Ab Initio Study of Substitutional Chlorine and Related
Complexes in GaN
G. Alfieri1), Y. Fujii2) and R. Micheletto2)
1) ABB Corporate Research Center, Segelhofstrasse, Baden­Dättwil, Swizerland
2) Yokohama City University, Dept. Of Nanosystem Sciences, Yokohama, Japan
Introduction

Computational Details

Reactive ion etching (RIE) is typically employed in device
processing, e.g., mesa etching. Cl-based RIE is known to:• Decrease Ohmic contact resistivity in n-type GaN [1,2]• Reduce carrier concentration in p-type GaN [3]This was attributed to the formation of nitrogen vacancies (VN)
or VN-complexes (VNVGa). However, VN behaves as a donor in ptype GaN and not in n-type GaN (neutral). In addition,divacancies appear only after high temperature treatment.
For these reasons, we investigated the role of Cl in GaN. This isdone by employing density functional theory (DFT).

SIESTA spin-polarized DFT calculations. We used:• a 96-atom supercell• LDA (CA)• Troullier-Martins norm-conserving pseudopotentials• double-z plus plus polarization as atomic orbital basis set• Charge density projection with an equivalent cutoff of 375 Ry• 8 Monkhorst-Pack special k-points
The d-orbital electrons were treated as valence electrons.
Equilibrium configurations of the pristine and defected supercell
were obtained by relaxing the atomic coordinates by a CGalgorithm• max atomic forces <0.025 eV/Å• stress tensor <0.5 GPa.
The formation energy (EFORM) was calculated by using ZhangNorthrup formalism.

Results
We first analyzed the MgGa acceptor and VN donor, by employing
the Makov-Payne [4] or the potential alignment [5] corrections
to EFORM. The latter gives better results and has been usedthrought the present work.
Correction

MgGaVN

Makov-Payne

EV+0.35 eV

No Negative-U

Potential alignment

EV+0.20 eV

Negative-U

Substitutional Cl is more stable at N- (ClN) rather than Ga-site(ClGa) by 2 eV.
ClN: after geometry relaxation, a Td symmetry is observed.
Average Cl-Ga bond length=2.4 ÅAverage Ga-Cl-Ga bond angle=110.1°
Number of electrons (Watkins model)=10
These can be accommodated in two singlet (a1) levels and one
triplet level (t2). It can be predicted that ClN is a double donor(oxygen is a single donor).
ClNMgGa: geometry relaxation shows that this defect has C1symmetry.
Cl-Mg bond length=2.27
Average Ga-Cl-Mg bond angle=108.3°
Average Ga-Cl bond length=2.34
Average Ga-Cl-Ga bond angle=110.5°
Number of electrons (Watkins model)=9These can be placed in singlet (a) levels, so there is one spareelectron. This defect can be a donor (or an acceptor).

ClN is a double donor for any Fermi energy (EF) value in the KohnSham band gap.
ClNMgGa is a single donor for any EF in the band gap.
For both defects, the EFORM is negative, especially in the case ofp-type GaN.

ClNVGa: the relaxed structure has C3v symmetry.
Average Ga-Cl bond length=2.34 Å
Average Ga-Cl-Ga bond angle=95.6°
Number of electrons (Watkins model)=7These electrons can be found in two singlet (a1) levels and one
doubly degenerate (e) level. Similarly to ClNMgGa, the level can bepredicted to be a donor (or an acceptor).

Conclusions

ClNVGa displays two transition levels: a donor level at EV+0.58 eV
and an acceptor level at EV+1.4 eV.

Cl in GaN (and complexes) behave as donors.
The relatively low EFORM of ClN can explain why Ohmic contacts
improve by RIE, in n-type GaN. Furthermore, the low EFORM ofClNMgGa can explain why RIE reduces the hole concentration in ptype GaN.
Investigation of ClNSiGa, Cli (and complexes) is in progress.

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Bibliography[1] Z. Fan et al., APL 68, 1672 (1996)[2] S. Joglekar et al., APL 109, 041602 (2016)[3] J.M. Lee et al., J. Vac. Sci. Technol. B 22, 479 (2004)[4] G. makov et al., PRB 51, 4014 (1995)[5] C.G. van de Walle et al., JAP 95, 3851 (2004)

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