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3. GaN-based blue light emitting device development by Akasaki and Amano
3.1 Selection of GaN
Akasaki undertook research to develop blue light emitting devices using GaN in the 1970s. He adopted a molecular beam epitaxy (MBE) method that can bond each molecule to the right place, in addition to the previously used hydride vapor phase epitaxy (HVPE) method. He succeeded in fabricating a GaN single crystal film using MBE method. In 1981, he achieved a light emission from a metal-insulator semiconductor structure with 0.12% efficiency using HVPE method1). However, he could not develop a thin film with good thickness uniformity. Also, he could not produce a p-type GaN film. Even on this macroscopically inferior quality crystal, he noticed a strong light emission from the very small spot of film that had thickness uniformity. This observation convinced him that GaN was, indeed, the material with which to develop a blue light-emitting device, despite the fact that many other researchers had abandoned GaN-based research. In 1981, Akasaki moved to Nagoya University to continue his research. In 1982, Amano joined Akasaki's laboratory and began his research work.
3.2 GaN thin film formation with superior uniformity
Akasaki had recognized the limitations of the HVPE method and the MBE method for GaN thin film fabrication. The problem with the HVPE method is that the crystallinity of thin film is bad under high film fabrication speed. On the other hand, the problems associated with the MBE method include slow film fabrication speed and difficulty in attaining a stoichiometric composition because nitrogen is easily pulled out by its high vapor pressure in an ultra-high vacuum ambient atmosphere. He decided to use a metal organic chemical vapor deposition method (MOCVD method) that brought an appropriate film fabrication speed in the same temperature region for each film. He selected a sapphire substrate, which can be used at a GaN film fabrication temperature of over 1000 °C in MOCVD method, and has a lattice symmetry near to that of GaN. However, because the lattice constant difference was 16 percent, it was a difficult challenge to fabricate hetero epitaxial thin film with good crystal quality and thickness uniformity.
Akasaki and Amano designed and developed the MOCVD equipment by themselves, because MOCVD fabrication equipment that could be used for GaN was not available at that time. They tried to identify the best fabrication conditions repeating experiments with various combinations of substrate temperature, vacuum pressure, material gas feed rate, inactive gas feed rate, fabrication time length, and so on. They performed over 1,500 experiments in two years, but they could not fabricate thin film with good crystal quality and thickness uniformity.
They conceived of locating a buffer layer fabricated at low temperature between the sapphire substrate and the GaN thin film after thorough investigation of their experimental results. They selected GaN, aluminum nitride (AlN), SiC, and zinc oxide as the candidates. They succeeded in fabricating GaN thin film with good crystal quality and thickness uniformity using AlN as a buffer layer in 19862). Good crystal quality of the film was confirmed using photoluminescence, x-ray diffraction, Hall effect measurement, transmission electron microscope and so on. From the result of Hall effect measurement, the electron mobility at room temperature was calculated to be improved from 50cm2/V s to 450cm2/Vs3). Moreover, they succeeded in realizing the n-type thin film with thickness uniformity and good conductivity4).
This success was achieved not just through one experiment, as above, but instead required a tremendous number of experiments to find the optimized conditions that made it a great breakthrough.
3.3 P-type GaN layer fabrication
GaN thin films usually became n-type semiconductors, because they contained smaller than stoichiometric numbers of nitrogen. Attempts were made to make a p-type layer using acceptor-doping material. This was also a big hurdle to making a GaN light emitting device. Akasaki and Amano initially used zinc as an acceptor dopant and tried to fabricate p-type GaN, but they did not achieve success. After they used magnesium as a dopant because of its higher electron affinity, they still did not achieve success. In 1988, Amano found that cathode luminescence light intensity increases with electron beam irradiation in other experiments to measure cathode luminescence of acceptor doped GaN. He thought from this result that the electrical and optical characteristics of acceptor doped GaN thin film were changed by electron beam irradiation5). Based on this, they measured the electrical characteristics of 10 kV electron irradiated Mg doped GaN thin film and found that the resistivity decreased by a factor of 10,000 to 35 Ohm-cm, and confirmed that GaN film is clearly p-type by its Hall effect measurement. Its hole mobility was 8 cm2/Vs. They made a p-n junction at the same time and measured current rising in the forward direction on the current-voltage curve and light emission -- both are characteristics of p-n junction6). This p-type layer fabrication method by electron beam irradiation was the historic first fabrication of p-type GaN. They made an important step toward developing a blue light emitting semiconductor device by inventing p-type GaN fabrication and GaN layer fabrication method.
3.4 Development of the blue LED
From 1987, Akasaki and Amano led blue LED development at Toyoda Gosei Co. Ltd., which received funding from the Japan Science and Technology Corporation. In 1992, they succeeded in developing a bright blue LED with 1 percent light emitting efficiency. Toyoda Gosei announced commercial production of blue LEDs in 1995.
3.5 Development of the blue LD
Akasaki and Amano also undertook the challenges of developing a blue LD. In 1990, they succeeded in measuring narrow bandwidth 374nm stimulated emission at room temperature with high density electron-hole pairs created by radiating 337nm nitrogen laser beam on a GaN thin film, which was fabricated on a low temperature buffer layer with good thickness uniformity7). Then they fabricated a multi-quantum well structure device and, decreasing the threshold power density, succeeded in measuring strong emission of 3nm half bandwidth on 1.0 kA/cm injection current density, which they reported in Japanese Journal of Applied Physics in November 19958). They reported achieving a laser oscillation of 405nm wavelength in June 19969).
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