The ternary system gaasi xpr now dominates thezed

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Unformatted text preview: in pocket calculators, wrist watches and the alpha-numeric output displays of many instruments; they are also used in infrared-emitting diodes, injection lasers, infrared detectors, photocathodes and photomultiplier tubes. An extremely elegant chemical solid-state technology has evolved in which crystals of the required properties are deposited, etched and modified to form the appropriate electrical circuits. The ternary system GaAsi -xP.r now dominates theZED market for a-numeric and graphic displays following the first report of this activity in 1961. GaAsl,P, is grown epitaxially on a single-crystal substrate of GaAs or GaP by chemical vapour deposition and crystal wafers as large as 20cmZ have been produced commercially. The colour of the emitted radiation is determined by the energy band gap E,; for GaAs itself E, is 138kl mol-' corresponding to an infrared emission (A 870 nm), but this increases to 184k.I mol-' for x -0.4 corresponding to red emission (A 650 nm). For x > 0.4 E, continues to increase until it is 218kJmol-' for GaP (green, A. 550nm). Commercial yellow and green LEDs contain the added isoelectronic impurity N to improve the conversion efficiency. A schematic cross-section of a typical GaAsl-,P, epitaxial wafer doped with Te and N is shown in the diagram: Te (which has one more valence electron per atom than As or P) is the most widely used dopant to give n-type impurities in this system at concentrations of 1016-10i8atoms cm-3 (0.5-50ppm). The p-n junction is then formed by diffusing Zn (1 less electron than Ga) into the crystal to a similar concentration. An even more recent application is the construction of semiconductor lasers. In normal optical lasers light is absorbed by an electronic transition to a broad band which lies above the upper laser level and the electron then drops into this level by a non-radiative transition. By contrast the radiation in a semiconductor laser originates in the region of a p-n junction and is due to the transitions of injected electrons and holes between the low-lying levels of the conduction band and the uppermost levels of the valence band. (Impurity levels may also be involved.) The efficiency of these semiconductor injection lasers is very much higher than those of optically pumped lasers and the devices are much smaller; they are also easily adaptable to modulation. As implied by the band gaps on p. 255, emission wavelengths are in the visible and near infrared. A heterostructure laser based on the system GaAs-Al,Gai_,As was the first junction laser to run continuously at 3 000K and above (1970). In the two types of device just considered, namely light emitting diodes and injection lasers, electrical energy is converted into optical energy. The reverse process of converting optical energy into electrical energy (photoconductivity and photovoltaic effects) has also been successfully achieved by 111-V semiconductor systems. For example, the small band-gap compound InSb is valuable as a photoconductive infrared detector, and several compounds are being actively studied for use in solar cells to convert sunlight into useful sources of electrical power. Th...
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This note was uploaded on 10/12/2012 for the course CHEMISTRY CHM1040 taught by Professor Jhon during the Spring '12 term at Florida A&M.

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