Mercury cadmium telluride
Hg1−xCdxTe or mercury cadmium telluride (also cadmium mercury telluride, MCT, MerCad Telluride, MerCadTel, MerCaT or CMT) is a chemical compound of cadmium telluride (CdTe) and mercury telluride (HgTe) with a tunable bandgap spanning the shortwave infrared to the very long wave infrared regions. The amount of cadmium (Cd) in the alloy can be chosen so as to tune the optical absorption of the material to the desired infrared wavelength. CdTe is a
Properties
Physical
Hg1−xCdxTe has a zincblende structure with two interpenetrating face-centered cubic lattices offset by (1/4,1/4,1/4)ao in the primitive cell. The cations Cd and Hg are statistically mixed on the yellow sublattice while the Te anions form the grey sublattice in the image.
Electronic
The electron mobility of HgCdTe with a large Hg content is very high. Among common semiconductors used for infrared detection, only InSb and InAs surpass electron mobility of HgCdTe at room temperature. At 80 K, 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.
The intrinsic carrier concentration is given by [1]
where k is Boltzmann's constant, q is the elementary electric charge, t is the material temperature, x is the percentage of cadmium concentration, and Eg is the bandgap given by [2]
Using the relationship , where λ is in μm and Eg. is in electron volts, one can also obtain the cutoff wavelength as a function of x and t:
Minority carrier lifetime
Auger recombination
Two types of
The Auger 1 minority carrier lifetime for intrinsic (undoped) HgCdTe is given by[3]
where FF is the overlap integral (approximately 0.221).
The Auger 1 minority carrier lifetime for doped HgCdTe is given by [4]
where n is the equilibrium electron concentration.
The Auger 7 minority carrier lifetime for intrinsic HgCdTe is approximately 10 times longer than the Auger 1 minority carrier lifetime:
The Auger 7 minority carrier lifetime for doped HgCdTe is given by
The total contribution of Auger 1 and Auger 7 recombination to the minority carrier lifetime is computed as
Mechanical
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.
Thermal
The
Optical
HgCdTe is transparent in the infrared at photon energies below the energy gap. The refractive index is high, reaching nearly 4 for HgCdTe with high Hg content.
Infrared detection
HgCdTe is the only common material that can
HgCdTe is a common material in
The main limitation of LWIR HgCdTe-based detectors is that they need cooling to temperatures near that of
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
In HgCdTe, detection occurs when an infrared
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.
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.
Epitaxial growth
Highly pure and crystalline HgCdTe is fabricated by
In recent years,
Toxicity
Mercury Cadmium Telluride is known to be a toxic material, with additional danger from the high vapor pressure of mercury at the material's melting point; in spite of this, it continues to be developed and used in its applications.[6]
See also
Related materials
Other infrared detection materials
- QWIP
Other
References
- Notes
- doi:10.1063/1.332153.
- doi:10.1063/1.330018.
- S2CID 95289400.
- S2CID 94762645.
- S2CID 121767039.
- ^ Bahram Zandi; Dragica Vasileska; Priyalal Wijewarnasuriya (November 2009). "Modeling Mercury Cadmium Telluride (HgCdTe) Photodiodes" (PDF). Apps.dtic.mil. Archived (PDF) from the original on December 29, 2021. Retrieved 2022-03-12.
- Bibliography
- Lawson, W. D.; Nielson, S.; Putley, E. H.; Young, A. S. (1959). "Preparation and properties of HgTe and mixed crystals of HgTe-CdTe". J. Phys. Chem. Solids. 9 (3–4): 325–329. .. (Earliest known reference)
- Properties of Narrow-Gap Cadmium-Based Compounds, Ed. P. Capper (INSPEC, IEE, London, UK, 1994) ISBN 0-85296-880-9
- HgCdTe Infrared Detectors, P. Norton, Opto-Electronics Review vol. 10(3), 159–174 (2002)
- Rogalski, A (2005). "HgCdTe infrared detector material: history, status and outlook". Reports on Progress in Physics. 68 (10): 2267–2336. S2CID 53975198.
- Chen, A B; Lai-Hsu, Y M; Krishnamurthy, S; Berding, M A (1990). "Band structures of HgCdTe and HgZnTe alloys and superlattices". Semiconductor Science and Technology. 5 (3S): S100. S2CID 250734000.
- Finkman, E.; doi:10.1063/1.326421..
- Finkman, E.; Schacham, S. E. (1984). "The exponential optical absorption band tail of Hg1−xCdxTe". Journal of Applied Physics. 56 (10): 2896. doi:10.1063/1.333828.
- Bowen, Gavin J. (2005). "HOTEYE: a novel thermal camera using higher operating temperature infrared detectors". In Andresen, Bjorn F; Fulop, Gabor F (eds.). Infrared Technology and Applications XXXI. Vol. 5783. pp. 392–400. S2CID 96808301..
- Semiconductor Quantum Wells and Superlattices for Long-Wavelength Infrared Detectors M.O. Manasreh, Editor (Artech House, Norwood, MA), ISBN 0-89006-603-5(1993).
- Hall, Donald N. B.; Atkinson, Dani (2012). "Performance of the first HAWAII 4RG-15 arrays in the laboratory and at the telescope". In Holland, Andrew D; Beletic, James W (eds.). High Energy, Optical, and Infrared Detectors for Astronomy V. Vol. 8453. pp. 84530W. .
- Hall, Donald N. B.; Atkinson, Dani; Blank, Richard (2016). "Performance of the first science grade λc=2.5μm HAWAII 4RG-15 array in the laboratory and at the telescope". In Holland, Andrew D; Beletic, James (eds.). Performance of the first science grade lambda_c=2.5 mum HAWAII 4RG-15 array in the laboratory and at the telescope. High Energy, Optical, and Infrared Detectors for Astronomy VII. Vol. 9915. pp. 99150W. .