The transmittance in the thin film can be expressed as (Baban et al. 2006):
T = — = (1 – R1) (1 – R2) (1 – R3) (1 – S) e-ad (1)
Where, a is the absorption coefficient antd d is the thickness of the semiconductor film. R1, R2 and R3 are the Fresnel power reflection coefficient and the Fresnel reflection coefficient at semiconductor – substrate and substrate – air interface. S measures the scattering coefficient of the surface.
UV Visible Spectrophotometer (Lambda 850) was used to measure the absorption and transmission data. This system covered the ultraviolet-visible range in 200 – 800 nm. The procedures in the Lambda 850 manual were followed. Figure 4 shows the transmittance spectra of the AlSb thin films. The films have an strong absorption in the visible spectral range up to 550 nm for film with Al:Sb ratio 2:5. Similarly for films with Al:Sb in the ratio of 1:3, 1:1 and 3:7 have strong absorption up to 700 nm. The films were transparent beyond these levels.
Fig. 4. Transmittance Spectra of AlSb films with different Al:Sb growth ratios.
The film with Al:Sb ratio of 7:3 didn’t have a clear transmittance spectra and thus not shown in the figure. This was because the increasing the content of aluminum would make the film metallic thus absorbing most of the light in visible spectrum.
Absorption coefficient of a film can be determined by solving equation 1 for absorption and normalizing the Transmittance in the transparent region as (Baban et al. 2006):
a = -2ln (normalized ) (2)
Optical band gap of the film was calculated with the help of transmission spectra and reflectance spectra by famous using Tauc relation (Tauc, 1974)
Where Eg is optical band gap and the constant n is 1/2 for direct band gap material and n is 2 for indirect band gap. The value of the optical band gap, Eg, can be determined form the intercept of (ahv’f1 Vs Photon energy, hv, at [ahv’^2 = 0.
Fig. 5. Bandgap estimation of AlSb semiconductor with Al:Sb growth ratios (a) 1:3, (b) 2:5,
(c) 1:1 and (d) 3:7.
The optical absorption coefficient of all the films was calculated from the transmittance spectra and was found in the range of 105 cm-1 for photon energy range greater than 1.2 eV. Fig. 5 shows the square root of the product of the absorption coefficient and photon energy (hv) as a function of the photon energy. The band gap of the film was then estimated by extrapolating the straight line part of the (ahv)1/2 vs hv curve to the intercept of horizontal axis.
This band gap for Al:Sb growth ratio 1:3, 2:5, 1:1 and 3:7 was found out to be 1.35 eV, 1.4 eV, 1.25 eV and 1.44 eV respectively. Since the ideal band gap of AlSb semiconductor is 1.6 eV we have taken the Al:Sb growth ratio to be 3:7 to characterize the film and fabricate the solar cells.