Alloying boron to InGaN: a promising route to high-efficiency visible LEDs

November 30, 2017 // By Julien Happich
Trying to circumvent the inherent efficiency limitations of narrow InGaN quantum well emitters (due to Auger recombinations at high current) as they are imperfectly grown on GaN (because of lattice mismatch), researchers from the University of Michigan used predictive atomistic calculations to identify novel alloy configurations that would present a better lattice match while offering a suitable band gap for emission in the visible spectrum.

In light emitters, the Auger recombination rate is equal to the carrier (electrons & holes) density cubed times a material-dependent Auger coefficient. Technically, the carrier density (and the Auger recombination rate) at a given current density could be reduced by simply increasing the volume of the active-region. But that requires growing dislocation-free thick quantum wells, only achievable with a near perfect lattice match with their substrate.

Basing their calculations on the hybrid density functional theory, the researchers investigated the thermodynamic, structural, and electronic properties of semiconducting BInGaN alloys and found that alloys with a boron:indium ratio of 2:3 were better lattice matched to GaN than InGaN.


Crystal structure of a BInGaN alloy.
Credit: Michael Waters and Logan Williams.

They calculated that their virtual alloy would be nearly lattice matched for indium fractions under 0.2. What they also found computationally was that by varying the Ga mole fraction of the alloy while keeping a constant B:In ratio, they would be able to adjust the semiconductor's direct band gap in the 1.75 to 3.39 eV range, covering the entire visible spectrum for the design of high-efficiency high-current LEDs.

The thermodynamics analysis published under the title "BInGaN alloys nearly lattice-matched to GaN for high-power high-efficiency visible LEDs" in the Applied Physics Letters also notes that boron is more easily incorporated into InGaN than into pure GaN, and such allows have already been made, which should be further investigated for droop-free LEDs.

University of Michigan – www.umich.edu

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