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Positron creation in semiconductors?

  1. Sep 21, 2006 #1
    I've posted a similar question to the high energy forum.

    As I understand it, LEDs emit photons when electrons and holes have sufficient energy to cross a particular "well". I'm sure this explaination is lacking in many key ways. Why can't we "tune" the gap to create electron-positron pairs and shape the crystalline structure to draw electrons to one side and positrons to the other? IOW, can a PED (positron emitting diode) be physically possible?
     
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  3. Sep 21, 2006 #2

    ZapperZ

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    Er... what "positron"? And what does producing positron has anything to do with the band gap?

    The band gap is the result of the collective interactions of all the ions of the material. The overlap of the local states of each ions with its nearest neighbor, next-nearest neighbor, next-next nearest neighbor, etc.. produces the band structure that we know of for almost all material. You "tune" the gap by varying the contituents of the material.

    In any case, that explanation is moot considering that it has nothing to do with producing positrons. Can you please cite a reference where you got the connection between the band gap and production of positrons?

    Zz.

    P.S. Please re-read the PF Guidelines before you proceed any further. Multiple posting is clearly not allowed.
     
  4. Sep 21, 2006 #3

    Astronuc

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    Visible light is around 2 - 3 eV, while the rest mass of an electron or positron is 0.511 MeV. A big difference!

    Positron-electron pairs are produced when a gamma-ray of at least 1.022 MeV interacts with the nucleus of an atom.
     
  5. Sep 21, 2006 #4
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  6. Sep 21, 2006 #5

    ZapperZ

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  7. Sep 21, 2006 #6
     
  8. Sep 21, 2006 #7
    OK, now we are in the ball park of the question. What would it take to cause spontaneous positron emission (with an electron mate of course)? We currently do it by slamming electrons into targets, but that is rather nasty and ineffcient. Surely there is a configuration of fields of sufficient intensity as to excite the local vacuum into pair emission when energy (a relatively large amount, granted) is pumped into the system?

    I used semiconductors as a basis because it is a very elegant use of quantum properties to stimulate real effects. In that case, the excitation of the semiconductor's "local vacuum" into spontaneous photon creation.

    If something similar, then what?
     
  9. Sep 21, 2006 #8

    ZapperZ

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  10. Sep 21, 2006 #9
     
  11. Sep 21, 2006 #10

    ZapperZ

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  12. Sep 21, 2006 #11
     
  13. Sep 21, 2006 #12
    Not sure if this is what you are looking for, but positronium (electron + positron) formation has been reported for semiconductors (see:
    Dannefaer S., Kerr D., Craigen D. (1996b): J. Appl. Phys. 79, 9110
    Itoh Y., Murakami H. (1994): Appl. Phys. A 58, 59)
    See also these:
    http://positron.physik.uni-halle.de/panet/text/intro/positronium.html
    http://www.springerlink.com/content/j3875jw28u516x11/
    http://www.springerlink.com/content/g740m38w7t2rlt76/
    And this:
    FRACTION OF POSITRONIUM FORMATION AT SEMICONDUCTOR SURFACE
    S.B. Shrivastava and A. Upadhyay
    School of Studies in Physics, Vikram University, Ujjain (M.P.), 456010 India
    Received: May 4, 1998; revised version December 8, 1998; in final form March 30, 1999
    ACTA Physica Polonica A

    The fraction of positronium formation (fps) has been calculated in Ge(110), Ge(111), Si(110) and Si(111) surfaces by solving the diffusion equation for positrons in semiconductors and by setting up the rate equation to describe the processes that are supposed to occur when a thermalised positron encounters the surface including the trapping of positrons in neutral and negative vacancies. Certain parameters used in the evaluation of fps, e.g., the bulk annihilation rate (\lambdas), the positron diffusion length (L+), the diffusion coefficient (D+) and the implantation profile parameter (A), have been taken from the experiments. The calculated values of fps as a function of incident positron energy and temperature in Ge(110) and Si(111) have been compared with the experimental results. It has been found that in general the calculated results are in good agreement with the experimental ones. The calculation also confirms that the trapping rate of positrons into negative vacancy has a T-1/2 dependence with respect to the temperature.
    PACS numbers: 78.70.Bj, 71.60.+z, 68.35.Fx
     
