Infrared Detection
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 8 to 12 µm (the long-wave window, LWIR). Detection in the MWIR and LWIR windows is obtained using 30% and 20% 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.
HgCdTe is a common material in photodetectors of Fourier transform infrared spectrometers. It is also found in military field, remote sensing and infrared astronomy research. Military technology has depended 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 relatively heavy compared to bolometers and require maintenance. On the other side, HgCdTe enjoys much higher speed of detection (frame rate) and is significantly more sensitive than some of its cheaper competitors.
HgCdTe is often a material of choice for detectors in Fourier-transform infrared (FTIR) spectrometers. 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 well in excess of 1 GHz 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 circuits (ROIC) and transformed into an electric signal. The physical mating of the HgCdTe detector array to the ROIC is often referred to as a "focal plane array".
In contrast, in a bolometer, light heats up a tiny piece of material. The temperature change of the bolometer results in a change in resistance which 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.
Read more about this topic: Mercury Cadmium Telluride