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2021 Fall Meeting

Nanoparticles and nanomaterials


Polytypism in semiconductors

Recent advances in the fabrication and characterization of polytypes semiconductor nanostructures have made crystal phase engineering a well-established tool to tailor material properties. This interdisciplinary symposium aims to identify challenges in the synthesis and characterization of new polytype semiconductors.


Semiconductors constitute the building blocks of the current  microelectronic and optoelectronic industry. The standard (low energy) crystal phases of most semiconductors have already revealed all their potential and limitations in the development of advanced devices. On the other hand, polytypes of most semiconductors possess very different and peculiar physical characteristics with respect to their lowest energy structures. Main scope of this symposium is to promote the progress in the fundamental understanding (in the broad sense of theory combined with experiments) of the role of crystal phase engineering in materials’ design. Specific topics and issues that will be carefully considered are:

Synthesis: The synthesis (pressure, indentation, epitaxy…) of polytype single crystals or heterostructures in a controlled manner represents a great challenge for a long time. The growth of nanowires has initiated a new impetus to this effort. Besides, new paradigms such as selective area growth or remote epitaxy open plenty of rooms to explore for original phase synthesis.

Experimental characterization: It is expected that a novel phase may alter remarkably the properties of the nanostructures (such as band gap, effective mass, phonon and electron scattering processes and excitonic properties) due to the presence of distinct crystal symmetry or of a significant interface between two phases. This stimulates the development and application of advanced experimental methods.

Theoretical modelling: Experimental investigation of novel phases in nanostructures requires deep quantitative understanding of condensed matter at nanoscale. Indeed, a significant uncertainty prevails in discerning the fundamental effect of crystal phase-dependent factors, from other factors (size, shape, composition, local strain, interface states…) that affect the main physical and chemical properties. This is the  role of theory, modeling and simulations in the description of semiconductors polytypes.

Hot topics to be covered by the symposium:

  • Phase transformation under extreme conditions or indentation
  • Appropriate growth strategies of new crystal phases (VLS growth, Van der Walls epitaxy, …) and synthesis of polytypic heterostructures
  • Modeling of critical processes during the synthesis of polytypes nanostructures in order to reach controlled composition, structure, geometry
  • Experimental methods for investigating the properties of polytypic structures
  • Theoretical methods for the description of electronic, optical and transport properties of novel polytypes
  • Devices for the exploitation of the properties of polytypic structures

Confirmed invited speakers:

  • Riccardo Rurali - Spain: Dopants, interfaces and alloys in crystal-phase engineering
  • Anna Fontcuberta i Morral - Switzerland: Recipes and properties of controlled polytypism in GaAs nanowires
  • Claes Thelander - Sweden: Quantum rings in polytypic nanowires
  • Vladimir Dubrovski – Russia:Controlling zincblende-wurtzite polytypism in III-V nanowires
  • Yann le Godec – France: High-Pressure Synthesis of Nanostructured Hexagonal Silicon 4H Polytype
  • Heinz Schmid - Germany: Selective Epitaxy of Metastable III-Vs
  • Sebastian Lehmann - Sweden: Polytypism in Au-seeded MOVPE-grown III-V nanowires
  • Claudia Rödl – Germany: ...
  • Federico Panciera – France: Real-time monitoring of crystal-phase switching in III-V nanowires
  • Jonas Laehnemann - Germany: Emission properties of quantum well shells on zincblende and wurtzite nanowire segments
  • Xavier Cartoixa - Spain: Thermal conduction in polytypes of 2D and 3D materials

Scientific committee:


Papers will be published in a special issue of Crystal Research and Technology (Wiley).

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08:50 Welcome message and introduction to the Symposium    
Session : tba
Authors : Anna Fontcuberta i Morral
Affiliations : Laboratory of Semiconductor Materials, Institute of Materials, EPFL, 1015 Lausanne, Switzerland

Resume : The high surface-to-volume ratio in nanostructures allows them to exist in crystal phases otherwise unstable in bulk. The ability to tune the crystal structure from zinc-blende to wurzite in III-V nanowires has been explained thanks to in situ observations in a transmission electron microscope. The nucleation point of every new layer, in or out of the triple-phase line, determines the crystal phase. In this presentation we will explain the interplay between the contact angle and the achievement of pure wurztite and zinc-blende GaAs by the Ga-assisted method [1]. We will also illustrate the optical properties by photoluminescence and absorption measurements [2,3]. [1] V. Dubrovskii et al Nano Lett. 21, 3139 (2021) [2] B. Ketterer et al Phys. Rev. B 83, 125307 (2011) [3] M. Swinkels et al Phys. Rev. Appl. 14, 024045 (2021)

