Polarization dependent photocurrents in thin films of the topological insulator Bi₂Se₃
Author(s)Lau, Claudia (Claudia M.)
Massachusetts Institute of Technology. Department of Physics.
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Topological insulators are a new class of three-dimensional quantum materials whose interior or bulk is an insulator but whose surface is a conductor. Bi₂Se₃ is a prototypical topological insulator that physicists at MIT are manufacturing and studying. Various interesting properties of the topological insulator include the flow of pure spin currents and topological protection. Pure spin current, distinct from electric current, is a net flow of spin without a net flow of charge. Recent research at MIT has revealed that shining circularly polarized laser light on a topological insulator turns its surface's pure spin current into a spin-polarized electrical current. The band structure of the bulk of a topological insulator resembles that of an ordinary insulator; the conduction band and valence band are separated, with the Fermi level falling between them. However, for Bi₂Se₃, the conducting surface's dispersion relation can be modeled by a Dirac cone, which crosses the Fermi level. Electrons with opposing spins reside on opposite sides of the Dirac cone. Illuminating a topological insulator with either left or right circularly polarized light depopulates one side of the Dirac cone, leaving on the other side the desired spin-polarized electrical current. In the experiment performed for this thesis, thin films of Bi₂Se₃ were grown on substrates of sapphire via molecular beam epitaxy (MBE). Electrical devices on a micron scale were then fabricated on the thin film surface and used to measure surface currents. Steps of this experiment included characterizing the surface quality of a sapphire substrate using atomic force microscopy (AFM), making electrical devices with Bi₂Se₃ via the processes of optical lithography, ion milling, and electron beam metal deposition. Photocurrents across these electrical devices were induced by the manipulation of optics and lasers and measured using low noise electronics. Experimental results revealed that it was indeed possible to induce spin-polarized electrical currents on thin films of MBE grown Bi₂Se₃. The desired photocurrent was observed when the laser beam spot size was enlargened to illuminate the entirety of the Bi₂Se₃ device simultaneously. These results were not replicable when the laser was more tightly focused onto a smaller area. Scanning the focused laser beam across the Bi₂Se₃ confirmed that different photocurrents were being induced at different points; these results led us to the conclude that there was something inhomogenous about our device. The reason behind this device inhomogeneity is still under investigation.
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 67-68).
DepartmentMassachusetts Institute of Technology. Department of Physics.
Massachusetts Institute of Technology