Wet-etching selectivity of Ag-photodoped AsGeSeS thin films and the fabrication of a planar corrugated one-dimensional photonic crystal by a holographic method

Characteristics of Ag photodoping into an amorphous ${\mathrm{As}}_{40}{\mathrm{Ge}}_{10}{\mathrm{Se}}_{15}{\mathrm{S}}_{35}$ thin film and its wet-etching selectivity containing edge-sharpening effect have been investigated. A planar corrugated one-dimensional photonic crystal (1D PC) has also been fabricated by a holographic method based on the Ag photodoping. Ag-doped films by an illumination of HeCd laser at 77 K (cooled Ag-doped films) exhibit a relatively uniformly distributed Ag profile over the entire depth of the chalcogenide and a strong resistance against an alkaline development solution. That is, the cooled Ag-doped film acts as a negative-type resist for wet-etching development using a solution of NaOH (selectivity >99%). In addition, its sensitivity and contrast, obtained from the exposure response curve, are very good to be approximately ${\text{3.3x10}}^{15}$ ${\mathrm{photons/cm}}^2$ and 6.46, respectively. Contrarily, Ag-doped films at 300 K (normally Ag-doped films) exhibit a Gaussian-formed Ag distribution, which has a peak at the interface (about 500 nm depth) between chalcogenide and substrate and a depletion at a depth of 0–400 nm. In particular, an edge-sharpening effect is observed in both developed films and this is discussed considering the model of photoinduced chemical modifications. This effect is thought to play an important role in fabricating a fine planar 1D PC structure by the holographic method. The fabricated 1D PC consisted of a space (s) of 350 nm and a linewidth (l) of 230 nm. For the holographic method based on Ag photodoping, a fabrication criterion for controlling the ratio s/l and the period Λ(=s+l) that are the important parameters in 1D PC structures is also proposed.