38.
The temperature dependence of the field effect response permits an unambiguous determination of the identity of those states responsible for electrostatic screening in the amorphous chalcogenides. We observe (1) in As
2Te
3, field effect screening by localized states at the Fermi level at low temperatures (~ 10
19 cm
?3 eV
?1) and by mobile charge carriers (~ 10
18 cm
?3 at 300 K) at high temperatures, and a transition from p-type to two-carrier (primarily n-type) conductivity as the temperature is raised above ~320 K; (2) in As
2SeTe
2, screening by mobile charge carriers (~ 10
18 cm
?3 at 300 K) with strongly type conductivity; (3) in As
2Se
2Te, screening by localized states at the Fermi level (~ 10
19 cm
?3 eV
?1) with strongly p-type conductivity; and (4) in Sb
2Te
3, a very high density of localized states at the Fermi level (~ 2 × 10
20 cm
?3 eV
?1) with both electron and hole contributions to the conductivity. Correlation with thermoelectric power results suggests that the p-type conductivity in As
2Te
3 is due to near-equal contributions from two processes: hopping in localized states plus extended state conduction. Aging and annealing behavior is described with the aid of a “chaotic potential model” that appears to be able to account for large changes in mobile carrier density that leave the conductivity unaltered.
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