Deep-level defects in Ga-doped silicon crystals

An important sector of the PV community is using gallium-doped silicon instead of boron-doped to prevent boron-oxygen-related light-induced degradation (BO-LID) in Si solar cells. However, the information available in the literature on electrically active defects, which can limit the minority carrier lifetime, in Ga-doped silicon crystals is rather limited. In this work, deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS have been used to characterize electrically active defects in Ga-doped silicon crystals grown by both Czochralski (Cz) and float-zone (FZ) techniques. Deep-level defects, which cause persistent photoconductivity at room temperature, have been detected by DLTS in both Cz-Si:Ga and FZ-Si:Ga crystals. We have carried out measurements of temperature dependencies of hole emission rate, equilibrium occupancy, and hole capture kinetics for the detected traps. The electronic characteristics of the detected traps have been determined and their configuration-coordinate diagrams constructed. It is argued that in Cz-Si:Ga the deep-level defect is formed by the interaction of mobile oxygen dimers with the substitutional Ga atoms in the temperature range 400-550 °C during cooling down the ingots or in heat-treated wafers. Atomic structures of the GaO2 defect, which are consistent with the observed electronic characteristics of the trap detected by DLTS, are proposed. The concentration of the GasO2 defects is found to be a few times 1013 cm-3 in.the as-grown Cz-Si:Ga crystals. Similarities and differences between the GasO2 center and BsO2 defect, which is responsible for light-induced degradation (LID) of solar cells made from Cz-Si:B, are discussed. The origin of the defect detected in FZ-Si:Ga remains unknown. Evidence of the bistability of this trap is presented.