Nanoscale Mapping of the 3D Strain Tensor in a Germanium Quantum Well Hosting a Functional Spin Qubit Device

A strained Ge quantum well, grown on a SiGe/Si virtual substrate and hosting two electrostatically defined hole spin qubits, is nondestructively investigated by synchrotron-based scanning X-ray diffraction microscopy to determine all its Bravais lattice parameters. This allows rendering the three-dimensional spatial dependence of the six strain tensor components with a lateral resolution of approximately 50 nm. Two different spatial scales governing the strain field fluctuations in proximity of the qubits are observed at 1 μm, respectively. The short-ranged fluctuations have a typical bandwidth of 2 × 10-4 and can be quantitatively linked to the compressive stressing action of the metal electrodes defining the qubits. By finite element mechanical simulations, it is estimated that this strain fluctuation is increased up to 6 × 10-4 at cryogenic temperature. The longer-ranged fluctuations are of the 10-3 order and are associated with misfit dislocations in the plastically relaxed virtual substrate. From this, energy variations of the light and heavy-hole energy maxima of the order of several 100 μeV and 1 meV are calculated for electrodes and dislocations, respectively. These insights over material-related inhomogeneities may feed into further modeling for optimization and design of large-scale quantum processors manufactured using the mainstream Si-based microelectronics technology

Ultrastrong Coupling of Si1−xGex Parabolic Quantum Wells toTerahertz Microcavities

Control and manipulation of quantum states by light are increasingly important for both fundamental research and applications. This can be achieved through the strong coupling between light and semiconductor devices, typically observed at THz frequencies in 2D electron gases embedded in lithographic optical cavities. Here, we explore the possibility of achieving ultrastrong coupling between conduction sub-band states in Si1–xGex heterostructures and THz cavity photons fabricated with a potentially silicon-CMOS-compliant process. We developed Si1–xGex parabolic quantum wells with a transition at ω0 = 3.1 THz and hybrid metal-plasmonic THz patch-antenna microcavities resonating between 2 and 5 THz depending on the antenna length. In this first demonstration, we achieved anticrossing around 3 THz with spectroscopically measured Rabi frequency ΩR ≃ 0.7 THz (ΩR/ω0 ≃ 0.2, i.e., ultrastrong coupling). The present group-IV semiconductor material platform can be extended to the 5–12 THz range, where these semiconductors are transparent, as opposed to the III–V compound semiconductors plagued by strong THz optical phonon absorption. Moreover, the intersubband transition in parabolic quantum wells hosted by the nonpolar Si1–xGex crystal lattice is robust against carrier density and temperature variations, making the strength of the coupling only weakly temperature-dependent from 10 to 300 K. These results pave the way for the employment of the Si1–xGex material platform to perform fundamental research in ultrastrong light–matter coupling, fully exploiting the plasmonic character of the cavity mirror, as well as in ultrafast modulators and saturable absorbers for THz laser research

THz Intersubband Emitter based on Silicon

We present THz quantum cascade emitters realized on a Si substrate. The emission centered at 3.4 and 4.9 THz originates from L-valley transitions in strain-compensated n-type Ge/SiGe heterostructures. This is an important step towards the realization of Si-based THz quantum cascade lasers

Terahertz intersubband electroluminescence from n-type germanium quantum wells

The Quantum Cascade Laser (QCL) has been demonstrated in polar III-V semiconductor materials employing transitions between conduction band states [1] . Harnessing intersubband transitions allows lasing at mid-infrared and far-infrared wavelengths. Buried InGaAs/InAlAs QCLs unlocked the mid-infrared application space, because they are operational at room-temperature and in continuous wave [2] . However, THz QCLs remain limited up to 250 K in pulsed operation with a large dissipation [3] . The quenching of the laser emission is related to ther-mally activated LO phonon emission in polar materials. Exploiting intersubband transitions in non-polar group IV materials with weaker electron-phonon interaction is an exciting approach to realize a Si-based THz QCL and to eventually elevate the operation temperature [4]

Scanning X-ray Diffraction Microscopy Reveals the Nanoscale Strain Landscape of Novel Quantum Devices

