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Peaks at reflectance graph

  1. Dec 23, 2013 #1
    Hi all

    When one measures the reflectance of say silicon, certain peaks appear in the graph. I am wondering, if anyone can explain the location of the peaks and their meaning.

    Here the graph is, I hope the picture thing works, else here is the link.


    The graph is for a polished silicon wafer.

    So I have a few ideas, which could make sense in my head:

    1: It is related to the band gap with respect to the transmission spectrum, however, the band gap of silicon is 1.1 eV, which is why I am confused.

    I still think it could be related to the band gap, so now I am thinking, it is because the band gap may have other "paths", where it can conduct a current, but that it needs even more energy, to conduct through those gaps. But then I am confused, as the transmission is lowered at these areas..

    2: It is related to the imaginary part of the refractive index, which is very high at the <500 nm wavelength. So why does the reflectance peak, when the imaginary refractive index is high?
    I could understand it, as it relates to the extinction coefficient perhaps? A higher imaginary part would make a smaller penetration depth, and thus lower the transmission? But then why does it peak?

    Anyway thanks for helping and merry x-mas!
  2. jcsd
  3. Dec 24, 2013 #2


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    Science Advisor

    E = h*u.
    Therefore voltage = 1239.84 / (wavelength in nm)
    Reading crude wavelengths from the graph and converting . . .
    270 nm = 4.592 volt
    360 nm = 3.444 volt
    These are greater than 1.1 volt, but there are other band gaps available.
    Since these differ by about 1.148 volt, I guess you have another band gap in series with the 1.1 volt gap and you are seeing the step between another level and the top or bottom of the commonly seen 1.1 volt band gap.
  4. Dec 25, 2013 #3
    Thanks that is what I was thinking as well, but I wasn't very sure. The numbers do help a lot to confirm it.
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