Electronic transport properties of hydrogenated and fluorinated graphene: a computational study.

Hydrogenation and fluorination have been presented as two possible methods to open a bandgap in graphene, required for field-effect transistor applications. In this work, we present a detailed study of the phonon-limited mobility of electrons and holes in hydrogenated graphene (graphane) and fluorinated graphene (graphene fluoride). We pay special attention to the out-of-plane acoustic (ZA) phonons, responsible for the highest scattering rates in graphane and graphene fluoride. Considering the most adverse cut-off for long-wavelength ZA phonons, we have obtained electron (hole) mobilities of 28 (41) cm2 V-1 s-1 for graphane and 96 (30) cm2 V-1 s-1 for graphene fluoride. Nonetheless, for a more favorable cut-off wavelength of ∼2.6 nm, significantly higher electron (hole) mobilities of 233 (389) cm2 V-1 s-1 for graphane and 460 (105) cm2 V-1 s-1 for graphene fluoride are achieved. Moreover, while complete suppression of ZA phonons can increase the electron (hole) mobility in graphane up to 278 (391) cm2 V-1 s-1, it does not affect the carrier mobilities in graphene fluoride. Velocity-field characteristics reveal that the electron velocity in graphane saturates at an electric field of ∼4 × 105 V cm-1. Comparing the mobilities with other two-dimensional (2D) semiconductors, we find that hydrogenation and fluorination are two promising avenues to realize a 2D semiconductor while providing good carrier mobilities.

Monte Carlo Study of Electronic Transport in Monolayer InSe.

The absence of a band gap in graphene makes it of minor interest for field-effect transistors. Layered metal chalcogenides have shown great potential in device applications thanks to their wide bandgap and high carrier mobility. Interestingly, in the ever-growing library of two-dimensional (2D) materials, monolayer InSe appears as one of the new promising candidates, although still in the initial stage of theoretical studies. Here, we present a theoretical study of this material using density functional theory (DFT) to determine the electronic band structure as well as the phonon spectrum and electron-phonon matrix elements. The electron-phonon scattering rates are obtained using Fermi's Golden Rule and are used in a full-band Monte Carlo computer program to solve the Boltzmann transport equation (BTE) to evaluate the intrinsic low-field mobility and velocity-field characteristic. The electron-phonon matrix elements, accounting for both long- and short-range interactions, are considered to study the contributions of different scattering mechanisms. Since monolayer InSe is a polar piezoelectric material, scattering with optical phonons is dominated by the long-range interaction with longitudinal optical (LO) phonons while scattering with acoustic phonons is dominated by piezoelectric scattering with the longitudinal (LA) branch at room temperature (T = 300 K) due to a lack of a center of inversion symmetry in monolayer InSe. The low-field electron mobility, calculated considering all electron-phonon interactions, is found to be 110 cm2V-1s-1, whereas values of 188 cm2V-1s-1 and 365 cm2V-1s-1 are obtained considering the long-range and short-range interactions separately. Therefore, the calculated electron mobility of monolayer InSe seems to be competitive with other previously studied 2D materials and the piezoelectric properties of monolayer InSe make it a suitable material for a wide range of applications in next generation nanoelectronics.

Electronic Transport Properties of Silicane Determined from First Principles.

Silicane, a hydrogenated monolayer of hexagonal silicon, is a candidate material for future complementary metal-oxide-semiconductor technology. We determined the phonon-limited mobility and the velocity-field characteristics for electrons and holes in silicane from first principles, relying on density functional theory. Transport calculations were performed using a full-band Monte Carlo scheme. Scattering rates were determined from interpolated electron-phonon matrix elements determined from density functional perturbation theory. We found that the main source of scattering for electrons and holes was the ZA phonons. Different cut-off wavelengths ranging from 0.58 nm to 16 nm were used to study the possible suppression of the out-of-plane acoustic (ZA) phonons. The low-field mobility of electrons (holes) was obtained as 5 (10) cm2/(Vs) with a long wavelength ZA phonon cut-off of 16 nm. We showed that higher electron (hole) mobilities of 24 (101) cm2/(Vs) can be achieved with a cut-off wavelength of 4 nm, while completely suppressing ZA phonons results in an even higher electron (hole) mobility of 53 (109) cm2/(Vs). Velocity-field characteristics showed velocity saturation at 3 × 105 V/cm, and negative differential mobility was observed at larger fields. The silicane mobility was competitive with other two-dimensional materials, such as transition-metal dichalcogenides or phosphorene, predicted using similar full-band Monte Carlo calculations. Therefore, silicon in its most extremely scaled form remains a competitive material for future nanoscale transistor technology, provided scattering with out-of-plane acoustic phonons could be suppressed.