Study of the 1D Scattering Mechanisms' Impact on the Mobility in Si Nanowire Transistors

Abstract

The extensive research of aggressively scaled nanoelectronic devices necessitates the inclusion of quantum confinement effects and their impact on performance. This work implements a set of multisubband phonon and impurity scattering mechanisms and the Kubo-Greenwood theory in order to study their impact on the mobility in Si nanowire transistors (NWTs). This 1D treatment has been coupled with a 3D Poisson-2D Schrödinger solver, which accurately captures the effect of confinement on charge dynamics. We also emphasize the importance of using the 1D models to evaluate the geometrical properties on mobility at the scaling limit.

Impact of the Effective Mass on the Mobility in Si Nanowire Transistors

“…The assumption herein considered for the mobility calculation, using the relaxation time approach, is based on the long-channel simulation model. Its capabilities have already been shown by studying the impact of the different scattering mechanisms and the cross-section dimensions on the NWT performance [7]- [8]. The innovation in this work is the precalculation of the effective masses (m eff ) from atomistic simulations, instead of using the bulk (m bulk ) ones.…”

“…The models for electron interactions with phonons (acoustic and optical) and impurities have been already described in Ref. [7], whereas the inclusion of the surface roughness scattering mechanism is the second innovation in this work. It plays an important role especially at high charge densities in realistic devices with strong confinement effects, where the electrons remain close to the non-ideal surfaces.…”

Mobility of Circular and Elliptical Si Nanowire Transistors Using a Multi-Subband 1D Formalism

“…Supplemental simulations with the ratio ranging from 1 to 2 (not shown) suggest that 1.5 presents the optimum ratio. We have taken into account this diameter range due to the well-established transition from fully-quantum behavior at 3 nm with strong confinement impact [16], and large effective mass deviation [17]; to near bulk-like electronic behavior at 8 nm [18]. The rest of the technological parameters remains identical: the gate oxide has an equivalent oxide thickness (EOT) of 0.8 nm, the metal gate work function is set to 4.35eV, and room temperature (300K) is assumed.…”

“…In order to accurately reproduce the quantum behavior for diameters smaller than 8 nm, we have extracted [19] the transport and the confinement effective masses from an empirical sp 3 d 5 s * tight-binding simulations with the Boykin's parameter set [20] implemented in QuantumATK from Synopsys [21]. Furthermore, as the surface roughness scattering mechanism dominates the mobility for very high sheet concentrations, all the mobility results are reported at the medium carrier concentration 2.8 × 10 12 cm −2 [16], [17], [22]. The gate bias is adjusted to obtain the sheet concentration (cm −2 ) being computed as the line density (cm −1 ) divided by the total perimeter.…”

“…Second, the different scattering rates, which expressions have been directly developed from the Fermi Golden rule accounting for the multi-subband quantization in the confinement plane, are calculated. The following scattering mechanisms have been used in this paper (more details of these mechanisms as well as their equations can be found in [16], [17], [22]): (i) acoustic phonon (Ac Ph) scattering; (ii) optical phonon (Op Ph) scattering, with fixed parameters for the different branches; (iii) surface roughness (SR) scattering, with root mean square (∆ RM S ) and correlation lengths 0.48 nm and 1.3 nm, respectively; and (iv) ionized impurity (II) scattering, with a constant effective ionized impurity concentration N I =10 18 cm −3 .…”