Transcript Slide - Computer Engineering Research Group

Temperature behaviour of
threshold on broad area
Quantum Dot-in-a-Well laser
diodes
By: Bhavin Bijlani
Why use quantum dots?
• The gain of a laser active region,
is proportional to its density-ofstates function (DOS).
• In bulk (a), layered (b) and wire
(c) materials, there are always
states populated which do not
contribute to gain. These are
parasitic states and contribute to
inefficiency.
• In quantum dot (d) materials, the
DOS is a set of discrete states.
Theory predicts this type of
material is ideal for the gain
region of a laser because fewer
parasitic states are occupied.
Ideal quantum dot lasers
From theory, it is predicted that using quantum dots as a
laser gain material has many beneficial properties.
• If the energy separation between
the ground and first excited state is
large enough, then all the dots will
have ground state population.
• Excited states are ‘parasitic’ to
ground state lasing. If an electron
in an excited state emits radiatively,
the photon would not be at the
correct lasing frequency and would
contribute to inefficiency.
Excited
States
Ground
State
Simplified
Quantum Dot
potential profile
• The threshold current is very
low and won’t vary with
temperature because the
excited state would not become
populated. This is again
assuming a large energy
separation.
• The differential efficiency
approaches the internal
quantum efficiency as dot
density increases. It is thus
possible to have very high
differential efficiency QD lasers.
Optical power output
Ideal quantum dot lasers
Threshold
Current
Slope is the
differential
efficiency
Injected Current
Dot-in-a-well lasers
• For a quantum dot (QD) to ‘capture’ an injected electron, the
electron energy and confined state energy must be close to
one another. Also, the spatial wavefunction of the electron
must cover a significant portion of the dot. This is not always
likely and causes typical QD lasers to deviate from the ideal.
• To remove this requirement, the concept of placing QD’s
within a quantum well (QW) was devised. The QW initially
captures the electron, confining it within its boundaries. Then,
the electron is captured and localized further by the QD’s.
Example DWELL TEM image taken
by a group at University of Sheffield.
These are InAs QD’s in InGaAs wells.
Materials Science and Engineering C 25 (2005) 779 – 783
Material and Band structure
• The lasers studied were Quantum-Dot-in-a-Well (DWELL) Broad
area lasers. InAs quantum dots (QD) are placed within InAlGaAs
quantum wells (QW), grown by molecular beam epitaxy onto InP.
InP
InAlGaAs
0.354eV
1.46eV 1.02eV
InAlAs
Simplified layer profile
QW
1.35eV

QD
InAlAs
Simplified band structure
Threshold characterization
• The temperature dependence of laser threshold between two
temperatures is usually defined by the characteristic temperature,
T0. This term is defined by the equation below.
I th2
 T2  T1 

 I th1  exp 
 T0 
• A larger T0 signifies a weak dependence of threshold on
temperature. Conversely, a small T0 signifies a strong variation of
the threshold current with temperature.
• Typical InGaAsP quantum well lasers have room temperature (RT)
T0 values around 60 K. GaAs quantum well lasers can have RT T0
values well over 100 K.
Threshold characterization
Pulsed
Current
Source
DWELL Laser
Computer
Temperature
Controller
Infrared Detector
Thermoelectric cooler and
thermistor
A pulsed current source drives the DWELL laser and simultaneously
measures the power output. A temperature controller sets the temperature of
a cooling chuck just below the laser while a computer collects the data.
Characteristic Temperatures
• We have determined the temperature dependence of the laser
threshold in the temperature range between 15 ºC and 40 ºC. The
characteristic temperature, To, was determined for five cavity
lengths ranging from 500 um to 2 mm.
Characteristic Temperature T0 (K)
15 - 30 °C
30 - 40 °C
Cavity Length
1.0 μs pulses
0.5 μs pulses
1.0 μs pulses
0.5 μs pulses
0.50 mm
62.3 ± 3.3
57.7 ± 3.0
56.5 ± 4.4
56.7 ± 4.5
0.75 mm
60.2 ± 3.1
60.0 ± 3.1
56.2 ± 4.3
57.1 ± 4.7
1.00 mm
62.5 ± 3.1
63.2 ± 3.2
59.1 ± 4.8
58.5 ± 4.5
58.1 ± 1.7
 from 15 to 40°C
54.8 ± 4.0
58.3 ± 4.2
1.50 mm
2.00 mm
64.0 ± 3.2
59.9 ± 2.9
Luminescence-current curves
Threshold versus Temperature
Summary
• We present the benefits of the Quantum-Dot-in-a-well structure as a
coherent light source. By using InP as a substrate, long wavelength
emission is possible (λ ~ 1.6 μm).
• The characterization of the threshold dependence on temperature
reveals T0 values ~ 60 K between 15 °C and 40 °C.
• These values are close to performance of other long wavelength InP
lasers.
• More spectroscopic studies of the dots and lasers are needed to
refine the performance towards ideal behaviour.