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Adsorption of ammonia on hydrogen covered GaN(0001) surface – Density Functional Theory study
Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Adsorption of ammonia on hydrogen covered GaN(0001) surface – Density Functional Theory study Paweł Kempisty a,n, Paweł Strak a, Konrad Sakowski a, Stanisław Krukowski a,b a b
Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland Interdisciplinary Centre for Mathematical and Computational Modelling, University of Warsaw, Pawinskiego 5a, 02-106 Warsaw, Poland
art ic l e i nf o
Keywords: A1. Computer simulation A1. Surface processes A3. Metalorganic vapor phase epitaxy B1. Nitrides B2. Semiconducting III–V materials
a b s t r a c t Density Functional Theory (DFT) simulations of ammonia adsorption at clean and H-covered surface confirmed that ammonia may dissociate into NH2 radical and H adatom or remain in the molecular form. The remaining hydrogen atoms are attached to Ga atoms where the charge transfer to the surface is possible. The calculations show that for the molecular process, the ammonia adsorption energy is close to 2.0 eV, independent of hydrogen coverage. The dissociative process is strongly H-coverage dependent, for low H-coverage the adsorption energy is close to 2.8 eV, for high coverage changes by more than 4 eV reaching negative values. Thus for low coverage the energetically preferred adsorption is dissociative, for high is molecular. The dissociation energy and preferred mode change are related to the change of the Fermi level pinning from Ga broken bond state to valence band maximum (VBM), confirming the decisive role of charge transfer in the adsorption processes. & 2013 Elsevier B.V. All rights reserved.
1. Introduction Due to their importance, physical properties of GaN(0001) surface and their change in different external conditions were investigated intensively, using both experimental and theoretical methods. These investigations led not only to the considerable increase of the basic knowledge but also to the development of the new methods of simulations. These new methods are now sufficiently mature to simulate the semiconductor (insulator's) surfaces fully accounting for the influence of the charged surface states and the field in the semiconductor interior, close to the surface [1,2]. Density Functional Theory (DFT) simulations of hydrogen covered GaN(0001) surface suggested that 0.75 ML H (hydrogen monolayer) coverage, with H atoms located directly on-top of Ga atoms is the stable energy configuration either with high [3–8] or low [9] bounding energy. Different results were obtained by Bermudez investigation using Ultraviolet Photoemission Spectroscopic (UPS) found the on-top position to be incompatible with the UPS data so that alternative Ga and H co-adsorbed structure was proposed [10,11]. Nevertheless, more recent charge-field DFT simulations confirmed the on-top position for the hydrogen coverage ranging from 0 to 1 ML coverage [12]. The surface state associated with the Ga–H bond is degenerate with the valence band. For the coverage up to 0.75 ML the Fermi level is pinned by the Ga broken bond. The 0.75 ML coverage was found to be n
Corresponding author. E-mail addresses: [email protected] (P. Kempisty), [email protected]. pl (P. Strak), [email protected] (K. Sakowski), [email protected] (S. Krukowski).
singular as the Fermi level is not pinned. For higher coverage the Fermi energy is pinned at the Ga–H surface state, which is degenerated in valence band [12]. These results are in the agreement with earlier report of Pignedoli et al. [5] that a 0.75 ML covered surface is free form of the surface states. The adsorption of ammonia on GaN(0001) surface was investigated by DFT calculations only in limited scope. Van de Walle and Neugebauer constructed GaN(0001) phase diagram in coordinates of chemical potentials of hydrogen and nitrogen, finding regions of stability of several surface structures [7,8]. Recently, these results were confirmed by extensive DFT calculations by Ito et al. [13] and the stable structures of polar and nonpolar GaN(0001) surfaces were determined. The reaction of ammonia with the bare and Hcovered surface was DFT-investigated by several groups [4–6]. Fritsch et al. [4] simulated several configuration of GaN(0001) surface and claimed that ammonia adsorption is dissociative and leads to decomposition of ammonia to H and NH2 radical and transformation of GaN(0001) flat surface into p(2 2) vacancy reconstruction. The NH2 radical was bound to gallium and H atom to gallium and nitrogen broken bonds respectively. This reconstruction was not confirmed by any other calculations. The adsorption energy barrier was found to be of order of 0.5 eV. The DFT calculations by Pignedoli et al. [5,6] also confirmed ammonia decomposition into H and NH2 species during adsorption at clean GaN(0001) surface. However, in these calculations the surface remained flat with both products adsorbed on-top of Ga surface atoms. Similarly, dissociative adsorption of ammonia was obtained by Bermudez [11] who found that the NH2 radical is adsorbed in the asymmetric bridge position at clean GaN(0001) surface. Recently, cluster approach was used by Cardelinos to
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.10.061
Please cite this article as: P. Kempisty, et al., Journal of Crystal Growth (2013), http://dx.doi.org/10.1016/j.jcrysgro.2013.10.061i
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investigate surface reaction of several species, including ammonia, with clean GaN(0001) surface [14]. They found that adsorption of ammonia at GaN(0001) surface leads to increase of the dissociation probability of the third hydrogen atom from the ammonia molecule. The experimental investigations of the ammonia adsorption at GaN(0001) are not numerous. Supersonic molecular beams investigation of the ammonia adsorption indicated that it is unactivated and it proceeds via precursor-mediated mechanism, leading to ultimate dissociative stage in which the molecule disintegrates to NHx radicals [15]. It is not clear whether the surface was clean or hydrogen covered. In all these investigations the hydrogen coverage of the initial GaN(0001) surface has not been investigated systematically. Thus the full overview of the ammonia adsorption is not available. Also the change of the electronic properties and the electric state of the surface were not obtained.
