Cadmium Zinc Telluride: The wonder material powering a medical 'revolution'

Posted by 1659447091 4 days ago

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Comment by analog31 4 days ago

This isn't much more than a factoid, but notice that many of the useful semiconductors are made from elements that straddle the column containing silicon and germanium. Making compounds whose outer shell electrons add up to be silicon-like lets you make semiconductors, but with electrical and optical properties that you can tune. GaAs is another one, and the LED's are made by choosing particular combinations that have specific bandgap energies corresponding to colors of photons.

Part of the "magic" involves finding ratios of elements that have relatively little mechanical strain, because the atoms "fit" just right, which introduce defects that degrade the semiconductor behavior.

Comment by viccis 4 days ago

FYI "factoid" means it's an incorrect piece of information passed off as a fact.

Comment by Semaphor 4 days ago

Nowadays it also refers to trivial facts: https://www.merriam-webster.com/dictionary/factoid

Comment by 4 days ago

Comment by empiko 4 days ago

FYI, what you said is one meaning. But it is also, surprisingly, defined as brief trivial fact.

Comment by viccis 3 days ago

In the same way literally is defined as "not literally", yes. Dictionaries are descriptive and capture colloquial usage, but "factoid" is a recently defined word with a specific definition that can be traced to a specific book. The alternative definition in the dictionary is simply recording common misuse.

Comment by tape_measure 4 days ago

In fact, your comment is a factoid (in your meaning or the other replies' interpretation)

Comment by viccis 3 days ago

Dictionaries are descriptive and do not prescribe what definition is correct. I am basing my definition on Norman Mailer's definition and I am defining "incorrect" as differing from a word's explicit definition. From the original definition: "facts which have no existence before appearing in a magazine or newspaper". I can think of no clearer "factoid" than to justify a meaning that didn't exist until a dictionary published it.

In a broader sense, I am always entertained at how Americans will literally change dictionaries before admitting they used a word incorrectly. Sometimes it is tedious, but sometimes when they do it to scientific jargon, it risks muddying the waters of discourse about scientific phenomena with that from "pop science" definition. Psychology in particular is prone to this, with "learned helplessness" and "trauma bonding" being two phrases used incorrectly probably 9 out of 10 times I see them, to the extent that the fake meanings (which are always just the most literal interpretation of the phrase) are incorrectly being treated with the scientific basis of the originals despite having no real clinical evidence.

Comment by sevensor 4 days ago

Additional factoid: these are known as III-V semiconductors after their columns in the periodic table / number of valence electrons. They all have different bandgaps and lattice constants, and interesting things happen when you modulate composition.

Also, you might actually want to introduce a lattice constant mismatch because the strained lattice has useful properties.

Comment by adrian_b 3 days ago

Close, but no match.

Cadmium zinc telluride is a II-VI semiconductor, not a III-V semiconductor, because Cd & Zn belong to the 2nd group, while Te belongs to the 6th group. (I find the habit of some modern authors of calling the group 2b as group 12 and the group 6a as group 16 extremely stupid, even if with the traditional approach it is debatable which should be group 2a and which should be group 2b, because for many properties Zn, Cd & Hg are more similar to Mg than Mg is similar to Ca, Sr & Ba. However this defect of the classic numbering is not solved, but it is made worse in the modern numbering.)

Both the III-V & the II-VI semiconductors, and also the few existing I-VII (made of Cu or Ag with halogens) and the few IV-IV semiconductors (e.g. silicon carbide) semiconductors, are compounds of chemical elements whose number of external electrons averages to 4, i.e. the same as in diamond, silicon or germanium, so they can form crystal structures of the same kind.

There are many other kinds of semiconductors, but those which have the cubic or hexagonal structures of diamond/lonsdaleite (more symmetric) or zinc sulfide (less symmetric) are much better understood than the other semiconductors and they are much more frequently used.

Comment by sevensor 3 days ago

Good point, I was responding to a the parent talking about GaAs, but CdZnTe is certainly II-VI.

