Abstract

The ability to accurately and consistently determine the surface electronic properties of polar materials is of great importance for device applications. Polar surface modelling is fundamentally limited by the spontaneous polarisation of these materials in a periodic boundary condition scheme. Surface data are sensitive to supercell parameters, including slab and vacuum thicknesses, as well as the non-equivalence of surface adsorbates on opposite surfaces. Using 4H–SiC as a specific case, this study explores calculation of electron affinities (EAs) of (000) and (0001) surfaces varying chemical termination as a function of computational parameters. We report the impact in terms of band-gap, electric fields across the vacuum and slab for single and double cell slab models, where the latter is constructed with inversional symmetry to eliminate the electric field in the vacuum regions. We find that single cells are sensitive to both slab and vacuum thickness. The band-gap narrows with slab thickness, ultimately vanishing and inducing charge transfer between opposite surfaces. This has a consequence for predicted EAs. Adsorbate species are found to play a crucial role in the rate of narrowing. Back to back cells with inversional symmetry have larger electric fields present across the slab than the single slab cases, resulting in a greater band-gap narrowing effect, but the vacuum thickness dependence is completely removed. We discuss the relative merits of the two approaches.

Highlights

  • Silicon carbide, and in particular its 4H-polytype, exhibits an array of attractive physical and electrical properties, including high thermal-conductivity, large breakdown-field and high carrier-mobility [1, 2]

  • We report the impact in terms of band-gap, electric fields across the vacuum and slab for single and double cell slab models, where the latter is constructed with inversional symmetry to eliminate the electric field in the vacuum regions

  • We have recently reported the electron affinities (EAs) of nonpolar surfaces of 4H–SiC [7], but there is a paucity of data relating to EAs of polar SiC surfaces

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Summary

Introduction

In particular its 4H-polytype, exhibits an array of attractive physical and electrical properties, including high thermal-conductivity, large breakdown-field and high carrier-mobility [1, 2]. A similar approach [13, 14] utilizes pseudo-hydrogen with fractional charges and/or altered surface-adatom distances in conjunction with a self-consistent Laplace correction scheme [13] to eliminate the electric-field in the vacuum. Such additional chemical components introduce challenges in systematically modelling different surface terminations. Due to the bulk polarization, increasing the slab thickness increases the potential difference between the surfaces, reducing and eliminating the band-gap This effect is not solely a property of the PBC. We report the results of density-functional modelling of polar 4H–SiC surfaces, presenting computed values of EAs for H-, F- and Cl-terminated surfaces

Methodology
Electron affinity calculations
Results
Si and C termination
Vacuum thickness
Slab thickness
EA dependence
Full Text
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