Jiseok Kim

Theoretical Study of Some Physical Aspects of Electronic Transport in nMOSFETs at the 10-nm Gate-Length

We discuss selected aspects of the physics of electronic transport in nMOSFETs at the 10-nm scale: Long-range Coulomb interactions, which may degrade performance and even prevent ballistic transport from occurring; scattering with high-k insulator interfacial modes, which depresses the electron mobility but is found to affect minimally the saturated transconductance of 15-nm devices; and the use of high-mobility small effective-mass substrates, which poses serious concerns related to performance limitations due to the density-of-states (DOS) bottleneck and to the band-to-band (Zener) leakage current. On the basis of our results, we argue that ballistic transport may not only be unachievable (because of unavoidable electron-electron collisions) but may also be undesirable, as it may enhance the DOS bottleneck. We also argue that the knowledge of low-field mobility is of little use in predicting quantitatively the performance of devices in the saturated region.

Electronic band structure calculations for biaxially strained Si, Ge, and III–V semiconductors

Electronic band structure and effective masses for relaxed and biaxially strained Si, Ge, III–V compound semiconductors (GaAs, GaSb, InAs, InSb, InP) and their alloys (InxGa1−xAs, InxGa1−xSb) on different interface orientations, (001), (110), and (111), are calculated using nonlocal empirical pseudopotential with spin-orbit interaction. Local and nonlocal pseudopotential parameters are obtained by fitting transport-relevant quantities, such as band gap and deformation potentials, to available experimental data. A cubic-spline interpolation is used to extend local form factors to arbitrary q and to obtain correct workfunctions. The nonlocal and spin-orbit terms are linearly interpolated between anions and cations for III–V semiconductors. The virtual crystal approximation is employed for the InxGa1−xAs and InxGa1−xSb alloys and deformation potentials are determined using linear deformation-potential theory. Band gap bowing parameters are extracted using least-square fitting for relaxed alloys and for strai...

Semiclassical and Quantum Electronic Transport in Nanometer-Scale Structures: Empirical Pseudopotential Band Structure, Monte Carlo Simulations and Pauli Master Equation

The study of electronic transport in nanometer-scale devices requires an accurate knowledge of the excitation spectrum (i.e., the band structure) of the systems and, for short devices, a formulation of transport which transcends the semiclassical Boltzmann formulation. Here we show that the use of ‘judiciously’ chosen empirical pseudopotentials, coupled to the supercell method, can provide a sufficiently accurate description of the band structure of thin semiconductor films, hetero-structures, nanowires, and carbon-based structures such as graphene, graphene nanoribbons, and nanotubes. We discuss semiclassical Monte Carlo simulations employing the supercell-pseudopotential band structure, considering transport in thin Si bodies. This example illustrates the importance of the full-band approach since in this case it yields the low value of the saturated high-field electron drift velocity, observed experimentally but never predicted when employing effective-mass band structures. Finally, we discuss a mixed envelope-supercell approach to deal with open systems within the full-band supercell scheme and review the Master-equation approach to quantum transport. Finally, we present some results of fully dissipative quantum transport using, for now, the effective mass approximation, emphasizing the role of impurity scattering in determining the ‘quantum access resistance’ in thin-body devices.

Empirical pseudopotential calculations of the band structure and ballistic conductance of strained [001], [110], and [111] silicon nanowires

The electronic band structure of hydrogen passivated, square cross-section, uniaxially strained [001], [110], and [111] silicon nanowires (Si NWs) has been calculated using nonlocal empirical pseudopotentials calibrated to yield the correct work function and benchmarked against first-principles calculations. We present results regarding the dependence and direct/indirect nature of the bandgap on wire diameter and uniaxial strain as well as the ballistic conductance and effective mass. As a result of practical interest, we have found that the largest ballistic electron conductance occurs for compressively strained large-diameter [001] wires while the smallest transport electron effective mass is found for larger-diameter [110] wires under tensile stress.

Resonance characteristics through double quantum dots embedded in series in an Aharonov-Bohm ring

Stimulated by recent intriguing experiments with a quantum dot in an Aharonov–Bohm (AB) ring, we investigate novel resonant phenomena by studying the total transmission probability of nanoscale AB rings with embedded double quantum dots in one arm and a magnetic flux passing through its centre. In this system, we show the overlapping and merging of both Breit-Wiger (BW) and Fano resonances as the interaction parameter between the dots changes. In the strong overlapping regime of Fano resonances, the transmission zeros leave the real-energy axis and move away in opposite directions in the complex-energy plane. The simultaneous swings (from Fano to BW and then back to Fano resonance) of a pair of Fano resonances in the overlapping regime are observed by modulating the magnetic flux threading the AB ring. The periodic tuning of the Fano resonance for a fixed interaction parameter is also discussed as the magnetic flux increases.

Empirical Pseudopotential Calculation of Band Structure and Deformation Potentials of Biaxially Strained Semiconductors

The electronic band structure of relaxed and biaxially strained Si, Ge, III-V semiconductors (GaAs, GaSb, InAs, InSb, InP) and their alloys (In x Ga 1-x As, In x Ga 1-x Sb) on different interface orientations, (001), (110) and (111), is calculated using the nonlocal empirical pseudopotential method (EPM) with spin-orbit interaction using cubic spline interpolations of the atomic form factors. For III-V alloys, the virtual crystal approximation (VCA) is employed to calculate the band gap bowing parameters. Calculated results such as band gap (direct and indirect), band gap bowing parameters, and deformation potentials are fitted to the experimental data when available. Deformation potentials are determined using linear deformation potential theory when the small biaxial strain (in-plane) is present.

Band structure and ballistic conductance of strained Si nanowires

The electronic band structure of hydrogen passi-vated, square cross-section, uniaxially strained [001], [110] and [111] silicon nanowires (Si NWs) has been calculated using nonlocal empirical pseudopotentials. Local pseudopotentials for bulk Si — calibrated to yield the correct workfunction and coupled to nonlocal corrections — yield results in good agreement with those of first-principles calculations, whenever available, and have been employed to calculate the electronic structure, the ballistic conductance and the effective mass of Si NWs, their dependence on wire diameter and strain and the corresponding variations of the band gap. We find the largest ballistic electron conductance for larger-diameter [001] wires under compressive strain while the smallest transport electron effective mass is found for larger-diameter [110] wires under tensile strain.