Aerospace & Defense Technology - December 2022 - 32

Tech Briefs
(a)
(b)
3
3
2
1
active
region
miniband
active
region
injector
QCL period
(a) Schematic of conduction band energy diagram and wavefunctions of a QCL structure. One QCL period consists of the injector
and active region, and the period is typically repeated 30-40 times in a full QCL structure. (b) Cross-section of buried heterostructure
QCL (right) and transmission electron microscopy cross-section of a portion of a QCL core.
Exacting epitaxial growth of QCL structures goes handin-hand
with optimization of its band-structure and wavefunction
modeling, advanced processing involving fabrication
and epitaxial regrowth of high-aspect ratio devices, and
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32
2
1
Emitting
Region
demanding heat-sinking
packaging.
Impressive progress
has been made
in each of these
areas and QCLs with
improved operating
temperature, wavelength
range, output
power, and efficiency
are routinely possible.
Record performance
at room temperature
is 5W cw output
power and 21 percent
wall plug efficiency
(WPE) in the MWIR
and 2W cw power
and 10 percent WPE
in the LWIR. Those
QCLs were grown by molecular beam epitaxy (MBE) or gassource
MBE, which are both high-vacuum growth processes.
Another viable growth technique for QCLs is metalorganic
vapor phase epitaxy (MOVPE), which operates at or slightly
below atmospheric pressure. It is the mainstream platform for
more conventional p-n optoelectronic devices and was shown
to be also suitable for QCL growth in the early 2000s. Comparable
performance between MOVPE and MBE-grown QCLs
was demonstrated in 2006, although the QCL structures were
different. With these encouraging results, numerous groups
have pursued MOVPE for development of QCLs, and have
achieved a high level of success.
Despite these accomplishments, the epitaxial growth of
QCL structures continues to be a challenge. In particular,
the emission wavelength from QCLs of the same structure
can not only be different when grown by MBE or MOVPE,
but also different when grown by MOVPE at different organizations.
For example, even though the same QCL structure
was used, MBE- and MOVPE-grown QCLs emitted at
different wavelengths. MBE-grown QCLs had emission at 9.3
µm, while MOVPE-grown QCLs from different organizations
were reported with emission at 8.4 µm and at 9.2 µm. QCLs
of that same structure grown elsewhere emitted around
10 µm. Thus, it is critical to gain additional knowledge of
the relationships between epitaxial growth, their materials
properties, QCL band structure modeling, and resulting QCL
performance in order for MOVPE to be a more predictable
growth process.
This work was performed by Christine A. Wang, Benedikt
Schwarz, Dominic F. Siriani, Leo J. Missaggia, Michael. K. Connors,
T. S. Mansuripur, Daniel R. Calawa, Daniel McNulty, M.
Nickerson, J.P. Donnelly, and F. Capasso for the Massachusetts
Institute of Technology. For more information, download
the Technical Support Package (free white paper) at mobilityengineeringtech.com/tsp
under the Lasers & Photonics
category. MIT-0007
mobilityengineeringtech.com
Aerospace & Defense Technology, December 2022
https://www.mobilityengineeringtech.com/tsp http://www.mobilityengineeringtech.com
Aerospace & Defense Technology - December 2022

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