Abrupt Dependence of Ultrafast Extrinsic Photoconductivity on Er Fraction in Gaas:Er

We present a study of room-temperature, ultrafast photoconductivity associated with a strong, sub-bandgap, resonant absorption around λ = 1550 nm in three MBE-grown GaAs epitaxial layers heavily doped with Er at concentrations of ≈2.9 × 1018 (control sample), 4.4 × 1020, and 8.8 × 1020 cm-3, respectively. Transmission-electron microscopy reveals lack of nanoparticles in the control sample, but abundant in the other two samples in the 1.0-to-3.0-nm-diameter range, which is consistent with the previously known results. We measure very high photoelectron (Hall) mobility (2.57 × 103 cm2/V-s) and terahertz power (46 μW average) in the 4.4 × 1020 sample, but then, an abrupt decay in these properties as well as the dark resistivity is seen as the Er doping is increased just 2 times. The Er doping has little effect on the picosecond-scale, 1550-nm photocarrier lifetime