This thesis provides also a detailed stepwise guideline on the data analysis for scanning X-ray diffraction experiments at a modern synchrotron radiation source.Halbleiterbasierte Spin-Qubits in elektrostatischen Quantenpunkten haben vor Kurzem ein technologisches Niveau erreicht, welches lange Kohärenzzeiten und hohe Fidelitäten ermöglicht. Diese Eigenschaften sind wichtig, um eine große Anzahl von Qubits zu realisieren, welche durch adiabatische Ladungstransporte miteiander verbunden werden sollen. Allerdings können lokale Fluktuationen der Gitterverspannung im aktiven Material die Spinzustände stören, da sie das elektrostatische Potential beeinflussen. Diese Arbeit untersucht die Gitterverspannung in funktionalen Loch-Spin-Qubits und in Bauelementen für kohärenten Elektronentransport, welche auf epitaktischen Ge/Si0.20Ge0.80 und Si/Si0.66Ge0.34 Heterostrukturen mit metallischen Elektroden basieren. Die experimentelle Herausforderung besteht darin, zugleich eine hohe Sensitivität für die Gitterdeformation und eine räumliche Auflösung auf der Nanometerskala zu erreichen. Dies wird durch rasternde Röntgenbeugungsmikroskopie an der Strahllinie ID01/ESRF ermöglicht, welche eine Abbildung des Verspannungstensors mit einer lateralen Auflösung von ≤ 50 nm in bis zu 10 nm-dünnen epitaktischen Quantentöpfen ermöglicht. Die Untersuchung von vier verschiedenen Quantenbauteilen zeigt Modulationen der Gitterverspannung von 10−4 − 10−3 auf, welche durch die Elektroden und die plastische Entspannung der Heterostruktur verursacht sind. Diese Modulationen werden in räumliche Fluktuationen der Bandkantenniveaus von einer Größenordnung von mehreren meV übersetzt, die damit ähnlich zu den Abständen der orbitalen Energieniveaus der Quantenpunkte sind. Folglich stellt diese Arbeit wichtige Informationen für die Realisierung eines skalierbaren Quantenprozessors durch eine Berücksichtigung der lokalen Materialeigenschaften bereitSemiconductor spin qubits featuring gate-defined electrostatic quantum dots have recently reached a maturity level enabling long spin coherence times and high fidelity. These characteristics are of paramount importance in the realization of large arrays of qubits interconnected by adiabatic charge shuttling. However, spin coherence can be strongly affected by local fluctuations of the lattice strain in the active material, since they impact the electrostatic potential. This work explores strain fluctuations in functional hole spin qubits and coherent electron shuttling devices based on epitaxial Ge/Si0.20Ge0.80 and Si/Si0.66Ge0.34 heterostructures with metallic electrodes. The main experimental challenge is to simultaneously achieve high sensitivity to the lattice deformation together with nanoscale spatial resolution. These requirements are met by Scanning X-ray Diffraction Microscopy at the synchotron beamline ID01/ESRF, which allows spatial mapping with ≤ 50 nm lateral resolution of the strain tensor in quantum well layers as thin as 10 nm. The analysis of four different devices highlights local modulations of the strain tensor components in the range of 10−4 − 10−3 induced by the gate electrodes and the plastic relaxation of the heterostructure. By means of band perturbation calculations, these strain fluctuations are translated into spatial modulations of the band edge energy levels. These perturbations are found to be of a few meV and thus on a similar magnitude as the orbital energy of the quantum dots. As such, this work provides important information for the realization of a scalable quantum processor with coherent interconnects by considering local material properties

The Interplay between Strain, Sn Content, and Temperature on Spatially Dependent Bandgap in Ge1−xSnx Microdisks

Germanium-tin (GeSn) microdisks are promising structures for complementary metal-oxide-semiconductor-compatible lasing. Their emission properties depend on Sn concentration, strain, and operating temperature. Critically, the band structure of the alloy varies along the disk due to different lattice deformations associated with mechanical constraints. An experimental and numerical study of Ge1-x Sn x microdisk with Sn concentration between 8.5 and 14 at% is reported. Combining finite element method calculations, micro-Raman and X-ray diffraction spectroscopy enables a comprehensive understanding of mechanical deformation, where computational predictions are experimentally validated, leading to a robust model and insight into the strain landscape. Through micro-photoluminescence experiments, the temperature dependence of the bandgap of Ge1-x Sn x is parametrized using the Varshni formula with respect to strain and Sn content. These results are the input for spatially dependent band structure calculations based on deformation potential theory. It is observed that Sn content and temperature have comparable effects on the bandgap, yielding a decrease of more than 20 meV for an increase of 1 at% or 100 K, respectively. The impact of the strain gradient is also analyzed. These findings correlate structural properties to emission wavelength and spectral width of microdisk lasers, thus demonstrating the importance of material-related consideration on the design of optoelectronic microstructures

Nanoscale Mapping of the 3D Strain Tensor in a Germanium Quantum Well Hosting a Functional Spin Qubit Device

A strained Ge quantum well, grown on a SiGe/Si virtual substrate and hosting two electrostatically defined hole spin qubits, is nondestructively investigated by synchrotron-based scanning X-ray diffraction microscopy to determine all its Bravais lattice parameters. This allows rendering the three-dimensional spatial dependence of the six strain tensor components with a lateral resolution of approximately 50 nm. Two different spatial scales governing the strain field fluctuations in proximity of the qubits are observed at &lt;100 nm and &gt;1 μm, respectively. The short-ranged fluctuations have a typical bandwidth of 2 × 10-4 and can be quantitatively linked to the compressive stressing action of the metal electrodes defining the qubits. By finite element mechanical simulations, it is estimated that this strain fluctuation is increased up to 6 × 10-4 at cryogenic temperature. The longer-ranged fluctuations are of the 10-3 order and are associated with misfit dislocations in the plastically relaxed virtual substrate. From this, energy variations of the light and heavy-hole energy maxima of the order of several 100 μeV and 1 meV are calculated for electrodes and dislocations, respectively. These insights over material-related inhomogeneities may feed into further modeling for optimization and design of large-scale quantum processors manufactured using the mainstream Si-based microelectronics technology. </p

High optical gain in InP-based quantum-dot material monolithically grown on silicon emitting at telecom wavelengths

Gefördert im Rahmen eines Open-Access-Transformationsvertrags mit dem Verla

THz intersubband absorption in n-type Si1−xGex parabolic quantum wells

High-quality n-type continuously graded Ge-rich Si1−xGex parabolic quantum wells with different doping levels were grown by using ultrahigh-vacuum chemical vapor deposition on Si(001) substrates. A thorough structural characterization study highlights an ideal parabolic compositional profile. THz intersubband absorption has been investigated in modulation-doped samples and samples directly doped in the wells. The comparison of experimental absorption data and theoretical calculations allowed us to quantify the impact of electron correlation effects on the absorption resonances in the different doping conditions and for electron sheet densities in the (1÷6)×1011 cm−2 range. A single optical resonance is present in modulation doped samples. Its peak energy and line shape are independent of temperature-induced variations of the electron distribution in the subbands up to 300 K, in agreement with the generalized Kohn theorem. This achievement represents a relevant step forward for the development of CMOS compatible optoelectronic devices in the THz spectral range, where thermal charge fluctuations play a key role.ISSN:0003-6951ISSN:1077-311