Researchers from the Low Energy Electronic Systems (LEES) interdisciplinary research group at the Singapore-MIT Alliance for Research and Technology (SMART), MIT’s research enterprise in Singapore, together with collaborators at MIT National University of Singapore and Nanyang Technological University, have discovered a new method of generating long-wavelength (red, orange and yellow) light using intrinsic defects in semi-conducting 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 colour 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 1990s and have since evolved to become ever-smaller while growing increasingly powerful, efficient and durable.
Ever-growing demand for such electronic devices has driven over two decades of research to LEDs that can emit different colours of light. Traditionally, InGaN material has been used in modern LEDs to generate purple and blue light with aluminium gallium indium phosphide (AlGaInP) – a different type of semi-conductor – 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 because of higher levels of indium required.
In addition, such InGaN LEDs with considerably high indium concentrations remain difficult to manufacture using conventional semi-conductor structures. The realisation of fully solid-state white-light-emitting devices, which require all three primary colours 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. The researchers describe a practical method to fabricate InGaN quantum dots with significantly higher indium concentration by using preexisting 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. Growing these structures on conventional silicon substrates further eliminates the need for patterning or unconventional substrates. 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, says: “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 developing future micro-LED arrays consisting of a single material.
“Our work could also have broader implications for the semi-conductor and electronics industry as the new method described here follows standard industry manufacturing procedures and can be widely adopted and implemented at scale,” says 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.”
