Die Gruppe erforscht innovative Integrationskonzepte elementarer Halbleitermaterialien der Gruppe IV (Kohlenstoff, Silizium, Germanium, Zinn und ihre Legierungen) in die Silizium-Technologieplattform. Ihre inhärenten Materialeigenschaften können genutzt werden, um die Leistung von Bauteilen zu verbessern und auf Quanteneffekten basierende Funktionen für Anwendungen in der Optoelektronik und im Quantencomputing hinzuzufügen.
Um tiefergehendes Wissen zu erlangen, das zur präzisen Kontrolle der Eigenschaften dieser Materialsysteme und ihrer möglichen Integration erforderlich ist, nutzt unsere Gruppe synergetische Kompetenzen in fortschrittlichen, hochmodernen experimentellen Techniken auf der Nanoskala und theoretische Modellierung.
Forschungsziele
- Entwickelung von Integrationsprozessen und innovativen Bauteilen in enger Zusammenarbeit mit der Technologieabteilung für Quantencomputing und Optoelektronik.
- Heterostrukturepitaxie von Gruppe IV Materialien mittels CVD in einer CMOS-kompatiblen Reinraumumgebung und MBE für explorative Forschung.
- Materialuntersuchung mit einem umfassenden Satz an modernsten Techniken zur Bestimmung inhärenter Struktureigenschaften und deren Korrelation mit anwendungsorientierter optischer und elektrischer Charakterisierung.
- Computergestützte Modellierung (FEM-Simulationen mit COMSOL und Lumerical, tight-binding & effektive mass theory) mechanischer, optischer, elektromagnetischer und elektrischer Eigenschaften.
- Anwendung und Förderung modernster auf Synchrotronstrahlung basierender Techniken für eine ausführliche und detaillierte Materialcharakterisierung.
Forschungsschwerpunkte
Die Arbeitsgruppe Halbleiter-Optoelektronik der Abteilung Materialforschung entwickelt gemeinsam mit der Abteilung Technologie halbleiterbasierte Quantenbits (Qubits) in SiGe-Heterostrukturen. Darüber hinaus bündeln das Forschungszentrum Jülich, die RWTH Aachen und das IHP ihre komplementären Kompetenzen im Bereich der Halbleiter- und Quantentechnologie. Wir arbeiten in einer zeitlich unbefristeten Kooperation im Rahmen eines Joint Labs gemeinsam an der Entwicklung skalierbarer Qubits, die Quantencomputer auf einer Halbleiterplattform ermöglichen. Das IHP bringt seine Expertise im Wachstum und der Charakterisierung von Heterostrukturen sowie in der Qubit-Herstellung auf Basis von Ge/SiGe- und Si/SiGe-Verbindungen ein. Darüber hinaus verfügen das Forschungszentrum Jülich und die RWTH Aachen im Rahmen des gemeinsamen JARA-Instituts für Quanteninformation über ausgewiesene Expertise auf dem Gebiet der Bauteilkonzeption, Charakterisierung und Qubit-Demonstration.
Des Weiteren wird das innovative quartäre Materialsystem CSiGeSn intensiv erforscht, das großes Potenzial für zukünftige Gruppe IV Halbleiter-Optoelektronik hat. Die flexible Halbleiterlegierung ermöglicht eine präzise Variation der adressierbaren Wellenlänge durch Anpassung der Sn-Konzentration. Das Wachstum von CSiGeSn-Schichtsystemen entsprechender Qualität auf Silizium ist eine große Herausforderung. Daher wollen wir Molekularstrahlepitaxieverfahren für die explorative Probenherstellung weiterentwickeln. In den Laboren der Abteilung Materialforschung werden die strukturellen, chemischen und optischen Eigenschaften von verspanntem CSiGeSn in einem Multiskalenansatz (von der atomaren bis zur Mikrometerskala) untersucht. Zudem untersuchen wir, ebenfalls in enger Zusammenarbeit mit dem Forschungszentrum Jülich, das auf Basis der gewonnenen Erkenntnisse „Proof-of-Concept“-Komponenten herstellt, strukturelle und optische Materialeigenschaften von GeSn-basierten optisch und elektrisch gepumpten Lasern, Fotodetektoren und thermoelektrischen On-Chip-Geräte.
