Photoinduced phase transformations in amorphous ZnSe thin films: amorphous-to-amorphous and amorphous-to-nanocrystalline transitions

摘要

Photoinduced characteristics of amorphous (a-) ZnSe thin films at 10 and 300 K have been investigated using real-time photoluminescence (PL) and X-ray diffraction. The structural phase of as-deposited film is evaluated to be predominantly amorphous with uniformly distributed nano-scale crystallites, and the optical energy gap and complex refractive index are approximately 2.928 eV and 3.04 + 10.35 (at λ= 325 nm), respectively. While the crystallite size is enlarged after illumination with HeCd laser at 300 K, a photodarkening effect without a change in crystallite size is observed in films illuminated at 10 K. That is, two types of temperature-dependent photoinduced changes are observed in a-ZnSe (i.e. amorphous-to-nanocrystalline transition at 300 K and amorphous-to-amorphous transition at 10 K). PL spectra of the photoinduced a-ZnSe measured at 10 K apparently show both the Stokes-shift and near band-edge broad peaks centered at ～1.5 and ～3.0 eV, respectively. In this work, we discuss a series of PL characteristics in a-ZnSe using a simple model based on valence-alternation pairs.