The third year of the Centre for Quantum Computer Technology has
seen the achievement of significant milestones in the fabrication
and measurement of solid-state quantum computer prototypes, together
with important new conceptual advances for solid-state quantum computation.
The Centre’s research is a nationally coordinated experimental
and theoretical effort directed towards the realisation of scalable
solid-state quantum computer prototype system based on a precisely
controlled array of phosphorus donors embedded in a silicon crystal.
This goal is being approached using two parallel strategies, a ‘top-down’
approach in which single phosphorus donors are implanted into the
device through masks and a ‘bottom-up’ approach that
offers the ultimate in atomically precise construction. Here the
devices are built atomic layer by atomic layer, using scanned probe
lithography and epitaxial semiconductor growth.
During 2002 the Centre achieved the following important milestones
in solid-state device fabrication and measurement.
• We have demonstrated controlled incorporation of single
phosphorus atoms in silicon at pre-defined sites—the first
step towards the realisation of atomic-scale doping of a semi-conductor.
• A fully configured device was fabricated using the top-down
ion implantation route with the low keV implant energies required
for a functioning prototype.
• The ability to read-out the charge state of these devices
at microsecond timescales was demonstrated using a radio-frequency
(rf) twin-SET simulation device.
• A new charge-based scheme for silicon quan-tum computing
was conceived and patented.
Bottom-up STM fabricated devices
Previously we have demonstrated a strategy to adsorb single phosphine
molecules to a silicon surface in precise locations using a hydrogen
resist. We have now managed to incorporate the phosphorus atoms
from these individual molecules into the silicon surface at atomically
precise locations using a critical anneal, see Figure 1.
Left: A hydrogen terminated silicon surface with a nanometer
sized area of bare silicon (circled) produced by hydrogen desorption
with an STM tip. Right: A single P atom has been incorporated
into the surface, to form a P-Si-H ‘hydrided heterodimer’,
at the precise position defined by the STM.
As a result the P atoms are now ‘locked in’ to the
surface, preventing them from desorbing or diffusing across the
surface during subsequent silicon encapsulation. This figure represents
an important step towards the ability to perform single atom doping
Secondly we have addressed the key problem of minimising P segregation
during silicon encapsulation. The influence of the Si growth temperature
on P segregation was investigated using Secondary Ion Mass Spectrometry
(SIMS) and STM measurements. SIMS depth profiling confirmed that
phosphorus segregated less than ~ 5 nm during Si encapsulation at
250°C (below the detection limit of SIMS). Using the STM itself
as a characterisation tool we were able to show that we could reduce
this by a factor of ~5 by encapsulating the phosphorus atoms at
Expansion of the Atomic Fabrication Facility
In 2002 the Atomic Fabrication Facility was significantly expanded
with the addition of a $3.1 million multi-chamber scanning tunnelling
microscope and molecular beam epitaxy system across two rooms of
the AFF laboratory — see figure 2. This new system contains
several unique design features that provide the necessary registration
capability and high purity silicon growth required for the fabrication
of the final multi-qubit device, and importantly for realising atomic-scale
devices in silicon.
Top-Down Ion-Implanted Devices
2 New combined UHV MBE/STM/SEM system, commissioned in November
2002, which is custom-designed to enable bottom-up fabrication
of buried P arrays with registration to surface control gates
The first implanted donor test devices were constructed during
2002. These devices were designed to demonstrate controlled single
electron transfer events between implanted donors. The structures
incorporate control gates for manipulating electron transfer, together
with integrated single electron transistors (SETs) for sensing charge
motion (see Figure 3). Their electrical characterisation is currently
underway and represents an exciting moment for the Centre. During
2002 researchers from a number of programs within the Centre also
conceived a novel charge-based qubit scheme that acts as a intermediate
stage for spin qubit development.
3 (a - c) SEM and optical images of complete buried atom device
showing critical dimensions. (d) Nanoapertures 24nm in dimension
for ion implantation. (e) Completed device design with optmised
Quantum Device Measurement
Amazing results were achieved by the quantum measurement team with
the development and demonstration of the twin radio frequency single
electron transistor (rf-SET), a key device for the fast and efficient
readout of solid-state qubits. They established the ability to sense
charge motion on sub-microsecond timescales (see figure 4) thereby
creating a world-class capability in efficient charge readout and
moving us significantly closer to the ultimate goal of single spin
4. Measurements of a rf twin single electron transistor device
at mK temperatures, showing periodic “Coulomb Blockade”
oscillations in the conductance(left). Radio frequency measurements
of the power reflected from the rf-SET (right) allow changes
in the the device conductance to be monitored on sub-ms timescales,
providing high speed detection of single electron motion.
to the work on the rf-SET, 2002 saw several other key projects achieve
success. This included the fabrication of MOSFET devices for the
characterization of interface trap densities and the development
of a fast pulse-gate capability aimed at delivering voltages pulses
to on-chip qubit gates with rise/fall times of the order of 40ps.
Finally, congratulations are due to the team whose results on spin
manipulation in ultra-low-disorder quantum wires were published
in Physical Review Letters.
the Centre published 80 papers with 5 Physical Review Letters and
28 Physics Review A,B or E. It also lodged 5 patents and gave over
100 conference presentations, 27 of which were invited. During the
year the Centre was involved in a nationally competitive bid to
reposition itself within the Australian Research Council’s
newly announced Centres of Excelllence (COE). The main thrust of
our COE application is to enable the Centre to address key issues
of fault tolerant quantum computing. In December 2002 it was announced
that our COE application was successful, making us one of the eight
COEs awarded nationally.
Michelle Simmons, Alex Hamilton and Robert Clark