(Invited) The Future of Nanoelectronics Devices from a Theoretical Point of View: The Search for New Channel Materials
Authors: Fischetti Massimo , Gaddemane Gautam , Gopalan Sanjay , Van de Put Maarten Ludo , Vandenberghe William 
Published: September 2023

Scaling electronic devices to the nanometer scale requires, among other constrains, a reduction of the thickness of the channel. This results in quantum confinement of the carriers which, in turn, causes an unavoidable degradation of the charge-transport properties. The advent of graphene seems to have opened a new avenue to circumvent the problem: The extremely high carrier mobility in such an atomically thin layer has caused an explosion of interest on other atomically-thin (“two-dimensional”, 2D) materials that, hopefully, may exhibit excellent transport-properties even when scaled to sub-nm thickness and replace Si in ULSI devices.

Since many of the 2D materials that have been considered are poorly known (even their existence is often questionable), theoretical studies of their electronic and charge-transport properties have become necessary to select, among the huge number of possible candidate materials, those that may exhibit the desired properties.

In this presentation, we will summarize theoretical work performed in our group regarding the search for these ‘new channel materials’.

We will first discuss how the outstanding properties of graphene are hard to replicate, since they originate from its Dirac-like dispersion and, even in other non-σh-symmetric Dirac-like materials, such as silicene or germanene, the interaction of electrons with acoustic flexural vibrations (the so-called “ZA phonons) may destroy such outstanding properties1, as shown in Fig. 1 in the case of silicene.

Next, taking phosphorene as an example, we will discuss how the ab initio methods (density functional theory, DFT) used to study these materials may provide a useful guideline but are still affected by severe uncertainties.2 In fact, theoretical predictions (namely, the carrier mobility) may depend strongly on different choices of pseudopotentials and exchange-correlation functionals employed when using DFT. This is an unfortunate situation dictated by the fact that transport calculations are sensitive to small changes of the band structures (of the order of the thermal energy, for example, when dealing with the energy of satellite valleys and intervalley scattering), a level of accuracy that remains beyond what DFT can offer. Table I shows the situation for transition metal dichalcogenides (TMD) monolayers.

Next, considering now TMD monolayers, we will consider how their charge-transport properties are affected by the ‘dielectric environment’ (insulating substrate, gate insulator, metallic gates).3 Indeed, electronic transport in two-dimensional (2D) materials is often studied theoretically assuming free-standing (mono)layers. However, in practical applications the monolayers are supported by an insulator and, often, top-gated. In this case, insulators with a high dielectric constant (‘high-κ insulators’) screen the interaction between charge carriers and phonons, resulting in a higher carrier mobility. However, this beneficial effect is negated by the additional scattering with interface modes resulting from the coupling of the insulator optical phonons and the 2D electron gas of the monolayer (2D plasmons), the so-called ‘remote phonon scattering’ (Table II). We shall finally present results obtained from Monte Carlo simulations of field-effect transistors employing various transition metal dichalcogenides (TMDs) as channel materials and discuss the role played by dielectric screening and remote-phonon scattering for a variety of insulators and TMDs.

A look at Table II shows that WeS2 appears is the most promising material, especially in complementary MOS applications. However, we should keep in mind that the phonon-limited electron mobility in Si slabs of a dielectric thickness comparable to TMD monolayers (approximately 1.2 nm) is about 500 cm2V−1s−1, being depressed by scattering with surface roughness. Thus, in a fair comparison among Si and many TMD monolayers, the advantage afforded by the latter 2D materials appears to be minimal. We shall conclude by discussing the possible broader cause of this conclusion4.

Massimo V. Fischetti and William G. Vandenberghe, Phys. Rev. B 93, 155413 (2016).

G. Gaddemane et al., Phys. Rev. B 98, 115416 (2018).

S. Gopalan et al., Phys. Rev. Appl. 18, 054062 (2022).

L. Cheng, C. Zhang, and Y. Liu, Phys. Rev. Lett. 125, 177701 (2020).

Figure 1