The realization of semiconductor-based devices which make use of the electron spin is of great academic and industrial interest, mainly due to the large spin lifetime in semiconductors. A spin polarization in these devices can be obtained by injection of spins from a ferromagnetic electrode. If the magnetization of the electrode is perpendicular to the surface, the majority spins have a perpendicular orientation, which makes optical detection of spins easier compared to an in-plane orientation. This study focuses both on the creation and optimization of a spin injection device, and on the optical creation and detection of spins in such device. In particular the development of perpendicularly magnetized electrodes was researched. Our spin injection device consists of a GaAs based semiconductor heterostructure, an AlOx tunnelbarrier and perpendicularly magnetized electrodes. The AlOx tunnelbarrier was grown in order to suppress the conductivity mismatch. The perpendicularly magnetized electrodes were created by sputter deposition of ultrathin (~1 nm) alternating Co/Pt layers. A detailed study was performed to establish optimal magnetic properties for a stack with the magnetic Co directly grown on top of the AlOx. The latter is a requirement for optimal spin injection. We succeeded in making these perpendicularly magnetized electrodes, resulting in a new and not previously reported material combination. Using devices with these optimized electrodes, we performed basic electronic measurements and photocurrent studies. Furthermore, time-resolved magnetization modulation spectroscopy was used to study spin relaxation and precession in the GaAs heterostructure. The influence of a magnetic electrode on the dynamic behavior of spins was investigated. The final step was hot electron injection from the perpendicularly magnetized electrode into the GaAs semiconductor. Due to the imperfect doping of the GaAs heterostructure, the very thin magnetic Co layers (in contrast to the Pt spacer layer) and the high barrier height, no spin injection was measured yet. Solutions to avoid some of the problems are discussed.
|Date of Award||31 Oct 2008|
|Supervisor||J.H.H. Rietjens (Supervisor 1) & Bert Koopmans (Supervisor 2)|