Publikationen 2024

Script list Publications

(1) 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 (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.

(2) SWCNT-Si Photodetector with Voltage-Dependent Active Surface
D. Capista, L. Lozzi, A. Di Bartolomeo, F. Giubileo, N. Martucciello, M. Passacantando
Nano Express 5(1), 015004 (2024)
DOI: 10.1088/2632-959X/ad12d9
New works on Carbon Nanotubes-Silicon MIS heterostructures showed that the presence of thickness inhomogeneities in the insulating layer across the device can be exploited increase its functionalities. In this work, we report the fabrication and the characterization of a device consisting of a Single-Walled Carbon Nanotube (SWCNT) film onto an n-type silicon substrate where the nitride interlayer between the nanotubes and the silicon has been intentionally etched to obtain different thickness. Three different silicon nitride thicknesses allow the formation of three regions, inside the same device, each with different photocurrents and responsivity behaviors. We show that by selecting specific biases, the photoresponse of the regions can be switched on and off. This peculiar behavior allows the device to be used as a photodetector with a voltage dependent active surface. Scanning photo response imaging of the device surface, performed at different biases highlight this behavior.

(3) Influence of En-APTAS Membrane on NO Gas Selectivity of HfO2-based Memristor Gas Sensors
M. Chae, D. Lee, H.-D. Kim
Japanese Journal of Applied Physics 63(3), 03SP07 (2024)
DOI: 10.35848/1347-4065/ad202d
Memristor-based gas sensors (gas sensor + memristor, gasistor) have gained popularity due to their high response characteristics and ability to operate at RT. In this paper, N-[3-(Trimethoxysilyl)propyl]ethylenediamine (en-APTAS), a commonly used membrane for NOx gas sensors, is applied in the gasistor with carbon nanotubes (CNTs)-top electrode (TE). As a result, we have demonstrated the response time was reduced by 104 s, and the response to 10 ppm Nitric oxide (NO) gas increased to 3.69, indicating an enhanced sensing property in a range of 10–50 ppm. Furthermore, when decorated with the proposed en-APTAS, the gasistor with CNTs-TE demonstrated a 3.76-fold increase in response to NO gas compared to NO2 gas, demonstrating remarkable selectivity. These improved features are attributed to the high adsorption energy of en-APTAS and the large kinetic diameter of NO2. The research proposal will be a foundational stage towards attaining selectivity in other gasistor studies.

(4) Low-Power Consumption IGZO Memristor-Based Gas Sensor Embedded in an Internet of Things Monitoring System for Isopropanol Alcohol Gas
M. Chae, D. Lee, H.-D. Kim
Micromachines 15(1), 77 (2024)
DOI: 10.3390/mi15010077
Low-power-consumption gas sensors are crucial for diverse applications, including environmental monitoring and portable Internet of Things (IoT) systems. However, the desorption and adsorption characteristics of conventional metal oxide-based gas sensors require supplementary equipment, such as heaters, which is not optimal for low-power IoT monitoring systems. Memristor-based sensors (gasistors) have been investigated as innovative gas sensors owing to their advantages, including high response, low power consumption, and room-temperature (RT) operation. Based on IGZO, the proposed isopropanol alcohol (IPA) gas sensor demonstrates a detection speed of 105 s and a high response of 55.15 for 50 ppm of IPA gas at RT. Moreover, rapid recovery to the initial state was achievable in 50 μs using pulsed voltage and without gas purging. Finally, a low-power circuit module was integrated for wireless signal transmission and processing to ensure IoT compatibility. The stability of sensing results from gasistors based on IGZO has been demonstrated, even when integrated into IoT systems. This enables energy-efficient gas analysis and real-time monitoring at ~0.34 mW, supporting recovery via pulse bias. This research offers practical insights into IoT gas detection, presenting a wireless sensing system for sensitive, low-powered sensors.

(5) 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⋅10cm^-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.

(6) 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.

