Hamamatsu News 2-2016 - page 8

News 2016 Vol. 2
R&D Interview
Quantum cascade laser (QCL)
The light emission point of a QCL is basically about 10 μm
square which means the optical axis is aligned on a point about one-
tenth the diameter of a hair, so this is no easy task. Moreover, even if
collimated, just a tiny deviation of the lens axis during shipping will
make it defective, so it has to be manufactured while taking its usage
environment into account.
Are there some measurement tasks that only QCL can accomplish?
One example is isotope measurement. For instance in CO
there are carbon and oxygen isotopes that have respectively different
mass numbers, and so each of them has their own different absorption
wavelength. Even within the same CO
, the isotope
will absorb
light at a wavelength of 4.329 μm while
will absorb light at a
wavelength of 4.328 μm. By finding the isotopic ratio, we can determine
the gas emission source such as plants, soil, and combustion, as well
as the generating factor. So this ability to measure isotopes can be
called the true value of the QCL.
Could you also tell us about the InAsSb photovoltaic detector
serving as the light sensor?
The InAsSb photovoltaic detector is a compound semicon­
ductor utilizing indium (In), arsenic (As), and antimony (Sb). The
mercury cadmium telluride or MCT photoconductive detector and
photovoltaic detector are widely used as conventional infrared detectors
in the 3 μm to 10 μm range. However, the MCT detectors include
mercury and cadmium which are prohibited under the RoHS directive
on use of hazardous substances, so we newly developed the InAsSb
detector that includes none of these hazardous substances.
Creating the InAsSb detector required developing new techniques
for crystal growth and wafer process. This was necessary because we
could not adapt the existing techniques directly to the fabrication of
the InAsSb detector and had to newly develop optimal growth methods
and process techniques to match the materials used. Film layers are
formed by crystal growth on a wafer serving as the substrate. The quality
of the crystal is a major factor that greatly affects the device character­
istics, so we had to make a lot of modifications to obtain a high quality
crystal. We finally achieved high sensitivity by applying new techniques
to the wafer process and improving the device structure. Results from
these efforts gave us new knowledge and skills for both crystal growth
and wafer process.
MCT detectors exhibit large individual variations. However,
the InAsSb photovoltaic detector offers advantages that it does not
contain hazardous substances such as mercury and cadmium and also
has highly stable characteristics and minimal variations between devices.
The InAsSb photovoltaic detector is also good for mass production once
the device specifications are settled.
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