TEM characterization of Cr-doped ZnS Thin Films for Solar Cell applications
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.
IdentifiersURN: urn:nbn:no:ntnu:diva-25057Local ID: ntnudaim:8686OAI: oai:DiVA.org:ntnu-25057DiVA: diva2:729429
Holmestad, Randi, ProfessorSæterli, RagnhildReenaas, Turid Worren