A spin- and angle-resolved photoemission study of the Rashba-Bychkov effect in lead quantum well states

Abstract

In the rapidly developing field of spintronics whose central issue is the utilization of the spin instead of the charge of the electron, the active control of spin-polarized carriers (electrons or holes) utilizing electric rather than magnetic fields has highest priority. Researchers world wide are searching for a realization of a spin-based field-effect transistor (spin-FET) that has the potential to revolutionize electronic devices and carries new prospects of data manipulation. The present thesis deals with the investigation of the Rashba-Bychkov effect as a possible can- didate for a spin-FET formed by an ultra-thin Pb film on a silicon substrate, using spin- and angle-resolved photoemission spectroscopy. The reduced dimensionality of such an epitaxially grown metallic film results in a two-dimensional (2D) electron gas associated with quantum well states (QWS). These states are free to move within the sample plane, while they are confined perpendicularly by the energy gap of the Si substrate on one side and by the repulsive image potential toward the vacuum on the other side, reminiscent of the particle-in-the-box model. Due to the broken inversion symmetry along the confinement direction and the high nuclear charge of Pb the QWS show a Rashba-type spin splitting which builds up throughout the whole metal layer along the growth direction as a result of competing effects between the metal-substrate and metal-vacuum interfaces. In this thesis the focus lies on altering the interface region between the film and the substrate to provide more insight into the origin of the Rashba effect and other phenomena such as the effective mass of the bands and Schottky barrier formation. In the first approach we have varied the chemistry of the interface by studying various interfactants √ √ such as Pb, Bi and Ag, which form highly regular structures of ( 3 × 3)R30◦ symmetry on Si. It is found that the size of the Rashba effect changes dramatically among these interfaces: replacing the Pb interface by a Bi layer reduced the Rashba parameter by 60%, whereas QWS in Pb films grown on a Ag reconstructed Si substrate showed no measurable spin splitting. Interestingly, the Schottky barrier of these systems, and the effective mass of the states varies in the same manner. For future device application it is necessary to control the Rashba parameter by external means. The results of our second approach, the study of the influence of the substrate doping concentration on the Rashba effect in Pb QWS, revealed a very promising pathway: by increasing the donor concentration by a factor of 20, we could tune the Rashba parameter by a factor of two. A simulation of the relevant parameter indeed showed that a gate voltage of only 12 V is enough to switch a 1 nm lateral spin-FET from the insulating to the conducting state. These findings are discussed in the framework of the interface dipole model and a doping dependent Schottky barrier. In order to gain more insight into the interplay of band dispersion, orbital character, and spin, we have focused on a wave vector region in Pb QWS where avoided crossing hybridization induced by spin-orbit coupling alters the band structure significantly.