Non-Equilibrium SiGeSn Group IV Heterostructures and Nanowires for Integrated Mid-Infrared Photonics

Le développement des nouvelles générations de dispositifs électroniques devient de plus en plus limité par la chaleur générée par effet Joule dans les puces électroniques à haute densité. Des circuits photoniques intégrés sur silicium (Si) compatibles avec les procédés CMOS ont été proposés comme solution rentable pour réduire le réchauffement des dispositifs tout en améliorant leur performance globale. Cependant, les émetteurs à base de Si sont jusqu’à présent les composantes les plus difficiles à concevoir pour ces circuits photoniques intégrés. La principale raison est la bande interdite indirecte qui limite sévèrement l’efficacité de la luminescence du Si. Récemment l’incorporation de l’étain (Sn) dans des alliages silicium-germanium représente une nouvelle direction de recherche qui mènera à des semiconducteurs de groupe IV ayant une bande interdite directe. Les semiconducteurs obtenus Ge1-x-ySixSny sont des alliages ternaires du groupe IV compatibles avec la technologie CMOS, et peuvent avoir une bande interdite directe ajustable en fonction de la composition et de la contrainte. Ces propriétés ont généré un grand intérêt pour développer ces semiconducteurs et mieux comprendre leurs propriétés. Dans cette perspective, ce travail présente une étude détaillée de la structure de bande de l’alliage ternaire Ge1-x-ySixSny contraint et relaxé en utilisant une approche théorique fondée sur le modèle des liaisons fortes. Cette méthode est basée sur une évaluation précise des constantes de déformation de Ge, Si et α-Sn en utilisant une approche stochastique de Monte-Carlo ainsi qu'une méthode d'optimisation basée sur le gradient. De plus, une nouvelle approche d'évolution différentielle efficace est également développée pour reproduire avec précision les masses effectives expérimentales et les énergies de bandes interdites. Sur la base de ces approches, nous avons élucidé l'influence du désordre dans la structure crystalline, de la contrainte et de la composition sur l'énergie de bande interdite de Ge1-x-ySixSny. Quand 0 ≤x ≤0.4 et 0 ≤y ≤0.2, nous avons trouvé que la contrainte élastique réduit la concentration critique de Sn nécessaire pour obtenir un semiconducteur à bande interdite directe avec des énergies de bande interdite correspondantes inférieures à 0.76 eV. Cette limite supérieure diminue à 0.43 eV pour les alliages ternaires à bande interdite directe complètement relaxés. La transition obtenue vers la bande interdite directe en fonction de la composition est décrite par y> 0.605x + 0.077 et y> 1.364x + 0.107 respectivement pour les alliages contraints et complètement relaxés. Les effets de la contrainte, à une composition fixe, sur la transition de bande interdite indirecte à directe ont également été étudiés et discutés.----------Abstract Progress in electronic devices has been increasingly limited by the heat generated due to Joule effect in high density electronic chips. Silicon (Si) integrated photonic circuits compatible with CMOS processing has been proposed as a viable solution to reduce the heating of devices while improving their overall performance. However, Si-based emitters are, until now, the most difficult components to design for these integrated photonic circuits. The main reason is the indirect band gap which severely limits the efficiency of Si emission and absorption of light. Recently, the incorporation of tin (Sn) into silicon-germanium alloys has been proposed to overcome this fundamental limit. The obtained semiconductors are Ge1-x-ySixSny ternary alloys of Group IV elements compatible with CMOS technology, and may have a band gap that is adjustable depending on the composition and the strain. These properties have generated a great interest to grow these semiconductors and to better understand their optoelectronic and physical properties. With this perspective, this work outlines detailed investigations of the band structure of strained and relaxed Ge1-x-ySixSny ternary alloys using a semi-empirical second nearest neighbors tight binding method. This method is based on an accurate evaluation of the deformation potential constants of Ge, Si, and a-Sn using a stochastic Monte-Carlo approach as well as a gradient based optimization method. Moreover, a new and efficient differential evolution approach is also developed to accurately reproduce the experimental effective masses and band gaps. Based on this, the influence of lattice disorder, strain, and composition on Ge1-x-ySixSny band gap energy and its directness were elucidated. For 0 ≤x ≤0.4 and 0≤y≤0.2, tensile strain lowered the critical content of Sn needed to achieve a direct band gap semiconductor with the corresponding band gap energies below 0.76 eV. This upper limit decreases to 0.43eV for direct gap, fully relaxed ternary alloys. The obtained transition to direct band gap is given by y>0.605x+0.077 and y>1.364x+0.107 for epitaxially strained and fully relaxed alloys, respectively. The effects of strain, at a fixed composition, on band gap directness were also investigated and discussed. Next, building upon the acquired knowledge from the band structure calculation, the analysis was extended toward quantifying the electron and hole confinement in a Ge1-ySny/Ge core/shell nanowire system. For that purpose, the conduction and valance band offsets were evaluated

