  The sub-bandgap absorption and ultrafast relaxation in a GaAs/ErAs/GaAs
heterostructure are reported. The infrared absorption and 1550 nm-excited
ultrafast photo-response are studied by Fourier transform infrared (FTIR)
spectrometry and time-domain pump-probe technique. The two absorption peaks
located at 2.0 um (0.62 eV) and 2.7 um (0.45 eV) are originated from the
ErAs/GaAs interfacial Schottky states and sub-bandgap transition within GaAs,
respectively. The photo-induced carrier lifetime, excited using 1550 nm light,
is measured to be as low as 190 fs for the GaAs/ErAs/GaAs heterostructure,
making it a promising material for 1550-nm-technology-compatible, high
critical-breakdown-field THz devices. The relaxation mechanism is proposed and
the functionality of ErAs is revealed.
