Novel Detectors For Single Atom Doping Of Quantum
Computer Devices
David N. Jamieson1, Changyi Yang1, Chris I. Pakes 1, Sean M. Hearne1, Steven
Prawer1, Fay E. Stanley2, Andrew S. Dzurak2,3 and Robert G. Clark2
1
Centre for Quantum Computer Technology, School of Physics, University of Melbourne, Victoria, 3010, Australia
2
Centre for Quantum Computer Technology, School of Physics and 3 School of Electrical Engineering &
Telecommunications, University of New South Wales, Sydney, 2052, Australia
Abstract. Devices that employ single atoms to store and manipulate information will be constructed in the near future.
For example a solid state quantum computer device has been proposed that encodes information in the nuclear spin of
shallow arrays of single 31P atoms (qubits) in a matrix of pure silicon. Construction of these devices presents formidable
challenges. We have proposed a strategy that employs single ion implantation, with an energy of 10 to 20 keV, to load
the qubits into prefabricated cells of the device that employs detector electrodes adjacent to the cells that can detect
single keV ion strikes appropriate for the fabrication of shallow arrays. Our method utilises a pure silicon substrate with
a very high resistivity, a thin (5 nm) SiO2 surface layer, biased electrodes applied to the surface and sensitive electronics
that can detect the charge transient from single keV ion strikes. A key feature of these detectors is the ultra-thin surface
dead layer. The charge collection efficiency of these detectors has been measured with MeV ions, keV x-rays and keV
ions. We show that our detectors have a near 100% charge collection efficiency for MeV ions and keV x-rays. We show
pulse height spectra from 15 keV H, He and P ion impacts allowing measurement of the pulse height defect for the keV
ions. We review the role of these detectors in the construction of a two qubit device that will test many of the essential
mechanisms of a revolutionary solid state quantum computer.
INTRODUCTION
Precision doping methods are under development.
A leading method involves use of an atomic Force
Microscope (AFM) to nanomachine hydrogen resists
for localization of surface doping sites, followed by
encapsulation in silicon by molecular Beam Epitaxy
(MBE) [6]. This precision method is evolving towards
long term applications. However we have developed
an alternative strategy for the insertion of single atoms
to precise locations in a substrate based on keV ion
implantation. This method offers the advantage of
simplicity, but the accuracy is constrained by ion
straggling, residual ion induced damage in the
substrate and limitations in the methods used to
localize the ion impacts to the required precision on
the substrate. Nevertheless it provides a rapid route
for single atom construction available immediately
As the transistor elements in ULSI Si devices
shrink below a scale of 100 nm the number of dopant
atoms per element will suffer severe statistical
fluctuations [1]. The time has therefore arrived where
the challenge of constructing devices that employ few
or single atoms as their basic functional element must
be addressed. Many proposals for such devices have
recently materialised. The motivation for the present
work is the project to construct a solid state quantum
computer [2-4] that employs individually addressable
single atoms as qubits with the potential to implement
algorithms that promise significantly increased
processing speeds. Kane [5] proposed a silicon based
device which employs arrays of 31 P atoms in Si that is
scalable,
compatible
with
Si
metal-oxidesemiconductor (MOS) technology and used single
electron transistors (SETs) for readout of the nuclear
spin orientation of the 31 P qubits.
Several groups have developed strategies for single
keV ion implantation. A common feature is the use of
a method for detection of single ion impacts triggering
a typical sequence of events to switch off the beam,
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
561
move the substrate to a new location, switch the beam
back on and repeat the sequence of events. Three
methods for detection of the single ions impacts have
been reported in the literature. These are based on ion
induced secondary electrons emitted from the surface
[7,8], scintillation of a surface layer [9] and, the
present method, electron-hole pairs induced in an
active substrate.
oriented silicon. The resistivity is therefore greater
than 5000 Ωcm. A 5 nm surface oxide isolates the
gates and ions must be implanted through this oxide.
This surface oxide also serves to stop “stray beam”
[10] often associated with low energy implantation
systems.
METHOD
A
J
A
Ion Implantation System
oxide
e–
The single ion implantation system is shown in
Figure 2. The ion source and accelerator consist of a
Colutron model 100-Q in a G-1 ion gun system fitted
with a modified velocity selector (Wein filter) and a
microchannel plate beam viewing system. A set of
MOD.3 Fischer precision micro -slits are used to
collimate the beam. These slits use a wedge shaped
aperture allowing continuous collimation from 150
microns down to less than 1 micron and were used to
collimate the beam flux to less than 100 ions per
second. Vacuum in the system was maintained by
three maglev turbopumps to better than 10-8 Torr.
e–
31P
31P
20 nm
FIGURE 1. Two qubit Kane quantum computer schematic.