Im Rahmen des gemeinsam mit der Universität Roma Tre gegründeten International Joint Lab betreiben wir auch unsere Forschung auf dem Gebiet der Ge/Si-basierten Quantenkaskadenstrukturen für Anwendungen in dem Bereich der THz-Optoelektronik. Insbesondere unterstützt unser Team die Forschungsbemühungen durch eine gründliche Charakterisierung der komplexen Heterostrukturen, die an der Roma Tre durch modernste Ultrahochvakuum-CVD abgeschieden werden.
Die Kontrolle der Verspannungs- und Segregationseigenschaften zur Erzeugung komplexer Heterostrukturen zur weiteren Leistungssteigerung des Materials spielt in dieser Arbeitsgruppe eine zentrale Rolle. Daher bestehen langjährige Kooperationen, insbesondere mit der European Synchrotron Radiation Facility (ESRF). Hier werden hochmoderne nanofokussierte Röntgenbeugungs- und Atomspektroskopie-experimente durchgeführt, um diese Struktureigenschaften zu analysieren, zu simulieren und zu verfeinern.
Forschungsergebnisse
Script list Publications
(1) Ultrastrong Coupling of Si1−xGex Parabolic Quantum Wells to Terahertz Microcavities
F. Berkmann, T. Venanz, L. Baldassarre, E. Campagna, E. Talamas-Simola, L. Di Gaspare, C. Corley-Wiciak, G.Capellini,G. Nicotra, G. Sfuncia, A. Notargiacomo, E. Giovine, S. Cibella, M. Virgilio, G. Scalari, M. De Seta, M. Ortolani
ACS Photonics 11(7), 2776 (2024)
DOI: 10.1021/acsphotonics.4c00641, (IHP- Roma Tre University Joint Lab)
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.
(2) High Quality CMOS Ccompatible N-Type SiGe Parabolic Quantum Wells for Intersubband Photonics at 2.5-5 THz
E. Campagna, E. Talamas Simola, T. Venanzi, F. Berkmann, C. Corley-Wiciack, G. Nicotra, L. Baldassarre, G. Capellini, L. Di Gaspare, M. Virgilio, M. Ortolani, M. De Seta
Nanophotonics 13(10), 1793 (2024)
DOI: 10.1515/nanoph-2023-0704, (FLASH)
A parabolic potential that confines charge carriers along the growth direction of quantum wells semiconductor systems is characterized by a single resonance frequency, associated to intersubband transitions. Motivated by fascinating quantum optics applications leveraging on this property, we use the technologically relevant SiGe material system to design, grow, and characterize n-type doped parabolic quantum wells realized by continuously grading Ge-rich Si1−x Ge x alloys, deposited on silicon wafers. An extensive structural analysis highlights the capability of the ultra-high-vacuum chemical vapor deposition technique here used to precisely control the quadratic confining potential and the target doping profile. The absorption spectrum, measured by means of Fourier transform infrared spectroscopy, revealed a single peak with a full width at half maximum at low and room temperature of about 2 and 5 meV, respectively, associated to degenerate intersubband transitions. The energy of the absorption resonance scales with the inverse of the well width, covering the 2.5–5 THz spectral range, and is almost independent of temperature and doping, as predicted for a parabolic confining potential. On the basis of these results, we discuss the perspective observation of THz strong light–matter coupling in this silicon compatible material system, leveraging on intersubband transitions embedded in all-semiconductor microcavities.
(3) Room Temperature Lattice Thermal Conductivity of GeSn Alloys
O. Concepción, J. Tiscareño-Ramírez, A.A. Chimienti, T. Classen, A.A. Corley-Wiciak, A. Tomadin, D. Spirito, D. Pisignano, P. Graziosi, Z. Ikonic, Q.T. Zhao, D. Grützmacher, G. Capellini, S. Roddaro, M. Virgilio, D. Buca
ACS Applied Energy Materials 7(10), 4394 (2024)
DOI: 10.1021/acsaem.4c00275, (SiGeSn TE)
CMOS-compatible materials for efficient energy harvesters at temperatures characteristic for on-chip operation and body temperature are the key ingredients for sustainable green computing and ultralow power Internet of Things applications. In this context, the lattice thermal conductivity (κ) of new group IV semiconductors, namely Ge1–xSnx alloys, are investigated. Layers featuring Sn contents up to 14 at.% are epitaxially grown by state-of-the-art chemical-vapor deposition on Ge buffered Si wafers. An abrupt decrease of the lattice thermal conductivity (κ) from 55 W/(m·K) for Ge to 4 W/(m·K) for Ge0.88Sn0.12 alloys is measured electrically by the differential 3ω-method. The thermal conductivity was verified to be independent of the layer thickness for strained relaxed alloys and confirms the Sn dependence observed by optical methods previously. The experimental κ values in conjunction with numerical estimations of the charge transport properties, able to capture the complex physics of this quasi-direct bandgap material system, are used to evaluate the thermoelectric figure of merit ZT for n- and p-type GeSn epitaxial layers. The results highlight the high potential of single-crystal GeSn alloys to achieve similar energy harvest capability as already present in SiGe alloys but in the 20 °C–100 °C temperature range where Si-compatible semiconductors are not available. This opens the possibility of monolithically integrated thermoelectric on the CMOS platform.
