The electric and non-volatile control of the spin texture in semiconductors would represent a fundamental step towards novel electronic devices combining memory and computing functionalities. Recently, GeTe has been theoretically proposed as the father compound of a new class of materials, namely ferroelectric Rashba semiconductors. They display bulk bands with giant Rashba-like splitting, due to the inversion symmetry breaking arising from the ferroelectric polarization, thus allowing for a ferroelectric control of the spin. Here we provide the experimental demonstration of the correlation between ferroelectricity and spin texture. A surface engineering strategy is used to set two opposite pre-defined uniform ferroelectric polarizations, inward and outward, as monitored by Piezoresponse Force Microscopy. Spin and angular resolved photoemission experiments show that these GeTe(111) surfaces display opposite sense of circulation of spin in bulk Rashba bands. Furthermore, we demonstrate the crafting of non-volatile ferroelectric patterns in GeTe films at the nanoscale, by using the conductive tip of an atomic force microscope. Based on the intimate link between ferroelectric polarization and spin in GeTe, ferroelectric patterning paves the way to the investigation of devices with engineered spin configurations.

Phase Change Materials (PCMs) are unique compounds employed in non-volatile random access memory thanks to the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical and optical properties. In addition to the amorphous-to-crystalline transition, experimental results on polycrystalline GeSbTe alloys (GST) films evidenced a Metal-Insulator Transition (MIT) attributed to disorder in the crystalline phase. Here we report on a fundamental advance in the fabrication of GST with out-of-plane stacking of ordered vacancy layers by means of three distinct methods: Molecular Beam Epitaxy, thermal annealing and application of femtosecond laser pulses. We assess the degree of vacancy ordering and explicitly correlate it with the MIT. We further tune the ordering in a controlled fashion attaining a large range of resistivity. Employing ordered GST might allow the realization of cells with larger programming windows.

We demonstrate optical orientation in $\mathrm{Ge}/\mathrm{SiGe}$ quantum wells and study their spin properties. The ultrafast electron transfer from the center of the Brillouin zone to its edge allows us to achieve high spin polarizations and to resolve the spin dynamics of holes and electrons. The circular polarization degree of the direct gap photoluminescence exceeds the theoretical bulk limit, yielding $\sim 37\%$ and $\sim 85\%$ for transitions with heavy and light holes states, respectively. The spin lifetime of holes at the top of the valence band is estimated to be $\sim 0.5\mathrm{ps}$ and it is governed by transitions between light and heavy hole states. Electrons at the bottom of the conduction band, on the other hand, have a spin lifetime that exceeds 5 ns below 150 K. Theoretical analysis of the spin relaxation indicates that phonon-induced intervalley scattering dictates the spin lifetime of electrons.

Direct-gap gain up to 850\,\,cm-1 at 0.74~eV is measured and modeled in optically pumped Ge-on-Si layers for photoexcited carrier densities of 2.0\ifmmode\times\else\texttimes\fi{}1020\,\,cm-3. The gain spectra are correlated to carrier density via plasma-frequency determinations from reflection spectra. Despite significant gain, optical amplification cannot take place, because the carriers also generate pump-induced absorption of $\approx${}7000\,\,cm-1. Parallel studies of III\char21{}V direct-gap InGaAs layers validate our spectroscopy and modeling. Our self-consistent results contradict current explanations of lasing in Ge-on-Si cavities.