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 $C_{3\nu }$ and $C_{2\nu }$ 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 $C_{3\nu }$ and $C_{2\nu }$. The crystal field is sufficiently strong to completely decouple the $\left|m_j\right|=\frac{1}{2} \mathrm{and} \frac{3}{2}$ levels of the acceptor ground state. The $\left|m_j\right|=\frac{3}{2}$ state is degenerate with the valence-band continuum.