(4) Thermal Expansion and Temperature Dependence of Raman Modes in Epitaxial Layers of Ge and Ge1-xSnx
A.A. Corley-Wiciak, D. Ryzhak, M.H. Zoellner, C.L. Manganelli, O. Concepción, O. Skibitzki, D. Grützmacher, D.Buca, G. Capellini, D. Spirito
Physical Review Materials 8(2), 023801 (2024)
DOI: 10.1103/PhysRevMaterials.8.023801, (GeSn Laser II)
Temperature dependence of vibrational modes in semiconductors depends on lattice thermal expansion and anharmonic phonon-phonon scattering. Evaluating the two contributions from experimental data is not straightforward, especially for epitaxial layers that present mechanical deformation and anisotropic lattice expansion. In this work, a temperature-dependent Raman study in epitaxial Ge and layers is presented. A model is introduced for the Raman mode energy shift as a function of temperature, comprising thermal expansion of the strained lattice and anharmonic corrections. With support of x-ray diffraction, the model is calibrated on experimental data of epitaxial Ge grown on Si and grown on Ge/Si, finding that the main difference between bulk and epitaxial layers is related to the anisotropic lattice expansion. The phonon anharmonicity and other parameters do not depend on dislocation defect density (in the range 7⋅106 - 4⋅108 cm^-2) nor on alloy composition in the range 5-14 at.%. The strain-shift coefficient for the main model of Ge and for the Ge-Ge vibrational mode of is weakly dependent on temperature and is around -500 . In , the composition-shift coefficient amounts to -100 , independent of temperature and strain.
(5) Deposition of Polymers on Titanium Nitride Electrodes
Y. Efremenko, A. Laroussi, A. Sengül, A.A. Corley-Wiciak, I.A. Fischer, V.M. Mirsky
Coatings (MDPI) 14(2), 215 (2024)
DOI: 10.3390/coatings14020215, (iCampus II)
An application of titanium nitride (TiN) as an electrode for electrochemical deposition or characterization requires the removing of an insulating layer from its surface. This process was studied and optimized, the conditions for complete removing of this layer by treatment with oxalic acid were formulated. The obtained TiN surfaces were used for deposition of various conducting and non-conducting polymers. Two different approaches were applied: (i) in-situ electrochemical synthesis of the main classes of conducting polymers including polyaniline, polypyrrole, polythiophene and few of their derivates, (ii) electrostatically driven Layer-by-Layer (LbL) deposition of multilayers of oppositely charged polyelectrolytes. The deposited polymers were characterized by electrochemical methods. Electrochemical properties of deposited conducting polymers and their deposition to the TiN surface were comparable to that to the metallic electrodes. The LbL deposited polymer films demonstrated strong influence of the charge of the last deposited polymer on the redox reaction of ferri/ferrocyanide thus confirming charge alteration with each deposited polymer layer. The studied deposition technologies can be used for surface modification of TiN surface required in the applications of this material in chemical sensors and other devices.
(6) Improving Epitaxial Growth of γ-Al2O3 Films via Sc2O3/Y2O3 Oxide Buffers
S. Gougam, M.A. Schubert, D. Stolarek, S.B. Thapa, M.H. Zoellner
Advanced Materials Interfaces 221(12), 2400228 (2024)
DOI: 10.1002/pssa.202400228, (GaN HEMT support)
Heteroepitaxial growth of γ-Al2O3 on Sc2O3/Y2O3/Si (111) is achieved with oxygen plasma-assisted molecular beam epitaxy in order to prevent polycrystalline grain boundary formation caused by lattice mismatch. Substrate temperature as well as oxygen flow are adjusted to optimize epitaxial growth conditions around 715–760 °C and 1.9 sccm, respectively. Epitaxial growth is monitored in situ by reflection high-energy diffraction, while surface morphology is studied by scanning electron microscopy ex-situ. X-ray diffraction indicates epitaxial out-of-plane 111 orientation with oxygen flow above 0.6 sccm. However, transmission electron microscopy shows stacking fault formation for high oxygen flows. Finally, nanobeam electron diffraction confirms Smrčok model of a spinel-like γ-Al2O3 crystal structure.
