Electronic Properties of Silicon Nanowires: Confined Phonons and Surface Roughness (original) (raw)
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Electron Mobility in Silicon Nanowires
IEEE Transactions On Nanotechnology, 2000
The low-field electron mobility in rectangular silicon nanowire (SiNW) transistors was computed using a self-consistent Poisson-Schrödinger-Monte Carlo solver. The behavior of the phonon-limited and surface-roughness-limited components of the mobility was investigated by decreasing the wire width from 30 nm to 8 nm, the width range capturing a crossover between twodimensional (2D) and one-dimensional (1D) electron transport. The phonon-limited mobility, which characterizes transport at low and moderate transverse fields, is found to decrease with decreasing wire width due to an increase in the electron-phonon wavefunction overlap. In contrast, the mobility at very high transverse fields, which is limited by surface roughness scattering, increases with decreasing wire width due to volume inversion. The importance of acoustic phonon confinement is also discussed briefly.
Electron mobility in gate all around cylindrical silicon nanowires: A Monte Carlo study
International Conference on Microelectronics, 2010
Electron mobility in gated silicon nanowires is calculated using a Monte Carlo simulation that considers phonon and surface roughness scattering. Surface roughness scattering rates are calculated using Ando's model. The eigenenergies and eigenfunctions required for scattering rate calculation are determined by self-consistent solution of the Schrödinger and Poisson equations. The effects of size quantization and transverse electric field on electron
Journal of Applied Physics, 2008
We investigate the effects of electron and acoustic-phonon confinement on the low-field electron mobility of thin square silicon nanowires (SiNWs) that are surrounded by SiO2 and gated. We employ a self-consistent Poisson-Schrödinger-Monte Carlo solver that accounts for scattering due to acoustic phonons (confined and bulk), intervalley phonons, and the Si/SiO2 surface roughness. The wires considered have cross sections between 3 × 3 nm 2 and 8 × 8 nm 2 . For larger wires, as expected, the dependence of the mobility on the transverse field from the gate is pronounced. At low transverse fields, where phonon scattering dominates, scattering from confined acoustic phonons results in about a 10% decrease of the mobility with respect to the bulk phonon approximation. As the wire cross-section decreases, the electron mobility drops because the detrimental increase in both electron-acoustic phonon and electron-surface roughness scattering rates overshadows the beneficial volume inversion and subband modulation. For wires thinner than 5 × 5 nm 2 , surface roughness scattering dominates regardless of the transverse field applied and leads to a monotonic decrease of the electron mobility with decreasing SiNWs cross section.
Electron dynamics in silicon nanowire using a Monte-Carlo method
Journal of Physics: Conference Series, 2009
We present a theoretical study of electron transport in silicon nanowire (SNW). A self-consistent 2D-Poisson-Schrödinger solver provides the band structure. Then, both electron velocity and low-field electron mobility along the SNW axis are computed with an ensemble Monte-Carlo method. Scattering mechanisms due to phonons (acoustic phonons, zero-order and first-order intervalley phonons) and surface roughness are taken into account. We investigate the effect of cross section size and transverse electric field on electron mobility in SNW.
Phys Rev B, 2011
A simulation framework that couples atomistic electronic structures to Boltzmann transport formalism is developed and applied to calculate the transport characteristics of thin silicon nanowires (NWs) up to 12 nm in diameter. The sp3d5s*-spin-orbit-coupled atomistic tight-binding model is used for the electronic structure calculation. Linearized Boltzmann transport theory is applied, including carrier scattering by phonons, surface roughness (SRS), and impurities. We present a comprehensive investigation of the low-field mobility in silicon NWs considering i) n- and p-type NWs, ii) [100], [110], and [111] transport orientations, and iii) diameters from D = 12 nm (electronically almost bulk-like) down to D = 3 nm (ultra-scaled). The simulation results display strong variations in the characteristics of the different NW types. For n-type NWs, phonon scattering and SRS become stronger as the diameter is reduced and drastically degrade the mobility by up to an order of magnitude depending on the orientation. For the [111] and [110] p-type NWs, on the other hand, large mobility enhancements (on the order of ˜4×) can be achieved as the diameter scales down to D = 3 nm. This enhancement originates from the increase in the subband curvatures as the diameter is scaled. It overcompensates for the mobility reduction caused by SRS in narrow NWs and offers an advantage with diameter scaling. Our results may provide understanding of recent experimental measurements, as well as guidance in the design of NW channel devices with improved transport properties.