Authors : Nicola Dengo, Federica Bertolotti, Dmitry N Dirin, Maksym V Kovalenko, Antonio Cervellino, Jan Skov Pedersen, Norberto Masciocchi, Antonietta Guagliardi
Affiliations : Dipartimento di Scienza e Alta Tecnologia and To.Sca.Lab, Università dell'Insubria, Via Valleggio 11, I-22100 Como, Italy. Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1, Zürich CH-8093, Switzerland. Empa-Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, Dübendorf CH-8600, Switzerland. SLS, Laboratory for Synchrotron Radiation-Condensed Matter, Paul Scherrer Institut, Villigen CH-5232, Switzerland. Department of Chemistry and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Gustav Wieds Vej 14, 8000 Aarhus, Denmark. Istituto di Cristallografia and To.Sca.Lab, Consiglio Nazionale delle Ricerche, Via Valleggio 11, I-22100 Como, Italy.

Resume : Semiconductors quantum dots (QDs) have been extensively studied in the last three decades because of their finely size- and shape- tunable optical and electronic properties. Metal chalcogenides is one of the most relevant class of QD materials, and, among these, CdSe played the most important role. For CdSe, the colloidal synthesis approaches already reached an advanced level of development, enabling ultra-precise control over size and morphology, and fine engineering of the surface structure. Despite this, the fine control over nanocrystal defectiveness and the fundamental investigation of the surface structure still remains a highly challenging task, that lacks established analytical approaches. We previously tackled this problem by combining X-ray total scattering techniques in the wide- (WAXS) and small-angle regions (SAXS) and using the Debye scattering equation (DSE) based modeling approach to achieve a detailed characterization of size, morphology, and planar defects in oleate-capped zincblende CdSe QDs. This DSE-based combined SAXS/WAXS method is now generalized for the atomic-to-nanometer scale characterization of allegedly wurtzite CdSe QDs. Three representative samples were synthetized, according to the approach described by Chen et al. (10.1038/nmat3539), by changing reaction times and temperatures. The resulting nanoparticles displayed a narrow size distribution and sizes that spanning from 2.3 to 5.6 nm, depending on the synthetic conditions. The nanoparticles morphology was investigated by building atomistic models and applying the DSE approach. The retrieved shapes can be described as a variation of hexagonal truncated bipyramids, exposing {001}, {100} and {011} facets, depending on the level of truncation. The relative extension of the different facets was found being both size- and fault- dependent. Noteworthily, obtaining high-quality SAXS experimental data in the Porod region was deemed crucial to reliably distinguish these fine morphological features. The same atomistic models were simultaneously employed to model the WAXS region, where the presence of planar defects was introduced using a layer-by-layer approach in the model construction. Here, the analysis pinpointed a remarkable high density of planar defects for the sample characterized by the largest particles’ sizes, which was also synthesized by employing longer reaction times and at the lowest operating temperature. Thus, this work shows the potential of the proposed analytical approach to untangle complex and intertwined features of crystal structure defectiveness and particles’ morphology, which might have an important impact on the final properties of CdSe and binary semiconductors QDs in general. This methodology, developed for archetypal binary systems, will be soon extended to more complex materials, such as ternary chalcogenides QDs. Financial support by Fondazione Cariplo, CubaGREEN Project 2020-4382 is acknowledged.

Authors : V.T. v. Lange, A. Dijkstra, E.M.T. Fadaly, M.A.J. v. Tilburg, M. A. Verheijen, J.R. Suckert, C. Rödl, J. Furthmüller, F. Bechstedt, S. Botti, D. Busse, J.J. Finley, E.P.A.M. Bakkers, J.E.M. Haverkort
Affiliations : Department of Applied Physics, Eindhoven University of Technology, Groene Loper 19, 5612AP Eindhoven, The Netherlands; Department of Applied Physics, Eindhoven University of Technology, Groene Loper 19, 5612AP Eindhoven, The Netherlands; Department of Applied Physics, Eindhoven University of Technology, Groene Loper 19, 5612AP Eindhoven, The Netherlands; Department of Applied Physics, Eindhoven University of Technology, Groene Loper 19, 5612AP Eindhoven, The Netherlands; Department of Applied Physics, Eindhoven University of Technology, Groene Loper 19, 5612AP Eindhoven, The Netherlands; Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany; Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany; Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany; Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany; Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany; Physik Department, Walter Schottky Institut, Technische Universität München, Am Coulombwall 4, 85748 Garching, Munich, Germany; Physik Department, Walter Schottky Institut, Technische Universität München, Am Coulombwall 4, 85748 Garching, Munich, Germany; Department of Applied Physics, Eindhoven University of Technology, Groene Loper 19, 5612AP Eindhoven, The Netherlands; Department of Applied Physics, Eindhoven University of Technology, Groene Loper 19, 5612AP Eindhoven, The Netherlands;

