High-brightness blue LED development (5): difficult breakthrough

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Introduction: The Great Invention of the 20th Century - High Brightness Blue Light Emitting Diode

(2): The equipment needs to be self-made, the parts are used repeatedly, and the development period is unbearable.

(3): With a frustrated return to China, everything starts from scratch and is busy with equipment renovation throughout the day.

(4): More than ten years of bitterness ushered in the darkness of the flowers, and the research and development began to advance rapidly.

The end of the story of high-brightness blue LED development. From the second to the fourth, we introduced the process of entering the company from Nakamura Shuji to the blue LED for the first time. This article will usher in the finale. High-brightness LEDs will be developed. Although GaN light emitting diodes emit light, the light is quite dark. Nakamura decided to change from a pn junction structure to a double Hetero structure. After that, the development speed is rapidly increasing. The development was also smooth, and impurities were incorporated into the double heterostructure to form a luminescent center, achieving a brightness of 1 cd and finally being put into production. Applications such as signal lights have also begun to appear.

In April 1992, Nakamura, who returned from the American Society to study, developed a double heterostructure to study the formation of InGaN films (Table 1). If a double heterostructure is introduced into a GaN LED, the brightness should be greatly increased.

Note 1) The pn junction type light-emitting diodes have been completed at that time. The pn junction only joins the p-type and n-type semiconductors, and the structure is simple. A forward bias is applied to the pn junction to inject electrons which are generated when electrons are rejoined in a hole. In the case of a double heterostructure light-emitting diode, a light-emitting layer is sandwiched between a semiconductor layer having a larger energy gap than the light-emitting layer. In terms of bonding between the light-emitting layer and the surrounding semiconductor layer, both sides are hetero-bonded (joining between different materials). When a forward bias is applied to the pn junction, the injected carriers are not all migrated from the energy band to the energy band (re-bonding). Most of the carriers will flow out to the electrodes, contributing to the luminescence. In the case of a double heterojunction type light emitting diode, the energy gap of the light emitting layer is smaller than the surrounding area. Therefore, the carriers are closed in the light-emitting layer, and the probability of re-engagement is increased. Therefore, if a double heterostructure is used, the brightness can be mentioned to be higher than the pn junction.

Table 1: Blue LED development chronology

Since this period, Nakamura's research team has received funding and talent input. This is the embodiment of the president's determination to productize. For the research of GaN light-emitting diodes, Nichia has invested hundreds of millions of dollars. From the company's point of view, this decision is like jumping from the stage of the Kiyomizu-dera Temple in Kyoto. I finally waited for the day when the GaN LED was illuminated. So the company hopes to turn it into a bestseller as soon as possible. As the object of the decision to invest, the president’s expectations for it are self-evident.

However, this expectation has become an obstacle to the progress of Nakamura. The president thinks that even if it is a little dark, it doesn't matter, and he is anxious to productize the pn junction LED. Nakamura has seen the limitations of pn junction LEDs and hopes to push the research to a deeper level. Because he has the confidence to produce results in a short time.

In spite of opposition from all, launch a tough battle

Nakamura decided to open the situation from the "central breakthrough." He tried to introduce to the president the necessity of switching to a double heterogeneous structure and hoped to get the understanding of the president. However, this effort did not work. The president advocated an early production and did not give in.

In this case, Nakamura had to change the "combat strategy." Nakamura decided to follow the advice of the president. However, just listen to it. At the company meeting, the president asked for “quickly put into production of pn junction products”. "Yes, I know," Nakamura promised. Although promised, but in fact, Nakamura has no intention to promote the pn junction productization. Nakamura completely ignored the company's ideas and shut himself down in the laboratory to start a study of double heterostructures.

Sure enough, Nakamura did not expect that the growth experiment of InGaN film took only 2 to 3 months. Later, in September 1992, the double-heterostructure GaN LED was finally successfully produced. Although the light was successfully emitted, it was still dark (Figure 1). Nakamura gave the president a look, and the evaluation was: "It was made by you, it is still very dark."

Figure 1: Realizing a double heterostructure

In September 1992, a blue light-emitting diode with double heterostructure was fabricated and successfully illuminated (a). However, at this time, it is not high brightness. However, this success is an important step in the development of high-brightness blue light-emitting diodes (b) that have now produced products. (a) Double-heterostructure blue light-emitting diodes that emit light for the first time; (b) Double-heterostructure blue light-emitting diodes that have been produced

Although it is still very dark, it is epoch-making to be able to make a successful double heterostructure on GaN materials and also emit light. Its future will have unlimited development potential. Nakamura decided to let the world judge in the form of papers. He took advantage of the company and continued to contribute papers when research and development made critical progress.

