Lattice Distortion and Metastability

FAs in a crystal can induce a local deformation of the lattice. When they are substitutional, this is caused by the difference between their atomic radii and those of the atoms they replace and also by their chemical affinity with the surrounding atoms. According to Vegard s law,1 substitutional atoms having a smaller (larger) atomic radius than the atom they replace should produce a uniform lattice contraction (expansion) of the crystal proportional to their concentration. With reference to the unperturbed lattice parameter ao of cubic crystal, the change Aa of the lattice parameter produced by a concentration Nf of FAs can be expressed as  

1 Vegard s law is an empirical rule which holds that an approximate linear relation exists between the crystal lattice parameter of an alloy and the concentration of its constituent elements (L. Vegard, Z. Kristaflogr. 67, 239 (1928). See also A.R. Denton, N.W. Ashcroft, Phys. Rev. A 43, 3161 (1991)). 

In crystals with a high concentration of shallow donors or acceptors, another contribution to the volume change at room temperature is the presence of free carriers in the continuum. This effect is related to the minimization of the total energy of the crystal compared to that of the undoped crystal ([24] and references therein). This purely electronic effect has been considered, together with the eventual presence of a high concentration of native defects, in the variation of the lattice parameter of Si-doped GaAs conducting substrates used in the electronic industry [3], 

It should be noted that, within the same global charge state of a centre, due to differences in electronic densities, lattice relaxation can also occur between the ground state and excited states, with the same consequences of equilibrium configuration coordinates. 

The change of configuration of a centre induced by its transition into a metastable state produces a lattice distortion which can result in a macroscopic volume change. Transient effects due to the photocreation of electron-hole pairs in n-type GaP and SI GaAs have been attributed to this effect [74]. 

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