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Influence of post-hydrogenation upon electrical, optical and structural properties of hydrogen-less sputter-deposited amorphous silicon

: Gerke, S.; Becker, H.W.; Rogalla, D.; Singer, F.; Brinkmann, N.; Fritz, S.; Hammud, A.; Keller, P.; Skorka, D.; Sommer, D.; Weiss, C.; Flege, S.; Hahn, G.; Job, R.; Terheiden, B.


Thin solid films 598 (2016), pp.161-169
ISSN: 0040-6090
Bundesministerium für Umwelt, Naturschutz, Bau und Reaktorsicherheit BMUB
Journal Article
Fraunhofer ISE ()

Amorphous silicon (a-Si) is common in the production of technical devices and can be deposited by several techniques. In this study intrinsic and doped, hydrogen-less amorphous silicon films are RF magnetron sputter deposited and post-hydrogenated in a remote hydrogen plasma reactor at a temperature of 370 degrees C. Secondary ion mass spectrometry of a boron doped (p) a-Si layer shows that the concentration of dopants in the sputtered layer becomes the same as present in the sputter-target. Improved surface passivation of phosphorous doped 5 Omega cm, FZ, (n) c-Si can be achieved by post-hydrogenation yielding a minority carrier lifetime of similar to 360 mu s finding an optimum for similar to 40 nm thin films, deposited at 325 degrees C. This relatively low minority carrier lifetime indicates high disorder of the hydrogen-less sputter deposited amorphous network. Post-hydrogenation leads to a decrease of the number of localized states within the band gap. Optical band gaps (Taucs gab as well as E-04) can be determined to similar to 1.88 eV after post-hydrogenation. High resolution transmission electron microscopy and optical Raman investigations show that the sputtered layers are amorphous and stay like this during post-hydrogenation. As a consequence of the missing hydrogen during deposition, sputtered a-Si forms a rough surface compared to CVD a-Si. Atomic force microscopy points out that the roughness decreases by up to 25% during post-hydrogenation. Nuclear resonant reaction analysis permits the investigation of hydrogen depth profiles and allows determining the diffusion coefficients of several post-hydrogenated samples from of a model developed within this work. A dependency of diffusion coefficients on the duration of post-hydrogenation indicates trapping diffusion as the main diffusion mechanism. Additional Fourier transform infrared spectroscopy measurements show that hardly any interstitial hydrogen exists in the post-hydrogenated a-Si layers. The results of this study open the way for further hydrogen diffusion experiments which require an initially unhydrogenated drain layer.