Aluminum Nitride Light-Emitting Diodes with the Shortest Wavelength


Yoshitaka Taniyasu and Makoto Kasu
Materials Science Laboratory

   Aluminum nitride (AlN) is a direct-bandgap semiconductor with a bandgap energy of 6 eV, the largest among semiconductors, and is therefore promising for light-emitting devices with the shortest wavelength for semiconductors.  Recently, we have succeeded in p-type and n-type doping of AlN and have fabricated a p-n homojunction AlN light-emitting diode (LED) [1].
 The AlN layers were epitaxially grown on SiC (0001) substrate by metalorganic vapor phase epitaxy (MOVPE).  To improve the crystal quality of the AlN layers, we suppressed the parasitic reaction of Al and N sources in the gas phase.  P-type and n-type AlN layers were obtained by Mg and Si doping during MOVPE growth, respectively [1, 2].  Figure 1 shows schematics of the AlN LED.  In this LED structure, between the p-type and n-type AlN layers, an undoped AlN emission layer was inserted to suppress Mg- and Si-impurity-related emission from the p-type and n-type AlN layers.  The p-type and n-type AlN/AlGaN superlattices (SLs) were used to reduce the contact resistance of the electrodes. 
 Figure 2 shows the emission spectrum of the AlN LED.  Deep-ultraviolet light emission was observed at 210 nm, the shortest wavelength ever reported for any kind of semiconductors.  On the basis of optical reflection and cathodoluminescence measurements of free-exciton transition in AlN, we assigned the light emission to a near-band-edge emission from AlN.
 Because light with a shorter wavelength has a higher energy, the AlN LEDs can be applied to light sources for decomposing very stable, harmful chemical substances, such as dioxin and polychlorinated biphenyls (PCBs), which cause serious environmental problems all over the world.  We will increase the emission efficiency of the AlN LEDs to a practical level by further improving their crystal quality and doping efficiency.

[1] Y. Taniyasu, M. Kasu, and T. Makimoto, Nature 441 (2006) 325.
[2] Y. Taniyasu, M. Kasu, and T. Makimoto, Appl. Phys. Lett. 89 (2006) 182112.

Fig. 1. Device structure of AlN LED
Fig. 2. Emission spectrum of AlN LED.

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