Search

Photodetectors based on a Ge-on-Si platform were widely studied over the last two decades, rapidly becoming the technology of choice for CMOS-integrated optoelectronic systems operating in the near infrared. Recently, we demonstrated a proof-of-concept device realized with a back-to-back, Ge-on-Si double photodiode with dual-band optical sensitivity and voltage-tunablity characteristics. Such a device represents the cornerstone for the development of integrated imaging systems operating both in the visible and in the near infrared spectral ranges. To achieve this ambitious goal, however, several technology improvements are needed starting from the fabrication of a multipixel array of dual-band photodetectors. Moreover, given the peculiar electronic behavior of the proposed device, specific readout electronics must be developed and integrated onto a CMOS platform. In this paper we demonstrate the operation of a Ge-on-Si multipixel array with a custom-developed readout integrated chip. We show a complete system characterization, also demonstrating its imaging capabilities with a simple experiment for the determination of the intensity profile of two different laser beams.

Germanium-based photodetectors represent the state-of-the-art in Near Infrared (NIR) detection systems when it comes to the monolithic integration with silicon electronics and CMOS compatibility. Germanium-on-silicon devices have been widely studied and several different photodetection schemes have been employed, such as Shottky junctions, pn diodes and phototransistors. Typically, germanium-based photodetectors provide a sensitivity spectrum ranging from the visible to the near infrared (NIR) and their responsivity at shorter wavelengths can be slightly tuned by the applied bias due to the varying extension of the electric field inside the Ge film. In this paper we propose a novel Ge-on-Si photodetector architecture whose spectral response can be completely switched from the visible to the NIR with the application of a proper bias. We provide Technology Computer Aided Design (TCAD) simulations of the device showing how the photodetection performance depend on different fabrication parameters. Finally, we show preliminary results obtained on sample devices.

In the last decade, silicon photonics has undergone an impressive development driven by an increasing number of technological applications. Plasmonics has not yet made its way to the microelectronic industry, mostly because of the lack of compatibility of typical plasmonic materials with foundry processes. In this framework, we have developed a plasmonic platform based on heavily n-doped Ge grown on silicon substrates. We developed growth protocols to reach n-doping levels exceeding 1020 cm-3, allowing us to tune the plasma wavelength of Ge in the 3-15 μm range. The plasmonic resonances of Ge-on-Si nanoantennas have been predicted by simulations, confirmed by experimental spectra and exploited for molecular sensing. Our work represents a benchmark for group-IV mid-IR plasmonics.

International audience; Midinfrared spectroscopy is a universal way to identify chemical and biological substances. Indeed, when interacting with a light beam, most molecules are responsible for absorption at specific wavelengths in the mid-IR spectrum, allowing to detect and quantify small traces of substances. On-chip broadband light sources in the midinfrared are thus of significant interest for compact sensing devices. In that regard, supercontinuum generation offers a mean to efficiently perform coherent light conversion over an ultrawide spectral range, in a single and compact device. This work reports the experimental demonstration of onchip two-octave supercontinuum generation in the mid-infrared wavelength, ranging from 3 to 13 μm (that is larger than 2500 cm −1) and covering almost the full transparency window of germanium. Such an ultrawide spectrum is achieved thanks to the unique features of Ge-rich graded SiGe waveguides, which allow second-order dispersion tailoring and low propagation losses over a wide wavelength range. The influence of the pump wavelength and power on the supercontinuum spectra has been studied. A good agreement between the numerical simulations and the experimental results is reported. Furthermore, a very high coherence is predicted in the entire spectrum. These results pave the way for wideband, coherent, and compact mid-infrared light sources by using a single device and compatible with large-scale fabrication processes.

Germanium is a very good candidate to host a versatile spintronics platform thanks to its unique spin and optical properties. Recently we focused on two approaches in order to tune the spin-orbit interaction (SOI) in this Ge-based platform. The first one relies on growing high quality epitaxial topological insulators (TIs) on a Ge (111) substrate, we developed an original method to probe the spin-to-charge conversion at the TI/Ge(111) interface by taking advantage of the Ge optical properties. The latter approach is to exploit the intrinsic SOI of Ge (111). By investigating the electrical properties of a thin Ge(111), we found a large unidirectional Rashba magnetoresistance, which we ascribe to the interplay between the externally applied magnetic field and the current-induced pseudo-magnetic field applied in the spin-splitted subsurface states of Ge (111). Both studies open a door towards spin manipulation with electric fields in an all-semiconductor technology platform.

