Does lattice expansion really occur in defected low-temperature GaAs?

A prominent example of lattice expansion caused by point defects is that of GaAs grown at low temperature (LT-GaAs). A linear correlation between the lattice expansion and concentration of As-antisite (As_Ga) point defect is obtained experimentally by XRD. However, using full first-principles method (DFT-LDA) on a large 512-atom supercell of GaAs bulk, we found that, in the limit of small concentrations of As_Ga (less than 1%), the lattice does not expand but the As-As bond length near the defect increased. DFT-LDA method is tested vis-a-vis the more sophisticated DFT-hybrid functionals and the former yields the same lattice changes as the latter. We propose that the observed expansion maybe due to the increased bonds near the defect. We also found that As_Ga point defects do not prefer isolated structure even at low concentration. Using molecular dynamics simulation, we confirm that the above results hold true even at room temperature. Study of electronic properties reveals that, in this low concentration regime, abrupt changes in density of states near the fermi level occur. These findings pose impact in the use of LT-GaAs for optoelectronic/terahertz devices, where fundamental understanding of defect concentration's relation with material structure/functionality is vital.