Papers by Keyword: Isotope Hetero-Structures

Abstract: Lithium transport through ultrathin silicon layers can be measured non-destructively by neutron reflectometry (NR) using a multilayer composed of silicon layers embedded between solid state Li reservoirs. An established model system is a multilayer with a repetition of five [Si / ^natLiNbO₃ / Si / ⁶LiNbO₃] units. Two types of Bragg peaks are detectable in reflectivity patterns. These Bragg peaks result from the interference of neutrons reflected at periodic interfaces. One type of Bragg peak originates from the periodicity of the LiNbO₃/Si chemical contrast (first order peak), while the other Bragg peak results from a superstructure with double periodicity. This superstructure may arise from ⁶Li/⁷Li isotope contrast or alternatively from periodic thickness variations, as shown by simulations based on the Parratt algorithm. The intention of the present paper was to elucidate the origin of the second Bragg peak. Experiments done by Secondary Ion Mass Spectrometry (SIMS) isotope sensitive depth profiling showed in a direct way that annealing at 360 °C destroys indeed the ⁶Li/⁷Li contrast, whereas the LiNbO₃/Si chemical contrast remains unchanged. This evidences that the experimentally observed decrease of the second Bragg peak in the reflectivity pattern during annealing is a measure for Li transport through the Si layer.

Abstract: The diffusion of lithium in amorphous lithium niobate layers is studied as a function of temperature between 293 and 423 K. About 800 nm thick amorphous ⁷LiNbO₃ layers were deposited on sapphire substrates by ion-beam sputtering. As a tracer source about 20 nm thin ⁶LiNbO₃ layers were sputtered on top. Isotope depth profile analysis is done by secondary ion mass spectrometry. Compared are amorphous samples which show a ratio of Li : Nb < 1 (Li-poor) and of Li : Nb > 1 (Li-rich) close to the stoichiometric composition of Li : Nb = 1 for crystalline LiNbO₃. The results reveal that the diffusivities of both types of samples obey the Arrhenius law with an activation enthalpy of 0.70 eV and 0.83 eV, respectively. The diffusivities of the sample containing a higher amount of Li are lower by a factor of about two to ten. This demonstrates that variation of the Li content in amorphous samples over the stability range of the crystalline LiNbO₃ phase has only a modest influence on diffusivities and activation enthalpies.

Abstract: We investigated lithium self-diffusion in amorphous and single crystalline lithium niobate at low temperatures of 323, 423 and 623 K. The diffusivity was studied by secondary ion mass spectrometry (SIMS), using ion beam sputtered ⁶LiNbO₃ as a tracer source. Our intention was to get information how structural disorder influences ionic diffusivity, while chemical composition remains unchanged. The results indicate an increase of the Li diffusivity by about eight orders of magnitude in the amorphous state.