(7) Three-Dimensional Reconstruction of Interface Roughness and Alloy Disorder in Ge/GeSi Asymmetric Coupled Quantum Wells using Electron Tomography
E. Paysen, G. Capellini, E. Talamas Simola, L. Di Gaspare, M. De Seta, M. Virgilio, A. Trampert
ACS Applied Materials & Interfaces 16(3), 4189 (2024)
DOI: 10.1021/acsami.3c15546, (FLASH)
Interfaces play an essential role in the performance of ever-shrinking semiconductor devices, making comprehensive determination of their three-dimensional (3D) structural properties increasingly important. This becomes even more relevant in compositional interfaces, as is the case for Ge/GeSi heterostructures, where chemical intermixing is pronounced in addition to their morphology. We use the electron tomography method to reconstruct buried interfaces and layers of asymmetric coupled Ge/Ge0.8Si0.2 multiquantum wells, which are considered a potential building block in THz quantum cascade lasers. The three-dimensional reconstruction is based on a series of high-angle annular dark-field scanning transmission electron microscopy images. It allows chemically sensitive investigation of a relatively large interfacial area of about (80 × 80) nm2 with subnanometer resolution as well as the analysis of several interfaces within the multiquantum well stack. Representing the interfaces as iso-concentration surfaces in the tomogram and converting them to topographic height maps allows the determination of their morphological roughness as well as layer thicknesses, reflecting low variations in either case. Simulation of the reconstructed tomogram intensities using a sigmoidal function provides in-plane-resolved maps of the chemical interface widths showing a relatively large spatial variation. The more detailed analysis of the intermixed region using thin slices from the reconstruction and additional iso-concentration surfaces provides an accurate picture of the chemical disorder of the alloy at the interface. Our comprehensive three-dimensional image of Ge/Ge0.8Si0.2 interfaces reveals that in the case of morphologically very smooth interfaces─depending on the scale considered─the interface alloy disorder itself determines the overall characteristics, a result that is fundamental for highly miscible material systems.
(8) On-Chip Refractive Index Sensors Based on Plasmonic TiN Nanohole Arrays
S. Reiter, A. Sengül, Ch. Mai, D. Spirito, Ch. Wenger, I.A. Fischer
Proc. IEEE Silicon Photonics Conference (SiPhotonics 2024), TuP10 (2024)
DOI: 10.1109/SiPhotonics60897.2024.10544048, (iCampus II)
(9) Lattice Dynamics in Chiral Tellurium by Linear and Circularly Polarized Raman Spectroscopy: Crystal Orientation and Handedness
D. Spirito, S. Marras, B. Martin-Garcia
Journal of Materials Chemistry C: Materials for Optical and Electronic Devices 12(7), 2544 (2024)
DOI: 10.1039/D3TC04333A
Trigonal tellurium (Te) has attracted researchers’ attention due to its transport and optical properties, which include electrical magneto-chiral anisotropy, spin polarization and bulk photovoltaic effect. It is the anisotropic and chiral crystal structure of Te that drive these properties, so the determination of its crystallographic orientation and handedness is key to their study. Here we explore the structural dynamics of Te bulk crystals by angle-dependent linearly polarized Raman spectroscopy and symmetry rules in three different crystallographic orientations. The angle-dependent intensity of the modes allows us to determine the arrangement of the helical chains and distinguish between crystallographic planes parallel and perpendicular to the chain axis. Furthermore, under different configurations of circularly polarized Raman measurements and crystal orientations, we observe the shift of two phonon modes only in the (0 0 1) plane. The shift is positive or negative depending on the handedness of the crystals, which we determine univocally by chemical etching. Our analysis of three different crystal faces of Te highlights the importance of selecting the proper orientation and crystallographic plane when investigating the transport and optical properties of this material. These results offer insight into the crystal structure and symmetry in other anisotropic and chiral materials, and open new paths to select a suitable crystal orientation when fabricating devices.
