Non-equilibrium molecular-dynamics for impurities in semiconductors: Vibrational lifetimes and thermal conductivities

Calculating from first principles the vibrational properties of impurities in semiconductors such as Si has kept theorists busy for more than a decade. The early focus has been on predicting IR- or Raman-active local vibrational modes, thus contributing to the identification of defects containing light impurities. However, the knowledge of the entire dynamical matrix of a supercell is precious. It allows the identification of all the localized modes in the system and the quantitative analysis of their degree of localization. It also allows the 'preparation' of a supercell in thermal equilibrium at non-zero temperatures without thermalization or thermostat. This allows non-equilibrium molecular-dynamics simulations to be performed with minimal temperature fluctuations. One application of this approach involves predicting the temperature dependence of vibrational lifetimes. Another application is the calculation of the impact of impurities on the thermal conductivity of semiconductor nanostructures. The approach involves preparing the supercell slightly off thermal equilibrium and then monitoring how its returns to equilibrium.