Ab initio study for molecular-scale adsorption, decomposition and desorption on AlN surfaces during MOCVD growth

Since AlGaN offers new opportunities for the development of the solid state ultraviolet (UV) luminescence, detectors and high-power electronic devices, the growth of AlN buffer substrate is concerned. However, the growth of AlN buffer substrate during MOCVD is regulated by an intricate interplay of gas-phase and surface reactions that are beyond the resolution of experimental techniques, especially the surface growth process. We used density-functional ab initio calculations to analyze the adsorption, decomposition and desorption of group-III and group-V sources on AlN surfaces during MOCVD growth in molecular-scale. For AlCH3 molecule the group-III source, the results indicate that AlCH3 is more easily adsorbed on AlN (0001) than (000\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline{1}$$\end{document}1¯) surface on the top site. For the group-V source decomposition we found that NH2 molecule is the most favorable adsorption source and adsorbed on the top site. We investigated the adsorption of group-III source on the reconstructed AlN (0001) surface which demonstrates that NH2-rich condition has a repulsion effect to it. Furthermore, the desorption path of group-III and group-V radicals has been proposed. Our study explained the molecular-scale surface reaction mechanism of AlN during MOCVD and established the surface growth model on AlN (0001) surface.

Scientific Reports | (2020) 10:17840 | https://doi.org/10.1038/s41598-020-72973-w www.nature.com/scientificreports/ atomic number and 1.25 atomic number for AlN (0001) and AlN (0001 ) surfaces, respectively. While those of the top four layers were variable 26 . The group-III and group-V sources were gradually moved from the vacuum space of the model to the adsorption sites shown in Fig. 2, and the adsorption energy was calculated for each case. In order to compare the stability of surface after adsorption, the surface formation energy was calculated using the chemical potential method [27][28][29] . The van der Waals (vdW) dispersion interactions are a key ingredient for molecule adsorption. For the calculation, the density functional theory (DFT)-D2/D3 with a generalized gradient approximation (GGA) for the exchange correlation energy 26,30,31 . The function used was a revised Perdew-Burke-Ernzerh of function (RPBE) 26,[32][33][34][35] . The wave functions were expanded in terms of numerical basis sets. The real-space cutoff energy was set as 600 eV, and 6 × 4 × 2 k-point sampling was used. The accuracy of this setting is sufficient to meet the requirements of AlN geometric optimization and energy optimization. The calculations were carried out using the program package CASTEP 36,37 . The adsorption energy of molecules adsorbed on the surface of AlN can be obtained by the total energy difference: where E total refers to the total energy of the optimized AlN layers with the adsorbed molecules. E molecules and E slab represent the energy of the molecules computed in the gas phase and the optimized slab of the AlN layers without adsorption, respectively 32 .
To study the desorption process of adsorbed molecules, we searched the transition states and found the most stable and the minimum energy by calculating reaction energy barrier used a LST/QST calculation method based on the transition state theory of DFT. During the deposition and growth on the surface of high temperature MOCVD, the reaction path direction was along the vertical direction to the growth surface (normal to the surface) 38,39 .
(1) E ad = E total − E molecules − E slab Figure 1. The models of (2 × 2) slab models with four AlN layers (a) AlN (0001) surface the top layer is Al atoms and (b) AlN (0001 ) surface the top layer is N atoms. The bottom layer of these models is terminated with fictitious hydrogen atoms, where pink, blue and white spheres denote aluminum, nitrogen and hydrogen atoms, respectively.

