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

Abstract

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 (Vdd) 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 Vdd 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-Vdd 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-Vdd operation. In this study, a hybrid self-assembled monolayer (SAM)/aluminum oxide (AlOx) gate dielectric is applied to a WSe2 CMOS inverter for high-gain low-Vdd 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/AlOx 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 Id–Vg 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 Id–Vg 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 Vdd in CMOS inverter. The gain of the CMOS inverter increased to as high as 9 at Vdd of 0.5 V.Figure 3 shows the benchmark of this study. Our proposed concepts enabled the operation with high gain at a low Vdd 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/AlOx gate dielectric has an important role in the low-Vdd operation of the WSe2 CMOS inverter. This study opens up interesting directions for the research and development of TMDC-based devices and circuits.