(7) Selective Epitaxy of GaP Islands Grown on CMOS Compatible Si Nanotips Wafer by Gas Source Molecular Beam Epitaxy
N. Kafi, S. Kang, C. Golz, A. Rodrigues-Weisensee, L. Persichetti, D. Ryzhak, G. Capellini, D. Spirito, M. Schmidbauer, A. Kwasniewski, C. Netzel, O. Skibitzki, F. Hatami
Crystal Growth & Design 24(7), 2724 (2024)
DOI: 10.1021/acs.cgd.3c01337, (NHEQuanLEA)
Gallium phosphide (GaP) is a III–V semiconductor with remarkable optoelectronic properties, and it has almost the same lattice constant as silicon (Si). However, to date, the monolithic and large-scale integration of GaP devices with silicon remains challenging. In this study, we present a nanoheteroepitaxy approach using gas-source molecular-beam epitaxy for selective growth of GaP islands on Si nanotips, which were fabricated using complementary metal–oxide semiconductor (CMOS) technology on a 200 mm n-type Si(001) wafer. Our results show that GaP islands with sizes on the order of hundreds of nanometers can be successfully grown on CMOS-compatible wafers. These islands exhibit a zinc-blende phase and possess optoelectronic properties similar to those of a high-quality epitaxial GaP layer. This result marks a notable advancement in the seamless integration of GaP-based devices with high scalability into Si nanotechnology and integrated optoelectronics.

(8) Enhanced Optical and Electrical Properties of Indium Tin Oxide for Solar Cell Applications via Post-Microwave Treatment
T. Kim, M. Chae, D. Lee, H.-D. Kim
Optical Materials 149, 115093 (2024)
DOI: 10.1016/j.optmat.2024.115093
The method to enhance current density in silicon heterojunction (SHJ) solar cells involves increasing surface roughness through higher temperature during indium thin oxide (ITO) deposition. Although cost-effective, the optoelectrical properties require further enhancement for solar cell application. Thus, post-heat treatment is necessary to improve their optoelectrical properties. Microwave treatment (MWT) emerges as a promising way to selectively generate heat in ITO briefly. Therefore, we propose a MWT process to enhance optoelectrical properties. After depositing ITO at 500 °C and then MWT, the average transmittance is over 98.7% in the visible range, with a sheet resistance of 81 Ω/□. Finally, the weighted reflectance was calculated using measured reflectance data to assess the applicability in solar cell applications. As a result, the calculation shows that a weighted reflectance value of 0.6% after MWT. These results indicate that MWT effectively enhances optoelectrical properties of ITO, which are essential for the development of solar cell applications.

(9) Response Characteristic in Discontinuous NO Gas Flows for Boron Nitride Memristor Gas Sensor Devices
D. Lee, M. Chae, H.-D. Kim
Sensors and Actuators B: Chemical 401, 135063 (2024)
DOI: 10.1016/j.snb.2023.135063
Most NO gas sensors are evaluated using continuous NO gas, making it difficult to accurately recognize discontinuous gas flow. Here, to reveal the response characteristics in discontinuous gas flows, we investigated a response in various NO gas flows using a boron nitride-based memristor gas sensor. In conventional continuous gas flow, the response characteristic of 16% showed for 5 ppm NO gas, while in the pulse like gas injection with a width of 1 second and an interval of 1 second, the response only increased to 8.13%, meaning that it is difficult to estimate the overall environment of NO gas using only continuous gases, as well as showing that a host of data is needed for discontinuous gases. As a result, we found that a neural network model trained by continuous/discontinuous NO gas data accurately predicts the concentration of discontinuous NO gas with a low error of 5.6%.

