Figure 2: The WLR method.
From: Evolution of opto-electronic properties during film formation of complex semiconductors
(a) An exemplary WLR spectrum, recorded at the end of the process after cool-down, shows the spectral regions which were used to deduct the surface roughness σRMS, minimum band gap Eg,min, sub-gap tail energy ESGT and layer thickness d. The deviations of the fit to the experimental spectrum arise from the linear approximation of the refractive index (see method section). The total reflected light intensity can be calculated from the envelope of the interference fringes by correcting it for parasitic light absorption at the metallic back contact and diffuse scattering at the interfaces. This allows the extraction of the CIGS absorption coefficient, which is shown in (b) Eg,min and ESGT are obtained by fitting the extracted absorption coefficient. The dashed blue lines are calculated absorption coefficients with two different sub-gap tail energies.
