What Is Indium?

Indium is a post-transition metal with atomic number 49 and the chemical symbol In. It is a soft, silvery-white metal with a melting point of 156.6 degrees Celsius, notable for its malleability down to near-absolute-zero temperatures and for the faint sound it produces when bent, a characteristic it shares with tin. Indium was identified in 1863 by Ferdinand Reich and Hieronymous Richter at the Freiberg School of Mines in Germany, who observed a distinctive violet spectral line in zinc blende mineral and named the element after the Latin word for that color. It belongs to Group 13 of the periodic table alongside aluminum, gallium, and thallium, and it shares their tendency to form compounds in the +3 oxidation state.

Indium ranks among the least abundant industrially used elements in the Earth's crust and is obtained almost exclusively as a by-product of zinc refining. As described by the Royal Society of Chemistry's periodic table entry for indium, China leads global indium production. The element's scarcity has prompted research into recovery from end-of-life electronics, given the concentration of indium in display and photovoltaic products.

Indium Compounds

The technological importance of indium derives primarily from its compounds. Indium tin oxide (ITO) is a solid solution of indium(III) oxide (In2O3) and tin(IV) oxide (SnO2), typically with tin comprising around 10 atomic percent. ITO combines high electrical conductivity with optical transparency exceeding 80 percent across the visible spectrum, making it the dominant transparent conducting oxide in electronics. As documented in a ScienceDirect overview of indium tin oxide, ITO thin films are deposited by magnetron sputtering, spray pyrolysis, and pulsed laser deposition, with magnetron sputtering achieving resistivity values as low as 115 microohm-centimeters. Indium phosphide (InP) and indium arsenide (InAs) are III-V compound semiconductors used in high-frequency and optoelectronic devices. Indium gallium arsenide (InGaAs) is a ternary alloy critical to near-infrared photodetectors, high-electron-mobility transistors (HEMTs), and laser structures. Copper indium gallium selenide (CIGS) is a quaternary compound used as the absorber layer in thin-film photovoltaic cells, offering high absorption coefficients and band gap tunability through compositional control.

Semiconductor and Electronic Applications

Indium-containing semiconductors occupy a range of specialized niches in the device landscape. InP substrates are used for telecommunications laser diodes and photodiodes operating at 1310 and 1550 nanometers, the wavelengths favored by optical fiber systems, as detailed in work on indium phosphide device structures. InGaAs detectors formed on InP substrates provide sensitivity from the visible into the short-wave infrared (SWIR) band, enabling applications in imaging, spectroscopy, and optical communications receivers. InAs-based materials are under investigation for quantum computing devices and topological insulator heterostructures because of their narrow band gap and strong spin-orbit coupling. ITO, representing approximately 45 percent of global indium consumption, is applied as a transparent electrode layer in liquid crystal displays, organic light-emitting diode (OLED) panels, touchscreens, and thin-film solar cells, where conductivity and optical transparency must coexist. The combination of properties that makes ITO valuable also presents a supply challenge, motivating research into alternative transparent conducting oxides such as aluminum-doped zinc oxide (AZO) and fluorine-doped tin oxide (FTO).

Applications

Indium and its compounds have applications across a range of electronics and materials contexts, including:

• Transparent conductive electrodes in flat-panel displays, touchscreens, and OLED devices
• Thin-film solar cells using CIGS absorber layers
• III-V semiconductor substrates and epitaxial layers for telecommunications lasers and photodetectors
• High-electron-mobility transistors for microwave and millimeter-wave amplifiers
• Low-melting-point solders and fusible alloys in electronics assembly