Resume : It has been a holy grail for several decades to demonstrate direct bandgap light emission in silicon. Here we study the light emission[1] from hexagonal crystal phase SiGe, which can be fabricated[2], [3] by growing a wurtzite GaAs nanowire core surrounded by a SiGe shell. Theory predicts Hex-Ge to have a direct bandgap while hex-Si remains indirect[4]. Extrapolations between Hex-Si and Hex-Ge predict that Hex-SiGe will be a direct bandgap semiconductor at Ge-compositions above 65%. An accurate analysis of the photoluminescence (PL) spectra as a function of temperature reveals the first indication that the emission is due to direct bandgap arrangement. In an indirect semiconductor, all carriers accumulate in the indirect valley at low temperature, yielding a low PL intensity. This is in contradiction with our measurements, which are showing bright PL at a temperature of 4K. In addition we observe a Fermi Dirac tail at high temperature and if the PL spectra are analyzed for increasing excitation density a clear Burstein-Moss shift is observed at high excitation, both reinforcing the argument in favor of band-to-band emission. A second proof for a direct bandgap is provided by investigating the optical lifetime of the hex-SiGe. The direct-bandgap transition is an efficient process with a short radiative lifetime in the order of nanoseconds. The indirect-bandgap however, additionally requires a phonon to overcome the mismatch in momentum increasing the lifetime dramatically to microseconds or even milliseconds. For the Hex-Si¬0.2Ge0.8 alloy we observe a lifetime around 0.9ns both at 4K and at room temperature corresponding to the high radiative efficiency expected from a direct bandgap semiconductor. Additionally, we will present a new initial estimation of the lifetime of hex-Ge using an analytical model for the Burstein-Moss bandfilling at 4K as a function of excitation density. Finally, we measured the PL spectra of Hex-Si1-xGex as a function of the composition. We observe a broad tunability of the direct bandgap between 1.8 µm at x=0.65 and 3.5 µm at x=1, measured at 4K. In conclusion, photoluminescence experiments can be used to characterize the change from indirect to a direct bandgap semiconductor when changing the crystal phase of SiGe from cubic to hexagonal. This project has received funding from the Horizon 2020 program under grant agreement No 735008 (SiLAS) and the Dutch Organization for Scientific Research (NWO). References: [1] E. M. T. Fadaly et al., “Direct-bandgap emission from hexagonal Ge and SiGe alloys,” Nature, vol. 580, no. 7802, pp. 205–209, Apr. 2020. [2] H. I. T. Hauge et al., “Hexagonal Silicon Realized,” Nano Lett., vol. 15, no. 9, pp. 5855–5860, Aug. 2015. [3] I. T. Hauge et al., “Single-Crystalline Hexagonal Silicon − Germanium,” Nano Lett, vol. 17, pp. 85–90, 2017. [4] C. Rödl et al., “Accurate electronic and optical properties of hexagonal germanium for optoelectronic applications,” Phys. Rev. Mater., vol. 3, no. 3, 2019.

