Overview of the process technology for the preparation of ultrahigh purity indium required for the fabrication of indium phosphide related epitaxial structures based devices needed for advanced electronic applications
Authors: Mani V. N. , Muthukumaran G. , Ramu A. G. , Kumar J. 
Published: September 2023

Indium, a rare metal, is created incidentally during the zinc refining process. When a zinc metal is produced, valuable elements, such as indium, are recovered and reused. The sulfide minerals, sphalerite, galena, and chalcopyrite, are all common hosts for indium metals. Indium metals of varying purities (from 99% to 99.9%) are used in many different commercial, other exclusive, specialty, dentistry, and research and development settings. In the production of indium phosphide and related select bulk single crystals, such as InP, InAs, InSb, etc., and select multilayered epitaxial material-systems based device structures, such as InGaAs/InP, InGaAsP/InP, etc., an ultrahigh purity (99.99999%) indium metal is used as one of the initial and primary input materials. light-emitting diodes, infrared detectors, lasers, and other components cannot be made without these device topologies. Triple junction solar cells made of GaInP, GaAs, and Ge with 40% conversion efficiency are being developed for use in space. Metal-organic and molecular beam epitaxial methods utilize trimethyl/triethyl-indium-epi-precursors, the high purity indium derivatives, as starting materials to develop and manufacture multilayered structures of InGaAs/InP, InGaAsP/InP, InGaN/InP AlInN, etc. The purpose of this review is to quickly touch on indium mineral sources, important uses for different indium metal grades, and the processes needed to refine, purify, and ultrahigh purify indium to higher purity levels using a zone refining–melting–leveling process, as well as impurity segregation considerations. The use of vacuum, inert gas environments, and an external electromagnetic field to efficiently segregate, levitate, stir, homogenize, and mix the molten zone/melt interface area (region) as well as purity analyses at ppb levels, class clean room, and packaging concepts were also discussed. This review also touched briefly on the use of ultrahigh purity indium in the preparation of TMIn, TEIn, and InCl precursors necessary for the growth of device structures by molecular beam, metal-organic vapor phase, atomic layer epitaxial, and chemical vapor deposition processes. Purifying and preparing polycrystalline indium to a type 7 N purity level as well as standardization and criticality testing for fine-tuning system parameters are essential parts of developing the purification process technology. It also highlights various compound semiconductors and epitaxial systems, such as high purity indium compounds, such as indium phosphide, for cutting-edge electronic applications. Material yield enhancement, impurity management (including C, O, N, and others), consistent results, impurity reduction (down to the ppb level), and class clean packaging are all active topics of research and development. There has been a rise in demand for ultrapure metals (7–10 N) with stringent purity criteria in the aerospace and defense sectors, where they are used in cutting-edge nanoelectronic applications. This literature review delves into these and related topics regarding the production of ultrahigh purity indium. The major objective of this review is to provide a concise summary of the research and development progress made toward the ultrahigh purity (7N-99.99999%) indium preparation and its epitaxial electronics application considerations as of the time of this writing.