The top-down approach limits the dimensions of devices to what is technically achievable using

determined to be ~600meV
22
. (© American Chemical Society 2002.)
Fig. 10 Tuning of tunnel barriers for design of DBRTs and SETs. (a) and (b) show the controlled formation and the digital nature of the thickness of single InP tunnel barriers in InAs
nanowires. (b) is a one-dimensionally integrated profile of the boxed area in (a), from which the width of the barrier can be determined to be 15-16 MLs.(c) shows the dynamical range of
the tunnel resistances that can be achieved by control of the thickness of the (single) tunnel barrier
27
. (© American Institute of Physics 2002.)

---

surrounded by tunnel barriers, one expects that the current
will be blocked for applied source-drain voltages smaller than
the charging energy 
E
C
= 
e
2
/
C, where C is the total
capacitance, which for these geometries are dominated by
the barrier capacitances. One also expects that a gate in an
ideal single-island SET will be able to lift the Coulomb
blockade completely, corresponding to the adjustment of the
‘next’ level (
N + 1) electrons on the island relative to the
emitter, and that the gate can periodically and subsequently
push the (
N + i) levels into resonance. This predicted
behavior is perfectly reproduced in the data
28
in Fig. 12,
which includes the so-called stability diagram, with the
conductivity gray-scale coded as a function of the 
source-drain bias (
V
SD
) and the gate bias (
V
G
), with perfect
diamonds corresponding to the blockade regions. By
following the periodic variation with respect to the gate, it
can be deduced that the island contains about 50 electrons 
at zero gate bias, which are all drained at a gate bias of 
about -0.8 V.
Photonic applications of nanowires
An important application of QDs is as an ideal quantum
emitter, possibly enabling the fabrication of a single-photon-
on-demand device. Such an ideal emitter would have
important consequences for quantum cryptography and
secure data transfer. 
Various attempts have been made to demonstrate this
concept, originally proposed by Imamoglu and Yamamoto
29
,
either using top-down etched columns or SK QDs in a mesa
containing resonant tunneling barriers around a QD layer.
Both these approaches have problems that arise from either
process-induced damage or the inability to address a single
QD. It is now quite clear that if one could use a QD created
inside a nanowire as the photon emitter with surrounding
tunnel barriers for injection of electrons and holes, this could
be the ideal candidate for a single-photon-on-demand device. 
Having already demonstrated that highly ideal DBRT and
SET devices can be fabricated using nanowire heterostructure
technology, the next step towards quantum optics
applications is to show optically active QDs in nanowires.
Very recently, we have grown optically active single QDs
inside nanowires
30
. As shown in Fig. 13, these InGaAs QDs,
positioned inside GaAs nanowires, emit characteristic
excitonic luminescence with spectral line-widths in the range
100-200 µeV, and show excitation level dependence as
expected for single exciton and biexciton emission
30
. These
results constitute the qualification tests of the potential of
nanowires as electrically addressable quantum emitters.
These results also promise the realization of one-dimensional
superlattice structures, which are interesting for studying
fundamental science as well as for infrared and terahertz
applications
31
. 
The right hand part of Fig. 13 demonstrates the highly
accurate technology that we recently developed for the
controlled deposition of nanowires at predetermined
positions
32
. This demonstrates the possibilities of a bottom-
REVIEW
FEATURE
October 2003
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