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I am currently studying the textbook Physics of Photonics Devices, Second Edition, by Shun Lien Chuang. In a section discussing The Invention of Semiconductor Lasers, the author says the following:

At the Solid State Device Research Conference in July 1962, an MIT Lincoln Laboratory group and RCA Laboratories reported extremely high efficiency (85% to 100%) electroluminescence from GaAs diffused junction diodes. Semiconductor lasers were invented during September to October 1962 by four groups within 30 days [9-12] (see the review article by Dupuis in Ref. 13). They were led by Robert N. Hall of General Electric Research Development Center, Schenectady, New York; Nick Holonyak Jr. of General Electric, Syracuse, New York; Marshall I. Nathan of the IBM Research Laboratory, Yorktown Heights, New York; and Robert Rediker of the MIT Lincoln Laboratory, Lexington, Massachusetts. Among the four groups, only Holonyak's laser diodes and light-emitting diodes (LEDs) were created from single-crystal GaAs$_{x}$P$_{1 - x}$ alloy material grown by vapor-phase transport and were the only devices emitting in the visible region.

I'm wondering what GaAs$_{x}$P$_{1 - x}$ is supposed to mean?

I would appreciate it if people would please take the time to clarify this.

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    $\begingroup$ Since it says “alloy”, my guess is that this notation indicates an alloy that is a fraction $x$ of gallium arsenide and a fraction $1-x$ of phosphorus. $\endgroup$
    – G. Smith
    Commented Feb 10, 2020 at 5:44
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    $\begingroup$ Confirmation at en.wikipedia.org/wiki/Gallium_arsenide_phosphide $\endgroup$
    – G. Smith
    Commented Feb 10, 2020 at 5:46
  • $\begingroup$ @G.Smith Thanks for the clarification! $\endgroup$
    – The Pointer
    Commented Feb 10, 2020 at 5:47

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I'm wondering what GaAs$_{x}$P$_{1 - x}$ is supposed to mean?

The $x$ indicates the fraction of the alloy that is gallium arsenide. The remaining fraction $1-x$ is gallium phosphide. See Wikipedia.

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  • $\begingroup$ It is slightly odd that they write it as GaAs$_{x}$P$_{1 - x}$ though. Normally $x=0$ corresponds to the undoped case, so it would be written more usually as GaAs$_{1-x}$P$_{x}$, unless they were actually doping gallium phosphide with arsenic. $\endgroup$
    – Clara Díaz Sanchez
    Commented Feb 10, 2020 at 5:55
  • $\begingroup$ @ClaraDiazSanchez I updated my answer. It is an alloy of gallium arsenide and gallium phosphide. $\endgroup$
    – G. Smith
    Commented Feb 10, 2020 at 5:57
  • $\begingroup$ OK, that makes sense. $\endgroup$
    – Clara Díaz Sanchez
    Commented Feb 10, 2020 at 6:23
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    $\begingroup$ In the III-V family of semiconductors, one can mix and match quite broadly. In this case it means that the III-sublattice has only Gallium on it, and the V-sublattice has a random mix of Arsenic and Phosphorous in the proportion given. That alloy can be made across the entire pseudo-binary, that is, any $x$ from 0 to 1 can be made. In the (Al,Ga,In)(N,P,As,Sb,Bi) there are many similar pseudo-binary alloys used for optoelectronics. $\endgroup$
    – Jon Custer
    Commented Feb 10, 2020 at 13:57

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