Glossary

III-Nitride

III-nitride semiconductors, comprised of alloys of (Al, In, Ga)N, are compound semiconductor materials which possess properties such as high breakdown voltage and a wide bandgap. They are well suited for optoelectronic devices emitting in the ultraviolet (UV) wavelength range and electronic devices operating at high voltages, frequencies and temperatures.

AlN: Aluminum Nitride

Aluminum Nitride (AlN) is a compound semiconductor with a direct, wide bandgap of 6.1 eV. It crystallizes in a wurtzite structure, featuring hexagonal (six-fold) symmetry around its crystallographic c-axis.

Despite the relative abundance of nitrogen and aluminum on planet Earth, AlN crystals do not occur in nature, which is, at least partly, due to the difficulty of dissociating N₂ molecules under natural conditions. However, AlN crystals can be grown in a laboratory environment utilizing extremely high temperatures. HexaTech's proprietary Physical Vapor Transport (PVT) technique produces AlN crystals of highest quality, and enables the production of AlN wafers for optoelectronic and electronic applications.

Nitride Alloys: (Al,In,Ga)N and AlGaN

III-nitride semiconductors generally are alloys of aluminum, indium, gallium and nitrogen, i.e., Aluminum Indium Gallium Nitride or (Al, In ,Ga)N.

AlGaN is the predominant alloy grown epitaxially on AlN substrates, as part of the semiconductor structure of optoelectronic and electronic devices.

Bandgap

The bandgap of a material refers to the energy difference, measured in electron Volts (eV) between the top of the valence band and the bottom of the conduction band in semiconductor materials. More simply, it is the energy required to promote a valence electron to a conduction electron, which is free to move and serve as a charge carrier to conduct electric current.

Silicon and other common semiconductor materials have a bandgap of ~1 to 1.5 eV. By comparison, wide bandgap materials typically have bandgaps >3 eV. Well known wide bandgap semiconductors include Silicon Carbide (SiC) and Gallium Nitride (GaN), which have bandgaps of ~3.2 - 3.4 eV. More novel materials, from a semiconductor standpoint, include Diamond (C) with a bandgap of 5.5 eV, and the widest bandgap semiconductor material, Aluminum Nitride (AlN), at 6.1 eV.

Wide bandgap semiconductors permit devices made from them to operate at much higher voltages, frequencies and temperatures than conventional semiconductor materials.

LED: Light Emitting Diode

A light emitting diode (LED) is a solid-state light source, consisting of semiconductor materials that are doped to form a p-n diode junction. When biased in the forward direction, charge carriers, i.e., electrons and holes, flow into the junction, where they recombine and release energy in form of a photon. The energy of the photons, i.e., the color of the emitted light, generally depends on the bandgap energy of the materials forming the p-n junction.

While infrared, visible, and near-ultraviolet wavelength LEDs are widely available, the production of efficient UV-C LEDs requires semiconductors with very wide bandgaps, and is generally more challenging than the established fabrication of LEDs emitting at longer wavelengths. HexaTech's AlN is an essential enabling technology for the fabrication of UV-C LEDs which are ideal for UV sterilization applications.

PVT: Physical Vapor Transport

Physical Vapor Transport (PVT )has emerged as the crystal growth technique of choice for materials that cannot easily be grown from the liquid phase (e.g., materials that have an impractically high melting point, or that decompose before melting, or that cannot be grown from a solution). The PVT process relies on the sublimation of a heated, solid source, transport of the sublimed species toward a relatively colder target (e.g., a seed crystal), and crystal growth at the target location driven by condensation from the vapor phase.

HexaTech grows AlN single crystals using a proprietary technique based on PVT. As AlN has a very low vapor pressure, extremely high process temperatures exceeding 2000°C are required to grow AlN crystals.

UV: Ultraviolet light

Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays. The UV spectrum ranges from 10 nm to 400 nm, which corresponds to photon energies from 3 eV to 124 eV. Ultraviolet light is so-named because the spectrum is adjacent to the high-energy end of the visible spectrum, which humans perceive as the color violet. UV light is invisible to humans.

The UV spectrum is subdivided into the following ranges:
400 - 300 nm: Near UV (NUV)
300 - 200 nm: Middle UV (MUV)
200 - 122 nm: Far UV (FUV)
122 -   10 nm: Extreme UV (EUV)

Alternatively, it is common practice to refer to the following UV bands:
400 - 315 nm: Ultraviolet A (UV-A)
315 - 280 nm: Ultraviolet B (UV-B)
280 - 100 nm: Ultraviolet C (UV-C)

UV-C: Ultraviolet C band

The ultraviolet C band (UV-C) corresponds to ultraviolet light with a wavelength of 280 to 100 nm, corresponding to photon energies of 4.4 eV to 12.4 eV. This spectral range is known for its germicidal action, as it coincides with the absorption band of DNA in microorganisms. UV-C irradiation allows for highly efficient water and air sterilization. Many microorganisms can be deactivated most efficiently if irradiated with UV-C light around 260 nm wavelength.