Authors : Jonas Lähnemann (1), Megan O. Hill (2), Jesús Herranz (1), Oliver Marquardt (3), Ville Havu (4), Guanhui Gao (1), Uwe Jahn (1), Chunyi Huang (2), Timo Streckenbach (3), Patrick Rinke (4), Stephan O. Hruszkewycz (5), Martin V. Holt (5), Irene Calvo-Almazán (5), Ali Al Hassan (6), Arman Davtyan (6), Ullrich Pietsch (6), Lincoln J. Lauhon (2), Lutz Geelhaar (1)
Affiliations : (1) Paul-Drude-Institut für Festkörperelektronik, Berlin, Germany; (2) Northwestern University, Evanston, IL, USA; (3) Weierstraß-Institut, Berlin, Germany; (4) Aalto University, Finland; (5) Argonne National Laboratory, IL, USA; (6) Universität Siegen, Germany

Resume : While the properties of wurtzite GaAs have been extensively studied during the past decade, little is known about the influence of the crystal polytype on ternary (In,Ga)As quantum well structures. We investigate this question with cathodoluminescence hyperspectral imaging on core-shell nanowires that contain extended segments of both the zincblende and wurtzite polytypes. To verify the structure, the emission properties are correlated to measurements by electron backscatter diffraction and nanoprobe x-ray diffraction, as well as atom probe tomography for the quantum well composition. The observed blueshift of the quantum well emission energy by 75 ± 15 meV in the wurtzite polytype segment can then be attributed to both a 30% drop in In mole fraction going from the zincblende to the wurtzite segment and to compressive strain on the quantum well, which has a much stronger impact on the hole ground state in the wurtzite than in the zincblende segment. The latter conclusion relies on k·p calculations for the specific sample geometry. To consolidate the simulations, we investigate the influence of improved material parameters for wurtzite (In,Ga)As. Our results highlight the role of the crystal structure in tuning the emission of (In,Ga)As quantum wells and pave the way to exploit the possibilities of three-dimensional bandgap engineering in core-shell nanowire heterostructures.

10:30 Q&A live session / Break    
Session : tba
Authors : Claudia Rödl, Pedro Borlido, Jens Renè Suckert, Friedhelm Bechstedt, Silvana Botti
Affiliations : Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany

Resume : Incorporation of microelectronics and optoelectronics is expected to revolutionize various fields of technology, such as communication, sensing, and imaging. A Si-compatible nanolaser would be the key to achieve integrated silicon photonics. However, Si as well as Ge in their diamond-structure equilibrium phases are known to be optically inactive due to the indirect nature of their band gaps. The hexagonal allotropes of Si and Ge in the lonsdaleite phase, which can now be grown in good quality, may overcome this limitation. Hexagonal Si is still indirect, whereas hexagonal Ge is a direct semiconductor. Unfortunately, the dipole matrix elements of the lowest optical transitions in hexagonal Ge are almost zero. Here, we show that it is possible to enhance the optical oscillator strength of hexagonal Ge at the absorption edge by applying tensile uniaxial strain or alloying it with Si. Upon structural modification, the two lowest conduction bands change order and the lowest optical transitions become strongly dipole active. We compare our results to recent data from our experimental collaborators. Using first-principles density-functional theory with hybrid functionals and meta-GGA, we calculate structural and electronic properties and show how the absorption and emission spectra are affected by strain and alloying, respectively.

Authors : Silvia Pandolfi[1*], Shiteng Zhao[2-3], John Turner[2], Peter Ercius[2], Chengyu Song[2], Rohan Dhall[2], Nicolas Menguy[1], Yann Le Godec[1], Alex Courac[1], Andrew Minor[2-3], Jon Eggert[4]; and Leora E. Dresselhaus-Marais[4#]
Affiliations : 1Sorbonne Université, Muséum National d'Histoire Naturelle, UMR CNRS 7590, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, IMPMC, Paris, France 2National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA, USA 3Department of Materials Science and Engineering, University of California, Berkeley, CA, USA 4Lawrence Livermore National Laboratory. 7000 East Avenue, Livermore, California 94550, USA *now at: SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA # now Assistant Professor of Materials Science & Engineering at Stanford University