2. Calculations details In the calculations reported below a freely accessible DFT code SIESTA [16,17], combining norm conserving pseudopotentials with the local basis functions, was used. Calculations were made in General Gradient Approximation with Wu–Cohen modification of Perdew, Burke and Ernzerhof exchange-correlation functional (GGA-WC) [18,19]. The norm-conserving Troullier–Martins pseudopotential, in the Kleinmann–Bylander factorized form were generated [20–22]. Gallium 3d electrons were included in the valence electron set explicitly. The basis functions have the following size: Gabulk – 4s: DZ (double zeta), 4p: DZ, 3d: SZ (single zeta), 4d: SZ; Gasurface – 4s: TZ (triple zeta), 4p: TZ, 3d: SZ, 4d: SZ; Nbulk – 2s: TZ, 2p: DZ; Nsurface – 2s: TZ, 2p: TZ, 3d: SZ; H – 1s: QZ (quadruple zeta), 2p: SZ. A grid in real space was obtained using an equivalent plane wave cutoff of 275 Ry. Integrals in k-space were performed using 3 3 1 Monkhorst–Pack grid for slab with lateral size 2 2 unit cell and only Γ point for 4 4 slab. The vacuum regions of about 48 Å were used to isolate slab replicas. The bottom artificial termination surface of the slab is passivated by pseudo-hydrogen atoms of the fractional atomic number Z¼0.75. The three topmost layers of atoms are fully relaxed and the all others are fixed at the ideal lattice positions. As a convergence criterion, terminating SCF loop, the maximum difference between the output and the input of each element of the density matrix was employed being equal or smaller than 10 4 . Relaxation of atomic position is stopped when the forces on the atoms become smaller than 0.04 eV/Å. The following values for the lattice constants of bulk GaN were obtained: a ¼b ¼3.2021 Å, c¼ 5.2124 Å, which are in good agreement with the experimental data for GaN: a ¼3.189 Å and c ¼5.185 Å [23]. The Born–Oppenheimer approximation was used for determination of the energy barriers in which an effective procedure, based on nudged elastic band (NEB) method was applied [24]. The NEB method finds minimum energy pathways (MEP) between two stable points, which has to be predetermined first. The MEP is characterized by at least one first order saddle point, finding the energy barrier corresponding to an activated energy complex approximation in chemical reaction kinetics. In the present formulation NEB module was linked to SIESTA package paving the way to fast determination of the energy and conformation of the ammonia species along the optimized pathways.