Results and discussion
Adsorption of group-III source on AlN surface. Firstly, the structures that group-III source adsorbed on the adsorption site on AlN (0001) and AlN (0001 ) surfaces (shown in Fig. 2) were optimized. In the case of AlCH 3 molecule, three configurations can be considered, which are shown in Fig. 3. Geometry optimization was performed for these configurations and it was found that AlCH 3 molecule adsorbed with the configuration which Al atom faces to surface. Based on the surface structures, the calculation for the adsorption process was performed for AlCH 3 , and the results are shown in Table 1. On AlN (0001) surface, it was found that the minimum distance of Al atom in AlCH 3 molecule with the topmost surface occurs in the case of top site with the value of 2.667 Å. For the angle between the C-Al bond and the topmost surface, the top site with the value of 1.097 degree shows the smallest angle. And the adsorption energy of the top site was the lowest with the value of − 4.58 eV. The results indicated that AlCH 3 will diffusion to the top site when it reaches on AlN (0001) surface. On the contrary, on the AlN (0001 ) surface, the adsorption energies of all the sites are positive, indicating that the adsorption process requires heat which is not stable.
The calculation of the potential-energy surfaces (PES) for AlCH 3 molecules on the 2 × 2 AlN (0001) and AlN (0001 ) surfaces are shown as Fig. 4. The most stable adsorption site on AlN (0001) surface is located above the topmost surface Al atom which is the top site ((arrow in Fig. 4a). This results in the formation of an Al-Al bond (bond length 2.66 Å) between Al atom in AlCH 3 molecule and topmost surface Al atom. The adsorption energy E ad = − 4.58 eV, corresponding to the energy gain to form an Al-Al bond. Similarly, the most stable adsorption site on the AlN (0001 ) surface is located above the center of the hexagonal structure on topmost surface which is the h3 site ((arrow in Fig. 4b). As shown in Fig. 4b, consistent with the adsorption energy calculated above, the adsorption energy of AlN (0001 ) surface is positive which indicates the adsorption process is not stable.
The adsorption of AlCH 3 molecule on AlN (0001) surface is bound to be accompanied by the transfer of charge between atoms and the change of electronic structure. Therefore, the Mulliken charge population of the adsorbed particles and the topmost surface atoms is analyzed, and the results are shown in Table 2. Due to the three H atoms in CH 3 are symmetrically distributed, the Mulliken charge population of one is listed only. As the electrons in the inner layer of an atom are stable, the s and p orbitals in Table 2 show the outermost atomic orbitals. In Table 2, there is a slight change of the charge population numbers of C and H atoms before and after adsorption, while the charge population numbers of s and p orbitals of Al atom in AlCH 3 molecule and Al atom on the top site of topmost surface changed greatly. The charge population of s and p orbitals of Al atom in AlCH 3 molecule changed from 1.81 and 0.59 before adsorption to 1.52 and 0.70 after adsorption, the electron transfer number is 0.29 and 0.11, respectively. Similarly, the electron transfer number of s and p orbitals of Al  www.nature.com/scientificreports/ atom on the top site is 0.12 and -0.05, respectively. Therefore, the adsorption of AlCH 3 molecule on the AlN (0001) surface mainly depends on the interaction between the s and p orbitals of Al atom in AlCH 3 molecule and the Al atom on the top site of topmost surface. The change of charge shows that the increase of positivity of Al atom in AlCH 3 molecule after adsorption is greater than that in the Al atom on the top site, indicating that Al atom in AlCH 3 loses electrons in the adsorption process, and the adsorption mechanism is that the adatoms transfer electrons to the surface atoms.
The results indicated that the adsorption energy and the stable adsorption site of group-III source AlCH 3 molecule on AlN surface are affected by the topmost surface atoms, and the AlCH 3 molecule is easier adsorbed on the top site of AlN (0001) surface which explained in theory why the epitaxial growth of AlN for devices has usually been grown along the [0001] direction.
Decomposition and adsorption of group-V sources on AlN surface. According to the above analysis, the AlN has been mainly grown along the [0001] direction, so of this part the growth surface is on AlN (0001) surface. First, the structures that NH n (n = 0-3) adsorbed on the adsorption sites on AlN (0001) surface (shown in Fig. 2) were optimized. The adsorption sites and adsorption energies for each adsorption species are shown in Table 3. From the calculations, it was shown that NH 3 , NH 2 , NH and N favorably were adsorbed on the top, top, bridge and t4 sites on AlN (0001) surface, respectively. It was found that the adsorption energy of NH 3 is positive, and d N−surf and θ were the maximum with the value of 2.461 Å and 0.069 degree which indicates the NH 3 is not stable species. From the adsorption energy, N is the most favorable adsorption species. However, these adsorption energies were obtained on the assumption that NH 2 , NH and N were present in the vapor phase from the beginning. In addition, we should consider the decomposition of nitrogen sources under high temperature gas phase. Therefore, we calculated the energy required for the decomposition process of the repeated dehydrogenation from the NH 3 .
(2)  Adsorption of group-III source on reconstructed AlN (0001) surface. Since surface reconstructions affect the crystals morphology and play an important role to fabricate high-quality crystals, understanding surface reconstructions is an important issue 21,22 . The adsorption on AlN (0001) surface under N-rich conditions is much easier than that under H-rich conditions, which the N-rich and H-rich conditions refer to the N atom and H atom coverage on the surface 10 . On the results of decomposition and adsorption of group V source on AlN (0001) surface, we investigate the adsorption on reconstruction AlN (0001) surface with NH 2 as coverage source. The structure that AlCH 3 molecule adsorb to reconstructed AlN (0001) surface (shown in Fig. 5) was optimized and the adsorption energy was calculated. As shown in the Fig. 5, we can see that the AlCH 3 moved from top site to h3 site, and the distance from AlCH 3 to the reconstructed AlN (0001) surface is 3.157 Å larger than that on the AlN (0001) surface. Meanwhile, the optimized adsorption energy of AlCH 3 molecule is 5.78 eV, indicating (4) NH = N + H Table 3. Structure parameters, stable adsorption sites at NH n adsorbed on the AlN (0001) surface and the corresponding adsorption energies. E ad represents the adsorption energy. d N−surf represents the average distance of N atoms of NH n with the surface atoms. θ represents the angle between the NH n and the AlN (0001) surface.