The paper has caused great repercussions among researchers in Europe and America. Letters of appreciation and requests for a copy of the papers written by Nakamura in the past are endless. However, "it is not recognized in Japan at all," Nakamura said when he recalled the situation at the time. “The Japanese researchers did not judge the credibility of the paper by the name of the company or the name of the university. I tried to ask a few researchers and answered that the people who didn’t believe at the time were mostly. High-brightness blue LEDs have become products, and it is estimated that there are many people who have rushed to read past papers" (Nakamura).

Lonely researcher

According to the company's regulations, publishing a paper at the Institute is prohibited. The secret submission of the paper will not be known by the company, but the words published in the society may not be able to hold the note 2). Because of the society held in Japan, the company's researchers will listen.

Note 2) At the time, the company stipulated that it was forbidden to publish speeches and submit papers. However, during this period, Nakamura submitted a number of English papers. In the company, because no one other than Nakamura orders English papers and magazines, there is no need to worry about the submission being known by the company. However, if published in a Japanese society, it may not be known by the company.

However, the significance of participating in the society has become smaller and smaller. This is because there is no in-depth discussion with other researchers. Although the result of the paper published in Nakamura is believed to be one of the reasons, the most important thing is that Nakamura is too advanced. Nakamura does not need to get any technology from the society. The moment of successful productization is approaching. Nakamura stopped all academic activities and paper submissions, focusing on the productization of GaN blue LEDs. Productization has entered the countdown phase.

The progress made after this is amazing. The reason why the light is dark is that the village is very clear. The first reason is that the emission wavelength is ultraviolet light. So first turn it into a blue light that is visible to the eye. For this reason, Nakamura decided to add an impurity as a luminescent center to the light-emitting layer InGaN (3). As a result, the wavelength of the emitted light jumps from 420 nm to 450 nm, and the visible brightness of the human eye is four times that of the previous one. This is the December 1992 thing. However, the pace of rapid advancement has not stopped due to this success. The crystallinity is gradually increased by further adjusting the formation conditions of the film. Brightness is increasing day by day (Figure 2).

Figure 2: Rapidly increasing brightness

After the double heterostructure is realized, the brightness of the blue LED is increasing day by day. After about half a year after the initial trial production, the brightness rose to 100 times before, reaching 1cd.

Note 3) The illuminating center is the impurity level (Fig.) set between the conduction band and the valence band, that is, the forbidden band. With this level, the electrons recombine with the positive holes to illuminate. By introducing the illuminating center, the wavelength of the illuminating light can be increased while using the semiconductor of the same energy gap.

Finally the brightness reaches 1cd

Finally came to the final adjustment stage before the production. This stage is mainly to increase the crystallographic grade of the GaN film and the InGaN film to improve the brightness. At the same time, improve mass production technology and improve the yield. Nakamura cut off all external contacts and locked himself in the lab.

Figure 3: High-brightness blue light-emitting diodes that are put into production at 1 cd

Nakamura displayed a display panel made of blue LEDs developed this time.

After such efforts, in October 1993, less than one year after the successful trial production of InGaN double-heterostructure light-emitting diodes, the conditions for productization were basically met. The brightness reached 1 cd. It was about 100 times that of commercially available blue light-emitting diodes using SiC.

Breaking the silence of nearly a year, the blue LED finally came to the fore (Figure 3). The product release date is scheduled for November 30th. Prior to this, Nakamura and his boss took a proud blue light-emitting diode and visited a major university and research institution in Japan. Among them, the greatest blessing to this success is the president of Nishizawa Ryuichi of Tohoku University in Japan (Figure 4). On the spot, President Nishizawa proposed to give the doctor a title to Dr. 4).

Figure 4: Appreciated by President Nishizawa Junichi

After seeing the high-brightness blue LED, President Nishizawa of Tohoku University in Japan immediately wrote an inscription.

Note 4) At that time, Nakamura had applied for the title of doctoral degree to his alma mater, Tokushima University, Japan. Nakamura said that he had to cut off the good intentions of President Nishizawa.

As a result, Nakamura gained confidence in the results of the chest release. Finally arrived the day before the product release. Although it is a product release, it does not rent a first-class hotel like a big manufacturer, and holds a grand press conference. Instead, the contents of the results were disclosed to the director of the Japan Economic News Agency. Nakamura said that because it was a shocking result of "the brightness reached 100 times that of the previous products," the director did not believe it at first. Although a small press conference was held with only internal staff, the news was published on the first edition of Nikkei Industry News on November 30.

There are still many goals for the next goal.

Since this day, Nichia Chemical Industry Co., Ltd., interview requests from the media, and consultations from users and other companies in the same industry have swarmed. You can receive 40 to 50 calls per day. This situation lasted for more than a week. "Is such an amazing achievement?" The president also panicked. After the phone tide, it was another wave of visits. With various proposals such as technical cooperation and funding, people visiting the chemical industry in Nichia are in constant stream.

