This article is the third in a series of high-brightness blue LED development stories. The first part describes the process of developer Nakamura Shuji from entering the Nichia Chemical Industry to selecting blue LEDs as a research topic. This article will continue to tell the story of Nakamura's research from the beginning to the successful growth of selected GaN materials into crystalline films. In order to study blue light-emitting diodes, it is first necessary to master the luminescent layer-film growth technique. To this end, Nakamura went to the United States to study, but in the United States wasted a year to manufacture equipment. After returning home, he continued to manufacture and rebuild the device. After a long and arduous effort, I finally achieved initial results...
In March 1988, Nakamura Shuji boarded a flight to Florida in the United States with excitement. He will study as a researcher at the University of Florida for one year (Table 1).
Table 1: Development chronology of blue LEDs.
I went to the United States to be an opportunity to visit a researcher. I came from Nakamura to talk to alumnus Sakai Shiro (professor of Tokushima University) who was studying at Tokushima University. To make a blue light-emitting diode, it is necessary to start with a single crystal film for a blue light-emitting diode. The technology includes MBE method (molecular beam epitaxy) 1 and MOCVD (metal organic chemical vapor deposition) 2). Nakamura did not hesitate to choose the MOCVD method. The reason is that the price of MBE devices is hundreds of millions of yen, and it is impossible for the company to consider purchasing.
Note 1) The MBE (molecular beam epitaxy) method is a method of growing a single crystal film on a substrate, and is a type of vapor phase growth method. While controlling the atomic (molecular) beam introduced into the high vacuum, the substrate is irradiated to deposit atoms. It can be called high precision vacuum deposition technology. This technique is required when manufacturing an element using a compound semiconductor such as silicon or GaAs.
Note 2) The MOCVD (metal organic chemical vapor deposition) method is one of CVD (chemical vapor deposition) methods for depositing a thin film on a substrate. Also known as OMCVD (organometal CVD). In the CVD method, a gas containing a deposition substance or a mixed gas of such a gas and an inert gas is introduced into a heated base plate to cause a chemical reaction such as thermal decomposition, redox and replacement, thereby forming or depositing on the substrate. The method of the required substance. Among them, a method in which a raw material gas is an organic metal (a compound in which an organic substance is directly combined with a metal, organometal) is called a MOCVD method. This technique is generally employed when a compound semiconductor single crystal film such as GaAs is grown on a substrate.
Although the MOCVD method was chosen, Nakamura was the first to contact this technology. So first you need to learn. He decided to consult with Sakai, who was well known at the time to study the MOCVD method. At this point, Sakai has decided to go to the University of Florida. He suggested to Nakamura, "The opportunity is rare, let's go together." This is a good opportunity to ask for it, but I don't know if the company will send it.
The company will certainly not agree, first apply to the company to say. With this kind of mentality, Nakamura decided to give it a try. So he asked Sakai to accompany him and explained his thoughts to the president and the president. Surprisingly, the company decided to send him to Florida on the spot.
Returning to the previous state
Everything is unimpeded! It feels like a dream. But it didn't last long. I was surprised to see Nakamura after arriving at the University of Florida. There is no MOCVD device here, and the situation is different from the imagination.
The research lab in Nakamura should have two MOCVD devices. One of them was removed by the next-door research room, while the other one needed to be manufactured from now on. In this way, in the United States, Nakamura is also busy making equipment (Figure 1). Every day I am busy with piping and welding, which is no different from when I was in Japan. He couldn't help but think, did he come to Florida for these jobs? What followed was the feeling of burnout. Time is ignored, and the mood of Nakamura continues to ruthlessly pass. When Nakamura finally finished manufacturing the device, it was already a month before he returned to China.
Figure 1: When the village visited the Florida plant in the fall of 1994, the village of MOCVD, which was built in the United States, will re-open with the device that is still in operation.
In addition to Nakamura, there were several researchers from countries such as Korea and China at the time. Nakamura accompanied the smile and begged: I have to go back to Japan after one month, the time is very tight, can the device give me priority. The answer I got was No!. Nakamura only carried out 3 or 4 crystal growth experiments, and it is necessary to end the study in the United States.
