Isotope Engineering and Lattice Disorder in Group IV Nanoscale and Quantum Semiconductors

L’ingénierie des isotopes stables est la manipulation artificielle de la composition et de la distribution des isotopes stables d’un élément dans la maille cristalline d’un matériau donné. Au cours des deux dernières décennies, de nombreuses études conduites sur des semi-conducteurs monocristallins ont montré que de telles modifications peuvent altérer considérablement leurs propriétés fondamentales comme les propriétés nucléaires, le comportement des phonons, le diagramme des bandes d’énergie et le paramètre de maille. Ces développements ont permis un nouvel élan d’innovation et d’applications potentielles exploitant l’ingénierie isotopique dans le transport thermique et thermoélectrique, dans l’optoélectronique, et dans le traitement quantique de l’information, parmi tant d’autres. L’essentiel de la littérature relative à l’ingénierie des isotopes à l’échelle quantique ou nanoscopique se concentre principalement sur des investigations théoriques. A ce jour, les études expérimentales demeurent absentes malgré leur importance dans l’élucidation d’un vaste éventail de phénomènes quantiques et nanoscopiques. Dans cette thèse, nous explorons ce paradigme méconnu en concentrant nos expérimentations sur les propriétés de base des structures dont la composition isotopique est contrôlée à l’échelle nanoscopique. Des nanofils isotopiquement pures de 29Si ou d’alliage isotopique 28Six30Si1-x ont été synthétisés à l’aide de la méthode vapeur-liquide-solide et leurs propriétés de transport des phonons ont été étudiées en utilisant la nanothermométrie Raman. La composition et la distribution isotopiques des nanofils individuels ont été déterminées à l’aide de la sonde atomique tomographique assistée par laser. Cependant, avant que la sonde atomique tomographique ne soit appliquée pour imager les isotopes dans un nanofil, l’utilisation de cette technique unique, mais néanmoins extrêmement délicate, a été d’abord optimisée grâce à deux systèmes additionnels. Le premier système de matériaux consiste en un super réseau isotopique de diamant, et le deuxième est une série d’alliages ternaires métastables de silicium-germanium-étain. Ces recherches nous ont permis non seulement de développer nos connaissances et notre maîtrise de la sonde atomique tomographique, mais également de faire des nouvelles découvertes intéressantes. En exploitant l’imagerie tridimensionnelle atomique d’alliages ternaires métastables, nous avons obtenu des preuves solides que ces alliages sont parfaitement monocristallins et croissent sans agrégats d’étain même pour des concentrations supérieures à la composition attendue de vii l’équilibre thermodynamique.----------Abstract Stable isotope engineering refers to the artificial manipulation of the content and distribution of the stable isotopes of an element within the lattice of a material. Over the last two decades, numerous studies conducted on bulk semiconductors have shown that exercising such a control can significantly alter the fundamental behavior of a material such as the nuclear properties, phonon behavior, electronic energy gaps, and lattice constant. Consequently, a myriad of opportunities emerged from this isotopic engineering of semiconductors enabling a variety of novel and potential applications such as thermal transport and thermoelectric, optoelectronics, and quantum information processing, to name a few. The body of literature related to isotope engineering in nanoscale materials is made primarily of theoretical investigations. Till date, the experimental investigations remain conspicuously missing, despite the fact that the combination of mass-related effects and size-related effects can provide a rich playground to uncover and harness a wide range of new nanoscale and quantum phenomena. In this thesis, we unfold this unexplored paradigm by focusing our experimental investigations on the basic lattice properties of isotopically programmed nanoscale structures. The isotopically pure Si 29 and mixed Six 28Si1−x 30 nanowires were synthesized using the metal catalysed vapor-liquid-solid method and the phonon transport in these nanowires was studied using Raman nanothermometry. The isotopic composition and distribution within an individual nanowire was investigated using laser-assisted atom probe tomography. However, before the atom probe tomography could be implemented to map the isotopes within a nanowire, the experimental capabilities of this unique yet extremely challenging technique were first optimized in two additional systems. The first material system consists of diamond isotopic superlattice and the second, a set of ternary metastable silicon-germanium-tin alloys. These investigations not only equipped us with the science and the practice of atom probe tomography, but also had some interesting revelations of their own. Based on the atom-by-atom three-dimensional mapping of ternary metastable alloys, we obtained clear evidence that these alloys grew without any tin clustering even at contents larger than the equilibrium composition. However, with the increase in tin content, the silicon distribution within these alloys was found to deviate from the ideal theoretical distribution. The root cause of this short-range atomic ordering is the presence of a repulsive interaction between silicon and tin x atoms

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

Reduction of Thermal Conductivity in Nanowires by Combined Engineering of Crystal Phase and Isotope Disorder

