Detailed investigation of surface passivation methods for lifetime measurements on silicon wafers

Abstract

The effect of five different common surface passivation techniques on the measured bulk lifetime values of multi- and monocrystalline p-type silicon wafers was investigated. Mono-[Czochralski (Cz) and floatzone (FZ)] and multicrystalline [mc and edge-defined film-fed growth (EFG)] silicon wafers were either deposited with a dielectric passivating layer of SiNx, Al2O3, or amorphous silicon (a-Si) or were passivated chemically with 0.08 M iodine-ethanol (IE) or 0.07 M quinhydrone-methanol (QM) solutions. The temporal stability of annealed and nonannealed Cz wafers that were passivated with QM and IE was tested. The lifetime values of EFG, mc, and FZ wafers that were subjected to repeated QM passivation and mc wafers that were subjected to IE passivation without a surface etching between passivations were found to decrease with each passivation. Lifetime values of a set of 11 mc wafers that were passivated with Al2O3 were found to decrease about 30% after a period of four weeks in darkness. The decrease was reversible by annealing the samples. The lifetime values of annealed Cz samples that were passivated with Al2O3 and a-Si were found to decrease by >;20% within 5 h of annealing. Subsequent tests on 200-Ω·cm FZ material did (for a-Si) and did not (for Al2O3) show surface passivation degradation over this time period. Neighboring mc wafers were passivated dielectrically or wet chemically with IE or QM and characterized with photoluminescence imaging. All mc wafers that were subjected to dielectric passivation methods that include annealing at 400 °C displayed a greater area of high lifetime values but fewer areas of very high lifetime values, providing visible evidence of internal gettering and/or defect redistribution.