  We report the effect of co-doping of In and Ga at low concentrations on the
structural, electronic, and thermoelectric properties of SnTe based
compositions $Sn_{1.03-2x}In_{x}Ga_{x}Te$ (x = 0, 0.01, 0.02, 0.04) prepared by
the solid-state route and spark plasma sintering (SPS). All compositions formed
in the fcc structure (Fm-3m) with no other impurity phase. The optical band gap
increased with the co-doping, indicative of band convergence effects. First
principle electronic structure calculations showed band convergence and the
formation of resonant levels, due to Ga and In doping respectively. The carrier
concentration increased on hole-doping by In and Ga ions while carrier mobility
decreased due to impurity scattering. The resistivity increased with
temperature, indicative of the degenerate semiconducting character of the
compounds. The Seebeck coefficient of the doped samples increased linearly with
temperature, reaching 85 - 95 ${\mu}$V/K at 783 K. Thermal conductivity
decreased sharply with co-doping, and the lattice thermal conductivity dropped
to 0.42 W$m^{-1}$ $K^{-1}$ above 750 K. The enhanced power factor and low
lattice thermal conductivity on doping resulted in a maximum figure of merit ZT
= 0.34 at 773 K, twice that of the pristine SnTe.
