Chun Fan

Strain distributions in lattice-mismatched semiconductor core-shell nanowires

The authors study the elastic deformation field in lattice-mismatched core-shell nanowires with single and multiple shells. The authors consider infinite wires with a hexagonal cross section under the assumption of translational symmetry. The strain distributions are found by minimizing the elastic energy per unit cell using the finite element method. The authors find that the trace of the strain is discontinuous with a simple, almost piecewise variation between core and shell, whereas the individual components of the strain can exhibit complex variations.

Impact of inhomogeneous strain on the valence band structures of Ge-Si core-shell nanowires

We report on a theoretical study of the valence band structures of germanium-silicon core-shell nanowires based on a 6times6 kldrp model. We take into account the inhomogeneous strain effects induced by the lattice mismatches between germanium and silicon. We find that the top subband ends drift back to Gamma point, and the effective masses of more subbands begin to decrease when the shell thickness increases.

Effects of Shell Strain on Valence Band Structure and Transport Properties of Ge/Si1-xGex Core-Shell Nanowire

Various Si1-xGex shell strains induced by changing the thickness or tuning the Ge and Si contents as well as by modulating the valence band structure and hole transport characteristics of core/shell nanowire field effect transistors (FETs) have been calculated. As Si1-xGex shell thickness increases, the strained valence subbands shift upwards and warp markedly. Most of the corresponding hole effective masses of the top five subbands decrease. Meanwhile, the hole mobility of the Ge(110) nanowire increases with increasing shell thickness. As the Ge concentration in the Si1-xGex shell decreases, the strained valence subbands and hole mobility show similar shifts. As a result, our calculation indicates the possibility of improving the nanowire performance of heterostructure nanowire FETs.

Performance Evaluation of GaAs–GaP Core–Shell-Nanowire Field-Effect Transistors

We evaluate the performance of GaAs-GaP core-shell (C-S)-nanowire (NW) field-effect transistors by employing a semiclassical ballistic transport model. The valence-band structures of GaAs-GaP C-S NWs are calculated by using a kldrp method including the strain effect. The calculations show that the strain causes substantial band warping and pushes valence subbands to move up. We demonstrate that the on current can be enhanced with the strength of strain induced in the core, but an extremely thin equivalent oxide thickness may suppress the effect of the strain-induced current improvement. The achieved results can provide a design guide for optimizing device performance.

AC conductance of finite-length carbon nanotubes

We propose a nonequilibrium Greens function approach to calculate the ac conductance of various finite-length carbon nanotubes. The simulated ac conductance differs significantly from that for infinite-length carbon nanotubes. At the low-frequency limit, the profiles of the quantized conductance are still observable in the finite-length carbon nanotubes, but many more peaks appear on the conductance curves. We also show that the conductance of finite-length carbon nanotubes oscillates as a function of the ac frequency. The dependence of the oscillation on the lengths, helicities and defects of the carbon nanotubes are also investigated. The knowledge we gain from this research will help us make carbon-nanotube-based interconnects or other ac devices in the future.

Surrounding Strain Effects on the Performance of Si Nanowires Grown in Different Axial Orientations

In this paper, we calculate the surrounding strain effects owing to gate dielectric on the device performance of Si nanowires (NWs) with different axial orientations. Surrounding strain effects from valence band structure to hole transport property of NW FETs are developed. The simulated results show that surrounding strain pushes the valence subbands upward. The upshifting trend of the valence subband maximum is (1 1 0) NW >; (0 0 1) NW >; (1 1 1) NW. The shift coincides with the εzz variation, which contributes the most to modulate the valence subbands. Compared to pure Si NWs, surrounding strain owing to HfO2 dielectric enhances the effective hole mobility. Effective hole mobility enhancement in HfO2 surrounding Si NW is (0 0 1) NW >; (1 1 1) NW >; (1 1 0) NW. However, Si(1 1 0) NW still has the largest effective hole mobility among three axial orientations.

Radial Boundary Forces-Modulated Valence Band Structure of Ge (110) Nanowire

For the radial boundary force induced in the process, the strain energy distribution and strain tensor components in Ge (110) nanowire (NW) are calculated by finite element method. Based on the strain distribution, we compute valence band structures with different radial forces. As increasing force values, top valence subbands shift downwards. The influence on the corresponding effective masses and density of states are also investigated.

GaAs-GaP core-shell nanowire transistors: A computational study

We evaluate the performance of GaAs-GaP core-shell nanowire field effect transistors by employing a semiclassical ballistic transport model and a k·p calculation of the valence band structures including the strain effect. We find that the strain will induce substantial modulation on the nanowire valence band structures and this modulation will push more conduction channels into the bias window as the shell thickness increases. We analyze its impact on the transistor performance, and our simulation results indicate that in order to achieve a good ON/OFF current ratio the epitaxial shell should be grown thin enough.

Impact of Thickness and Deposition Temperature of Gate Dielectric on Valence Bands in Silicon Nanowires

a The strain distribution and strained valence band structure in silicon nanowire with varied thicknesses and deposition temperatures of gate dielectric are discussed in detail in this work. Our calculation indicates that valence subbands are dependent on the structure and process parameters. Strain has little effects in (001) orientation. But in Si (110) nanowire, the valence subbands shift upper and warp remarkably as the gate dielectric becomes thicker. Taking thermal residual strain into consideration, the strained valence subbands go to higher energy positions compared to NW without the residual strain. The different deposition temperature by a certain process slightly influences the valence bands. Strain effects on densities of states and effective masses are also investigated.

Band structure calculations of Ge-Si core-shell nanowires

We have modeled and calculated the band structure of Ge-Si core-shell nanowire, with both subband interactions between Ge core and Si shell and the inhomogeneous strain effects taken into account. Our results show that the effective masses of subbands, the densities of states and quantum conductance will undergo significant changes and converge to saturated values, as the Si shell turns thicker and provides more band modulations.