Analysis of the effects of hydrostatic pressure on the electronic emission and capture properties of the (0/+ ) and ( + /+ + ) deep levels of the ELI defect in GaAs leads to the following conclusions: (I) Both levels move higher in the band gap with pressure; (2) relatively large inward (outward) lattice relaxations accompany electron emission (capture) from (by) these levels; and (3) the magnitudes of the relaxations agree quantitatively with theoretical results which identify EL2 as the As antisite defect. These results which emphasize the antibonding character of the orbitals which describe EL2 are consistent with this identification.PACS numbers: 7l.55.EqThe midgap donor known as EL2 is the dominant deep electronic level in melt-grown and vapor-phase, epitaxially grown GaAs. It controls the electronic properties of semi-insulating GaAs by pinning the Fermi level, and its microscopic structure and metastability have been the subject of very extensive research [1][2][3][4][5][6]. There are two in-gap deep levels associated with the stable atomic configuration of EL2 [6]. The first corresponds to the first donor ionization state (0/+), is observed in «-type GaAs, and has an electron emission activation energy of 0.815 eV and a thermally activated electron capture cross section with energy barrier Eh =66 meV. Thus, this level is located at E^ -0.75 eV (where E^ is the conductionband edge) as is also deduced from Hall measurements [6]. The second level corresponds to the second donor ionization state (-l-/-f + ) and is observed in p-type GaAs. It has a hole emission activation energy of 0.54 eV and, presumably, no capture barrier [7], so that it is located at £",,4-0.54 eV (where £",, is the valence-band edge).Although the preponderance of the evidence definitively points to the involvement of the As antisite defect (ASGU) in its stable configuration, uncertainties still remain as to EL2\ exact microstructure. Currently, the two leading models [1-6] associate EL2 with either (i) the isolated Asca or (ii) a loosely bound As-antisite-Asinterstitial (i.e., Asoa-As,) pair. The formation of either defect can be expected to lead to relaxation of the neighboring atoms, and additional relaxation should result from the emission or capture of electrons from the center. Both the signs and magnitudes of these relaxations are important to understanding the physics and microstructure of EL2. Recent theoretical calculations [2,4] have evaluated one or both of these relaxations for the Asca defect, but there are no experimental measurements.Here we report the effects of hydrostatic pressure on the electron emission and capture properties of the (0/+) state, and use these results along with complementary earlier data to quantitatively evaluate the breathing-mode relaxations associated with the emission and capture processes for both the (0/-f) and (-!-/+ + ) states. The principal findings of the work are as follows: (i) There are relatively large inward (outward) lattice relaxations accompanying electron emission (capture) from (b...