Spin-polarized transport in Si-nanostructures

Many quantum computer concepts have been developed over the last decades. The calculating capacity of such computers could be much higher than that of today’s CMOS-based silicon-chip-computers. Basically, quantum computers work by using the superposition of wavefunctions of several so called qubits (quantum bits). Recently silicon has attracted attention as basic material for the realization of spin-based qubits, as its main isotope 28Si has no nuclear spin. Therefore a reduced scattering probability with the base material can be expected. Spin-polarized electrons can be obtained in the edge-states of two-dimensional electron gases at high magnetic fields, where the different spin orientations are individually accessible by introducing suitable constrictions, thus enabling the investigation of scattering between the spin-states by measuring electric current.

To probe the properties of such a system, Hall-bar devices consisting of a silicon MOS field effect transistor with embedded split-gates (forming a point-contact) below the top gate were fabricated and characterized at 1.5 Kelvin and in magnetic fields up to 8 Tesla. Transport through the constriction induced by the split-gates shows fluctuations, which can be interpreted as the effect of transmission resonances in a one-dimensional channel of a length comparable with the split-gate dimensions. Furthermore, a step in the differential conductance of the point-contact could be observed, which changes its position when the magnetic field is varied. This behaviour is interpreted as the observation of quantized conductance, as it is well-known from measurements on quantum point-contacts in heterostructures like GaAs/AlGaAs.

Wire-bonded Hall-bar structure on silicon