I study a-SiOx:H thin films (500 - 1000 nm) by FTIR spectroscopy. And try to find oxygen and hydrogen contention in this films. One of the problems is the bleaching effect (the IR transmission of the film is more than 100%). It means that the absorption coefficient is negative. How to deal with this problem?
It is not unusual the parameter that you measure, Transmittance = I (film+substrate)/I(substrate) to be higher than 1. In the case n(film)
I think that there is something wrong with your experimental set-up. The transmission is defined as the ratio It/Io, where It and Io are the intensities of transmitted and incident light. Then T>1 (i.e >100%) does not have physical meaning.
One can indeed observe photobleaching effects in strong fields (i.e., with a laser) or samples where one fills sub-band gap states. In the second case, it would be rather difficult to monitor filling and escape processes with cw excitation, so one would usually adapt some sort of pulsed techniques including pump-probe. Usually, FTIR techniques don't use lasers so I'd assume you're using some relatively weak cw excitation. While it still can be possible that your sample responds to this excitation in a special way, I'd double check the set-up and measurement protocol making sure that what you see is real and not an artifact. BTW, is it something people report routinely with this kind of sample? Are you talking about amorphous Si that has an exponential (or so) DOS?
P.S. Tsvetanka's made a very good point - if you've judged on possible photobleaching by comparing incident and transmitted light, then measuring more light that you sent on the sample would indicate a presence of an active structure with light generation.
Thank you for your answeres.
I used two different set-ups to measure IR transmission of the films (at vacuum and at the air). Transmittance = I (film+substrate)/I(substrate). Substrate is cryst. silicon wafer. I assume that the presence of bleaching effect is due to the presence of SiO2 in the bulk of a-SiOx:H films.
There are many instrumental effects which can bring ironic results. Can you be sure about the instrument? What you can say about: geometry of the light beam and its modification due to sample under investigation, interferogram position (fixed or not), air absorption? Are you the first who observed the bleachnig effect? I complitly agree with previews answers.
I am not the first who obserwed this effect. In paper http://journals.aps.org/prb/pdf/10.1103/PhysRevB.50.18046 , figure 1 is shown this effect.
"However, it involves some undesirable sects;
for instance, the film-substrate transmittance Ty(~) results
in some samples, larger than that of the bare substrate,
T,(tu), in the transparent region (i.e., 2400—3600
cm ) of the spectra. This may be due to imperfect
matching of the sample substrate with the c-Si reference,
to the presence of air in the measurement chamber, and
to a low-re&active index difference between the substrate
and the film, which causes interference fringes. Nevertheless,
the absorption spectra of the a-Si:H can be recovered
if an appropriate base line subtraction is adopted in the
treatment of the transmittance data"
Is it what you also observe?
Figure 1 in PhysRevB.50.18046 shows interference effect, which is a part of instrumental setup. Is there any place with bleaching effect?
It is not unusual the parameter that you measure, Transmittance = I (film+substrate)/I(substrate) to be higher than 1. In the case n(film)
Tsvetanka? thank you, I understand this fact. But I don't know how to obtain true absorbtion of films.
Alexandr, when I use UV-VIS-NIR spectroscopy I always measure the sample (film on known substrate) relative to air, not to bare substrate. Is it possible you to do the same in your experiment?
What is the substrate you use for the deposition of your SiOx films ?
Maybe you already mentioned. Which IR technique do you apply for the characterization of your SiOx films deposited onto silicon wafers ?
*Transmission / ATR / External Reflection
Best regards,
Martin Müller
I guess you take the uncoated silicon wafer as the reference (I0) and the coated (SiOx film) one as the sample (I). How does your absorbance spectrum (A) look like, if you apply
A = -log(I/I0) ?
Do you have positive or also negative signals ?
Wavenumber positions of your signals ?
Best regards,
Martin Müller
Yes, you are right. You can see the spectrum of the film in attach file. Could you tell me how to eliminate the interference, to use the formula presented in article A.A. Langford; M.L. Fleet; A.H. Mahan - Correction for multiple reflections in infrared spectra of amorphous silicon, Solar Cells, Vol. 27, issue 1-4, 1989 to calculate the absorption coefficient of the film?
Sincerely,
Alexandr Zamchiy
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