Superfast Detection Of UV Light By Uniting SiC And Ga2O3
A new type of photodiode composed of silicon carbide and gallium oxide shows promise for space-based communication and monitoring ozone depletion
Researchers in Japan have developed a new photodiode that can detect in just milliseconds, a certain type of high-energy ultraviolet light, called 'UVC'.
They say that UVC is powerful enough to break the bonds of DNA and harm living creatures. The researchers describe their new device in the Journal of Applied Physics Letters.
Although this radiation doesn't normally reach the Earth's surface, it can leak through to just below the hole in the ozone layer. Monitoring this radiation is a way of tracking the hole in the ozone layer, and photodiodes that measure UVC are also used as flame sensors and for communication in space.
Now, Shinji Nakagomi and colleagues at Ishinomaki Senshu University in Japan have built a new kind of photodiode that can detect the whole range of UVC light while remaining insensitive to visible light from the sun - two features that have eluded designers of current devices. By being "solar blind," photodiodes are more sensitive to the UVC range and are therefore more useful.
Building a Better Photodiode
Some photo detectors consist of vacuum tubes on sale, but they have short lifetimes and are relatively large and unwieldy. Today, many photodiodes are instead based on a p-n junction, in which a semiconductor that carries positive charge (the absence of electrons called holes) is put in contact with the same semiconductor that instead carries negative charge (electrons).
When light with sufficient energy (short wavelength) strikes atoms near the interface between the two semiconductors - the p-type and n-type, respectively - it generates mobile electrons and holes, boosting electrical current across the p-n junction and signalling the presence of light.
Other photodiodes made from materials like AlN and diamond are sensitive only to a limited range of UVC light. But recently, the material gallium oxide (in the form of beta-Ga2O3, (β-Ga2O3)) has shown promise because it's sensitive to the entire UVC range and is solar blind. The problem, however, is that it's difficult to make p-type Ga2O3.
A crystal model of β-Ga2O3 (Credit: Ishinomaki/S.Nakagomi)
Instead of a photodiode based on a conventional p-n junction, the researchers built one based on what's called a heterojunction, which is a p-n junction that incorporates two different semiconductors.
The team used β-Ga2O3 and SiC, and found that their device responds within milliseconds to UV light and exhibited little dark current. This is the intrinsic electrical current that flows through the device even in the absence of light. Minimal dark current is important for a sensitive photodiode.
"The most important aspect about our device is that it is based on a heterojunction between gallium oxide and silicon carbide," says Nakagomi. "This gallium oxide and silicon carbide photodiode is promising for the detection of UV light."
This work is described in detail in the paper, "Deep ultraviolet photodiodes based on β-Ga2O3/SiC Heterojunction," by Shinji Nakagomi et al in the Journal of Applied Physics Letters. http://dx.doi.org/10.1063/1.4818620