Gallium self-interstitial relaxation in GaAs: An ab initio characterization

Ga interstitials in GaAs $\left(I_{\mathrm{Ga}}\right)$ are studied using the local-orbital ab initio code SIESTA in a supercell of $216+1$ atoms. Starting from eight different initial configurations, we find five metastable structures: the two tetrahedral sites in addition to the $110\text{−}{\text{split}}_{\left[\mathrm{Ga}\text{−}\mathrm{As}\right]}$, $111\text{−}{\text{split}}_{\left[\mathrm{Ga}\text{−}\mathrm{As}\right]}$, and $110\text{−}{\text{split}}_{\left[\mathrm{Ga}\text{−}\mathrm{Ga}\right]}$. Studying the competition between various configuration and charges of $I_{\mathrm{Ga}}$ at $T=0\mathrm{K}$, we find that predominant gallium interstitials in GaAs are charged $+1$, neutral, or at most $-1$ depending on doping conditions and prefer to occupy the tetrahedral configuration where it is surrounded by Ga atoms. Our results are in excellent agreement with recent experimental results concerning the dominant charge of $I_{\mathrm{Ga}}$, underlining the importance of finite size effects in the calculation of defects.

Figure 1

(Color online) Top: The eight initial configurations considered in this study for neutral self-interstitial $I_{\mathrm{Ga}}$. The six first configurations, going from left to right, are viewed near the ⟨110⟩ direction, while the remaining two are viewed along the ⟨100⟩ direction. Bottom: The metastable configurations obtained after full relaxation of $I_{\mathrm{Ga}}$. Full arrows connect the initial configuration to its metastable counterpart, while dashed arrows mean that the initial configuration is unstable and converged to the pointed configuration. Gallium atoms are red, while arsenic atoms are white; the interstitial Ga atom is colored yellow. For splits, regular sites of the displaced lattice atoms are highlighted by a dotted circle.

Figure 2

(Color online) The two distinct (a) 110 and (b) 111 splits seen from a 110 view. Dashed lines correspond to ⟨111⟩ and ⟨110⟩ crystalline axes (indicated in blue) and dotted circles refer to the regular crystalline position of the displaced arsenic atom. Gallium atoms are red, while arsenic atoms are white; the interstitial Ga atom is colored yellow. Some lattice atoms have been removed for clarity.

Figure 3

(Color online) Formation energies as function of Fermi level for various charge states of the ${\mathrm{tetra}}_{\left[\mathrm{Ga}\text{−}\mathrm{Ga}\right]}$ configuration at $0\mathrm{K}$. Dotted lines point at the ionization level locations delimiting the three stability domains labeled by (I), (II), and (III).

Figure 4

(Color online) Formation energies as function of Fermi levels for various charge states of the $100\text{−}{\text{split}}_{\left[\mathrm{Ga}\text{−}\mathrm{Ga}\right]}$ configuration at $0\mathrm{K}$.

Figure 5

(Color online) Formation energies as function of Fermi levels for various charge states of the 110- $(q=-2,-1,0)$ and 111- $(q=+1,+2,+3)$ -${\text{split}}_{\left[\mathrm{Ga}\text{−}\mathrm{As}\right]}$ configuration at $0\mathrm{K}$.