Far-infrared spectroscopy of shallow acceptors in semi-insulating GaAs: evidence for defect interactions with EL2

Far-infrared FTIR absorption measurements are carried out at 10 K in order to assess the uncompensated fraction of shallow acceptors in semi-insulating GaAs after optical bleaching of EL2. In carbon-doped material a linear correlation of the G line at 122.3 cm(-1) with the C-As, concentration is found in the whole concentration range between 1 x 10(14) and 2 x 10(16) cm(-3). In samples co-doped with zinc in the range around 1 x 10(16) cm(-3) and 1 to 2 x 10(15) cm(-3) carbon, both, Zn-Ga- and C-As-related far-infrared absorption lines are observed. Taking into consideration the Fermi level position after bleaching of EL2, it can be concluded that no other donors than EL2 are present above a total concentration of 1 x 10(15) cm(-3). In addition to the far-infrared measurements, the intracentre transition of EL2 (A(1) --> T-2) at 1.0389 eV is studied. The results are compatible only with an increase of the total EL2 concentration with the shallow acceptor concentration. Possible mechanisms for the interaction between shallow- and deep-level defects are discussed.