Combined Iodine- and Sulfur-based Treatments for an Effective Passivation of GeSn Surface

GeSn alloys are metastable semiconductors that have been proposed as building blocks for silicon-integrated short-wave and mid-wave infrared photonic and sensing platforms. Exploiting these semiconductors requires, however, the control of their epitaxy and their surface chemistry to reduce non-radiative recombination that hinders the efficiency of optoelectronic devices. Herein, we demonstrate that a combined sulfur- and iodine-based treatments yields effective passivation of Ge and Ge0.9Sn0.1 surfaces. X-ray photoemission spectroscopy and in situ spectroscopic ellipsometry measurements were used to investigate the dynamics of surface stability and track the reoxidation mechanisms. Our analysis shows that the largest reduction in oxide after HI treatment, while HF+(NH4)2S results in a lower re-oxidation rate. A combined HI+(NH4)2S treatment preserves the lowest oxide ratio <10 % up to 1 hour of air exposure, while less than half of the initial oxide coverage is reached after 4 hours. These results highlight the potential of S- and I-based treatments in stabilizing the GeSn surface chemistry thus enabling a passivation method that is compatible with materials and device processing

3-D Atomic Mapping of Interfacial Roughness and its Spatial Correlation Length in sub-10 nm Superlattices

The interfacial abruptness and uniformity in heterostructures are critical to control their electronic and optical properties. With this perspective, this work demonstrates the 3-D atomistic-level mapping of the roughness and uniformity of buried epitaxial interfaces in Si/SiGe superlattices with a layer thickness in the 1.5-7.5 nm range. Herein, 3-D atom-by-atom maps were acquired and processed to generate iso-concentration surfaces highlighting local fluctuations in content at each interface. These generated surfaces were subsequently utilized to map the interfacial roughness and its spatial correlation length. The analysis revealed that the root mean squared roughness of the buried interfaces in the investigated superlattices is sensitive to the growth temperature with a value varying from about 0.2 nm (+/- 13%) to about 0.3 nm (+/- 11.5%) in the temperature range of 500-650 Celsius. The estimated horizontal correlation lengths were found to be 8.1 nm (+/- 5.8%) at 650 Celsius and 10.1 nm (+/- 6.2%) at 500 Celsius. Additionally, reducing the growth temperature was found to improve the interfacial abruptness, with 30 % smaller interfacial width is obtained at 500 Celsius. This behavior is attributed to the thermally activated atomic exchange at the surface during the heteroepitaxy. Finally, by testing different optical models with increasing levels of interfacial complexity, it is demonstrated that the observed atomic-level roughening at the interface must be accounted for to accurately describe the optical response of Si/SiGe heterostructures.Comment: 17 A4 pages of main manuscript, 2 table, 5 figures, 20 A4 pages of supplementary informatio

Mid-infrared emission and absorption in strained and relaxed direct bandgap GeSn semiconductors

By independently engineering strain and composition, this work demonstrates and investigates direct band gap emission in the mid-infrared range from GeSn layers grown on silicon. We extend the room-temperature emission wavelength above ~4.0 {\mu}m upon post-growth strain relaxation in layers with uniform Sn content of 17 at.%. The fundamental mechanisms governing the optical emission are discussed based on temperature-dependent photoluminescence, absorption measurements, and theoretical simulations. Regardless of strain and composition, these analyses confirm that single-peak emission is always observed in the probed temperature range of 4-300 K, ruling out defect- and impurity-related emission. Moreover, carrier losses into thermally-activated non-radiative recombination channels are found to be greatly minimized as a result of strain relaxation. Absorption measurements validate the direct band gap absorption in strained and relaxed samples at energies closely matching photoluminescence data. These results highlight the strong potential of GeSn semiconductors as versatile building blocks for scalable, compact, and silicon-compatible mid-infrared photonics and quantum opto-electronics

