Supporting Information Direct n- to p-type Channel Conversion in Monolayer/Few-Layer WS 2 Field-Effect Transistors by Atomic Nitrogen Treatment Baoshan Tang 1,2,, Zhi Gen Yu 3,, Li Huang 4, Jianwei Chai 1, Swee Liang Wong 1, Jie Deng 1, Weifeng Yang 1, *, Hao Gong 2, *, Shijie Wang 1, *, Kah-Wee Ang 4, Yong-Wei Zhang 3, and Dongzhi Chi 1, * 1 Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, Singapore 138634, Singapore, 2 Department of Materials Science and Engineering, National University of Singapore, Singapore 117576, Singapore, 3 Institute of High Performance Computing, A*STAR (Agency for Science, Technology and Research), 1 Fusionopolis Way, Connexis North, Singapore 138632, Singapore, and 4 Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore. Corresponding Author These authors contributed equally to this work. * Address correspondence to: yangw@imre.a-star.edu.sg; msegongh@nus.edu.sg; sj-wang@imre.a-star.edu.sg; dz-chi@imre.a-star.edu.sg
Figure S1. Schematic diagram of the combined system capable of sputtering deposition and in-situ nitrogen doping treatment. The design of the system allows for the doping process without breaking vacuum. Figure S2. Characterizations of the 2D WS 2 grown by sputtering deposition. Raman of pure WS 2 1 samples on SiO 2 /Si substrate. The two characteristic Raman modes (E 2g and A 1g ) of WS 2 with various thickness are labelled.
Figure S3. Raman spectra collected from five random spots of monolayer N-WS2. The inset is the photography of monolayer N-WS2 sample on SiO2/Si substrate. The film shows high uniformity over a large area. The scale bar is 1 cm. Figure S4. HRTEM images of exfoliated WS2 flakes on TEM grid before (a, b) and after (c, d) nitrogen doping, indicating that no obvious lattice distortion was present in the WS2 after nitrogen doping. Atomic nitrogen treatment of the WS2 FETs was performed at the RF power of 400 W for 30 minutes.
Figure S5. Investigation of long-term stability in monolayer N-WS 2 via XPS. The N-WS 2 sample was left in dry box after a time span of 1 month. The XPS results show no change in nitrogen concentration (~1.8 %), indicating that the high stability of nitrogen in WS 2. The N 1s spectral of monolayer WS 2 is presented as reference. Figure S6. Investigation of thermal stability in monolayer N-WS 2 via XPS. (a) N 1s spectra after post annealing of N-WS 2 at high temperature for one hour. (b) N 1s spectra of N-WS 2 post annealed
at 500 C for different duration. The XPS results show gradual reduction in nitrogen concentration after thermal annealing above 600 C. Nitrogen concentration remains almost unchanged at 500 C for 90 min, indicating the high thermal stability of nitrogen in WS 2. N-related peaks maintain the same positions (~397.8 ev, related to N-W bonds), indicative of the stable substitution of S by N in the WS 2 host lattice. Figure S7. AFM image and line profile (inset) of (a) monolayer WS 2, (b) few-layer WS 2, and (c) multilayers WS 2. Figure S8. Electrical properties of monolayer and few-layer WS 2 and N-WS 2 FETs. Re-plot (linear scale) of drain current vs. gate voltage transfer characteristics at a fixed drain voltage of 1 V for the (a) monolayer and (b) few-layer WS 2 and N-WS 2 FETs.
Figure S9. Electrical properties of multilayer WS 2 and N-WS 2 FETs. Drain current vs. gate voltage transfer characteristics at drain voltages of 0.1, 0.5 and 1 V for the multilayer WS 2 FET with gate voltage sweep from 100 V to -100 V (a) before and (b) after the atomic nitrogen treatment. The inset of (a) is the optical image of WS 2 FET with a channel width of 22.4 µm and length of 16.3 µm, scale bar is 10 µm. For the multilayer WS 2 FET which exhibits a bipolar transport characteristics with a much higher (by ~ 10 3 ) ON-currents in n-fet mode than that in p-fet mode, a significant boost (by ~ 10 2 ) of ON-currents under p-fet operation is obtained after atomic nitrogen treatment while the ON-currents under n-fet operation remain almost the same. Also, we obtained an electron mobility of 41.63 cm 2 /Vs for multilayer WS 2, while a hole mobility of 7.27 cm 2 /Vs for multilayer N-WS 2 at a fixed drain voltage of 1 V.
Figure S10. Hysteresis loops in transfer characteristics of (a) monolayer (1L), (b) few-layer (FL), and (c) multilayer (ML) WS 2 FETs before (top) and after (bottom) and nitridation. Sweep direction for (a) is colored blue for forward sweep and red for backward sweep while for (b) and (c), the sequence begins with blue, followed by red and ends with black. Figure S11. Atomic configurations of possible N embedding sites in monolayer WS 2. (a) Substitutional N site at S site (N S ). (b) Substitutional N at 2S sites (N 2S ). (c) N adatom above S atom (N T S). (d) N adatom above W atoms (N T W). (e) Substitutional N at W site (N W ). The yellow, gray and dark balls represent S, N and W atoms, respectively.
Figure S12. The calculated total density of states (TDOS) and projected DOS (PDOS). (a) Pristine, (b) 1N embedded, (c) 2N embedded, (d) 3N doped monolayer WS 2.