  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.
