ZnO-based LEDs begin to show full-color potential

Start-up company MOXtronics has recently produced the first colored ZnO-based LEDs. According to a recent report in Compoundsemiconductor.net. Although the efficiency of these LEDs is not high, improvements are rapid and the emitters have the potential to outperform their GaN rivals, say Henry White and Yungryel Ryu from MOXtronics.

The attractiveness of ZnO LEDs stems from the potential for phosphor-free spectral coverage from the deep ultraviolet to the red, coupled with a quantum efficiency that could approach 90% and a compatibility with high-yield low-cost volume production. These LEDs could even one day outperform their GaN-based cousins, which offer a narrower spectral range, thanks to three key characteristics – superior material quality, an effective dopant and the availability of better alloys.

The superior material quality is seen in the low defect densities of ZnO layers. At MOXtronics, our development of a viable p-type dopant has provided hole-conducting layers for ZnO-based devices. And our growth of BeZnO layers has shown that it is possible to fabricate ZnO-based high-quality heterostructures (see "The advantages of ZnO over GaN").

ZnO also promises very high quantum efficiencies, and ultraviolet detectors based on this material have produced external quantum efficiencies (EQE) of 90%, three times that of equivalent GaN-based detectors. The physical processes associated with detection suggest that similarly high efficiency values should be possible for the conversion of electrical carriers to photons. So it is plausible that ZnO LEDs will have an EQE upper limit that is three times higher than that of GaN-based devices.