Stability evaluation of n- and p- type (Bi,Sb)2(Te,Se)3 solid solutions

The properties change of solid solutions n-Bi
₂
Te
_2.79
Se
_0.15
S
_0.06
(alloyed by I), and p-Bi
_2-x
Sb
_x
Te
_3-y
Se
_y
(x=1.5-1.56, y=0.06-0.3), (alloyed by excess Te), during long-term annealing at 150degC. The samples were obtained by directional crystallization method in graphite containers in argon atmosphere. The measurements of electrical conductivity in samples were carried out along cleavage planes direction The n-type material showed itself as stable one at specified temperatures. The change of electrical conductivity did not exceed 1-2%. It could be explained by homogeneity area spreading to both sides of stoichiometric composition and by absence of any composition and properties changes during the annealing process. The investigation of p-type samples composition influence on their thermal stability showed that electrical conductivity was droningly decreasing during the annealing process to 30% for Bi
_0.44
Sb
_1.56
Te
₃
, to 15% for Bi
_0.5
Sb
_1.5
Te
_2.94
Se
_0.06
, and for Bi
_0.5
Sb
_1.5
Te
_2.7
Se
_0.3
the electrical conductivity changed no more than 5%. In these solid solutions the homogeneity area was displaced from stoichiometric composition, and at that the tellurium solubility was increasing at temperature decrease. The concentration of anti-structural defects and carriers decreased correspondingly. The introduction of Se into composition caused lower displacement from stoichiometric composition and made the composition and properties more stable, but the material effectiveness Z was decreasing at that. The influence of stabilizing annealing process at 300-350degC on properties and stability of p-type material was investigated. It was shown that change of electrical conductivity in this case (for those compositions) made up 12%, 8% and 2% correspondingly. Therefore the stabilizing annealing together with Se introduction causes essentially increased thermal stability of p-type material and losses of effectiveness Z does not exceed 2% at that.