TEM characterization of Cr-doped ZnS Thin Films for Solar Cell applications

Abstract

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 beenstudied by transmission electron microscopy (TEM). Investigations of the poly-morphic crystal structure of ZnS have been done by analysis of diffraction, brightfield and high resolution images. Both similarities and differences in morphologybetween the three samples have been discovered. An unambiguous determinationof the crystal structure could not be done due to the similarities of the possiblephases of ZnS. However, evidence show that the MBE sample contains more zincblende than wurtzite, and that the PLD samples are containing more wurtzitethan zinc blende. It is also found that the cross section PLD sample is texturedin 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-dispersiveX-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). EDXanalysis revealed relatively large variations in Cr concentration, and a connectionbetween increases in Cr and decreases in Zn was discovered, indicating that Cr issubstituting Zn in the ZnS lattice. Determination of the valence state of Cr wasdone by EELS analysis, and found to be +II, a result supporting the indicationsof the EDX analysis.ZnS:Cr is an interesting material for realizing the intermediate band solar cellconcept using ultrahigh doping levels. Still, the three samples show that thematerial is underdeveloped at this stage for solar cells, as the thin films arepolycrystalline with small grains, and have features that increase recombination.