6 - Defect reactions, thermal equilibrium and metastability

Summary

In Chapter 5, the doping properties of a-Si:H are described in terms of the alternative bonding structures of the dopants and the silicon network. The doping efficiency, η, is derived from the defect reaction, Eq. (5.27), by assuming thermal equilibrium between the different bonding configurations. This chapter applies the ideas of defect equilibrium to several phenomena, including a more detailed analysis of doping and compensation, the intrinsic defect density and the impurity distribution coefficients. The phenomenon of metastability, in which an external electronic excitation, such as light illumination, causes a reversible change in the density of electronic states, are also explained in the context of the defect reactions of the structure. The equilibration of the material represents a considerable simplification in our understanding of a-Si: H, because thermodynamic models can be used to predict the electronic properties.

It may seem surprising to apply thermal equilibrium concepts to amorphous silicon, because the amorphous phase of a solid is not the equilibrium phase. However, a subset of bonding states may be in equilibrium even if the structure as a whole is not in its lowest energy state. The attainment of equilibrium is prevented by bonding constraints on the atomic structure. The collective motion of many atoms is required to achieve long range crystalline order and the topological constraints are formidable. On the other hand the transformation of point defects requires the cooperation of only a few atoms. Therefore any partial thermal equilibrium may be expected at point defects or impurities.