Fabrication of In₂(Se, Te)₃ chalcogenide thin films by thermal co-evaporation

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2001.Cataloged from PDF version of thesis. "May 2001."Includes bibliographical references (pages 46-47).In₂(Se, Te)₃ III-VI chalcogenides belong to a unique class of phase change materials that have interesting optical and electrical properties, making them suitable for a wide variety of applications, including absorbers for solar cells and storage materials in memory devices. A greater understanding of how different growth parameters influence the crystallinity and microstructure of such chalcogenide thin films can lead to an enhanced ability to manipulate the materials for desired optoelectrical characteristics. The purpose of the following thesis was threefold. The first was to fabricate homogeneous, single-phase thin films of In2(Se, Te)₃ using thermal co-evaporation of elemental In and (Se, Te) in a high vacuum vapor deposition chamber. The In₂(Se, Te)₃ samples prepared by this method were found to be single phase textured films. Since re-evaporation of (Se, Te) from the films has previously resulted in deviations from the stoichiometric In₂(Se, Te)₃ compound [1], the second element of this thesis involved the microstructural characterization of films deposited with an excess of (Se, Te). The results from XRD and AFM reveal that after annealing the films the excess material does not manifest itself in any observable manner. Preliminary results from RBS and EDS reveal that some of the excess material may actually be evaporating through the 50 Å A1₂O₃ capping layer deposited on the film's surface, though further analysis with Auger and XPS will be necessary to enhance the understanding of what happens to the excess material. The third element was to assess how temperature and duration of post deposition thermal treatment influenced the crystal structure and surface morphology of the films. The films were annealed at temperatures ranging from 473 to 673K for 5 minutes, 1 hour, and 4 hours. Results from XRD showed that vacuum annealing of the samples at temperatures above 623K for times above 1 hour consistently produced well-oriented thin films of high crystalline quality. Higher annealing temperature resulted in films with higher crystallinity, whereas annealing durations longer than 1 hour did not contribute significantly to the film phase or crystallinity. AFM measurements of surface morphology before and after annealing showed that the roughness of the films before annealing was on the order of a few angstroms, whereas large, distinct grains and surface inhomogeneites were present on the sample surface after annealing. Again, no observable change was reported for films with excess (Se, Te) indicating that the single-phase compound that formed was very stable.by Shikha Gupta.S.B