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Materialien/Werkstoffe Poster: Do., 13:00–15:30 D-P383
In situ X-Ray Diffraction Study of the Growth of the Layer-Type Semiconductor
WS2
Klaus Ellmer 1 , Stephan Brunken 1 , Rainald Mientus 2 , Stefan Seeger 1
1 Hahn-Meitner-Institut, Abt. Solare Energetik, Glienicker Str. 100, 14109 Berlin, Germany
– 2 Opto-Transmitter-Umweltschutz-Technologie e.V., 12555 Berlin, Köpenicker
Str.325b Germany
Transition metal dichalcogenides like WS2 are layer-type semiconductors with energy
band gaps and absorption coefficients which make them candidates for absorber layers
in thin film solar cells. In order to use the advantageous (weak) van der Waals bonding
of the sulfur-terminated (001) lattice planes in these materials thin films have
to be prepared with a strong (001) texture. By time-resolved energy-dispersive X-ray
diffraction (EDXRD) at the beamline F3-HASYLAB, we found that the films always
nucleate with the (001) planes, i.e. the van der Waals planes, parallel to the substrate
surface. For high deposition rates and/or low substrate temperatures a texture crossover
from the (001) to the (100) crystallite orientation occurs during the growth. High
deposition rates, low substrate temperatures or low sputtering pressures lead to a significant
lattice expansion of the crystallites in c direction (up to 3 %). This is most
probably caused by a disturbed or turbostratic film growth induced by the energetic
bombardment during film deposition. Reflected and neutralized energetic ions (Ar 0 ,
S 0 ) from the tungsten target and negative ions (S − ) accelerated in the cathode dark
space constitute the main sources of the energetic bombardment leading to crystallographic
defects. The energy of these particles can be tailored by (i) thermalization
between target and substrate in the sputtering gas or (ii) by a reduction of the discharge
or target voltage, respectively, by a high frequency excitation (13 or 27 MHz) of the
plasma. Another thin film preparation route, which avoids ion-bombardment-induced
defects, is the crystallization of an amorphous sulfur-rich WS3+x (0