A Giorgioni

Spin and energy relaxation in germanium studied by spin-polarized direct-gap photoluminescence

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.

Photoluminescence decay of direct and indirect transitions in Ge/SiGe multiple quantum wells

We present a detailed experimental study of the photoluminescence decay of direct Γ-Γ and indirect L-Γ transitions in compressively strained Ge/Si0.15Ge0.85 type I multiple quantum wells. The lifetime of the fundamental L-Γ indirect-gap related transition is in the 6 to 13 ns range at the lattice temperature of 14 K. These values are just one order of magnitude higher than those typical of type-I direct gap III-V quantum wells and are significantly shorter than those characteristic of type-II indirect gap SiGe/Si quantum wells. The measured decay times show a clear dependence on the quantum well width and lattice temperature. The decay of the Γ-Γ direct-gap related transition is dominated by the ultrafast electron scattering from Γ-type to L-type states of the conduction band.

Optical spin injection and spin lifetime in Ge heterostructures

We studied spin properties of Ge heterostructures by optical orientation and Hanle measurements. The circular polarization of the direct gap photoluminescence is shown to exceed the theoretical bulk limit, yielding about 37% and 85% for transitions with heavy and light holes, respectively. The energetic proximity of Γ and L valleys and ultrafast scattering of electrons from Γ to L states allowed us to resolve the spin dynamics of holes and to observe the polarization of electrons after scattering to L valleys. The spin relaxation analysis indicates that the spin lifetime of electrons exceeds 5 ns below 150 K, whereas it is in the 500 ps range for holes.

Valley-dependent spin polarization and long-lived electron spins in germanium

Spin orientation and relaxation of conduction band electrons in bulk Ge are addressed by studying the steady-state circular polarization of the indirect gap photoluminescence (PL) at low temperatures. This provides a direct experimental proof of recently predicted spin-dependent selection rules for phonon-mediated optical transitions in Ge. In addition, we observe valley-dependent circularly polarized emission, and map the concomitant redistribution of electron spins within the multi-valley conduction band of Ge by gaining simultaneous access to the circular dichroism of light emitted across the direct and the indirect gap transitions. Finally, the lifetime of L-valley electrons is measured by means of decay curves of the indirect gap PL emission, yielding spin relaxation times in the order of hundreds of ns.

Temperature-Dependent Photoluminescence Characteristics of GeSn Epitaxial Layers

Ge1–xSnx epitaxial heterostructures are emerging as prominent candidates for the monolithic integration of light sources on Si substrates. Here we propose a suitable explanation for their temperature-dependent photoluminescence (PL) that is based upon the so far disregarded optical activity of dislocations. By working at the onset of plastic relaxation, which occurs whenever the epilayer releases the strain accumulated during growth on the lattice-mismatched substrate, we demonstrate that dislocation nucleation can be explicitly seen in the PL data. Notably, our findings point out that a monotonic thermal PL quenching can be observed in coherent films, in spite of the indirect nature of the Ge1–xSnx band-gap. Our investigation, therefore, contributes to a deeper understanding of the recombination dynamics in this intriguing group IV alloy and offers insights into crucial phenomena shaping the light emission efficiency.

Optical tailoring of carrier spin polarization in Ge/SiGe multiple quantum wells

The polarization of the direct gap emission from Ge/SiGe multiple quantum wells has been studied in the 4 K to 300 K temperature range, and in samples with different well thicknesses. Our results demonstrate that the polarization type and degree strongly depend on the excitation of electrons from the heavy hole and the light hole subbands, thus providing an effective degree of freedom to control the polarization of the direct interband emission. In addition, the analysis of the temperature dependence of the polarization degree highlights spin depolarization mechanisms.

Disentangling nonradiative recombination processes in Ge micro-crystals on Si substrates

We address nonradiative recombination pathways by leveraging surface passivation and dislocation management in μm-scale arrays of Ge crystals grown on deeply patterned Si substrates. The time decay photoluminescence (PL) at cryogenic temperatures discloses carrier lifetimes approaching 45 ns in band-gap engineered Ge micro-crystals. This investigation provides compelling information about the competitive interplay between the radiative band-edge transitions and the trapping of carriers by dislocations and free surfaces. Furthermore, an in-depth analysis of the temperature dependence of the PL, combined with capacitance data and finite difference time domain modeling, demonstrates the effectiveness of GeO2 in passivating the surface of Ge and thus in enhancing the room temperature PL emission.

Strong confinement-induced engineering of the g factor and lifetime of conduction electron spins in Ge quantum wells

Control of electron spin coherence via external fields is fundamental in spintronics. Its implementation demands a host material that accommodates the desirable but contrasting requirements of spin robustness against relaxation mechanisms and sizeable coupling between spin and orbital motion of the carriers. Here, we focus on Ge, which is a prominent candidate for shuttling spin quantum bits into the mainstream Si electronics. So far, however, the intrinsic spin-dependent phenomena of free electrons in conventional Ge/Si heterojunctions have proved to be elusive because of epitaxy constraints and an unfavourable band alignment. We overcome these fundamental limitations by investigating a two-dimensional electron gas in quantum wells of pure Ge grown on Si. These epitaxial systems demonstrate exceptionally long spin lifetimes. In particular, by fine-tuning quantum confinement we demonstrate that the electron Landé g factor can be engineered in our CMOS-compatible architecture over a range previously inaccessible for Si spintronics.

Optically reconfigurable polarized emission in Germanium

Light polarization can conveniently encode information. Yet, the ability to tailor polarized optical fields is notably demanding but crucial to develop practical methods for data encryption and to gather fundamental insights into light-matter interactions. Here we demonstrate the dynamic manipulation of the chirality of light at telecom wavelengths. This unique possibility is enrooted in the multivalley nature of the conduction band of a conventional semiconductor, namely Ge. In particular, we demonstrate that optical pumping suffices to govern the kinetics of spin-polarized carriers and eventually the chirality of the radiative recombination. We found that the polarized component of the emission can be remarkably swept through orthogonal eigenstates without magnetic field control or phase shifter coupling. Our results provide insights into spin-dependent phenomena and offer guiding information for the future selection and design of spin-enhanced photonic functionalities of group IV semiconductors.

Spin-resolved study of direct band-gap recombination in bulk Ge

Recent investigations have demonstrated how temperature and doping can be employed as effective turning knobs to fully control the angular momentum of the photons emitted at the direct gap recombination of carriers with optically oriented spin in bulk Germanium. Here we emphasize how cooling of hot electrons via Coulomb collisions and intervalley scattering affect spin distribution within the conduction band, and explore the role of an additional degree of freedom, namely the excitation power density, in contributing to the electron spin relaxation.