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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

Spin physics of carriers in germanium (Ge) has been a hot topic of the last years, due to the expected very long spin lifetime in centrosymmetric semiconductors. The thesis reports about an all-optical investigation of the spin properties of carriers in Ge-based systems. The results of polarization resolved photoluminescence measurements on bulk Ge and Ge quantum wells (QWs) provided the experimental demonstration of optical spin injection and detection in these structures. The comparison of the measured polarization of the direct gap emission in bulk Ge samples with the results of a Monte Carlo simulation allowed us to outline the rich spin dynamics of the photoexcited electrons. On the other hand, in Ge QWs the study of the polarization of direct transitions between confined states demonstrates that strain and confinement can be used as tools to tailor the optical spin polarization. Information on the spin lifetime of conduction electrons are provided by the study of the indirect gap emission both in the bulk material and in the nanostructures. In particular, for intrinsic bulk Ge our results constitute the first experimental evidence of a predicted very long spin lifetime in the low temperature regime. The study of the spin lifetime of conduction electrons in Ge QWs is enriched by Electron Spin Resonance measurements, which in addition allowed us to address the g factor of confined electrons, demonstrating the effect of confinement on the spin-orbit coupling of conduction electrons.

Germanium and silicon-germanium alloys have found entry into Si technology thanks to their compatibility with Si processing and their ability to tailor electronic properties by strain and band-gap engineering. Noticeably, the quasi-direct band structure of Ge allows for optical spin orientation bridging the fields of spintronics and Si photonics where Ge is increasingly applied [3,4]. Despite recent progress in the investigation of spin dynamics in Ge, very little is known about spin flip scattering by dopants and the role played by impurities in determining spin properties in different temperature regimes. Here we present a combined experimental and theoretical investigation of the spin and energy relaxation of electrons in Ge. By taking advantage of polarization-resolved photoluminescence measurements and Monte Carlo simulations of carrier dynamics we unveil unique and very rich spin physics and highlight the importance of the multi-valley conduction band of this material. We demonstrate full control over the angular momentum of the direct gap emission with a complete reversal between right- and left-handed circular polarization. This detailed investigation of spin properties of Ge is expected to open new opportunities connected to photonics and spintronics on mainstream CMOS-compatible platforms.

Germanium and silicon-germanium alloys have found entry into Si technology thanks to their compatibility with Si processing and their ability to tailor electronic properties by strain and band-gap engineering. Noticeably, the quasi-direct band structure of Ge allows for optical spin orientation bridging the fields of spintronics and Si photonics where Ge is increasingly applied. Despite recent progress in the investigation of spin dynamics in Ge, very little is known about spin flip scattering by dopants and the role played by impurities in determining spin properties in different temperature regimes. Here we present a combined experimental and theoretical investigation of the spin and energy relaxation of electrons in Ge. By taking advantage of polarization-resolved photoluminescence measurements and Monte Carlo simulations of carrier dynamics we unveil a unique and very rich spin physics and highlight the importance of the multi-valley conduction band of this material. We demonstrate full control over the angular momentum of the direct gap emission with a complete reversal between right- and left-handed circular polarization. In the low temperature regime, the observed state of light polarization is ascribed to the so-far overlooked carrier cooling process that stems from the Coulomb scattering originated within the X-valleys. In the high temperature range, we disclose the role of backscattering from the L-valley in defining the depolarization of direct gap luminescence. This detailed investigation of spin properties of Ge is expected to open new opportunities connected to photonics and spintronics on a CMOS-compatible platform