Comment by AnimalMuppet 4 days ago

Additional additional factoid: "Gallium Arsenide" would be a great name for a speed metal band.

Comment by gsf_emergency_6 4 days ago

Factoids are facts without citation, I suppose the other factoid to be mentioned is the direct band gap (which CZT has?)

https://en.wikipedia.org/wiki/Direct_and_indirect_band_gaps

Comment by adrian_b 4 days ago

For those unfamiliar with this, when a semiconductor has a direct band gap that means that it is likely to be suitable for devices that emit or detect photons, because when photons are absorbed, they generate electron-hole pairs, and when electron-hole pairs combine, their energy is released as photons.

In semiconductors with indirect band gap, when electron-hole pairs combine they usually just heat the material, instead of emitting light, which is why silicon, for instance, is not suitable for making LEDs.

While a direct band gap is desirable in LEDs, lasers and photodetectors, an indirect band gap is preferable in other applications where you do not want electrons and holes to recombine easily, e.g. in bipolar transistors or SCRs and in many kinds of diodes.

Comment by PhotonHunter 4 days ago

Re strain, sometimes you want that! It can be used to tune nanoparticles for example.

Comment by adrian_b 3 days ago

True. All MOS transistors used in the modern CMOS processes used for instance to make CPUs are doped with germanium in their gate regions, in order to produce a strain in the silicon lattice.

While a little strain can be beneficial in some cases, the large strain caused by the mismatches in crystal lattice cell size between various semiconductor layers that must be deposited one over the other in order to make some semiconductor device can cause great problems during manufacturing, by generating various defects that may make the process yield unacceptable.

Because of this, when researching new semiconductor materials a lot of effort is dedicated for finding compositions that can have matched lattice cell sizes.

Comment by PhotonHunter 3 days ago

I can't speak to the meso or macro regimes, but for nano (and in my case, colloids) dislocations are certainly a problem.

Comment by summa_tech 4 days ago

A few years ago, CZT detectors made by eV Products showed up in quantity on eBay. Pretty much everyone interested in radioactivity seemed to snap one up back then. It took a fairly long time for folks to figure out how to use them well! But they're really not bad, especially for the size.

Here's some spectra with 3% FWHM @ 662 keV:

https://maximus.energy/index.php/2020/05/01/gamma-spectrosco...

Comment by ziofill 4 days ago

You mean they’re good because the measurements are precise?

Comment by summa_tech 4 days ago

3% FWHM for something you could, at one time, buy for under $100 on eBay is very good. A typical scintillator + photomultiplier detector will get you about 6-7% (NaI:Tl scintillator). The Radiacode, which is super cute and all, gets about 7-9% depending on model.

Narrower FWHM means you will miss fewer energy peaks from isotopes.

Comment by perihelions 4 days ago

> "Whenever a high energy photon strikes the CZT, it mobilises an electron and this electrical signal can be used to make an image. Earlier scanner technology used a two-step process, which was not as precise."

I understand the unnamed alternative is the scintillation-type detector, where high-energy photons induce fluorescence, emitting secondary photons of lower energy. Detecting the secondary photons (converting them to electrons) is the second step.

https://en.wikipedia.org/wiki/Scintillator

https://en.wikipedia.org/wiki/Scintillation_counter

https://en.wikipedia.org/wiki/Scintillation_(physics)

Comment by neutronicus 4 days ago

Huh, I worked on a CZT radiation detector in undergrad back in 2007.

Comment by adrian_b 4 days ago

The article says that the use of CZT is not new, but now the material has become much more affordable, due to improved production techniques, which has opened up a lot of application fields for it, which were previously prevented by its scarcity and cost.

There are plenty of materials that have been known for a long time to be better than those normally used in certain applications, but which still do not replace the inferior alternatives due to excessive cost, so discovering any new process that can make them cheaply is as important as knowing the properties of the material.

Comment by MangoToupe 4 days ago

> pulmonary embolism

Ahh

Comment by gsf_emergency_6 4 days ago

Probably not related

https://en.wikipedia.org/wiki/Gamma-ray_laser