William Angerer - Department of Physics and Astronomy - University ...

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sample was mounted on a goniometer, which had an accuracy of 0.010 in the angle
of incidence. A laser beam was aligned with the center of the goniometer, optically
chopped at 2kHz, and p-polarized with a Newport Glan-Thompson calcite polarizer
model 10GT04AR.14. The reflected light was detected by a Thor Labs silicon photodiode
model FDSIOO and measured by a Stanford Research Systems lock-in amplifier
model 530. The absolute reflection coefficient was determined by measuring the light
transmitted through the optical system without the sample, mounting the sample on
the goniometer, measuring the light reflected from the sample as a function of angle.
and calculating the ratio of the reflected light to transmitted light. ~ote, the effect of
temporal fluctuations on the intensity of reflected light was removed by measuring a
fraction of the incident beam and the light reflected from the sample simultaneously.
The angular spectrum was then renormalized to remove temporal fluctuations.
There are two experimental difficulties in measuring the thickness of a ,...., 1 pm
GaN film grown on a sapphire substrate. First, the effective film thickness. nd. where
n is the index of refraction and d is the film thickness, is several times the incident
wavelength. In theory, a reflection measurement can unambiguously determine a film
thickness only if the effective thickness is less than the wavelength of incident light.
The measured intensity is produced by the interference of a wave that is reflected off of
the front surface and a wave that is transmitted into the thin film where it undergoes
a series of reflections and is partially transmitted out of the film. If the effective
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