J. Phys. Chem. B 2001, 105, 7619-7623
7619
Influence of Adsorbed Species on the Reconstruction of 4H-SiC(0001) Surfaces J. Olander* and K. Larsson Department of Materials Chemistry, The Ångstro¨ m Laboratory, Uppsala UniVersity, Box 538, S-751 21 Uppsala, Sweden ReceiVed: February 7, 2001; In Final Form: April 13, 2001
Surface reconstructions of unterminated 4H-SiC(0001) surfaces have been investigated theoretically using the first principal density functional theory. A (2 × 1) reconstruction was found for the Si(0001) surface, whereas the C(0001h) surface retained its initial (1 × 1) structure. The downward relaxation was, on the other hand, much larger for the C surface than for the Si surface. The effects of adsorption of C2H2 (or Si) on the two surfaces were also studied. The adsorbates were then observed to bond strongly to the surfaces of Si(0001) and C(0001h), respectively. They also influenced the surfaces in the direction of bulk parameters.
I. Introduction Silicon carbide (SiC) has a large technological potential for high-power devices and semiconductor materials. This is mainly due to properties such as extreme hardness, thermal conductivity, and chemical stability. To produce high-quality SiC by chemical vapor deposition (CVD) methods,1 it is important to find optimal growth parameters. It is then essential to achieve a deeper knowledge at the atomic level of the surface processes occurring during growth of the deposited material. This knowledge, in part, can be obtained from quantum mechanical calculations. Silicon carbide exists in many polytypes that differ only in one dimension.2 More specifically, this means different stacking variants of the Si and C planes. The only cubic polytype is called β-SiC, whereas the numerous polytypes representing hexagonal and rhombohedral modifications of SiC are collectively referred to as R-SiC.3 4H-SiC differ from 6H-SiC, and from β-SiC, only in the sixth layer in the direction perpendicular to the (0001) plane. The SiC(0001) surface contains one dangling bond/ surface atom. It is the most stable one of the crystallographic planes and it is also the conventional growth direction of R-SiC. The (0001) and the (0001h) surfaces consist of only Si or C atoms, respectively, whereas both the (101h0) and (112h0) planes contain equal numbers of C and Si atoms. Because of the expected partial charge transfer from Si to C, crystals of SiC thus possess both polar and nonpolar surfaces.4 These circumstances make the conditions for SiC growth quite different for different directions. For most electronic purposes, 4H-SiC is the preferred polytype because of properties such as large band gap and high electron mobility.5 The stability of unterminated R-SiC surfaces is very sensitive to preparation conditions. Experimental investigations of the Siface of hexagonal SiC(0001) have among others shown the (1 × 1), (x3 × x3 )-R30°, (3 × 3), or the (6x3 × 6x3)-R30° reconstructions.6 Corresponding C faces have shown the (1 × 1), (2 × 2), (3 × 3), or (x3 × x3)-R30° reconstructions.6 Theoretical investigations have resulted in the (1 × 1) and (x3 × x3)-R30° reconstruction for both the Si and the C faces of 6H-SiC(0001).7 One of the purposes with this study was to theoretically investigate unterminated 4H-SiC(0001) surfaces. The structural * E-mail:
[email protected].
relaxation and reconstruction of the Si and C planes were in focus. Another purpose was to study the effect of adsorption of C2H2 and Si, respectively, on these relaxed and reconstructed surface structures. The gaseous C2H2 and Si species were chosen because they have been important experimentally as silicon carbide growth species.8 It is possible to consider the adsorption of these species to the investigated surfaces as the first steps initializing further SiC growth. II. Method The calculations have been performed within the framework of density functional theory (DFT),9 using the Cambridge Sequential Total Energy Package (CASTEP) computer program package from Biosym/Molecular Simulation Technologies in San Diego. The adsorption energies were calculated by using:
E[ads, A] ) E[A] + E[surf] - E[surf-A],
(1)
where E[A], E[surf], and E[surf-A] are the total energies for the adsorbing species and for the surface with and without the adsorbed species, respectively. DFT methods use various approximations to describe the exchange and correlation interactions between the electrons. The commonly used local density approximation (LDA) results in rather accurately described local properties such as bond lengths and vibration frequencies. However, this method tends to overestimate the bond energies. The more accurate generalized gradient approximation (GGA) methods involve improved treatment of inhomogeneities in the electron density, resulting in improved results of the global changes in energy (e.g., bond energies) compared with the LDA methods. In this study, the LDA method developed by Perdew and Zunger10 was used for the initial geometry optimization calculations. In subsequent single-point calculations, a GGA method developed by Perdew and Wang (PW91)11 was applied. These single-point calculations were performed to obtain a good description of the electronic structure of the geometry-optimized surfaces. Complete geometry optimizations using the GGA were computationally too demanding to perform. In all calculations the atoms were represented by nonlocal pseudopotentials in the KleinmanBylander separable form.12 Furthermore, the electronic wave functions were expanded in terms of plane waves. An all-bands
10.1021/jp010499z CCC: $20.00 © 2001 American Chemical Society Published on Web 07/19/2001
7620 J. Phys. Chem. B, Vol. 105, No. 32, 2001
Olander and Larsson TABLE 1: Interlayer Distances Obtained as a Result of Geometry Optimization Calculations of 4H-SiC(0001) Surfaces (1 × 1) ideal
Si-face (2 × 1) relaxed
C-face (1 × 1) relaxed
z12a (Å)
0.63
0.35
z23 (Å)
1.89
z34 (Å)
0.63
z45 (Å)
1.89
0.24 vs 0.78b 1.85 vs 1.94b 0.62 vs 0.70b 1.85
2.00 0.55 1.92
a
zij is the distance between the layers i and j. b Every second surface Si atom was uplifted from the surface, while the others were somewhat lowered.
