NATURE|Vol 441|18 May 2006 NEWS & VIEWS

DEVICE PHYSICS light with a wavelength in the UVB and the UVC portion of the solar spectrum, respec- tively. But unfortunately, as the aluminium A bug-beating diode fraction increases, so too does the difficulty of Asif Khan . It is hardest of all for aluminium — which is, in fact, an insulator. A diode that emits light at a shorter wavelength than ever before shows There are several reasons why it is difficult huge — albeit destructive — technological promise. But further work is to turn aluminium nitride or aluminium gal- lium nitride with large aluminium fractions needed to ensure that this promise is fulfilled. into conductors. It requires the incorporation of many impurity atoms to generate additional Earth’s ozone layer completely blocks solar in all three primary colours — red, green and positive and negative current carriers, but light of very low wavelengths. blue — became available, opening up a multi- thermal vibrational effects caused by the high- Biological organisms on Earth have therefore billion-dollar market for the lighting and dis- temperature conditions required for growing never developed a tolerance for this ‘UVC’ play industries. LEDs that emit at even lower, high-quality crystalline aluminium nitride radiation, and artificially generated UVC light ultraviolet wavelengths, divided into UVA, layers work against this incorporation process. has become a useful tool in the treatment and UVB and UVC, soon followed3,4 (Fig. 1). In addition, the layers are deposited over non- destruction of bacteria, yeast, viruses and The main obstacle to the development of nitride materials, such as sapphire, generating fungi. Mercury, xenon and deuterium lamps both blue–green and ultraviolet LEDs has been a large number of defects that hinder doping. are currently the main sources of UVC light, the difficulty in manipulating the conduction Defects are also generated if the number of but the high operating voltages and large size of properties of various AlInGaN materials that incorporated dopant species becomes too such lamps, together with the environmental possess the appropriate optical properties. The numerous, resulting in a self-compensation dangers of using mercury, preclude their use in level and type of conductivity of such materials effect that actually reduces conductivity. techniques for disinfection, for air and water can be controlled by incorporating impurities Finally, the effect of doping material on the purification and in biomedicine. On page 325 into them such as and magnesium, in a conduction properties can be compensated by of this issue, Taniyasu et al.1 report the poten- process known as doping. nitride (with that of other materials such as hydrogen gas, tially revolutionary production of UVC light no aluminium or ) absorbs visible used as part of the manufacturing process. using light-emitting diodes and ultraviolet light, and is typically a good Taniyasu and colleagues’ crucial contribu- (LEDs) based on aluminium nitride, which conductor. Manipulating its conductivity by tion1 is to overcome these obstacles by using a could be powered by low-voltage solar cells. doping is also relatively easy. The same is true variation of the standard growth conditions. An LED generally comprises a junction for , but this compound only They control the amount of dopant precisely between semiconductor materials with two dif- absorbs ultraviolet radiation, and is completely to avoid self-compensation, and use annealing ferent types of conductivity, electron conduc- transparent to visible light. procedures to avoid compensation effects tivity and hole conductivity, that are equivalent Typically, the wavelength of light emitted from the reactant gases such as hydrogen. to the movement of negative and positive from a semiconductor LED is nearly the same They are thus able to impart to aluminium charge carriers, respectively. At the junction, as that of the light that it absorbs. And as the nitride sufficient conductivity for both posi- electrons and holes recombine, emitting energy proportion of aluminium in the alloy increases, tive and negative current carriers. By combin- in the form of light. Highly efficient blue–green so does its transparency down to lower ultra- ing layers of the two conducting types, they LEDs, using semiconducting aluminium violet wavelengths. Thus, whereas an LED based fabricated LEDs emitting at a UVC wave- indium gallium nitride (AlInGaN), were ini- on indium gallium nitride emits visible radia- length of 210 nanometres — at present the tially reported2 in the early 1990s. Thus, for the tion, those based on aluminium gallium nitride shortest reported wavelength of any LED first time, miniature low-voltage light sources and aluminium nitride generate ultraviolet when injected with electrical current. Developments are needed in two areas to improve such UVC LEDs to the point where they can be used in devices: first, an increase in their efficiency by a factor of at least a million; and second, a reduction in their operating volt- age to well below the 25 volts used by Taniyasu and colleagues1. The first development will require substantial improvements in the crys-

UVB talline quality of the aluminium nitride layers; UVC UVA Visible Infrared the second will necessitate more-efficient dop- ing to bring about a near thousandfold increase in their room-temperature conductivity. Both AlN AlGaN InGaN challenges are difficult: further bold and inno- Intensity of light at Earth'sIntensity surface vative research will be required if the promise of UVC LEDs is to be fulfilled. ■ Asif Khan is in the Photonics and Microelectronics 100 280 315 400 700 Laboratory, Department of Electrical Engineering, Wavelength (nanometres; log scale) University of South Carolina, 301 South Main Figure 1 | Diodes in the solar radiation spectrum. The blue line indicates the approximate intensity of Street, Columbia, South Carolina 29208, USA. radiation that penetrates Earth’s atmosphere at wavelengths from the ultraviolet (UV) through the e-mail: [email protected] visible to the infrared. Light at low UVC wavelengths is completely absorbed by the atmospheric ozone layer, so organisms on Earth have developed no tolerance to it. A small-scale, low-power light source 1. Taniyasu, Y., Kasu, M. & Makimoto, T. Nature 441, 325–328 emitting in the UVC range could therefore be useful for sterilization applications. Previous work had (2006). led to indium gallium nitride and aluminium gallium nitride light-emitting diodes (LEDs) that emit at 2. Nakamura, S. et al. Jpn J. Appl. Phys. 34, L797–L799 (1995). blue2 and UVB (ref. 4) wavelengths, respectively. But, until now1, difficulties controlling the conduction 3. Kinoshita, A. et al. Appl. Phys. Lett. 77, 175–177 (2000). properties of aluminium nitride prevented the production of LEDs at UVC wavelengths. 4. Sun, W. et al. Jpn J. Appl. Phys. 43, L1419–L1421 (2004).

299 © 2006 Nature Publishing Group