Wurtzite AlN (0001) surface oxidation: hints from ab-initio calculations

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Surfaces, Interfaces, and Applications

Wurtzite AlN (0001) surface oxidation: hints from ab-initio calculations Zhi Fang, Enhui Wang, Yafeng Chen, Xinmei Hou, KuoChih Chou, Weiyou Yang, Junhong Chen, and Ming-Hui Shang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08242 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 25, 2018

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ACS Applied Materials & Interfaces

Wurtzite AlN (0001) surface oxidation: hints from ab-initio calculations Zhi Fang1, Enhui Wang1, Yafeng Chen1, Xinmei Hou1*, Kuo-Chih Chou1, Weiyou Yang2, Junhong Chen3 and Minghui Shang2* 1

Collaborative Innovation Center of Steel Technology, University of Science and

Technology Beijing, Beijing 100083, China. 2

Institute of Materials, Ningbo University of Technology, Ningbo City, 315016,

China. 3

School of Material Science and Technology, University of Science and Technology

Beijing, 100083, China.

KEYWORDS: AlN, oxygen adsorption, surface oxidation, nitrogen vacancy, phase transformation, first-principle.

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ABSTRACT: With superior electrical and thermal properties, aluminum nitride (AlN) has exhibited wide application. However, AlN is rather oxygen-sensitive and tends to be oxidized at high-temperature. The surface oxidation of AlN remains the major challenge while the underlying physics of AlN surface oxidation is still elusive. First-principle calculations were performed to study wurtzite AlN (0001) surface oxidation process. The adsorption energy of oxygen was calculated to be site-dependent on the surface with varying O-coverage. Calculation indicates that oxygen atoms are preferentially adsorbed at the hollow site (H3) of the AlN (0001) surface regardless of the O-coverage. N2 is calculated to be the dominant gas product. The procedure of N3- removal and the formation of N vacancies (VN) take place step by step. VN plays an acceleration role in the oxidation of AlN and O2- prefers to occupy the site of VN via consuming the Al p lone-pair electrons and passivating the dangling bond states of Al. An O-Al-O layer is formed when the first Al-N bilayer is fully oxidized, which could be regarded as a precursor of γ-Al2O3. Based on our atomic level simulation, a possible phase transformation mechanism from γ-Al2O3 to α-Al2O3 was further proposed.

1. INTRODUCTION Among the III-V groups of nitrides, aluminium nitride (AlN) has attracted a great attention owing to its excellent physical properties such as high electrical resistivity (>4×108 Ω·cm), high thermal conductivity (3.2 W·cm-1·K-1) and low coefficient of thermal expansion (4.03~6.0×10-6 K-1).1-2 Therefore it is widely applied as 2 ACS Paragon Plus Environment

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high-temperature ceramics and refractories,3-6 for instance, the belly of the blast furnace. However, AlN is sensitive to oxygen and tends to be oxidized or corroded under high-temperature,2 which seriously restricts its applications. Therefore, the oxidation of AlN ceramics remains the major challenges for its practical applications. Numerous efforts have been made to investigate the oxidation of AlN with aid of a series of experimental observations.7-9 The oxidation of AlN ceramics under two different oxidizing atmospheres was studied by Hou et al.,10 which indicate that the oxidation rate of AlN strongly depends on temperature and oxygen partial pressure. It is widely believed that the main oxidation products of AlN are Al2O3 and N2.11-12 The formation and escape of N2 lead to produce a porous oxide layer on the surface of AlN, which promotes the infiltration of oxygen molecules. The oxidation kinetics is thus rather fast.10, 12 It is well known that the compact Al2O3 layer can prevent further oxidation of Al metal.13 While porous Al2O3 layer cannot protect AlN,12 which is one of the reasons for weakening the surface oxidation of AlN. Compared with the mechanism of surface oxidation of AlN explored experimentally,2, 8, 12 less success has been achieved on theoretical studies. In this work, we investigated the adsorption and reaction of oxygen on wurtzite AlN (0001) surface via a first-principle approach. The (0001) plane was chosen because it has the lowest density of surface energy and is easiest to be exposed on the surface.14-16 The adsorption energies suggest that the hollow site (H3) is energetically preferred for the adsorption of oxygen on AlN (0001) surface, which is in agreement with the reported result.17 As for the gas products, the formation of N2 is more 3 ACS Paragon Plus Environment


