  Spin-charge interconversion (SCI) phenomena have attracted a growing interest
in the field of spintronics as means to detect spin currents or manipulate the
magnetization of ferromagnets. The key ingredients to exploit these assets are
a large conversion efficiency, the scalability down to the nanometer scale and
the integrability with opto-electronic and spintronic devices. Here we show
that, when an ultrathin Bi film is epitaxially grown on top of a Ge(111)
substrate, quantum size effects arising in nanometric Bi islands drastically
boost the SCI efficiency, even at room temperature. Using x-ray diffraction
(XRD), scanning tunneling microscopy (STM) and spin- and angle-resolved
photoemission (S-ARPES) we obtain a clear picture of the film morphology,
crystallography and electronic structure. We then exploit the Rashba-Edelstein
effect (REE) and inverse Rashba-Edelstein effect (IREE) to directly quantify
the SCI efficiency using optical and electrical spin injection.
