Candidate:
Teppei
Miyoshi
Title:
Theoretical
Studies
on
Terahertz
Quantum
Cascade
Lasers
for
Device
Design
Guideline
Date:
January
16,
2023
Time:
9:00
AM
Place:
REMOTE
ATTENDANCE
Supervisor(s):
Nielsen,
Christopher
(Acting
Supervisor)
Abstract:
Terahertz
(THz)
quantum
cascade
laser
(QCL)
is
the
most
promising
THz
light
source
for
the
next-generation
THz
technologies
of
imaging,
spectroscopy,
and
wireless
communication.
However,
room-temperature
operation
of
THz
QCLs,
which
is
necessary
for
these
applications,
has
yet
been
demonstrated,
and
the
record
value
of
the
highest
operation
temperature
of
THz
QCLs
remains
250K.
Thus,
this
issue
has
been
tackled
theoretically
and
experimentally
by
many
researchers
and
institutions
around
the
world
nowadays.
In
this
thesis,
three
theoretical
research
topics
related
to
device
design
guideline
for
high-performance
devices
are
presented.
(1)
development
of
fast
and
accurate
device
simulator,
(2)
investigation
of
the
influence
of
impurity
doping
on
device
performance,
and
(3)
investigation
of
the
influence
of
barrier
height
on
device
performance.
First,
a
fast
and
accurate
device
simulator
is
developed
based
on
a
rate
equation
model
introduced
by
Razavipour.
This
development
implements
speed-up
of
the
calculation
of
electron-electron
scattering
rate,
careful
selection
of
a
screening
effect
model,
introduction
of
theoretical
models
of
pure
dephasing
time,
optical
linewidth,
and
a
leakage
current.
In
calculation,
the
developed
device
simulator
can
reproduce
the
device
characteristics
of
previously
published
devices
accurately,
and
the
simulated
highest
operation
temperature
of
previously
published
devices
are
reasonable
compared
to
the
experimental
values.
Second,
the
influence
of
ionized
impurities
on
device
performance
is
investigated
based
on
a
two-well
resonant-phonon
THz
QCL
designed
by
Khalatpour
et
al.
Seven
doping
conditions
determined
by
doping
positions
and
distributions
and
an
undoped
condition
are
simulated
over
a
range
of
sheet
doping
density
from
1.0×1010
to
1.0×1012cm-2
for
a
single
module.
By
this
simulation,
optical
linewidth
is
found
to
be
small
in
the
undoped
condition
due
to
absence
of
ionized-impurity
scattering,
resulting
in
the
highest
optical
gain
among
all
doping
patterns.
Among
doped
conditions,
wide
doping
in
a
phonon-well
shows
the
highest
optical
gain,
and
this
is
attributed
to
mitigation
of
band-bending
effect.
Based
on
these
results,
a
modulation
doping
scheme
is
proposed.
Lastly,
the
influence
of
barrier
height
on
device
performance
of
two-well
resonant-phonon
THz
QCLs
is
investigated
through
two
steps
of
research,
and
a
series
of
comprehensive
research
discovers
a
new
device
design
concept
featured
by
a
thick
radiation
barrier
which
is
around
40Å
(Type-B).
Furthermore,
around
15%
of
Al-composition
is
found
to
provide
the
optimal
barrier
height
conditions
for
two-well
resonant-phonon
structures
under
both
the
conventional
design
(Type-A)
and
Type-B
concepts
in
device
structure
exploration.
The
reason
for
this
feature
is
clarified
by
careful
analysis
of
calculation
data,
and
the
optimal
conditions
are
found
to
happen
by
complex
behavior
of
componential
parameters
of
optical
gain,
which
varies
with
barrier
height.
Monday, January 16, 2023 9:00 am
-
9:00 am
EST (GMT -05:00)