Foreword
When we look at the colorful colors on the screen of a mobile phone or computer, we know that the colors on the display are composed of tiny pixels, and each individual pixel is composed of LED lamp beads of the three primary colors of light (R-red, G-green, B-blue). But did you know that the development of blue LED stumped the top scientists and engineers at the time. Red LED was first invented by American electrical engineer Nick Holonyak Jr. in 1962. In the 1970s, Fairchild Semiconductor expanded the light spectrum of LED to orange, yellow and green with the help of new technologies and new materials, but blue LED has never been able to break through. Even if it can emit light, it is only weak blue light, which is completely unusable for commercial applications. It was not until 1993 that it was broken by an unknown Japanese engineer, Shuji Nakamura, who also won the 2014 Nobel Prize in Physics for Isamu Akasaki, Hiroshi Amano and Shuji Nakamura.
Ⅰ. LED light-emitting principle
The core of the light-emitting diode is that the holes and electrons of the diode flow from the electrode to the PN junction under the action of voltage. When the holes and electrons meet and recombine, the electrons will fall to a lower energy level and release energy in the form of light. As shown below:
Ⅱ. What determines the color of LED light?
The color of light is determined by the energy of photons, which in turn depends on the band gap of semiconductor materials:
Large band gap → high energy photons (blue/purple light)
Small band gap → low energy photons (red/yellow light)
Ⅲ.Why is blue LED difficult to manufacture?
1. Challenges in material selection
Suitable semiconductor materials are scarce:
Red and green LEDs used materials such as gallium phosphide (GaP) in the early days, but the materials that can produce efficient blue light are very limited. Eventually, gallium nitride (GaN) was confirmed as the best choice, but it was extremely difficult to prepare under the technology at the time.
Crystal growth problems of gallium nitride:
GaN requires high temperature (about 1000°C) and high pressure environment to grow, and lacks a suitable substrate (substrate material). The common sapphire substrate does not match the GaN lattice, resulting in many crystal defects and low efficiency.
2. Bottleneck of doping technology
Breakthrough problem of p-type doping:
GaN naturally tends to conduct n-type (electron-rich), while LEDs require p-n junctions to emit light. It was not until the 1990s that the team of Isamu Akasaki and Hiroshi Amano discovered that p-type GaN could be achieved by doping with magnesium (Mg) and combining it with electron beam activation, a process that took many years.
3. Physical limitations of short wavelengths
High energy requirements:
Blue light has a shorter wavelength (~450 nanometers), which requires a wider bandgap of semiconductor materials (GaN has a bandgap of 3.4 eV). Large bandgap materials have extremely high requirements for purity and crystal quality, and any defects will cause a sharp drop in efficiency.
4. Lagging behind in technological accumulation
Red LEDs (1960s) and green LEDs (1970s) were developed earlier, while blue LEDs were not commercialized until 1994 by Nakamura Shuji's team (Nichia Chemical). In the decades before that, scientists even believed that GaN was "impossible to be practical."
Ⅳ. The advent of blue LEDS
1 .Early research by Isamu Akasaki and Hiroshi Amano (1980s)
The team of Isamu Akasaki and Hiroshi Amano at Nagoya University in Japan used sapphire (Al₂O₃) as a substrate and successfully grew high-quality GaN crystals through low-temperature buffer layer technology (first depositing an AlN buffer layer on sapphire). In addition, they found that magnesium (Mg) doping combined with electron beam irradiation can achieve P-type GaN, solving the key problem of PN junction preparation.
2.Commercialization breakthrough of Shuji Nakamura (1990s)
Based on Isamu Akasaki and Hiroshi Amano, Shuji Nakamura, who was working at Nichia Corporation at the time, further optimized the GaN growth process:
Dual-flow MOCVD (metal organic chemical vapor deposition): improved the quality and uniformity of GaN films.
InGaN (indium gallium nitride) quantum well structure: high-efficiency blue light emission was achieved by adjusting the In component.
Thermal annealing technology: greatly improved the conductivity of P-type GaN.
In 1993, Shuji Nakamura successfully developed a high-brightness blue LED and achieved commercial production in 1994. This breakthrough made white light LED (blue light LED + yellow phosphor) possible, which completely changed the lighting industry.