Ab Initio Study of Substitutional Chlorine and Related Complexes in GaN
G. Alfieri1,*, Y. Fujii2, R. Micheletto2
1
ABB Corporate Research Center, Segelhofstr. 1K, Baden-Dättwil 5405, Japan
2
Yokohama City University, Dept. of Nanosystem Sciences Yokohama 236-0027, Japan*E-mail: giovanni.alfieri@ch.abb.com
Chlorine based reactive ion etching (RIE) is known to decrease ohmic contact resistivity inn-type GaN [1,2] and to reduce carrier concentration in p-type GaN [3]. The improvement ofcontact resistivity was attributed to nitrogen vacancies (VN) [1, 3]. However, the formationenergy (Eform) is such [4] that, VN may not be responsible for the increased surface donor
density in n-type GaN. On the other hand, the decreased carrier concentration, in p-type GaN,
was attributed to defects other than VN [3]. In order to shed light on these two effects, wecarried out a theoretical study of Cl, substitutional and related defects, in wurtzite GaN.
An ab-initio study was carried out by using SIESTA (LDA). 3d-electrons of Ga were treatedas valence electrons. Formation energies were calculated by using the Makov-Payne
correction or the potential alignment [5] correction.
First, we found that Cl is more stable in an N-site (ClN), than in a Ga-site, by ~2 eV. After
geometry relaxation, ClN has Td symmetry and our Watkins model predicts the presence of ten
electrons (three from VN and seven from Cl). These are accommodated in two singlets andone triply degenerate level. Analysis of its Eform reveals that ClN is a double donor for every
value of EF in the Kohn-Sham Egap (~2.0 eV). The defect-molecule model of ClNMgGa (C1)consists of nine electrons with a spare electron in a singlet level (a). We found that ClNMgGa isa single donor for each value of the EF.
For ClNVGa (C3v), we predict the presence of seven electrons, five from VGa and two from theClN donor. This suggests that a spare electron is present in a singlet level (a1) and our results
show the presence of a (0/+) and (-/0) level, located at EV+0.20 eV and EV+1.43 eV,
respectively. By applying the potential alignment corrections [5], these transition levels arefound at EV+0.41 eV and EV+1.22 eV.
ClN possesses a lower Eform than VN in the upper part of the Kohn-Sham Egap. This can explain
why Cl-based RIE is beneficial in terms of ohmic contacts in n-type GaN [1]. Furthermore,
the observed reduction of carrier concentration in Cl-plasma etched p-type GaN [3] can bealso explained in terms of lower Eform, of both ClN and ClNMgGa, compared to VN.
[1] T. Fujishima et al., Appl. Phys. Lett. 103, 083508 (2013)[2] S. Joglekar et al., Appl. Phys. Lett. 109, 041602 (2016)[3] J.M. Lee et al., J. Vac. Sci. Technol. B 22, 479 (2004)[4] J.L. Lyons et al., Computational Materials 3, 12 (2017)[5] C.G. Van de Walle et al., J. Appl. Phys. 95, 3851 (2004)

Supplementary information
Table.1. Test for evaluating the methodology employed in our study. With the Potentialalignment correction the transition level of the MgGa acceptor is found at 0.20 eV above thevalence band edge (EV) and VN shows a negative-U behavior (the +2 charge state is notstable). By using the Makov-Payne correction, MgGa is at EV+0.22 eV and no negative-Ubehavior is found for VN.
Correction usedMgGa

VN

Potential Alignment

EV+0.20 eV

negative-U behavior

Makov-Payne

EV+0.22 eV

no negative-U behavior

Fig.1. Formation energies of VN, ClN, ClNMgGa and ClNVGa, for Ga-rich conditions, adjusted
by using (a) Madelung correction and (b) the potential alignment. Unlike for the Madelungcorrection, the negative-U behavior of VN is observed by employing the potential alignment.

Fig.2. Isosurfaces of the highest occupied orbitals of (a) ClN, (Td) (b) ClNMgGa (C1) and (c)ClNVGa (C3v). Red is for the positive values of the wave function, green for negative ones.
Isovalues were set to 0.05 e-/Å3 for ClN, 0.09 e-/Å3 for ClNMgGa and 0.07 e-/Å3 for ClNVGa.