3. Results and discussion 3.1. Adsorption energy – a role of electron transfer For the processes involving charge transfer the adsorption energy dependence on the doping of the semiconductor is drastically
different for the surface with Fermi level pinned and nonpinned by the surface states in which the difference originates from the energetic effect of the electron transfer between the surface state and the bulk. Since all surface state energies are shifted by the surface band bending equally, therefore the electron transferred from the bulk states at the Fermi level to the emerging surface state gains the energy equal to the energy gain in transfer from the pinning surface state. Thus the energy difference is the same for ntype and p-type material as both surface states are shifted by the same energy by surface band bending. The simulations of the adsorption of hydrogen confirmed that the adsorption energy is virtually independent on the Fermi level in the sample [25]. Naturally, the question arises whether the adsorption energy is independent on the Fermi level pinning surface state. Thus this issue involves the two different states of the surface in which the Fermi level is pinned by the two different states, i.e. having different energy with respect to valence and conduction bands at the surface. This could be realized by two different coverages of the surface. A notable example is the GaN(0001) surface under small and high hydrogen coverage [26,27]. For the small hydrogen coverage, the Fermi level is pinned by Ga broken bond state located at 0.45 eV below CBM. Since the Ga–H state is degenerate with the valence band, for high hydrogen coverage, the Fermi level is pinned at the VBM. Thus the energy difference is about 3 eV, which in DFT implementation it is reduced to the value close to 1.6 eV. The adsorption of ammonia on H-covered GaN(0001) surface, investigated below is used in detailed studies of the various contributions to adsorption energy. The ammonia molecule may be adsorbed in molecular form or dissociated upon adsorption into separate NH2 radical and H adatom [5,6,11]. Thus the emerging species have different bonding that entails different charge transfer to the surface states. The comparison of the different H-coverage allowed to study influence of different pinning states and verified description presented above. In addition, the systematic studies of the adsorption energies are provided which are of considerable importance to the thermodynamic stability of the surface coverages and the molecular mechanism of crystal growth from the vapor in general and MOVPE and HVPE in particular. 3.2. Adsorption of ammonia at clean GaN(0001) surface As discussed in the Introduction, the adsorption of ammonia may lead to two different atomic configurations: first in which the entire molecule was attached on the surface, and the second presenting dissociation to NH2 radical and H atom which were attached in the bridge and on-top positions respectively. These two positions are presented in Fig. 1. The adsorption energyposition dependence, obtained from NEB simulations is shown on the same diagram. From the above data it follows that both these processes are barrierless, leading to energy minima at 1.87 eV and 2.71 eV for molecular and dissociative adsorption, respectively. Thus the dissociative process leads to stable configuration, the present result being in accordance with the earlier results of Pignedoli et al. [5,6] and Bermudez [11]. According to the latter report, the NH2 radical is attached to the surface in the asymmetric bridge position. The present data confirm that, the ammonia adsorbed molecularly at the surface is in a metastable state, 0.84 eV above the stable dissociated configuration. The dissociative adsorption is favorable energetically which reflects general tendency of the surface to reduce occupation of the surface states pinning the Fermi level. For the case of the molecular adsorption only single Ga broken bond state is saturated while in the case of dissociative adsorption, the three states are saturated which overcompensates the reduction of the Ga–N overlap in the skewed bond configuration and energetically favors
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Fig. 1. Ammonia adsorption at clean GaN(0001) surface: adsorption energy vs distance dependence, inset – atomic configurations at the surface, dissociated (left), molecular (middle) and at far distance (right).
the bridge configuration. It is therefore expected that for low H-coverage the ammonia adsorption scenario will be identical to that of clean surface. 3.3. Adsorption of ammonia at H-covered GaN(0001) surface The adsorption of ammonia on partially or fully covered surface depends drastically on the hydrogen coverage. Since the initial and final states change in the slab representation by finite amount of coverage, the process analysis is based on 4 4 slabs, which allow investigation of the partial coverage of the surface in 1/16 ML intervals, starting from low coverage where the dissociation of the ammonia molecule and creation of a bridge configuration dominates in direct analogy to the clean surface discussed above. The alternative is molecular adsorption leaving two more Ga broken bonds unsaturated. For low hydrogen covered surface both processes are barrierless (as shown in Fig. 2), which indicates on the strong tendency to saturate broken Ga bonds, dominating the whole process. The molecular process is characterized by the adsorption energy close to 2.0 eV, virtually independent of the coverage up to 0.8750 ML, as shown in Fig. 3. In contrast to that, the dissociative process is characterized by the radical change from the 0.6875 ML initial coverage. For higher hydrogen coverage, the adsorption energy is drastically reduced, and for 0.75 ML the dissociative adsorption becomes energetically unfavorable. The mechanism leading to such drastic difference is related to different charge transfer in these two processes [25]. The dissociative adsorption leads to attachment of additional hydrogen at the surface which pins the Fermi level at the valence band maximum (VBM). The energy of such state is dramatically increased by pinning and the field at the surface. Thus dissociative adsorption of ammonia at the coverage of 0.75 ML of hydrogen or higher is energetically unfavorable, confirming that Fermi level pinning is energetically costly. Ammonia molecule can adsorb on the surface also in the site covered with hydrogen adatom. In the process two species are formed: NH2 radical, remaining on the surface in position above Ga atom and hydrogen molecule, released into the gas phase as schematically shown in Fig. 4. Since the change of energy in the process is negative (Table 1), so this mechanism enables to transform surface coverage into a mixture of NH3 molecules and NH2 radicals.
