We propose to study devices fabricated from sub-50 nm diameter nanowires with axial Si/Ge heterojunctions grown by the vapor-liquid-solid technique in collaboration with T. Picraux's epitaxy group at Los Alamos. Previous work has shown that sidewall strain relaxation greatly expands the available bandgap and strain engineering, allowing segments of essentially arbitrary SiGe composition to be incorporated into the axial nanowire. We plan to study the following classes of devices: i) axial Si/Ge heteronanowire tunneling transistors, which promise a combination of sharper switching together with higher tunneling currents due to bandgap and strain engineering along the nanowire combined with the gate-all-around device geometry; ii) Si/Ge double-barrier tunneling structures with high conduction and valence band barriers made possible by the relaxation of critical thickness limits in narrow heteronanowires; iii) Si/Ge/Si pn heterojunctions with lateral size confinement that promise stronger optical transitions due to narrower and pseudo-direct bandgap in the Ge region. Experimental results will be compared to numerical band structure simulations from both continuum elasticity and molecular dynamics (again, in collaboration with the Picraux group).
BROADER IMPACT Heteronanowire tunneling transistors with gate-all-around geometry comprise one of the more realistic candidates for low-voltage switching compatible with silicon technology. Also, our work will provide experimental data for atomistic simulations of inhomogeneous strain at ~10 nm length scale. The Los Alamos collaboration research will have added graduate education impact due to extended student visits to Los Alamos. Undergraduates will work on sub-sections, including REU students from minority institutions (identified through the Brown MRSEC).
