Substrate influence on the X-ray diffraction patterns of amorphous chalcogenide thin films deposited on silicon wafers
Introduction
Structural changes induced in amorphous chalcogenide thin films due to different factors (changes in composition, metal-doping or -alloying, annealing, exposure to radiation) have been widely studied and reported in the literature [1], [2], but the topic is still under investigation.
Silicon substrate plays a significant role in crystallization, photo-vitrification and photo-oxidation of As50Se50 layers [3]. To judge the effect of the substrate is not easy because a perfect crystallographical orientation of the Si substrate can contribute to large maxima in the region of the first sharp diffraction peak (FSDP).
The possibility of avoiding false peaks for films having low texture was shown by Zevin [4]. He observed that using a weak asymmetry in the angle θ, between the incident X-ray beam, and the analyzed sample surface during a measurement, it is possible to eliminate the diffraction peaks arising from the single crystal.
The maxima in the diffraction pattern arising from silicon substrate have been remarked before by Collard and Hoyer [5] for carbon-nitride films. They indicated the possibility of rotating the sample around the axis perpendicular to the film surface, in order to eliminate spurious peaks.
During X-ray diffraction (XRD) measurements, in many cases a very strong escape-peak was observed, situated in the FSDP's region. The escape-peak exhibits an asymmetric shape, and is very sensitive in intensity to the wafer's position in its own plane.
This paper carefuly analyzed the effect of rotation on the non-sample peak in the angular region of the FSDP. Two methods were proposed to minimize or to eliminate the distorsion of the FSDP, induced by the silicon wafer substrate.
Section snippets
Experimental
Thin amorphous films with the composition Ge27Sb13Se60 and thickness between 2 and 3 μm, as well as tin-doped AsSe amorphous films of approximately 5 μm in thickness, deposited on silicon (100) wafers were investigated.
In the case of the AsSe:Sn thin films, three samples have been studied: fresh As50Se50 (A0), fresh (As50Se50)90Sn10 (A10N) and UV-exposed (As50Se50)90Sn10 (A10L). Ge27Sb13Se60 thin films have been investigated through four samples: fresh film (FN), fresh and UV-exposed film (FL),
Results and discussion
According to the disordered layer-like amorphous model, the first sharp diffraction peak (FSDP) region of the X-ray diffractogram of an amorphous material is the most sensitive for any modification suffered by the material in its medium range order (MRO) structure.
The initial X-ray diagrams recorded for several samples have shown very large differences in intensity and peak position, in the FSDP region (13–16° in 2θ, corresponding to a quasi-periodic spacing in the range of 5.5–6.8 Å), even for
Conclusions
XRD measurements of the MRO characteristics of glassy or amorphous materials require accurate settings of the diffractometer conditions, in order to obtain correct and reproducible diagrams. This is especially valid in the case of amorphous thin films, which give a significantly lower diffraction intensity, because of the small amount of material, which interacts with the X-ray beam. There are cases when the used substrate of the thin film gives a strong broad peak near to the FSDP, or even
Acknowledgments
Thanks for the discussions, ideas and support to Dr. M. Popescu from the National Institute of Materials Physics, Bucharest, as well as to Prof. Dr. Hab. W. Hoyer, Dr. H. Giegengack, Dr. S. Collard, and F. Holl from the TU-Chemnitz. Thanks to Dr. Hab. M. Iovu from the Institute of Applied Physics, Chishinau, Moldova, for preparing the AsSe:Sn samples, and to Dr. V. Pamukchieva from the ‘G. Nadjakov’ Institute of Solid State Physics, Sofia, Bulgaria for preparing the Ge27Sb13Se60 samples. The
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