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I7.9.1

BANDGAP ENERGIES AND REFRACTIVE INDICES OF Pb1-xSrxSe A. Majumdar, H.Z. Xu, F. Zhao, L. Jayasinghe, S. Khosravani, X. Lu, V. Kelkar, Z. Shi. School of Electrical and Computer Engineering, University of Oklahoma, Norman, Oklahoma 73019 ABSTRACT Optical transmission measurements were carried out on Pb1-xSrxSe samples with different Sr compositions (x ranging from 0 to 1) grown by molecular beam epitaxy (MBE). Refractive indices were calculated at room temperature and at 77K by fitting the transmittance data. The bandgap energies were determined by fitting the absorption curve with direct and indirect energy bandgap transitions. A distinct bandgap inversion from the direct to the indirect transition is observed at x ~ 0.20 as the Sr composition increases. This is the first observation of such transition in the Pb1-xSrxSe material system. Both direct and indirect bandgap energies were determined by fitting the experimental results. PbSe and lead-alkaline-earth-chalcogenide materials Pb1-xSrxSe have attracted considerable attention in optoelectronic applications especially in the mid-infrared (mid-IR) lasers and midIR/ultraviolet (UV) detectors.1 The bandgap energies and the refractive indices of this material system vary widely with the change of Strontium composition. Such properties have been used to grow quantum wells and diffraction Bragg reflectors (DBR) for vertical cavity surface emitting lasers2 (VCSELS) and detectors. Yet only a narrow range of the whole material system has been explored and the basic electronic and optical properties are widely unknown for Pb1xSrxSe with high Sr compositions. The primary aim of this research is to determine, experimentally, the electronic and optical properties of these ternary compounds for different compositions. PbSe which is a narrow bandgap material, have been studied widely and are know to have a direct narrow bandgap of 0.3eV at L point.3 SrSe on the other hand is a wide bandgap material which have an indirect bandgap between X-Γ.4 As the Sr composition in the ternary compound Pb1-xSrxSe increases, its electronic structure changes from a narrow direct bandgap material to a wide indirect bandgap material. However, the composition at which the material becomes indirect was unknown, even though it is an important parameter in designing optoelectronics devices. The other properties of this material system on which we have investigated upon are the refractive index and absorption coefficient, which were largely unknown. In this research, we have grown epitaxial layers of Pb1-xSrxSe by molecular-beam epitaxy (MBE), on BaF2 substrate, with nine different Sr compositions (x) ranging from zero to one. The crystal growth is characterized by high-resolution X-ray diffraction measurements and the Sr composition is determined by assuming that the material follows the Vegard’s law. Transmissions, in the wavelength range mid-IR to UV, were measured at different temperatures. The refractive indices and absorption coefficients of Pb1-xSrxSe with different Strontium composition w