Why Reflectance and Transmittance add up to more than 100% in TiN Thin Film (Using Uv-vis spectrophotometry)?

Good question

We would require some more details from you. First of all, is it a freestanding film or a film on a substrate? If the latter, what kind of substrate? Also, in the UV/Vis spectral range you would normally take an aluminum mirror as reference for reflectance, knowing that it would not really have itself a reflectance of 100 % and an empty sample holder as reference for transmittance. Did you do that? Finally, how large are the errors we are taking about? It would be best, if you could provide us with both, the reflectance as well as the transmittance spectrum.

P.S.: Absorbance is in this connection definitely useless. 1-R-T is called absorptance.

Dear Fernando,

welcome, Adding to the above answer, even if the absorptance is equal to zero, the summation of the reflectance and transmittance will be equal to one. So, it is physically impossible that (R+T) can be greater than one.

So, if they are greater than one, they may be measured with different reference other than the total incident light power. Also, one has to consider that R and T are total reflected fraction and total transmitted fraction of the total incident light power. The last condition is important the light is reflected and refracted are directive.

So, in summary you have to review your measurements is you are interested in the absolute values. If you are interested in the spectral response of the material you can use relative transmittance units.

Best wishes

Dear Fernando,

1.I agree with Thomas : More experimental details needed. In particular, what substrate did you use ? In these type of problems (R+T>1 - IMPOSSIBLE, it contradicts energy conservation law !!), I would definitely start with single crystal sapphire substrate, optically polished as a proper substrate.

2. I also agree with Abdelhalim that the problem is in the calibration of the incident light power as seen in both the Transmission and the Reflection. As a reflection standard I would NOT use Al mirror (even in commercially available products, the absolute Reflectance can vary appreciably) but again optically flat and optically polished, single crystal sapphire. If I remember correctly, the reflectance is about 15% , which is plenty.

3. You will need to find out what are the precise optical paths (in your spectrometer) when measuring Reflection and Transmission. Is there only one detector, does the system mechanically move when changing from Transmission to Reflection measurement, etc., ..etc. That will give you an idea where the problem might be.

4. I would use sapphire substrate to calibrate my system. Below sapphire band gap (> 5 eV) and with thicknesses of the order of ~1 mm, you will observe interference fringes and the sum of R and T has to give you 1.00. That will be your absolute calibration precision of the incoming light power intensity.

With best regards

Petr

Hello everyone

I am having a similar issue. I measured T and R for few substrates (500 microns thick). I repeated few times but I always got same results. I am doing batch of samples not isolated measures (in a row). In the figure you can see the T +R for these substrates. I even perform a T for air (green line) that shows 100 % for the whole range. I am using a Perkin Lambda 1050. Autozero performed at 0%T and 100 %T (internal beam on beam off). These are bare commercial substrates. I am investigating now some deposits on the substrates but I guess the situation will be the same when I will try to extract the substrate absorptance. Any ideas?

Dear Petru,

when looking at your data on various "substrates", I would again urge you to use optically polished single crystal sapphire as "golden reference" to start with. Here the sum of Reflection and Transmission intensity adds to 100 % to within an experimental error which can be as small as 1-2 % over the entire 0.5 to 5.0 eV.

Once the question of energy conservation is settled, I would start investigating more "exotic" substrates.

With best regards

Petr