500-period epitaxial Ge/Si0.18Ge0.82 multi-quantum wells on silicon

Ge/SiGe multi-quantum well heterostructures are highly sought-after for silicon-integrated optoelectronic devices operating in the broad range of the electromagnetic spectrum covering infrared to terahertz wavelengths. However, the epitaxial growth of these heterostructures at a thickness of a few microns has been a challenging task due the lattice mismatch and its associated instabilities resulting from the formation of growth defects. To elucidates these limits, we outline herein a process for the strain-balanced growth on silicon of 11.1 nm/21.5 nm Ge/Si0.18Ge0.82 superlattices (SLs) with a total thickness of 16 {\mu}m corresponding to 500 periods. Composition, thickness, and interface width are preserved across the entire SL heterostructure, which is an indication of limited Si-Ge intermixing. High crystallinity and low defect density are obtained in the Ge/Si0.18Ge0.82 layers, however, the dislocation pile up at the interface with the growth substate induces micrometer-longs cracks on the surface. This eventually leads to significant layer tilt in the strain-balanced SL and in the formation of millimeter-long, free-standing flakes. These results confirm the local uniformity of structural properties and highlight the critical importance of threading dislocations in shaping the wafer-level stability of thick multi-quantum well heterostructures required to implement effective silicon-compatible Ge/SiGe photonic devices

Germanium-tin semiconductors: A versatile silicon-compatible platform

Compound semiconductor alloys have been successfully used for a precise and simultaneous control of lattice parameters and bandgap structures bringing to existence a variety of functional heterostructures and low-dimensional systems. Extending this paradigm to group IV semiconductors will be a true breakthrough that will pave the way to creating an entirely new class of silicon-compatible ultra-fast/low-power electronic, optoelectronic, and photonic devices. With this perspective, germanium-tin (Ge1-xSnx) and germanium-silicon-tin (Ge1-x-ySixSny) alloys have recently been the subject of extensive investigations as new material systems to independently engineer lattice parameter and bandgap energy and directness. The ability to incorporate Sn atoms into silicon and germanium at concentrations about one order of magnitude higher than the equilibrium solubility is at the core of these emerging potential technologies. In this presentation, we will address the epitaxial growth and stability of these metastable semiconductors. We will also discuss the optical and electronic properties as well as the nature of the atomic order in Sn-rich group IV semiconductors. We will show that lattice strain engineering is critical to facilitate the incorporation of Sn at concentrations reaching, for in stance, nearly 20at.% in GeSn while suppressing Sn surface segregation and composition gradient. The basic properties of these GeSn layers will be discussed in the light of extensive optical and microscopic investigations. Moreover, we will also demonstrate that GeSn can be effectively used as a template to grow highly tensile strained Ge quantum wells. Results of the investigations of electronic properties of these new family of low-dimensional systems will be discussed. This includes the effects on strain level and nature (compressive vs. tensile) on charge carriers confinement and mobility. Finally, new concepts involving Ge/GeSn core-shell nanowires will be presented and their potential as versatile building blocks for electronics, integrated photonics, and quantum information will be addressed. Please click Additional Files below to see the full abstract

All-Group IV membrane room-temperature mid-infrared photodetector

Strain engineering has been a ubiquitous paradigm to tailor the electronic band structure and harness the associated new or enhanced fundamental properties in semiconductors. In this regard, semiconductor membranes emerged as a versatile class of nanoscale materials to control lattice strain and engineer complex heterostructures leading to the development of a variety of innovative applications. Herein we exploit this quasi-two-dimensional platform to tune simultaneously the lattice parameter and bandgap energy in group IV GeSn semiconductor alloys. As Sn content is increased to reach a direct band gap, these semiconductors become metastable and typically compressively strained. We show that the release and transfer of GeSn membranes lead to a significant relaxation thus extending the absorption wavelength range deeper in the mid-infrared. Fully released Ge$_{0.83}$Sn$_{0.17}$ membranes were integrated on silicon and used in the fabrication of broadband photodetectors operating at room temperature with a record wavelength cutoff of 4.6 $\mu$m, without compromising the performance at shorter wavelengths down to 2.3 $\mu$m. These membrane devices are characterized by two orders of magnitude reduction in dark current as compared to devices processed from as-grown strained epitaxial layers. The latter exhibit a content-dependent, shorter wavelength cutoff in the 2.6-3.5 $\mu$m range, thus highlighting the role of lattice strain relaxation in shaping the spectral response of membrane photodetectors. This ability to engineer all-group IV transferable mid-infrared photodetectors lays the groundwork to implement scalable and flexible sensing and imaging technologies exploiting these integrative, silicon-compatible strained-relaxed GeSn membranes

Optical Behavior of Group-IV Semiconductors in Relation to Atomic Interfacial Disorder, Quantum Confinement, and Fano Resonance

