Transcript Investigating the Effects of a Microwave on a Quantum Dot Device

INVESTIGATING THE
EFFECTS OF A
MICROWAVE ON A
QUANTUM DOT DEVICE
Marco Cruz-Heredia
Outline
■ Background
– Semiconductors
■
Properties of Semiconductors
■
Semiconductor Devices
– Percolation Pathways
– Quantum Dots
■
Properties of Quantum Dots
■
Applications of Quantum Dots
■ Methods
– Device fabrication
– Measurement Methods
■ Results
■ Conclusion
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Semiconductors
■ Solid materials with
electrical resistance
between conductors
and insulators
■ The valenceconduction bandgap
is such that it can be
overcome by some
biasing, e.g. thermal
or electric potential
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Properties of Semiconductors
■ Variable conductivity – altering the electron density in the
material—called doping—alters the conductivity
– n-type – excess of electrons
– p-type – shortage of electrons
■ Electron-hole pairs can be created or annihilated in the
presence of a temperature gradient or incident photons
■ Photon emission can occur as a way to relax excited state
electrons as opposed to heat production
■ Heterojunctions – n-type and p-type materials joined together
– One directional conduction (n-type to p-type)
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Semiconductor Devices
■ Diode
– Single p-n junction.
– One directional current.
– Can be light absorbing or emitting
■ Transistor
– Bipolar junction (p-n-p or n-p-n)
– Field-Effect
■ Integrated Circuits
– Many electrical devices on a single
semiconducting substrate
■ Quantum Dots
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Trapped Charges
■ Semiconductor devices often
have an oxide layer between the
substrate and the proper
device.
■ The semiconductor-oxide layer
interface can have defects that
form charge traps.
■ These charge traps affect the
conductivity of their neighboring
regions and give rise to what we
call percolation pathways
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Percolation Pathways
Contour diagram
Typical cross section
Potential
e
microwave
Potential
e
e
Maekawa Lab meeting 20160520
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Quantum Dots
Semiconductor devices of
size of the same order as
the Bohr exciton.
Johnston, Physics World (2015)
e
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Properties of QDs
■ Photoluminescence (PL)
– Narrow Gaussian spectra; high resolution
■ Discrete excitonic states
– Similar to states between two potential barriers
■ Tuneable
– Size and shape affect:
■
PL spectrum
■
Transition energy (HOMO  LUMO)
■
Conductivity
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Applications of QDs
■ Biomedical devices
– Alternative to organic dyes used in particle tracking
– Medical Imaging
■ LEDs and LCD displays
– Since the Sony XBR X900A in 2013
■ Diode Lasers
■ Transistors
■ Photodetectors and Photovoltaic cells
■ Quantum computing
– Use of discrete energy states
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Electron Pumping
Potential well illustration of the electronic transitions
in a quantum dot during four stages of an RF cycle
Van der Wiel et.al. Photon Assisted Tunneling in Quantum Dots (2002)
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Device Fabrication
Charge sensor (CS)
CS side gate
Quantum dot (QD)
Silicon – SOI layer (~20nm)
SiO2 – BOX layer (380nm)
Silicon – substrate
QD side gate
Back gate
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Measurement Methods
■ The conductivity of the device was observed by looking at the
source-drain current which is a proportional quantity.
■ The effect of three independent variables was investigated:
– backgate (offset) voltage
– source-drain (biasing) voltage
– microwave frequency
■ Each variable was swept while keeping the others constant
■ 2 parameter and 3 parameter sweeps were performed as well
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Experimental Setup
■ The quantum dot device is mounted on a measurement stage
which is submerged in a liquid helium tank.
■ The stage runs connections from the device to an SMB
connector which then connects to a voltage source and
measuring device
■ An RF wave emitter is connected via optical wire and is
attenuated to minimize reflection
■ A computer program is used to sweep the parameters and
each one can be set manually if a constant value is desired
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Coulomb Peaks
■ Peaks in sourcedrain current as a
function of backgate
due to the matching
of the energy
eigenvalues of the
quantum dot
■ This measurement
was done without
the presence of a
microwave
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Coulomb Peaks
■ This measurement
was made with an
incident microwave
away from resonance
frequency
■ Notice the dips at the
end of the coulomb
peaks, this is due to
electron pumping
■ Notice that the peaks
are broadened and
higher
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Coulomb Peaks
■ This measurement
was made with the
microwave at
resonant frequency
■ We see that we no
longer have electron
pumping and the
peaks reduce in
height again.
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MW Frequency sweep
VSD=0.06 V
VBg=5.7 V
Low-Q
High-Q
Due to
hopping and tunneling
Due to
two-level system
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High Q Resonance
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Coulomb Diamonds
■ This maps the sourcedrain current
(conductance) as a
function of backgate
voltage (x) and sourcedrain voltage (y)
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Coulomb Diamonds
■ This maps the sourcedrain current as a
function of backgate
voltage (x) and sourcedrain voltage (y) with no
microwave
■ Should form more of a
diamond shape but sweep
durations are long relative
to the drift velocity of the
energy levels due to
thermal fluctuations.
21
Coulomb Diamonds
■ This maps the sourcedrain current as a
function of backgate
voltage (x) and sourcedrain voltage (y) off
resonant frequency
■ Should form more of a
diamond shape but
sweep durations are
long relative to the drift
velocity of the energy
levels due to thermal
fluctuations.
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Coulomb Diamonds
■ This maps the sourcedrain current as a
function of backgate
voltage (x) and
source-drain voltage
(y) at resonant
frequency
■ Should form more of
a diamond shape but
sweep durations are
long relative to the
drift velocity of the
energy levels due to
thermal fluctuations.
23
Resonance Map
■ Color map of
source-drain current
as a function of
frequency and
backgate voltage
■ Note that the
frequency
resonance at the
32.8 BGV coulomb
peak is not aligned
with the frequency
at the 32.3 BGV,
this is due to the
resonance drift.
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Resonance Map
■
This figure is
interesting despite
the error due to the
very clear,
discernible coulomb
peaks
■ The quantum dot
dominates the
conductivity of this
device which makes
it very well behaved
25
Resonance Map
■ This is a
resonance map
of the same
frequency
resonance and
coulomb peaks
taken with a
resonance
tracking
algorithm, note
that the
resonance is now
aligned.
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Resonance Map
■ We see that in
this case we have
that the
conductance
drops sharply
right after the
coulomb peaks
(as you increase
the backgate)
which agrees
with our plots for
off resonance
coulomb peaks
27
Conclusion
■ Semiconductor quantum dots show promise for the
implementation of quantum computing
■ The charge-based approach taken in this experiment is still in
the early stages, but results are not fully discouraging
■ Better control over the temperature of the device would help
resolve charge states and discrete quantum mechanical
behavior
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Questions?
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