Hole Accumulation in Ge/Si Core/Shell Nanowires Studied by Electron Holography

Abstract

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Thermoelectrics of Nanowires

“…In the case of Si-core and Ge-shell NWs, the alignment of the Fermi levels between Ge and Si results in large concentration of free holes accumulated in the Ge core (Figure a,b), as indicated by the lower valence band below the Fermi energy, akin to the generation of electron or hole gases in 2D QWs, but now in the quasi-1D geometry. This hole accumulation has been directly observed from electron holography and Raman scattering, , and has been theoretically studied using first-principles calculations by Amato et al Core–shell Group IV NWs with sharp Ge–Si interfaces (Figure c) have been grown by the common VLS approach, where the intrinsic Si (i-Si) shells were deposited on i-Ge core NWs through the decomposition of Si-containing precursor gases (e.g., SiH 4 ). ,, This dopant-free strategy to create free carriers can result in very high carrier mobility in the NWs, due to the absence or mitigation of the ionized impurity scattering, which often is a strong scattering mechanism for charge carriers. The high mobility has been exploited for a number of electronic applications, such as high-performance NW transistors with high mobility (∼730 cm 2 /V·s) or transconductance, high on-current, and short intrinsic delay time .…”

Gated silicon nanowire for thermo-electric power generation and temperature sensing

“…However, increased doping leads to increased impurity scattering [24]. Significant improvement in σS 2 has been reported via modulation doping [24], applying a gate voltage on gated TEMs [25][26][27][28][29], or generation of a strong internal electric field (E int ) as the result of core/shell structures [30][31][32][33]. Reproducibility and precision of doping profiles become challenging as NWs are downscaled [34] while applying a gate voltage requires additional circuitry and power.…”

“…Reproducibility and precision of doping profiles become challenging as NWs are downscaled [34] while applying a gate voltage requires additional circuitry and power. Previously, E int at interfaces of i-Ge (core)/i-Si (shell) [33] and i-GaN (core)/i-AlN (shell)/i-AlGaN (shell) [32] structures have led to accumulation of holes and electrons in the core respectively, where i stands for intrinsic. However, one has to find an approach to accumulate carriers on a dopant-free abundant material with lowcost fabrication processes, like Si [9].…”