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exothermic than any other gases, in line with our previous experiments.8, 10 Further, the stepwise processes of oxidation on AlN (0001) surface were investigated till the first Al-N bilayer was completely oxidized, where all N3- in sublayers were replaced by O2- with all H3 sites on the surface occupied by O2-. The possible mechanism of phase transformation from γ-Al2O3 to α-Al2O3 was proposed by comparing the fully oxidized configuration of AlN (0001) surface with the crystal structure of α-Al2O3.

2. COMPUTATIONAL METHODS AND MODELS The calculations were carried out employing the general gradient approximation (GGA) and the Perdew-Burke-Ernzerhof (PBE) exchange and correlation functional18 within the framework of density-functional theory (DFT). All the DFT calculations were performed by the plane-wave pseudopotentials method implemented in the CASTEP code.19 The ionic cores and valence electron interactions were described by the Vanderbilt ultrasoft pseudopotentials20 which can reduce the number of plane waves required in the expansion of the Kohn-Sham orbitals. The AlN (0001) surface was modeled by a slab geometry with a 10 Å vacuum region between two consecutive slabs. The slab supercell consisted of 2×2 unit cells with six AlN bilayers where the lower four bilayers were fixed for simulating bulk phase during the structure optimization. The upper two bilayers and any adatoms were allowed to relax until all the force components decreased to or less than 0.01 eV/Å and the self-consistent-field (SCF) iteration converges with total energy variation not more than 1.0×10-6 eV/atom. The plane wave cutoff energy was set to 450 eV and a 6×6×2 Monkhorst-Pack mesh

21

was adopted for the first Brillouin zone integration 4

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based on convergence tests. To prevent the unphysical charge transfer and decouple the surfaces of the slab, pseudohydrogens with fractional charges were employed to terminate the dangling bonds on the bottom side and the H atoms were frozen during the structure optimization. For the Al-terminated (0001) surface, there are three possible adsorption sites, i.e. H3, T4 and atop, as shown in Figure 1. Regarding the fact that the total number of each site at this 2×2 AlN surface is four, configurations with 1, 2, 3 and 4 O2adsorbed on the surface at homogeneous sites are respectively defined as 0.25, 0.5, 0.75 and 1 monolayer (ML) O-coverage.

Figure 1. (a) Configurations of AlN (0001) slab model; (b) the top view of the (0001) surface and three possible adsorption sites. T4 is the site above the sublayer N atom, H3 is the hollow site and atop is the site above the surface Al atom.

3. RESULTS AND DISCUSSION 3.1. Adsorption energy The adsorption energies at three different adsorption sites were calculated as 5 ACS Paragon Plus Environment


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below:

EnO = En O − E( n −1) O − µO

(1)

where n is the number of O2– adsorbed on the AlN surface, EnO is the total energy of the oxidized system with n O2–, µ O is the chemical potential of an O atom, namely the half of the total energy of an O2 gas phase molecule.

Figure 2. The optimized configurations (upper panel) and adsorption energies (lower) of oxygen at H3, T4 and atop sites (labeled by dashed red circles) over different O-coverages.

The optimized configures and corresponding adsorption energies at different sites with increasing oxygen(O-) coverages were investigated and plot as function of 6 ACS Paragon Plus Environment

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O-coverage (Figure 2). For 0.25 ML O-coverage, the atom is respectively added at H3, T4 and atop sites of AlN surface for searching the most energetically preferential oxidation sites. After optimization, the O atom adsorbed at atop site is unstable and relaxed spontaneously to the H3 site, then the configurations H3 and atop have quite close energies ( EnO ) for 0.25 ML in Figure 2. The adsorption energy at H3 site is the most negative among these sites, indicating that the oxidation of the AlN (0001) surface starts from H3 site. At H3 site, an O atom bonds with three ligand Al3+ ions forming Al-O bonds (bond length = 1.93 Å). With increased O-coverage, the adsorption energy at H3 site is always the most negative. For instance, in the case of 1 ML coverage, there are already three O at H3 sites and the fourth O is respectively added at H3, T4 and atop sites. And the structure with the fourth O occupying H3 site is proved to be ground state. Therefore oxygen atoms energetically prefer the H3 sites on the AlN (0001) surface regardless of the O-coverage. The optimized ground state configures with all O adsorbed at H3 sites over each coverage and their adsorption energies are displayed in Figure 3. The adsorption energy rises from -6.2407 eV to -0.2510 eV accompanying with the O-coverage increaseing from 0.25 to 1 ML. Coulomb energy that gains from repulsive interaction between O2- ions becomes larger at denser O-coverage. Therefore, the adsorption energy of oxygen on the AlN (0001) surface increases with coverage, implying the oxidation occurs easily for the clean AlN (0001) surface or less dense O-coverage.



