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3 Domain Charge Structure of Amorphous Semiconductor We have seen that a new quality in the system emerges when one curve is touched to another, as shown in Figs. 5. It means that light-driven self-organization is indeed a threshold effect according to the third statement of Prigogine. Light intensity is a governing parameter: self-organization appears at μ = I σ0 /γ ≈ 1. Here we shall give an example of the system where γ → 0 and light-driven selforganization is observed at weak pumping (HeNe-laser).

This is a very important, but as-yet unsolved problem. Theoretical study of the simplest self-organized systems (presented in Chap. 2) shows that light-driven ordering is governed by the pumping parameter μ = I σ0 /γ (the ratio of excitation and decay rates). Self-organization arises when light power exceeds some threshold and excitation becomes more intense than the decay (μ > 1). The decay rate of trapped electrons and holes discussed in this chapter being dependent on electron–hole distances are exponentially small for long-distance pairs.

Trapped electron states near the mobility threshold are involved (|εi | → V , |εf | → V ) with slow decaying wave functions (κi → 0, κf → 0, κf i → 0) that increase the matrix elements of the transitions. The factor γ0 is of the order of the inverse lifetime for excited electron states in an atom, so that γ0 ≈ 108 s−1 . Light-driven kinetics are governed by dimensionless pumping constant I σ0 μ= , γ0 the other parameters are: Δ, the absorption line width; and a, the average distance between traps (N a 3 is volume of a sample, N is number of traps).