Question

In the figure below the absorption coefficient as a function of the wavelength for several semiconductor materials is presented. Let's consider monochromatic light of photons with energy of Eph=1.55eV that incidents a film with thickness d. If we ignore possible reflection losses at the rear and front interfaces of the film, what thickness d (in μm) is required to achieve a light absorption of 90%?
Since it was very difficult to accurately read off the absorption coefficient values from the graph above, we have chosen to provide you with these values.

Answer 1

The absorption coefficients for the different semiconductor materials at α(800nm) are:

αGaAs=2∗104cm−1
αInP=4∗104cm−1
αGe=6∗104cm−1
αSi=1∗103cm−1.
Calculate:
The thickness d (in μm) required to achieve a light absorption of 90% is:
a) For GaAs
b) For InP
c) For Ge
d) For Si

Answer 2

Using Lambert-Beer Formula I(lamda,x)=I0(lamda)exp^(-α(lamda)x), exp^(-α(lamda)x) = remaining light after absorption of 90%, which is 10%. So When x is the thickness you are looking for, find x=ln(0.1)/(given absorption coefficients).

Hope this helps ;)

Answer 3

Thank you!