Lead tin telluride, also referred to as PbSnTe or Pb1−xSnxTe, is a ternary alloy of lead, tin and tellurium, generally made by alloying either tin into lead telluride or lead into tin telluride. It is a IV-VI narrow band gap semiconductor material. The band gap of Pb1−xSnxTe is tuned by varying the composition in the material. SnTe can be alloyed with Pb in order to tune the band gap from 0.29 eV to 0.18 eV. It is important to note that unlike II-VI chalcogenides, e.g. cadmium, mercury and zinc chalcogenides, the band gap in Pb1−xSnxTe does not changes linearly between the two extremes. In contrast, as the composition is increased, the band gap decreases, approaches zero in the concentration regime and further increases towards bulk band gap of SnTe. Therefore, the lead tin telluride alloys have narrower band gaps than their end point counterparts making lead tin telluride an ideal candidate for mid infrared, 3–14 μm opto-electronic application.
Properties
Lead tin telluride is p-type semiconductor at 300 K. The hole concentration increases as the tin content is increased resulting in an increase in electrical conductivity. For composition range x = 0 to 0.1, electrical conductivity decreases with increase in temperature up to 500 K and increases beyond 500 K. For composition range, x ≥ 0.25, electrical conductivity decreases with increases in temperature. The Seebeck coefficient of Pb1−xSnxTe decreases with increases in Sn content at 300 K. For composition x > 0.25, thermal conductivity of Pb1−xSnxTe increases with increase in Sn content. Thermal conductivity values decreases with increase in temperature over the entire composition range, x > 0. For Pb1−xSnxTe, the optimum temperature corresponding to maximum thermoelectric power factor increases with increase in composition x. The pseudo binary alloy of Lead tin telluride acts as a thermoelectric material over 400–700 K temperature range. Lead tin telluride has a positive temperature coefficient i.e. for a given composition x, band gap increases with temperature. Therefore, temperature stability has to be maintained while working with lead tin telluride based laser. However, the advantage is that the operating wavelength of the laser can simply be tuned by varying the operating temperature. The optical absorption coefficient of lead tin telluride is typically ~750 cm−1 as compared to ~50 cm−1 for the extrinsic semiconductors such as doped silicon. The higher optical coefficient value not only ensures higher sensitivity but also reduces the spacing required between individual detector elements to prevent optical cross talk making integrated circuit technology easily accessible.
Application
Due to tunable narrow band gap and relatively higher operating temperature of lead tin telluride as compared to mercury cadmium telluride, it has been a material of choice for commercial applications in IR sources, band pass filters and IR detectors. It has found applications as photovoltaic devices for sensing radiation in 8-14 μm window. Single Crystal Pb1−xSnxTe diode lasers have been employed for detection of gaseous pollutants like sulfur dioxide. Lead tin tellurides have been used in thermoelectric devices.
