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Kavli Institute Delft and
Philips demonstrate integration of semiconductor and superconductor electronics
on the nanoscale
Delft,
Eindhoven, the Netherlands
-- In the July 8 issue of Science, scientists from the Kavli Institute of
Nanoscience Delft and Philips present the first superconducting transistors
based on semiconductor nanowires. These nanoscale superconductor/semiconductor
devices enable the fabrication of new nanoscale superconducting electronic
circuits and at the same time they provide new opportunities for the study of
fundamental quantum transport phenomena.
After the invention of the first solid-state transistor (Bardeen, Brattain and
Shockley, 1947), semiconductors have become the reference material system for
electronics. This success results from the possibility to control the resistance
of a semiconductor with an electrical voltage applied to a nearby gate
electrode. Despite the astonishing number of different types of semiconductor
devices it has always been difficult to combine semiconductors with
superconducting materials, i.e. materials with vanishing resistance at low
temperatures. This exotic combination has captured the attention of both
experimental and theoretical physicists already since the 80s. It enables new
technology for electronic circuits based on dissipation-less superconducting
elements which could be exploited for advanced applications where the
requirement of low-temperature operation is not a limiting factor.
The results presented in the Science article show that the combination of indium
arsenide semiconductor nanowires with aluminum-based superconducting contacts
results in very reproducible superconducting transistors. In these devices a
supercurrent (i.e. a current without resistance) can flow through the nanowire
from one superconducting contact to the other. This quantum effect can be
described as the “leakage” of Cooper pairs (i.e. paired electrons responsible
for superconductivity) from the superconducting contacts into the semiconductor
nanowire. Moreover, this supercurrent can be controlled by a gate voltage making
it a supercurrent transistor.
The use of a recently developed method to grow semiconductor nanowires plays a
central role in this achievement. The nanowires are made in a “bottom up”
technology, i.e. instead of growing layers of material and removing the regions
that are not needed, a device is constructed from small building blocks. In this
case the nanowires grow from small gold particles by a vapor-liquid-solid (VLS)
process. The size of these nanoparticles is in the range between 10 and 100 nm
and this sets the diameter of the nanowires. The length of the nanowires is
proportional to the growth time and can easily reach tens of microns providing a
convenient aspect ratio for post-growth device fabrication.
The demonstrated high yield of the superconducting devices is an important
requirement for the successful up scaling to small superconducting circuits
incorporating multiple nanowire devices. For instance, two nanowire devices
could be used to build an electrically tunable superconducting quantum
interference device (SQUID). Such a device could be useful in solid-state
quantum computer architectures as a switchable coupling element between
superconducting quantum bits. Another possibility could be the combination of a
nanowire light-emitting diode (LED; this can be made by alternating the
semiconductor vapor between n- and p-doped during growth) with superconductivity
in order to transfer quantum information from electrons to photons.
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