Multiscale ensemble Monte Carlo of transport and gas sensing in monolayer MoS 2

Two-dimensional semiconductors, such as monolayer MoS 2 , combine a technologically useful band gap with a thickness of three atomic planes. The entire carrier density then lies within a few Å of the surface, which makes them useful as field-effect-transistor channels and as chemically sensitive devices. Device design for MoS 2 needs a transport model whose scattering rates are computed rather than calibrated, whose predictions are tested against experiment, and that spans the scales from electron-phonon coupling to terminal current and sensor response. We present a self-consistent ensemble Monte Carlo (EMC) framework for monolayer MoS 2 built on the ViennaEMC solver, which couples a Boltzmann transport model to Poisson's equation with degenerate free-carrier statistics. The transport model carries wavevector-dependent acoustic, polar-optical, homopolar and intervalley phonon scattering within and between the K valleys and the spin-orbit-split Q valleys, together with remote substrate phonons, screened charged impurities and surface roughness. Every intrinsic input of the transport model is computed from first principles for this work, namely the band structure of the K and Q valleys, the phonon energies, sound velocities and dielectric response, and the electron-phonon couplings from density-functional perturbation theory (DFPT) matrix elements at 34 explicit phonon wavevectors and from Wannier interpolation with the two-dimensional long-range treatment. The model therefore contains no fitted transport parameter and no coupling taken from another calculation, and its scattering rates are checked against an independent solution of the linearised Boltzmann equation with the same couplings. The EMC velocity-field characteristic feeds a one-dimensional channel model, which is verified against a direct self-consistent three-dimensional EMC simulation of the gated channel and reproduces the measured transfer characteristics of a CVD monolayer device in air and vacuum. An ambient/adsorbate extension reproduces the measured conductivity response to oxygen partial pressure and the concentration dependence of NO 2 and NH 3 sensing of independent monolayer devices.