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TEM characterization of Cr-doped ZnS Thin Films for Solar Cell applications
Norwegian University of Science and Technology, Faculty of Natural Sciences and Technology, Department of Physics.
2014 (English)MasteroppgaveStudent thesis
Abstract [en]

The morphology of three Cr-doped zinc sulfide thin films, one deposited by molec- ular beam epitaxy (MBE) and two by pulsed laser deposition (PLD), have been studied by transmission electron microscopy (TEM). Investigations of the poly- morphic crystal structure of ZnS have been done by analysis of diffraction, bright field and high resolution images. Both similarities and differences in morphology between the three samples have been discovered. An unambiguous determination of the crystal structure could not be done due to the similarities of the possible phases of ZnS. However, evidence show that the MBE sample contains more zinc blende than wurtzite, and that the PLD samples are containing more wurtzite than zinc blende. It is also found that the cross section PLD sample is textured in the growth direction of the film. The MBE sample does not show this texture, but here twin stacking faults are commonly found. The cross section PLD sample was studied in greater detail using energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). EDX analysis revealed relatively large variations in Cr concentration, and a connection between increases in Cr and decreases in Zn was discovered, indicating that Cr is substituting Zn in the ZnS lattice. Determination of the valence state of Cr was done by EELS analysis, and found to be +II, a result supporting the indications of the EDX analysis. ZnS:Cr is an interesting material for realizing the intermediate band solar cell concept using ultrahigh doping levels. Still, the three samples show that the material is underdeveloped at this stage for solar cells, as the thin films are polycrystalline with small grains, and have features that increase recombination.

Place, publisher, year, edition, pages
Institutt for fysikk , 2014. , 82 p.
URN: urn:nbn:no:ntnu:diva-25057Local ID: ntnudaim:8686OAI: diva2:729429
Available from: 2014-06-25 Created: 2014-06-25 Last updated: 2014-06-25Bibliographically approved

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