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Figure 3. The adsorption energy and corresponding top and side views with 0.25, 0.50, 0.75 and 1 ML O-coverage on the AlN (0001) surface.

3.2. Formation energy of different gas products Oxidation of AlN is found to be accompanied by the removal of N3- and production of N-based gas. NO, NO2 and N2 are regarded as the gas products candidates.11 To clarify the lowest energy cost of gas emission, we calculated the formation energies of NO, NO2 and N2 at different O-coverage as following

EnNO = Entotal + µNO − En O O-NO

EnNO2 = E ntotal + µNO2 − En O O-NO 2

(2)

,

(3)

,

EnN ( = EnN 2 2 ) = Entotal O − N + µ N − EnO

,

(4)

NO NO where En , En 2 or EnN respectively represent the formation energy of NO, NO2,

and N in the form of N2 from the AlN(0001) surfaces with n O2- adsorbed, and EnO-NO ,

EnO-NO2 or EnO-N denote the total energy of the oxidized AlN(0001) surfaces after the gas emission of NO, NO2 or N in the form of N2, respectively. µ NO and µNO2 were calculated from the gas phase of NO and NO2 molecule respectively. µ N is the


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chemical potential of a N atom from the total energy of gas phase N2 molecule. The formation energies of NO and NO2 for all coverages (except NO at 1 ML coverage) are more positive, suggesting the emission of NO and NO2 are strongly endothermic reactions. While the emission of N2 becomes more exothermic with further negative formation energy. Therefore, N2 emission is the most energetically preferred below 1 ML O-coverage. O2- prefers to bond with Al3+ instead of N3-, which leads to larger formation energies of NO or NO2. It is noteworthy that the formation energy EnN decreases while the adsorption energy EnO of O increases progressively with the O-coverage increasing from 0.25 ML to 0.75 ML, indicating that the repulsive interaction between O2- promotes the formation of N2. Interestingly, the EnN turns to be negative only at the case of 0.75 ML coverage. Figure 4a shows that there is a N vacancy (VN) remained in the optimized structure once the two N3- are removed to form a N2 molecule at 0.75 ML coverage, followed by one of the O2- transferring to the VN. The total energies of the system increase in presence of VN, which causes the formation energies of N2 positive at 0.25 and 0.50 ML coverages. However, in the case of 0.75 ML, the occupation of O2- at the site of VN reduces the total energy of the system, leading the formation energy of N2 to be negative. Therefore, the N2 emission can occur spontaneously with the O-coverage achieving 0.75 ML. However, the interval between 0.5 and 0.75 ML is rather large and the emission of N2 may occur at certain O-coverage between 0.5 and 0.75 ML. To investigate N2 emission with finer O-coverage, we’ve conducted N2 formation energy estimation employing larger 3×3 slab model (Figure S1). The adsorption energy of oxygen increases and the formation 9 ACS Paragon Plus Environment


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energy of N2 decreases with coverage increasing. This is in good agreement with the result concluded based on 2×2 slab model. N2 emission tends to happen at 5/9 ML O-coverage. Similarly, a N vacancy is formed after the N3- removal, and then oxidation spontaneously occurs with O2- moves to the site of VN.

Figure 4. (a) Optimized structures after emission of NO, NO2 and N2 at different O-coverages. (b) Formation energies of O2 adsorption, NO, NO2 and N2 emission. N vacancies are labeled by dashed blue circles.

