Intracenter transitions in the major deep level EL2 in GaAs were identified for the first time by superimposing photocurrent measurements on those of optical absorption. These transitions were found to be responsible for the characteristic EL2 absorption band between 1.0 and 1.3 eV. At low temperatures (<60 K) intracenter absorption exhibits a fine structure involving the zero phonon line and replicas at energies close to those of transverse acoustic phonons (TA). This coupling with TA phonons is a strong indication that EL2 is an extrinsic self-trapping center.
The concentration of the major electron trap (0.82 eV below the conduction band) in GaAs (Bridgman grown) was found to increase with increasing As pressure during growth. It was further found that (for a given As pressure) the concentration of this trap decreased with increasing concentration of shallow donor dopants (Si, Se, and Te). Donor concentrations above a threshold of about 1017 cm−3 led to the rapid elimination of the trap. On the basis of these findings, the 0.82-eV trap was attributed to the antisite defect AsGa formed during the postgrowth cooling of the crystals.
Current oscillations thermally activated by the release of electrons from deep levels in undoped semi-insulating GaAs were observed for the first time. They were attributed to electric field-enhanced capture of electrons by the dominant deep donor EL2 (antisite AsGa defect). This enhanced capture is due to the configurational energy barrier of EL2, which is readily penetrated by hot electrons.
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