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

Abstract

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. © 2003 Elsevier B.V. All rights reserved.