Enchanted Journeys in Blue Light
Abstract of the lecture
I gained an interest in luminescence when I was assigned to a group working on the fluorescent screens of Braun tubes for televisions at my first job after university. Then, I was involved in research work at Nagoya University on single crystal growth of germanium (Ge) and physical properties of Ge and several other semiconductors. In 1961, I succeeded in growing a Ge single crystal film by what is now known as the “vapor-phase epitaxial growth method.” This achievement led me to a position at the then newly-established Matsushita Research Institute Tokyo, Inc. in 1964, where I began my research on crystal growth and light-emitting devices of III-V compound semiconductors.
In the 1960s, red and yellow-green light-emitting diodes (LEDs) and infrared semiconductor laser had already been realized, but there was no prospect for practical blue light-emitting devices even in the 1970s.
Both of the two essential requirements for creating high-performance blue light-emitting devices, namely, the growth of high-quality single crystals of semiconductors with wide bandgap energy, such as gallium nitride (GaN), and realization of their p-n junction, were extremely difficult to achieve. I set for myself the goal of overcoming these difficulties in some way so that I could develop GaN p-n junction blue light-emitting devices.
As expected, the task of crystal growth was tremendously difficult, and I was forced to go through a continuous process of trial and error. By the late 1970s, many researchers had withdrawn from studies on this “unexplored semiconductor,” but day in and day out I continued my research of GaN crystal growth simple-mindedly, feeling as if I was “exploring the wilderness alone.” Then in 1978, I successfully caught a glimpse of tiny yet high-quality crystals through my microscope, when I sensed the potential of GaN. And again, I decided to go back to the basics, i.e., “crystal growth.” In retrospect, this was a major turning point both in my research and in the R&D history of GaN in general. In 1979, I chose to adopt “metalorganic vapor-phase epitaxy (MOVPE)” which I believed to be the optimal crystal growth method for GaN. The correctness of my own choice has been attested by the fact that even today this method is almost invariably chosen to fabricate GaN-based devices, including blue LEDs.
Since 1981, with the most generous cooperation of graduate students and co-researchers at Nagoya University, I achieved a series of “firsts” in the world, including high-quality GaN using the low-temperature buffer layer technology, p-type conduction by electron beam irradiation of high-quality GaN doped with magnesium, and GaN p-n junction blue LEDs. In my lecture, I would like to speak more about the subsequent developments of my research.