Extending and controlling the spectral range of light detectors is very appealing for several sensing and imaging applications. Here we report on a normal incidence dual band photodetector operating in the visible and near infrared with a bias tunable spectral response. The device architecture is a germanium on silicon epitaxial structure made of two back-to-back connected photodiodes. The photodetectors show a broad photoresponse extending from 390nm to 1600nm with the capability to electronically select the shorter (400-1100 nm) or the longer (1000-1600 nm) portion with a relatively low applied voltage. Devices exhibit peak VIS and NIR responsivities of 0.33 and 0.63 A/W, respectively, a low optical crosstalk (120dB) and, thanks to their low voltage operation, maximum specific detectivities of 7·1011cmHz1/2/W and 2·1010cmHz1/2/W in the VIS and NIR, respectively.

Germanium-on-silicon technology has been thoroughly studied in the last 20 years and it now reached a high readiness level and it is mature for industrial development. In particular, germanium has been employed in a more-than-Moore pathway for the enhancement of silicon optical properties and the extension of the sensitivity spectrum of Si-based photodetectors. Typically, Ge-on-Si photodetectors exploit the extended visible to near-infrared absorption spectrum of Ge; we propose a dual diode Ge/Si structure where both semiconductors are used for the realization of photodetectors with a tunable sensitivity spectrum that can be completely electrically shifted from the visible to the near infrared wavelength range.

A novel spectroscopy technique to enable the rapid characterization of discrete mid-infrared integrated photonic waveguides is demonstrated. The technique utilizes lithography patterned polymer blocks that absorb light strongly within the molecular fingerprint region. These act as integrated waveguide detectors when combined with an atomic force microscope that measures the photothermal expansion when infrared light is guided to the block. As a proof of concept, the technique is used to experimentally characterize propagation loss and grating coupler response of Ge-on-Si waveguides at wavelengths from 6 to 10 µm. In addition, when the microscope is operated in scanning mode at fixed wavelength, the guided mode exiting the output facet is imaged with a lateral resolution better than 500 nm i.e. below the diffraction limit. The characterization technique can be applied to any mid-infrared waveguide platform and can provide non-destructive in-situ testing of discrete waveguide components.

The integration in Si technology of highly doped of Ge layers with a controlled amount of strain is crucial for nanoelectronic and photonic applications. N-type doping of Ge layers epitaxially grown on Si by P ion-implantation and pulsed laser melting is reported. In particular, samples with or without a post-growth annealing cycle in order to reduce the amount of threading dislocations (TDs) have been studied, in comparison with bulk Ge. Samples have been characterized by Secondary Ion Mass Spectrometry, Van der Paw-Hall and High-Resolution X-ray Diffraction. A very low out-diffusion, 1 × 1020 cm−3 carrier concentration with 100% P activation, as well as an increase of the tensile strain together with an improvement of the crystalline quality is reported, regardless of the as-grown TD density.

We introduce a new experimental strategy to investigate the transient resonant behavior of plasmonic nanostructures. Our approach allows to access their full-time field-resolved response in amplitude and phase.

We experimentally investigate the smallest germanium waveguide cavity resonators on silicon that can be designed to work around 1.55 mu m wavelength and observe an almost 30-fold enhancement in the collected spontaneous emission per unit volume when compared to a continuous germanium film of the same thickness. The enhancement is due to an effective combination of (i) excitation enhancement at the pump wavelength, (ii) emission enhancement (Purcell effect) at the emission wavelength, and (iii) effective beaming by the nanoresonators, which act as optical antennas to enhance the radiation efficiency. Our results set a basis for the understanding and engineering of light emission based on subwavelength, CMOS-compatible nanostructures operating at telecommunication wavelengths.

