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Group II-VI Compound Semiconductors

  • II-VI semiconductors
    • Cadmium selenide (CdSe)
    • Cadmium sulfide (CdS)
    • Cadmium telluride (CdTe)
    • Zinc oxide (ZnO)
    • Zinc selenide (ZnSe)
    • Zinc sulfide (ZnS)
    • Zinc telluride (ZnTe)
  • II-VI ternary alloy semiconductors
    • Cadmium zinc telluride (CdZnTe, CZT)
    • Mercury cadmium telluride (HgCdTe)
    • Mercury zinc telluride (HgZnTe)


Cadmium selenide (CdSe) is a solid, binary compound of cadmium and selenium. Common names for this compound are cadmium(II) selenide, cadmium selenide, and cadmoselite (a very rare mineral).

Cadmium selenide is a semiconducting material, but has yet to find many applications in manufacturing. This material is transparent to infra-red (IR) light, and has seen limited use in windows for instruments utilizing IR light.

Much current research on cadmium selenide has focused on nanoparticles. Researchers are concentrating on developing controlled syntheses of CdSe nanoparticles. In addition to synthesis, scientists are working to understand the properties of cadmium selenide, as well as apply these materials in useful ways.

Cadmium sulfide is a chemical compound with the formula CdS. Cadmium sulfide is yellow in colour and is a semiconductor[1]. It exists in nature as two different minerals, hexagonal greenockite[2] and cubic hawleyite[3]. Cadmium sulfide is a direct band gap semiconductor (gap 2.42 eV[4]) and has many applications for example in light detectors. It forms thermally stable pigments and with the addition of e.g. CdTe, HgS colours ranging from deep red to yellow are formed.

Cadmium telluride (CdTe) is a crystalline compound formed from cadmium and tellurium. It is used as an infrared optical window and a solar cell material. It is usually sandwiched with cadmium sulfide to form a p-n junction photovoltaic solar cell. Typically, CdTe cells use a n-i-p structure

Zinc oxide is an inorganic compound with the formula ZnO. It usually appears as a white powder, nearly insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber (e.g. car tyres), lubricants, paints, ointments, adhesives, sealants, pigments, foods (source of Zn nutrient), batteries, ferrites, fire retardants, first aid tapes, etc. ZnO is present in the Earth crust as a mineral zincite; however, most ZnO used commercially is produced synthetically.

Zinc selenide (ZnSe), is a light yellow binary solid compound. It is an intrinsic semiconductor with a band gap of about 2.82 eV at 25 °C. ZnSe rarely occurs in nature. It is found in the mineral stilleite named after Hans Stille.

Zinc sulfide (or zinc sulphide) is a chemical compound with the formula ZnS. Zinc sulfide is a white- to yellow-colored powder or crystal. It is typically encountered in the more stable cubic form, known also as zinc blende or sphalerite.[6] The hexagonal form is also known both as a synthetic material and as the mineral wurtzite.[7] A tetragonal form is also known as very rare mineral polhemusite (Zn,Hg)S. Both sphalerite and wurtzite are intrinsic, wide-bandgap semiconductors. The cubic form has a band gap of 3.54 eV at 300 K whereas the hexagonal form has a band gap of 3.91 eV. The transition from the sphalerite form to the wurtzite form occurs at around 1020 ºC.

Zinc telluride is the chemical compound with the formula ZnTe. This solid is an intrinsic semiconductor material with band gap of 2.23–2.25 eV. It is usually a P-type semiconductor. Its crystal structure is cubic, like that for sphalerite and diamond.

Cadmium zinc telluride, (CdZnTe) or CZT, is a compound of cadmium, zinc and tellurium or, more strictly speaking, an alloy of cadmium telluride and zinc telluride. A direct bandgap semiconductor, it is used in a variety of applications, including radiation detectors, photorefractive gratings, electro-optic modulators, solar cells, and terahertz generation and detection. The band gap varies from approximately 1.4 to 2.2 eV, depending on composition.

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.

Mercury zinc telluride (HgZnTe, MZT) is a telluride of mercury and zinc, an alloy of mercury telluride and zinc telluride. It is a narrow-gap semiconductor material. Mercury zinc telluride is used in infrared detectors and arrays for infrared imaging and infrared astronomy. Mercury zinc telluride has better chemical, thermal, and mechanical stability than mercury cadmium telluride. The bandgap of MZT is more sensitive to composition fluctuations than that of MCT, which may be an issue for reproducible device fabrication. MZT is less amenable than MCT to fabrication of complex heterostructures by molecular beam epitaxy.


Refrences:
  1. Pradyot Patnaik (2002). Handbook of Inorganic Chemicals. McGraw-Hill. ISBN 0070494398. 
  2. Egon Wiberg, Arnold Frederick Holleman (2001) Inorganic Chemistry, Elsevier ISBN 0123526515
  3. Traill R J, Boyle R W (1955). "Hawleyite, isometric cadmium sulphide, a new mineral". American Mineralogist 40: 555–559. 
  4. Charles Kittel Introduction to Solid State Physics- 7th Edition (1995) Wiley-India ISBN 1081-265-1045-5
  5. Greenwood, Norman N.; Earnshaw, A. (1997), Chemistry of the Elements (2nd ed.), Oxford: Butterworth-Heinemann, ISBN 0-7506-3365-4
  6. Wells, A. F. (1984), Structural Inorganic Chemistry (5th ed.), Oxford: Clarendon Press, ISBN 0-19-855370-6 
  7. Sidot, T. (1866). ". Sur les propriétés de la blende hexagonale". Compt. rend. 63.: 188–189. http://gallica.bnf.fr/ark:/12148/bpt6k30204.image.f188.langFR.