The effects of applied and internal strain on the electronic propertiesof amorphous silicon

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Abstract
The elasto conductivity has been investigated in a series of doped and undoped glow discharge (g.d.) a-Si specimens under applied tensile and compressive linear strains, S. A periodic bending-couple was applied to the glass substrate and conductance changes were recorded by phase-sensitive detection. The elastoconductivity parameter K= — (1/S)(δ[Sgrave]/[Sgrave]) was studied as a function of Fermi-level position, temperature, specimen thickness and angle between current and strain. The experiments were also extended to specimens in which phonon-assisted hopping predominated. It was found that irrespective of the transport mechanism, the strain causes a small unidirectional shift δϵf (∼ meV) of the Fermi energy. δϵfdepends critically on the position of ϵfwithin the mobility gap. A plot of the dependence of K on ϵc-ϵf shows peaks at 0–6eV and about 1.0eV which are related to the dependence of the pre-factor [sgrave]o on ϵc-ϵf. It is concluded that the δϵf produced by the strain is linked to the movement of the defect distributions with respect to the band edges, most likely the dangling-bond states in the case of a-Si. The final section deals with the related problem of the internal strain S i produced in the specimens during deposition. It is shown that the high compressive strain in good g.d. material, deposited at T d ≃ 300°C, is relaxed by the introduction of defects as T d is reduced. It is suggested that the observed tensile Si in low-T d and evaporated specimens is associated with the changing defect structure.