(10) Blooming and Pruning: Learning from Mistakes with Memristive Synapses
K. Nikiruy, E. Perez, A. Baroni, K.D.S. Reddy, S. Pechmann, Ch. Wenger, M. Ziegler
Scientific Reports 14, 7802 (2024)
DOI: 10.1038/s41598-024-57660-4, (KI-IoT)
Blooming and pruning is one of the most important developmental mechanisms of the biological brain in the first years of life, enabling it to adapt its network structure to the demands of the environment. The mechanism is thought to be fundamental for the development of cognitive skills. Inspired by this, Chialvo and Bak proposed in 1999 a learning scheme that learns from mistakes by eliminating from the initial surplus of synaptic connections those that lead to an undesirable outcome. Here, this idea is implemented in a neuromorphic circuit scheme using CMOS integrated HfO2-based memristive devices. The implemented two-layer neural network learns in a self-organized manner without positive reinforcement and exploits the inherent variability of the memristive devices. A combined experimental and simulation-based parameter study is presented to find the relevant system and device parameters leading to a compact and robust memristive neuromorphic circuit that can handle association tasks.

(11) 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.

(12) A Current Mirror Based Read Circuit Design with Multi-Level Capability for Resistive Switching Devices
S. Pechmann, E. Perez, Ch. Wenger, A. Hagelauer
Proc. International Conference on Electronics, Information, and Communication (ICEIC 2024), (2024)
DOI: 10.1109/ICEIC61013.2024.10457188, (KI-IoT)

(13) Thermal Compact Modeling and Resistive Switching Analysis in Titanium Oxide-Based Memristors
J.B. Roldán, A. Cantudo, D. Maldonado, C. Aguilera-Pedregosa, E. Moreno, T. Swoboda, F. Jimenez-Molinos, Y. Yuan, K. Zhu, M. Lanza, M.M. Rojo
ACS Applied Electronic Materials 6(2), 1424 (2024)
DOI: 10.1021/acsaelm.3c01727, (KI-IoT)
Resistive switching devices based on the Au/Ti/TiO2/Au stack were developed. In addition to standard electrical characterization by means of I–V curves, scanning thermal microscopy was employed to localize the hot spots on the top device surface (linked to conductive nanofilaments, CNFs) and perform in-operando tracking of temperature in such spots. In this way, electrical and thermal responses can be simultaneously recorded and related to each other. In a complementary way, a model for device simulation (based on COMSOL Multiphysics) was implemented in order to link the measured temperature to simulated device temperature maps. The data obtained were employed to calculate the thermal resistance to be used in compact models, such as the Stanford model, for circuit simulation. The thermal resistance extraction technique presented in this work is based on electrical and thermal measurements instead of being indirectly supported by a single fitting of the electrical response (using just I–V curves), as usual. Besides, the set and reset voltages were calculated from the complete I–V curve resistive switching series through different automatic numerical methods to assess the device variability. The series resistance was also obtained from experimental measurements, whose value is also incorporated into a compact model enhanced version.

(14) Stochastic Resonance in 2D Materials Based Memristors
J.B. Roldán, A. Cantudo, J.J. Torres, D. Maldonado, Y. Shen, W. Zheng, Y. Yuan, M. Lanza
Nature Nanotechnology 8, 7 (2024)
DOI: 10.1038/s41699-024-00444-1, (KI-IoT)
Stochastic resonance is an essential phenomenon in neurobiology, it is connected to the constructive role of noise in the signals that take place in neuronal tissues, facilitating information communication, memory, etc. Memristive devices are known to be the cornerstone of hardware neuromorphic applications since they correctly mimic biological synapses in many different facets, such as short/long-term plasticity, spike-timing-dependent plasticity, pair-pulse facilitation, etc. Different types of neural networks can be built with circuit architectures based on memristive devices (mostly spiking neural networks and artificial neural networks). In this context, stochastic resonance is a critical issue to analyze in the memristive devices that will allow the fabrication of neuromorphic circuits. We do so here with h-BN based memristive devices from different perspectives. It is found that the devices we have fabricated and measured clearly show stochastic resonance behaviour. Consequently, neuromorphic applications can be developed to account for this effect, that describes a key issue in neurobiology with strong computational implications.

