Optical characterization of aluminium gallium nitride thin films

Abstract

Optical characterization methods have proved to be the most efficient for structural description and molecular identification of thin film samples, due to their high sensitivity, excellent resolution and non-destructive technique. Infrared Spectroscopy describes resonance with vibrational phonons, and can be used to extract the index of refraction and dielectric function profiles. Ultraviolet-Visible Spectroscopy is very efficient in calculating the value of the bandgap of the material studied since it describes the interaction with excited electrons. We have developed a MATLAB code that describes the theoretical model used to characterize the AlGaN thin films. This reflectivity model is based on equations derived from the behavior of the electric field at each interface, and the Kramers-Kronig analysis technique. The solutions of Maxwell’s equations in each layer are matched using boundary conditions resulting in a complex expression for the reflectivity. The properties of the substrate are obtained by a theoretical model that was already developed in a previous study to describe the intrinsic optical response of a material to infrared excitation. Due to the complexity of the AlGaN alloy, this model is not applicable to the top layer. This led us to develop our new model to characterize the AlGaN samples which showed to be very efficient. The refraction and extinction coefficients are obtained for the Si and Mg doped AlGaN samples for different Aluminum molar fractions. The most important outcome is the detection of plasmon modes observed at specific frequencies. Our AlGaN thin film samples are grown on a Sapphire substrate with GaN and AlN nucleation layers in between. They are separated into two groups: Silicon doped (n-type) and Magnesium doped (p-type). The Si-doped samples are either slightly or heavily doped. The Aluminium molar fraction ranges from 0 to 1. This provides us with a wide range of samples with different types of doping and Al molar fractions that can be compared and a certain trend ca