Introduction Nowadays, the scaling of silicon very large-scale integration(VLSI) technologies has reached channel thicknesses of 7nm. However, further downscaling becomes increasingly difficult as the gate electrostatics require a substantial reduction of the channel thickness to about a fourth of the gate length. As a result, the actual gate length of high-performance(HP) FETs at the current state-of-the-art amounts to 16nm and is projected to stall at 12nm for the 1.5nm node and the following nodes, according to the International Roadmap for Devices and Systems(IRDS)[1]. For these nodes, two-dimensional(2D) materials could have the potential to replace silicon as the channel material. They can maintain sizable mobilities at atomic thicknesses, thus providing enhanced gate control in stacked channel nanosheet transistor structures[2]. Even though FETs based on 2D materials hold the promise to serve as FETs at the front end of the line, numerous challenges still need to be overcome. Downscaling Dimensions For ultra-scaled device designs, 2D materials can be used as a channel in stacked nanosheet FETs. In such a structure, the 2D material is gated from both sides, as depicted in Fig.1(a). By stacking multiple double-gated channels on top of each other, the required on-current density of around 1mA/μm can be achieved[3]. The first experimental demonstrations have achieved sub-5nm gate lengths[4] or, in a different design, a gate pitch of only 42nm[5], meeting the requirements for the 1.5nm node. While these prototypes show that 2D FETs can be scaled down, they have been fabricated in a laboratory. Many challenges must be overcome before such small devices can be fabricated in industry-compatible batch processes which require a high yield and low variability. Contact Engineering In order to enable sufficiently high on-current densities, small contact resistances to 2D semiconductors are required, which are inhibited by the formation of Schottky barriers at the metal to semiconductor interface. Schottky contacts to 2D materials are so prevalent because there is, on one side, a lack of stable doping schemes, and on the other side, pronounced Fermi-level pinning at the metal-to-semiconductor interface. Semi-metallic bismuth contacts can provide low-resistive contacts to n-type MoS2 FETs, as the bismuth suppresses the metal-induced gap states which pin the Fermi-level[6]. This way, a small contact resistance of 123Ωμm has been achieved, reaching the IRDS limit for the 1.5nm node. While comparable results have been realized for antimony contacts to n-type FETs, low-resistive contacts to p-type FETs are more challenging. Gate Stack Design A good gate stack for 2D FETs must provide excellent gate control while maintaining small gate leakage currents, which requires a capacitive equivalent thickness(CET) smaller than 0.9 nm. At these small CETs, a small gate leakage can only be obtained with a high-dielectric constant gate insulator and sizable band offsets, rendering some insulators, e.g. hexagonal boron nitride, unsuitable. Simultaneously, the gate insulator should form a van-der Waals interface with the 2D semiconductor, as otherwise, interface traps degrade the carrier mobility and the sub-threshold slope[7]. Charge traps within the insulator are the root cause of the limited stability and reliability of 2D FETs, as seen by a large hysteresis in the transfer characteristics or in pronounced Bias Temperature Instabilities(BTI). The electrical stability can be improved, if the defect bands in the gate insulator are energetically far away from the semiconductor’s conduction and valence band edges, see Fig.1(b)[8]. Conclusions Despite all the progress achieved within the last decade, the challenges for VLSI integration of 2D materials are enormous. First, stacked 2D FETs with scaled dimensions still need to be realized at an industrial scale. Second, despite low contact resistance demonstrations for n-type FETs, the resistances for p-type FETs are one order of magnitude too high. Finally, one of the most serious obstacles is identifying a suitable gate stack. Even though promising combinations of insulators to semiconductors have been suggested, like Bi2O5Se/Bi2O2Se, CaF2/MoS2 or SrTiO3/MoS2, it is at the moment unclear, which of them provides the best performance. Acknowledgments The authors thank for funding from the European Research Council under grant agreement no. 101055379.
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Nov 8, 2024
Jiulin Li 
Open Access