Not even meeting
I don't know if I feel pitiful in Nakamura, or I have taken a fancy to Nakamura's excellent welding and piping technology. The professor in the research room has retained him: "I will pay you a salary, and stay for another year." But during the United States, there were too many unpleasant memories left for Nakamura.
Nakamura did not write a paper before going to the United States. Because the company does not allow it. It is for this reason that he finally went to the United States as a researcher, but the other party did not treat him as a researcher, and he did not even inform him of the meeting. The university also has people who study LEDs, but when Nakamura wants to ask questions, people ignore them.
During his studies in the United States, Nakamura also first experienced the "race disorder" that he had only heard before. Americans will naturally be with Americans, and Asians will form a circle with Asians. Although it was hard to get the opportunity to work with researchers from all over the world, there was no communication between them.
Nakamura recalled the days of studying in the United States and said, "There is no good memory." But waiting for him after returning to China is still a painful day. He was troubled by "no post after returning." I didn't learn technology in the United States. After I came back, I didn't have a job. Nothing was there. Nakamura can only start from scratch.
Unable to implement GaN film
Even so, Nakamura began research. Although in the workplace, Nakamura is like Taro Ura, but the president who sent him to the United States remembered him. The company was assigned to two new employees in Nakamura and began manufacturing equipment. He decided to purchase a commercially available MOCVD device and then retrofit it. In addition, he also let the company purchase a crystalline film evaluation device. The company has spent hundreds of millions of yen on all the devices together.
When the GaAs single crystal was developed, almost no device was purchased. Even if it is good to say that you agree to pay, it is only about 1 million yen. The sudden increase in investment to hundreds of millions of yen is very rare for Nakamura, and it also creates a kind of pressure.
After returning to Japan in April 1989, Nakamura began research. One month, two months, or even half a year has passed, but research has not made any progress. The light-emitting layer of the blue light-emitting diode - the GaN film is never formed. Even before it reached the GaN film, it fell.
The MOCVD method is a method in which a material gas is introduced into a substrate heated at a high temperature, and then the gas is decomposed on the surface of the substrate to form a crystalline film. It is necessary to place the bottom plate in a container for introducing gas and heat it at a high temperature, and the problem is here.
The first problem is that Nakamura chose GaN as a luminescent material for blue light-emitting diodes. In principle, several materials can achieve blue light. Among them, GaN is a material that is left out of the cold. Note 4). Only because "other people did not adopt", Nakamura decided to choose this material. After beginning to challenge the growth of the crystal film, he understood why the material was unpopular. That is, GaN film formation is very difficult. If only a commercially available device is slightly modified, film growth cannot be achieved at all.
Note 3) GaN (gallium nitride) is a type of III-V compound semiconductor. It is a direct migration type with an Energy Gap of 3.4 eV. By forming a mixed crystal with InN (energy gap 2.0 eV) and AlN (energy gap 6.3 eV), the energy gap can be between 2.0 eV and 6.3 eV.
Note 4) Materials for blue light-emitting diodes include ZnSe, SiC, and GaN. In 1989, SiC was the fastest-growing research in the use of blue light-emitting diodes, and light-emitting diodes with lower brightness have been produced. Research on ZnSe is also very popular, and has attracted attention as a strong candidate for materials for blue light-emitting diodes and blue semiconductor lasers. However, GaN has rarely been studied. At that time, there were cases in which the ZnSe seminar was held in Japan and the GaN seminar was less than 10 participants.
Known as the weirdo
In order to grow a GaN single crystal film on the substrate by MOCVD, the substrate must be heated to a high temperature of +1000 ° C or higher. It is very difficult to achieve this. What's even worse is that the other choice of Nakamura has further aggravated the situation. That is to use a method of heating the bottom plate with a heater.
The Nagoya University Research Group, which started researching GaN films very early, 5) heated the bottom plate by applying a high-frequency electromagnetic field from the outside of the device (Fig. 2). Nakamura still chose heater heating on the grounds that “I don’t want to use the same method as othersâ€.
Note 5) In addition to the Nichia Chemical Industry, other Japanese research groups that study GaN include the Toyota Synthetic Research Group and the group of Professor Akasaka Akasaka (at the time, Professor of the famous City University) of Nagoya University. The research group of Toyota Synthetic and Nagoya University successfully developed GaN single crystal film in 1989. In the early 1990s, GaN blue light-emitting diodes were successfully produced. It can be said that they are one step ahead of Nichia.
