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Aluminum nitride is a solid nitride with high thermal conductivity and a wide band gap of about six eV at room temperature, making it suitable for optoelectronic devices. In particular, its relatively large band gap makes it a promising candidate for group III-nitride-based LEDs spanning the UVA, UVB and UVC spectrum. However, the main obstacle to practical AlN LEDs has been the existence of extremely high densities of threading dislocations in both the Al-rich AlGaN and the p-AlN layers. This is due to the lattice mismatch between AlN and various available sapphire substrates, which results in a large magnesium (Mg) activation energy at room temperature.
By developing technology for high-purity AlN fabrication and the p-type and n-type doping technologies essential to LEDs, NTT has successfully produced low resistance AlN LEDs displaying strong band-edge electroluminescence at 210 nm (Fig. 3). The power output increases nearly linearly with the injection current density, showing excellent LED device reliability.
Moreover, NTT has also succeeded in increasing the maximum power level of a single-chip LED to 160 mW at 24 V bias. This represents a significant increase in light output, which will make AlN LEDs a viable alternative to traditional blue LEDs.
In addition to the superior performance and reliability, NTT’s new AlN substrate material has a number of attractive features such as a coefficient of thermal expansion that is close to Si’s, which is essential for high-power LED base plates. We expect that this will allow AlN to gradually replace traditional high-power LED substrate materials such as alumina and sapphire in HBLED, RF and power semiconductor applications.