Ball and stick model of the Zr adatom at the T4 site. The dark goldenrod (big) balls represent Al atoms and the green (small) balls represent N atoms. (a) Top view of the H3, T4, Br and T1 adsorption sites. (b) Side view with the notation used for the interlayer distances and the Zr incorporation simulation.

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... for AlN wurtzite structure were used to build the slab for the AlN(0001)-(2 Â 2) surface calculations. The equilibrium surface atomic positions were calculated via structural relaxation of a clean AlN(0001) surface. The structure parameters d Al-Zr , d 12 and d 23 characterizing the relaxations of surfaces (with zirconium adatom) are shown in Fig. 1. The calculated parameters of structure relaxations for a clean AlN(0001) surface are given in Table 1. When the clean AlN(0001) surface is created, the topmost Al layer moves outward by 0.059 A ˚ with respect to its ideal bulk position, while the uppermost AlN bilayer moves outward by 0.002 A ˚ with respect to its ideal bulk position. ...

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... zirconium atomic adsorption, a variety of adatom positions were investigated in order to determine the most stable structure. There are three different high-symmetry points on the clean Al-terminated AlN(0001)-2 Â 2 surface as shown in Fig. 1. A zirconium atom is placed at the H3, T4 and T1 positions and the atomic positions were relaxed. Table 1 shows the geometric data of the Zr adatom on the AlN(0001) surface after relaxation, as defined in the Fig. 1. The adsorption energies E ads were calculated as the difference between the total energy of the AlN(0001) slab with ...

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... most stable structure. There are three different high-symmetry points on the clean Al-terminated AlN(0001)-2 Â 2 surface as shown in Fig. 1. A zirconium atom is placed at the H3, T4 and T1 positions and the atomic positions were relaxed. Table 1 shows the geometric data of the Zr adatom on the AlN(0001) surface after relaxation, as defined in the Fig. 1. The adsorption energies E ads were calculated as the difference between the total energy of the AlN(0001) slab with adsorbed Zr atom and the sum of the total energies of the clean surface and the isolated Zr atom. The bond lengths d Al-Zr were obtained from the average distance between Zr adatom and nearest Al surface atoms. The E ads ...

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... of connected configurations (7 from T1 to H3, and 7 from H3 to T4) between the initial and final geometries was allowed to relax. Relaxation during SMD minimization is performed for all atoms, except the bottom two layers that were kept fixed. Figure 2 shows the energy pathway through high-symmetry points for the Zr adatom, such as those that Fig. 1 shows. The local minima from Fig. 2 give the possible adsorption sites. The calculations demon- strate that the Zr adatom avoids the Br and T1 positions and prefers to adsorb at the H3 and T4 positions. The diffusion path exhibits a high-energy site T1 with a barrier of 1.501 eV. The most preferable pathway for Zr diffusion would ...

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... the surface coverage (u Zr ) as the ratio of the amount of adsorbed Zr adatoms to the monolayer (ML) sites' capacity, we built monolayer coatings with u Zr ¼ 1=4, 1/2, 3/4 and 1 ML for the coverage at the AlN(0001) surface. In Fig. 3, we plot the adsorption energy (in eV per Zr atom) vs. coverage for the high-symmetry positions as shown in the Fig. 1. From Fig. 3, we found the T4 position to be the energetically lowest adsorption site for low and high coverages (from 1/4 up to 1 ML), but the H3 position is very close in energy. The preference for adsorption at the T4 site over the H3 site can be attributed to the electrostatic attraction of the Zr adsorbate with the N atoms in the second layer ...

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... Fig. 3, we found the T4 position to be the energetically lowest adsorption site for low and high coverages (from 1/4 up to 1 ML), but the H3 position is very close in energy. The preference for adsorption at the T4 site over the H3 site can be attributed to the electrostatic attraction of the Zr adsorbate with the N atoms in the second layer (see Fig. 1) of the AlN(0001) surface, at low and high Zr concentration. Additionally, in Fig. 3 we can observe a increase of the magnitude of adsorption energy with increasing coverage at the H3 and T4 positions, which reflects the attractive interaction between Zr adatoms at adjacent surface sites on the AlN(0001) ...

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... aluminium chemical potential are shown in Fig. 4. A large number of configurations with different amounts of Zr atoms in substitutional Al sites were studied and denoted as (n 1 =n 2 =n 3 /), where n 1 , n 2 and n 3 are the numbers of substitutional Zr impurities in the first, second and third bilayer, respectively, starting from the surface (see Fig. 1). In Fig. 4, we showed the energetically most stables configurations for each concentration of Zr- doped AlN surface. In order to study the high concentration of Zr atoms, we have chosen m Zr ¼ m ZrðZrÀbulkÞ for the value of Zr chemical potential. First, we observed that in the Phys. Status Solidi B 250, No. 8 (2013) ...

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... Figure 2 Relative energy profile for Zr adatom at different sites on AlN(0001) surface. All the values are in eV. The energy zero corresponds to that of the Zr-T4 surface configuration. The notation is introduced in Fig. 1. www.pss-b.com absence of Zr impurities, we reproduce the results for the clean AlN(0001) surface reported in reference [48]. Under moderately Al-rich conditions the most favorable recon- struction is the Al adatom in a T4 position (Al-T4), while under N-rich conditions the N adatom in the H3 position is most stable (N-H3). Under ...