Band Gap Energies and Refractive Indices of Epitaxial Pb_1-xSr_xTe Thin Films

Pb_1-xSr_xTe thin films with different strontium (Sr) compositions are grown on BaF₂(111) substrates by molecular beam epitaxy (MBE). Using high resolution x-ray diffraction (HRXRD), we obtain Pb_1-xSr_xTe latticeconstants, which vary in the range 6.462--6.492 AA. According to the Vegard law and HRXRD data, Sr compositions in Pb_1-xSr_xTe thin films range from 0.0--8.0%. The Pb_1-xSr_xTe refractive index dispersions are attained from infrared transmission spectrum characterized by Fourier transform infrared (FTIR) transmission spectroscopy. It is found that refractive index decreases while Sr content increases in Pb_1-xSr_xTe. We also simulate the Pb_1-xSr_xTe transmission spectra theoretically to obtain the optical band gap energies which range between 0.320eV and 0.449eV. The simulated results are in good agreement with the FTIR data. Finally, we determine the relation between Pb_1-xSr_xTe band gap energies and Sr compositions (E_g=0.320+0.510x-0.930x² +184x³(eV)).