The recent advances in the development of quantum cascade laser with room temperature operation in the mid infrared paved the way for the realization of wideband communication systems. Particularly, two mid-infrared atmosphere transparency windows lying between 3-5 μm and between 8-14 μm exhibit great potential for further implementation of wideband free space communications. Additionally this wide unregulated spectral region shows reduced background noise and low Mie and Rayleigh scattering. Despite the development of a plethora of photonic components in mid infrared such as sources, detectors, passive structures, less efforts have been dedicated to investigate polarization management for information transport. In this work, the potential of Ge-rich SiGe waveguides is exploited to build a polarization insensitive platform in the mid-infrared. The gradual index evolution in SiGe alloys and geometric parameter optimization are used to obtain waveguides with birefringence below 2×10-4 and an unprecedented bandwidth in both atmosphere transparency windows i.e. near 3.5 μm and 9 μm. Following waveguide birefringence optimization an ultra-wideband and polarization insensitive multimode interference coupler was designed. The optimized structure shows a 4.5 μm wide bandwidth in transverse electric and transverse magnetic polarization at 9 μm wavelength. The developed ultra-wideband polarization insensitive photonic building blocks presented in this work pave the way for further implementation of free space communication systems in the mid infrared spectral region., Silicon Photonics XIV, February 2-7, 2019, San Francisco, USA, Series: Proceedings of SPIE; no. 10923

We demonstrate the use of plasmonic effects to boost the near-infrared sensitivity of metal-semiconductor-metal detectors. Plasmon-enhanced photodetection is achieved by properly optimizing Au interdigitated electrodes, micro-fabricated on Ge, a semiconductor that features a strong near IR absorption. Finite-difference time-domain simulations, photocurrent experiments and Fourier-transform IR spectroscopy are performed to validate how a relatively simple tuning of the contact geometry allows for an enhancement of the response of the device adapting it to the specific detection needs. A 2-fold gain factor in the Ge absorption characteristics is experimentally demonstrated at 1.4 µm, highlighting the potential of this approach for optoelectronic and sensing applications.

In this work we present model calculations and experimental verification of the giant nonlinear coefficients provided by intersubband transitions in hole-doped Ge/SiGe and Si/SiGe asymmetric quantum wells in the mid-infrared. We develop a model for the valence band-structure of the SiGe alloy including heavy-hole, light-hole and split-off bands for any value of Ge concentration, so as to obtain heterojunction parameters for the calculation of the wavefunction of confined hole states in quantum wells.

The n-type doping of Ge is self-limiting process due to formation of the vacancy-donor complexes (Dn V with n≤4). Here we report on experiments and density functional theory (DFT) calculations solving the basic problem of donor deactivation in heavily doped Ge. The self-healing process of heavily doped n-type Ge is achieved by rear-side flash lamp annealing (r-FLA) for 20 ms with the peak temperature of about 1050 K. The positron-annihilation lifetime spectroscopy (PALS) reveals that the P4V clusters are main defects in the as-grown Ge:P samples. Millisecond range high-temperature treatment dissociates the phosphorus-vacancy cluster (P4V) and, as shown by SIMS, fully supress the P diffusion. The electrochemical capacitance-voltage (ECV) profiling shows that the effective carrier concentration in P doped Ge (P concentration - 1×1020 cm-3) increases from about 3×1019 cm-3 in as-grown sample to above 8×1019 cm-3 after r-FLA. For the first time using structural (PALS, SIMS) and electrical (ECV) characterization combined with DFT calculations we were able to addressed, explained and solved the fundamental problem hindering the full integration of Ge with CMOS technology.

International audience; Recent works towards the development of Ge-rich SiGe photonic integrated circuits for on-chip mid-IR spectroscopy will be presented. First, the demonstration of ultra-wideband passive circuits will be discussed, followed by the first proofs of concepts towards the realization of efficient wideband active devices. The combination of on-chip integrated spectrometers with on-chip mid-IR sources will provide a solid basis for the development of a competitive mid-IR integrated platform from 3 to 15 µm wavelength.

In this report we present a novel strategy in selective epitaxial growth on top of Si pillars, which results in a tessellated Ge film, composed by self-aligned micron-sized crystals in a maskless process. Modelling by rate equations the morphology evolution of fully facetted crystal profiles is extensively outlined, showing an excellent prediction of the peculiar role played by flux shielding among microcrystals, in the case of dense array configuration. Crack formation and substrate bending, caused by the mismatch in thermal expansion coefficients, are eliminated by the mechanical decoupling among individual microcrystals, which are also shown to be dislocation- and strain-free. The method has been also tested for Si 1− x Ge x alloys, with compositions ranging from pure silicon to pure germanium. There are ample reasons to believe that this approach could be extended to other material combinations and substrate orientations, actually providing a technology platform for several device applications.