4. Conclusions The adsorbing ammonia may dissociate into NH2 radical and H adatom or remain in the molecular form. It is shown that the 0.75 ML hydrogen coverage of GaN(0001) surface, thermodynamically preferable, is the border line between the two different
Fig. 2. The energy change vs distance dependence during adsorption of NH3 molecule on partially hydrogen covered GaN(0001) surface: (a) molecular and (b) dissociative process.
dissociation withhout dissociation
Fig. 3. Adsorption energies of ammonia molecule on relatively highly hydrogen covered GaN(0001) surface: blue – dissociative and red – molecular process.
adsorption scenarios. In the case of low hydrogen coverage up to 0.75 ML, the ammonia molecule dissociates to NH2 radical attached in the bridge position and the H adatom located in the on-top position. In this case the adsorption energy is close to 2.7 eV. For the case of relatively high hydrogen coverage, above 0.75 ML, the ammonia is adsorbed in the molecular form. The process leads to energy decrease of about 2.0 eV. The dissociation of NH3 is energetically unfavorable due to associated increase of hydrogen coverage which is energetically costly in this case. The molecular process is independent on the coverage of the neighboring sites by hydrogen adatoms. Even the process of adsorption of ammonia at single vacant state leads to virtually identical adsorption energy, close to 2.0 eV. It is therefore expected that
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Fig. 4. Adsorption of NH3 molecule on hydrogen covered site of GaN(0001) surface covered with 25% NH3 admolecules and 75% H adatoms. As a result of this process a NH2 radical is formed on the surface and H2 molecule is detached.
Table 1 Energy differences after adsorption of ammonia molecule on hydrogen covered site resulting in NH2 radical formation and desorption of H2 molecule. Initial coverage
Final coverage
ΔE (eV)
12-H 4-NH3 11-H 4-NH3 1-NH2 10-H 4-NH3 2-NH2
11-H 4-NH3 1-NH2 10-H 4-NH3 2-NH2 9-H 4-NH3 3-NH2
0.47 0.46 0.45
exposure of partially hydrogen covered surface leads to complete filling by the ammonia admolecules with no vacant Ga sites. Finally, the surface coverage can be converted to a mixture of NH3 molecules and NH2 radicals by adsorption of ammonia at the sites containing hydrogen.
Acknowledgement The research was supported by funds of National Science Center of Poland granted by decision No. DEC-2011/01/N/ST3/ 04382. This research was supported in part by PL-Grid Infrastructure. Images of atomic configurations were generated using XCRYSDEN shareware [28]. References [1] P. Kempisty, S. Krukowski, J. Appl. Phys. 112 (2012) 113704. [2] S. Krukowski, P. Kempisty, P. Strak, J. Appl. Phys. 114 (2013) 143705.
[3] J. Elsner, M. Haugk, G. Jungnickel, T. Frauenheim, Solid State Commun. 106 (1998) 739. [4] J. Fritsch, O.F. Sankey, K.E. Smith, J.B. Page, Phys. Rev. B 57 (1998) 15360. [5] C.A. Pignedoli, R.D. Felice, C.M. Bertoni, Phys. Rev. B 64 (2001) 113301. [6] C.A. Pignedoli, R.D. Felice, C.M. Bertoni, Surf. Sci. 547 (2003) 63. [7] C.G. Van de Walle, J. Neugebauer, Phys. Rev. Lett. 88 (2002) 066103. [8] C.G. Van de Walle, J. Neugebauer, J. Cryst. Growth 248 (2003) 8. [9] R.M. Feenstra, J.E. Northrup, J. Neugebauer, MRS Internet J. Nitride Semicond. Res. 7 (2002) 3. [10] V.M. Bermudez, Surf. Sci. 565 (2004) 89. [11] V.M. Bermudez, Chem. Phys. Lett. 317 (2000) 290. [12] P. Kempisty, P. Strak, S. Krukowski, Surf. Sci. 605 (2011) 695. [13] T. Ito, T. Akiyama, K. Nakamura, Semicond. Sci. Technol. 