Compact and portable sources of ultraviolet (UV) radiation are important for applications such as purification and sterilization, particularly in the developing world. Semiconductors offer such portability without using the harmful materials found in many conventional UV sources. Yoichi Kawakami and co-workers from Kyoto University in Japan have now constructed the most efficient semiconductor UV source obtained to date — an achievement made possible by rethinking the way charge carriers are injected into the device.1

Fig. 1: An electron beam injects carriers directly into a wide-bandgap semiconductor to create an efficient source of ultraviolet light.From Ref. 1. © 2010 Y. Kawakam

The properties of a semiconductor are largely determined by an energy barrier known as the band gap. Aluminum nitride is of particular interest because it has a larger band gap than other semiconductors such as gallium arsenide, making it useful as a source of short-wavelength, UV light.

Light-emitting devices, like most semiconductor components, operate when injected with a flow of electrons. Such an electron flow is usually delivered by an electrical current that passes into and out of the device via metallic contacts. The problem is that metal contacts do not make an efficient connection with aluminum nitride. “We have taken an alternative approach,” explains Kawakami. “We fire electrons directly at the structure using an electron-beam gun.”

Their UV source comprises thin layers of aluminum gallium nitride sandwiched between aluminum nitride barriers. Substituting gallium for some of the aluminum atoms in the aluminum nitride crystal allows the emission wavelength of the structure to be tuned, in this case resulting in a device that outputs light with a peak wavelength of 240 nm. Using the electron-beam technique, Kawakami and his colleagues generated 100 mW of UV radiation at a record efficiency of 40%. The team attributes the improvement in part to the high quality of their material, which was grown with atomic meticulousness by a process known as migration-enhanced epitaxy.

The development of semiconductor technology over the past 60 years has brought successive advances in miniaturization — electronic appliances that would once have filled entire rooms can now be slipped into a pocket. With further improvements in efficiency and optical output, Kawakami and his team believe that ultrasmall sources of UV light based on their technology will contribute to further device miniaturization.