Effects of the c-Si/a-SiO₂ interfacial atomic structure on its band alignment: an ab initio study

Abstract

The crystalline-Si/amorphous-SiO (c-Si/a-SiO) interface is an important system used in many applications, ranging from transistors to solar cells. The transition region of the c-Si/a-SiO interface plays a critical role in determining the band alignment between the two regions. However, the question of how this interface band offset is affected by the transition region thickness and its local atomic arrangement is yet to be fully investigated. Here, by controlling the parameters of the classical Monte Carlo bond switching algorithm, we have generated the atomic structures of the interfaces with various thicknesses, as well as containing Si at different oxidation states. A hybrid functional method, as shown by our calculations to reproduce the GW and experimental results for bulk Si and SiO, was used to calculate the electronic structure of the heterojunction. This allowed us to study the correlation between the interface band characterization and its atomic structures. We found that although the systems with different thicknesses showed quite different atomic structures near the transition region, the calculated band offset tended to be the same, unaffected by the details of the interfacial structure. Our band offset calculation agrees well with the experimental measurements. This robustness of the interfacial electronic structure to its interfacial atomic details could be another reason for the success of the c-Si/a-SiO interface in Si-based electronic applications. Nevertheless, when a reactive force field is used to generate the a-SiO and c-Si/a-SiO interfaces, the band offset significantly deviates from the experimental values by about 1 eV.

Computational Study on Interfaces and Interface Defects of Amorphous Silica and Silicon

“…Thus, the transition layer is also called suboxide layer. The XPS data also indicate that the ratio of the partially oxidized silicon atoms [Si(I), Si(II), Si(III)] is 1:2:3 for a‐SiO 2 /Si(100) interfaces and 1:2:1 for the a‐SiO 2 /Si(111), which may also be sensitive to sample preparation and probing techniques . The amorphous nature of a‐SiO 2 and the transition layer have challenged the modeling of the interfaces.…”

“…The MC simulation of a‐SiO 2 /Si interface usually starts with a prepared initial structure, in which crystalline SiO 2 (c‐SiO 2 ) is stacked onto c‐Si, and periodic boundary condition is applied in both normal and lateral directions . The supercell lengths in the lateral directions are fixed and match the lattice constant of c‐Si.…”

“…The supercell lengths in the lateral directions are fixed and match the lattice constant of c‐Si. The length in the normal direction is either determined by the experimental density of a‐SiO 2 or free to relax in the simulation . The 3‐5 central layers of Si atoms are held fixed to simulate the c‐Si region …”

“…Recently, Zheng et al adopted the bond switching MC approach to generate reasonable a‐SiO 2 /Si interfaces without coordination defects. They introduced a potential [Equation ] without the “suboxide” term but constrained the max number ( n ) of Si atoms connected in a chain of Si‐Si bonds.…”

“…One is based on the amorphous structure of SiO 2 , which is generated by using molecular dynamics (MD) or Monte Carlo (MC) simulation . The models generated by this category of approaches may reproduce the key experimental feature of the interface, an amorphous transition layer between a‐SiO 2 and Si that is characterized of the density and local structure parameters different from bulk a‐SiO 2 and, more importantly, the transitions of Si oxidation state and bandgap . The other category is based on crystalline structures of SiO 2 .…”