News Article

Humphreys Rocks The InGaN Boat

Working at the esteemed department of materials science and metallurgy at the University of Cambridge, UK, Colin Humphreys is arguably the GaN community's most candid researcher. Jon Cartwright visited his laboratory – the Cambridge Centre for Gallium Nitride.
How did your controversial views on light generation in GaN originate?


When GaN was first brought out as an LED, the most remarkable thing was that it emitted bright light despite having a huge dislocation density. Dislocations quench light emission, so people couldn t understand this.The belief developed that indium-rich clusters form in the InGaN quantum wells. These clusters would be a couple of nanometers across and would be away from where the dislocations were. So the light emission would also be separated from the dislocations. Everyone believed it because you could see these clusters in an electron microscope and analyze them as being indium-rich.




However, one day I had a research student who said: "Look, I ve just turned the electron beam on to a very low intensity and when I study the first few frames there are no clusters at all!" This meant that these clusters must have developed under the beam in the electron microscope – they re not normally there in the material.

How did people react to this finding?


When we presented the work at an international nitrides conference, there was a stunned silence at the end. "There s something wrong with your materials," someone said. So we looked at material grown at other [fabs], like Nichia s, who claimed to make very bright LEDs by engineering the clusters. But again we found no clusters at all.
What is your alternative explanation?


Along with Phil Dawson at Manchester University, we proposed another mechanism, which was based on atomic structure. If you conduct a high-resolution electron micrograph of the quantum wells you find that the lower interface – the interface between the GaN and the InGaN – is sharp, but the upper interface has many more steps of atomic height on it.




These steps create extra strain energy in the quantum wells. And because the piezoelectric effect is high in these materials the strain creates a potential variation, which confines the electrons and holes. So it is nano-scale confinement that makes it unlikely for a dislocation to quench the light emission.

Is this mechanism now accepted?


When we published a paper at the end of 2003, we were alone in saying this. The nitrides semiconductor conference in [September] 2005 at Bremen had an evening symposium devoted to debating "do clusters exist?". I said they didn t, some said they did and others said maybe. But people are slowly coming around to it. Now, I think, 70–80% of the world agrees with us.

Shuji Nakamura s recent Nature Materials paper suggested that localized chains of indium atoms cause the localization. Does this fit your theory?


They say there is no gross indium clustering, which is in agreement with what we re saying. The paper proposes that there are indium fluctuations giving rise to localized chains of two or three indium and nitrogen atoms, and it s those chains that are causing the localization.


There are two things I think about this: one – that in a random alloy you will indeed get these localized chains, so the analysis is fine. But what is actually causing the localization of the electron-hole pairs? I don t think that they have actually calculated what the localization energy is in their paper. We can show [in our mechanism] by calculation that the steps on the InGaN quantum wells can create sufficient potential difference to cause localization.


I think in principle both [of our suggested mechanisms] could cause localization, but we re not yet sure which one is dominating. We need to find that out.


Is it vital to have such a fundamental understanding?


In blue LEDs, dislocations don t seem to quench the light emission. But when you go into the UV it is important because there is much weaker localization and dislocations have an enormous effect on light emission. If we can understand the localization then we may be able to increase it. Nobody has successfully done that yet.

You have just secured a joint £1 million ($1.97 m) grant. How do you plan to spend it?



One application will be to develop the emission of UV-LEDs. But we also want to promote solid-state lighting. If we could get them into houses faster, LEDs would save a lot of energy. We have a contract with the Department of Trade and Industry on the reliability of LEDs.


People have assumed the lifetime of blue and green LEDs is 100,000 h, but it s based on the fact that red LEDs have been around a long time and they last for 100,000 h. In Singapore, where they have LED traffic lights, all the LEDs in the red lights work. But in the green ones a third of them aren t working at all and that s because they re InGaN. A Chinese solid-state lighting delegate whom I met said: "Many public buildings in Beijing are lit by white LEDs. But now they re dark – they re dead!" The white LEDs last for a few 1000 h or less and the problem is all in the packaging – the chip itself is fine. If you have the right packaging they could last for 100,000 h.


Now, Japan s government has got a national program in solid-state lighting, as has Korea and Taiwan. Even George Bush s America has a national program! But we [in the UK] have nothing. Lord Jenkin [a baron in the UK House of Lords] said to me: "It s a scandal!" We re doing world-class work here and the funding other people get is so high because the returns are so great. The lighting market is worth more than $12 billion a year. So to put in a few million dollars is a good investment.

What industrial collaborations do you have?



Thomas Swan Scientific Equipment donate us the growth equipment and maintain it free of charge. And another good collaboration is with Forge Europa located in Cumbria, who sell LED-based products. We re their research wing. Recently we ve been talking to another UK company with whom we have a project to grow GaN on 6 inch silicon substrates that could each contain 100,000 LEDs. If our work is successful for growing on six-inch silicon, they will manufacture in this country. There will, at last, be a UK LED manufacturer.

What specific projects are you focusing on?


The first project under the grant is studying localization, from the deep-UV right through to the green. The second is to reduce dislocation density. The third project is non-polar and semi-polar growth. All [commercial] blue LEDs and lasers feature hexagonal GaN grown along the c-axis. This is called the "polar direction" and is the [direction of the] built-in electric field across the quantum well.


This electric field keeps the electrons and holes apart and so it delays their recombination. But there are other crystallographic directions you can grow along in GaN where there aren t these electric field effects and so you should get brighter devices.


We have been growing this material for about nine months and it is at least as good as other published results. Now we have got a major project grant to improve the growth, which should boost the intensity in blue, white and green LEDs.

What are the main applications for UV-LEDs?


One is water purification. Life has developed in the absence of deep-UV radiation because the Earth s atmosphere cuts it out. So life has not developed any defence mechanism against it.


There s a particular wavelength at approximately 270 nm that just kills everything by destroying the nucleic acid in DNA. So if you irradiate impure water with this wavelength it kills off all the bacteria and the viruses, mosquito larvae – everything. In the developing world you could have pipes to people s houses with a ring of deep-UV LEDs that kill all the bugs as the water flows through.


In 2004, the British Medical Journal stated that more than half the hospital beds in the world were taken up by patients suffering from diseases related to impure water supplies. These diseases will, essentially, kill more people than global warming. They will probably kill more people than AIDS. Deep-UV LEDs could help to solve what is probably the biggest problem in the developing world.

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