RECENT ADVANCES IN THE THEORY OF PHONONS IN SEMICONDUCTORS

Calculations of structural energies of solids have recently reached a new stage, in which it has been shown to be feasible to calculate directly crystal structures of minimum energy and distortion energies that determine harmonic phonon eigenmodes and anharmonic terms. In this paper are discussed 1) calculational methods, 2) recent results, which have been shown by several groups to give accurate results for Si, Ge, GaAs and Se with no adjustable parameters, 3) the corresponding electronic charge densities, which show graphically the bonding and suggest interpretations for the origins of the atomic forces. Comparisons of direct computations of total energy differences with second-order perturbation calculations of harmonic coefficients shows the advantages and disadvantages of each method and how each can complement the other