(15) Selective Epitaxy of Germanium on Silicon for the Fabrication of CMOS Compatible Short-Wavelength Infrared Photodetectors
D. Ryzhak, A.A. Corley-Wiciak, P. Steglich, Y. Yamamoto, J. Frigerio, R. Giani, A. De Iacovo, D. Spirito, G. Capellini
Materials Science in Semiconductor Processing 176, 108308 (2024)
DOI: 10.1016/j.mssp.2024.108308, (VISIR2)
Here we present the selective epitaxial growth of Ge on Si using reduced pressure chemical vapor deposition on SiO2/Si solid masks realized on 200 mm Si wafers, aiming at manufacturing integrated visible/short-wavelength infrared photodetectors. By a suitable choice of the reactants and of the process conditions, we demonstrated highly selective and pattern-independent growth of Ge microstructure featuring high crystalline quality. The Ge “patches” show a distinct faceting, with a flat top (001) facet and low energy facets such as e.g. {113} and {103} at their sidewalls, independently on their size. Interdiffusion of Si in to the Ge microcrystals is limited to an extension of ∼20 nm from the heterointerface. The Ge patches resulted to be plastically relaxed with threading dislocation density values better or on par than those observed in continuous two-dimensional Ge/Si epilayer in the low 107 cm−2 range. A residual tensile strain was observed for patches with size >10 μm, due to elastic thermal strain accumulation, as confirmed by μ-Raman spectroscopy and μ-photoluminescence characterization. Polarization-dependent Raman mapping highlights the strain distribution associated to the tridimensional shape. On this material, Ge photodiodes were fabricated and characterized, showing promising optoelectronic performances.

(16) 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.

(17) High Gain Graphene Based Hot Electron Transistor with Record High Saturated Output Current Density
C. Strobel, C.A. Chavarin, M. Knaut, S. Völkel, M. Albert, A. Hiess, B. Max, Ch. Wenger, R. Kirchner, T. Mikolajick
Advanced Electronic Materials 10(2), 2300624 (2024)
DOI: 10.1002/aelm.202300624, (FFLEXCOM (D020))
Hot electron transistors (HETs) represent an exciting frontier in semiconductor technology, holding the promise of high-speed and high-frequency electronics. With the exploration of two-dimensional materials such as graphene and new device architectures, HETs are poised to revolutionize the landscape of modern electronics. This study highlights a novel HET structure with a record output current density of 800 A/cm² and a high current gain α, fabricated using a scalable fabrication approach. The HET structure comprises two-dimensional hexagonal boron nitride (hBN) and graphene layers wet transferred to a germanium substrate. The combination of these materials results in exceptional performance, particularly in terms of the highly saturated output current density. The scalable fabrication scheme used to produce the HET opens up opportunities for large-scale manufacturing. This breakthrough in HET technology holds promise for advanced electronic applications, offering high current capabilities in a practical and manufacturable device.

(18) 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 (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.

(19) Thin and Locally Dislocation-Free SiGe Virtual Substrate Fabrication by Lateral Selective Growth
Y. Yamamoto, W.-C. Wen, M.A. Schubert, A.A. Corley-Wiciak, S. Sugawa, Y. Ito, R. Yokogawa, H. Han, R. Loo, A. Ogura, B. Tillack
Japanese Journal of Applied Physics 63(2), 02SP53 (2024)
DOI: 10.35848/1347-4065/ad189d
Locally dislocation-free SiGe-on-insulator (SGOI) is fabricated by chemical vapor deposition. Lateral selective SiGe growth of ~30%, ~45% and ~55% is performed around ~1 µm square Si(001) pillar located under the center of a 6.3 µm square SiO2 on Si-on-insulator substrate which is formed by H2-HCl vapor phase etching. The selective SiGe is deposited by H2-SiH2Cl2-GeH4-HCl. In the deposited SiGe layer, tensile strain is observed by top-view. The degree of strain is slightly increased at the corner of the SiGe. The tensile strain is caused by the partial compressive strain of SiGe in lateral direction and thermal expansion difference between Si and SiGe. Slightly higher Ge incorporation is observed in higher tensile strain region. At the peaks formed between the facets of growth front, Ge incorporation is reduced. These phenomena are pronounced for SiGe with higher Ge contents. Dislocation-free SGOI is formed along <010> from the Si pillar by lateral aspect-ratio-trapping.

(20) 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.

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