3.3. Stepwise reactions Based on the calculation, N2 is the dominant gas product. Thus we only take the N2 emission as the N removal into consideration in the next oxidation process. Then 10 ACS Paragon Plus Environment

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we investigated the oxidation process step by step. For instance, in the case of three O2- adsorbed on the AlN (0001) surface, one N3- was removed and nearby O2occupied its initial position subsequently. We defined the adsorption steps at H3 sites of 1st, 2nd and 3rd O2- as the step 1, 2 and 3 of the whole oxidation process, respectively. The subsequent N3- removal was defined as the step 4. To better understand the mechanism of the whole oxidation process, the further reaction steps had been investigated until O2- occupied all H3 sites and N sites in the first Al-N bilayer as shown in Figure 5(12).

Figure 5. The optimized structures of each reaction step. The sign “+O” means adsorbing an O2- in this reaction step and the sign “−N” means removing a N3- in the form of N2 gas. (N vacancies are labeled by dashed blue circles, and the dashed red circles represent the O2- occupying the N vacancies).

In each step, we consider two possible circumstances, one is the adsorption of O2- (the sign “+O” in the Figure 5) and the other is the removal of N3- (the sign “−N” in the Figure 5). The oxidation energy for the i-th step reaction is defined as: E iox = E itotal − µ O − E itotal -1

(5)

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E iox = E itotal + µ N − E itotal -1

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(6)

where Eiox is the oxidation energy of step i, E itotal and E itotal are the total energies of -1 the optimized structures in step i-1 and i, respectively. Formulas (5) and (6) were used in circumstance of the O adsorption or the N removal, respectively. Then, all E iox during the whole oxidation process are plotted in Figure 6.

Figure 6. The energy evolution of each step and corresponding oxidation energy of the whole oxidation process (the initial energy of AlN (0001) system is normalized to zero). The red spheres represent the main reactions while the black spheres represent some secondary possible reactions.

For the Al-adlayer-terminated (0001) surface, there are many dangling bonds of Al that tend to be saturated by O2-. As a result, several O2- adsorb on the surface at the beginning of oxidation and the O2- occupying H3 site instead of the other sites can reduce more surface energy by saturating the Al dangling bonds. Thus O preferentially adsorb at the H3 site of the AlN (0001) surface. During the process of O adsorption, electrons of N3– will transfer to O adatoms owing to its higher 12 ACS Paragon Plus Environment

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electronegativity. Thus the valence state of O changes from 0 to –2 and AlON as the intermediate product is formed.22 With more O2- incorporation, the valence states of N atoms change from –3 to 0 and then N2 molecule is formed. After the emission of N2, three new dangling bonds of Al come up with a residual VN. As a result, the O2- tends to transfer to the site of VN in order to saturate dangling bonds and balance charge distribution. Hence, the removal of N3- must be followed by the occupation of O2- on VN. Figure 6 depicts the oxidation energy varying path during the process of O adsorption and oxidation. At the first three steps, three O2- are adsorbed at H3 sites on the AlN (0001) surface. At step 4, one of the O adatoms transfers to the site of VN after the removal of N3-. Thus there are two empty H3 sites for two more O2- to adsorb at step 5 and 6, which the O adsorptions turn to the dominant reactions. At step 7, the N removal becomes the unique circumstance owing to the lack of H3 site. Then the next O atom fills up the VN at step 8. Steps 7 and 8 can be regarded as the substitution of N atom by O atom. The N removal forming VN and the occupation of VN by O2occur alternatively at the latter steps, until all the N3- are substituted by O2- as shown in Figure 7.

Figure 7. The top (a) and side view (b) of the final structure (2×2 supercell) where the 13 ACS Paragon Plus Environment


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first Al-N bilayer of Al (0001) surface is completely oxydized: all H3 sites and N3sites are replaced by O2-.

