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Application and technology news for general and architectural SSL |
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The Very Basics of Compound Semiconductor Epitaxy April 4, 2001... Most of today's compound semiconductor devices are grown, atom-by-atom, "epitaxially" on a semiconductor wafer, which is made from a semiconductor boule, or substrate. A very simple analogy is to think of a starting substrate boule, or "crystal" as a hunk of bologna which is thinly sliced into wafers. When properly sliced and polished, a wafer becomes the surface for epitaxial growth in an epitaxy chamber, or reactor... sometimes referred to as a production platform. As a size comparison, boules of silicon are at least 8 inches in diameter, gallium arsenide (GaAs) boules can be as large as 6 inches, indium phosphide (InP) at between 2 and 4 inches, silicon carbide at approximately 3 inches, and gallium nitride (GaN) is still virtually nonexistent as a bulk substrate. Some very important compound semiconductor combinations are only small chunks of bulk material and those combinations can only be produced for use as devices by epitaxial growth. The term "epitaxy" is derived from the Greek word meaning "ordered upon." Of the various epitaxial growth techniques in popular use today, the primary methods are LPE (Liquid Phase Epitaxy), VPE (Vapor Phase Epitaxy), MBE (Molecular Beam Epitaxy) and MOCVD (defined below). LPE is the most simple and oldest method (a bit like dipping candles until they are the size and composition desired), MBE has long been favored for its simplicity and flexibility, thus especially popular at the research level. MOCVD, which has been viable as a manufacturing technique since the mid-1980s, tends to be today's most popular choice in large scale manufacturing environments. Whereas conventional LEDs are still grown by LPE, the vast majority of today's high brightness LEDs (HB-LEDs) and laser diodes (LDs) and are grown by MOCVD. Complex heterostructure electronic devices, such as the HBTs and HEMTs (defined below) which are used in broadband communications, and advanced solar cells are grown by either MBE and MOCVD. The term MOCVD stands for Metal Organic Chemical Vapor Deposition. MOCVD technology is also known in some circles as OMVPE (Organo-Metal Vapor Phase Epitaxy) and MOVPE (Metal Organic Vapor Phase Epitaxy). Although "OMVPE" is technically the most accurate term in academic circles, all three essentially mean the same thing in the field, and are often used interchangeably. Typical devices produced by MOCVD and their primary applications include: high efficiency solar cells as use in next-generation satellites, the high brightness LEDs in all colors of the spectrum (including white) that are rapidly replacing incandescent and other forms of conventional lighting, and as solid state semiconductor lasers which are rapidly replacing larger, more expensive lasers in a variety of applications, especially as optical components in broadband communication, especially fiberoptic networks. Laser Diodes (LDs) are also used in everything from laser pointers and common barcode readers to sophisticated medical applications. LDs are the primary component used to read and write CD-ROMs, Digital Video (or Versatile) Discs (DVD) and the hard disk drives in computers. Blue spectrum laser diodes (which include green, blue, violet) show excellent promise as replacements for conventional CDs, DVDs and disk drives because they can write in a smaller space, thus significantly condense the amount of space needed for storing information on a single disk. HBTs (heterobiopolar transistors) and HEMTs (high electron mobility transistors) are prevelant in cordless portable telephones and cellphones as well as other wireless communication devices. Essentially all these devices are compound semiconductors, created by epitaxial growth techniques, reaching performance levels not possible with silicon-based devices. Return to CompoundSemi Return to SSLnet/LIGHTimes Return to Solid State Lighting Design |
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