Nanowires are a versatile platform to investigate and harness phonon and thermal transport phenomena in nanoscale systems. With this perspective, we demonstrate herein the use of crystal phase and mass disorder as effective degrees of freedom to manipulate the behavior of phonons and control the flow of local heat in silicon nanowires. The investigated nanowires consist of isotopically pure and isotopically mixed nanowires bearing either a pure diamond cubic or a cubic-rhombohedral polytypic crystal phase. The nanowires with tailor-made isotopic compositions were grown using isotopically enriched silane precursors SiH, SiH, and SiH with purities better than 99.9%. The analysis of polytypic nanowires revealed ordered and modulated inclusions of lamellar rhombohedral silicon phases toward the center in otherwise diamond-cubic lattice with negligible interphase biaxial strain. Raman nanothermometry was employed to investigate the rate at which the local temperature of single suspended nanowires evolves in response to locally generated heat. Our analysis shows that the lattice thermal conductivity in nanowires can be tuned over a broad range by combining the effects of isotope disorder and the nature and degree of polytypism on phonon scattering. We found that the thermal conductivity can be reduced by up to ∼40% relative to that of isotopically pure nanowires, with the lowest value being recorded for the rhombohedral phase in isotopically mixed Si Si nanowires with composition close to the highest mass disorder (x ∼ 0.5). These results shed new light on the fundamentals of nanoscale thermal transport and lay the groundwork to design innovative phononic 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

Intersubband Transition Engineering in the Conduction Band of Asymmetric Coupled Ge/SiGe Quantum Wells

n-type Ge/SiGe asymmetric coupled quantum wells represent the building block of a variety of nanoscale quantum devices, including recently proposed designs for a silicon-based THz quantum cascade laser. In this paper, we combine structural and spectroscopic experiments on 20-module superstructures, each featuring two Ge wells coupled through a Ge-rich SiGe tunnel barrier, as a function of the geometry parameters of the design and the P dopant concentration. Through a comparison of THz spectroscopic data with numerical calculations of intersubband optical absorption resonances, we demonstrated that it is possible to tune, by design, the energy and the spatial overlap of quantum confined subbands in the conduction band of the heterostructures. The high structural/interface quality of the samples and the control achieved on subband hybridization are promising starting points towards a working electrically pumped light-emitting device. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

n-type Ge/SiGe multi quantum-wells for a THz quantum cascade laser

Exploiting intersubband transitions in Ge/SiGe quantum cascade devices provides a way to integrate terahertz light emitters into silicon-based technology. With the view to realizing a Ge/SiGe Quantum Cascade Laser, we present the optical and structural properties of n-type strain-symmetrized Ge/SiGe asymmetric coupled quantum wells grown on Si(001) substrates by means of ultrahigh vacuum chemical vapor deposition. We demonstrate the high material quality of strain-symmetrized structures and heterointerfaces as well as control over the inter-well coupling and electron tunneling. Motivated by the promising results obtained on ACQWs, which are the basic building block of a cascade structure, we investigate, both experimentally and theoretically, a Ge/SiGe THz QCL design, optimized through a non-equilibrium Green's function formalism

Phonon engineering in isotopically disordered silicon nanowires

The introduction of stable isotopes in the fabrication of semiconductor nanowires provides an additional degree of freedom to manipulate their basic properties, design an entirely new class of devices, and highlight subtle but important nanoscale and quantum phenomena. With this perspective, we report on phonon engineering in metal-catalyzed silicon nanowires with tailor-made isotopic compositions grown using isotopically enriched silane precursors ²⁸SiH, ²⁹SiH, and ³⁰SiH with purity better than 99.9%. More specifically, isotopically mixed nanowires ²⁸Si ³⁰Si with a composition close to the highest mass disorder (x ∼ 0.5) were investigated. The effect of mass disorder on the phonon behavior was elucidated and compared to that in isotopically pure Si nanowires having a similar reduced mass. We found that the disorder-induced enhancement in phonon scattering in isotopically mixed nanowires is unexpectedly much more significant than in bulk crystals of close isotopic compositions. This effect is explained by a nonuniform distribution of ²⁸Si and ³⁰Si isotopes in the grown isotopically mixed nanowires with local compositions ranging from x = ∼0.25 to 0.70. Moreover, we also observed that upon heating, phonons in ²⁸Si ³⁰Si nanowires behave remarkably differently from those in ²⁹Si nanowires suggesting a reduced thermal conductivity induced by mass disorder. Using Raman nanothermometry, we found that the thermal conductivity of isotopically mixed ²⁸Si Si nanowires is ∼30% lower than that of isotopically pure ²⁹Si nanowires in agreement with theoretical predictions. (Figure Presented)

Inferring causal molecular networks: empirical assessment through a community-based effort

Inferring molecular networks is a central challenge in computational biology. However, it has remained unclear whether causal, rather than merely correlational, relationships can be effectively inferred in complex biological settings. Here we describe the HPN-DREAM network inference challenge that focused on learning causal influences in signaling networks. We used phosphoprotein data from cancer cell lines as well as in silico data from a nonlinear dynamical model. Using the phosphoprotein data, we scored more than 2,000 networks submitted by challenge participants. The networks spanned 32 biological contexts and were scored in terms of causal validity with respect to unseen interventional data. A number of approaches were effective and incorporating known biology was generally advantageous. Additional sub-challenges considered time-course prediction and visualization. Our results constitute the most comprehensive assessment of causal network inference in a mammalian setting carried out to date and suggest that learning causal relationships may be feasible in complex settings such as disease states. Furthermore, our scoring approach provides a practical way to empirically assess the causal validity of inferred molecular networks