Direct evidence for the acceptorlike character of the Cu-relatedCandFbound-exciton centers in GaAs

Zeeman measurements have been performed on Cu-doped GaAs in order to reveal the nature of the centers binding the Cu-related excitons $C_0$ at 1.5030 eV and $F_0$ at 1.4839 eV. Thermalization between the magnetic subcomponents in Zeeman transmission spectra shows that a splitting occurs in the ground state of the bound-exciton system in each case. In photoluminescence Zeeman spectra no thermalization is observed. From the diamagnetic shift observed for both centers it is concluded that each excitonic complex contains a weakly bound electron, localized by the Coulomb field of more tightly bound holes. Consequently, we definitely identify the C and F centers as neutral acceptor centers, which has hitherto been arbitrarily, albeit correctly assumed. The magnetic measurements lead to effective g values for the $C_0$ and $F_0$ bound excitons, $g_{\mathrm{eff}=2.44\mathrm{}}$ and $g_{\mathrm{eff}=2.30\mathrm{}}$, respectively. In both cases the splitting is almost isotropic. A model is proposed for the electronic configurations of these excitons in which the isotopic g value is explained by a quenching of the orbital angular momentum of the bound hole states through the action of a strong compressive strain field locally at the defect site. In this model the bound exciton consists of two spinlike holes with their spins coupled off, and an electron. The bound-exciton transition occurs between the S=(1/2) state of the exciton to a spin-only $j_z$=±(1/2) hole state at the neutral acceptor. This state has a hole g value close to the electron value g=2 for both acceptor centers. In addition we report a new unrelated line in the transmission spectra close to the $C_0$ line. We assign this line to an unidentified neutral isoelectronic donor complex from the thermalization behavior and the diamagnetic shift measured for this center.