Band gap study of lead chalcogenide alloys

Lead chalcogenides (PbQ, Q = Te, Se and S) have been applied in many devices as a result of their narrow band gaps, low electrical resistivity and high carrier mobility. The band gaps of lead chalcogenides have shown to change energy upon alloying and altering carrier concentration. This technique is known as band engineering and has been researched thoroughly as a method of improving performance in thermoelectrics, thermophotovoltaics, solar cells, UV-LEDs, laser, transistors and much more. However systematic research into the band gap energies of lead chalcogenide solid solution alloys has received little attention. Here, the intrinsic solid solution ternary (PbTe)1-x(PbSe)x and quaternary (PbTe)0.9- y(PbSe)0.1(PbS)y and (PbTe)0.65-y(PbSe)0.35(PbS)y alloys are fabricated to determine their band gap by optical absorption edge measurements using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). A positive band gap bowing occurs which challenges the accepted linear relationship with composition. A small band gap bowing parameter of -0.052 ± 0.011 eV was obtained for the PbS-free ternary (PbTe)1-x(PbSe)x alloy as a result of the disparity in electronegativity and valence electron potential of the Te and Se atoms. A significantly larger band gap bowing of -0.21813 ± 0.01561 and -0.08326 ± 0.02352 eV was acquired for quaternary solid solution (PbTe)0.9- y(PbSe)0.1(PbS)y and (PbTe)0.65-y(PbSe)0.35(PbS)y alloys respectively. The considerably larger bowing parameter for the PbS-alloyed quaternary alloys compared to the ternary system was determined to be associated with increased mismatch of ionicity and lattice parameters. Whereas, the difference in bowing parameters between the two quaternary systems is originated from the larger miscibility gap that occurs in (PbTe)0.9-y(PbSe)0.1(PbS)y system compared to (PbTe)0.65-y(PbSe)0.35(PbS)y system. The temperature dependent thermal conductivity of PbSe0.1Te0.9, PbSe0.1S0.9 and PbSe0.35S0.65 revealed that the contribution of thermally activated minority charge carrier in bipolar conductivity is reduced in PbSe0.1S0.9 and PbSe0.35S0.65 alloys as a result of band gap widening.