Computational simulation of the effects of oxygen on the electronic states of hydrogenated 3C-porous SiC
© Trejo et al.; licensee Springer. 2012
Received: 16 May 2012
Accepted: 4 August 2012
Published: 22 August 2012
A computational study of the dependence of the electronic band structure and density of states on the chemical surface passivation of cubic porous silicon carbide (pSiC) was performed using ab initio density functional theory and the supercell method. The effects of the porosity and the surface chemistry composition on the energetic stability of pSiC were also investigated. The porous structures were modeled by removing atoms in the  direction to produce two different surface chemistries: one fully composed of silicon atoms and one composed of only carbon atoms. The changes in the electronic states of the porous structures as a function of the oxygen (O) content at the surface were studied. Specifically, the oxygen content was increased by replacing pairs of hydrogen (H) atoms on the pore surface with O atoms attached to the surface via either a double bond (X = O) or a bridge bond (X-O-X, X = Si or C). The calculations show that for the fully H-passivated surfaces, the forbidden energy band is larger for the C-rich phase than for the Si-rich phase. For the partially oxygenated Si-rich surfaces, the band gap behavior depends on the O bond type. The energy gap increases as the number of O atoms increases in the supercell if the O atoms are bridge-bonded, whereas the band gap energy does not exhibit a clear trend if O is double-bonded to the surface. In all cases, the gradual oxygenation decreases the band gap of the C-rich surface due to the presence of trap-like states.
Nanoscale engineering of silicon carbide (SiC) allows for considerable modification of its basic physicochemical properties. For example, SiC nanostructures have shown greater elasticity and strength than bulk SiC , and SiC nanowires have stable emission properties and an electron field emission threshold comparable to those of carbon nanotube-based materials. Various SiC nanostructures, such as nanospheres, nanowires, nanorods, nanopowders, and even nanoflowers, have been developed [2–5] with interesting technological applications. Among the multiple SiC nanostructures, porous silicon carbide (pSiC) is particularly interesting for the development of engineering technologies and applications, such as LEDs, photodetectors, and hydrocarbon gas sensors , because of its high strength and hardness, low expansion coefficient, chemical and thermal stability at elevated temperatures, and good thermal shock resistance and thermal conductivity . Other interesting applications of pSiC can be found in the biotechnology field, where pSiC could be used as a membrane in implantable biosensors because it exhibits less protein adhesion than porous silicon . Additionally, pSiC exhibits highly efficient blue-to-violet photoluminescence at room temperature , which makes it suitable for optoelectronic applications.
SiC is a material with multiple polytypes, and porous structures, such as 6H and 4H porous SiC, have been developed in these polytypes. The cubic SiC (3C-SiC) porous structures have not received as much attention because of the special conditions required to grow crystals of this polytype ; however, cubic pSiC exhibits promising properties for use in fast-response hydrogen (H) sensors . However, only a few theoretical investigations of the electronic structure of 3C-pSiC have been reported. For example, in our previous work , we addressed the effects of quantum confinement and, to a lesser scale, the effect of the C or Si richness in the surface of H-terminated pSiC on the electronic structure. In addition, we studied the effect of oxygen (O) atoms on the surface passivation of pSiC with mixed surface configurations . However, we did not perform a detailed study on the effects of O at the pSiC surface with only C or Si atoms, or a study on the difference between a bridge bond (X-O-X, X = Si or C) and an O double bond at the surface of the pores. This research could be fundamental to understanding the properties of these materials to enhance their possible applications.
Motivated by the experimental development of the synthesis and characterization of pSiC , we performed a study of the structural and electronic properties of pSiC using density functional theory (DFT) as described by the generalized gradient approximation. In particular, we have used a revised version of the Perdew-Burke-Ernzerhof (RPBE) exchange-correlation functional. The energetic stability, structure, and dispersion of the electronic states were investigated for full surface passivation of the dangling bonds with H and, as a first model of surface oxidation, for surfaces created by replacing H with O atoms in certain positions at the pore surface. Two different types of bonding, including double and single bonds between O and Si or C atoms, were explored.
Because of the large surface area of pSiC, there are many potential sites for the attachment of various chemical species, such as O atoms, which were studied in this work. We analyzed the effects of the interaction of O atoms with the pore surface on the electronic structure by replacing pairs of H atoms on the passivated surface with O atoms. The changes in the electronic structure that arise due to two types of O bonding environments at the surface, single Si-O-Si or C-O-C bonds (bridge-bonded) and double Si = O or C = O bonds, were compared. The calculations of the electronic band structure and density of states (DOS) of pSiC were performed using an ab initio density functional theory generalized gradient approximation scheme as described by a revised version of the RPBE  functional and norm-conserving pseudopotentials  as implemented in the CASTEP code . We used a cutoff energy of 750 eV and a highly converged k-point set according to the Monkhorst-Pack scheme  with grids of up to 3 × 3 × 6 k points. All structures were optimized to their basal state by modifying the atomic coordinates and the supercell shape through the BFGS algorithm . DFT is known to underestimate the band gap energy. This problem can be overcome using other approximations, such as GW; however, in this work, we focused on the relative energetic stability and electronic property differences between the pores and, hence, did not apply any correction.