The polarization of the photoluminescence (PL) from the indirect gap of bulk Ge is studied after optical orientation by absorption of circularly polarized light through the direct gap. The spin relaxation time ts is then estimated by combining the polarization degree and the decay time of the indirect gap emission. Ge is a promising material for spin-optoelectronics due to its feasible integration on Si, its noticeable spin properties and the possibility of achieving room temperature lasing action. In order to merge the spin degree of freedom and the photonics functionalities, the knowledge of properties such as the electron spin lifetime is of practical and fundamental importance. In this work we address this issue reporting the first evidence that low temperature PL from the indirect gap of bulk Ge is circularly polarized. This is the smoking gun proof that spin-polarized electrons preserve their orientation during the long dwell time in the L valley. We experimentally verified the selection rules for phonon mediated transitions showing an excellent agreement with the theory. Finally, to estimate ts we applied time-resolved PL, yielding in addition accurate data for the carrier lifetime. Hundreds of ns long ts is found providing the longest estimation reported at T<30K. Our results demonstrate the prominent and viable role of Ge for merging photonics and spintronics on the same materials platform.

We report micron-sized Ge crystal arrays grown on deeply patterned Si substrates that yield a surge of the interband photoluminescence intensity by more than 2 orders of magnitude with respect to that typical for epitaxial layers directly grown on planar substrates. This finding is ascribed to the strongly modified internal quantum efficiency induced by controlling the nonradiative recombination at dislocations and to the improved light extraction offered by the array architecture. By spectrally resolving the interband and the dislocation-related luminescence, we address the parasitic activity of extended defects and its impact on the optical properties of the heterosystem. Such results are then exploited along with band gap engineering to design SiGe reflectors and Ge quantum wells that are effective in further amplifying the emission yield

We performed spin-resolved photoluminescence measurements on Ge/SiGe multiple quantum wells with different well thickness and using different exciting power densities. The polarization of the direct emission strongly depends on the relative weight of electrons photoexcited from the light and the heavy hole subbands. The study of the polarization as a function of the exciting power highlights the role of the carrier-carrier interactions in determining spin depolarization. © 2013 AIP Publishing LLC.

The carrier dynamics that occurs in Ge/SiGe QWs when electrons are excited to confined states at Gamma is studied by means of optical spectroscopy at different lattice temperatures. The typical times for the different relaxation and recombination processes are given and discussed

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

Spin orientation of photoexcited carriers and their energy relaxation are 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 a 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 multivalley 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. © 2013 American Physical Society.

We studied the plastic deformation of an ultrathin silicon-on-insulator with epitaxial Si1−xGex by transmission electron microscopy, Raman spectroscopy, and finite-element method. We analyzed a top Si layer of 10 nm (testing also a 2 nm layer) with epitaxial Si0.64Ge0.36 stressors of 50 and 100 nm. SiGe plastically deforms the top Si layer, and this strain remains even when Si1−xGex is removed. For low dislocation densities, dislocations are gettered close to the Si/SiO2 interface, while the SiGe/Si interface is coherent. Beyond a threshold dislocation density, interactions between dislocations force additional dislocations to position at the Si1−xGex/Si interface.

A systematic study of carrier relaxation in Ge quantum wells is performed applying photoluminescence and pump-probe spectroscopy. Intersubband-relaxation on a 100 fs time-scale and the presence of a non-thermal carrier distribution are identified. © 2012 OSA.

Ge is emerging as an interesting material for spintronic applications. We demonstrate that in SiGe heterostructures strain and quantum confinement effects can be used to tailor spin related properties enhancing the spin polarization of injected carriers above the bulk limit. Moreover the fast ?-L electron scattering and the strain induced removal of the heavy-hole, light-hole degeneracy, make SiGe heterostructures an ideal material platform for the study of hole dynamics. © The Electrochemical Society

We demonstrate optical orientation in Ge/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 ∼37% and ∼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 ∼0.5ps 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

We present a dephasing time analysis of the excitonic resonances in Ge/SiGe quantum wells for various lattice temperatures by coherent oscillation spectroscopy (COS). The results are compared to the linewidths of the excitonic resonances determined from linear absorption measurements. Additionally, COS is applied to different samples with varying linewidth, identifying one sample with a dominating homogeneously broadened 1s excitonic resonance down to 7 K