bonds of the atoms in the lowest layer were saturated with hydrogen atoms. III. Results and Discussion
Figure 1. (a) A side view of the ideal bulk structure model of 4H-SiC(0001) initially used during the calculations. The black color denotes the atoms that were kept fixed during the geometry optimizations. The larger, medium (grey) and small circles represent Si, C and H atoms, respectively. By interchanging the Si and C atoms one obtains the 4H-SiC(0001h) model cell. (b) A top view of the bulk structure model of 4H-SiC(0001). In this figure, all but the uppermost two layers, are colored black in order to clarify the bonding situation of the surface atoms.
minimization method was applied during the electronic minimization process.13 The Monkhorst-Pack scheme generated 1 k-point in representing the electronic wave functions of the system.14 To verify the quality of this representation, the adsorption energy of H to a H-terminated SiC(0001) surface was calculated using either 1 or 3 k-points. Because the obtained energies differed by less than 1%, 1 k-point was considered to be adequate for these calculations. An additional check of the influence of k-points on the geometrical structure was also made. The top layer of the SiC(0001) surface, described in section III.B, was geometry optimized using 3 k-points. As a result, a geometry identical with the results obtained using 1 k-point was obtained. The slabs modeling the 4H-SiC surfaces were constructed by periodically repeated unit cells, usually referred to as super cells. The unit cells were 12.36 Å in the a- and b-directions and 15 Å in the c-direction. The final reconstructed and relaxed surface structures were achieved by performing geometry optimizations of cells with initial bulk structure (Figure 1). The two lowest layers of atoms in these calculations were maintained in fixed order to hold the characteristics of the bulk structure. Six layers of atoms were used during the initial geometry optimization calculations. In subsequent single-point calculations, and in the calculations involving an adsorbate, the cells were reduced to include the four upper geometry-optimized layers. The resulting vacuum layer between the slabs was 7.5 Å for the geometry optimization calculations. In the single-point calculations, the corresponding distance was 10 Å. The dangling
A. General. Among the various chemical steps involved in CVD, the adsorption of gaseous species on the solid surface is frequently a rate-determining step, either by contributing to the growth rate or by blocking available surface sites. During SiC deposition, SiH4 and C3H8 are generally used as gas-phase precursors. C2H4 and SiH2 have then been found to be the most important of the resulting growth species at temperatures below 1700 K, whereas C2H2 and Si are most important at higher temperatures.8 Because the long-term goal of the present authors is to investigate SiC growth at higher temperatures, the gaseous species C2H2 and Si were allowed to adsorb to the Si face and C face, respectively, in these calculations. To achieve a deeper understanding of the growth processes it is important to study the prerequisites of adsorption, such as the structure of the surface before adsorption. One aspect of the structure is surface saturation, that is, whether the growing surface is unterminated or terminated with, for example, hydrogen. For this reason, it is of interest to study both types of SiC surfaces (H-terminated vs nonterminated). The focus of this article is the structure of unterminated 4H-SiC surfaces. When creating a 4H-SiC(0001) surface with an idealized bulk structure, a surface with unsaturated dangling bonds is formed. Because these bonds are not energetically stable, the atoms will reposition themselves to minimize the surface free energy. Most theoretical investigations regarding reconstructions of all the different SiC(0001) surfaces have been performed by initially building differently reconstructed models that suit experimentally obtained results.7 It has also been common to allow the model to relax downward with a fixed structure in the x- and y-directions. In this investigation, a 4H-SiC surface with an ideal bulk geometry has been used as the initial model from which the final reconstructed surface has been obtained theoretically. B. 4H(0001) Surface. B.1. Relaxation. As seen in Table 1, the downward relaxation obtained as a result of the geometry optimization was significant for the uppermost layer of the Si(0001) surface. The distance between the first and second atomic layers decreased by 0.13 Å () 21%) as a result of the geometry optimization, whereas the distances between the lower layers were only slightly changed (