27 (2012) 024010. [14] B.H. Cardelino, C.A. Cardelino, J. Phys. Chem. C 115 (2011) 9090. [15] A. McGinnis, D. Thomson, R. Davis, E. Chen, A. Michel, H. Lamb, Surf. Sci. 494 (2001) 28–42. [16] P. Ordejón, E. Artacho, J.M. Soler, Phys. Rev. B 53 (1996) R10441. [17] J.M. Soler, E. Artacho, J.D. Gale, A. Garcia, J. Junquera, P. Ordejón, D. SánchezPortal, J. Phys.: Condens. Matter 14 (2002) 2745. [18] Z. Wu, R.E. Cohen, Phys. Rev. B 73 (2006) 235116. [19] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [20] N. Troullier, J.L. Martins, Phys. Rev. B 43 (1991) 1993. [21] N. Troullier, J.L. Martins, Phys. Rev. B 43 (1991) 8861. [22] L. Kleinman, D.M. Bylander, Phys. Rev. Lett. 48 (1982) 1425. [23] M. Leszczynski, H. Teisseyre, T. Suski, I. Grzegory, M. Bockowski, J. Jun, S. Porowski, K. Pakula, J. Baranowski, C.T. Foxon, T.S. Cheng, Appl. Phys. Lett. 69 (1996) 73. [24] H. Jónsson, G. Mills, K.W. Jacobsen, in: B.J. Berne, G. Ciccotti, D.F. Coker (Eds.), Classical and Quantum Dynamics in Condensed Phase Simulations, World Scientific, 1998, p. 385. [25] S. Krukowski, P. Kempisty, P. Strak, J. Appl. Phys. 114 (2013) 063507. [26] P. Kempisty, S. Krukowski, J. Cryst. Growth 358 (2012) 64. [27] S. Krukowski, P. Kempisty, P. Strak, Phys. Status Solidi C 9 (2012) 826. [28] A. Kokalj, J. Mol. Graph. Modell. 17 (1999) 176–179.
Please cite this article as: P. Kempisty, et al., Journal of Crystal Growth (2013), http://dx.doi.org/10.1016/j.jcrysgro.2013.10.061i
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Adsorption of ammonia on hydrogen covered GaN(0001) surface – Density Functional Theory study Paweł Kempisty a,n, Paweł Strak a, Konrad Sakowski a, Stanisław Krukowski a,b a b
Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland Interdisciplinary Centre for Mathematical and Computational Modelling, University of Warsaw, Pawinskiego 5a, 02-106 Warsaw, Poland
art ic l e i nf o
Keywords: A1. Computer simulation A1. Surface processes A3. Metalorganic vapor phase epitaxy B1. Nitrides B2. Semiconducting III–V materials
a b s t r a c t Density Functional Theory (DFT) simulations of ammonia adsorption at clean and H-covered surface confirmed that ammonia may dissociate into NH2 radical and H adatom or remain in the molecular form. The remaining hydrogen atoms are attached to Ga atoms where the charge transfer to the surface is possible. The calculations show that for the molecular process, the ammonia adsorption energy is close to 2.0 eV, independent of hydrogen coverage. The dissociative process is strongly H-coverage dependent, for low H-coverage the adsorption energy is close to 2.8 eV, for high coverage changes by more than 4 eV reaching negative values. Thus for low coverage the energetically preferred adsorption is dissociative, for high is molecular. The dissociation energy and preferred mode change are related to the change of the Fermi level pinning from Ga broken bond state to valence band maximum (VBM), confirming the decisive role of charge transfer in the adsorption processes. & 2013 Elsevier B.V. All rights reserved.
1. Introduction Due to their importance, physical properties of GaN(0001) surface and their change in different external conditions were investigated intensively, using both experimental and theoretical methods. These investigations led not only to the considerable increase of the basic knowledge but also to the development of the new methods of simulations. These new methods are now sufficiently mature to simulate the semiconductor (insulator's) surfaces fully accounting for the influence of the charged surface states and the field in the semiconductor interior, close to the surface [1,2]. Density Functional Theory (DFT) simulations of hydrogen covered GaN(0001) surface suggested that 0.75 ML H (hydrogen monolayer) coverage, with H atoms located directly on-top of Ga atoms is the stable energy configuration either with high [3–8] or low [9] bounding energy. Different results were obtained by Bermudez investigation using Ultraviolet Photoemission Spectroscopic (UPS) found the on-top position to be incompatible with the UPS data so that alternative Ga and H co-adsorbed structure was proposed [10,11]. Nevertheless, more recent charge-field DFT simulations confirmed the on-top position for the hydrogen coverage ranging from 0 to 1 ML coverage [12]. The surface state associated with the Ga–H bond is degenerate with the valence band. For the coverage up to 0.75 ML the Fermi level is pinned by the Ga broken bond. The 0.75 ML coverage was found to be n
Corresponding author. E-mail addresses: [email protected] (P. Kempisty), [email protected]. pl (P. Strak), [email protected] (K. Sakowski), [email protected] (S. Krukowski).