International audience; Midinfrared plasmonic sensing allows the direct targeting of unique vibrational fingerprints of molecules. While gold has been used almost exclusively so far, recent research has focused on semiconductors with the potential to revolutionize plasmonic devices. We fabricate antennas out of heavily doped Ge films epitaxially grown on Si wafers and demonstrate up to 2 orders of magnitude signal enhancement for the molecules located in the antenna hot spots compared to those located on a bare silicon substrate. Our results set a new path toward integration of plasmonic sensors with the ubiquitous CMOS platform.

International audience; We report the design of a Bragg-mirror based Fabry-Perot cavity integrated on SiGe waveguides working at 7.25 µm. The demonstration of such resonant structures will be a major step forward for sensing applications in mid-infrared.

Mid-infrared (mid-IR) silicon photonics is becoming a prominent research with remarkable potential in several applications such as in early medical diagnosis, safe communications, imaging, food safety and many more. In the quest for the best material platform to develop new photonic systems, Si and Ge depart with a notable advantage over other materials due to the high processing maturity accomplished during the last part of the 20th century through the deployment of the CMOS technology. From an optical viewpoint, combining Si with Ge to obtain SiGe alloys with controlled stoichiometry is also of interest for the photonic community since permits to increase the effective refractive index and the nonlinear parameter, providing a fascinating playground to exploit nonlinear effects. Furthermore, using Ge-rich SiGe gives access to a range of deep mid-IR wavelengths otherwise inaccessible (λ ~2-20 μm). In this paper, we explore for the first time the limits of this approach by measuring the spectral loss characteristic over a broadband wavelength range spanning from λ = 5.5 μm to 8.5 μm. Three different SiGe waveguide platforms are compared, each one showing higher compactness than the preceding through the engineering of the vertical Ge profile, giving rise to different confinement characteristics to the propagating modes. A flat propagation loss characteristic of 2-3 dB/cm over the entire wavelength span is demonstrated in Ge-rich graded-index SiGe waveguides of only 6 μm thick. Also, the role of the overlap fraction of the confined optical mode with the Si-rich area at the bottom side of the epitaxial SiGe waveguide is put in perspective, revealing a lossy characteristic compared to the other designs were the optical mode is located in the Ge-rich area at the top of the waveguide uniquely. These Ge-rich graded-index SiGe waveguides may pave the way towards a new generation of photonic integrated circuits operating at deep mid-IR wavelengths.

International audience; Recent works towards the development of Ge-rich SiGe photonic integrated circuits will be presented, such as the demonstration of low-loss waveguides and ultra-wideband Mach Zehnder interferometer from 5.5 to 8.6 µm wavelength, as well as the first steps towards the realization of efficient wideband optical sources.

International audience; Mid-infrared (mid-IR) silicon photonics is expected to lead key advances in different areas including spectroscopy, remote sensing, nonlinear optics or free-space communications, among others. Still, the inherent limitations of the silicon-on-insulator (SOI) technology, namely the early mid-IR absorption of silicon oxide and silicon at λ~3.6 µm and at λ ~8.5 µm respectively, remain the main stumbling blocks that prevent this platform to fully exploit the mid-IR spectrum (λ ~2-20 µm). Here, we propose using a compact Ge-rich graded-index Si1-xGex platform to overcome this constraint. A flat propagation loss characteristic as low as 2-3 dB/cm over a wavelength span from λ = 5.5 µm to 8.5 µm is demonstrated in Ge-rich Si1-xGex waveguides of only 6 µm thick. The comparison of three different waveguides design with different vertical index profiles demonstrates the benefit of reducing the fraction of the guided mode that overlaps with the Si substrate to obtain such flat low loss behavior. Such Ge-rich Si1-xGex platforms may open the route towards the implementation of mid-IR photonic integrated circuits with low-loss beyond the Si multi-phonon absorption band onset, hence truly exploiting the full Ge transparency window up to λ ~15 µm.

We show that the photon energy dependence of the photo-induced inverse spin-Hall effect (ISHE) signal at Pt/semiconductor junctions can be reproduced by a model that explicitly accounts for the electron spin diffusion length Ls in the semiconductor. In particular, we consider the Pt/GaAs, Pt/Ge, and Pt/Si systems: although optical spin injection and transport of spin-polarized electrons in the conduction band of these semiconductors are ruled by different mechanisms, a simple one dimensional analytical diffusion model, where Ls is the free parameter, can reproduce the ISHE data in all cases. This highlights the potentialities of the photo-induced ISHE spectra as a tool to directly address fundamental spin transport properties in semiconductors.

