When transmission data of films less than 300 nm deposited on glass substrate is taken form spectroscopic ellipsometry the transmission edge lies at 300-350nm where as it doesn't give a correct estimation of film band gap. The transmission edge corresponds to the substrate used. How can we avoid this problem if I want to use a thin film as in the case of thick film this difficulty doesn't exist.
What are you using - presumably glass microslides or fused silica?
I think in this case you would need to consider using a different substrate for your ellipsometry measurements, this is not my area of specialty but possibly silicon wafer might be a suitable candidate in this case.
Sorry, but i don't really understand the real problem. First of all, we have to clarify that transmission mode isn't really SE. You cannot measure ellipsometric angles in transmission mode. So I have to agree with Matthew. It's better to use c-Si wafers as a substrate and have real ellipsometric measurements, or you can be more specific in what you're looking for, so i can help with the transmission process. The use of a UV lamp will be helpful!
I am attaching up a file hope it helps in understanding my point...............
In transmission your spectrum will be dominated by the substrate. In reflection you should have more sensitivity to thin films on the surface. You will need to correctly model your film/substrate and possibly roughen the back surface of your substrate to avoid reflections from there.
I want to study the transmission properties................. But here i am getting is the dominant effect of substarte
You can also use some substrate made of quartz (it is transparent on the UV, haigher 200 nm and visible).
You do not get transmission data from spectroscopic ellipsometer unless you have special ellipsometer that collects spectra in transmission separately; the two measurements completely unrelated. Having said that you do get bandgap information from multiangle SE (mostly in semiconductors using various available models one can derive such parameters). If substrate absorption is creating problem in reflection / ellipsometry data and you are bound to use glass, an easy way to circumvent is to use white playdough or a black tape (carbon tape works better) underneath the glass substrate to get rid of all reflections other than the film on top. Otherwise as suggested you can use silicon / quartz -- similar substrate change will help you in transmission measurements as well.
-Gregory M. Matenoglou
It IS possible to get SE data from transmission measurements. The sample has to be held at an angle to the beam, backside reflections must be accounted for in the model, and the sample can't be too thick (not sure what the limit is here).
When referring to transmission and ellipsometry, it is important to specify whether you mean transmission intensity or transmission ellipsometry data (Psi and Delta).
I know the Woollam M-2000 can do both, not sure about other ellipsometers.
Generalized rule is that when measuring the transmittance of a thin layer, the substrate must be transparent (i.e. its extinction coefficient, k = 0) or k must be well-known beforehand for the respective spectral region in which the measurements are made. In order to monitor the correct position of the transmission edge of the spectrum of transmission of thin films is required k = 0. These rules are valid for the both ellipsometric and spectrophotometric transmission measurements.
For the UV spectral region can be used the fused quartz, sapphire (Al2O3) and some halides such as NaCl, BaF2, KBr.
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