<em>(Invited) </em>Low Voltage Operation of CMOS Inverter Based on WSe<sub>2</sub> n/p FETs

Kawanago, Takamasa

Transition metal dichalcogenides (TMDC) have remarkable properties for the next generation of electronic devices [1]. A complementary metal-oxide-semiconductor (CMOS) inverter consisting of pairs of p-type and n-type field-effect transistor (FET) is a fundamental building block in modern digital electronics [2]. Therefore, realization of both p-type and n-type FETs based on semiconducting TMDC is of particular importance. Among various semiconducting TMDC, tungsten diselenide (WSe₂) is a promising candidate for constructing a CMOS inverter because of its high mobility, symmetric electron and hole effective mass and ambipolar transport [3]. These prominent features of WSe₂ motivates the fabrication and characterization of a CMOS inverter. One of the significant challenges is to develop a high-gain CMOS inverter for operation at a low power supply voltage (V_dd) for future low power digital electronics. Previous studies on a WSe₂ CMOS inverter were limited to operation with a low gain of 3 or a high V_dd of more than 1 V under conditions of relatively low gate capacitance and degraded interfacial properties [3-5]. This study addressed these issues by developing a doping technique and gate stack technology to demonstrate the high-gain with low-V_dd operation of a WSe₂ CMOS inverter [6]. The focus is on the doping technique of both electrons and holes to a WSe₂ layer by simple spin-coating. Of particular interest in this study is the impact of gate stack technology on the CMOS inverter characteristics in low-V_dd operation. In this study, a hybrid self-assembled monolayer (SAM)/aluminum oxide (AlO_x) gate dielectric is applied to a WSe₂ CMOS inverter for high-gain low-V_dd operation since superior interfacial properties with high gate capacitance have been reported in molybdenum disulfide (MoS₂) FETs.

For p-type WSe₂ FETs, the fluoropolymer CYTOP (AGC, CTX-809A) was applied since polarization in CYTOP due to the difference in electronegativity between C–F bonds generates holes in WSe₂. On the other hand, poly(vinyl alcohol) (PVA) was utilized for n-type WSe₂ FETs because positive charges in PVA accumulates electrons in WSe₂. The resin solutions of both CYTOP and PVA were purchased from a commercial supplier. A heavily doped p-type silicon substrate (p⁺-Si) was thermally oxidized to form a 60 nm thick silicon dioxide (SiO₂) layer for the global back-gate architecture. Palladium (Pd) and gold (Au) as the source and drain contacts for p-type and n-type WSe₂ FETs were fabricated by the lift-off process, respectively. An aluminum (Al) gate was prepared and a hybrid SAM/AlO_x gate dielectric was formed by oxygen (O₂) plasma and an immersion process. After that, mechanically exfoliated WSe₂ was transferred with a poly(dimethylsiloxane) (PDMS) stamp. Finally, CYTOP and PVA were deposited by spin-coating for p-type and n-type WSe₂ FETs, respectively.

Figures 1 (a) and (b) show the I_d–V_g characteristics of fabricated FETs. For the p-type WSe₂ FET, a small SS of 78 mV/dec and an on/off ratio of about 10⁶ order were observed. On the other hand, the I_d–V_g characteristics of the n-type WSe₂ FET were degraded compared with those of the p-type WSe₂ FET because of the Schottky contacts.

Figures 2 (a) and (b) show the transfer characteristics and gain as a function of V_dd in CMOS inverter. The gain of the CMOS inverter increased to as high as 9 at V_dd of 0.5 V.

Figure 3 shows the benchmark of this study. Our proposed concepts enabled the operation with high gain at a low V_dd as low as 0.5 V for the WSe₂ CMOS inverter, which had not been realized in previous studies. The dangling-bond-free and ultrathin SAM/AlO_x gate dielectric has an important role in the low-V_dd operation of the WSe₂ CMOS inverter. This study opens up interesting directions for the research and development of TMDC-based devices and circuits.

References

[1] Q. H. Wang, K. K. Zadeh, A. Kis, J. N. Coleman, and M. S. Strano, Nat. Nanotechnol. 7, 699 (2012).

[2] Y. Taur and T. H. Ning, Fundamentals of Modern VLSI Devices (Cambridge University Press, Cambridge, 1998).

[3] L. Yu, A. Zubair, E. J. G. Santos, X. Zhang, Y. Lin, Y. Zhang, and T. Palacios, Nano Lett. 15, 4928 (2015).

[4] C.-S. Pang and Z. Chen, 2018 76th Device Research Conf. (DRC), p. 1, 2018.

[5] M. Tosun, S. Chuang, H. Fang, A. B. Sachid, M. Hettick, Y. Lin, Y. Zeng, and A. Javey, ACS Nano 8, 4948 (2014).

[6] T. Kawanago, T. Matsuzaki, R. Kajikawa, I. Muneta, T. Hoshii, K. Kakushima, K. Tsutsui, and H. Wakabayashi, Jpn. J. Appl. Phys. 61, SC1004 (2022).

(Invited) Low Voltage Operation of CMOS Inverter Based on WSe2 n/p FETs

(Invited) Low Voltage Operation of CMOS Inverter Based on WSe₂ n/p FETs