3.4 Electronic structure variation during AlN (0001) surface oxidation The geometrical change of material always rises from the electronic structure variation. To achieve a better insight into the trend for O adsorption and reaction on the AlN (0001) surface, the calculated densities of states (DOS) for several surfaces are plotted in Figure 8 and all the DOS for each oxidation step are shown in Figure S2. For this Al-terminated AlN (0001) surface, each Al in the first bilayer contributes one dandling bond creating surface donor states. It is widely accepted that surface donor states play an important role in the surface oxidation of AlN and the development of two-dimensional electron gas at AlN-based heterostructures.23-26 Miao et al.27 have reported that surface states arising from Al dangling bonds were unoccupied and such dangling bonds gave rise to states in the upper part of the band gap. This is consistent with our assumption (Figure 8a) that Al surface states are indeed close the conduction band minimum (CBM) as labeled by a dashed blue rectangle. It was also mentioned that adding oxygen to the surface will provide a means of pouring electrons into surface states, so partial unoccupied Al surface states will become occupied. After 0.25 ML of oxygen adsorbing on the surface, dangling bonds of Al are thus partially saturated by O2-, and a weaker dangling bond state of O is then observed as labeled by dashed red circle in Figure 8a near the bulk valence band maximum (VBM). In principle, 3/8 ML of O-coverage is required to passivate the AlN (0001) surface for removing all the dangling bond states of Al because each Al dangling bond donates 14 ACS Paragon Plus Environment

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3/4 electrons.17 With the O-coverage increasing from 0.25 ML to 0.50 ML, dangling bond states of Al are fully passivated and the intensity of O dangling bond state becomes higher. The AlN (0001) surface with further adsorption of oxygen might fall into an acceptor-like feature. What’s more, as indicted by the first-principle calculation by Kempisty et al.,28-29 electron transferring and change of Fermi energy level are responsible for the adsorption energy change. For the AlN free surface, the Fermi energy level is pinned by Al dangling bond state in the midgap under the electron-rich condition (Figure 8a). With 0.25 ML O adsorbing on the surface, the Fermi energy level is shifted down close to VBM owing to the presence of O dangling bond states and partial passivation of those Al dangling bond states (Figure 8a) via electron compensation. Then at 0.5 ML, Fermi energy level is pinned to VBM by O dangling bond states as Al dangling bond states are fully passivated (Figure 8a), where all Al p lone-pair electrons are bonded to O2- anions. With the O-coverage further increasing, Fermi energy level is still in VBM. As mentioned above, the Fermi energy level is shifted from the bandgap (middle) to the top of the valence band and the adsorption energy attains different value for the Fermi energy level differently pinned.30 Figure 8b shows the DOS of AlN surface with and without N vacancy (VN) at oxidation step 4. In presence of VN, more dangling bonds of Al3+ arise due to the breaking of Al–N bonds. Several dangling bond states of Al are thus formed around the bandgap and the surface becomes unstable (Figure 8b(i)). These dangling bond states are passivated when one of the O2- transfers to the VN (Figure 8b(ii)), leading to 15 ACS Paragon Plus Environment


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significantly decreased free energy of the AlN surface (see Figure 6). That is the innermost physics of O adatoms prefer to occupy the site of VN during the whole oxidation process. What’s more, there is only O surface state near the VBM when the AlN (0001) surface is completely oxidized (Figure 8c(ii)). Compared with AlN bulk (Figure 8c(i)), an upshift VBM and reduced bandgap is then obtained. The electric field in the slab is responsible for band bending and reduction of effective bandgap. Krukowski et al. reported that semiconductor surfaces could be clarified as charge categories, i.e., surface acceptor, donor, and neutral site.31 For AlN free surface and fully oxidized surface, the surface sites can be considered as donor and acceptor category, respectively. The electric potential variation between surface and inner layers may rise and the field may affect some surface properties. The electric fields at the surface cause band bending, and then a reduced band gap can be expected.

Figure 8. (a) DOS for the AlN (0001) free surface and surface covered by 0.25, 0.50, 16 ACS Paragon Plus Environment

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0.75 and 1 ML of oxygen (The Fermi energy is set to zero); (b) (partial-)DOS for the surface before and after N vacancy occupied by O2- at step 4 of the oxidation process; (c) (partial-)DOS for the AlN unit cell and AlN (0001) surface after full oxidation. The surface states of O and Al are labeled by dashed red circles and dashed blue rectangles.

3.5. Mechanism of AlN phase transformation The oxidation of AlN under dry oxidizing condition can be represented by the following reaction,12 4AlN+3O2→2Al2O3+2N2.

