Bipolaron mechanism of DX center in AlxGa1-xAs:Si

The free-carrier concentration in Si-doped AlxGa1-xAs has been calculated by grand-canonical-ensemble statistics without any fitting parameters. Our results are quantitatively in agreement with various experimental data in the temperature range 77-300 K, which indicates that the physical picture of the ground state of DX center (DX-) is of an electronic bipolaron due to the interaction between excess electrons and lattice. When exposed to light, one bipolaron can turn into a polaron, meantime release one electron to the conduction band accompanied by lattice relaxations. Our calculations also prove that DX0 is unstable at thermal equilibrium, which further confirms our bipolaron mechanism.