TABLE OF CONTENTS In this chapter we are concerned with the effect of structure on the optical properties of thin films. In the past the optical properties of thin films that were important consisted of the solar and thermal reflectances and transmittances inasmuch as these films were used as passive coatings of lenses, windows, etc. With the advent of the possibility of integration of optical and electrical devices in telecommunication and computing another group of optical properties became significant for thin films, such as luminescence, wave guide transmission, and various non-linear optical properties. Unfortunately, there are few, if any, quantitative theoretical relationships between structure and optical properties. Thus, the discussion to follow is based on qualitative concepts and phenomenology. Nevertheless, the influence of structure on optical properties will be made apparent.
Structure affects both the wavelength and intensity of the light emitted in the luminescence displayed by various materials. The wavelength of light emitted in devices made from III-V semiconductors corresponds closely to the band gap energy, which, in turn, is dependent on the lattice parameter of the common sphalerite crystal structure, as illustrated in Figure 3.1. As shown, the band-gap energy decreases (i.e. the wavelength of the emitted light increases) as the lattice parameter increases. However, even at a constant lattice parameter there exists a range of band gap energies. Thus, other parameters besides the lattice parameter must affect the band-gap energy.
