Tailoring Surface Phonon Polariton on SiO₂ by Ion-Beam Irradiation

Synchrotron infrared nanospectroscopy (SINS) is used to study the effect of disorder on the phonon polariton in amorphous SiO2. For SiO2, it is known that the mid-infrared scattering spectrum is dominated by a surface phonon polariton mode associated with antisymmetric (AS) stretching. This excitation has a benchmark role in SINS studies. In this work, an amorphous 50 nm SiO2 film, on a crystalline Si(100) substrate, is irradiated with 25 keV fluorine anions and fluence of 1.3 × 1015 ions/cm2, modifying the two asymmetric stretches (AS1 and AS2) present in the SINS spectrum. The dominating peak corresponding to the SiO2 surface phonon polariton mode, AS1, has a maximum intensity that is affected by irradiation and shows a shoulder identified with a suboxide SiOx component (with x < 2) due to defects. On the other hand, the surface phonon polariton mode AS2 is completely suppressed by irradiation. In addition, calculations based on the finite dipole model (FDM) indicate that the polariton measured in the irradiated sample corresponds to the light–matter interaction at the surface of an effective medium combining the SiO2 and SiO regions.