Inverted Structure Yields Solar-Blind UV Detector

Michael D. Wheeler

A team from Cree Lighting Co. has devised an AlGaN photodiode that may find application as a solar-blind UV detector. Whether locating faults on power lines or tracking incoming missiles, numerous applications call for the detection of UV light, but standard, silicon-based photodetectors don't measure up for outdoor applications because they can be overwhelmed by the sun. Cree's inverted heterostructure device escapes this problem.

Solar-blind detectors respond to light in a narrow wavelength range, roughly between 260 and 300 nm.

Atmospheric ozone absorbs sunlight in this range, so it never reaches the surface of the Earth. Thus, detectors that are sensitive in this range see only terrestrial sources of UV light.

Solar-blind photodiodes based on AlGaN have been limited by some fundamental difficulties. As the aluminum content of an AlGaN layer increases, cracking problems arise. In traditional designs, the N- and P-type layers have a larger bandgap than the absorbing layer sandwiched between them. In AlGaN-based solar-blind detectors, these layers must have very high aluminum content.

Inverted heterostructure

"In this design we replace one or both of the wide-bandgap cladding layers with a layer [that] has a smaller bandgap than the absorbing layer," said Peter Kozodoy, a member of the Cree research team. Thin cladding layers minimize absorption, and the heterobarrier at the interface of the layers prevents long-wavelength light from contributing to the photodiode's response

"By sidestepping a number of thorny materials issues, this design has allowed us to greatly facilitate the production of AlGaN-based solar-blind UV photodiodes," he said.

The research appeared in the July 17 issue of Applied Physics Letters.