This paper describes the pilot experiments for the
single ion implantation system. The Kane quantum
computer, Figure 1, proposes to couple the 31 P nuclear
spins via the overlap of the 31 P valence electrons,
themselves coupled to the nuclear spin by spin-orbit
coupling, induced by appropriate potentials from gates
registered to the individual qubits. The distance
between qubits needs to be greater than the Bohr
radius of the 31 P valence electron which is about 3 nm
in Si. In the prototype device a 31 P separation of 60
nm was feasible, but this will be reduced in the near
future. The donors need to be implanted to a depth of
the order of 20 nm, which constrains the initial beam
energy to be close to 15 keV.
The ion beam covered an area of diameter 200
microns when collimated to the required 100 ions per
second. The ion current was measured with an
AMTEK channel electron multiplier (CEM) unit that
was sensitive to single ion impacts. A front viewing
zoom microscope, with a field of view of about 1 mm
in size at maximum magnification, is used to locate the
implant region under the ion beam by first locating the
beam position in the chamber with reference to a 200
micron diameter collimator on the front of the CEM
detector.
The substrate for construction of the quantum
computer is to be high-purity single crystal <100>
Modified Colutron System
Specimen Chamber
Active
substrate
Ion gun
Einzel Lens
system
Beam
Velocity filter
deflector (E/B filter)
Micro-slits
FIGURE 2. The single ion implantation system.
562
CEM
detector
Retractable Microscope LN2
cooling
beam viewer
(electronic stopping) of which only the latter produces
a signal in the external circuit.
Ion Registration System
The present ion registration system differs from the
previous methods because it employs a single ion
impact detector integrated into the substrate being
implanted. The electron-hole pairs induced in the high
resitivity substrate have a long lifetime and are
detected by two Al electrodes. These electrodes are
deposited directly onto the surface oxide using a mask.
The as-deposited electrodes display I-V curves
characteristic of Schottky barriers. This is highly
desirable for our application because it means that
biasing one electrode relative to the other produces one
reverse biased Schottky junction. This leads to a
leakage current of less than 50 pA and hence high
sensitivity for the detection of the current transient in
the electrodes from single ion impacts.
500 µm
The electrodes, shown in Figure 3, are spaced 10
microns apart and ion impacts between the electrodes
can be detected with close to 100 % efficiency. The
detection efficiency was measured by first mapping
the charge collection efficiency with the Ion Beam
Induced Charge (IBIC) technique [11] in the
Melbourne nuclear microprobe [12]. The IBIC image,
also shown in Figure 3, was obtained by mapping the
charge collected as a function of position by scanning
the device with a focused 2 MeV He + nuclear
microprobe focused to about 1 micron in diameter
[13]. The image reveals that the strong electric field
region between the electrode fingers has a charge
collection close to 100%. In the context of this paper
100 % charge collection efficiency means that all of
the electron-hole pairs created by ion impact below the
oxide layer are collected.
80 µm
FIGURE 3. The ion detector system integrated with the
quantum computer substrate. Top: overview of device.
Centre: Close-up view of central region showing the two
detector electrode fingers with 10 micron gap where the
construction zone is located. Bottom: Nuclear microprobe
IBIC image of the central regions showing 100 % charge
collection efficiency from the construction zone.
Also visible in the IBIC image are regions where
charge can be collected from beneath auxiliary
electrodes used to make contact with the control gates
of the quantum computer that were not part of the
detection system. These regions are masked and will
not be sensitive to the 15 keV 31 P ions which
themselves cannot penetrate the mask.
The loss of energy to the nuclear collisions is
known as the pulse height defect and has previously
been measured for high energy ions [14-16], He ions
between 15 and 30 keV [17] and 25.7 keV H and He
ions [18], but not previously for 15 keV 31 P ions. The
contribution to the energy loss are listed in Table 1
which was calculated from stopping powers obtained
from SRIM [19]. The fact that only a few keV worth
of ionisation is available requires a high sensitivity,
low noise, detection system.
Detection of keV 31 P ion impacts represents a
significant challenge. This is because of significant
competing effects for dissipation of the kinetic energy
of keV ions that are not so significant for MeV ions.
First, the surface of the device is covered with a 5 nm
oxide dead layer where the energy deposited by the
passage of the ion is lost. Second, once the ions pass
through the deadlayer and enter the substrate, energy is
lost to both nuclear collisions and ionisation
The charge collection efficiency was also modeled
with numerical simulations based on the TCAD
package [20]. These simulations [21] were used to
determine the bias voltage required to obtain the
maximum charge collection efficiency consistent with
563
the dielectric strength of the substrate.
These
simulations revealed that most charge was collected
within a time interval of a few ns (see Figure 4). The
collected charge can be converted to keV by assuming
it takes 3.6 eV ionization to produce one electron-hole
pair in silicon [22].
