Wednesday, January 25, 2012

Journal publications in the blue LED "droop" area

Within a story titled -- The LED's Dark Secret -- in IEEE Spectrum, one finds some allusions to difficulties in publishing science papers:

As to droop, the loss of LED efficiency at high power:

Another theory [for droop] was proposed as far back as 1996 by Nakamura. He argued that everything could be explained by the structure of the quantum well. Nakamura and his colleagues looked at LEDs with a transmission electron microscope and were surprised to find light and dark areas within the quantum well, suggesting that the material there was not uniform. They then investigated the crystalline structure more closely, using X-ray diffraction, and found that the quantum well had indium-rich clusters (bright) next to indium-poor areas (dark).
Nakamura conjectured that because the indium clusters were free from defects, the electrons and holes would be trapped in them, making bright emission possible, at least at low currents. Continuing with this line of reasoning, Nakamura’s team argued that LEDs’ high efficiency at low currents stemmed from a very high proportion of electron-hole recombination in defect-free clusters. At higher currents, however, these clusters would become saturated, and any additional charge carriers would spill over into regions having defects dense enough to kill light emission. The saturation at high current, they suggested, accounted for the observed droop.
This theory has fallen out of favor in recent years.

In 2003, Humphreys presented that jaw-dropping finding at the Fifth International Conference on Nitride Semiconductors, in Nara, Japan. It wasn’t well received. Many delegates contended that something must have gone wrong with the Cambridge samples. So Humphreys’s group went back and studied a wider variety of specimens, including LEDs supplied by Nichia. Their work only reinforced their view that the clusters were formed by electron-beam damage.
In 2007, Humphreys’s Cambridge team, together with researchers at the University of Oxford, described how they had attacked the problem with what’s known as a three-dimensional atom probe. This device applies a high voltage that evaporates atoms on a surface, then sends them individually through a mass spectroscope, which identifies each one by its charge-to-mass ratio. By evaporating one layer after the other and putting all the data together, you can render a 3-D image of the surface with atomic precision.
The resulting images confirmed, again, what the electron microscope had shown: There is no clustering. Discrediting the cluster theory was an important step, even though it left the research community without an alternative explanation for droop.
Then, on 13 February 2007, the California-based LED manufacturing giant Philips Lumileds Lighting Co. made the stunning claim that it had ”fundamentally solved” the problem of droop. It even said that it would soon include its droop-abating technology in samples of its flagship Luxeon LEDs.
Lumileds kept the cause of droop under wraps for several months. Then, at the meeting of the International Conference of Nitride Semiconductors, held September 2007 in Las Vegas, it presented a paper putting the blame on Auger recombination—a process, named after the 20th-century French physicist Pierre-Victor Auger, that involves the interaction of an electron and a hole with another carrier, all without the emission of light.

**Of publication of this work:

The idea was pretty radical, and it has had a mixed reception. Applied Physics Letters published Lumileds’ paper only after repeated rejections and revisions. ”In my experience, it was one of the most difficult papers to get out there,” says Mike Krames, director of the company’s Advanced Laboratories.

**Of criticism of the Philips work:

”All [Lumileds] showed was that they can fit the results with a dependence that is like Auger,” claims Hader. ”It’s a fairly weak argument to see a fit that fits, and see what might correspond to that fitting.” In his view, there’s a good chance that the Lumileds data could also be fitted with other density dependencies, as well as the cubed dependence that is classically associated with Auger recombination.

The first Philips Auger paper was: “Auger Recombination in InGaN Measured by Photoluminescence,” by Y. C. Shen, G. O. Mueller, S. Watanabe, N. F. Gardner, A. Munkholm, and M. R. Krames, Applied Physics Letters 91 141101, 1 October 2007,

One notes US 20070262342 (filed May 15, 2006 ; published November 15, 2007 ) titled P-Type Layer For A III-Nitride Light Emitting Device.

As historical background:

The first-ever report of light emission from a semiconductor was by the British radio engineer Henry Joseph Round, who noted a yellowish glow emanating from silicon carbide in 1907. However, the first devices at all similar to today’s LEDs arrived only in the 1950s, at Signal Corps Engineering Laboratories, at Fort Monmouth, in New Jersey. Researchers there fabricated orange-emitting devices; green, red, and yellow equivalents followed in the ’60s and ’70s, all of them quite inefficient.
The great leap toward general lighting came in the mid-1990s, when Shuji Nakamura, then at Nichia Corp., in Tokushima, Japan, developed the first practical bright-blue LED using nitride-based compound semiconductors. (Nakamura’s achievement won him the 2006 Millennium Technology Prize, the approximate equivalent in engineering of a Nobel Prize.) Once you’ve got blue light, you can get white by passing the blue rays through a yellow phosphor. The phosphor absorbs some of the blue and reradiates it as yellow; the combination of blue and yellow makes white.

**Note the reference to the Readers Digest article predicting LEDs for lighting in LBE's 8 JMRIPL 80 (2008)


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