Journal of Applied Physics, 2007
We present a theoretical study of electron mobility in cylindrical gated silicon nanowires at 300 K based on the Kubo-Greenwood formula and the self-consistent solution of the Schrödinger and Poisson equations. A rigorous surface roughness scattering model is derived, which takes into account the roughness-induced fluctuation of the subband wave function, of the electron charge, and of the interface polarization charge. Dielectric screening of the scattering potential is modeled within the random phase approximation, wherein a generalized dielectric function for a multi-subband quasi-one-dimensional electron gas system is derived accounting for the presence of the gate electrode and the mismatch of the dielectric constant between the semiconductor and gate insulator. A nonparabolic correction method is also presented, which is applied to the calculation of the density of states, the matrix element of the scattering potential, and the generalized Lindhard function. The Coulomb scattering due to the fixed interface charge and the intra-and intervalley phonon scattering are included in the mobility calculation in addition to the surface roughness scattering. Using these models, we study the low-field electron mobility and its dependence on the silicon body diameter, effective field, dielectric constant, and gate insulator thickness.
Carrier-phonon interaction in small cross-sectional silicon nanowires
Journal of Applied Physics, 2008
Using first-order perturbation theory and deformation potential approximation, the interaction of electrons and holes with acoustic and optical phonons is investigated in silicon nanowires ͑SiNWs͒ with different diameters and crystallographic axis orientations. The electronic band structures for ͓110͔ and ͓100͔ SiNWs are obtained from a sp 3 d 5 s ء tight-binding scheme, while a continuum model is assumed for phonon dispersion. The influence of confined and bulk phonons on carrier transport is investigated.
Strong anisotropy and diameter effects on the low-field mobility of silicon nanowires
2011 International Conference on Simulation of Semiconductor Processes and Devices, 2011
We describe a method to couple the sp 3 d 5 s * -spin-orbitcoupled (SO) atomistic tight-binding (TB) model and linearized Boltzmann transport theory for the calculation of low-field mobility in Si nanowires (NWs). We consider scattering mechanisms due to phonons and surface roughness. We perform a simulation study of the low-field mobility in n-type and p-type Si NWs of diameters from 3nm to 12nm, in the [100], [110] and [111] transport orientations. We find that the NW mobility is a strong function of orientation and diameter. This is a consequence of the large variations in the electronic structure with geometry and quantization. Especially in the case of p-type [111] and [110] NWs, large phonon-limited mobility improvements with diameter scaling are observed.
Bandstructure and mobility variations in p-type silicon nanowires under electrostatic gate field
Solid-State Electronics, 2013
The sp 3 d 5 s * -spin-orbit-coupled atomistic tight-binding (TB) model is used for the electronic structure calculation of Si nanowires (NWs), self consistently coupled to a 2D Poisson equation, solved in the cross section of the NW. Upon convergence, the linearized Boltzmann transport theory is employed for the mobility calculation, including carrier scattering by phonons and surface roughness. As the channel is driven into inversion, for [111] and [110] NW devices of diameters D>10nm the curvature of the bandstructure increases and the hole effective mass becomes lighter, resulting in a ~50% mobility increase. Such improvement is large enough to compensate for the detrimental effect of surface roughness scattering. The effect is very similar to the bandstructure variations and mobility improvement observed under geometric confinement, however, in this case confinement is caused by electrostatic gating. We provide explanations for this behavior based on features of the heavy-hole band. This effect could be exploited in the design of p-type NW devices. We note, finally, that the "apparent" mobility of low dimensional short channel transistors is always lower than the intrinsic channel diffusive mobility due to the detrimental influence of the so called "ballistic" mobility.
Quantum transport length scales in silicon-based semiconducting nanowires: Surface roughness effects
Physical Review B, 2008
We report on a theoretical study of quantum charge transport in atomistic models of silicon nanowires with surface roughness-based disorder. Depending on the nanowires features (length, roughness profile) various conduction regimes are explored numerically by using efficient real space order N computational approaches of both Kubo-Greenwood and Landauer-Büttiker transport frameworks. Quantitative estimations of the elastic mean free paths, charge mobilities and localization lengths are performed as a function of the correlation length of the surface roughness disorder. The obtained values for charge mobilities well compare with the experimental estimates of the most performant undoped nanowires. Further the limitations of the Thouless relationship between the mean free path and the localization length are outlined.