Adsorption species
Adsorption site E ad (eV) d N−surf (Å) θ (degree)  Figure 5. (a) Side-view and (b) Top-view of (2 × 2) slab model of reconstructed AlN (0001) surface, where pink, blue and white spheres denote aluminum, nitrogen and hydrogen atoms, respectively. The digital signs show the distance from AlCH 3 to the NH 2 coverage layers and the distance from the NH 2 coverage layers to the AlN (0001) surface, the unit is Å.  10 . In summary, we have found that when N and H atoms cover the surface in the form of NH 2 molecular structure, they will inhibit the adsorption of Al source on AlN (0001) surface, which also indicated that the desorption of H atom has a greater impact on the initial surface growth process on AlN (0001) surface than the adsorption of N atom.
Desorption path of group-III and group-V sources on AlN surface. Our study shows that the adsorption of group-III and group-V sources on the AlN (0001) surface is in the form of AlCH 3 and NH 2 , and the stable adsorption site is top site. In order to established the surface growth model of AlN film, we also modeled the desorption process of group-III and group-V sources on the AlN (0001) surface. The model for the desorption path of group-III and group-V sources on AlN (0001) surface were performed as AlCH 3 and NH 2 , and the results are shown in Fig. 6. The reaction path (5) Fig. 6a. Second, due to the molecular interaction, the molecules stable adsorption sites will be optimized, as shown in Fig. 6b. Finally, the desorption path is the demethylation of AlCH 3 and dehydrogenation of NH 2 , which the CH 3 and H 2 in the gas phase, as shown in Fig. 6c. The charge difference diagram is shown in Fig. 7. The Al atom in AlCH 3 molecule and N atom in NH 2 form a Al-N covalent bond with value of 1.74 Å (arrow in Fig. 7), which is shorter than the Al-N bond on AlN (0001) surface. In addition, the N atom in NH 2 and Al atom on the top site of topmost surface forms a Al-N covalent bond with value of 1.83 Å (arrow in Fig. 7). Therefore, we proposed the surface growth model on AlN (0001) surface as follows: after AlCH 3 and NH 2 molecules adsorbed on the top site respectively, they will be follow the desorption path (5) and the initial bonding process is given.

Conclusions
In summary, the adsorption, decomposition and desorption process of group-III and group-V sources on AlN surfaces during MOCVD growth were investigated using density-functional ab initio calculations in molecularscale. We have found that AlCH 3 and NH 2 molecules prefer to be adsorbed on AlN (0001) surface on the top site. By the study of the adsorption on reconstruction AlN (0001) surface, it follows that AlCH 3 molecules growth to be prominent under ideal surface rather than NH 2 -rich conditions. Moreover, we have proposed the desorption path of the group-III and group-V sources on AlN (0001) surface. To sum up, the study in this paper explained the molecular-scale surface growth mechanism of AlN during MOCVD, and established the surface growth model on AlN (0001) surface. The results are helpful for the future calculation concerning more detailed growth process of AlN process during MOCVD growth.