The detection of optically-injected spin-polarized holes by means of inverse spin Hall effect (ISHE) in a Pt/n-doped semiconductor junction is challenging because of the faster spin relaxation of holes compared to electrons at room temperature. Nevertheless, electric fields at the junction arising from the contact potential or an externally-applied bias voltage can favor the transfer of spin-polarized holes repelling electrons from Pt. Here, we report on photo-induced ISHE measurements where spin-polarized holes are detected using two different configurations, namely, i) at low temperature in a Pt/lightly P-doped Si junction, and ii) at 300K in a non-local architecture leveraging graphene as a spin interconnect between Pt and lightly As-doped Ge. Spin-polarized holes are optically oriented with a confocal microscopy setup in the valence band of the semiconductors illuminated with circularly polarized photons with energy above the band gap. In the first device, at T < 22K in Si the spin-relaxation time of holes increases and the majority of phosphorus dopants are not ionized, hence the built-in electric field originating from the potential difference between the work functions of Si and Pt extends to the whole substrate fostering (hampering) the diffusion of holes (electrons) towards Pt. The combination of these two phenomena allows one to measure spin-polarized holes at low temperatures. In the second device, photo-generated spin-polarized holes are successfully transferred to graphene by applying a bias voltage to the graphene/Ge junction. Since graphene is characterized by a significantly-long spin-relaxation time, holes diffuse with negligible spin losses towards Pt where their spin is revealed by means of photo-induced ISHE.
The realization of a regular distribution of defects or step edges with a specific orientation at the surface of a semiconductor or a semimetal, such as Bi, might have interesting implications for both fundamental studies and applications, due to the electronic properties stemming from their peculiar topology. Here we present an accurate comparison of the morphological and electronic structure of thin Bi film (with a thickness of 10 nm) grown on Ge(111) and on a high index Ge(111) vicinal surface, Ge(223). We make use of low energy electron diffraction (LEED) and spin-resolved photoemission spectroscopy (SR-PES) for the crystallographic and electronic characterization, respectively. We show that on both substrates it is possible to grow thin Bi films showing the hexagonal Bi(111) surface orientation, whose spin-resolved electronic structure is reminiscent of the one characteristic of bulk Bi(111). At variance with the films grown on Ge(111), those grown on Ge(223) present some specific features, namely the presence of a splitting in the LEED diffraction spots and a reduced momentum dispersion of the electronic states. We interpret these features as evidences that the peculiar morphology of the substrate can be indeed used to modulate the growth of the Bi film leading to the formation of a stepped Bi surface.
Germanium is a very good candidate to host a versatile spintronics platform thanks to its unique spin and optical properties. Recently we focused on two approaches in order to tune the spin-orbit interaction (SOI) in this Ge-based platform. The first one relies on growing high quality epitaxial topological insulators (TIs) on a Ge (111) substrate, we developed an original method to probe the spin-to-charge conversion at the TI/Ge(111) interface by taking advantage of the Ge optical properties. The latter approach is to exploit the intrinsic SOI of Ge (111). By investigating the electrical properties of a thin Ge(111), we found a large unidirectional Rashba magnetoresistance, which we ascribe to the interplay between the externally applied magnetic field and the current-induced pseudo-magnetic field applied in the spin-splitted subsurface states of Ge (111). Both studies open a door towards spin manipulation with electric fields in an all-semiconductor technology platform.
The aim of semiconductor spintronics is to exploit the spin degree of freedom of electrons to add new functionalities to electronic devices and boost their performances. The development of assets with the ability of efficiently injecting, transferring and detecting spins is a first step towards this goal. In this sense, a well established spin injection/detection scheme relies on an heavy metal grown on the top of a Ge substrate. The semiconductor is exploited to photogenerate spin-polarized carriers making use of the optical orientation technique. These carriers are then transferred to the heavy-metal layer where spin detection occurs by means of the inverse spin-Hall effect. A key point to get quantitative information from the investigation of such a platform is the knowledge of the total spin transferred from the semiconductor to the heavy-metal layer. Here, we address this problem by employing both an analytical and a numerical spin drift-diffusion model.
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