Optical Measurements of Spin/Valley Polarization in Transition Metal Dichalcogenides

Abstract

Finding a fundamentally novel way to generate and store information is one of the most critical topics in semiconductor research during the past several decades. Spin electronics (spintronics), using the spin degree of freedom to store and transfer information in solid state devices, offers one possible solution. In this dissertation, we use optical ways to study the spin dynamics in semiconductors which includes generating/detecting spin polarization and measuring spin lifetime.

Monolayer transition metal dichalcogenides (TMDs), a category of direct bandgap semiconductors, are great candidates for spintronics devices due to their unique optical properties and a new degree of freedom called valley pseudospin. We use optical orientation to generate spin/valley polarization in monolayer tungsten diselenide WSe2 and measure its lifetime. A very long ~ 80 ns spin/valley polarization lifetime is observed which is much longer than the radiative recombination time. We propose two possible physical mechanisms for this long-lived polarization. In the single particle picture, this long-lived polarization signal can be explained by the transfer of polarization from photo-excited carriers to resident carriers. Alternatively, in the quasi-particle picture, it can be attributed to the dark trions, which cannot radiatively recombine due to spin or momentum mismatch. Also, the spin/valley polarization is insensitive to an external transverse magnetic eld because of the large spin splitting in the valence band. We also find that the excitation energy that maximizes the Kerr rotation follows the free exciton emission energy, although the localized exciton emission dominates the photoluminescence spectrum at low temperature.

To study the spin/valley polarization transport in TMDs, such as spin diffusion length and spin/valley Hall effect, a high-resolution (3-micron resolution) time-resolved Kerr rotation microscope (TRKM) has been designed and built. The Kerr rotation microscope has a 4-f scanning system which allows us to do a relatively large spatial scan with a ne spatial resolution. Test measurements on bulk GaAs and InGaAs samples show high signal-to-noise data on the TRKM setup.