One-dimensional hole gas in germanium/silicon nanowire heterostructures

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 germanium͞silicon (Ge͞Si) core͞shell nanowire heterostructure. Room temperature electrical transport measurements clearly show hole accumulation in undoped Ge͞Si nanowire heterostructures, in contrast to control experiments on singlecomponent nanowires. Low-temperature studies show wellcontrolled 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.ballistic transport ͉ bandstructure design ͉ conductance quantization ͉ nanoscience ͉ single-electron transistor C arbon nanotubes and semiconductor nanowires have attracted considerable attention as 1D structures for fundamental studies and also as potential building blocks for nanodevices (1-3). Current synthetic methods can reproducibly yield nanotubes and nanowires with diameters of a few nanometers (4, 5), comparable with the de Broglie wavelength of the carriers. In this regime, quantum confinement may affect significantly transport through these materials, thus making them model platforms to study and use potentially unique properties of 1D systems (6, 7). Indeed, ballistic transport and conductance quantization have been observed in metallic (8) and semiconducting carbon nanotubes (9, 10). Similar effects have not been observed in semiconductor nanowires, although the ability to vary size, material composition, and electronic properties of semiconductor nanowires in a controlled manner (2, 3) offers substantial potential for creating designed 1D systems.Band structure engineering has been widely used in the past to yield interesting planar systems, including 2D electron gases in GaAs͞AlGaAs heterostructures (11) and 2D hole gases in Ge͞SiGe heterostructures (12). 2D electron and 2D hole gases have been central to mesoscopic physi...

Electronic transport through Si nanowires: Role of bulk and surface disorder

“…8 ͑solid line͒, revealing a strong energy dependence. In the interval −0.15 eVϽ E Ͻ −0.05 eV, we find l e ϳ 50 nm, while for energies around −0.35 eV the MFP in on the order of 1 m. Comparing with estimated phonon scattering MFP's of more than 500 nm, 16 the application of the elastic scattering model applied in this work is justified for most of the energies. Moreover, at several energies the vacancy scattering might be the dominant even at room temperature.…”

“…Compared to estimated phonon scattering MFP's of more than 500 nm, 16 the elastic scattering model applied in this work is justified. Comparing the results obtained in this work with the estimated long phonon MFP we suggest that impurity and defect scattering could be the dominant scattering source even at room temperature.…”

“…Also, electron-phonon scattering might be suppressed in thin wires. Indeed, recent experiments by Lu et al 16 indicated ballistic transport in undoped Si/ Ge core-shell wires at room temperature with an estimated phonon scattering mean free path ͑MFP͒ l ph Ͼ 500 nm. This might imply that even at room temperature defects could be the most important scattering source, and a thorough understanding of the scattering processes is thus required.…”

Energy production and conversion applications of one-dimensional semiconductor nanostructures

“…Th ey also rapidly exclude materials forming near the growing walls, terminating propagation through the crystal. Th e LED, which is a forward-biased junction of a p-type (hole-transporting) and n-type (electron-transporting) material, can be downsized by the use of 1D NWs and also benefi t, in effi ciency, from the superior electronic [2][3][4] and thermal [5] transport properties compared with their bulk counterparts. One of the challenges is doping the NWs, and the other is, of course, achieving a good electrical p-n junction.…”

Andreev tunneling enhanced by Coulomb oscillations in superconductor-semiconductor hybrid Ge/Si nanowire devices

“…The devices reported were fabricated on undoped Ge/Si core/shell nanowires grown by a two-step chemical vapor deposition 24 . Due to the large valence band offset between the Ge core and the Si shell, one-dimensional hole gas can be confined in the Ge channel.…”