Adsorption of Group-I and -VII Atoms on Silicon-Carbide Polytypes
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ACES, INTERFACES, AND THIN FILMS
Adsorption of Group-I and -VII Atoms on Silicon-Carbide Polytypes S. Yu. Davydova,* and O. V. Posrednikb a Ioffe
b
Institute, St. Petersburg, 194021 Russia St. Petersburg State Electrotechnical University (LÉTI), St. Petersburg, 197376 Russia *e-mail: [email protected] Received March 20, 2020; revised July 1, 2020; accepted July 13, 2020
Abstract—In the context of Haldane–Anderson’s model, the transferred charge and the energy of adsorption of alkali metal and halogen atoms on the C and Si faces of the 3C-, 6H-, and 4H-SiC polytypes are estimated. The contributions of the band and local states to the formation of the charge of adatoms and the metallic and ionic components of the adsorption energy are clarified. Keywords: adatom, substrate, charge transfer, adsorption energy DOI: 10.1134/S1063782620110275
1. INTRODUCTION Silicon carbide (SiC) distinguished by a large number of polytypes with substantially different band gaps and by a high stability to corrosive media still attracts particular interest [1–6]. However, despite the fact that the adsorbability of materials is an important characteristic from both the fundamental and technological standpoints [7–10], the adsorption properties of silicon carbide remain barely known. In [11–13], we showed the possibility of describing these properties in the context of simple models of adsorption that allow one to derive analytical expressions appropriate for further use not only by theoreticians. The above-mentioned studies were stimulated to a large extent by experimental results [6, 14, 15]. With this study, we begin to conduct systematic investigations of the adsorbability of silicon carbide. Here we consider the adsorption of Group-I and -VII atoms, i.e., the most electrically positive and negative elements, on the Si and C polar faces of the 3C, 6H, and 4H polytypes of SiC. In this study, emphasis is placed on estimations concerning charge transfer and the adsorption energy. 2. MODEL AND NUMERICAL ESTIMATES The model approach to the problem of adsorption was described in [16, 17]. The density of states on an adatom, ρa(ω), can be represented as the Lorentz contour
Γ(ω) (1) . ρa (ω) = 1 π [ω − εa − Λ(ω)]2 + Γ2(ω) Here, ω is the energy variable, εa is the energy level of the adsorbed atom, Γ(ω) = πV2ρsub(ω) is the function of broadening of the quasi-level (ρsub(ω) is the density
of states of the substrate, V is the matrix element of adatom–substrate interaction), and Λ(ω) is the function of the shift of the quasi-level (the Hilbert transform of the function Γ(ω). To describe the density of states of silicon carbide, we make use of Haldane– Anderson’s model [16, 17]: ρsub(ω) = ρs at |ω – E0| ≥ Eg/2 and ρsub(ω) = 0 at |ω – E0| < Eg/2, where E0 = χ + Eg/2 is the energy at the middle of the band gap with respect to the vacuum level, and Eg and χ are the band gap and the electron affinity of the silicon-carbide polytype. Assuming E0 = 0, we obtain Λ(ω) = (Γ/π)ln|(ω – Eg/2)/(ω + Eg/2)|, where Γ ≡ πV2ρs = const. For the 3C, 6H, and 4
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