Elsevier

Applied Surface Science

Volume 493, 1 November 2019, Pages 882-888
Applied Surface Science

Full length article
Oxidation simulation study of silicon carbide nanowires: A carbon-rich interface state

https://doi.org/10.1016/j.apsusc.2019.07.016Get rights and content

Highlights

  • SiC|C|SiOx core-shell structure can be obtained through thermal oxidation of SiCNWs.

  • A saturation emit mechanism is proposed to account for generation of C-rich layer.

  • SiO2 layer narrows the band gaps of the SiCNWs matrix.

  • Accumulated C at the interface could offset the CB and VB.

Abstract

Silicon carbide nanowires (SiCNWs) have attracted increasing attention due to their excellent properties and wide range of potential applications. SiCNWs covered with oxide layer, which can be simply prepared by thermal oxidation, show a number of fascinating functional prospects. However, the oxidation mechanism of the SiCNWs is barely explored. Here we studied oxidation of SiCNWs at 300–1500 K by using the reactive molecular dynamics simulations with ReaxFF potential. The temperature-dependent oxidation processes and the obtained structures were discussed in detail. The results reveal that self-limiting oxidation of the SiCNWs can switch from the rate-determining step of chemical reaction at the interface to diffusion limited oxidation after a definite time. In addition, the SiC|C|SiOx core-shell structure was proved to be formed during the oxidation from a theoretical view. A saturation emit mechanism was proposed to explain why the C-rich layer is too thin to be detected. DFT calculations were employed to study electronic structure of the core-shell structures, indicating the SiO2 layer apparently narrows band gaps of the SiCNWs, and the accumulated C at the interface could offset conduction band and valence band, resulting in instability problem during application of the SiCNWs.

Introduction

As a post silicon (Si) functional material, silicon carbide (SiC) has been recognized to be promising for electronic and optoelectronic devices in harsh environments due to its superior physical properties over the Si, such as wide band gap, high breakdown electric field and high thermal conductivity, as well as for biocompatible material due to its chemical inertness and hydrophilic surface [[1], [2], [3], [4]]. As compound semiconductor, SiC is reported to be the only one that can be thermally oxidation to form silica (SiO2) layer, which benefits for the fabrication of metal-oxide-semiconductor field-effect transistors (MOSFETs) because the oxide layer is a natural insulation and passivation layers [5,6]. In particular, SiC nanowires (SiCNWs), as a one-dimension (1D) nanometer materials, not only inherit excellent intrinsic properties of SiC, but also feature high mechanical properties, direct band gap, and optical activity due to the quantum confinement effects [[7], [8], [9]]. For SiCNWs, the coated SiO2 layer could offer a host of advantages and modified possibilities, making the SiC/SiO2 core-shell structure a potential candidate in many fields, such as field emission [8,10], electromagnetic wave absorbing [7,11,12], optical [[13], [14], [15], [16], [17]], superhydrophobic [18], chemo-/biosensing [19], biomedical applications [20], and so forth. However, the existence of SiO2 oxide layer generates huge interface state densities, causing electron mobility of SiC-based power devices is far lower than the theoretical value [1,3,21]. This results the energy waste and impedes application of the SiC-based power devices. In contrast, Si-based MOSFET does not form localized electron traps at the interface between the Si and its native SiO2 layer, namely, it is inherently free from the trapping and scattering of carriers. Therefore, understanding the interface components and cause of interface state densities in the SiCNW core-shell structure is crucial for effective passivation of defect state at the interface, which will promote application of the SiCNWs in the MOSFET devices.

Experimental and theoretical efforts have found that the carbon (C) related defects are the main reason for the trapping of carries [[22], [23], [24], [25], [26], [27], [28], [29], [30], [31]], which acts as a killer of the carrier mobility. Thus, to figure out whether accumulated C exists at the SiC/SiO2 interface has attracted great attentions worldwidely [1]. Some experimental results have proven there are C-related species (e.g., sp2-bonded and graphite-like clusters) near the interface [22,23,28,29]; while the C-rich layers are not detected in other certain researches [32,33]. The conflicting conclusions are probably because the accumulated C near the interface is out of experimental detection range. Another effective way to understand the interface nature of SiC/SiO2 core-shell structure at an atomic level is to perform in situ oxidation study of SiCNW by computer simulations, which is crucial but has not been reported yet as far as we know.

In the present work, reactive molecular dynamics (RMD) simulations were performed to investigate the oxidations of 3C-SiCNWs at various temperatures. The temperature-dependent oxidation process and the obtained oxidized structures were discussed in detail. In order to characterize the effect of SiO2 layer and C related defects on the SiCNWs, periodic DFT calculations were used to study electronic structure of the slab models. The results are helpful for prompting applications of the SiCNWs in the MOSFET devices.

Section snippets

MD simulations

MD simulations were performed by using the reactive force field (ReaxFF) for C/Si/O system. This ReaxFF potential developed by van Duin et al. [34] has been tested and used successfully in the field of pyrolysis and oxidation for C/Si/H/O system [[34], [35], [36], [37], [38]]. Therefore, the ReaxFF potential from Ref. 34 was used to describe the forces on the atoms in our C/Si/O system. According to our previous work, 3C-SiCNWs oriented along the [111] directions are energetically stable [39].

Oxidation and oxide layer growth process of the SiCNWs

Fig. 2(a) illustrates the O2 consumptions of the SiCNW 10 nm in diameter during the oxidation simulations at different temperatures. Total O2 consumptions are <10% in our MD simulations to keep the O2 environment as stable as possible. The slope of O2 consumption over oxidation time could reflect the violence degree of the oxidation reaction and show a positive correlation between them. The O2 consumptions at low temperatures are a little higher than those at high temperatures in the very

Conclusions

In this work, we studied the oxidation mechanism of the SiCNWs with diameters of 4, 6, and 10 nm over a temperature range of 300–1500 K by using RMD simulations. The MD results reveal that the self-limiting oxidation of SiCNWs changes from rate-determining step of chemical reaction at the interface to the diffusion limited oxidation after a definite time, e.g., 50 ps oxidation time (0.7 nm oxide thickness). Besides, we provided a theoretical evidence that a C-rich layer is formed at the

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51772237), and Shaanxi Science & Technology Co-ordination & Innovation Project (No. 2015KTCL01-13), and Shaanxi Innovation Capacity Support Program (2018TD-031).

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