Germanium impurities

As mentioned in Sec. 7.5.5, the Ge(Li) detectors have been replaced by Ge detectors, which are devices that use hyper pure germanium (impurity concentration 10 atoms/m or less). The main advantage of Ge over Ge(Li) detectors is that the former should be kept at low temperatures only when in use the latter must be kept cool at all times. 

The element is a gray-white metalloid. In its pure state, the element is crystalline and brittle, retaining its luster in air at room temperature. It is a very important semiconductor material. Zone-refining techniques have led to production of crystalline germanium for semiconductor use with an impurity of only one part in lOio. 

Germanium tetrachloride refined for use in making optical fibers is usually specified to contain less than 0.5 to 5 ppb of each of eight impurities vanadium, chromium, manganese, iron, cobalt, nickel, copper, and zinc. Limits are sometimes specified for a few other elements. Also of concern are hydrogen-bearing impurities therefore, maximum limits of 5 to 10 ppm are usually placed on HCl, OH, CH2, and CH contents. 

The standard electrode potential for zinc reduction (—0.763 V) is much more cathodic than the potential for hydrogen evolution, and the two reactions proceed simultaneously, thereby reducing the electrochemical yield of zinc. Current efficiencies slightly above 90% are achieved in modem plants by careful purification of the electrolyte to bring the concentration of the most harmful impurities, eg, germanium, arsenic, and antimony, down to ca 0.01 mg/L. Addition of organic surfactants (qv) like glue, improves the quaUty of the deposit and the current efficiency. 

The technology of silicon and germanium production has developed rapidly, and knowledge of die self-diffusion properties of diese elements, and of impurity atoms has become reasonably accurate despite die experimental difficulties associated widi die measurements. These arise from die chemical affinity of diese elements for oxygen, and from die low values of die diffusion coefficients. 

The metlrod of zone refining which was first used in the production of very pure germanium depends for its success on the difference between the thermodynamic properties of an impurity, present as a dilute constiment dissolved in... 

Germanium is a semiconductor. If small amounts of the elements In, P, Sb, and Ga are present as impurities, which of them will make germanium into (a) a p-type semiconductor ... 

Germanium was not known until 1886 and has the distinction of having been predicted by Mendeleev before it was discovered (see Box 1.2). It is recovered from the flue dust of industrial plants processing zinc ores (in which it occurs as an impurity). It is used mainly—and increasingly—in the semiconductor industry. 

Ultra-High Vacuum Reactors. CVD reactions at extremely low pressures (i.e., 10 Torr) are being developed for the deposition of semiconductor materials, such as silicon-germanium and some optoelectronic materials. Advantages appear to be better control of the deposit structure and reduction of impurities. 

The relatively large band gaps of silicon and germanium limit their usefulness in electrical devices. Fortunately, adding tiny amounts of other elements that have different numbers of valence electrons alters the conductive properties of these solid elements. When a specific impurity is added deliberately to a pure substance, the resulting material is said to be doped. A doped semiconductor has almost the same band stmeture as the pure material, but it has different electron nonulations in its bands. 

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