Both n-type and p-type ZnO will be required for development of homojunction light-emitting diodes (LEDs) and laser diodes (LDs). It is easy to obtain strong n-type ZnO, but very difficult to create consistent, reliable, high-conductivity p-type material. The most natural choice of an acceptor dopant is N, substituting for O, and indeed several groups have been able to obtain p-type material by such doping. Surprisingly, however, other groups have also been successful with P and As, elements with much larger ionic radii than that of O. Although ZnO substrates are now available, most of the epitaxial p-type layers so far have been grown on sapphire, or other poorly-matched materials. The lowest p-type resistivity obtained up to now is about 0.5 Omega-cm, which should be sufficient for LED fabrication. In spite of the present availability of p-type ZnO, very few homojunction LEDs have been reported so far, to our knowledge; however, several good heterojunction LEDs have been demonstrated, fabricated with p-type layers composed of other materials. One such structure, with fairly strong 389-nm emission at 300 K, involves n-type ZnO and p-type AlGaN, grown on an SiC substrate. Also, an N+-ion implanted ZnO layer, deposited by chemical vapor deposition on Al9O3, exhibits 388-nm emission at 300 K and could be economical to produce. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
