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2. Light emitting devices
2.1 Light emitting diode
A LED is a compound semiconductor device that emits visible or infrared light when an electric current passes through it. It contains a p-n junction with a direct transition band structure. Injected electrons and holes recombine near the p-n junction area to emit light. In order to increase light emitting efficiency, a double heterojunction is widely used, which is made by covering an active layer by p-type and n-type semiconductor layers that have larger band gaps. The wavelength of the emitted light is determined by the band gap of the semiconductor material. Gallium arsenide (GaAs) based material and gallium phosphide (GaP) based material are used to produce red and green LEDs. In the 1970s, red and green LEDs were commercialized and used for a variety of applications, such as indicators and signs.
2.2 Laser diode
A laser diode is a light emitting diode that uses an optical cavity to amplify the light emitted from the energy band gap that exists in semiconductors. For a laser to work, there needs to be a population inversion between the conduction and valence band of the semiconductors, which is created by applying an electrical current to the p-n junction. First generation LDs used homogenous p-n junctions and the same material as LEDs. These were relatively inefficient and required a high current density to achieve lasing. Laser operation was limited only to pulse mode at low temperatures. Today, double heterojunction lasers are widely used. They produce light more efficiently, have a lower current density threshold for operation, and can provide stable and continuous laser oscillation at room temperature. Since the 1980s, red and infrared LD have been used for optical fiber communications, as read/write devices for optical storage, and in other applications.
2.3 Blue light emitting semiconductor device
Silicon carbide (SiC), zinc selenide (ZnSe) and gallium nitride (GaN) were all potential semiconductor materials for the development of blue light emitting semiconductor devices, based on the width of their energy band gaps. However, SiC has an indirect transition band structure, so it was not a suitable candidate. ZnSe was considered a good candidate and active research was undertaken from the 1970s. Although LEDs and LDs using ZnSe were developed in the laboratory, commercial products could not be developed because of the short lifespan caused by, for example, the electrode metal migration to the light emitting semiconductor layer.
Research on GaN also started in the 1970s. There were no good substrates possessing a lattice constant near that of GaN. A sapphire substrate with a 16 percent different lattice constant had to be used, and it was almost impossible to fabricate a GaN thin film with good crystal quality and uniform thickness. Even if such a high quality GaN thin film could be fabricated, it was also almost impossible to make p-type GaN film. Considering these challenges, many researchers thought it was impossible to develop a blue light-emitting device using GaN and they abandoned this line of research.
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