Studies of spin-polarized electrons in gallium arsenide-based quantum well LEDs.

摘要

In this thesis spin injection of electrons from a spin polarizing contact into GaAs-based heterostructures is investigated. The structures known as “spin-LEDs”, consist of AlGaAs(n)/GaAs(i)/AlGaAs(p) quantum well light emitting diodes (LEDs) on top of which two different spin polarizing contact materials were grown.; The first material, a Zn_1−xMn_xSe(n) layer, is a paramagnetic diluted magnetic semiconductor (DMS). The second contact material, an Fe film, is a ferromagnetic metal. Spin polarized electrons are electrically injected into the GaAs quantum well from the AlGaAs(n) side while unpolarized holes are injected from the AlGaAs(p) barrier. The carriers recombine in the GaAs quantum well and emit photons through a variety of recombination channels. The confined free exciton ground state (e₁h₁) channel is used to determine the spin polarization of electrons from the degree of circular polarization of the emitted photons using the Wigner-Eckert theorem.; In ZnMnSe based spin-LEDs, ZnMnSe acts as a spin filter due to its large (several meV at modest magnetic fields) conduction band spin splitting which is well above kT at the temperature of our experiments (T = 4.5 K). As a consequence, practically all the electrons injected in the device are in their lowest (m _s = −1/2) spin state and the emitted electroluminescence is circularly polarized predominantly as σ₊. The maximum optical polarization achieved in these low temperature devices is 83% which corresponds to a high electron spin polarization of 92%.; In Fe spin-LEDs, spin injection is based on the unequal populations of spin up and spin down d electrons of Fe. The Fe contacts inject predominantly m_j = −1/2 electrons as in the case of ZnMnSe, so that the excitonic emission exhibits a σ₊ circular polarization. The differences of Fe with ZnMnSe-based spin-LEDs is that the maximum electron spin polarization achieved in the former is lower (59%) and that the spin injection itself persists at higher temperatures due to the high Curie temperature of Fe.