(10) Asymmetric-Coupled Ge/SiGe Quantum Wells for Second Harmonic Generation at 7.1 THz in Integrated Waveguides: A Theoretical Study
E. Talamas Simola, M. Ortolani, L. Di Gaspare, G. Capellini, M. De Seta, M. Virgilio
Nanophotonics 13(10), 1781 (2024)
DOI: 10.1515/nanoph-2023-0697, (FLASH)
We present a theoretical investigation of guided second harmonic generation at THz frequencies in SiGe waveguides embedding n-type Ge/SiGe asymmetric coupled quantum wells to engineer a giant second order nonlinear susceptibility. A characteristic of the chosen material system is the existence of large off-diagonal elements in the χ2 tensor, coupling optical modes with different polarization. To account for this effect, we generalize the coupled-mode theory, proposing a theoretical model suitable for concurrently resolving every second harmonic generation interaction among guide-sustained modes, regardless of which χ2 tensor elements it originates from. Furthermore, we exploit the presence of off-diagonal χ2 elements and the peculiarity of the SiGe material system to develop a simple and novel approach to achieve perfect phase matching without requiring any fabrication process. For a realistic design of the quantum heterostructure we estimate second order nonlinear susceptibility peak values of ∼7 and ∼1.4 × 105 pm/V for diagonal and off diagonal χ2 elements, respectively. Embedding such heterostructure in Ge-rich SiGe waveguides of thicknesses of the order of 10–15 μm leads to second harmonic generation efficiencies comprised between 0.2 and 2 %, depending on the choice of device parameters. As a case study, we focus on the technologically relevant frequency of 7.1 THz, yet the results we report may be extended to the whole 5–20 THz range.
(11) The Interplay between Strain, Sn Content, and Temperature on Spatially-Dependent Bandgap in Ge1-xSnx Microdisks
I. Zaitsev, A.A. Corley-Wiciak, C. Corley-Wiciak, M.H. Zoellner, C. Richter, E. Zatterin, M. Virgilio, Beatriz Martín-García, D. Spirito, C.L. Manganelli
Physica Status Solidi - Rapid Research Letters 18(3), 2300348 (2024)
DOI: 10.1002/pssr.202300348
Germanium-tin microdisks are promising structures for CMOS-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 the different lattice deformation associated with the mechanical constraints in the microstructures. We report an experimental and numerical study of Ge1-xSnx microdisk with Sn concentration between 8.5 and 14 at.%. Combining finite element method calculations, micro-Raman spectroscopy 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-photo-luminescence experiments, the temperature dependence of the band gap of Ge1-xSnx is parametrized using the Varshni formula with respect to strain and Sn content. These results are the input for a spatially-dependent band structure calculation based on the deformation potential theory. We observe 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. We also find that the strain gradient impacts the band structure in the whole volume of the microdisk. These findings correlate structural properties to the emission wavelength and spectral width of Ge1-xSnx microdisk lasers, thus demonstrating the importance of material-related consideration on the design of optoelectronic microstructures.
F. Berkmann, T. Venanz, L. Baldassarre, E. Campagna, E. Talamas-Simola, L. Di Gaspare, C. Corley-Wiciak, G.Capellini,G. Nicotra, G. Sfuncia, A. Notargiacomo, E. Giovine, S. Cibella, M. Virgilio, G. Scalari, M. De Seta, M. Ortolani
ACS Photonics 11(7), 2776 (2024)
DOI: 10.1021/acsphotonics.4c00641, (IHP- Roma Tre University Joint Lab)
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.
(2) High Quality CMOS Ccompatible N-Type SiGe Parabolic Quantum Wells for Intersubband Photonics at 2.5-5 THz
E. Campagna, E. Talamas Simola, T. Venanzi, F. Berkmann, C. Corley-Wiciack, G. Nicotra, L. Baldassarre, G. Capellini, L. Di Gaspare, M. Virgilio, M. Ortolani, M. De Seta
Nanophotonics 13(10), 1793 (2024)
DOI: 10.1515/nanoph-2023-0704, (FLASH)
A parabolic potential that confines charge carriers along the growth direction of quantum wells semiconductor systems is characterized by a single resonance frequency, associated to intersubband transitions. Motivated by fascinating quantum optics applications leveraging on this property, we use the technologically relevant SiGe material system to design, grow, and characterize n-type doped parabolic quantum wells realized by continuously grading Ge-rich Si1−x Ge x alloys, deposited on silicon wafers. An extensive structural analysis highlights the capability of the ultra-high-vacuum chemical vapor deposition technique here used to precisely control the quadratic confining potential and the target doping profile. The absorption spectrum, measured by means of Fourier transform infrared spectroscopy, revealed a single peak with a full width at half maximum at low and room temperature of about 2 and 5 meV, respectively, associated to degenerate intersubband transitions. The energy of the absorption resonance scales with the inverse of the well width, covering the 2.5–5 THz spectral range, and is almost independent of temperature and doping, as predicted for a parabolic confining potential. On the basis of these results, we discuss the perspective observation of THz strong light–matter coupling in this silicon compatible material system, leveraging on intersubband transitions embedded in all-semiconductor microcavities.
