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

76
of :UN is deposited on the Ah03 substrate prior to deposition of GaN. The Al~ film
serves as a buffer layer and facilitates nucleation of GaN on the substrate. All of our
calculations of linear and nonlinear optical transmission and reflection approximate
the GaN/AIN/Ah03 sample as a GaN/AI 2 0 3 sample. Using the reported values for
the index of refraction of AIN [54] and the nonlinear susceptibility of AIN [55], we
calculated that this simplified two film model introduces an error of less than l.-1o/c
(6%) in the linear (nonlinear) transmission measurements as compared with the three
slab model. Therefore, we are justified in ignoring the contribution of the thin .-\1,:\
buffer layer in our calculations, and adopt the simpler model.
4.1 Linear Measurement of Film Thicknesses
Because linear spectroscopic measurements inherently encompass multiple interference.
knowledge of the thin film thickness is required to determine the optical constants.
In this section, I describe the theory and experimental techniques we used to
determine the thickness of thin films grown on transparent substrates.
4.1.1 Experimental Techniques
The experimental apparatus used to measure thin film thickness is displayed in
Fig. 4.1. The experiment consisted of measuring the reflection coefficient as a function
of the angle between the GaN / Al 2 0 3 surface normal and the incident beam. The
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.