One-Dimensional Hole Gas in Germanium/Silicon Nanowire Heterostructures

TL;DR

This paper reports the creation and study of a one-dimensional hole gas in germanium/silicon nanowire heterostructures, demonstrating ballistic transport, conductance quantization, and spin polarization phenomena at low and room temperatures.

Contribution

It introduces a novel 1D hole gas system in Ge/Si nanowires, showing controlled transport properties and quantum effects not previously demonstrated in such nanostructures.

Findings

Observation of conductance quantization indicating ballistic transport.

Detection of a 0.7 structure suggesting spin polarization.

Ballistic transport persists at room temperature.

Abstract

Two-dimensional electron and hole gas systems, enabled through band structure design and epitaxial growth on planar substrates, have served as key platforms for fundamental condensed matter research and high performance devices. The analogous development of one-dimensional (1D) electron or hole gas systems through controlled growth on 1D nanostructure substrates, which could open up opportunities beyond existing carbon nanotube and nanowire systems, has not been realized. Here we report the synthesis and transport studies of a 1D hole gas system based on a free-standing germanium/silicon (Ge/Si) core/shell nanowire heterostructure. Room temperature electrical transport measurements show clearly hole accumulation in undoped Ge/Si nanowire heterostructures, in contrast to control experiments on single component nanowires. Low-temperature studies show well controlled Coulomb blockade oscillations when the Si shell serves as a tunnel barrier to the hole gas in the Ge channel. Transparent contacts to the hole gas also have been reproducibly achieved by thermal annealing. In such devices, we observe conductance quantization at low temperatures, corresponding to ballistic transport through 1D subbands, where the measured subband energy spacings agree with calculations for a cylindrical confinement potential. In addition, we observe a 0.7 structure, which has been attributed to spontaneous spin polarization, suggesting the universality of this phenomenon in interacting 1D systems. Lastly, the conductance exhibits little temperature dependence, consistent with our calculation of reduced backscattering in this 1D system, and suggests that transport is ballistic even at room temperature.