EL2--like defects in InP nanowires

We have performed an {\it ab initio} total energy investigation, within the density functional theory (DFT), of antisite defects in InP nanowires (InP$NW$s) grown along the [111] direction. Our total energy results indicate that, (i) P antisites (P$_{\rm In}$) are the most likely antisite defect compared with In antisites (In$_{\rm P}$), and (ii) the formation energies of P and In antisites do not depend on the $NW$ diameter. In particular, thin InP$NW$s, diameters of $\sim$13 \AA, the P$_{\rm In}$ antisite exhibits a trigonal symmetry, lying at 0.15 \AA from the $T_d$ site, followed by a metastable configuration with P$_{\rm In}$ in an interstitial position (1.15 \AA from the $T_d$ site). We find a P$_{\rm In}$--P dissociation energy of 0.33 eV, and there is no EL2--like center for such a thin InP$NW$. However, EL2--like defects occur by increasing the $NW$ diameter. For diameters of $\sim$18 \AA, the P$_{\rm In}$--P dissociation energy increases to 0.53 eV, which is 0.34 eV lower compared with the P$_{\rm In}$--P dissociation energy for InP bulk phase, 0.87 eV. We mapped the atomic displacements and calculated the relaxation energy, Franck--Condon shift, upon single excitation of P$_{\rm In}$ induced states in InP$NW$. The formation (or not) of EL2--like defects, P$_{\rm In}$--P dissociation energy barrier, and the Franck--Condon (FC) energy shift, can be tuned by the $NW$ diameter.