Suggesting applications in the elimination of environmental toxins, investigators at NTT Basic Research Laboratories in Atsugi, Japan, have reported the development of an AlN PIN homojunction LED that emits 210-nm radiation. With the proper choice of materials and the optimization of their approach to doping AlN, they hope to realize a practical version of the emitter within five yearsTreatment with ultraviolet C radiation — typically defined as radiation with wavelengths of 100 to 280 nm — is an environmentally friendly alternative to the use of substances such as chlorine or ozone for the elimination of harmful microorganisms from drinking water. It also may be used to break down and render harmless persistent contaminants such as polychlorinated biphenyls and dioxins.With an emission wavelength of 210 nm, the AlN LED suggests potential applications in the ultraviolet decomposition of environmental contaminants. The AlN PIN homojunction is sandwiched between P- and N-type AlN/AlGaN superlattices. A Pd/Au semitransparent P-type electrode and a Ti/Al/Ti/Au N-type electrode overlay the superlattices. Courtesy of Yoshitaka Taniyasu. Unfortunately, mercury lamps and gas lasers are the best sources of such radiation today. The former present environmental problems of their own, and the latter lack the form factor and simplicity of operation to make them an appropriate solution for many such applications. A compact, low-energy-consumption LED source of ultraviolet, in contrast, would be ideal.Yoshitaka Taniyasu of the NTT lab said that AlN, with its wide direct bandgap of approximately 6 eV, always has been theorized to be able to yield LEDs that produce ultraviolet C radiation. The problem has been that, although the bandgap enables the emission of very short wavelengths, it makes it difficult to dope AlN to produce a semiconductor diode. By exploring the mechanisms behind this limitation, however, the scientists discovered a means of overcoming it.“We noticed that unintentionally incorporated crystal defects and impurities limit the doping control in AlN,” he explained. “When the defects or impurities exist in doped AlN, they capture generated holes or electrons, and the P-type or N-type layer is not formed. So we focused on improving the quality of AlN to obtain N-type and P-type doping.”To do so, the scientists raised the growth temperature of metallorganic vapor phase deposition to 1100 °C and increased the flow of the Al(CH3)3 and NH3 source gases, resulting in a decrease in defect density and impurity concentration of an order of magnitude. This enabled them to reliably produce N-type and P-type AlN by doping with silicon and magnesium, respectively, and thus to realize a functional LED.In the researchers’ proof-of-principle demonstration, the 210-nm LED displayed an external quantum efficiency on the order of 10–6 percent, which they note is millions of times less efficient than typical visible LEDs. Taniyasu said that the most significant limiting factor remains the defect density, and he suggested that moving from an SiC substrate to AlN would address this by eliminating defects attributable to lattice mismatch.He added that they are exploring alternatives to magnesium as the P-type dopant to improve the doping efficiency and thereby increase the external quantum efficiency and decrease the operating voltage.Nature, May 18, 2006, pp. 325-328.
UV LEDs