Figure 2: Heating method of the bottom plate
In the method of using a high-frequency electromagnetic field, it is necessary to place a bottom plate on a heating table (ssusceptor) made of a conductor, and to heat the bottom plate by raising the temperature of the susceptor by a high-frequency electromagnetic field applied from the reaction chamber (a). There is no need to provide a mechanism for heating in the reaction chamber, so the construction is relatively simple. However, it is not possible to form a reaction chamber using a metal as a conductor, and a reaction chamber is generally made of quartz glass. By the method of (b) of the heater, a heating table equipped with a heater can be placed in the reaction chamber, and then the bottom plate is placed thereon, and the bottom plate is heated by this method. With this method, the material of the reaction chamber can be freely selected.
The raw material gas for producing a GaN film, NH3, is corrosive. No heater is resistant to high temperatures and corrosion. Therefore, the heater is quickly corroded and the film cannot grow.
At that time, Nakamura was very depressed every day. Come to the company in the morning and open the device. Did you create a real film today and the heater burned out again? The afternoon job is to renovate and repair the equipment. He went to work the first time in the morning and got off work at 6 pm. Repeat this monotonous day without end.
Nakamura’s words became less and less, the phone did not pick up, and the people around him began to treat him as a weirdo. One of the two new employees under the ministry resigned because of "the hope of not seeing success at all."
The goddess of victory once smiled, but it was fleeting
Things suddenly turned around. After many failures and constant exploration, Nakamura finally developed a heater that will not burn out. 6). After the heating of the bottom plate is successful, all that remains is to modify the device and improve the access method of the raw material gas.
Note 6) The top secret in top secret is how to avoid heater burnout. It is still a technical flaw that does not leak. According to reports, due to the development of such a heater, Nakamura "has become a heater design expert." This is the same as the welding technique and piping technology for Nakamura.
Nakamura has absolute confidence in the transformation of the device. After entering the company's development department, all the devices were made by themselves, and the gas piping skills were fully mastered in the United States for one year. Although people around him advised him that it is dangerous to arbitrarily transform the MOCVD device, it did not allow Nakamura to retreat. When he was in the company's development department, he experienced several explosions, so he was not afraid at all.
After the heater was developed successfully, the method of heating with the heater really worked well. When heating with a high-frequency electromagnetic field, it is necessary to use a quartz glass to manufacture a reaction chamber, an indoor pipe, an air outlet, and the like of the MOCVD apparatus. Although Nakamura's welding technology is very high, it is not easy to modify the device made of quartz parts.
However, if heated by a heater, the reaction chamber, the piping, and the gas outlet can be made of metal. The processing is easy, the installation and disassembly are also very convenient, and the transformation becomes very easy (Fig. 3(a)).
Figure 3: Method of introducing a modified gas
According to Nakamura's experimental notes at the time, four gas introduction methods were tried at the end of August 1990. It was found that the Two-Flow method of introducing gas from the side and above the bottom plate was effective in early September. (a) Experimental notes of August 27, 1990; (b) Experimental notes of September 10, 1990.
In September 1990, the moment when the GaN film was launched was finally ushered in. Nakamura invented the "Two-Flow method" in which gas was blown from both directions of the bottom plate, and a crystal film was successfully grown (Fig. 3(b)). He was pleased to evaluate the film formed. This film has the highest mobility in previously released films (Figure 4). awesome! Finally succeeded! Nakamura rushed to start the growth of the second and third batch of crystalline films. I plan to produce a higher quality film...
Figure 4: High mobility GaN film growth is successful
In September 1990, a GaN film was grown using the Two-Flow method. It achieved the highest migration rate at that time, and was higher than the number published by the leading research team at Nagoya University. Excerpted from the experimental notes of September 17, 1990.
But after entering October, the incredible thing happened, and the GaN film suddenly could not grow. Nakamura rushed to inspect the device, but found no problems. Once succeeded, it did form a film, but now it can't grow, and the reason is unknown. There must be something wrong with it. (To be continued)
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