Optical Properties and Composition of Fe:ZnS Films
Iron doped ZnS thin films were deposited using the physical vapor deposition technique over a concentration range of 0-12% Fe. The thickness and composition of the films were measured with a profilometer and EDS, respectively, and confirmed a desired gradient composition of the films obtained by the geometrical separation of the zinc sulfide and iron sources. XPS revealed that Fe2+ had successfully substituted Zn2+ atoms, incorporating Fe in the ZnS structure while XRD and TEM indicated that the incorporation of Fe stabilizes small grain growth in the films. The latter technique confirmed a hexagonal structure.
Based on initial results, films made with Fe and ZnS were thought to be sulfur deficient and were annealed in a sulfur atmosphere. Additional Fe:ZnS films were deposited using FeS as iron source. However, poorer quality films were produced with this method since the films had a greater absorption and had non-uniform refractive indices.
The optical properties of the PVD films were measured and compared with those of films made under UHV conditions. Absorption studies of transmission measurements and various theoretical models were used to reveal an increase in absorption with increased Fe content, in addition to a broadened Urbach tail. The real part of the refractive index at a wavelength of 1000nm and the direct band gap of the films seemed to be unaffected by the doping level with estimated values of 2.29 and 3.5eV, respectively. While the real part of the refractive index is in agreement with published values, the direct band gap is lower than the theoretical values for ZnS. This is attributed to nano-crystalline grains found in the films.
A well isolated Fe2+ absorption at 2 700 nm was observed for the thick films made under UHV conditions. This suggests that Fe:ZnS thin films may be useful material for saturable absorbers or mid-IR waveguide lasers.
Place, publisher, year, edition, pages
Institutt for fysikk , 2014. , 149 p.
IdentifiersURN: urn:nbn:no:ntnu:diva-25657Local ID: ntnudaim:11239OAI: oai:DiVA.org:ntnu-25657DiVA: diva2:738677
Gibson, Ursula, Professor