Morphology and chemical composition of cobalt germanide islands on Ge(001): in-situ nanoscale insights into contact formation for Ge-based device technology

TL;DR

This study investigates the growth, morphology, and composition of cobalt germanide islands on Ge(001) using in-situ and ex-situ microscopy and spectroscopy techniques, revealing diverse island shapes, phases, and epitaxial relationships relevant for Ge-based device contacts.

Contribution

It provides detailed nanoscale insights into the morphology, stoichiometry, and phase composition of cobalt germanide islands grown on Ge(001), including identification of multiple phases and epitaxial relationships.

Findings

CoGe₂ is the dominant phase with unique epitaxial orientation.

Multiple island morphologies and facets are observed.

Intermediate phases Co₅Ge₇ and CoGe are present, influenced by substrate defects.

Abstract

The reactive growth of cobalt germanide on Ge(001) was investigated by means of in-situ x-ray absorption spectroscopy photoemission electron microscopy (XAS-PEEM), micro-illumination low-energy electron diffraction ($\mu$-LEED), and ex-situ atomic force microscopy (AFM). At a Co deposition temperature of 670{\deg}C, a rich morphology with different island shapes and dimensions is observed, and a correlation between island morphology and stoichiometry is found. Combining XAS-PEEM and $\mu$-LEED, we were able to identify a large part of the islands to consist of CoGe$_2$, with many of them having an unusual epitaxial relationship: CoGe$_2$[$\bar110$](111) $\parallel$ Ge[$\bar110$](001). Side facets with (112) and (113) orientation have been found for such islands. However, two additional phases were observed, most likely Co$_5$Ge$_7$ and CoGe. The occurrence of these intermediate phases is promoted by defects, as revealed by comparing growth on Ge(001) single crystals and on Ge(001)/Si(001) epilayer substrates.