MECHANISMS OF PHOTOSENSITIVITY IN GERMANOSILICA FILMS

Mechanisms underlying the refractive index changes in germanosilica fi lms deposited by hollow cathode plasma enhanced chemical vapor deposit ion and subjected to UV irradiation are proposed based on observed cha nges in film thickness, stress, and structure. An increase in refracti ve index after UV exposure is observed in films deposited under low io n bombardment conditions. This increase in refractive index is accompa nied by a reduction in film thickness which is an order of magnitude l arger than that expected from the Lorentz-Lorentz relation. This behav ior is shown to result from: (i) a significant degree of porosity in t he as-deposited material; (ii) oxygen deficiency of the as-deposited m aterial. Upon UV irradiation, the porous structure is compacted, thus accounting for the large decrease in thickness, while the oxygen defic iency is reduced causing a decrease in the material polarizability and counteracting the effect of the thickness reduction. On the other han d, germanosilica deposited under high ion-bombardment conditions is of normal optical quality and exhibits a decrease in refractive index af ter exposure to UV. This refractive index reduction is shown to be the result of three processes: structural dilation and stress relief on o ne side; and an increase in material polarizability on the other, with structural dilation having the largest effect. Annealing of the expos ed samples has shown that most of the polarizability increase is likel y to be annealed out at 500 degrees C, while the refractive index chan ge caused by structural dilation is stable up to 800 degrees C. Finall y, it is shown that during plasma enhanced chemical vapor deposition, germanosilica is more prone to nucleation and columnar growth than pur e silica and therefore a higher level of ion bombardment is required i n the former case in order to obtain a high quality homogeneous materi al. (C) 1997 American Institute of Physics.