Author:

Keywords:

Ge1-xSnx (GeSn), Epitaxial growth, Ge, Chemical vapor deposition, Ge2H6, SnCl4

Abstract:

Thanks to their unique crystalline, optical and electrical properties, Ge1-xSnx alloys have emerged as group IV semiconductor materials with potential major impacts on future CMOS and photonic applications. However, the growth of high quality Ge1-xSnxnbsp;is challenged by numerous factors,nbsp;an equilibrium solid solubilitynbsp;Sn in Ge below 1 at.%. This Ph.D. work proposes a novel chemical vapor deposition approach for Ge1-xSnx growth basednbsp;the pioneering combination of two stable commercially-available Ge and Sn precursors, namely Ge2H6 and SnCl4 . This low-temperature atmospheric pressure approach allows the growth of metastable Ge1-xSnx films with substitutionalnbsp;contents as high as 12.4 at.%, whose quality is demonstrated by anbsp;of typical implementations for CMOS and photonic devices. The objective of this thesis isnbsp;On the one hand, this work focuses on the investigation ofnbsp;kinetics and the chemicalnbsp;involvednbsp;the epitaxial growth mechanism ofnbsp;chemical vapornbsp;In this context, Ge2H6 is characterizednbsp;a promising Ge precursor for low temperature chemical vapor deposition and a specific surface reaction is proposed to explain the growth mechanism on H passivated surfaces. For each SnCl4 partial pressure, a minimum critical Ge2H6 partial pressure exists in order to grow continuous monocrystallinenbsp; layers devoid of any phase separation.nbsp;physical origin of such criticalnbsp; partial pressure is thoroughly investigated. In addition, this work aims at gaining a profound understanding of the Ge1-xSnx materials properties.nbsp;X-ray absorption fine structure measurements are used to probe the local environment of Sn atoms in strained and relaxed Ge1-xSnxnbsp;layers with different compositions, revealing that they are covalently bonded to four Ge atoms in a tetrahedral configuration. The analysisnbsp;the crystalline properties ofnbsp;grown layers indicates a positive deviation from Vegard’s law and allows the extraction of a new experimental bowing parameter in agreement with density functional theory predictions. The principal strain relaxation mechanism through whichnbsp;excess strain energy stored in the growing Ge1-xnbsp;layers is relieved is the formation of misfit dislocations at its interface with Ge. However,nbsp;growth of thick strain-relaxed Ge1-xnbsp;layers - which are particularly interesting for their expected direct bandgap and for their ability to induce tensile stress in a Ge overlayer - is complicated by Sn precipitation and by the development of particular island-type features with annbsp;core.