer technique. Briefly, samples are coated with MMA (8.5) MAA (6% concentration in ethyl lactate), baked at 18 C for 5 min then coated with PMMA (2% concentration in anisole) and exposed using an electron-beam lithography system. nature nanotechnology www.nature.com/naturenanotechnology 1
supplementary information doi: 1.138/nnano.21.279 After development in MIBK:IPA 1:3 solution for 3 min, we evaporate contact materials (Au, 5-1nm thick or Ti/Au, 1/5 nm thickness ) and lift-off in acetone. After fabricating >5 devices involving 1-1 layers in this manner we have established that the lowest device resistances can be obtained in the case of annealed Au contacts or Ti/Au contacts without annealing. Contact annealing In case of Au contacts, we anneal devices in a vacuum tube furnace with temperature of 2 C, 1 sccm Ar flow and 1 sccm H 2 for two hours. 3 After annealing, devices typically show a factor of 1 resistance decrease. The I ds -V bg curve for the device presented in the main manuscript is shown in Fig. S1a. a b 25 V ds = 1mV 35 Current Ids (1-9 A) 2 15 1 5 =.2 cm 2 /Vs Current Ids (1-9 A) 3 25 2 15 1 5 = 217 cm 2 /Vs V ds = 1mV -2 V bg (V) 2 4-1 -5 5 1 Back Gate Voltage V bg (V) Figure S1. a I ds V bg curve for the device presented in the main manuscript, after annealing and before ALD deposition. We can extract a mobility of.2 cm 2 /Vs b I ds - V bg curve for the same device after deposition of 3nm HfO 2 by ALD. Typical mobility extracted from two-contact measurements for single layers are in the.1-1 cm 2 /Vs. We note that these numbers represent the lower limit because of contact resistance. 2 nature nanotechnology www.nature.com/naturenanotechnology
doi: 1.138/nnano.21.279 supplementary information ALD growth of HfO 2 ALD of HfO 2 on the contacted MoS 2 monolayers was performed at 2ºC using tetrakis (dimethylamido) hafnium(iv), Hf(N(CH 3 ) 2 ) 4 (Sigma-Aldrich CAS Number:19782-68-4) and H 2 O vapor as precursors. The Hf precursor container was heated in the range from 75 to 95 C, corresponding to a vapor pressure of.5 to.27 Torr, 4 while the H 2 O container was kept at room temperature, corresponding to a vapor pressure of 2 Torr. Each cycle included 1Hf precursor pulse and 2 H 2 O vapor pulses that have been set to 1s in duration, followed by 5s purging time for Hf and 1s for H 2 O. This results in a growth rate of 1.9Å/cycles. The reactor pressure was kept at 9 mtorr and high purity nitrogen was used as carrier and purge gas. AFM images of the device presented in the manuscript before and after ALD growth is shown on Fig. S2. Figure S2. Atomic force microscope images of the device presented in the manuscript before (left) and after ALD growth of 3nm of HfO 2. After ALD growth all our devices show a dramatic improvement in mobility, sometimes by more than three orders of magnitude, Fig. S1b. We also note that due to the small thickness of HfO 2 compared to SiO 2, the capacitive coupling between the back-gate the the MoS 2 channel is not likely to significantly change due to HfO 2 deposition. nature nanotechnology www.nature.com/naturenanotechnology 3
supplementary information doi: 1.138/nnano.21.279 Local gates After the HfO 2 deposition by ALD, local gates are fabricated using another electron beam lithography step followed by deposition of Cr/Au or Ti/Au. On most of the devices we find negligible gate leakage currents, less than 2pA/µm 2 in the ±3V voltage range. Leakage currents through the top and bottom gate for the top gate sweep shown on Fig 3. in the manuscript are shown here on Fig. S3. a 2. b 3 Top Gate Leakage I tg (1-12 A) 1.5 1..5. -.5 Bottom Gate Leakavge I bg (1-12 A) 2 1-1 -2-1. -3-2 -1 1 2 3-3 -3-2 -1 1 2 3 Top Gate Voltage V tg (V) Top Gate Voltage V tg (V) Figure S3. Top and bottom gate leakage during the top gate voltage sweep from the main manuscript. Even though the device presented in the main manuscript was fabricated using Au electrodes, we have also reproduced similar high mobility and current on/off ratio in device with Ti/Au contacts, shown here on Fig S4. 4 nature nanotechnology www.nature.com/naturenanotechnology
doi: 1.138/nnano.21.279 supplementary information a I ds (A) 1-7 1-8 1-9 1-1 1-11 1-12 1-13 1-14 1-15 V ds =1mV V bg = I On /I Off ~ 5.6 1 6-1. -.5..5 1. V tg (V) b I ds (1-9 A) 6 4 2-2 -4 V tg =-.4 V V tg =.4 V tg =.8 V tg =1.2 V tg =1.6 V bg = -6-4 -2 2 4 V ds (1-3 V) Figure S4. Single-layer device with Ti/Au contacts. a I ds-v tg characteristics. b I ds-v ds curves for different top gate voltages. Characteristics of other functioning devices are summarized in the following table. Device N Mobility label layers Contacts ALD growth (cm 2 /Vs) I On /I Off 73-5i 5-7 1 Au July 28, 21 78 6 1 6 73-5j 1-16 1 Au July 28, 21 78 >15 73-5j 14-16 719-11-1 1-2 719-21-1 1-2 1 Au July 28, 21 313 >47 1 Ti/Au July 3, 21 47 5.6 1 6 4-5 Ti/Au July 3, 21 571 > 3 1 6 1971 1 Ti/Au July 3,21 983 3.3 1 6 nature nanotechnology www.nature.com/naturenanotechnology 5
supplementary information doi: 1.138/nnano.21.279 References 1 Novoselov, K. S. et al. Electric Field Effect in Atomically Thin Carbon Films. Science 36, 666-669 (24). 2 Benameur, M., Radisavljevic, B., Sahoo, S., Berger, H. & Kis, A. Visibility of dichalcogenide nanolayers. Cond-mat, 16.148 (21). 3 Ishigami, M., Chen, J. H., Cullen, W. G., Fuhrer, M. S. & Williams, E. D. Atomic Structure of Graphene on SiO2. Nano Lett. 7, 1643-1648, doi:153-6984 (27). 4 Hausmann, D. M., Kim, E., Becker, J. & Gordon, R. G. Atomic layer deposition of hafnium and zirconium oxides using metal amide precursors. Chem. Mat. 14, 435-4358, doi:doi 1.121/Cm2357x (22). 6 nature nanotechnology www.nature.com/naturenanotechnology