singular as the Fermi level is not pinned. For higher coverage the Fermi energy is pinned at the Ga–H surface state, which is degenerated in valence band [12]. These results are in the agreement with earlier report of Pignedoli et al. [5] that a 0.75 ML covered surface is free form of the surface states. The adsorption of ammonia on GaN(0001) surface was investigated by DFT calculations only in limited scope. Van de Walle and Neugebauer constructed GaN(0001) phase diagram in coordinates of chemical potentials of hydrogen and nitrogen, finding regions of stability of several surface structures [7,8]. Recently, these results were confirmed by extensive DFT calculations by Ito et al. [13] and the stable structures of polar and nonpolar GaN(0001) surfaces were determined. The reaction of ammonia with the bare and Hcovered surface was DFT-investigated by several groups [4–6]. Fritsch et al. [4] simulated several configuration of GaN(0001) surface and claimed that ammonia adsorption is dissociative and leads to decomposition of ammonia to H and NH2 radical and transformation of GaN(0001) flat surface into p(2 2) vacancy reconstruction. The NH2 radical was bound to gallium and H atom to gallium and nitrogen broken bonds respectively. This reconstruction was not confirmed by any other calculations. The adsorption energy barrier was found to be of order of 0.5 eV. The DFT calculations by Pignedoli et al. [5,6] also confirmed ammonia decomposition into H and NH2 species during adsorption at clean GaN(0001) surface. However, in these calculations the surface remained flat with both products adsorbed on-top of Ga surface atoms. Similarly, dissociative adsorption of ammonia was obtained by Bermudez [11] who found that the NH2 radical is adsorbed in the asymmetric bridge position at clean GaN(0001) surface. Recently, cluster approach was used by Cardelinos to
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.10.061
Please cite this article as: P. Kempisty, et al., Journal of Crystal Growth (2013), http://dx.doi.org/10.1016/j.jcrysgro.2013.10.061i
P. Kempisty et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
investigate surface reaction of several species, including ammonia, with clean GaN(0001) surface [14]. They found that adsorption of ammonia at GaN(0001) surface leads to increase of the dissociation probability of the third hydrogen atom from the ammonia molecule. The experimental investigations of the ammonia adsorption at GaN(0001) are not numerous. Supersonic molecular beams investigation of the ammonia adsorption indicated that it is unactivated and it proceeds via precursor-mediated mechanism, leading to ultimate dissociative stage in which the molecule disintegrates to NHx radicals [15]. It is not clear whether the surface was clean or hydrogen covered. In all these investigations the hydrogen coverage of the initial GaN(0001) surface has not been investigated systematically. Thus the full overview of the ammonia adsorption is not available. Also the change of the electronic properties and the electric state of the surface were not obtained.
2. Calculations details In the calculations reported below a freely accessible DFT code SIESTA [16,17], combining norm conserving pseudopotentials with the local basis functions, was used. Calculations were made in General Gradient Approximation with Wu–Cohen modification of Perdew, Burke and Ernzerhof exchange-correlation functional (GGA-WC) [18,19]. The norm-conserving Troullier–Martins pseudopotential, in the Kleinmann–Bylander factorized form were generated [20–22]. Gallium 3d electrons were included in the valence electron set explicitly. The basis functions have the following size: Gabulk – 4s: DZ (double zeta), 4p: DZ, 3d: SZ (single zeta), 4d: SZ; Gasurface – 4s: TZ (triple zeta), 4p: TZ, 3d: SZ, 4d: SZ; Nbulk – 2s: TZ, 2p: DZ; Nsurface – 2s: TZ, 2p: TZ, 3d: SZ; H – 1s: QZ (quadruple zeta), 2p: SZ. A grid in real space was obtained using an equivalent plane wave cutoff of 275 Ry. Integrals in k-space were performed using 3 3 1 Monkhorst–Pack grid for slab with lateral size 2 2 unit cell and only Γ point for 4 4 slab. The vacuum regions of about 48 Å were used to isolate slab replicas. The bottom artificial termination surface of the slab is passivated by pseudo-hydrogen atoms of the fractional atomic number Z¼0.75. The three topmost layers of atoms are fully relaxed and the all others are fixed at the ideal lattice positions. As a convergence criterion, terminating SCF loop, the maximum difference between the output and the input of each element of the density matrix was employed being equal or smaller than 10 4 . Relaxation of atomic position is stopped when the forces on the atoms become smaller than 0.04 eV/Å. The following values for the lattice constants of bulk GaN were obtained: a ¼b ¼3.2021 Å, c¼ 5.2124 Å, which are in good agreement with the experimental data for GaN: a ¼3.189 Å and c ¼5.185 Å [23]. The Born–Oppenheimer approximation was used for determination of the energy barriers in which an effective procedure, based on nudged elastic band (NEB) method was applied [24]. The NEB method finds minimum energy pathways (MEP) between two stable points, which has to be predetermined first. The MEP is characterized by at least one first order saddle point, finding the energy barrier corresponding to an activated energy complex approximation in chemical reaction kinetics. In the present formulation NEB module was linked to SIESTA package paving the way to fast determination of the energy and conformation of the ammonia species along the optimized pathways.
