In this thesis, the photoluminescece and the photoconductivity excitation of GaAs heavily doped with Si are studied. The GaAs:Si samples were prepared by liquid phase epitaxy technique. The doping concentrations of the samples are in the range of $8×10^{17}$ to $4×10^{19} cm^{-3}$. Three broad lines are observed. The peak energies of the three broad lines are measured to be 1.463, 1.400 and 1.372 eV, respectively. We investigate the change of peak energies, the integrated intensity and FWHM(full width half maximum) while varing the temperature and the excitation laser intensity. The peak observed at 1.463 eV is designed to be the transition from the conduction band to the Si aqcceptor level. The mechanism of the 1.400 eV and the 1.372 eV PL peak are interpreted in terms of the configuration coordinate (CC) model, which explains the coupling of the electronic transition with the lattice vibration. The PL peak at 1.400 eV is estimated to be the transition between the As vacancy and the valence band. The 1.372 eV PL peak is attributed to the recombination between two states of the complex defect involving the Si acceptor and the As vacancy.
