Tillmann Kubis

Engineering Nanowire n-MOSFETs at L-g < 8 nm

As metal-oxide-semiconductor field-effect transistors (MOSFETs) channel lengths (Lg) are scaled to lengths shorter than Lg <; 8 nm source-drain tunneling starts to become a major performance limiting factor. In this scenario, a heavier transport mass can be used to limit source-drain (S-D) tunneling. Taking InAs and Si as examples, it is shown that different heavier transport masses can be engineered using strain and crystal-orientation engineering. Full-band extended device atomistic quantum transport simulations are performed for nanowire MOSFETs at Lg <; 8 nm in both ballistic and incoherent scattering regimes. In conclusion, a heavier transport mass can indeed be advantageous in improving ON-state currents in ultrascaled nanowire MOSFETs.

Engineering Nanowire n-MOSFETs at

As metal-oxide-semiconductor field-effect transistors (MOSFETs) channel lengths (Lg) are scaled to lengths shorter than Lg <; 8 nm source-drain tunneling starts to become a major performance limiting factor. In this scenario, a heavier transport mass can be used to limit source-drain (S-D) tunneling. Taking InAs and Si as examples, it is shown that different heavier transport masses can be engineered using strain and crystal-orientation engineering. Full-band extended device atomistic quantum transport simulations are performed for nanowire MOSFETs at Lg <; 8 nm in both ballistic and incoherent scattering regimes. In conclusion, a heavier transport mass can indeed be advantageous in improving ON-state currents in ultrascaled nanowire MOSFETs.

L_{g}

Empirical tight binding(ETB) methods are widely used in atomistic device simulations. Traditional ways of generating the ETB parameters rely on direct fitting to bulk experiments or theoretical electronic bands. However, ETB calculations based on existing parameters lead to unphysical results in ultra small structures like the As terminated GaAs ultra thin bodies(UTBs). In this work, it is shown that more reliable parameterizations can be obtained by a process of mapping ab-initio bands and wave functions to tight binding models. This process enables the calibration of not only the ETB energy bands but also the ETB wave functions with corresponding ab-initio calculations. Based on the mapping process, ETB model of Si and GaAs are parameterized with respect to hybrid functional calculations. Highly localized ETB basis functions are obtained. Both the ETB energy bands and wave functions with subatomic resolution of UTBs show good agreement with the corresponding hybrid functional calculations. The ETB methods can then be used to explain realistically extended devices in non-equilibrium that can not be tackled with ab-initio methods.

Tight-binding analysis of Si and GaAs ultrathin bodies with subatomic wave-function resolution

Transistor designs based on using mixed Γ-L valleys for electron transport are proposed to overcome the density of states bottleneck while maintaining high injection velocities. Using a self-consistent top-of-the-barrier transport model, improved current density over Si is demonstrated in GaAs/AlAsSb, GaSb/AlAsSb, and Ge-on-insulator-based single-gate thin-body n-channel metal-oxide-semiconductor field-effect transistors. All the proposed designs successively begin to outperform strained-Si-on-insulator and InAs-on-insulator (InAs-OI) in terms of ON-state currents as the effective oxide thickness is reduced below 0.7 nm. InAs-OI still exhibits the lowest intrinsic delay (τ) due to its single Γ valley.

Simulation Study of Thin-Body Ballistic n-MOSFETs Involving Transport in Mixed

It is critical to capture the effect due to strain and material interface for device level transistor modeling. We introduce a transferable

\Gamma

s{p}^{3}{d}^{5}{s}^{*}

-L Valleys

tight-binding model with nearest-neighbor interactions for arbitrarily strained group IV and III-V materials. The tight-binding model is parametrized with respect to hybrid functional (HSE06) calculations for varieties of strained systems. The tight-binding calculations of ultrasmall superlattices formed by group IV and group III-V materials show good agreement with the corresponding HSE06 calculations. The application of the tight-binding model to superlattices demonstrates that the transferable tight-binding model with nearest-neighbor interactions can be obtained for group IV and III-V materials.

Transferable tight-binding model for strained group IV and III-V materials and heterostructures

The hole surface roughness and phonon limited mobility in the silicon 〈100〉, 〈110〉, and 〈111〉 square nanowires under the technologically important conditions of applied gate bias and stress are studied with the self-consistent Poisson-sp3d5s*-SO tight-binding bandstructure method. Under an applied gate field, the hole carriers in a wire undergo a volume to surface inversion transition diminishing the positive effects of the high 〈110〉 and 〈111〉 valence band nonparabolicities, which are known to lead to the large gains of the phonon limited mobility at a zero field in narrow wires. Nonetheless, the hole mobility in the unstressed wires down to the 5u2009nm size remains competitive or shows an enhancement at high gate field over the large wire limit. Down to the studied 3u2009nm sizes, the hole mobility is degraded by strong surface roughness scattering in 〈100〉 and 〈110〉 wires. The 〈111〉 channels are shown to experience less surface scattering degradation. The physics of the surface roughness scattering dependence...

Does the low hole transport mass in 〈110〉 and 〈111〉 Si nanowires lead to mobility enhancements at high field and stress: A self-consistent tight-binding study

Future VLSI devices will require low CV_dd²/2 switching energy, large on-currents (I_on), and small off-currents (I_off). Low switching energy requires a low supply voltage V_dd, yet reducing V_dd typically increases /off and reduces the I_on/I_off ratio. Though tunnel FETs (TFETs) have steep subthreshold swings and can operate at a low V_dd, yet their I_on is limited by low tunneling probability. Even with a GaSb/InAs heterojunction (HJ), given a 2nm-thick-channel (001)-confined TFET, [100] transport, and assuming V_dd=0.3V and I_oFF=10^-3A/m, the peak tunneling probability is <;3 % (fig. 1 a) and I_on is only 24 A/m (fig. 1b) [1]. This low I_on will result in large CV_dd/I delay and slow logic operation. Techniques to increase /on include graded AlSb/AlGaSb source HJs [2,3] and tunneling resonant states [4]. We had previously shown that tunneling probability is increased using (11 0) confinement and channel heterojunctions [1], the latter increasing the junction built-in potential and junction field, hence reducing the tunneling distance. Here we propose a triple heterojunction TFET combining these techniques. The triple-HJ design further thins the tunnel barrier to 1.2 nm, and creates two closely aligned resonant states 57meV apart. The tunneling probability is very high, >50% over a 120meV range, and the ballistic I_on is extremely high, 800A/m at 30nm Lg and 475 A/m at 15nm Lg, both with I_off=10^-3 A/m and V_dd=0.3 V. Compared to a (001) GaSb/InAs TFET, the triple-HJ design increases the ballistic /on by 26:1 at 30nm L_g and 19:1 at 15nm L_g. The designs may, however, suffer from increased phonon-assisted tunneling.

Extremely high simulated ballistic currents in triple-heterojunction tunnel transistors

The Semi-Empirical tight binding model developed in Part I Hegde et al. [J. Appl. Phys. 115, 123703 (2014)] is applied to metal transport problems of current relevance in Part II. A systematic study of the effect of quantum confinement, transport orientation, and homogeneous strain on electronic transport properties of Cu is carried out. It is found that quantum confinement from bulk to nanowire boundary conditions leads to significant anisotropy in conductance of Cu along different transport orientations. Compressive homogeneous strain is found to reduce resistivity by increasing the density of conducting modes in Cu. The [110] transport orientation in Cu nanowires is found to be the most favorable for mitigating conductivity degradation since it shows least reduction in conductance with confinement and responds most favorably to compressive strain.