Cadmium telluride semiconductor detector

Phases formed on semiconductor surfaces can change the electrical properties in an uncontrolled, deleterious fashion. Oxide passivation layers on compound semiconductors (e.g., mercury cadmium telluride IR detectors or gallium arsenide solar cells) can be grown to impart protection to the surfaces and to stabilize electrical properties by preventing uncontrolled reactions. 

W.F.H. Micklethwaite, The Crystal Growth of Cadmium Mercury Telluride Paul E. Petersen, Auger Recombination in Mercury Cadmium Telluride R.M. Broudy and V.J. Mazurczyck, (HgCd)Te Photoconductive Detectors M.B. Reine, A.K. Sood, and T.J. Tredwell, Photovoltaic Infrared Detectors M.A. Kinch, Metal-Insulator-Semiconductor Infrared Detectors... 

Mercury-cadmium-telluride is the principal semiconductor now being used in advanced infrared systems, both for military and other surveillance applications. Its preparation and use in infrared detectors and arrays was the subject of Volume 18 of this treatise. New generations of detectors and arrays require sophisticated epitaxial growth, which in turn requires precise phase diagram data. 

The cadmium chalcogenide semiconductors (qv) have found numerous applications ranging from rectifiers to photoconductive detectors in smoke alarms. Many Cd compounds, eg, sulfide, tungstate, selenide, telluride, and oxide, are used as phosphors in luminescent screens and scintillation counters. Glass colored with cadmium sulfoselenides is used as a color filter in spectroscopy and has recendy attracted attention as a third-order, nonlinear optical switching material (see NONLINEAR OPTICAL MATERIALS). Dialkylcadmium compounds are polymerization catalysts for production of poly (vinyl chloride) (PVC), poly(vinyl acetate) (PVA), and poly(methyl methacrylate) (PMMA). Mixed with TiCl4, they catalyze the polymerization of ethylene and propylene. 

A photoconductive detector is a semiconductor whose conductivity increases when infrared radiation excites electrons from the valence band to the conduction band. Photovoltaic detectors contain pn junctions, across which an electric field exists. Absorption of infrared radiation creates electrons and holes, which are attracted to opposite sides of the junction and which change the voltage across the junction. Mercury cadmium telluride (Hg,. Cd/Te, 0 < x < 1) is a detector material whose sensitivity to different wavelengths is affected by the stoichiome-try coefficient, x. Photoconductive and photovoltaic devices can be cooled to 77 K (liquid nitrogen temperature) to reduce thermal electric noise by more than an order of magnitude. 

The evolution of imager structures using Mercury Cadmium Telluride (HgCdTe) is described based on published patents and patent applications. The book is divided into two parts. The first describes monolithic arrays, with the detector elements and read-out means integrated in the same semiconductor body. The second part describes hybrid arrays, with the detector elements and the read-out means formed in separate semiconductor bodies. 

A problem with the monolithic arrays is that the techniques for building metal-oxide-semiconductor (MOS) devices in silicon cannot be transferred intact to narrow bandgap materials such as mercury cadmium telluride, mainly due to tunneling and avalanche breakdown occuring at very low voltages. A monolithic array, in which read-out electronics is integrated in the same mercury cadmium telluride chip as the infrared detectors, is therefore difficult to achieve. 

Cadmium telluride — A II—IV compound -> semiconductor frequently employed in infrared systems (active component in infrared detectors) and -> photovoltaic devices. Electrochemical - passivation has been employed to improve surface recombination behavior. 

Quantum detectors are usually made of semiconductor materials or mixtures. Some commonly used quantum detectors are made of lead sulfide (PbS), lead selenide (PbSe), indium antimony (InSb), or mercury cadmium telluride (MCT, HgTe-CdTe). The absorption of infrared radiation in quantum detectors excites electrons... 

The other potential semiconductor detector materials have a larger band gap than germanium and consequently would have the advantage of room temperature operation assuming that their other properties were satisfactory. Of these, only cadmium telluride, cadmium zinc telluride (CZT) and mercuric iodide have found their way into... 

CASCADE SUMMING See True Coincidence Summing. CdTe Cadmium Telluride A semiconductor detector for X rays and low energy gamma-rays. Good resolution but low efficiency due to small size. 

Detector. The radiation is detected by infrared-sensitive semiconductors, such as indium gallium arsenide (InGaAs) or mercury cadmium telluride (HgCdTe, but often abbreviated MCT). Infrared light striking these materials can promote electrons into conduction, allowing the radiation to be detected as an electrical current. 

Operating wavelength of the detector should be as close to the cutoff wavelength (Ico = hc/Eg) as possible. This requirement is easiest to meet in three-compound semiconductor materials with continually adjustable bandgap, e.g., mercury cadmium telluride (Hgi- cCd cTe) [8], mercury zinc telluride (Hgi- cZn cTe) [69-71], lead tin telluride Pbi Sn Te [72, 73], and indium arsenide antimonide (Ini - cAS cSb) [74] which for x = 0 reduces to indium antimonide, InSb. 

For infrared microspectroscopy, single-element detectors are used for point and mapping measurements. More recently, array detectors have been applied for spectroscopic imaging in the infrared. In infrared focal plane arrays, the monolithic silicon design used in CCDs is replaced by a hybrid construction. In a hybrid detector, photon detection occurs in a semiconductor layer (indium antimonide, mercury cadmium telluride, and doped-silicon are typical detector materials), while the readout and amplification stages are carried out in a silicon layer. The two layers are electrically connected at each pixel through indium bump-bonds . Other innovations such as microbolometer arrays also show promise for spectroscopic imaging applications. 

In quantum detectors, IR radiation causes electrons to be excited to a higher energy level. In an n-type semiconductor, electrons in the valence band are unable to increase the conductivity. If IR radiation excites the electrons to the conduction band, they can act as current carriers. For good sensitivity, it is necessary to cool the detector. For certain detectors, such as PbS and PbSe, it may be sufficient to use a thermoelectric cooler to maintain their temperature just below ambient. Mercury cadmium telluride detectors must usually be maintained at liquid-nitrogen temperature (77 K), whereas others may require cooling to the temperature of liquid helium (4.2 K). 

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