Methods to enhance absorption signals in infrared reflectance spectroscopy: A comparison using optical simulations

Abstract

We use optical simulations to determine the substrate and experimental geometry that should be used to enhance infrared absorption signals from thin films in a reflectance geometry. Our goal is to highlight those configurations which are suitable for the real-time study of surface and subsurface chemical bonding during growth or processing. The enhancement techniques reviewed are the use of metal substrates, multiple internal reflection (MIR), and an optical cavity substrate. For each technique, we simulate the absorption strength for Si–H vibrational modes in and on hydrogenated amorphous silicon films as a typical example. The techniques are discussed in terms of (i) the choice of the substrate and flexibility in measurement geometry; (ii) the sensitivity in both s and p polarizations; (iii) the effect of film refractive index (n) and extinction coefficient (k) on the line shape and absorption intensity; and (iv) the sensitivity as a function of film thickness. For the real-time study of high-refractive-index films, the optical cavity substrate and MIR are best suited. MIR is most applicable to study surface reactions during epitaxial chemical vapor deposition (CVD) on high-refractive-index crystals having low bulk absorption in the infrared region. The optical cavity substrate is best suited to study film growth on low-refractive-index material. Metal substrates work well for low-refractive-index materials, e.g., studies of adsorbate layers and metal CVD. These techniques are complementary to each other and provide a means of studying a wide variety of material–substrate combinations.