Ultraviolet pulse laser induced modifications of native silicon/silica interfaces analyzed by optical second harmonic generation

Abstract

Native silicon/silica (Si∕SiO2) interfaces are investigated by electric field induced second harmonic (EFISH) generation employing near infrared femtosecond laser pulses (782.8nm, 80fs, 10nJ, 80MHz repetition rate). Here, the temporal EFISH evolution induced by the femtosecond laser irradiation is recorded with a time resolution of 0.2s. Comparative EFISH studies are performed with Si∕SiO2 interfaces, preexposed to ultraviolet (UV) laser pulses (308nm, 16ns, 0.45–2.8J∕cm2) as well as virgin Si∕SiO2. After UV irradiation the femtosecond laser induced electron injection and trapping in the ultrathin oxide is found to be drastically accelerated in contrast to the essentially unaffected interfacial hole dynamics. This result is explained by an enhanced interfacial electron trap density caused in most cases by the UV laser induced melting and recrystallization of the near-interface silicon. Furthermore, three-dimensional second harmonic imaging reveals a saturation effect of the UV induced sample modification due to the melting of the near-interface silicon, for which a threshold fluence of 0.47J∕cm2 is extracted for xenon chloride (XeCl) laser irradiation. Below this threshold an accumulative behavior of the UV induced modification is observed for repeated exposure to several UV laser pulses. This is pointing to a dose dependent modification process presumably due to thermally driven interface chemistry and/or microscopic structural and electronic changes of the Si∕SiO2 interface.