Researchers from the Low Energy Electronic Systems Interdisciplinary
Research Group at Singapore-MIT Alliance for Research and Technology, MIT's
research enterprise in Singapore, together with collaborators at the MIT,
National University of Singapore and Nanyang Technological University have
discovered a new method of generating long-wavelength (red, orange, and
yellow) light through the use of intrinsic defects in semiconducting
materials, with potential applications as direct light emitters in
commercial light sources and displays. This technology would be an
improvement on current methods, which use phosphors, for instance, to
convert one color of light to another.
A type of group-III element nitride-based light-emitting diode (LED), indium
gallium nitride (InGaN) LEDs were first fabricated over two decades ago in
the 90s, and have since evolved to become ever smaller while growing
increasingly powerful, efficient and durable. Today, InGaN LEDs can be found
across a myriad of industrial and consumer use cases, including signals
& optical communication and data storage—and are critical in high-demand
consumer applications such as solid state lighting, television sets,
laptops, mobile devices, augmented (AR) and virtual reality (VR) solutions.
Ever-growing demand for such electronic devices has driven over two decades
of research into achieving higher optical output, reliability, longevity and
versatility from semiconductors—leading to the need for LEDs that can emit
different colors of light. Traditionally, InGaN material has been used in
modern LEDs to generate purple and blue light, with aluminum gallium indium
phosphide (AlGaInP) – a different type of semiconductor—used to generate
red, orange, and yellow light. This is due to InGaN's poor performance in
the red and amber spectrum caused by a reduction in efficiency as a result
of higher levels of indium required.
In addition, such InGaN LEDs with considerably high indium concentrations
remain difficult to manufacture using conventional semiconductor structures.
As such, the realization of fully solid-state white-light-emitting
devices—which require all three primary colors of light—remains an
unattained goal.
Addressing these challenges, SMART researchers have laid out their findings
in a paper titled "Light-Emitting V-Pits: An Alternative Approach toward
Luminescent Indium-Rich InGaN Quantum Dots", recently published in the
journal ACS Photonics. In their paper, the researchers describe a practical
method to fabricate InGaN quantum dots with significantly higher indium
concentration by making use of pre-existing defects in InGaN materials.
In this process, the coalescence of so-called V-pits, which result from
naturally-existing dislocations in the material, directly forms indium-rich
quantum dots, small islands of material that emit longer-wavelength light.
By growing these structures on conventional silicon substrates, the need for
patterning or unconventional substrates is further eliminated. The
researchers also conducted high spatially-resolved compositional mapping of
the InGaN quantum dots, providing the first visual confirmation of their
morphology.
In addition to the formation of quantum dots, the nucleation of stacking
faults—another intrinsic crystal defect—further contributes to emissions of
longer wavelengths.
Jing-Yang Chung, SMART graduate student and lead author of the paper said,
"For years, researchers in the field have attempted to tackle the various
challenges presented by inherent defects in InGaN quantum well structures.
In a novel approach, we instead engineered a nano-pit defect to achieve a
platform for direct InGaN quantum dot growth. As a result, our work
demonstrates the viability of using silicon substrates for new indium-rich
structures, which along with addressing current challenges in the low
efficiencies of long-wavelength InGaN light emitters, also alleviate the
issue of expensive substrates."
In this way, SMART's discovery represents a significant step forward in
overcoming InGaN's reduced efficiency when producing red, orange and yellow
light. In turn, this work could be instrumental in the future development of
micro LED arrays consisting of a single material.
Dr. Silvija Gradečak, co-author and Principal Investigator at LEES, added,
"Our discovery also has implications for the environment. For instance, this
breakthrough could lead to a more rapid phasing out of non-solid-state
lighting sources—such as incandescent bulbs—and even the current
phosphor-coated blue InGaN LEDs with a fully solid-state color-mixing
solution, in turn leading to a significant reduction in global energy
consumption."
"Our work could also have broader implications for the semiconductor and
electronics industry, as the new method described here follows standard
industry manufacturing procedures and can be widely adopted and implemented
at scale," said SMART CEO and LEES Lead Principal Investigator Eugene
Fitzgerald. "On a more macro level, apart from the potential ecological
benefits that could result from InGaN-driven energy savings, our discovery
will also contribute to the field's continued research into and development
of new efficient InGaN structures."
Reference:
Jing-Yang Chung et al, Light-Emitting V-Pits: An Alternative Approach toward
Luminescent Indium-Rich InGaN Quantum Dots, ACS Photonics (2021).
DOI: 10.1021/acsphotonics.1c01009
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