RÉSUMÉ: Les semi-conducteurs du groupe IV sont omniprésents dans nos activités quotidiennes. Les alliages binaires silicium-germanium (SiGe) en particulier sont à la pointe de la technologie des transistors, grâce à une réduction constante du nombre de nœuds. Le procédé technologique actuel en microélectronique de 5 nm dépend de deux éléments: les hétérostructures et les interfaces. Ces deux configurations sont fortement interconnectées. Les hétérostructures forment une plateforme riche pour concevoir une variété de structures et de dispositifs à faible dimension. Dans de tels systèmes, la nature des interfaces est un facteur crucial qui définit leurs propriétés et leurs performances de base. En outre, la connaissance précise de la rugosité de l’interface est également devenue de plus en plus critique dans les conceptions des transistors introduites pour les procédés technologiques de 5 à 7 nm et au-delà. Ces architectures sont basées sur des super-réseaux SiGe/Si. Cependant, malgré leur importance, l’évaluation directe des interfaces dépend encore largement d’approches microscopiques sophistiquées, retardant l’épitaxie, telles que la tomographie à sonde atomique et la microscopie à force atomique. La caractérisation optique de la rugosité des interfaces n’est pas à la hauteur des techniques microscopiques. Dans les années 1990, la spectroscopie de photoluminescence à basse température s’est imposée comme la technique de référence pour l’étude des interfaces. Néanmoins, les approches spectroscopiques à température ambiante manquent toujours dans la littérature. Par conséquent, en utilisant des super-réseaux SiGe/Si avec différentes périodicités, nous avons développé une technique de métrologie à température ambiante, pour évaluer la qualité des interfaces. Cette technique est basée sur l’ellipsométrie spectroscopique (SE), ainsi que sur la modélisation théorique semi-empirique pour quantifier l’effet des interfaces sur la structure électronique. On a constaté que l’élargissement de l’interface induit une localisation des états électroniques à l’intérieur de la bande interdite de SiGe. Cette localisation favorise les transitions optiques interbandes, qui sont détectées en mesurant le coefficient d’absorption. Cette approche, simpliste dans sa mise en œuvre expérimentale, a soulevé des questions fondamentales liées à la physique des états localisés dans les hétérostructures du groupe IV. Toute l’expertise acquise lors de ces mesures ellipsometrique a été exploitée pour analyser une nouvelle classe d’hétérostructures basée sur les semi-conducteurs Ge et germanium-étain (GeSn). Des puits quantiques de Ge/GeSn déformés par traction ont été développés par dépôt chimique en phase vapeur et caractérisés optiquement par SE pour étudier l’interaction entre le confinement quantique et l’élargissement interfacial. ABSTRACT: Group-IV semiconductors are prevalent in our day-to-day activities. Silicon-germanium (SiGe) binary alloys in particular are at the forefront of transistor technology, driven by relentless node reduction. The current 5 nm technology node depends on two elements: heterostructures and interfaces. These two components are interconnected, as one cannot obtain the latter without the former. Heterostructures have been a rich platform to engineer a variety of low-dimensional structures and devices. In such systems, the nature of the interfaces is a crucial factor that ultimately defines their basic properties and performance. Additionally, the precise knowledge of the buried interface roughness also becomes increasingly critical in silicon (Si) gate-all-around designs introduced for the 5 − 7 nm technology nodes and beyond. These architectures are based on SiGe/Si superlattices (SLs). Despite their importance, direct assessment of interfaces still relies heavily on destructive and time-consuming microscopic approaches, such as transmission electron microscopy. Optical characterization of interface abruptness is not up to par with microscopic techniques. During the 1990s, low-temperature photoluminescence spectroscopy surged as the go-to technique to investigate interfaces and is nowadays well-established. Nevertheless, room temperature spectroscopic approaches are still conspicuously missing in the literature. Through use of SiGe/Si superlattices with different periodicity, we developed a room temperature, metrology technique to evaluate the quality of the interfaces. The technique is based on spectroscopic ellipsometry (SE), as well as rigorous semi-empirical theoretical modeling to quantify the effect of the interfaces on the electronic structure. Interface broadening induces electronic state localization inside the SiGe bandgap. This localization promotes interband optical transitions, which are detected by measuring the absorption coefficient. This approach, simplistic in its experimental implementation, raises fundamental questions related to the physics behind state localization in group-IV heterostructures. All the expertise gained from these SE measurements were employed to analyze a new class of group-IV heterostructures, based on Ge and germanium-tin (GeSn) semiconductors. Tensile strained Ge/GeSn quantum wells were grown with chemical vapor deposition, and optically characterized with SE to study the dynamics between interfacial broadening and quantum confinement. The investigations revealed that SE can indeed gauge quantum confinement. These salient findings shed a new light on the interplay between tensile strain and light-hole (LH) state confinement. In fact, a sophisticated methodology to examine quantum confinement is developed in multilayered heterostructures, a class of structures renowned for its inaccessibility to SE due to the intricacies of the associated optical modeling