Surface steps on Si-terminated SiC(111)-√3&times√3 surfaces: A first-principles calculation

抄録

Among more than 200 polytypes of SiC, the cubic zinc blende (3C) polytype and he hexagonal 4H and 6H polytypes have been most intensely studied. The polar surfaces of SiC are technologically important because they are commonly used for SiC epitaxial growth. Although there is a variation of the fundamental energy gap by about 1 eV between the 3C and 4H polytypes, the 3C-SiC(111) and hexagonal nH-SiC(0001) with n Si-C bilayers in a unit cell exhibit many similarities. The (√3&times√3)R30^&ordm reconstruction is one of the intrinsic surface phases of 3C-SiC(111) and nH-SiC(0001) surfaces and is observed experimentally [1]. The model with Si adatom on the T4 site which is the on-top site of an underlying second layer is most favorable for the Si-terminated (√3&times√3)R30^&ordm reconstruction [2]. Our previous calculations have shown energetics for surface steps with the Si-terminated (1&times1) reconstructed SiC(111) surface and the microscopic atomic structures have been identified [3,4]. We considered the five types of steps appearing on the (111) vicinal Si-terminated surfaces inclined toward either the <-12-1> or <-101> direction. We found that the <-12-1> straight step is energetically more favorable than the straight <-101> step, and that either the straight or the meandering step edge, depending on the inclined direction of the vicinal SiC surfaces, emerges. We report here first-principles calculations that clarify the dependence for the surface steps on Si-terminated 3C-SiC(111) surfaces on the surface reconstructions.

The calculations are performed using RSDFT code [5] which is based on the real-space finite-difference pseudopotential method and is the most suitable for the current massively parallel architecture. Figure 1 is a schematic illustration of five distinct atomic steps (Si2, Si3, C1, C2, and SC) on the Si-terminated 3C-SiC(111) surface with an (1&times1) reconstruction. In the case of the (√3&times√3) reconstruction, we use the periodic-array slabs containing 400–600 atoms to model Si2+Si3 steps, Si2+C1 steps, and 2 SC steps as the step-edge configurations. There are several distinct structural arrangements for each pair of steps, depending on the distances between the step edge and the Si adatoms on the terrace. We have found that the geometries in which the Si adatoms are located at off-edge sites are generally lower in energy. We observe the rebonding of the Si bonds between the upper- and lower-terrace Si edge atoms, which is also observed in the case of the (1&times1) reconstruction. The calculated formation energies for the Si-rich and Si-poor conditions are considered. We have found that the order of the formation energy for each step pair is identical to that obtained for the case without the (√3&times√3)R30^&ordm terrace reconstruction. The formation-energy differences between the most and the next stable step pairs are the same in the range of less than 70 meV/&Aring. We conclude that the structural feature and the energetics of steps on 3C-SiC(111) surfaces are essentially the same for (1&times1) and (√3&times√3)R30^&ordm reconstructions on the terrace.

[1] P. M&aringrtensson, F. Owman, and L. I. Johansson, Phys. Status Solidi B 202, 501 (1997).

[2] M. Sabisch, P. Kr&uumlger, and J. Pollmann, Phys. Rev. B 55, 10561 (1997).

[3] K. Seino and A. Oshiyama, Appl. Phys. Exp. 13, 015506 (2020).

[4] K. Seino and A. Oshiyama, Phys. Rev. B 101, 195307 (2020).

[5] J.-I. Iwata et al., J. Comput. Phys. 229, 2339 (2010).