The simulations from SRIM and TCAD show that
the ion transient detection system must be sensitive to
a few hundred electron-hole pairs induced by ion
impact. Also the electronic noise level must be less
than 0.5 keV and preferably lower. Therefore, for the
experimental data reported here, the detector was
cooled to liquid nitrogen temperatures to reduce
leakage current and noise.
15 keV 31P ion impact
Several combinations of preamplifier were tested.
With the Amptek [23] A250 preamplifier the
electronic noise level was around 4 keV, believed to be
limited by the feedback resistor. With a Moxtek [24]
MP103 preamplifier, which employs a MX20 JFET
incorporating an integral transistor reset system, the
noise was found to be 1.5 keV, believed to be limited
by residual leakage current in the substrate and
environmental noise. Developments are underway to
improve the noise shielding and reduce the noise level
further.
Electrodes
Two donor device
10µm
The detector system was integrated with a
nanomachined mask in order to fabricate a prototype
two donor device. The configuration of the system is
shown in figure 6.
FIGURE 4: Drift of electrons (light) and holes (dark)
towards the electrodes following ion impact
15 keV 31P Ions
0.1
Charge (fC)
PMMA
0.08
–
0.06
+
5 nm
oxide
0.04
Impact
signal
Silicon
0.02
Bias
voltage
0
0
5
10
15
FIGURE 6. Single ion implantation detection system (not
to scale) combined with a nanomachined 2-aperture mask.
(see text).
Electrode Potential (V)
FIGURE 5. Charge collection as a function of electrode
bias for electrode gap of 10 microns and 14 keV 31P ion
impact. From TCAD.
The mask comprised a 60 nm thick layer of PMMA
deposited over the surface of the substrate into which
two apertures had been fabricated with electron beam
lithography. The apertures were 28 nm in diameter,
with a centre-to-centre spacing of 60 nm [25] and the
PMMA thickness was sufficient to stop 15 keV 31 P
ions from reaching the substrate. Upon ion irradiation,
two ion impacts were detected. This scheme allows
construction of a two donor device with one donor
implanted through each aperture with a probability of
50%. The other 50% comprises both ions implanting
through the same aperture.
Although the IBIC measurements had shown close
to 100 % charge collection efficiency with MeV ions,
the TCAD simulations showed a charge collection
efficiency of 100 % could be achieved also with keV
31
P ion impacts (see Figure 5) with an electrode bias
voltage of less than 10 V, leading to a maximum
electric field in the substrate of 105 V/m, which is
comfortably less than the dielectric strength of Si and
SiO2 which are 3×107 and 6×108 V/m respectively.
564
RESULTS
spectrum with considerable low energy stray signals,
see Figure 8, the peak from the primary ion beam is
significant and has an energy consistent with the
expected pulse height defect.
Detector Tests
The single ion detection system was tested with
several particles, including x-rays, summarized in
Table 1. In each case the detector was cooled with a
cold finger cooled by liquid nitrogen which also
cooled the MX20 JFET.
14 keV 1H
To calibrate the system, x-ray irradiation was done
with the characteristic x-rays from 55 Mn (produced by
the radioactive decay of 55 Fe) and the pulse height
spectrum is shown in Figure 7. For testing the device,
x-rays have the advantage that they produce pure
ionization in the substrate and do not induce lattice
damage. This allows the pulse height spectrum to be
calibrated with respect to energy by assuming that the
full energy peak corresponded to the x-ray energy.
This is justified because within the shallow surface
region of the detector, where the electric field is
strong, the ionization is collected with 100 %
efficiency. However the x-rays are also absorbed
below the sensitive region and this produces a signal
below the full energy peak due to incomplete charge
collection.
FIGURE 8: Pulse height spectrum from a 14 keV 1H ion
beam using a beam with poor mass and energy purity.
The detector configured without the nanomachined
PMMA mask was also tested with 14 keV 31 P ions.
The detector was otherwise masked so that only the
region of high sensitivity was exposed to the ion beam.
The resulting pulse height spectrum is shown in Figure
9. The result is very promising for this method. First,
the noise level is about 1.1 keV, believed to arise
mainly from environmental noise in the laboratory and
residual leakage current. Second, the full width at half
maximum of the full energy peak is only about 2.5
keV. This is considerably less than might be expected
from the proportion of energy (8.5 keV) lost to the
statistically variable nuclear stopping processes.
Incomplete charge collection
owing to the large excitation
volume
14 keV 31P
55
FIGURE 7: Pulse height spectrum from M n Kα and Kβ
characteristic x-rays. The Kβ line is not resolved.