(3) Room Temperature Lattice Thermal Conductivity of GeSn Alloys
O. Concepción, J. Tiscareño-Ramírez, A.A. Chimienti, T. Classen, A.A. Corley-Wiciak, A. Tomadin, D. Spirito, D. Pisignano, P. Graziosi, Z. Ikonic, Q.T. Zhao, D. Grützmacher, G. Capellini, S. Roddaro, M. Virgilio, D. Buca
ACS Applied Energy Materials 7(10), 4394 (2024)
DOI: 10.1021/acsaem.4c00275, (SiGeSn TE)
CMOS-compatible materials for efficient energy harvesters at temperatures characteristic for on-chip operation and body temperature are the key ingredients for sustainable green computing and ultralow power Internet of Things applications. In this context, the lattice thermal conductivity (κ) of new group IV semiconductors, namely Ge1–xSnx alloys, are investigated. Layers featuring Sn contents up to 14 at.% are epitaxially grown by state-of-the-art chemical-vapor deposition on Ge buffered Si wafers. An abrupt decrease of the lattice thermal conductivity (κ) from 55 W/(m·K) for Ge to 4 W/(m·K) for Ge0.88Sn0.12 alloys is measured electrically by the differential 3ω-method. The thermal conductivity was verified to be independent of the layer thickness for strained relaxed alloys and confirms the Sn dependence observed by optical methods previously. The experimental κ values in conjunction with numerical estimations of the charge transport properties, able to capture the complex physics of this quasi-direct bandgap material system, are used to evaluate the thermoelectric figure of merit ZT for n- and p-type GeSn epitaxial layers. The results highlight the high potential of single-crystal GeSn alloys to achieve similar energy harvest capability as already present in SiGe alloys but in the 20 °C–100 °C temperature range where Si-compatible semiconductors are not available. This opens the possibility of monolithically integrated thermoelectric on the CMOS platform.
(4) Thermal Expansion and Temperature Dependence of Raman Modes in Epitaxial Layers of Ge and Ge1-xSnx
A.A. Corley-Wiciak, D. Ryzhak, M.H. Zoellner, C.L. Manganelli, O. Concepción, O. Skibitzki, D. Grützmacher, D.Buca, G. Capellini, D. Spirito
Physical Review Materials 8(2), 023801 (2024)
DOI: 10.1103/PhysRevMaterials.8.023801, (GeSn Laser II)
Temperature dependence of vibrational modes in semiconductors depends on lattice thermal expansion and anharmonic phonon-phonon scattering. Evaluating the two contributions from experimental data is not straightforward, especially for epitaxial layers that present mechanical deformation and anisotropic lattice expansion. In this work, a temperature-dependent Raman study in epitaxial Ge and layers is presented. A model is introduced for the Raman mode energy shift as a function of temperature, comprising thermal expansion of the strained lattice and anharmonic corrections. With support of x-ray diffraction, the model is calibrated on experimental data of epitaxial Ge grown on Si and grown on Ge/Si, finding that the main difference between bulk and epitaxial layers is related to the anisotropic lattice expansion. The phonon anharmonicity and other parameters do not depend on dislocation defect density (in the range 7⋅106 - 4⋅108 cm^-2) nor on alloy composition in the range 5-14 at.%. The strain-shift coefficient for the main model of Ge and for the Ge-Ge vibrational mode of is weakly dependent on temperature and is around -500 . In , the composition-shift coefficient amounts to -100 , independent of temperature and strain.
