Alberto Ferrari

Photoinduced inverse spin Hall effect in Pt/Ge(001) at room temperature

We performed photoinduced inverse spin Hall effect (ISHE) measurements on a Pt/Ge(001) junction at room temperature. The spin-oriented electrons, photogenerated at the direct gap of Ge using circularly polarized light, provide a net spin current, which yields an electromotive field EISHE in the Pt layer. Such a signal is clearly detected at room temperature despite the strong Γ to L scattering, which electrons undergo in the Ge conduction band. The ISHE signal dependence on the exciting photon energy is in good agreement with the electron spin polarization expected for optical orientation at the direct gap of Ge.

Photon energy dependence of photo-induced inverse spin-Hall effect in Pt/GaAs and Pt/Ge

We report the photon energy dependence of photo-induced inverse spin Hall effect (ISHE) in Pt/GaAs and Pt/Ge Schottky junctions. The experimental results are compared with a spin drift-diffusion model, which highlights the role played by the different spin lifetime in the two semiconductors, in determining the energy dependence of the ISHE signal detected in the Pt layer. The good qualitative agreement between experiments and modelling indicates that photo-induced ISHE can be used as a tool to characterize spin lifetime in semiconductors.

Ge/SiGe heterostructures as emitters of polarized electrons

The spin polarization of electrons photoemitted from Ge-on-Si and strained Ge epilayers is measured by means of Mott polarimetry. Our results indicate that efficient electron spin polarization can be obtained using SiGe heterostructures deposited on Si(001). Mismatch strain is seen to increase spin polarization above the 50% bulk limit by removing the heavy-hole/light-hole degeneracy in the valence band.

Optical spin orientation in group-IV heterostructures

We investigate the electron spin polarization upon photoemission from different Si1−xGex heterostructures by means of Mott polarimetry. We demonstrate the possibility to lower the vacuum energy level below the bottom of the conduction band at the Γ point of the Brillouin zone in compressively strained Si1−xGex alloys and we show that the optimization of the stoichiometry of group-IV heterostructures leads to a spin polarization of the electrons in the conduction band up to P=72%±3%. Such a value is not only greater than those attainable in compressively strained pure Ge heterostructures, but it is also comparable to the typical electron spin polarization values of III-V semiconductor heterostructures.

Research data supporting “p-wave triggered superconductivity in single layer graphene on an electron-doped oxide superconductor”

Measurements data collected in several institutions including the Racah Institute of Physics (STM data), at the Cambridge Graphene Centre (Raman spectroscopy data) and Department of Materials Science and Metallurgy (XRD and electronic transport data).

Ultrafast valley depolarization dynamics in monolayer MoS 2

We study the ultrafast valley relaxation dynamics in monolayer MoS2 by time resolved Faraday rotation and circular dichroism. We find that the intervalley scattering process is ultrafast and display a peculiar bi-exponential behavior.

Spin-polarized photoemission from SiGe heterostructures

We apply the principles of Optical Orientation to measure by Mott polarimetry the spin polarization of electrons photoemitted from different group-IV heterostructures. The maximum measured spin polarization, obtained from a Ge/Si0.31Ge0.69 strained film, undoubtedly exceeds the maximum value of 50% attainable in bulk structures. The explanation we give for this result lies in the enhanced band orbital mixing between light hole and split-off valence bands as a consequence of the compressive strain experienced by the thin Ge layer.

Transport properties related to spin-orbit interaction

We propose a novel set of boundary conditions, based on the continuity of a generalized velocity and on the continuity of the probability current at the interface of heterojunctions, which is well suited to construct the solution of the tunneling problem when spin-orbit interaction is taken into account. We illustrate this procedure in a model case: tunneling of conduction electrons through a [110]-oriented GaAs barrier. In that case, the new boundary conditions reduce to two set of equations: the first one expresses the discontinuity of the envelope function at the interface while the other one is close to the standard condition on the derivative of the envelope function.

Photoinduced inverse spin-Hall effect in Pt/GaAs and Pt/Ge

We performed photoinduced inverse spin Hall effect (ISHE) measurements on Pt/GaAs(001) and Pt/Ge(001) junctions at room temperature. The spin-oriented electrons, photogenerated at the direct gap of the semiconductor using circularly polarized light, provide a net spin current which yields an electromotive field EISHE in the Pt layer. Such a signal is clearly detected in GaAs, where electrons diffuse around the Γ point of the Brillouin zone, and also in Ge, despite the strong Γ to L scattering which electrons undergo in the Ge conduction band. The ISHE signal dependence on the exciting photon energy is investigated in both junctions.

Spin polarized photoemission from strained Ge epilayers grown by low-energy plasma-enhanced CVD (LEPECVD)

We present spin polarized photoemission experiments (SPPE) on strained Ge/SiGe/Si(001) layers. SPPE indicates that compressive bi-axial strain effectively lifts the HH-LH degeneracy raising the polarization of injected electrons above the P=50% limit of bulk material.