Further tests with 14 keV H ions were done to
measure the noise level in the detector, the width of
the energy signal with the aim of studying the role of
straggling and the peak position so that the ion PHD
could be measured. The detector was configured
without the nanomachined PMMA mask for the tests
with ions. Unfortunately, the temporary configuration
of the ion source system used to produce 14 keV 1 H
ions resulted in a beam of poor mass and energy
purity.
Although this produced a pulse height
FIGURE 9: Pulse height spectrum for 14 keV 31P ions.
In this prototype device, the tail of the 31 P
distribution is buried in the noise. But if the FWHM
of the distribution is assumed to be the sum of the
electronic noise and the contribution from the statistics
of the ion stopping processes, then future reduction in
565
the electronic noise level should allow close to 100%
efficiency of detection for 15 keV 31 P ions.
TABLE 1. Theoretical and experimental pulse heights for keV particles in silicon detectors with 5 nm deadlayer
Particle and Energy
Deadlayer loss
Nuclear loss (PHD)
Electronic loss
Experimental
(keV)
(keV)
(keV)
pulse height (keV)
0
0
5.89
5.89
5.89 keV x-rays (Mn K α)
14 keV H
0.34
0.61
13.05
13.0
14 keV 31P
4.14
8.56
1.99
1.67
Nuclear and electronic losses apply to the active substrate alone and do not include the loss in the 5 nm deadlayer. The
electronic loss is equal to the theoretical pulse height.
Two Donor Construction System
CONCLUSION AND FUTURE
DEVELOPMENTS
The system was used to demonstrate the
construction of a two donor device. In this case a
detector was used coated with the PMMA mask
containing the nanomachined apertures located
between the detector fingers shown in Figure 6.
Electronics connected to the detector we set to switch
off the ion beam when two ion impacts were detected.
In this case an ion impact was defined as a signal
above the noise threshold of the detector. The
resulting “pulse height spectrum” from these two
signals is shown in Figure 10.
The integration of the ion detector system with the
substrate has shown to be compatible with the
construction of devices which require single ion
doping in high purity silicon. In the near future, better
environmental shielding and improved detector
electrodes will reduce the electronic noise threshold to
allow an even greater fraction of the 15 keV 31 P ions to
be detected.
To scale up the array of donors from two, as here, it
is necessary to use a method for targeting sites on the
substrate for single ion implantation to high precision.
This may be accomplished with Focused Ion Beam
(FIB) systems and 50 nm has been achieved with 30
keV Ga + ions [26]. Even higher precision may be
possible with a “step-and-repeat” system based on the
nanostencil of Lüthi et al. [27]. Shown in Figure 11,
this system comprises a nanomachined aperture in an
AFM cantilever that is positioned in the desired
location while a focused 31 P beam dwells on the
aperture until an ion impact in the substrate is detected.
Such system may allow the construction of large-scale
arrays of single ions.
Although the noise level of the device used for
these tests was around 3.2 keV, meaning that the
device was only sensitive to the high energy tail of the
pulse height spectrum from 15 keV 31 P ions, the results
nevertheless demonstrate the technique.
Following implantation, this device was subject to
rapid thermal annealing to repair implantation damage.
Future devices will be subject to focused laser
annealing at the implant site. Further measurement
details will be reported in a later paper.
FIGURE 10: “Pulse height spectrum” from two 15 keV 31P
ion impacts in the device of Figure 6 compared to the noise
spectrum over a similar time period with the beam off.
FIGURE 11: Integration of FIB and moveable mask in
AFM cantilever with active substrate for “step and repeat
construction of arrays of single ions.
566
ACKNOWLEDGMENTS
12. .Jamieson, D.N., Nucl. Instr. and Meth., B 136-138, 1
(1998).
This work was supported by the Australian
Research Council Special Research Centre scheme
through the Centre for Quantum Computer
Technology and the USA Army Research Office under
contract number DAAD19-01-1-0653.
13. Yang, C.J., Jamieson, D.N., Hearne, S.M, Pakes, C.I.,
Rout, B., Gauja, E., Dzurak, A.S. and Clark, R.G., Nucl.
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14. Haines, E.L. and Whitehead, A.B., Rev Sci. Instr. 37 190
(1966).
The assistance of Robert Short, Roland Szymanski
and Paul Spizzirri in construction, maintenance and
operation of the ion implantation system is gratefully
acknowledged as is the skill of Eric Gauja in
fabrication of the detector devices. We appreciate the
work of Damien George for performing the TCAD
simulations of Figures 4 and 5.
15. Lee., C. and Fletcher, N.R., Nucl. Instr, and Meth., A 432
313 (1999).
16. Funaki, H., Mashimo, M., Shimizu, M., Oguri, Y. and
Arai, E., Nucl. Instr. and Meth. B 56-57, 975 (1991).
17. Hsieh, K.C., Nucl. Instr. and Meth., 138 677 (1976).
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