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Ultraviolet and blue LEDs
Current bright blue LEDs are based on the wide band gap semiconductors GaN (gallium nitride) and InGaN (indium gallium nitride). They can be added to existing red and green LEDs to produce the impression of white light, though white LEDs today rarely use this principle.
The first blue LEDs using gallium nitride were made in 1971 by Jacques Pankove at RCA Laboratories. These devices had too little light output to be of practical use and research into gallium nitride devices slowed. In August 1989, Cree Inc. introduced the first commercially available blue LED based on the indirect bandgap semiconductor, silicon carbide. SiC LEDs had very low efficiency, no more than about 0.03%, but did emit in the blue portion of the visible light spectrum.
In the late 1980s, key breakthroughs in GaN epitaxial growth and p-type doping ushered in the modern era of GaN-based optoelectronic devices. Building upon this foundation, in 1993 high-brightness blue LEDs were demonstrated. Efficiency (light energy produced vs. electrical energy used) reached 10%. High-brightness blue LEDs invented by Shuji Nakamura of Nichia Corporation using gallium nitride revolutionized LED lighting, making high-power light sources practical.
By the late 1990s, blue LEDs had become widely available. They have an active region consisting of one or more InGaN quantum wells sandwiched between thicker layers of GaN, called cladding layers. By varying the relative InN-GaN fraction in the InGaN quantum wells, the light emission can be varied from violet to amber. AlGaN aluminium gallium nitride of varying AlN fraction can be used to manufacture the cladding and quantum well layers for ultraviolet LEDs, but these devices have not yet reached the level of efficiency and technological maturity of the InGaN-GaN blue/green devices. If the active quantum well layers are GaN, instead of alloyed InGaN or AlGaN, the device will emit near-ultraviolet light with wavelengths around 350–370 nm. Green LEDs manufactured from the InGaN-GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems.
With nitrides containing aluminium, most often AlGaN and AlGaInN, even shorter wavelengths are achievable. Ultraviolet LEDs in a range of wavelengths are becoming available on the market. Near-UV emitters at wavelengths around 375–395 nm are already cheap and often encountered, for example, as black light lamp replacements for inspection of anti-counterfeiting UV watermarks in some documents and paper currencies. Shorter-wavelength diodes, while substantially more expensive, are commercially available for wavelengths down to 247 nm. As the photosensitivity of microorganisms approximately matches the absorption spectrum of DNA, with a peak at about 254 nm, UV LED emitting at 250–270 nm are to be expected in prospective disinfection and sterilization devices. Recent research has shown that commercially available UVA LEDs (365 nm) are already effective disinfection and sterilization devices.
Deep-UV wavelengths were obtained in laboratories using aluminium nitride (210 nm), boron nitride (215 nm) and diamond (235 nm).
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