Physicists in Japan have made a diode that emits light at the shortest wavelength ever. The device, made by Yoshitaka Taniyasu and colleagues at NTT Basic Research Laboratories in Atsugi, is made from aluminium nitride and emits deep in the ultraviolet part of the spectrum at 210 nanometres (Nature 441 325). The work represents an important step towards the development of very low-wavelength light emitters that could find use in a wide variety of applications, including medicine, photolithography and to destroy bacteria in contaminated water.
A light-emitting diode (LED) generally consists of a junction between two types of semiconducting materials: an “n-type” layer in which current is carried by mobile electrons and a “p-type” layer where the carriers are positively charged holes. The electrons and holes recombine at the junction to emit light. Following the development of blue-green LEDs based on aluminium indium gallium nitride in the early 1990s, low-voltage light sources in all three primary colours – red, green and blue – were available for the first time, opening up a multi-billion dollar market for the lighting and display industries. This material has a wide band-gap, which means the wavelength of emitted light is low.
Although LEDs based on indium gallium nitride emit light in the visible range, those based on aluminium gallium nitride and aluminium nitride emit ultraviolet light. However, as the amount of aluminium in the alloy increases, it becomes more and more difficult to “dope” the material. Doping is necessary to improve the electronic properties of semiconducting materials and it works by increasing the number of charge carriers (electrons and holes) in the material. However, aluminium nitride itself is notoriously difficult to dope because it has the widest band-gap of any semiconductor at 6 eV and is, in fact, an insulator.
Taniyasu and co-workers have now overcome this problem by adapting the standard growth conditions traditionally used to make this compound. Aluminium nitride usually contains many crystalline defects and large amounts of impurities, but the new method produces high-quality aluminium nitride in which both n- and p-type doping can be precisely controlled. This ensures that both the n and p-layers have sufficient conductivity so that enough electrons and holes can recombine to produce light.
The researchers made their LED by sandwiching an undoped layer of aluminium nitride between n- and p-type layers. When current is passed though the structure, it emits ultraviolet light with a wavelength of 210 nm.
“The devices could be used in biomedical research and water purification,” says Taniyasu. “Moreover, micro-fabrication technology and environmental science both demand light sources with shorter emission wavelength: the former for improved resolution in photolithography and the latter for sensors that can detect minute toxic particles.”
Before such applications become a reality, however, the researchers say they need to improve the device’s efficiency at least a million-fold as well as its output power, which is just 0.02 microwatts at present. In contrast, state-of-the art LEDs have an operating power level of 1 to 10 milliwatts. They must also reduce the LED’s huge operating voltage of 25 V.