Self-Diffusion of Ge in Amorphous Ge x Si1-x Films Studied In Situ by Neutron Reflectometry

Ge x Si1-x alloys are gaining renewed interest for many applications in electronics and optics, especially for miniaturized devices showing quantum size effects. Point defects and atomic diffusion play a crucial role in miniaturized and metastable systems. In the present work, Ge self-diffusion in sputter deposited amorphous Ge x Si1-x alloys is studied in situ as a function of Ge content x = 0.13, 0.43, 0.8, and 1.0 by neutron reflectometry. The determined Ge self-diffusivities obey the Arrhenius law in the investigated temperature ranges. The higher the Ge content x, the higher the Ge self-diffusivity at the same temperature. The activation enthalpy decreases with x from 4.4 eV for self-diffusion in pure silicon films to about 2 eV self-diffusion in Ge0.8Si0.2 and Ge. The decrease of the activation enthalpy for amorphous Ge x Si1-x is similar to the case of crystalline Ge x Si1-x . Possible explanations are discussed.

Analysis of the Diffusion in a Multilayer Structure under a Constant Heating Rate: The Calculation of Activation Energy from the In Situ Neutron Reflectometry Measurement

Understanding mass transport in micro- and nanostructures is of paramount importance in improving the performance and reliability of the micro- and nanostructures. In this work, we solve the diffusion problem in a multilayer structure with periodic conditions under a constant heating rate via a Fourier series. Analytical relation is established between the coefficients of eigenfunctions and the intensity of X-ray or neutron Bragg peak. The logarithm of temporal variation of the intensity of X-ray or neutron Bragg peak is a linear function of the nominal diffusion time, with the nominal diffusion time being dependent on the heating rate. This linear relation is validated by experimental data. The Taylor series expansion of the linear relation to the first order of the diffusion time yields an approximately linear relation between the logarithm of temporal variation of the intensity of X-ray or neutron peak and the diffusion time for small diffusion times, which can be likely used to calculate the activation energy for the diffusion in a multilayer structure. The validation of such an approach is subjected to the fact that the characteristic time for heat conduction is much less than the characteristic time for the ramp heating as well as the characteristic time for diffusion.

Neutron reflectometry to measure in situ the rate determining step of lithium ion transport through thin silicon layers and interfaces

Li ion transport through thin (14-22 nm) amorphous silicon layers adjacent to lithium metal oxide layers (lithium niobate) was studied by in situ neutron reflectometry experiments and the control mechanism was determined. It was found that the interface between amorphous silicon and the oxide material does not hinder Li transport. It is restricted by Li diffusion in the silicon material. This finding based on in situ experiments confirms results obtained ex situ and destructively by secondary ion mass spectrometry (SIMS) depth profiling investigations. The Li permeabilities obtained from the present experiments are in agreement with those obtained from ex situ SIMS measurements showing similar activation enthalpies.