Substrate- and interface-mediated photocrystallization in a-Se films and multi-layers

Photocrystallization in a-Se films and layered a-Se structures is studied by Raman scattering as a function of temperature for photon energies near or slightly below the band gap. The samples are $\sim $16.5 $\mu $m thick films of a-Se grown i) directly on glass, ii) on indium tin oxide (ITO) coated glass, iii) on glass that is spin coated with 800nm polymide, and iv) on a Capton sheet. A low As-concentration ($<$ 0.2 {\%}) is present in several of the a-Se films. We compare the results on these samples to prior findings on a-Se HARP targets, and on a polymer-encapsulated a-Se film [1]. We observe strong evidence that the interface between the a-Se film and the underlying substrate and/or multi-layers plays an important role in the onset time and growth rate of photocrystallized Se domains. In some samples a discontinuous increase in the onset time with increasing temperature occurs near the glass transition ($\sim $310K), and there is a surprising ``dead zone'' of no crystallization in this region. Other samples merely show a minimum in the onset time at similar temperatures, but no discontinuity and no region where crystallization is absent. Soft intermediate layers appear to increase stability against crystallization in an overlying a-Se film. The competing effects of substrate shear strain and thermal driving forces on the photocrystallization process are considered to account for these findings. \\[4pt] [1] R.E. Tallman et. al. J. Non-crystalline Sols. \textbf{354,} 4577-81 (2008)