Sub-picosecond electron relaxation of near-infrared intersubband transitions in n-doped (CdS/ZnSe)/BeTe quantum wells

We report on intersubband (ISB) absorption and ultrafast ISB energy relaxation of carriers in n-type doped (CdS/ZnSe)/BeTe quantum wells (QWs), grown by molecular-beam epitaxy. The highly n-type doped QW samples were obtained by introducing a few monolayer CdS into a ZnSe/BeTe QW, and ISB absorption with a peak wavelength as short as 1.62 mum, covering 1.55 mum within its absorption bandwidth, was achieved. The ISB carrier relaxation was investigated by means of femtosecond (similar to150 fs) one-color pump and probe technique at the ISB absorption peak. The ISB carrier relaxation time of 270 fs was observed in the sample with the absorption peak at 1.82 mum. The slow decay component with a time constant of a few ps, which has been observed in ZnSe/BeTe QWs, was not observed in the (CdS/ZnSe)/BeTe QWs, indicating that the Gamma(ZnSe)-X(BeTe) electron transfer is suppressed, as expected from the band alignment. (C) 2002 American Institute of Physics.