(5) Deposition of Polymers on Titanium Nitride Electrodes
Y. Efremenko, A. Laroussi, A. Sengül, A.A. Corley-Wiciak, I.A. Fischer, V.M. Mirsky
Coatings (MDPI) 14(2), 215 (2024)
DOI: 10.3390/coatings14020215, (iCampus II)
An application of titanium nitride (TiN) as an electrode for electrochemical deposition or characterization requires the removing of an insulating layer from its surface. This process was studied and optimized, the conditions for complete removing of this layer by treatment with oxalic acid were formulated. The obtained TiN surfaces were used for deposition of various conducting and non-conducting polymers. Two different approaches were applied: (i) in-situ electrochemical synthesis of the main classes of conducting polymers including polyaniline, polypyrrole, polythiophene and few of their derivates, (ii) electrostatically driven Layer-by-Layer (LbL) deposition of multilayers of oppositely charged polyelectrolytes. The deposited polymers were characterized by electrochemical methods. Electrochemical properties of deposited conducting polymers and their deposition to the TiN surface were comparable to that to the metallic electrodes. The LbL deposited polymer films demonstrated strong influence of the charge of the last deposited polymer on the redox reaction of ferri/ferrocyanide thus confirming charge alteration with each deposited polymer layer. The studied deposition technologies can be used for surface modification of TiN surface required in the applications of this material in chemical sensors and other devices.
(6) Improving Epitaxial Growth of γ-Al2O3 Films via Sc2O3/Y2O3 Oxide Buffers
S. Gougam, M.A. Schubert, D. Stolarek, S.B. Thapa, M.H. Zoellner
Advanced Materials Interfaces 221(12), 2400228 (2024)
DOI: 10.1002/pssa.202400228, (GaN HEMT support)
Heteroepitaxial growth of γ-Al2O3 on Sc2O3/Y2O3/Si (111) is achieved with oxygen plasma-assisted molecular beam epitaxy in order to prevent polycrystalline grain boundary formation caused by lattice mismatch. Substrate temperature as well as oxygen flow are adjusted to optimize epitaxial growth conditions around 715–760 °C and 1.9 sccm, respectively. Epitaxial growth is monitored in situ by reflection high-energy diffraction, while surface morphology is studied by scanning electron microscopy ex-situ. X-ray diffraction indicates epitaxial out-of-plane 111 orientation with oxygen flow above 0.6 sccm. However, transmission electron microscopy shows stacking fault formation for high oxygen flows. Finally, nanobeam electron diffraction confirms Smrčok model of a spinel-like γ-Al2O3 crystal structure.
(7) Three-Dimensional Reconstruction of Interface Roughness and Alloy Disorder in Ge/GeSi Asymmetric Coupled Quantum Wells using Electron Tomography
E. Paysen, G. Capellini, E. Talamas Simola, L. Di Gaspare, M. De Seta, M. Virgilio, A. Trampert
ACS Applied Materials & Interfaces 16(3), 4189 (2024)
DOI: 10.1021/acsami.3c15546, (FLASH)
Interfaces play an essential role in the performance of ever-shrinking semiconductor devices, making comprehensive determination of their three-dimensional (3D) structural properties increasingly important. This becomes even more relevant in compositional interfaces, as is the case for Ge/GeSi heterostructures, where chemical intermixing is pronounced in addition to their morphology. We use the electron tomography method to reconstruct buried interfaces and layers of asymmetric coupled Ge/Ge0.8Si0.2 multiquantum wells, which are considered a potential building block in THz quantum cascade lasers. The three-dimensional reconstruction is based on a series of high-angle annular dark-field scanning transmission electron microscopy images. It allows chemically sensitive investigation of a relatively large interfacial area of about (80 × 80) nm2 with subnanometer resolution as well as the analysis of several interfaces within the multiquantum well stack. Representing the interfaces as iso-concentration surfaces in the tomogram and converting them to topographic height maps allows the determination of their morphological roughness as well as layer thicknesses, reflecting low variations in either case. Simulation of the reconstructed tomogram intensities using a sigmoidal function provides in-plane-resolved maps of the chemical interface widths showing a relatively large spatial variation. The more detailed analysis of the intermixed region using thin slices from the reconstruction and additional iso-concentration surfaces provides an accurate picture of the chemical disorder of the alloy at the interface. Our comprehensive three-dimensional image of Ge/Ge0.8Si0.2 interfaces reveals that in the case of morphologically very smooth interfaces─depending on the scale considered─the interface alloy disorder itself determines the overall characteristics, a result that is fundamental for highly miscible material systems.
