Properties of hydrogen molecules in semi-conductors
Final Report Abstract
Hydrogen is an important, highly reactive impurity in many semiconductor materials. The project aimed at gaining a better understanding of the properties of hydrogen incorporated in semiconductors. For this purpose, hydrogen-related defects were created in single-crystalline germanium and silicon by exposure of the samples to a hydrogen plasma and studied mainly by means of Raman scattering, concentrating on the special case of hydrogen interacting with itself forming H2 molecules. Hydrogen is the only atmosphere suitable for the growth of high purity Ge single crystals. In the first part of the project it was shown that exposure of single crystalline Ge to a hydrogen plasma can result in the formation of extended planar defects called platelets. A Raman signal at 1980 cm"1 was assigned to local vibrational modes (LVMs) of Ge-H bonds constituting these platelets, whereas a mode at 4155 cm"1 was shown to result from molecular hydrogen trapped within these platelets. The influence of the temperature during plasma treatment was examined and the thermal stability of the platelets was studied by isochronal annealing. Based on isotope substitution experiments and comparison of the measured frequencies to the results of ab initio calculations, two Raman signals at 3826 and 3834 cm""1 were assigned to ortho- and para-H2 interstitially trapped at the T site of the Ge host lattice. Preliminary Raman studies had suggested that, contrary to ortho-H2 (nuclear spin 1), the para-H2 (nuclear spin 0) species is unstable against room temperature annealing and band gap illumination, which was tentatively explained by different diffusivities of the two species. In the present project, Si wafers were exposed to a remote RF hydrogen and/or deuterium plasma. Two Raman signals at 3199 and 3193 cm^1 were identified as ro-vibrational transitions of interstitial HD. In Cz Si, three lines at 3727, 3733, and 3740 cm"1 were assigned to ro-vibrational modes of hydrogen molecules trapped near interstitial oxygen forming an O-H2 complex. By monitoring the trapping kinetics of interstitial H2 at oxygen it was found that ortho- and para-H2 have similar diffusivities. The temperature dependence of the ortho-para ratio of interstitial H2 lead to the conclusion that the previously reported absence of the para species in Raman spectra at elevated temperatures is due to thermally activated excitation of the molecule from the J ~ 0 to the J = 2 rotational state. Due to the Pauli principle, the conversion from the ortho state with J = 1 to the ground state with J = 0 is not allowed for isolated H2- However, the presence of a nearby magnetic moment renders this transition allowed and it is well documented for hydrogen in the solid and liquid phases, but far less has been done in relation to isolated hydrogen trapped in semiconductors. In our experiments, FZ and Cz Si samples were stored at 77 K in liquid nitrogen after exposure to a RF hydrogen plasma to create interstitial H2. With increasing storage time at 77 K, the concentration of the ortho-H2 species decreases, whereas the concentration of the para-H2 species increases. This suggests that an ortho-to-para conversion of ortho-H2 to the ground state with J = 0 occurs. The orthopara conversion was also observed for H2 trapped within O-H2 complexes in Cz Si and is suggested to be caused by interaction of H2 with the nuclear magnetic moment of the silicon isotope 29Si. At 300 K, the reverse para-to-ortho transition was observed.
Publications
- M. Miller, E.V. Lavrov, and J. Weber Hydrogen-induced platelets in Ge determined by Raman scattering, Phys. Rev. B 71, 045208 (2005).
- M. Hiller, E.V. Lavrov, and J. Weber, Raman scattering study of H2 in Si, Phys. Rev. B 74, 235214 (2006).
- M. Hiller, E.V. Lavrov, and J. Weber, Raman spectroscopy of hydrogen molecules in germanium, Physica B 376-377, 142 (2006).
- M. Hiller, E.V. Lavrov, and J. Weber, Ortho-Para Conversion of Interstitial H2 in Si, Phys. Rev. Lett. 98, 055504 (2007).
- H.F.W. Dekkers, G. Beaucarne, M. Hiller, H. Charifi, and A. Slaoui, Molecular hydrogen formation in hydrogenated silicon nitride, Appl. Phys. Lett. 89, 211914 (2006).
- J. Weber, M. Hiller, and E.V. Lavrov, Hydrogen in germanium, Mater. Sei. Semicond. Proc, 9, 564 (2006).
- J. Weber, M. Hiller, and E.V. Lavrov, Hydrogen molecules in semiconductors, Physica B 401-402, 91 (2007).
- J. Weber, T. Fischer, E. Hieckmann, M. Killer, and E.V. Lavrov, Properties of hydrogen induced voids in silicon, J. Phys. Cond. Matter 17, 2303 (2005).
- M. Hiller, E.V. Lavrov, and J. Weber, A Raman scattering study of HZ trapped near O in Si, Physica B 401-402, 97 (2007).
- M. Killer Properties of Interstitial H2 in Silicon and Germanium - A Raman Scattering Study, Dissertation, Technische Universität Dresden (2007).
- M. Killer, E.V. Lavrov, J. Weber, B. Hourahine, R. Jones, and P.R. Briddon, Interstitial H2 in germanium by Raman scattering and ab initio calculations, Phys. Rev. B 72, 153201 (2005).
