When was the first LED made?

Blue light emitting diodes

The invention of efficient blue LEDs paved the way for energy-saving white light sources - and brought Isamu Akasaki, Hiroshi Amano and Shuji Nakamura the 2014 Nobel Prize in Physics. Henning Riechert from the Paul Drude Institute for Solid State Electronics in Berlin explains why the development of blue light-emitting diodes was so difficult and how the three Nobel Prize winners succeeded.

They shine towards us from smartphone displays and television screens and are increasingly illuminating our homes and streets. They are durable, handy and efficient. We are talking about white LEDs, i.e. light-emitting diodes. The researchers Isamu Akasaki, Hiroshi Amano and Shuji Nakamura from Japan developed the first powerful blue light-emitting diodes in the 1990s and thus the basis for efficient white LED light. For this they received the Nobel Prize in Physics in 2014.

Henning Riechert:“It is quite a technologically and material science-oriented achievement. Whereas otherwise fundamental physical discoveries are often rewarded. The enormous energy saving potential of the light emitting diodes is well the central advantage that the award committee ultimately has convinced. "

Red and green diodes had been around since the 1960s, but they were not enough to produce white LED light. It was only with the discovery of Akasaki, Amano and Nakamura that the third basic color blue was available in addition to red and green. On the one hand, the three colors could now be superimposed to form white light. On the other hand, other processes were soon developed with the help of blue LEDs to generate white light.

“There are phosphors that absorb blue light and emit yellow light on it. With the right balance, the human eye can get two signals: the blue light from the light-emitting diode and the yellow light from the fluorescent material. If these mix together, the brain uses them to construct the sensation of white light. "

The right band gap

Light emitting diodes consist of two different semiconductor layers. The electrons in it can assume different energy states. Unlike in an atom, however, not only individual energy values ​​are permitted in a solid, but entire energy ranges or energy bands. In semiconductors, the so-called valence band is fully occupied by electrons. The higher energy conduction band, however, is only partially filled with electrons. Therefore, they can move and thus transport cargo.

“The basic parameter is what we call the band gap in semiconductor physics. That is the energy gap between the conduction band and the valence band of such a semiconductor structure. "

Henning Riechert

The band gap is also crucial for the LEDs. There are many free electrons in one of the two semiconductor layers, which can migrate through the layer in the conduction band. In the other layer, on the other hand, electrons in the valence band are missing. The resulting positive vacancies or holes can - similar to the negatively charged electrons - migrate through the material. If the two layers are brought together and an electrical voltage is applied from the outside, the electrons and holes meet at the boundary layer. The electrons fill the empty spaces, emitting energy in the form of light. How much energy is released depends on the size of the band gap between the conduction and valence bands.

“Depending on the material, these band gaps are different. In principle it is possible to find a material with a suitable band gap for every visible color as well as for the adjacent spectral ranges infrared and ultraviolet. "

Because the larger the band gap, the more energy the electrons give off in the form of light. Blue light is very short-wave and therefore rich in energy. A particularly large band gap is therefore required for its generation.

“With the semiconductors that were introduced in the 1960s and 1970s-JIf they were available, it just doesn't work. These were usually gallium arsenide and similar materials. This class of material allows very efficient red emission. But blue or green just don't work. "

Although the first blue LEDs made of the semiconductor material silicon carbide existed before the 1990s, they were faint and inefficient due to their material properties.

“Silicon carbide LEDs are the ones that used to be seen as high beam bulbs in cars. That was the only useful application one could think of for the inefficient light emitting diodes. There if they were mostly operated in the dark, it was also not necessary to generate special brightness.

The right crystal

The semiconductor material gallium nitride has been considered theoretically well suited for blue light-emitting diodes since the 1970s. By introducing foreign atoms, known as doping, the aim was to produce a gallium nitride layer with many free electrons and one with many holes. These thin layers should have been vapor-deposited onto a gallium nitride crystal, as they can only be produced on a substrate that has a similar structure to the layers themselves. However, it is simply not possible to grow sufficiently large crystals for this project.

“These semiconductor crystals are normally pulled from the melt in a crystal growing process. That means you have to melt the material. However, the melting point of gallium nitride is 2500 degrees Celsius. So it is hardly manageable from a purely thermal point of view. "

Isamu Akasaki and Hiroshi Amano from Nagoya University worked around this problem. Instead of gallium nitride, they used a different material as a base and tried to apply the doped gallium nitride layers to it.

“You used sapphire for this. The sapphire crystal has basically the same symmetry as the gallium nitride. But the distance between the atoms in the lattice differs by 16 percent. If these two lattices are connected, this difference in the atomic distances must be absorbed by crystal defects. "

The gallium nitride layers on the sapphire surface initially showed a lot of defects in the otherwise periodic crystal lattice. However, semiconductor crystals must be near perfect in order to function efficiently. Getting this problem under control was the great challenge that Akasaki and Amano eventually mastered.

“Amano and Akasaki achieved this by inventing a very specific pretreatment process for the sapphire substrates. In a multi-step growth process, they first deposit a very imperfect but very smooth layer of aluminum nitride on this sapphire, on which they then grow the gallium nitride. "

With warmth to success

With an intermediate layer of aluminum nitride, gallium nitride could now be vapor-deposited very evenly on sapphire. But there was another problem: The introduction of magnesium atoms should create holes in one of the layers, which then move through the material as positive charge carriers, so to speak. But this so-called p-line just didn't work.

“The two of them were able to solve this problem by chance. you examine the Surface of the Gallium nitride crystals doped with magnesium routinely under the electron microscope. One day they discovered that this magnesium-doped gallium nitride really did become p-type if they did it for a while viewed in the electron microscope.“

Function and structure of blue LEDs

A little later, Shuji Nakamura found an explanation for this effect. At that time he was working at Nichia Chemical and was closely following the results of his colleagues. He suspected that it was not the bombardment with electrons that made the samples p-conductive, but the heating of the sample during this process.

“His hypothesis was: If you heat a sample using other methods, it should also become p-conductive. After they have grown in the reactor, they can be kept at a higher temperature instead of being turned offsluice. And he had Law: The layers actually became p-conductive. "

This process developed by Nakamura made the production of the components much easier. Today it is known that hydrogen atoms are transported out of the material as a result of the heating, which would otherwise prevent the p-conduction. In the years that followed, Nakamura contributed significantly to the further development of diodes and continually discovered new methods to make them more efficient.

“It was initially a scientific race that Akasaki and Amano won in the end. They are the first to have such light-emitting diodes from the material family of gallium nitride and indium-gallium nitride produced. But the entire technological and economic implementation was essentially done by Nakamura with his research group. "

A little later, blue semiconductor lasers followed the blue semiconductor diodes. Today they enable the storage of large amounts of data on blue-ray disks. The light-emitting diodes themselves have undergone enormous further development since the 1990s. The quality of the material has been improved, new processes have been developed for better light output, and the efficiency of the diodes has thus been multiplied.

“At the moment it depends on the cost. All companies involved are working feverishly to scale up the manufacturing processes so that costs can be kept down. The prices of such white light LEDs are falling increasingly. If you consider how long these light-emitting diodes live, how seldom you have to replace them, then you already have an economic advantage today. "