Symmetry and structure of carbon-nitrogen complexes in gallium arsenide from infrared spectroscopy and first-principles calculations

TL;DR

This study combines infrared spectroscopy and first-principles calculations to analyze the structure and symmetry of carbon-nitrogen complexes in gallium arsenide, revealing their stability, vibrational modes, and charge states.

Contribution

It provides the first combined experimental and theoretical characterization of CN$_2$ and C$_2$N complexes in GaAs, including their symmetry, vibrational frequencies, and stability.

Findings

Identified characteristic IR absorption modes at 1973 and 2060 cm^{-1}.

Supported the stability of linear N-C-N and C-C-N complexes in various charge states.

Showed CN$_2$ is thermodynamically more stable than C$_2$N in GaAs.

Abstract

Molecular-like carbon-nitrogen complexes in GaAs are investigated both experimentally and theoretically. Two characteristic high-frequency stretching modes at \num{1973} and \SI{2060}{cm^{-1}}, detected by Fourier transform infrared absorption (FTIR) spectroscopy, appear in carbon- and nitrogen-implanted and annealed layers. From isotopic substitution it is deduced that the chemical composition of the underlying complexes is CN$_2$ and C$_2$N, respectively. Piezospectroscopic FTIR measurements reveal that both centers have tetragonal symmetry. For density functional theory (DFT) calculations linear entities are substituted for the As anion, with the axis oriented along the \hkl<100> direction, in accordance with the experimentally ascertained symmetry. The DFT calculations support the stability of linear N-C-N and C-C-N complexes in the GaAs host crystal in the charge states ranging from $+3$ to $-3$. The valence bonds of the complexes are analyzed using molecular-like orbitals from DFT. It turns out that internal bonds and bonds to the lattice are essentially independent of the charge state. The calculated vibrational mode frequencies are close to the experimental values and reproduce precisely the isotopic mass splitting from FTIR experiments. Finally, the formation energies show that under thermodynamic equilibrium CN$_2$ is more stable than C$_2$N.