Optical Properties of Excitons Bound to Copper-Complex Centers in Gallium Arsenide

The photoluminescence of copper-doped high-purity epitaxial GaAs in the near-gap region is investigated as a function of excitation intensity, temperature, and an external magnetic field up to 5.7 T. Sharp emission lines are identified as originating from the recombination of excitons bound to neutral-copper-complex centers of C_3ν and C_2ν symmetry with ionization energies of 156 and about 450 meV, respectively. The spectrum exhibits replicas of these lines, which are due to the simultaneous excitation of resonant modes of 3.6- and 6.1-meV energy. The relative intensities obey a Poisson distribution law. The dissociation of the bound excitons takes place in a two-step process: First a free single particle is liberated, whereas at higher temperatures free-electron-hole pairs are created. The linear dependence of the luminescence on the excitation intensity leads to the conclusion that photocreated coupled electron-hole pairs are trapped directly by the binding center. A group-theoretical analysis of the Zeeman pattern attributes the different lines to the appropriate electronic transitions between states of the double groups C_3ν and C_2ν. The crystal field is sufficiently strong to completely decouple the |mⱼ|=1/2 and 3/2 levels of the acceptor ground state. The |mⱼ|=3/2 state is degenerate with the valence-band continuum.