9:00 AM - 9:15 AM
▲ [16a-3A-1] [Young Scientist Presentation Award Speech] Band structure of GaMnAs near the Fermi level studied by ultrafast time-resolved light-induced reflectivity measurements
Keywords:Ferromagnetic semiconductor,pump-probe,band
There have been intense discussions over the position of the Fermi level (EF) in ferromagnetic semiconductor GaMnAs. While a number of studies have indicated that EF exists in the impurity band (IB) in the band gap (Eg), recent time-resolved measurements in terms of the light-induced reflectivity have indicated that EF exists in the valence band (VB); however, the pump fluence in this study is rather high, and the accumulation of the photo-carriers induced by the pump pulse irradiation may shift the absorption edge. In this case, the definition of both Eg and EF is obscure. In our study, we have carefully performed the pump and probe reflectivity measurements with the pump pulse having very low fluence to suppress the accumulation of the photo-induced carriers.
We used 20-nm-thick Ga1-xMnxAs films with the Mn concentration x of 1%, 3%, and 6% grown on semi-insulating (S.I.) GaAs (001) substrates. Time-resolved reflectivity measurements were performed by using degenerate pump-probe technique. The photon energies of the pump and probe pulses were set in the vicinity of the resonance of the band-to-band transition in the GaMnAs samples. At the delay time t after the irradiation of the pump pulse with the fluence of 0.16 μJ/cm2, the low-power probe pulse with 1 nJ/cm2 detects the reflectivity change ΔR/R. By assuming that the pump-pulse light is absorbed only in the GaMnAs layer, the density of the photo-excited carriers is estimated to be below 3×1017 cm-3, which is two orders of magnitude smaller than that in the previous study. The photon energy resolution of our set-up is estimated to be around 0.5 meV, which is much higher than that in the previous works.
In this study, we determine the position of the EF by analyzing the time-resolved reflectivity spectrum. We measured ΔR/R at 5 K for the S.I. GaAs substrate and for the Ga1-xMnxAs films at t=166 ps. We assigned Eg and EF by the Kramers-Kronig analysis of the reflectivity spectrum. We found that Eg-EF increases with increasing x from 1%.
We used 20-nm-thick Ga1-xMnxAs films with the Mn concentration x of 1%, 3%, and 6% grown on semi-insulating (S.I.) GaAs (001) substrates. Time-resolved reflectivity measurements were performed by using degenerate pump-probe technique. The photon energies of the pump and probe pulses were set in the vicinity of the resonance of the band-to-band transition in the GaMnAs samples. At the delay time t after the irradiation of the pump pulse with the fluence of 0.16 μJ/cm2, the low-power probe pulse with 1 nJ/cm2 detects the reflectivity change ΔR/R. By assuming that the pump-pulse light is absorbed only in the GaMnAs layer, the density of the photo-excited carriers is estimated to be below 3×1017 cm-3, which is two orders of magnitude smaller than that in the previous study. The photon energy resolution of our set-up is estimated to be around 0.5 meV, which is much higher than that in the previous works.
In this study, we determine the position of the EF by analyzing the time-resolved reflectivity spectrum. We measured ΔR/R at 5 K for the S.I. GaAs substrate and for the Ga1-xMnxAs films at t=166 ps. We assigned Eg and EF by the Kramers-Kronig analysis of the reflectivity spectrum. We found that Eg-EF increases with increasing x from 1%.