Magneto-optische Untersuchungen des elektrisch induzierten Spintransports und des Laser-induzierten Spin-Seebeck-Transports in n-GaAs

摘要

The topic of this Ph.D. thesis is the magneto-optical investigation of the electrically-induced spin transport and of the laser-induced spin Seebeck transport in n-GaAs. This work is part of the research project "Spin Caloritronics in III-V-Semiconductors" of the DFG priority programm "Spin Caloric Transport" (SPP 1538).Measurements are performed by spatially and time-resolved Faraday spectroscopy. Here, a local spin polarisation is generated by ultrafast laser pulses and detected via the magneto-optical Faraday effect with variable time delay. The material system under investigation, i.e. n-GaAs doped to the metal-to-insulator transition at $n = 2 cdot 10^{16}$ cm$^{-3}$, is optimally suited for spin transport experiments due to long spin dephasing times $au_s geq 100$ ns at low temperatures, which result in large spin transport length of $sim 10,mu$m.The measurement and analysis method used in this work allows to study the temporal evolution of the optically generated spin polarisation by Fourier transformation of spatially resolved magnetic field scans (RSA scans) with very high spatial resolution and to determine the spin drift velocity $v_s$, the spin diffusion constant $D_s$ and the spin dephasing time $au_s$.Spin transport in an applied electric field shows Ohmic transport behaviour $v_s = - mu_s E$ with spin mobility $mu_s = (1159 pm 4)$ cm V$^{-1}$ s$^{-1}$ at $T = 10$ K. Spin transport measurements in dependence of the excitation energy and of the excitation density indicate that, under intense excitation at low temperatures, spin drift and diffusion are influenced by a strong electron-hole interaction. The temperature dependence of the electrically-induced spin transport shows the first experimental verification of the generalized Einstein relation for semiconductors at the metal-to-insulator transition at low temperatures.Spin Seebeck transport in n-GaAs is investigated with laser-induced temperature gradients, which are characterized by photoluminescence measurements. While the spin diffusion constant $D_s$ and the spin dephasing time $au_s$ seem to be dominantly influenced by the large, optically excited electron and hole density, the spin drift velocity $v_s$ shows the expected behaviour for spin Seebeck transport in laser-induced temperature gradients in all the measurements. Particularly, the simultaneous vanishing of $v_s$ and of the temperature gradient $rac{dT}{dx}$ at a lattice temperature of $T = 30$ K confirms the conclusion that these experiments show the first evidence of lateral spin Seebeck transport in a non-magnetic semiconductor.