The optical properties and stoichiometry of evaporated bismuth oxide thin films

The optical properties of bismuth oxide thin films were measured for samples prepared by direct evaporation of Bi2O3. The as−evaporated low−rate (1−5 Å/sec) films are microcrystalline, bismuth rich (relative to Bi2O3), and their optical absorption edge broadens and shifts to lower energies. High−rate (15−25 Å/sec) films are amorphous and oxygen−rich with an absorption edge shifted to higher energies. Thermal decomposition of the Bi2O3 during evaporation causes these variations in film stoichiometry. We show that shifts in the absorption edge may be correlated directly with departures from stoichiometry. The refractive index and absorption edge change with thermal treatment in oxygen; the former varies linearly with the partial oxygen pressure over the range of pressures observed during evaporation in this study. The refractive index and extinction coefficient are compared with published data on Bi2O3 thin films. Using the Lorenz−Lorentz equation, the dependence of the refractive index on the bismuth/oxygen ratio is derived. Application to as−evaporated films shows a good correlation with our data.