A Ge-SiGe multiple quantum well structure created by low energy plasma enhanced chemical vapour deposition, with nominal well thickness of 5.4 nm separated by 3.6 nm SiGe spacers, is analysed quantitatively using scanning transmission electron microscopy. Both high angle annular dark field imaging and electron energy loss spectroscopy show that the interfaces are not completely sharp, suggesting that there is some intermixing of Si and Ge at each interface. Two methods are compared for the quantification of the spectroscopy datasets: a self-consistent approach that calculates binary substitutional trends without requiring experimental or computational k-factors from elsewhere and a standards-based cross sectional calculation. Whilst the cross section approach is shown to be ultimately more reliable, the self-consistent approach provides surprisingly good results. It is found that the Ge quantum wells are actually about 95% Ge and that the spacers, whilst apparently peaking at about 35% Si, contain significant interdiffused Ge at each side. This result is shown to be not just an artefact of electron beam spreading in the sample, but mostly arising from a real chemical interdiffusion resulting from the growth. Similar results are found by use of X-ray diffraction from a similar area of the sample. Putting the results together suggests a real interdiffusion with a standard deviation of about 0.87 nm, or put another way—a true width defined from 10%–90% of the compositional gradient of about 2.9 nm. This suggests an intrinsic limit on how sharp such interfaces can be grown by this method and, whilst 95% Ge quantum wells (QWs) still behave well enough to have good properties, any attempt to grow thinner QWs would require modifications to the growth procedure to reduce this interdiffusion, in order to maintain a composition of ≥95% Ge.

Germanium (Ge) has played a key role in silicon photonics as an enabling material for datacom applications. Indeed, the unique properties of Ge have been leveraged to develop high performance integrated photodectors, which are now mature devices. Ge is also very useful for the achievement of compact modulators and monolithically integrated laser sources on silicon. Interestingly, research efforts in these domains also put forward the current revolution of mid-IR photonics. Ge and Ge-based alloys also present strong advantages for mid-infrared photonic platform such as the extension of the transparency window for these materials, which can operate at wavelengths beyond 8 μm. Different platforms have been proposed to take benefit from the broad transparency of Ge up to 15 μm, and the main passive building blocks are now being developed. In this review, we will present the most relevant Ge-based platforms reported so far that have led to the demonstration of several passive and active building blocks for mid-IR photonics. Seminal works on mid-IR optical sensing using integrated platforms will also be reviewed.

Recent works towards the development of Ge-rich SiGe photonic integrated circuits will be presented, such as the demonstration of low-loss waveguides and ultra-wideband Mach Zehnder interferometer from 5.5 to 8.6 μm wavelength, as well as the first steps towards the realization of efficient wideband optical sources.

Despite the recent introduction of heavily doped semiconductors for mid-infrared plasmonics, it still remains an open issue whether such materials can compete with noble metals. A whole set of figures of merit are employed to thoroughly assess the use of heavily doped Ge on Si as a mid-infrared plasmonic material and benchmark it against standard noble metals such as Au. A full-wave electrodynamics framework is used to model and design high-performance, silicon-foundry compatible mid-infrared plasmonic sensors based on experimental material data reaching plasma wavelengths down to λp ∼ 3.1 μm. It is finally shown that Ge sensors can provide signal enhancements for vibrational spectroscopy above the 3 orders of magnitude, thus, representing a promising alternative to noble metals, leveraging the full compatibility with the silicon foundry microfabrication processes.

Topological insulators (TIs) like Bi$_2$Se$_3$ are a class of material with topologically protected surface states in which spin-momentum locking may enable spin-polarized and defect-tolerant transport. In this work, we achieved the epitaxial growth of Bi$_2$Se$_3$ thin films on germanium, which is a key material for microelectronics. Germanium also exhibits interesting properties with respect to the electron spin such as a spin diffusion length of several micrometers at room temperature. By growing Bi$_2$Se$_3$ on germanium, we aim at combining the long spin diffusion length of Ge with the spin-momentum locking at the surface of Bi$_2$Se$_3$. We first performed a thorough structural analysis of Bi$_2$Se$_3$ films using electron and x-ray diffraction as well as atomic force microscopy. Then, magnetotransport measurements at low temperature showed the signature of weak antilocalization as a result of two-dimensional transport in the topologically protected surface states of Bi$_2$Se$_3$. Interestingly, the magnetotransport measurements also point out that the conduction channel can be tuned between the Bi$_2$Se$_3$ film and the Ge layer underneath by means of the bias voltage or the applied magnetic field. This result suggests that the Bi$_2$Se$_3$/Ge junction is a promising candidate for tuning spin-related phenomena at interfaces between TIs and semiconductors., 18 pages, 8 figures

International audience; We report on the first third order nonlinear experimental characterization of Ge-rich Si 1-x Ge x waveguides, with Germanium concentrations x ranging from 0.7 to 0.9. The experimental values will be compared with theoretical models. These results will provide helpful insights to assist the design of nonlinear integrated optical based devices in both the near-and mid-IR wavelength ranges.

