Trouble Collecting IR Spectrum of a-Si:H

  1. I am working on my thesis for my masters in physics, and am running into some issues while trying to gather an IR spectrum of my hydrogenated amorphous silicon (a-Si:H). I am depositing my films using PECVD onto a glass microscope slide substrate. The films I have used so far have been 0.2-0.4 μm in thickness. The piece of equipment I am using right is a Thermo Scientific Nicolet iS10 FTIR Spectrometer.

    What I am encountering is essentially zero signal. I am not seeing any absorption of IR by my film.

    The professor I am working with and I think the reason may be because I do not have good contact with the surface of my film and the Ge crystal in the machine because the films and substrate are to rigid or because my film so far have been to thin.

    I would like to know if anyone has any suggestions for me on taking FTIR spectrum of amorphous silicon thin films or of thin film materials in general. Thank you very much for your time.
  2. jcsd
  3. ehild

    ehild 12,940
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    The glass microscope slide is opaque in the infrared. In principle, you can try the ATR method, but your professor is right, it is not easy to make a good contact with the Ge prism and sample, which is necessary to get a reliable spectrum. Try to deposit the layer on a polished silicon wafer, better on a silicon window of about 1-2 mm thick, and measure transmittance. Also, the layer thickness should be over 1 μm.

  4. Thanks for your help. I am using the ATR method and once I get back into the lab next week I plan on growing thicker films and depositing a silicon wafer.
  5. Si-H bond has fundamental frequency near 4.4 μm, second harmonic - 2.25 μm, third - 1.54 μm...
  6. Ehild,

    I read over your comment again today. What exactly do you mean by a silicon window?
  7. ehild

    ehild 12,940
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    It is a thick silicon plate 1-2 mm thick, both sides polished. Its transmittance is about 54% above 2000 cm-1, (wavelength less than 5μm) and free from interference waves at moderate resolution. The spectrum of a commercial one-side polished and 0.5 mm thick or thinner silicon wafer is influenced by light scattering and interference, that you need to take into account when evaluating a spectrum.
    The thick silicon plates are expensive, the commercial silicon wafers are cheep, you can use them, but choose 0.5 mm thick ones and select those which do not show too big interference waves.

    Last edited: Aug 27, 2013
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