Thermoelectric power in phosphorous doped amorphous silicon

Abstract

Thermoelectric power and conductivity measurements have been made as a function of temperature on a series of a-Si specimens doped substitutionally with phosphorous. The samples were prepared by the glow discharge decomposition of silane containing predetermined concentrations of phosphine. The thermoelectric power of all the samples investigated was negative as expected. For the lightly doped specimens the thermoelectric and conductivity results support the model developed from previous work. Electron transport is in the extended states, but below room temperature there is an increasing contribution from tail state hopping. At higher doping concentrations the data can be interpreted in terms of transport through the extended states and the localized donor states, as suggested by the analysis of recent Hall effect results. The value of the intercept S ₀ on the S axis at 1/T = 0 depends strongly on the position of the Fermi energy ∊_F with respect to the mobility edge at ∊_c. The movement of ∊_F with temperature arising from the statistical overflow of carriers to energies above ∊_F is discussed in some detail. For most specimens it is possible to explain S ₀ values in terms of the temperature shift of ∊_F, but in the case of heavily doped samples this effect cannot completely account for the magnitude of S ₀.