(7)

Based on the recent reported results in the literatures,2,32 the rhombohedral α-Al2O3 is identified as the thermodynamically stable oxidation phase of AlN. However, there are many metastables, so called “transition alumina” structures exiting, one of which is cubic γ-Al2O3.33 In this study, γ-Al2O3 is considered to be the transient phase during oxidation process, prior to the formation of α-Al2O3. Due to the difference of crystal system and atom number between α-Al2O3 and γ-Al2O3 (Figure 9), it is rather hard to investigate the phase transformation from γ-Al2O3 to α-Al2O3 via complementary first-principle calculations. To gain a better insight into the phase transition, the crystal structures of α-Al2O3 and γ-Al2O3 are analyzed in detail. As shown in Figures 9a and b, the Al3+ are located in tetrahedral or octahedral positions irregularly in the γ-Al2O3 crystal, while the Al3+ are highly symmetrically distributed in octahedral

positions in α-Al2O3. In γ-Al2O3 cell unit, there are four nonequivalent types of Al3+ in special positions 8a (tetrahedral), 16d (octahedral), 16c (octahedral) and 48f (tetrahedral), labeled as Al1, Al2, Al3 and Al4, respectively. The Al ions and vacancies 17 ACS Paragon Plus Environment


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are distributed over these octahedral and tetrahedral positions. The occupation of Al1, Al2, Al3 and Al4 are 0.863, 0.816, 0.028 and 0.019, respectively. As shown in Figure 9c, the Al–O bond distances are 1.8112 Å, 1.9326 Å, 2.0394 Å as well as 1.7432 Å and 1.7000Å for Al1, Al2, Al3 and Al4 atoms, respectively.34 In α-Al2O3 cell unit, all Al3+ reside in the same octahedral (12c) positions whose sublattices are fully occupied and the Al–O distances are 1.8764 Å and 2.0257 Å. As for the O2- in both two units, all O2- are closest packed and the oxygen close-packed planes of γ-Al2O3 and α-Al2O3 are stacked in fcc sequence (ABCABC…) and hcp sequence (ABAB…), respectively, as shown in Figure 9d and e. According to the stacking of O and occupation of Al, we implicitly assume that the partial structure of γ-Al2O3 (Figure 9d) could be transformed to that of α-Al2O3 (Figure 9e) by migrating several atom layers along the direction parallel to the close-packed planes.


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Figure 9. The crystal structure of γ-Al2O3 (a) and α-Al2O3 (b) and their fcc (ABC) (d) and hcp (ABA) (e) stacking of O2-. (c) There are four kinds of Al3+ sites in γ-Al2O3 labeled as Al1, Al2, Al3 and Al4 and only one type of Al3+ in α-Al2O3 labeled as Al0.

Based on above illumination about the crystal structures, a potential reconstruction to explain the γ-Al2O3 to α-Al2O3 phase transformation is proposed from the perspective of oxygen atomic stacking. In view of AlN (0001) surface, the whole oxidation of the first bilayer is calculated assuming the N3- in second bilayer 19 ACS Paragon Plus Environment


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will be completely substituted by O2- in the subsequent oxidation as shown in Figures 10a, c and e. Three layers of O planes are stacked in fcc (ABC) sequence with Al3+ occupying the center of octahedral and tetrahedral, which is similar to the geometrical network of γ-Al2O3. This kind of structure can thus be regarded as the precursor of γ-Al2O3. α-Al2O3 can be formed by reconstructing lattice sites of γ-Al2O3. For

