Thermally stimulated current (TSC) and photo induced current transient
spectroscopy (PICTS) were used to investigate deep traps in oxygen-rich
medium resistivity GaAs. We observed an abnormal behaviour of the TSC
signal at a temperature of about 200 K, where the TSC current is lower
than the dark current. This results in a negative peak occurring in the
temperature range where the signal of the well known EL3 defect in
undoped semi-insulating GaAs is present. For verification, comparative
PICTS-measurements were performed, which exhibit the same negative peak
at similar temperatures. The activation energy for this negative peak
was determined to be ~ 0.6 eV (TSC). This is in good agreement with
published values for the EL3 ranging from 0.56 to 0.68 eV. Furthermore,
we illustrate that the depth of the peak increases with increasing oxygen
content. Our results support the assumption, that oxygen on an arsenic
site – the so called off centre oxygen oc-OAs – should be the
origin of the EL3.