In my previous blog post, I wrote about the general development of the LED. Focus was in the development of LED component. How it was transformed from a laboratory experiment to a mass production of commercial product. Different colors of LEDs were produced by applying different semiconductor layers on certain substrate materials.
Still, there was one obstacle to overcome. LEDs emitting blue color couldn’t be produced with reliability good enough for mass production. In this post I tell how LEDs with blue light were developed. And how it finally made possible of creating the white LED. Blue LED was the base for the current white LED revolution. This invention was awarded with the Nobel prize in Physics in September 2014.
Although LED components could produce red, green and yellow light, blue was unreachable. The main reason for early stage difficulties related to quality of materials. Materials like zinc selenide (ZnSe) and silicon carbide (SiC) produced inefficient light emission. So they were not suitable to use. Part of this was because of the quantum mechanical feature called indirect bandgap. The indirect bandgap allows only small part of the energy go to light production. Most of it turns to heat. In direct bandgap materials, the energy distribution is opposite. A greater part of the supplied energy goes to the light production. This enables more efficient light-emitting components.
One solution was gallium nitride (GaN). First experiments started already on the end of 1950s at Philips Research Laboratories. But, at that time growing larger crystals was difficult. There was also another problem with GaN at that time. Growing semiconductor structures having p-n junction for diode operation was not possible. The researchers concentrated on gallium phosphide (GaP) material instead.
The next solution on the line was to use the growing method Hybride Vapour Phase Epitaxy (HVPE). Yet, there were many problems with the method. For example the presence of hydrogen passivated p-type doping. So forming good enough p-n junctions was impossible. Also, surface roughness was not controllable enough. So there were two main reasons for lack of progress. One being material growth problems and other problem in semiconductor doping, especially p-type doping.
In the 1970s, scientists developed new growing methods: MBE and MOVPE. After the development of these methods, the material quality problems could be solved. This was thanks to good enough manufacturing techniques or growing methods. Also material stess/strain problems could be solved with new inventions on growing temperatures. Isamu Akasaki and a PhD student Hiroshi Amano were conducting research on GaN in Nagoya University. At the same time Shuji Nakamura was doing research on the same material in Nichia Chemical Corporation.
Both groups used a so-called spacer layer between the sapphire and the rest of the semiconductor structure. The difference between two research groups was the spacer layer material. Akasaki and Amano used aluminium nitride (AlN) while Nakamura’s groups used GaN. Another crucial thing was the growing temperature. The Spacer layer was grown at much lower temperature than the rest of the layers in semiconductor structure.
After solving the biggest problem, two more existed:
- How to get a proper p-n junction, by solving p-type doping issue, to produce good diode performance.
- How to get better light emission efficiency to develop more efficient blue light emitting LED diode.
The solution for the first challenge above was found by accident. While studying the zinc-doped GaN with a scanning electron microscope. Akasaki and Amono noticed that the material emitted more light This meant that in p-n junction p-type doping was higher thus enabling more efficient light emission. The same effect was also noticed when GaN was doped with magnesium.
The effect was later explained by Shuji Nakamura. The role of hydrogen in passivating p-type doping mentioned earlier in this post was essential. Electron irradiation in scanning tunnel microscope prevents this passivation of hydrogen. This leads to activation of p-type dopants. The same effect can also be achieved with thermal treatment, also known as thermal annealing.
Then there was a target for better light emission efficiency. How to collect electrons and holes, into small volume to generate efficient light. In light generation, a process called electron-hole recombination, generates light of certain wavelength. In this post we deal with blue light generation. Blue light was the last obstacle that had to be won to finally create a white light LED.
The solution to efficient light production was to use so-called double heterostructures. The special case for these is quantum well structure that is used for example in manufacturing of semiconductor lasers. The structure means that there are two interfaces with two different semiconductor materials. They form a “sandwich” structure. Usually, the middle layer has a smaller band gap than the layers above and below it. This makes it possible to collect electrons and holes into the middle layer. This enables more efficient recombination and more efficient light generation. In the case of blue LED the sandwich structure example is p-type AlGaN/InGaN/n-type AlGaN. Thus there is:
- p-n junction to create diode structure electrically
- double heterostructure to create suitable band gap structure to enable efficient light generation
This way both the problems in material mechanical quality and optical quality had been solved. Efficient blue LED was able to being produced. These advancements led later to the production of blue laser diodes.
Soon after the invention of blue LED, white LED was developed.
There are two ways to produce a white light LED:
- combining red, green and blue LEDs to create white light
- use blue LED with phosphor material that converts blue light to white light by adding red and green regions into the spectrum of light
These advancements in manufacturing processes and semiconductor structures were huge. This made possible further advancements in creating more and more powerful LEDs.
Isamu Akasaki, Hiroshi Amano and Shuji Nakamura won the Nobel Physics Prize in 2014 for “the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources”. Additionally, Shuji Nakamura has won the Millennium Technology Prize on 2006 for his contribution in developing blue and white LED light.