  14. Sep 21, 2006 #13
     
  15. Sep 21, 2006 #14

    jtbell

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    These experiments let positrons from an external source strike a semiconductor, where they meet up with electrons to produce positronium. Definitely not what FieldIntensity is looking for.
     
  16. Sep 22, 2006 #15

    ZapperZ

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  17. Sep 22, 2006 #16
     
  18. Sep 22, 2006 #17

    ZapperZ

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  19. Sep 22, 2006 #18
    OK, I've snipped the entire thing to make a general statement. First, thank you Zapper for all your responses to my question, and also thank you for pushing me to clarify the language of my question. That helps me understand what I am asking a little better, I think.

    Second, on semiconductors and band gaps: I started with this because it is a very familiar instance of a material that is engineered to a precision that allows the direct control of the quantum level interaction of electrons and holes. In LEDs, we can manipulate the material to generate photons from electron-hole annihilation.

    Using this as a jump off point, I envisioned that the energy of electrons "falling" into the "well" of a semiconductor might be converted (if enough energy is present) into e-p pairs instead of photons. Now I understand that semiconductors just can't do that.

    Third, on standard positron creation: Energetically speaking, if enough energy is present in a system, and the right conditions exist (more on that in a moment), any particle-antiparticle pair can be generated. The usual way is to slam electrons into a dense metal target, creating e-p pairs which are then seperated by magnetic fields into individual beams of positrons and electrons. The reason why this is the standard way is because it is just brute force and easy, though horribly ineffcient.

    Other energetic systems should be able to do this more elegantly and efficiently, if enough energy is present. The reason why we don't do it now is because we lack the precision to accurately manipulate matter on the scale necessary to create an environment conducive to pair creation.

    Physicists know that the quantum vacuum is affected in different ways by different arrangements of matter and fields. Virtual particles are constantly popping in and out of existence even in empty space. The presence of a single electron causes the vacuum to boil with fleeting e-p pairs. Sometimes virtual particles become real if a sufficient amount of energy is added to account for the rest masses of the particles; 1.022 MeV for an e-p pair.

    We should be able, given enough precision, to create a vacuum condition in which pairs spontaneously appear when energy is pumped into the system. With the right setup, electrons will go in on direction and positrons in the other.

    I guess it boils down to precision. Will we ever acheive that level of manipulation?
     
  20. Sep 22, 2006 #19

    Astronuc

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    A vacuum is simply the absence of matter in a particular volume. There will be no spontaneous generation of e-p pairs in a vacuum, even if there are 1.022 MeV photons passing through.

    In matter, photons scatter off electrons, whereas in nuclei they can create e-p pairs, or photons could eject neutrons from the nucleus - so-called photo-neutrons, but the energy has to exceed some threshold like 1.6 MeV or so, i.e. the binding energy of a neutron.

    In the case of LED's, the band widths are low energy - on the order of eV. If the band energy was on the order of 100's of keV, this is a huge energy and would require huge pressures to hold the matter together.

    Some perspective - the binding energy of the K-electrons in U is 115.606 keV -
    ref: http://xdb.lbl.gov/Section1/Table_1-1c.htm - which is way short of 511 keV, the rest energy of an electron. The band electrons in LED or any semiconductor are the 'outermost' electrons which have very low energies (~ eV).
     
    Last edited: Sep 22, 2006
  21. Sep 22, 2006 #20
    That is the classical view of the vacuum. Quantum mechanics puts a whole new spin on it. The quantum vacuum is very dynamic and fluid.
     
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