The region beneath the interface of an InGaAs layer grown onto an undo
ped InP substrate by molecular beam epitaxy was investigated before an
d after rapid thermal annealing. Capacitance-voltage measurements reve
aled a depletion region in the underlying InP due to donor compensatio
n that is mainly caused by the Fe-related E(c)-0.63 eV-deep level obse
rved by deep level transient spectroscopy.