3. Results and discussion 3.1. Adsorption energy – a role of electron transfer For the processes involving charge transfer the adsorption energy dependence on the doping of the semiconductor is drastically
different for the surface with Fermi level pinned and nonpinned by the surface states in which the difference originates from the energetic effect of the electron transfer between the surface state and the bulk. Since all surface state energies are shifted by the surface band bending equally, therefore the electron transferred from the bulk states at the Fermi level to the emerging surface state gains the energy equal to the energy gain in transfer from the pinning surface state. Thus the energy difference is the same for ntype and p-type material as both surface states are shifted by the same energy by surface band bending. The simulations of the adsorption of hydrogen confirmed that the adsorption energy is virtually independent on the Fermi level in the sample [25]. Naturally, the question arises whether the adsorption energy is independent on the Fermi level pinning surface state. Thus this issue involves the two different states of the surface in which the Fermi level is pinned by the two different states, i.e. having different energy with respect to valence and conduction bands at the surface. This could be realized by two different coverages of the surface. A notable example is the GaN(0001) surface under small and high hydrogen coverage [26,27]. For the small hydrogen coverage, the Fermi level is pinned by Ga broken bond state located at 0.45 eV below CBM. Since the Ga–H state is degenerate with the valence band, for high hydrogen coverage, the Fermi level is pinned at the VBM. Thus the energy difference is about 3 eV, which in DFT implementation it is reduced to the value close to 1.6 eV. The adsorption of ammonia on H-covered GaN(0001) surface, investigated below is used in detailed studies of the various contributions to adsorption energy. The ammonia molecule may be adsorbed in molecular form or dissociated upon adsorption into separate NH2 radical and H adatom [5,6,11]. Thus the emerging species have different bonding that entails different charge transfer to the surface states. The comparison of the different H-coverage allowed to study influence of different pinning states and verified description presented above. In addition, the systematic studies of the adsorption energies are provided which are of considerable importance to the thermodynamic stability of the surface coverages and the molecular mechanism of crystal growth from the vapor in general and MOVPE and HVPE in particular. 3.2. Adsorption of ammonia at clean GaN(0001) surface As discussed in the Introduction, the adsorption of ammonia may lead to two different atomic configurations: first in which the entire molecule was attached on the surface, and the second presenting dissociation to NH2 radical and H atom which were attached in the bridge and on-top positions respectively. These two positions are presented in Fig. 1. The adsorption energyposition dependence, obtained from NEB simulations is shown on the same diagram. From the above data it follows that both these processes are barrierless, leading to energy minima at 1.87 eV and 2.71 eV for molecular and dissociative adsorption, respectively. Thus the dissociative process leads to stable configuration, the present result being in accordance with the earlier results of Pignedoli et al. [5,6] and Bermudez [11]. According to the latter report, the NH2 radical is attached to the surface in the asymmetric bridge position. The present data confirm that, the ammonia adsorbed molecularly at the surface is in a metastable state, 0.84 eV above the stable dissociated configuration. The dissociative adsorption is favorable energetically which reflects general tendency of the surface to reduce occupation of the surface states pinning the Fermi level. For the case of the molecular adsorption only single Ga broken bond state is saturated while in the case of dissociative adsorption, the three states are saturated which overcompensates the reduction of the Ga–N overlap in the skewed bond configuration and energetically favors
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Fig. 1. Ammonia adsorption at clean GaN(0001) surface: adsorption energy vs distance dependence, inset – atomic configurations at the surface, dissociated (left), molecular (middle) and at far distance (right).
