Dr. Bortrum
11/01/2013
More on Light-emitting Diodes
CHAPTER 39 Some See It, Others Do Not
"Trick or treat!" It was three years ago that I last heard those words from children standing at our front door on Halloween. I must admit that I've not been much of a fan of Halloween. However, yesterday, I was quite happy when about a hundred costumed youngsters rang our doorbell to collect their treats. It was comforting to realize that, at least so far, we in our area of New Jersey have come through 2013 untouched by weather events as shattering as last year's Super Storm Sandy or the 2011 Super Snowstorm, both of which canceled Halloween in our area. This year our October nightmare wasn't a weather event but a self inflicted event that caused many people throughout the land to suffer in various ways. Take, for example, the case of one of our Old Guard members who took his family to Arizona on vacation but was unable to visit a prime objective of the trip - Grand Canyon. It was closed!
Personally, I was not directly affected by the insane shutdown of our government. There was one bit of annoyance. Before the shutdown, I received an email from NASA with a link to a press release on clouds that have been observed on a planet outside our solar system. This, I thought, would be a possible topic for inclusion in November's column. However, when I tried to open the link I couldn't do it; NASA was shut down. Normally, I receive emails from NASA virtually every day, sometimes as many as five or so in a single day. Not a single one during the shutdown. When the shutdown ended, I was able to open the link to the cloudy planet and NASA emails once again fill my inbox.
I was hoping that the cloudy planet was one of the roughly Earth-sized planets discovered by the Kepler space telescope but no such luck. Combining data from both the Kepler and Spitzer space telescopes, the planet Kepler-7b is a Jupiter-like planet with temperatures in the range of about 1,500 to1,800 degrees Fahrenheit and zips around its sun in a mere five days. NASA describes it as one of the "puffiest" planets known. If you could put the planet in a humongous tub of water, it would float. I suspect that if you could find such a tub, it would be a pretty spectacular explosion putting such a hot body in a liquid that boils at only a couple hundred degrees Fahrenheit!
During the shutdown, I was perusing my online October 7 issue of Chemical and Engineering News. Having spent a decade or so working on light-emitting diodes (LEDs) at Bell Labs, I was attracted to an article titled "Enlightening Technologies" by Celia Henry Arnaud. The article deals with recent work on LEDs and organic light-emitting diodes (OLEDs) reported at a recent meeting of the American Chemical Society. Much of the article was devoted to the work of P. Daniel Dapkus and his group at the University of Southern California (USC) on LEDs. As I was reading the article it suddenly occurred to me, "Wait a minute. Didn't I work with a Dan Dapkus more than 40 years ago when I supervised a group supplying a device area with gallium phosphide materials for their LEDs? Also, didn't the two of us play golf in an evening league and didn't we play in a match against each other for the top spot in that league one year?"
Well, I Googled Dapkus and sure enough, found that he had been at Bell Labs. Naturally, I emailed him asking if he were the Dan Dapkus I knew and, within the hour, received a very nice reply. He said that my ears must have been burning because he had recently invited one Jerry Woodall to give a seminar at USC and he and Jerry were discussing me and others who worked on LEDs back in the old days. Jerry, who worked for IBM, and I were both very active in The Electrochemical Society and were good friends in those days of friendly competition among researchers from Bell Labs, IBM, RCA, GE, Lincoln Labs and various universities. I can't help thinking that our legislators could learn from those types of interactions among highly competitive individuals who could also come together at meetings and run (govern?) societies such as The Electrochemical Society, the American Physical Society, the Materials Research Society and others.
Dan Dapkus, the director of the Center for Energy Nanoscience at USC, and the work of his group on LEDs, notably gallium nitride LEDs. I've discussed before the work of Shuji Nakamura in Japan, who came up with the first commercially viable blue LEDs using gallium nitride some 20 years ago. Nakamura is now at nearby University of California, Santa Barbara. Gallium nitride is much harder to work with than the other compounds used for LEDs. A major problem is the fact that the compound is generally grown via a vapor deposition process that forms layers of gallium nitride on a substrate. Ideally, the substrate has the same structure as the compound being deposited on it from the vapor. With the materials used for other LEDs the substrates are cut from crystals of the same material grown by pulling from the melt. This has not possible with gallium nitride and as a result the substrate commonly used is sapphire, which has a somewhat close match to the structure of gallium nitride. Even so, the mismatch of the structures results in defects known as dislocations in the gallium nitride layers.
These dislocations reduce the efficiency of light emission and hence the brightness of the LEDs. Furthermore, gallium nitride itself does not emit blue light; it emits light in the ultraviolet region of the spectrum. We can't see ultraviolet light; so what do we do? We add indium to the gallium nitride forming an alloy - indium gallium nitride. Add a little indium and the light is blue. Add some more indium and the light emitted is green. The problem is that the more indium you add the greater the mismatch with the sapphire substrate and the lower the efficiency of the LEDs.
Dapkus and his group are trying to address this problem. They're growing LEDs on tiny nanowires or nanorods. It turns out that when you go to very small dimensions the strains are less and/or the dislocations are fewer. The efficiency is greater than in larger LEDs . This is especially relevant to green gallium nitride LEDs, which because of the added indium, have more dislocations and hence, lower efficiencies. There are two ways to get white light. Combine red, green and blue LEDs or, alternatively, use coatings of phosphors which convert blue light to red and yellow-green light and by mixing these colors white light can be obtained. The phosphor approach, however, results in lower efficiency so it would be nice to have bright green LEDs mix with the red and blue LEDs to get the white light.
As I mentioned, we can't see ultraviolet light. However, there is at least one critter that can - the sea turtle. And, apparently, fish cannot. At least that's what I gather from an article by Rachel Nuwer in the November issue of Scientific American. It seems that fishermen find in their nets not only the fish they're trying to catch but also sea turtles; it's estimated that millions of turtles end up in the nets. Many of the turtle species caught inadvertently are in danger of extinction.
In order to address this problem John Wang , a fisheries researcher at the University of Hawaii at Manoa, has teamed up with fishers in Baja California Sur in Mexico. They placed ultraviolet LEDs at intervals on the fishers' gill nets. The LEDs were battery-powered. The fishers found that the number of sea turtles entrapped in these nets fell by 40 percent compared with the catches from control nets similarly equipped but with the LEDs not turned on. The number of fish caught in the UV lighted nets was down a bit compared to the control nets but the fishers didn't suffer any significant financial loss. I'm assuming the UV LEDs were gallium nitride LEDs. At the same time, I'm wondering why anyone would be making UV LEDs. I'm guessing that they are used with phosphors to make white LEDs.
Incidentally, Dapkus did remember playing golf in the evening league at Bell Labs and cited a round in which he defeated a member of my group. I didn't mention to Dan that I remembered beating him. All of which leads to my last official round in our Old Guard golf group this week. I finished the round sinking a 5- or 6-foot putt for a par on the final, hardest-to-putt hole on our par-3 course. A very satisfying finish to a hip replacement surgery-shortened golfing year.
Next column should be posted on or about December 1.
Allen F. Bortrum
