Transition from anomalous kinetics towards Fickian diffusion for Si dissolution into amorphous Ge

TL;DR

This study investigates the diffusion of amorphous silicon into amorphous germanium, revealing a transition from anomalous to Fickian diffusion behavior as the process progresses.

Contribution

First experimental observation of the transition from anomalous to Fickian diffusion in amorphous materials during Si-Ge interdiffusion.

Findings

Initial diffusion coefficient exponent kc ≈ 0.7 indicating anomalous diffusion

Transition to classical Fickian diffusion with kc ≈ 0.5 after several monolayers

Diffusion behavior differs from previous crystalline-based models

Abstract

Over the last years several experimental and theoretical studies of diffusion kinetics on the nanoscale have shown that the time evolution differs from the classical Fickian law (kc=0.5). However, all work was based on crystalline samples or models, so far. In this letter, we report on the diffusion kinetics of a thin amorphous-Si layer into amorphous-Ge to account for the rising importance of amorphous materials in nanodevices. Employing surface sensitive technics, the initial kc was found at 0.7+-0.1. Moreover, after some monolayers of Si dissolved into the Ge, kc changes to the generally expected classical Fickian law with kc=0.5.