First-principles theoretical study of hydrolysis of stepped and kinked Ga-terminated GaN surfaces
© Oue et al.; licensee Springer. 2013
Received: 31 December 2012
Accepted: 18 March 2013
Published: 16 May 2013
We have investigated the initial stage of hydrolysis process of Ga-terminated GaN surfaces by using first-principles theoretical calculations. We found that the activation barrier of H2O dissociation at the kinked site of the Ga-terminated GaN surface is about 0.8 eV, which is significantly lower than that at the stepped site of about 1.2 eV. This is consistent with the experimental observation where a step-terrace structure is observed after the etching process of Ga-terminated GaN surfaces with catalyst-referred etching method. Detailed analysis on the nature of the chemical interaction uring the hydrolysis processes will be discussed.
GaN has been attracting enormous attention because it is one of the most promising materials for short-wavelength optoelectronic devices such as light-emitting diodes, blue laser diodes, and high-power, high-frequency electronic devices [1, 2]. The performance of these semiconductor devices depends on the quality of GaN crystals, and it is important to prepare atomically smooth, damage-free surfaces for homoepitaxial growth of high-quality GaN layers. Recently, catalyst-referred etching (CARE) has been proposed as a new finishing method. By using this method, atomically smooth surfaces with step-terrace structure were obtained [3–5]. GaN surfaces can be etched even by pure water with Pt as a catalyst [6, 7]. However, the remaining problem in this method is its low removal rate. To find a clue on how to improve the removal rate, it is important to clarify the etching process at the atomic level and find determinant factors in the process. Because step-terrace surfaces were observed in the CARE-processed surfaces, the etching reactions at step edges are considered to be important. In this paper, we analyzed elementary reaction processes and their activation barriers of dissociative adsorption of water and hydrolysis of Ga-terminated GaN surfaces as the initial stage of etching processes by means of first-principles calculations.
Calculation method and model
All calculations were performed using STATE program package  based on density functional theory within a generalized gradient approximation, and we employed an exchange-correlation energy functional proposed by Perdew et al. . We used ultrasoft pseudopotentials to describe the electron-ion interactions . Wave functions are expanded by a plane-wave basis set, and cut-off energies for wave function and charge density are set to be 25 and 225 Ry, respectively. The reaction barriers of dissociative adsorption of water are calculated by a climbing image nudged elastic band (NEB) method .
Results and discussions
Termination of the GaN surface
Figure 2b shows that OH termination is more stable than H termination for all coverages. Moreover, the differential adsorption energy becomes positive for Θ>0.75 ML for both H and OH termination. This can be understood by counting the number of electrons in the surface dangling bonds. Each surface Ga atom has one dangling bond, and on average, three-fourth of the electrons are accommodated in each dangling bond. Therefore, if the coverage of H or OH is 0.75 ML, their dangling bonds are fully occupied by paired electrons, and the remaining 25% of surface dangling bonds become empty, forming a closed-shell electronic structure. A closed-shell electronic structure can be also formed by terminating the remaining 25% dangling bonds with H2O. As seen in Figure 2b, the differential adsorption energy of H2O is −1.93 eV, further stabilizing the OH-terminated GaN surface. An empty Ga dangling bond attracts the lone pairs of H2O as observed at the water/GaN(100) interface . Therefore, in the following calculations, we terminated 75% of surface Ga dangling bonds with OH and 25% with H2O.
Dissociative adsorption of H2O
Barrier height and the energy of the final state relative to the initial state
It is found that the dissociative adsorption of water in the back bond process at the kinked structure is the most energetically favorable path we have investigated so far. Therefore, we think that etching reactions take place predominantly at kinked sites. Note that our kinked model represents an extreme case, and the activation barriers of dissociative adsorption of H2O should be somewhat larger than our calculated values but still smaller than those calculated for stepped sites.
Before closing our discussion, we mention about roles of additional water molecules terminating empty Ga dangling bonds. As discussed above, 75% of surface Ga dangling bonds are terminated by OH and 25% are by H2O. These additional H2O molecules initiate proton transfer on the GaN surfaces and promote chemical reactions at surfaces as discussed by Wang and co-workers . Actually, additional water molecules play an active role in two step processes of H2O dissociation, in which H2O molecule is dissociated, OH is bound to surface Ga, and H is bound to neighboring H2O (MO et al., unpublished results). Following this process, proton transfer takes place to terminate a dangling bond at subsurface N. However, in the direct H2O dissociation we have investigated in the present study, it seems that the additional water molecules are spectator of the reaction, and they play a rather minor role.
In summary, we have investigated the initial stage of hydrolysis process of Ga-terminated GaN surfaces by using first-principles theoretical calculations. The activation barrier of H2O dissociation at kinked sites of the Ga-terminated GaN(0001) surface is about 0.8 eV, which is significantly lower than that at stepped sites of about 1.2 eV, suggesting that etching reactions take place predominantly at kinked sites of GaN surfaces; and this is consistent with the experimental observation where a step-terrace structure is observed after the etching process of Ga-terminated GaN(0001) surfaces with CARE method. The origin for the activation barriers are ascribed to the Pauli repulsion in the early stages of hydrolysis process, while they are ascribed to the bond switching between OH bond of H2O and NH bond at the edge of a stepped site.
This work was partly supported by Grant-in-Aid for Scientific Research (c) from the Ministry of Education, Culture, Science, Sports, and Technology (MEXT), Japan. The numerical calculations were carried out at the computer centers of Osaka University, Tohoku University, and the Institute for Solid State Physics, the University of Tokyo.
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