(8) On-Chip Refractive Index Sensors Based on Plasmonic TiN Nanohole Arrays
S. Reiter, A. Sengül, Ch. Mai, D. Spirito, Ch. Wenger, I.A. Fischer
Proc. IEEE Silicon Photonics Conference (SiPhotonics 2024), TuP10 (2024)
DOI: 10.1109/SiPhotonics60897.2024.10544048, (iCampus II)
(9) Lattice Dynamics in Chiral Tellurium by Linear and Circularly Polarized Raman Spectroscopy: Crystal Orientation and Handedness
D. Spirito, S. Marras, B. Martin-Garcia
Journal of Materials Chemistry C: Materials for Optical and Electronic Devices 12(7), 2544 (2024)
DOI: 10.1039/D3TC04333A
Trigonal tellurium (Te) has attracted researchers’ attention due to its transport and optical properties, which include electrical magneto-chiral anisotropy, spin polarization and bulk photovoltaic effect. It is the anisotropic and chiral crystal structure of Te that drive these properties, so the determination of its crystallographic orientation and handedness is key to their study. Here we explore the structural dynamics of Te bulk crystals by angle-dependent linearly polarized Raman spectroscopy and symmetry rules in three different crystallographic orientations. The angle-dependent intensity of the modes allows us to determine the arrangement of the helical chains and distinguish between crystallographic planes parallel and perpendicular to the chain axis. Furthermore, under different configurations of circularly polarized Raman measurements and crystal orientations, we observe the shift of two phonon modes only in the (0 0 1) plane. The shift is positive or negative depending on the handedness of the crystals, which we determine univocally by chemical etching. Our analysis of three different crystal faces of Te highlights the importance of selecting the proper orientation and crystallographic plane when investigating the transport and optical properties of this material. These results offer insight into the crystal structure and symmetry in other anisotropic and chiral materials, and open new paths to select a suitable crystal orientation when fabricating devices.
(10) Asymmetric-Coupled Ge/SiGe Quantum Wells for Second Harmonic Generation at 7.1 THz in Integrated Waveguides: A Theoretical Study
E. Talamas Simola, M. Ortolani, L. Di Gaspare, G. Capellini, M. De Seta, M. Virgilio
Nanophotonics 13(10), 1781 (2024)
DOI: 10.1515/nanoph-2023-0697, (FLASH)
We present a theoretical investigation of guided second harmonic generation at THz frequencies in SiGe waveguides embedding n-type Ge/SiGe asymmetric coupled quantum wells to engineer a giant second order nonlinear susceptibility. A characteristic of the chosen material system is the existence of large off-diagonal elements in the χ2 tensor, coupling optical modes with different polarization. To account for this effect, we generalize the coupled-mode theory, proposing a theoretical model suitable for concurrently resolving every second harmonic generation interaction among guide-sustained modes, regardless of which χ2 tensor elements it originates from. Furthermore, we exploit the presence of off-diagonal χ2 elements and the peculiarity of the SiGe material system to develop a simple and novel approach to achieve perfect phase matching without requiring any fabrication process. For a realistic design of the quantum heterostructure we estimate second order nonlinear susceptibility peak values of ∼7 and ∼1.4 × 105 pm/V for diagonal and off diagonal χ2 elements, respectively. Embedding such heterostructure in Ge-rich SiGe waveguides of thicknesses of the order of 10–15 μm leads to second harmonic generation efficiencies comprised between 0.2 and 2 %, depending on the choice of device parameters. As a case study, we focus on the technologically relevant frequency of 7.1 THz, yet the results we report may be extended to the whole 5–20 THz range.
(11) The Interplay between Strain, Sn Content, and Temperature on Spatially-Dependent Bandgap in Ge1-xSnx Microdisks
I. Zaitsev, A.A. Corley-Wiciak, C. Corley-Wiciak, M.H. Zoellner, C. Richter, E. Zatterin, M. Virgilio, Beatriz Martín-García, D. Spirito, C.L. Manganelli
Physica Status Solidi - Rapid Research Letters 18(3), 2300348 (2024)
DOI: 10.1002/pssr.202300348
Germanium-tin microdisks are promising structures for CMOS-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 the different lattice deformation associated with the mechanical constraints in the microstructures. We report an experimental and numerical study of Ge1-xSnx microdisk with Sn concentration between 8.5 and 14 at.%. Combining finite element method calculations, micro-Raman spectroscopy 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-photo-luminescence experiments, the temperature dependence of the band gap of Ge1-xSnx is parametrized using the Varshni formula with respect to strain and Sn content. These results are the input for a spatially-dependent band structure calculation based on the deformation potential theory. We observe 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. We also find that the strain gradient impacts the band structure in the whole volume of the microdisk. These findings correlate structural properties to the emission wavelength and spectral width of Ge1-xSnx microdisk lasers, thus demonstrating the importance of material-related consideration on the design of optoelectronic microstructures.
