Gallium occurs in very low concentrations in the earth crust. It is not present in pure metallic form but is usually extracted as a by-product during processing of aluminium from Bauxite by Bayor process on the other hand; gallium can hardly form its own mineral deposit, and is often recovered as by-product from other ores moreover, from the semiconductor fabrication waste. The majority and the initial concentration of the impurities in gallium will depend on by which technique it has been extracted. Thus gallium obtained might usually contain many metallic impurities such as Al, B, Fe, Hg, In, Cd, Zn, Cu, Pb, Co, Ni, etc. The specific targeted impurities such as Zn, Pb, Fe, Ni, B, Cr, Mn, Cu and Al which may tamper the device performance by spoiling the electronic and optical properties. 3N/4N pure gallium obtained by extraction, electro refining processes has to be further purified to 6Nlevel for its use in growing device quality crystals and multilayered epitaxial structures.
The most useful compounds of gallium are gallium arsenide (GaAs) and gallium Phosphide (GaP) which are used in the manufacture of electronic devices. GaP is used to fabricate light emitting diodes (LEDs). Since GaAs has the ability to convert electrical energy into optical energy and vice-versa, it is used in the fabrication of optoelectronic devices such as laser diodes, photo diodes, and solar (Photo voltaic) cells. GaAs-based ICs, although developed in the recent years, are primarily used in applications such as satellites, supercomputers, and defence because they can send information about five times faster, withstand more radiation and operate at higher temperatures than silicon-based ICs.
4N gallium used as raw material for preparation of 6N Gallium by a number of advanced separation and fine purification techniques: Hydrochemical processing, Vacuum heat treatment, zone-refining and finally fractional crystallization technique.
Hydrochemical processing of gallium with acids makes it possible to remove oxide and hydroxide inclusions and metallic impurities. Note that low-melting metals, including gallium, are capable of dissolving water. This Hydrochemical processing may take care of the metallic impurities especially Na, K, Al, Mg and Zn.
Vacuum refining of gallium:
This process step is used to remove residual moisture, dissolved gases and a no of impurities that are more volatile than gallium not only metals (alkali metals, Cd, Zn, Mg and others) but also their compounds. The process will be run in a dynamic vacuum (10-5 mbar) in a quartz reactor. Gallium will be taken in a graphite boat, which ensured a large vaporization area. The process temperature and duration will be 900-1000°C and 2 hrs, respectively. After vacuum heat treatment gallium will be cooled, transferred to a special container in an inert atmosphere and then analyzed for the impurities. The heat treatment under vacuum will reduce the Cd, As, P, Te, S, Se, Zn, Hg and other low boiling metals at that particular temperature and vacuum.
Purification of gallium by zone-refinig:
Research methodology include, creation of narrow and uniform solid-liquid interface, maintenance of proper ultra low temperature conditions through circulation of coolant (+5 to -20°C), optimization of zone travel rate (crystallization rate) during controlled zone-refining (crystallization) of gallium, elemental purity analysis, RRR measurements. Importantly, segregation study pertaining to the impurities (Al, Fe, Cu, Zn, Sn, Pb, In, Bi, Cd) present in the gallium (major) matrix will form one of the major segments of this process step.
Fractional crystallization of gallium:
Fractional crystallization technique is very simple in operation and apparatus and industrially advantageous. Here the starting raw gallium which is in molten stage will be crystallized fractionally by using a coolant condenser. The trace metal impurities whose distribution coefficient is less than unity will start solidifying on the coolant condenser, leaving the impurities in the molten state gallium. The solidified gallium metal has to be removed before its external walls touches the inner walls of the liquid gallium vessel. As the solidification is slower the purity will be better. We have to optimize the solidification rate, where it depends upon the flow rate and cooling media temperature.
S. NoElements in Cd matrixImpurity (in ppb)1Zn<0.12Pb<0.13Fe<0.14B<0.15Cr<0.16V<0.17Cu<0.18Al<0.19Hg<0.110Mn<0.111Ag<0.1