Optical characterization of semiconductor films using transmittance and reflectance spectra

• Grant number: 12/01395-6
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• Start date: May 01, 2012
• End date: December 31, 2012
• Field of knowledge: Physical Sciences and Mathematics - Physics - General Physics
• Principal Investigator: Jose Humberto Dias da Silva
• Grantee: Carlos Guilherme Gonçalves de Azevedo
• Host Institution: Faculdade de Ciências (FC). Universidade Estadual Paulista (UNESP). Campus de Bauru. Bauru , SP, Brazil

Abstract

Some existing methods allow the determination of the refractive index, absorption coefficient, and thickness of thin films. However, many of them are limited by approximations and others are not based on good physical grounds. In this project, the development of a method for calculating the optical constants of semiconductor thin films from transmittance and reflectance spectra is proposed. Complete expressions for the transmittance and reflectance derived directly from Maxwell's equations will be used in the calculations. The expressions are used to take into account the coherent multiple reflections in the films and incoherent multiple reflections in the substrate. The aim is to obtain an accuracy of order 1% when the method is applied to homogeneous films with flat parallel faces, in the 0.5 and 6.0 eV photon energy range, and widths between 100 and 2000 nm. The transmittance and reflectance spectra will be measured in a Perkin-Elmer L1050 spectrophotometer, equipped with an integrating sphere, over a wavelength range between 200 nm and 3300 nm. The calculation method will rely on in the fitting of the optical constants in the region of the interference fringes, and in the iterative calculation in the high absorption spectral range. A subsequent refinement will be done using comparisons between the calculated and experimental spectra of reflectance and transmittance, enabling convergence consistent with the measured spectra. The method shall be applied initially for the determination of the optical constants in GaSe films obtained by thermal evaporation, and Mn-doped GaN and TiO2 films produced in the laboratory by reactive sputtering. These materials are currently applied in photoconductivity devices, as diluted magnetic semiconductor devices, and in photocatalytic processes respectively. The determined optical constants will allow the analysis of the relationship between optical properties and the electronic structure of the materials of interest. (AU).