We report on the nonlocal detection of optically oriented spins in lightly $n$-doped germanium at room temperature. Localized spin generation is achieved by scanning a circularly polarized laser beam ($\ensuremath{\lambda}=1550$ nm) on an array of lithographically defined Pt microstructures. The in-plane oriented spin generated at the edges of such microstructures, placed at different distances from a spin-detection element, allows for a direct imaging of spin diffusion in the semiconductor, leading to a measured spin diffusion length of about $10\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$. Two different spin-detection blocks are employed, consisting of either a magnetic tunnel junction or a platinum stripe where the spin current is converted in an electrical signal by the inverse spin-Hall effect. The second solution represents the realization of a nonlocal spin-injection/detection scheme that is completely free from ferromagnetic functional blocks.

Silicon-rich SiGe alloys represent a promising platform for the development of large-area single-mode optical waveguides to be integrated in silicon-based optical circuits. We find that SiGe layers epitaxially grown on Si successfully guide radiation with a 1.55 μm wavelength, but, beyond a critical core thickness, their optical properties are strongly affected by the clustering of misfit dislocations at the interface between Si and SiGe, leading to a significant perturbation of the local refractive index. Transmission electron microscopy and micro-Raman spectroscopy, together with finite-element simulations, provide a complete analysis of the impact of dislocations on optical propagation. © 2009 Elsevier B.V. All rights reserved.

The paper reports on the developments of Ge/SiGe quantum well (QW) waveguide modulators operating at 1.3 μm. Two modulator configurations have been studied: The first one is based on QW structures grown on a 13-μm SiGe buffer on bulk silicon. Light was directly coupled and propagated in Ge/Si0.35Ge0.65 QWs. Using a 3-μm-wide and 50-μm-long modulator, an extinction ratio (ER) up to 6 dB was obtained at 1.3 μm. In the second configuration, the aim is to integrate Ge/SiGe QW on standard silicon-on-insulator (SOI) waveguides. A reduction of buffer thickness is then required to allow the light coupling from Si waveguide to Ge/SiGe QW. To this purpose, first we demonstrated QCSE with a thin (360-nm-thick) Si0.08Ge0.92 buffer on silicon using the well-known Ge/Si0.15Ge0.85 QWs (operated at a wavelength of 1.4 μm). Based on these promising experimental results, we theoretically investigated properties of a Ge/Si0.65Ge0.35 QW modulator integrated on SOI waveguides. 7.7 dB ER were predicted with 4 dB optical insertion loss and an estimated energy consumption of 59 fJ/bit for a modulator length as short as 69 μm.

We have investigated the electron spin diffusion length at room temperature in bulk n-doped germanium as a function of the doping concentration. To this purpose, we exploit a nonlocal spin injection/detection scheme where spins are optically injected at the direct gap of Ge and electrically detected by means of the inverse spin-Hall effect (ISHE). By optically generating a spin population in the conduction band of the semiconductor at different distances from the spin detector, we are able to directly determine the electron spin diffusion length Ls in the Ge substrate. We experimentally observe that Ls > 20 μm for lightly doped samples and, by taking into account the electron diffusion coefficient, we estimate electron spin lifetime values τs larger than 50 ns. In contrast, for heavily doped Ge substrates, the spin diffusion length decreases to a few micrometers, corresponding to τs ≈ 20 ns. These results can be exploited to refine spin transport models in germanium and reduce the experimental uncertainties associated with the evaluation of Ls from other spin injection/detection techniques.

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., Ge/Si heterojunctions are prominent candidates for spintronics, but the spin-dependent phenomena have been elusive. Here, Giorgioni et al. report long spin relaxation and coherence times in a two dimensional electron gas confined in quantum wells of pure Ge grown on SiGe-buffered Si substrates.