High Temperature Spintronic Devices.

Abstract

Spintronics is a rapidly developing multidisciplinary field which investigates avenues of exploiting the spin degree of freedom in charge carriers and the nucleus of atoms to design novel devices that would outperform existing ones based on complementary-metal-oxide-semiconductor (CMOS) technology. Spin-polarized semiconductor based coherent light sources promise an ability to predict, stabilize and control orthogonal polarization states with reduced input power requirements. This doctoral thesis investigates different aspects of design, epitaxial growth, fabrication, characterization and modeling of spin based electronic and optoelectronic devices working near room temperature. We have demonstrated the modulation of magnetoresistance in an InAs/ In0.53Ga0.47As/ In0.52Al0.48As lateral spin valve with a gate electrode placed alongside the MnAs polarizer contact and outside the current transport channel. The results indicate that the change in magnetoresistance is caused, in part, by Rashba spin-orbit coupling due to the gate bias. In order to achieve higher working temperatures devices for real world applications MnAs/GaAs/MnAs based vertical spin valves have been realized. We have attributed the near room temperature operation of these devices to valence band electron tunneling of spin polarized carriers in and out of a heavily p-doped GaAs:Mn layer. Peak magnetoresistance of 40 % and 1 % have been observed at 10 K and 300 K respectively. The continuous wave, transient and high frequency dynamics of spin-polarized carriers and photons in a spin laser have been studied. Besides lowering of threshold currents, it has been theoretically estimated that these devices can show larger small-signal modulation bandwidth and 100% output polarization, independent of the injected carrier spin polarization, under appropriate biasing conditions. Measurements were done at 230 K on a InAs/GaAs quantum dot spin vertical cavity surface emitting laser (VCSEL). A time-averaged output polarization of 55% is measured with an active region spin polarization of 5-6%. A peak threshold current reduction of 4.5 % and a polarization modulation index of 0.6 have been measured in these devices under continuous wave bias. The measured output characteristics, both DC and transient, match very well with those calculated from theory.