Current white light generation techniques rely on color mixing from red, blue, and green light. While reasonable red and blue light-emitting diodes (LEDs) are available for this scheme, there currently exists a gap in the wavelength spectral window from roughly 530 nm to 570 nm — known as the “green gap” — where realization of efficient green LEDs is proving to be very difficult.

For over a decade, wide bandgap III-V alloys that utilize Ga, Al, and In as cations, and N as the anion, have been extensively researched for synthesis of various wavelength LEDs, including green LEDs. The primary obstacle for green LEDs is the poor material quality due to the alloy phase separation that occurs when substantial amounts of In is added to GaN to shift the emission into the green gap, or substantial amounts of Ga is added to the InN to shift the emission towards the red.

Device under forward bias emitting in the green gap.

The present invention provides a solution to this problem by approaching the green gap not from the lower limit (530 nm) but from the upper limit (570 nm) using the GaInP alloy grown on conventional GaAs substrates. Typically, lattices created by molecular gases don’t match up with the lattices of the layer below, resulting in very poor efficiency of the device. National Renewable Energy Laboratory’s solution to this problem is to gradually bridge the gap between these mismatched layers with intermediate layers that have lattice patterns close to, but not exactly matching, the layers below. The difference in lattice between these intermediate layers is within the elastic limit of the material. By building iterations of these layers, the overall lattice mismatch can be bridged.

A p-i-n diode structure device has been synthesized which, when forward biased, functions as an LED that emits green light well within the green gap (see image). This device essentially operates a solar cell in reverse, and is not optimized for production of light. Regardless, the device produces a deep green at the peak of the green gap.

Visit the U.S. Department of Energy for information concerning a license to use the technology, or a partnership to further develop it.

Lighting Technology Magazine

This article first appeared in the November, 2010 issue of Lighting Technology Magazine.

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