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LED atomic structure opens up to reveal defect

21 Apr 2016  | Carolyn Mathas

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A team of researchers from University of California, Santa Barbara (UCSB) has revealed that there is a type of defect that exists in the atomic structure of an LED that drags performance, and removing this would provide more longer-lasting and efficient LEDs.

The research team, led by UCSB professor Chris Van de Walle, published their findings as a featured article in Applied Physics Letters. The team found that it is possible to assess whether the defects are present in the LED material, and that knowledge can be used to improve the quality of the material. In LEDs performance relies heavily on semiconductor material quality at the atomic level.

According the Van de Walle, "In an LED, electrons are injected from one side, holes from the other. As they travel across the crystal lattice of the semiconductor, in this case gallium-nitride-based material, the meeting of electrons and holes (the absence of electrons) is what is responsible for the light that is emitted by the diode: As electron meets hole, it transitions to a lower state of energy, releasing a photon along the way."

LED atomic structure

Figure 1: This schematic energy band diagram depicts SRH recombination at a (+1/0) defect level in a material with band gap Eg. If the defect is initially in a+1 charge state, recombination proceeds via (1) electron capture with rate Rn and (2) hole capture with rate Rp. Cn (Cp) are capture coefficients, N+ (N0) the density of defects in the +1 (0) charge state, and n (p) the density of electrons (holes). (Source: Applied Physics Letters)

The problem arises in the LEDs when the charge carriers meet and don't produce light, resulting in Shockley-Read-Hall (SRH) recombination. The charge carriers are captured at defects in the lattice where they combine, but don't emit light.

The defects involve complexes of gallium vacancies with oxygen and hydrogen, which were previously observed in nitride semiconductors. Now, according to Cyrus Dreyer, lead paper author, their detrimental effects are finally understood.

According to Van de Wall, co-author Audrius Alkauskas is responsible for the development of a theoretical formalism that was necessary to calculate the rate at which defects capture electrons and holes. His work will be used to identify other defects by which SRH recombination occurs.

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