whereTis the transmission, t is the thickness of the film, and
A/cm is the absorbance per centimeter. Using multiple samples of
different film thickness, one can then solve the resulting system
of equations and determine the reproducibility and standard
deviation of the optical absorbance per centimeter[44].
The calculation of optical absorbance assumes that the film is
of homogenous composition and uniform thickness. Therefore, an
integrating reflectance accessory is required and the reflectance
measured is predominately specular[45]. Transmission-based
measurements also require that the film thickness of the sample
on the substrate be optimized for the dynamic range of the
technique so that the transmittance of the film falls in the range
from 3% to 90%. If the transmittance falls much below 1%, the
accuracy of the measurement is severely degraded, and erroneous
results appear.
Once the extinction coefficient,k, and the base 10 optical
absorbance per cm, A/cm, have been determined, the base e
optical absorption coefficient,a, can be determined using Eq (3).
The absorption coefficient,a, corresponds to the attenuation of
the light transmitted through the sample and is calculated on a
natural logarithm basis. Since the absorbance per cm,A/cm, is
determined from the base 10 logarithm of the optical density
given in Eq. (3), a value of ln(10), or 2.302585, is introduced into
Eq. (4). Both k and a are inherent optical properties of the
material. On the other hand, absorbance per centimeter is
frequently based only on transmission measurements, and thus
neglects effects arising from the index mismatch between the film
and substrate, thin film interference effects, and film non-uniformity effects.
The fundamental absorption edge spectra have been determined by