Spin orientation of photoexcited carriers and their energy relaxation is investigated in bulk Ge by studying spin-polarized recombination across the direct band gap. The control over parameters such as doping and lattice temperature is shown to yield high polarization degree, namely larger than 40%, as well as a fine-tuning of the angular momentum of the emitted light with a complete reversal between right-and left-handed circular polarization. By combining the measurement of the optical polarization state of band-edge luminescence and Monte Carlo simulations of carrier dynamics, we show that these very rich and complex phenomena are the result of the electron thermalization and cooling in the multi-valley conduction band of Ge. The circular polarization of the direct-gap radiative recombination is indeed affected by energy relaxation of hot electrons via the X valleys and the Coulomb interaction with extrinsic carriers. Finally, thermal activation of unpolarized L valley electrons accounts for the luminescence depolarization in the high temperature regime.

A unique spin depolarization mechanism, induced by the presence of g-factor anisotropy and intervalley scattering, is revealed by spin-transport measurements on long-distance germanium devices in a magnetic field longitudinal to the initial spin orientation. The confluence of electron-phonon scattering (leading to Elliott-Yafet spin flips) and this previously unobserved physics enables the extraction of spin lifetime solely from spin-valve measurements, without spin precession, and in a regime of substantial electric-field-generated carrier heating. We find spin lifetimes in Ge up to several hundreds of nanoseconds at low temperature, far beyond any other available experimental results.

Injection of spins into semiconductors is essential for the integration of the spin functionality into conventional electronics. Insulating layers are often inserted between ferromagnetic metals and semiconductors for obtaining an efficient spin injection, and it is therefore crucial to distinguish between signatures of electrical spin injection and impurity-driven effects in the tunnel barrier. Here we demonstrate an impurity-assisted tunneling magnetoresistance effect in nonmagnetic-insulator-nonmagnetic and ferromagnetic-insulator-nonmagnetic tunnel barriers. In both cases, the effect reflects on/off switching of the tunneling current through impurity channels by the external magnetic field. The reported effect is universal for any impurity-assisted tunneling process and provides an alternative interpretation to a widely used technique that employs the same ferromagnetic electrode to inject and detect spin accumulation.For the realization of semiconductor spintronic devices [1][2][3][4][5][6][7][8], the conductivity mismatch problem [9][10][11][12] and the difficulty of manipulating semiconductors at the nanoscale are the main issues delaying the progress of this research field. Employing the so-called three-terminal (3T) setup and making use of a single ferromagnetic/insulator contact for both injection and detection of spin-polarized currents was a big step towards this purpose [13]. Due to the simplicity of the micron-sized structures employed, this setup has gained popularity in semiconductor spintronics [13][14][15][16][17][18][19][20][21][22]. The Lorentzian-shaped magnetoresistance (MR) effect measured in 3T-semiconductor devices has been often attributed to spin injection on accounts of the resemblance to the celebrated Hanle effect in optical spin injection experiments [23]. However, it has been increasingly realized that the MR reported depends much on the tunneling process and too little on the semiconductor [13][14][15][16][17][18][19][20][21][22]. Furthermore, the typical junction working conditions employed for these measurements, with bias voltage settings much larger than the Zeeman energy, render the signal detection prone to subtle effects driven by impurities embedded in the tunnel barrier [14,24].In this Letter, we elucidate the physics behind such experiments by focusing on the tunnel barrier. Accordingly, our devices render a compact geometry with an aluminumoxide tunnel barrier created between metallic electrodes, M 1 /AlO x /M 2 , as sketched in Fig. 1(a). The M 1 /AlO x /M 2 devices were fabricated in-situ in a UHV electron-beam evaporation chamber with integrated shadow masks. The base pressure of the chamber is below 10 −9 mbar. The thickness of the top and bottom metallic electrodes, M 1 and M 2 , ranged between 10 nm and 15 nm. To decisively probe the role of impurities in the oxide, a series of devices were fabricated with 1) O 2 plasma exposure at 10 −1 mbar at a power ranging from around 24 to 40 W for 120 seconds to 210 seconds to minimize the impurity density, or 2) n−step (n...

We show that the electric-field-induced thermal asymmetry between the electron and lattice systems in pure silicon substantially impacts the identity of the dominant spin relaxation mechanism. Comparison of empirical results from long-distance spin transport devices with detailed Monte Carlo simulations confirms a strong spin depolarization beyond what is expected from the standard Elliott-Yafet theory even at low temperatures. The enhanced spin-flip mechanism is attributed to phonon emission processes during which electrons are scattered between conduction band valleys that reside on different crystal axes. This leads to anomalous behavior, where (beyond a critical field) reduction of the transit time between spin-injector and spin-detector is accompanied by a counterintuitive reduction in spin polarization and an apparent negative spin lifetime.

At low temperatures, electrons in semiconductors are bound to shallow donor impurity ions, neutralizing their charge in equilibrium. Inelastic scattering of other externally-injected conduction electrons accelerated by electric fields can excite transitions within the manifold of these localized states. Promotion of the bound electron into highly spin-orbit-mixed excited states drives a strong spin relaxation of the conduction electrons via exchange interactions, reminiscent of the Bir-Aronov-Pikus process where exchange occurs with valence band hole states. Through lowtemperature experiments with silicon spin transport devices and complementary theory, we reveal the consequences of this previously unknown spin depolarization mechanism both below and above the impact ionization threshold.