The president rejected these proposals one by one. Nakamura’s belief that “people are not for people, I am for it” has finally achieved results that transcend large enterprises. The president also adheres to his belief that he is "not dependent on others." Previously, the company has been relying on its own strength for research and development. The president decided to continue to be self-reliant in the future.

After throwing those cockroaches into the brain, Nakamura continued to conduct research. Although blue light-emitting diodes have produced products, semiconductor lasers as research and development targets have not yet been completed.

In addition, after the completion of the blue LED, consumer demand for green and blue-green LEDs has become more intense. If you combine the completed blue LED with the already-productive red and green LEDs, you can create a full-color display (Figure 5). However, the brightness of the green diode is lower than the high brightness of the blue and red diodes. A green LED with higher brightness is required.

Figure 5: Large full-color display with LEDs available

The photo was developed by Kinki Nippon Railway and was installed in the central hall of the Uehonmachi Train Station in Japan in January 1995. The number of pixels is 320×240

In addition, red, yellow and blue three-color light-emitting diodes have been manufactured so far, and the door to the signal light is thus opened. However, the color of the Japanese green signal light is blue-green. Therefore, it is also necessary to develop a light-emitting diode that matches this.

Strive to achieve inter-band illumination

Nakamura first completed the blue-green LED. At present, the signal lamp using such a light-emitting diode has been available and has been put to practical use (Fig. 6). The rest are lasers and high-brightness green LEDs.

Figure 6: Blue-green LEDs used on signal lights

At that time, experimental setup was carried out in Aichi Prefecture and Tokushima Prefecture, Japan. The signal light using the light-emitting diode can prevent misidentification because it is colorless when it is not illuminated.

There are two challenges in creating a blue semiconductor laser using GaN materials. The first problem is the need to achieve the inter-band illumination required for semiconductor lasers 5). In the case of sandwiching the energy gap of InGaN, blue can be obtained by band-to-band illumination. By further clamping the energy gap, green light-emitting diodes and semiconductor lasers can also be realized (6).

Note 5) Light-emitting diodes produced today emit ultraviolet light if they emit light between the bands. Therefore, blue light emission has been achieved by introducing the light-emitting center into the light-emitting layer. However, semiconductor lasers cannot be fabricated by this method. However, if the In concentration of the light-emitting layer InGaN can be increased and the energy gap of InGaN is sandwiched, blue can be obtained by band-to-band illumination. If the In concentration can be further increased, the energy gap can be clamped accordingly, thereby realizing blue-green and green light-emitting diodes and semiconductor lasers.

Note 6) In InGaN, the energy gap can be changed by changing the ratio of In to Ga. If Ga is gradually increased, the energy gap can be expanded to a maximum of 6.3 eV, and if the In is gradually increased, the energy gap can be reduced to a minimum of 2.0 eV. By changing the energy gap in this way, a light-emitting diode of any light color can be produced in the wavelength range of red to ultraviolet light. However, the more In is added, the more difficult it is to produce an InGaN film having better crystallinity.

Another difficulty is to fabricate the construction necessary for laser oscillation on GaN materials. Current semiconductor lasers employ a configuration in which a light-emitting layer of a double-heterostructure light-emitting diode is mirror-covered, thereby confining light and resonating light. The cleavage plane* of the crystal film is used as a mirror surface. However, GaN cannot be opened. It is necessary to make resonance surfaces by other means.

*劈面面 = smooth surface where the crystal is easily split and formed in a certain direction. Making such a split surface is called splitting. When a material such as a semiconductor single crystal is cut, a split method is used. However, depending on the type of crystal, there are points that are easy to open and cannot be opened.

The light between the bands is expected to be realized. Blue LEDs and green LEDs that are brighter than current blue LEDs will be manufactured in the near future.

In the remaining problem, the resonance surface, the technology of forming a resonance surface by etching is currently being accelerated. Nakamura confidently stated that it successfully achieved room temperature oscillations during the year of 1995.

The people around me finally understand themselves.

Everything is going smoothly. Without being recognized by the society, Nakamura firmly believes that it can succeed, and therefore conducts research with one mind. The hard work he has done has produced fruitful results. Since then, the situation has completely changed. The invitation to invite him to give a speech has continued. His life has been hiding in the research room all the time, and has become flying to speak.

The research environment has also changed a lot. Previously, I was alone in research, and now the number of researchers has increased to five. Nakamura began to take the initiative to increase the proportion of management work in the work content. Although he said "Because I am a personally intimate personality, so there is still some loneliness", but seeing the growth of young technicians who have been able to stand alone, Nakamura has raised his eyes with satisfaction.

"But the biggest change is the recognition of the people around. In the past, although the LEDs were made and the light was emitted, the people around you still don't believe in you. Is it a waste of money, is it really successful? Can you make money? This is everywhere. Now this situation has changed. Everyone has begun to respect me." In this way, Nakamura smiled embarrassedly. (To be continued...)

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