Spin transport in ferromagnet-semiconductor heterostructures

We have used optical pumping to generate spin-polarized carriers in epitaxially prepared ferromagnet-GaAs Schottky diodes, which have In_0.1Ga _0.9As quantum wells in the depletion region. We used polarization-sensitive photocurrent measurements to establish a small spin transport signal of 0.5% for excitations near the band edge, and an upper bound for spin transport in the range between 2–9% for excitations near the quantum well ground state. We also demonstrate that the signal from the p-doped samples is stronger, in agreement with the fact that the electron spin lifetime is larger than hole spin lifetime. A series of control measurements established strong background contributions that make the quantification of the spin ejection process difficult. We studied this process in two different geometries, Faraday and Voigt (side pumping), and point out the practical and fundamental differences between them. The side pumping (Voigt) geometry is convenient, because the signal is free of large background contributions present in high magnetic fields used in Faraday geometry.; In the second project, we have used polarized electroluminescence to establish the feasibility of spin-sensitive injection of electrons from the ferromagnet into the semiconductor Al_0.1Ga_0.9As/GaAs/Al _0.1Ga_0.9As PIN diode heterostructure with a quantum well in its depletion region. We establish that the spin injection signal is between 1% and 7% after comparing field-dependent electroluminescence polarization with the results of field-dependent control measurements. I also report on the strong bias dependence of the spin injection signal, which shows a rise at low bias and a decrease at high bias. I propose a rate equation model to explain the rise at low bias, and a cascade-like model at high bias to explain the decrease of the signal due to the high kinetic energy of injected electrons. The results strongly suggest a further need for the improvements of the heterostructure design, with the goal of finding the best doping profile in the drift region, and improving the quantum well detectors.