Rebirth of Northland Technology

Not long after Cheng Yongxing returned to Chuncheng from Fengtian, the summer vacation was over.During the whole vacation, he didn't stay at home for a few days at all, almost all of them were spent in Fengtian.

And his fellow classmates are all overwhelmed with addiction.

The meaning of the word "returning home in clothes" has been deeply understood by everyone.

Of course, the one who has gained the most from this should be Cheng Yongxing, the youngest.With word of mouth, slowly, his role was also recognized by classmates and parents.

Cheng Yongxing even saw something wrong in the eyes of the parents of several female classmates.Instead, it was their daughter, still ignorant.

When returning to school, everyone still started in groups.

This group of young people, leaving their hometown this time is different from the past, they are generally full of longing for the future.Young people are at the age of self-confidence, and they are touted by their hometown fathers and elders. They are just waiting for the beauty of the Premier League!

Everyone’s parcels are usually filled with food this time.Even if they had any clothes to wash, they went to Cheng Yongxing to solve them, and no one came back with dirty clothes.

So, on the train back to school, the small table was full of food.Everyone ate snacks while riding in the car.When I arrived at school, I was not hungry at all.

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Back at school, Cheng Yongxing's first thing to do is to sort out the information and prepare to apply for a patent.

In Fengtian, with the full cooperation of the scientific research personnel of the Science Instrument Factory, he quickly broke through two important nodes.

As a designated factory for semiconductor equipment, Keyi Instrument Factory does not have a full range of products, but in general, the conditions are much better than those in the laboratories of the University of Technology.

In addition, the processing methods in the factory are complete, and some minor repairs and modifications are very convenient.

Had it not been for the start of school, Cheng Yongxing even felt that doing research at the Science Instrument Factory would definitely make the progress of the project faster.

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The third pass is the improvement of the two-step process.

Here, let's talk about the material selection of blue LED first.

There are three types of suitable blue LED materials, one is SiC (silicon carbide) material, the other is ZnSe (zinc selenide) material, and the other is GaN (nitridation) material.

Due to the physical principle of SiC, its luminous efficiency cannot be high, so people naturally turn their attention to ZnSe-based and GaN-based materials.

The synthesis of GaN is very difficult, and the grown material has a high density of line defects (dislocations). According to the understanding of traditional semiconductor physics, it is impossible for GaN to emit strong light with such a high dislocation density.

Therefore, among the world’s scientists who study blue LEDs, more than 10,000 people choose ZnSe, while fewer than 10 choose GaN.

In order to solve the GaN synthesis problem, the remaining ten scientists started their own attempts.

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Professor Akasaki chose sapphire with a crystal structure close to GaN but mismatched lattice constants as the substrate material for heteroepitaxial growth (growth) of GaN.

Due to the lattice mismatch, there is a mismatch stress between the GaN epitaxial layer and the sapphire substrate, and the release of the stress will cause a large number of defects in the GaN.Such materials cannot be applied to devices.

But this problem was solved in 1985 by Professor Akasaki's disciple, Hiroshi Amano. This is the famous two-step method.

The step is to grow an AlN buffer layer on the sapphire substrate, and then increase the temperature to grow GaN.

Since the buffer layer releases the mismatched stress between GaN and sapphire, this "two-step" growth technology significantly improves the crystal quality of GaN and meets the basic requirements for device fabrication.

The discovery of this method took about five years before the teacher and student relayed the effort.

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It stands to reason that the general scientific research worker's thinking for a certain research is to follow the footsteps left by the predecessors.Especially before the end is reached.

Because changing the predecessors, or even overturning the work of the predecessors, is completely outweighed by the gains.

For example, the discovery of the two-step method took about four to five years.Is it necessary to prepare for several years if I change my research?

How to choose this is not clear at a glance!

But the hero is the hero. After Nakamura received this paper, he did not continue to study, but let himself go.

He decided to try the method of using GaN instead of AlN in the buffer layer.

The specific idea is to grow a GaN single crystal film under high temperature conditions on an amorphous GaN film grown at a low temperature.As long as this is successful, the same structure as the direct growth of a single crystal GaN film on the substrate can be produced.

Following this line of thinking, Nakamura tried.

The result is a success!

The core of this method is the use of a low-temperature GaN buffer layer (around 500°C) instead of the AlN buffer layer.This "two-step" process based on a low-temperature GaN buffer layer has become a standard process for growing GaN-based LEDs in the industry in the future.

Of course, the reason for this improvement is also very strange.The explanation given by Nakamura is that I don’t need a method that others have used!

Cheng Yongxing has also used this "two" way of speaking!

Isn't it just a strong word!

Who can't!

You have kind!

Don't use other people's right methods!

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The fourth level is the annealing process.

The LED is essentially a diode, and the core structure of the diode is a semiconductor pn junction.The pn junction is an interface composed of an n-type semiconductor (containing a large amount of free electrons inside) and a p-type semiconductor (containing a large amount of positively charged free carriers-holes).

For GaN, n-type doping is relatively easy to achieve, but p-type doping is very difficult.The p-type dopant often used in GaN is Zn or Mg, but after doping these impurities, GaN often still exhibits high resistance characteristics, which means that the p-type dopant is not activated and has no effect.

This problem has puzzled the scientific community for a long time, and finally it was solved by Hiroshi Amano.The solution is to use low-energy electron beam irradiation to obtain p-GaN.

The discovery of this method, Amano Hiroshi also took a long time.He has been trying since 1986, until 1989 when he suddenly got his luck.

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At this step, Nakamura's "second" disease has occurred again!

He overthrew the research results of the previous scientists again and changed it to heating!

This is the origin of the so-called annealing process.

According to Nakamura's own explanation, he got this unexpected result under very accidental circumstances.

Before repeating the electron beam irradiation experiment, he accidentally heated the worktable!Thus, he found that heating under the sample during the electron beam irradiation process can achieve better results.

He continued to study this phenomenon and confirmed that p-GaN can be obtained only by heating.The principle of the annealing process, Nakamura Heroes did not give a reasonable explanation.

Since then, thermal annealing has become the standard process for making blue LEDs, and it is still used today.

Of course, no one knows whether things are really accidental.

Who can tell stories?

...

The principle behind the annealing process was not revealed until a long time later.

The Mg in p-GaN will be passivated by H introduced in the MOCVD epitaxy process to form a Mg-H complex.Whether it is low-energy electron irradiation or thermal annealing, Mg impurities are activated by destroying the Mg-H bond with external energy.

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The realization of these two process steps is enough to illustrate Nakamura's unlucky luck!

The results of the predecessors took five or six years, he overthrew them all in a short time, and found a better way!

And the process steps he discovered, even after thirty years, no one can change!

Is there such a possibility:

No one can change these processes because they are actually mature processes thirty years later!But it was received early in 1990!