Experimental Research Of Spin Diffusion And Relaxation In Semiconductor Quantum Wells

Spintronics aims to exploit a new generation of electronic devices, which isbased on the electron spin. Spintronics has an important impact on not only theinformation industries, but also the fundamental researches in physics. It is animportant field of condensed matter physics. To finish spin based electric device, theelectron spin polarization should persist enough long time and transport to enough far.It is necessary to understand the mechanism of the spin relaxation and the spintransport. We build up the transient spin grating (TSG) system, and with the help ofwhich we study the spin relaxation and spin diffusion in GaAs/AlGaAs quantum wells(QW) grown on different direction, assisted by time resolved Kerr rotation.The main results studied in the thesis are followed:The spin grating system is built up. With the help of heterodyne detection andlocked-in amplification, the grating signal is very well. The spin diffusion coefficientDs in (110)-oriented GaAs/AlGaAs quantum wells is measured. In intrinsic quantumwells, the spin diffusion coefficient decreases from ¹02cm²/s to ⁵0cm²/s, while thepump power increases from 2mW to 18mW. It is due to the excited carriers'concentration dependent momentum relaxation. In doped quantum wells, the spindiffusion coefficient increases with pump power. It is due to the reduced boundingeffect of localized center while the pump power or excited carriers increased.Spin diffusion coefficients are also measured in (100)- and (111)-orientedGaAs/AlGaAs quantum wells, and they are independent on pump power for bothoriented QW. In (100) QW with the well width 8nm, the is ⁸5cm²/s; in the (100)QW with the well width 12nm, the is ¹05cm²/s; in (111) QW, the is⁸0cm²/s. The results indicate that the spin diffusion coefficient is directly related tothe growth direction of the samples.We perform the experimental study on the spin relaxation in doped Besides (110) (110)-oriented GaAs/AlGaAl quantum well at low temperature (T=3K). The spindiffusion coefficientDs is linear to 1/ s, and obtained L_SO 3.7 0.2 m. Thevalue is in good agreement with the result obtained by theoretical calculation due tothe exchange interaction relaxation mechanism.The pump power dependent spin lifetimes are observed in (100)-, (110)- and(111)-oriented GaAs/AlGaAs quantum wells by time resolved Kerr rotation. In theintrinsic (100) and (111) QW, the spin lifetimes increase with the increased pumppower; while in one-side modulation doped (100) QW, the spin lifetimes decreases;and in (110) quantum wells, increase.The ambipolar diffusion coefficient of (110) GaAs/AlGaAs quantum wells wasmeasured by the transient spin grating technique. The ambipolar diffusion coefficientand carrier lifetime areD_a 13.0cm²/ sand R 1.9ns, respectively.Da keeps aconstant when the photoexcited carrier concentration increased up to1.2 10¹¹ cm²/ s.The result is in good agreement with theoretical calculation by Young s group.