instance, if the upper O-Al-O layer migrates along the direction of [010] as indicated by arrow in Figure 10b, the α-Al2O3-like structure (Figures 10b, d and e) can be achieved. The three-layer O planes are stacked in hcp (ABAB) sequence with the first two layers of Al3+ occupying the octahedral interstices, which can be regard as the precursor of α-Al2O3. Thus the transformation of γ-Al2O3 to α-Al2O3 phase can be explained by the composite atom layer migration model with modified oxygen atomic stacking way. To catch physics of the transformation of γ-Al2O3 to α-Al2O3 phase, the total energies of γ-Al2O3 (Figure 9a) and α-Al2O3 crystal cell (Figure 9b) have been calculated. Owing to the different atom number in these two cells, we use the average total energy per atom as the descriptor of the geometry stability. The calculated total energies per atom are –286.38 eV/atom and –157.67 eV/atom for α-Al2O3 and γ-Al2O3, respectively, indicating that α-Al2O3 is more stable with lower total energy. Recently, Dycus et al. reveals the structure of ultrathin oxides that form on AlN surfaces by experiment.6 There are some similarities between our simulated structures and the experimental results, as shown in Figure S3. In both our and Dycus’s study, the oxide layers are consisted of tetrahedral-ortahedra Al-O units, i.e. an O-Al-O trilayer and Al-O bilayer. The O-Al-O trilayer is outer (Figure S3a) in our work. 20 ACS Paragon Plus Environment

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While the Al-O bilayer is outer in Dycus’s work (Figure S3b). The oxide layers are in an arrangement similar to bulk γ-Al2O3 and θ-Al2O3 in our and Dycus’s study, respectively. As we have already mentioned, γ-Al2O3-like structure can be transformed into α-Al2O3-like structure when the upper O-Al-O trilayer migrates along the direction of [010]. Similarly, the θ-Al2O3-like structure can also be transformed into α-Al2O3-like structure by migrating Al-O bilayer along the [010] direction. Based on our recent work,2 the intermediate phases of alumina such as γ-Al2O3 and θ-Al2O3 appear simultaneously during the oxidation of AlN and they all

tend to change to more stable α-Al2O3 at high temperature. As proved by experiments and simulations, γ-Al2O3 and θ-Al2O3 can both be transformed to α-Al2O3 via atom layer migration.

Figure 10. (a, b) Three-dimensional, (c, d) side and (e, f) top views of (a, c, e) the precursor of γ-Al2O3 and (b, d, f) the precursor of α-Al2O3 structures. Red atoms are the O2- at H3 sites; Green and grey atoms represent the O2- substituting the N3- in first and second AlN bilayers, respectively. The atoms labeled by same numbers are the 21 ACS Paragon Plus Environment


same atoms in (e) and (f).

4. CONCLUSION The oxidation mechanism of AlN in dry oxidizing environment was investigated first-principally. It consists of the adsorption of oxygen and oxidation at the atomic level. With the lowest adsorption energy, hollow site (H3) of the AlN (0001) surface is the most preferential site for oxygen atoms to be adsorbed regardless of coverages. The adsorption energy at H3 site gets less negative with O-coverage further increasing up to 1 monolayer (ML). During the oxidation process, N2 is proved to be the dominant gas product with lowest formation energy. With the N3- being removed, a N vacancy can form, followed by the occupation of nearby O2- at the site of VN. The dangling bond states of Al caused by the removal of N3- and the breaking of Al–N bonds are passivated when the O2- transferred to the site of VN. The formation of VN and the migration of O2- are the main factors to promote the oxidation of AlN. The oxidized surface gets more stable owing to the Al p lone-pair electrons balanced out via O adsorption and removal of the Al dangling bonds. Therefore, AlN can be promisingly kept free of oxidation by controlling the surface defects of VN. The γ-Al2O3-like finally configuration with oxidized H3 and N atomic sites can be transformed to α-Al2O3 by composite atom layer migration.

ASSOCIATED CONTENT

Supporting Information. Adsorption energies of O adsorption and formation of N2 remission employing 3×3 model (Figure S1), DOS on the AlN (0001) surface in every 22 ACS Paragon Plus Environment

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step (Figure S2) and transformation of γ-Al2O3 and θ-Al2O3 to α-Al2O3 (Figure S3) are provided as Supporting Information.

AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected]; (X. H.), [email protected] (M. S.) Fax: +86 10 6233 2570; Tel: +86 10 6233 2570.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare that they have no competing financial interests.

ACKNOWLEDGMENT The authors express their appreciation to the National Nature Science Foundation for Excellent Young Scholars of China (No. 51522402). The authors also appreciate the National Nature Science Foundation of China (No. 51572019, No. 11547033 and No. U1460201) and the Central Universities of FRF-TP-15-006C1 for financial support.

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Graphical Table of Contents


Wurtzite AlN (0001) surface oxidation: hints from ab-initio calculations

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