A comparative study of EL2 and other deep centers in undoped SI GaAs using optical absorption spectra and photoconductivity measurements

The performance of radiation detectors fabricated from semi-insulating (SI) GaAs is highly sensitive to EL2+-concentration in the material. Near-infrared optical absorption measurements are commonly used to determine the EL2-concentration and to roughly estimate the EL2+-concentration under the assumption that the optical absorption is mainly determined by the photoionization and the photoneutralization of EL20 and EL2+, respectively. However, the presence of different native defects can contribute to optical absorption and reduce the precision of determination of EL2-concentration. In this work, we evaluate the contributions into optical absorption from EL2 and other deep center namely EL3 defect (0.55 eV) using near-infrared optical absorption and photoconductivity (PC) measurements in the photon energy interval 0.5–1.4 eV for SI GaAs crystals grown by the liquid encapsulated Czochralski method from melts with As content changing from 50% to about 46%. The photoelectrical spectra were measured on p–i–n structure detectors with heavily doped p+ and n+ layers grown by Liquid Phase Epitaxy and on Schottky diodes. The short circuit photocurrent spectra were registered for all detectors in the energy interval 0.65–1.4 eV. Unexpectedly, the current sensitivities in the regions of the extrinsic and intrinsic absorption were comparable. A comparative study of optical absorption, PC and short circuit photocurrent spectra resulted in determination of EL2+-concentration. It was concluded that contribution of additional deep centers, particularly the ionized EL3+ defect could be comparable to the EL2-contribution. The EL3 centers were attributed to oxygen-related defects based on published results and on some indirect evidence in our experimental data.