“The work reported in this paper was possible only because of the outstanding development of 2-inch ZnO wafers, and then MBE homoepitaxy on these wafers, by Gene Cantwell and Dave Eason of the Eagle-Picher Corporation.”

This paper is highly cited because it involves a hot topic, the development of p-type ZnO, which might enable ZnO (rather than GaN) to be the material of choice for the next generation of UV light-emitting diodes (LEDs) and laser diodes (LDs). Furthermore, it is the first report of p-type ZnO grown by molecular beam epitaxy (MBE), and also the first report of p-type ZnO grown by homoepitaxy, i.e., ZnO on ZnO. Finally, in light of the predictions by many that p-type ZnO could never be made, we took great pains to show that our derived electrical parameters (hole concentration and mobility) were accurate and believable. Since this paper was published, many other groups have also produced genuine p-type ZnO, and even p-n junctions, that emit light. In addition, some excellent density-functional theory is being carried out to understand the nature of the acceptors which produce the holes.

The significance of our paper lies in the convincing demonstration of p-type ZnO, especially after some theoreticians had predicted that this material was forever destined to be n-type only. For decades, scientists have known that ZnO would make an ideal light emitter, if only a viable p-type form could be made. A few papers on p-type ZnO had appeared before ours, but some of them seemed to have unrealistic electrical parameters, and thus were discounted by many researchers. The next step for the ZnO community is to identify the most reliable and manufacturable p-type technology, and then the natural advantages of ZnO will likely propel this material into the marketplace for DVDs, laser printers, and solid-state white lights.

The work reported in this paper was possible only because of the outstanding development of 2-inch ZnO wafers, and then MBE homoepitaxy on these wafers, by Gene Cantwell and Dave Eason of the Eagle-Picher Corporation. This technology has now been transferred to ZN Technology, Inc., at Brea, CA. We—Cole Litton, Don Reynolds, and I—got involved with ZnO research in about 1996, when we joined with Eagle-Picher (Gene Cantwell and Bill Harsch) on a Defense Advanced Research Projects Agency (DARPA) project to find suitable wafers for the epitaxial growth of GaN, which does not have its own large-area-wafer technology. While performing photoluminescence measurements on the new Eagle-Picher ZnO wafers (to be used for GaN epitaxy), we realized that this material was interesting in its own right, and had excitonic emission properties superior to those of GaN. However, nobody up to this time had been able to make p-type ZnO, so it appeared that ZnO-based LEDs and LDs were but a dream. In 1999 we held the First International Workshop on ZnO in Dayton, and I think we were all surprised by the many scattered bits of research which were being carried out all over the world. The second workshop was held in 2002, but this year (2004) there have been at least five international workshops on ZnO, showing how rapidly the field is developing. Interestingly, while ZnO is indeed used as a substrate for GaN in some cases, the opposite is also true, and some ZnO/GaN or ZnO/AlGaN structures have even demonstrated UV light emission.