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Mercury(II) cadmium(II) telluride
HgCdTe or Mercury cadmium telluride (also Cadmium Mercury Telluride, MCT or CMT) is an alloy of CdTe and HgTe and is sometimes claimed to be the third semiconductor of technological importance after Silicon and Gallium(III) arsenide. The amount of cadmium (Cd) in the alloy (the alloy composition) can be chosen so as to tune the optical absorption of the material to the desired infrared wavelength. CdTe is a semiconductor with a bandgap of approximately 1.5 eV at room temperature. HgTe is a semimetal, hence its bandgap energy is zero. Mixing these two substances allows one to obtain any bandgap between 0 and 1.5 eV.
HgCdTe is usually referred to as MerCad Telluride, or simply MerCad in the Infrared sensors community.
Additional recommended knowledge
The electron mobility of HGCdTe with a large Hg content is very high. At room temperature only InSb and InAs of common semiconductors used for infrared detection surpass HgCdTe's electron mobility. At 80K the electron mobility of Hg0.8Cd0.2Te can be several hundred thousand cm²/V/s. Electrons also have a long ballistic length at this temperature; their mean free path can be several micrometres.
HgCdTe is a soft material due to the weak bonds Hg forms with tellurium. It is a softer material than any common III-V semiconductor. The Mohs hardness of HgTe is 1.9, CdTe is 2.9 and Hg0.5Cd0.5Te is 4. The hardness of lead salts is lower still.
The thermal conductivity of HgCdTe is low; at low cadmium concentrations it is as low as 0.2W.K-1m-1. This means that it is unsuitable for high power devices. Although, infrared light-emitting diodes and lasers have been made in HgCdTe they must be operated cold to be efficient. The specific heat capacity is 150 J.kg-1K-1. 
HgCdTe is transparent in the infrared at photon energies below the energy gap. The refractive index is high, reaching nearly 4 for high Hg content HgCdTe.
HgCdTe is the only common material that can detect infrared radiation in both of the accessible atmospheric windows. These are from 3 to 5 µm (the mid-wave infrared window, abbreviated MWIR) and from 10 to 12 µm (the long-wave window, LWIR). Detection in the MWIR and LWIR windows is obtained using 30% [(Hg0.7Cd0.3)Te] and 20% [(Hg0.8Cd0.2)Te] cadmium respectively. HgCdTe can also detect in the short wave infrared SWIR atmospheric windows of 2.2 to 2.4 µm and 1.5 to 1.8 µm.
Owing to its cost, the use of HgCdTe has so far been restricted to the military field and infrared astronomy research. Military technology depends on HgCdTe for night vision. In particular, the US air force makes extensive use of HgCdTe on all aircraft, and to equip airborne smart bombs. A variety of heat-seeking missiles are also equipped with HgCdTe detectors. HgCdTe detector arrays can also be found at most of the worlds major research telescopes including several satellites. Many HgCdTe detectors (such as Hawaii and NICMOS detectors) are named after the astronomical observatories or instruments for which they were originally developed.
The main limitation of LWIR HgCdTe-based detectors is that they need cooling to temperatures near that of liquid nitrogen (77K), to reduce noise due to thermally excited current carriers (see cooled infrared camera). MWIR HgCdTe cameras can be operated at temperatures accessible to thermoelectric coolers with a small performance penalty. Hence, HgCdTe detectors are heavy and require maintenance. On the other side, HgCdTe enjoys much higher speed of detection and is much more sensitive than some of its cheaper competitors.
HgCdTe is often a material of choice for detectors in Fourier Transform Infrared Spectrometer (FTIR) instruments. This is because of the large spectral range of HgCdTe detectors and also the high quantum efficiency.
HgCdTe can be used as a heterodyne detector, in which the interference between a local source and returned laser light is detected. In this case it can detect sources such as CO2 lasers. In heterodyne detection mode HgCdTe can be uncooled, although greater sensitivity is achieved by cooling. Photodiodes, photoconductors or photoelectromagnetic (PEM) modes can be used. A bandwidth in excess of 1GHz can be achieved with photodiode detectors.
The main competitors of HgCdTe are less sensitive Si-based bolometers (see uncooled infrared camera), InSb and photon-counting superconducting tunnel junction (STJ) arrays. Quantum Well Infrared Photodetectors (QWIP), manufactured from III-V semiconductor materials such as GaAs and AlGaAs, are another possible alternative, although their theoretical performance limits are inferior to HgCdTe arrays at comparable temperatures and they require the use of complicated reflection/diffraction gratings to overcome certain polarization exclusion effects which impact array Responsivity. In the future, the primary competitor to HgCdTe detectors may emerge in the form of Quantum Dot Infrared Photodetectors (QDIP), based on either a colloidal or type-II superlattice structure. Unique 3-D Quantum Confinement effects, combined with the unipolar (non-exciton based photoelectric behavior) nature of quantum dots could allow comparable performance to HgCdTe at significantly higher operating temperatures. Initial laboratory work has shown promising results in this regard and QDIPs may be one of the first significant nanotechnology products to emerge.
In HgCdTe, detection occurs when an infrared photon of sufficient energy kicks an electron from the valence band to the conduction band. Such an electron is collected by a suitable external readout integrated circuit (ROIC) and transformed into an electric signal.
In a bolometer, light heats up a tiny piece of material. The temperature change is measured and transformed into an electric signal.
Mercury zinc telluride has better chemical, thermal, and mechanical stability characteristics than HgCdTe. It has a steeper change of energy gap with mercury composition than HgCdTe, making compositional control harder.
HgCdTe growth techniques
Bulk crystal growth
The first large scale growth method was bulk recrystallization of a liquid melt. This was the main growth method from the late 1950s to the early 1970s.
Highly pure and crystalline HgCdTe is fabricated by epitaxy on either CdTe or CdZnTe substrates. CdZnTe is a compound semiconductor, the lattice parameter of which can be exactly matched to that of HgCdTe. This eliminates most defects from the epilayer of HgCdTe. CdTe was developed as an alternative substrate in the '90s. It is not lattice-matched to HgCdTe, but is much cheaper, as it can be grown by epitaxy on silicon (Si) or germanium (Ge) substrates.
Liquid phase epitaxy (LPE), in which a substrate is repeatedly dipped into a liquid melt, gives the best results in terms of crystalline quality, and is still a common technique of choice for industrial production.
In recent years, molecular beam epitaxy (MBE) has become widespread because of its ability to stack up layers of different alloy composition. This allows simultaneous detection at several wavelengths. Furthermore, MBE, and also MOVPE, allow growth on large area substrates such as CdTe on Si or Ge, whereas LPE does not allow such substrates to be used.
Research laboratories working on HgCdTe
Republic of Korea
This list is not exhaustive.
Other infrared detection materials
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Mercury(II)_cadmium(II)_telluride". A list of authors is available in Wikipedia.|