the bridge configuration. It is therefore expected that for low H-coverage the ammonia adsorption scenario will be identical to that of clean surface. 3.3. Adsorption of ammonia at H-covered GaN(0001) surface The adsorption of ammonia on partially or fully covered surface depends drastically on the hydrogen coverage. Since the initial and final states change in the slab representation by finite amount of coverage, the process analysis is based on 4 4 slabs, which allow investigation of the partial coverage of the surface in 1/16 ML intervals, starting from low coverage where the dissociation of the ammonia molecule and creation of a bridge configuration dominates in direct analogy to the clean surface discussed above. The alternative is molecular adsorption leaving two more Ga broken bonds unsaturated. For low hydrogen covered surface both processes are barrierless (as shown in Fig. 2), which indicates on the strong tendency to saturate broken Ga bonds, dominating the whole process. The molecular process is characterized by the adsorption energy close to 2.0 eV, virtually independent of the coverage up to 0.8750 ML, as shown in Fig. 3. In contrast to that, the dissociative process is characterized by the radical change from the 0.6875 ML initial coverage. For higher hydrogen coverage, the adsorption energy is drastically reduced, and for 0.75 ML the dissociative adsorption becomes energetically unfavorable. The mechanism leading to such drastic difference is related to different charge transfer in these two processes [25]. The dissociative adsorption leads to attachment of additional hydrogen at the surface which pins the Fermi level at the valence band maximum (VBM). The energy of such state is dramatically increased by pinning and the field at the surface. Thus dissociative adsorption of ammonia at the coverage of 0.75 ML of hydrogen or higher is energetically unfavorable, confirming that Fermi level pinning is energetically costly. Ammonia molecule can adsorb on the surface also in the site covered with hydrogen adatom. In the process two species are formed: NH2 radical, remaining on the surface in position above Ga atom and hydrogen molecule, released into the gas phase as schematically shown in Fig. 4. Since the change of energy in the process is negative (Table 1), so this mechanism enables to transform surface coverage into a mixture of NH3 molecules and NH2 radicals.
4. Conclusions The adsorbing ammonia may dissociate into NH2 radical and H adatom or remain in the molecular form. It is shown that the 0.75 ML hydrogen coverage of GaN(0001) surface, thermodynamically preferable, is the border line between the two different
Fig. 2. The energy change vs distance dependence during adsorption of NH3 molecule on partially hydrogen covered GaN(0001) surface: (a) molecular and (b) dissociative process.
dissociation withhout dissociation
Fig. 3. Adsorption energies of ammonia molecule on relatively highly hydrogen covered GaN(0001) surface: blue – dissociative and red – molecular process.
adsorption scenarios. In the case of low hydrogen coverage up to 0.75 ML, the ammonia molecule dissociates to NH2 radical attached in the bridge position and the H adatom located in the on-top position. In this case the adsorption energy is close to 2.7 eV. For the case of relatively high hydrogen coverage, above 0.75 ML, the ammonia is adsorbed in the molecular form. The process leads to energy decrease of about 2.0 eV. The dissociation of NH3 is energetically unfavorable due to associated increase of hydrogen coverage which is energetically costly in this case. The molecular process is independent on the coverage of the neighboring sites by hydrogen adatoms. Even the process of adsorption of ammonia at single vacant state leads to virtually identical adsorption energy, close to 2.0 eV. It is therefore expected that
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Fig. 4. Adsorption of NH3 molecule on hydrogen covered site of GaN(0001) surface covered with 25% NH3 admolecules and 75% H adatoms. As a result of this process a NH2 radical is formed on the surface and H2 molecule is detached.
Table 1 Energy differences after adsorption of ammonia molecule on hydrogen covered site resulting in NH2 radical formation and desorption of H2 molecule. Initial coverage
Final coverage
ΔE (eV)
12-H 4-NH3 11-H 4-NH3 1-NH2 10-H 4-NH3 2-NH2
11-H 4-NH3 1-NH2 10-H 4-NH3 2-NH2 9-H 4-NH3 3-NH2
0.47 0.46 0.45
exposure of partially hydrogen covered surface leads to complete filling by the ammonia admolecules with no vacant Ga sites. Finally, the surface coverage can be converted to a mixture of NH3 molecules and NH2 radicals by adsorption of ammonia at the sites containing hydrogen.
Acknowledgement The research was supported by funds of National Science Center of Poland granted by decision No. DEC-2011/01/N/ST3/ 04382. This research was supported in part by PL-Grid Infrastructure. Images of atomic configurations were generated using XCRYSDEN shareware [28]. References [1] P. Kempisty, S. Krukowski, J. Appl. Phys. 112 (2012) 113704. [2] S. Krukowski, P. Kempisty, P. Strak, J. Appl. Phys. 114 (2013) 143705.
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