Characterization of X-ray radiation damage in Si/SiO2 structures using second-harmonic generation

We report the first application of second-harmonic generation (SHG) measurements for the characterization of X-ray radiation damage in Si/SiO2 structures. The main advantage of this experimental technique is that it is noninvasive, contactless, and sensitive to the electric field at the interface. Interaction of intense 800 mn femtosecond laser pulses with Si/SiO2 structures results in electron-hole pair creation in the Si, multiphoton carrier injection and second-harmonic generation. The time-dependent second-harmonic (doubled frequency) signal is a measure of the dynamic electric field at the interface. This dynamic field is created and altered by unequal electron-hole injection into the oxide, trapping/detrapping of charges, and carrier recombination processes. We find that the SHG response from Si/SiO2 samples before and after X-ray irradiation is significantly different. Thus, SHG is a promising technique for the characterization of radiation damage in Si/SiO2 structures. In particular, SHG is especially useful in characterizing damage in ultrathin oxide layers, for which conventional electrical measurements may not be sufficiently sensitive to the kinds of defects observable via optical methods.