Results and discussion
For the double-bonded oxygen systems, a decrease in the formation energy with increasing amounts of oxygen at the surface that is similar to that observed in the bridge-bonded cases shown in Figure 2b is observed. However, when four oxygen atoms are present at the surface, an increase in the formation energy is observed with the calculated energies for both porosities almost converging at the same value. This behavior can be attributed to the large lattice deformations caused by the O atom interactions in the 15.6% porosity case. This structure shows that it is possible for an O2 molecule to be released during the formation of this pore (inset in Figure 2b). Notably, after structural relaxation, the double-bond character is lost at almost all Si-rich surfaces, and a tendency to create Si-O-Si bridge bonds is observed instead. The double-bond character is preserved in all of the C-rich surface cases along with the original symmetry, which indicates the stability of this type of bond at the pore surface. The energetically favorable formation energies of the systems with C = O bonds also support the results of some experimental studies, which suggest that C oxides form at the pore surface  in contrast to the SiC/SiO2 interface and mixed oxides that are mainly formed in the bulk . A slight increase in the formation energy of the 4-oxygen double-bonded C-rich surface is observed and is explained in terms of the surface bond distortions in the proximity of the C-H bonds due to the presence of the neighboring C = O double bonds.
Calculated results of Ω (in eV) for O defects in the H-pSiC passivated surface
O atoms added per supercell (n)
H atoms removed per supercell (m)
Defect formation energy (Ω)
C = O
Si = O
The preferential configurations involve bridge-bonded O species at the Si-rich pSiC surface because these configurations have the lowest defect formation energy values. The most stable bridge-bonded substitution in the Si-rich pores is the one with four O atoms replacing eight H atoms for both porosities, as shown by their defect formation energies (−24.308 eV for 15.6% porosity and −22.056 eV for 40.6% porosity). These results indicate that as the number of bridge-bonded O atoms replacing H atoms increases, less energy is needed to stabilize the defects in the passivation, most likely because fewer interactions occur between O and H. For the Si-rich double-bonded substitution case, although all the defects in the surface passivation are energetically stable, the defect formation energies do not decrease monotonically. A sharp increase is observed in the formation energy for the 4-oxygen case, which might be due to interactions between H and O atoms at the surface of the nanopores that can lead to the creation of OH groups and a concomitant change in the surface chemistry of the pores. These changes in the surface structure mean that the formation energy might represent a third type of defect in the passivation. Notably, the deformation energy of the 40.6% porosity case is lower than that of the 15.6% case for the 4-oxygen double-bonded substitution at the Si-rich surface, in contrast to the trend observed for all other Si-rich surfaces. This particular shift might also be due to the formation of OH defects, which change the formation energy behavior, instead of the double-bonded O defects that are preserved to some degree in the 15.6% case.
In the C-rich case, the defect formation energy is almost constant for all surfaces, with the double-bonded defects being more energetically favorable than the bridge-bonded defects, in contrast to the Si-rich case. The greater energetic values of the C-rich surfaces compared to those of the Si-rich surfaces show that the inclusion of O at the C-rich surface is energetically less favorable, most likely in part due to a stronger C-H bond, which requires more energy to be broken than the Si-H bond. However, the double-bonded impurities seem to be stabilized better by the 40.6% porosity than by the 15.6% porosity, which suggests that greater stabilization of these impurities in the surface passivation of C-rich pSiC is possible at larger porosities.
In summary, we have studied the energetic stability, electronic band structures, and densities of states of pSiC with different chemical surface compositions within the framework of density functional theory. The results from the energetic stability analysis show an increased energetic stability in the H-passivated Si-rich surface when some of the H atoms are replaced by O atoms, most likely due to the formation of highly stable Si-O-Si bonds. These bridging bonds form irrespective of the initial configuration of the Si-O bond. At the C-rich surface, the structures seem to lose some energetic stability; however, all porous structures are still energetically favorable. Another remarkable result is that the highly deformed structures with Si = O bonds have formation energies similar to those with C = O bonds. However, the electronic structure calculations show that the C-rich surface has a greater band gap than does the Si-rich surface when the surfaces are fully passivated by H, most likely due to the greater strength of the C-H bond compared to that of the Si-H bond. As the oxygenation of the surface increases, the bridge-bonded Si-O-Si configuration creates a broadening in the band gap energy. In contrast, no clear trend for the Si = O-bonded structures is discernible because the O atom tends to create Si-O-Si bonds in some cases and -OH groups, which reduce the band gap energy, in other cases. For all of the C-rich surfaces, the presence of the O atom reduces the band gap energy by creating trap-like states near the valence band maximum and the conduction band minimum. These states are most likely due to the extra p orbitals introduced by the O atoms. In comparison with our previous results , we see a similar behavior in the electronic band gap in the mixed Si and C surface scheme as in the bridge-bonded case. However, some key differences exist, such as the irregular changes in the band gap caused by a change in the chemical environment during the geometry optimization. The double bond is also shown to affect the band gap energy and surface constitution of the nanopores, which was not addressed previously. This result shows that there is a possibility of band gap engineering through the surface manipulation of pSiC for applications in sensors and optoelectronics.
AT and MC are PhD students. ER has a PhD degree in materials science and engineering and is a research associate at the Universidad Nacional Autónoma de México (UNAM). MCI has a PhD degree in materials science and is a professor at the Instituto Politécnico Nacional (IPN).
This work was partially supported by the multidisciplinary project 2012–1439 from SIP-Instituto Politécnico Nacional, PICSO12-085 from Instituto de Ciencia y Tecnología del Distrito Federal (ICyTDF) and IB101712-2 from PAPIIT-UNAM.
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