Modeling the Initial Stages of Si(100) Thermal Oxidation: An Ab-initio Approach

Silicon together with its native oxide SiO$_2$ was recognized as an outstanding material system for the semiconductor industry in the 1950s. In state-of-the-art device technology, SiO$_2$ is widely used as an insulator in combination with high-$k$ dielectrics such as HfO$_2$, demanding fabrication of ultra-thin interfacial layers. The classical standard model derived by Deal and Grove accurately describes the oxidation of Si in a progressed stage, however, strongly underestimates growth rates for thin oxide layers. Recent studies report a variety of oxidation mechanisms during the growth of oxide films in the range of \SI{10}{\angstrom} with various details still under debate. This paper presents a first-principles based approach to theoretically assess the thermal oxidation process of the technologically relevant Si(100) surfaceduring this initial stage. Our investigations range from the chemisorption of single O$_2$ molecules onto the $p(2\times2)$ reconstructed Si surface to oxidized Si surface layers with a thickness of up to \SI{20}{\angstrom}. The initially observed enhanced growth rate is assigned to barrierless O$_2$ chemisorption events upon which the oxygen molecule dissociate. We present strong evidence for an immediate amorphization of the oxide layer from the onset of oxidation. Surface reactions dominate until the surface is saturated with oxygen and separated from the Si substrate by a \SI{5}{\angstrom} transition region. The saturated surface becomes inert to dissociative reactions and enables the diffusion of molecular oxygen to the \interface interface as assumed within the Deal-Grove model. Further oxidation of the Si substrate is then provided by O$_2$ dissociations at the interface due to the same charge transfer process responsible for the chemisorption at the surface.