Indium is a soft, low-melting, slightly bluish scattered (rare) metal. It has almost no independent ore deposits in the crust and is recovered entirely as a by-product of zinc and lead-zinc smelting, so output is small and price high. Its best-known use is in touchscreens and displays: indium tin oxide (ITO) transparent conductive films are the only transparent electrode material produced at large scale for flat-panel LCDs, OLEDs, touch panels, and plasma TVs. For any application requiring a material that is both transparent and conductive, almost no other material offers a comparable price-performance ratio; behind the screen glass of smartphones, tablets, laptops, and self-service terminals lies an ITO film only tens of nanometers thick. In semiconductors, compound semiconductors of indium with phosphorus and arsenic are key materials for infrared detection, fiber-optic communications, and high-frequency devices: indium phosphide is the substrate for lasers and detectors in long-haul fiber-optic communications, while indium arsenide is used in infrared sensors and terahertz devices. Soldering is another important market for indium: indium-based soft solders are low-melting, wet well, and buffer thermal stress between materials with different coefficients of thermal expansion, used in semiconductor packaging, low-temperature vacuum seals, and photovoltaic modules; they are especially irreplaceable in aerospace and cryogenic electronics that require extremely low-temperature joints. It is also used in low-melting alloys (fire sprinklers, fusible plugs), alloy bearings, and nuclear-radiation detectors.
Indium sesquioxide (In2O3) is one of the main constituents of ITO transparent conductive films, a white or pale yellow powder; when doped with tin it becomes a conductive transparent film, the core electrode material for touchscreens, LCD panels, and OLED displays. Indium arsenide (InAs) is a narrow-bandgap semiconductor with extremely high electron mobility, used mainly to make infrared detectors, quantum-well devices, Hall sensors, and high-speed electronic devices, and in terahertz imaging research. Indium phosphide (InP) has a direct bandgap and high electron mobility and is the substrate material for 1310-nm and 1550-nm lasers, photodetectors, and high-speed HEMTs in fiber-optic communications, underpinning today's backbone internet optical networks. ITO, short for Indium Tin Oxide, is a solid solution of In2O3 doped with about 10% SnO2; it combines high transparency in the visible with high reflectance in the near infrared and, besides display electrodes, is used as low-emissivity glass, anti-icing heater films on aircraft, and transparent electromagnetic-shielding coatings.