Resume : Silicon (Si) has a wide variety of allotropes. Among them, hexagonal Si polytypes are promising photovoltaic materials, as they have enhanced efficiency in absorbing visible light and a band structure that is predicted to shift with strain and pressure [1-2]. We recently established the first synthesis of pure hexagonal Si with Si-4H structure through high-pressure synthesis, demonstrating a substantial nanostructuration set by the high density of stacking faults [3]. Here, using multi-scale imaging, we reveal a hierarchical structure in the morphology of Si-4H obtained from high-pressure synthesis. We demonstrate discrete structural units, platelets, at an intermediate length-scale between the bulk pellet and the flake-like crystallites inferred previously [3]. The platelets are crystalline units only tens of nanometers thick which appear to be flexible at the single-domain level, and they can be dispersed and manipulated to design novel optoelectronic and solar devices. Furthermore, it will now pbe possible to assess the influence of the hierarchical nanostructure on Si-4H optoelectronic, as Si-4H single-crystals growth has recently been demonstrated [4]. References: [1] C. Raffy, J. Furthmüller, and F. Bechstedt, Properties of Hexagonal Polytypes of Group-IV Elements from First-Principles Calculations, Phys Rev B 66, 075201 (2002). [2] C. Rödl, T. Sander, F. Bechstedt, J. Vidal, P. Olsson, S. Laribi, and J.-F. Guillemoles, Wurtzite Silicon as a Potential Absorber in Photovoltaics: Tailoring the Optical Absorption by Applying Strain, Phys Rev B 92, 045207 (2015). [3] S. Pandolfi, C. Renero-Lecuna, Y. L. Godec, B. Baptiste, N. Menguy, M. Lazzeri, C. Gervais, K. Spektor, W. A. Crichton, and O. O. Kurakevych, Nature of Hexagonal Silicon Forming via High-Pressure Synthesis: Nanostructured Hexagonal 4H Polytype, Nano Lett 18, 5989 (2018). [4] T. B. Shiell, L. Zhu, B. A. Cook, J. E. Bradby, D. G. McCulloch, and T. A. Strobel, Bulk Crystalline 4H-Silicon through a Metastable Allotropic Transition, Phys Rev Lett 126, 215701 (2021).

Authors : Sumit Kumar, Frédéric Fossard, Gaëlle Amiri, Jean-Michel Chauveau and Vincent Sallet
Affiliations : S.Kumar, G. Amiri, JM Chauveau, V. Sallet GEMaC, CNRS - UVSQ, Université Paris-Saclay, 45 avenue des Etats-Unis, 78035 Versailles, France F. Fossard LEM, ONERA-CNRS, Université Paris-Saclay, 29 avenue Division Leclerc, 92322 Chatillon, France

Resume : Unique growth mechanisms involved in semiconductor nanowires (NWs) pave the way to the achievement of new crystallographic phases and remarkable material properties. Interestingly, in the case of 1D nanostructures, polytypism can arise due to the particular growth mode below a catalyst droplet, that may induce stacking faults along the length of the NWs. Moreover, these stacking faults can be correlated and form ordered arrays, until giving rise to new phases (polytypes) with distinct properties [1,2]. Hence, 4H, 6H, 8H, and 10H (so-called high order polytypes) can been observed in NWs [3]. Hence, studying polytypism in semiconductor NWs arouses a strong interest for the next generation of electronic and photonic applications. In this framework, ZnS is an important II-VI semiconductor which has a wide range of optoelectronic applications including luminescent devices, infrared windows, and UV-photodetectors. In this work, Au-assisted ZnS NWs were grown by MOCVD, directly on GaAs (111B) substrate (VLS, vapor-liquid-solid mode), and on ZnS (buffer)/GaAs (111B) (VSS, vapor-solid-solid mode). The idea is to provide a change in the growth mechanism via the physical state of catalyst droplet (liquid or solid) and hence, study the induced polytypism in ZnS NWs. ZnS NWs with length up to 1.4 µm and diameter in the range 10?34 nm was successfully achieved. The obtained morphologies and densities of the NWs has been systematically inspected by scanning electron microscopy (SEM) directly on the substrate. Transmission Electron Microscopy has been also used to investigate the crystallographic structures and compositions of both catalysts and NWs. NWs grown directly on GaAs (VLS mode) induced periodic stacking faults, and the resulting structure was accurately identified as 3 sequences of 5 planes ABCBA-BCACB-CABAC, giving rise to an astonishing 15R crystal structure [4]. This structure is highlighted for the first time in ZnS nanowires. Additional conventional TEM has been performed to identify the signature of the 15R phase and its peculiar pattern (i.e. a 5th order superstructure). Additionally, we modeled this 15R structure and plotted its formation probability in the framework of the classical nucleation theory and axial-next-nearest-neighbour-Ising model (ANNNI). Interestingly, in contrast with the VLS case, in nanowires grown on ZnS buffer (i.e. VSS mode, with solid catalyst), a different crystal structure made of pure ZB and WZ phases was observed. References: [1] G. Priante et al. Phys Rev B. 89 (2014) 241301 [2] F.J. Lopez et al. Nano Lett. 9 (2009) 2774?2779. [3] Y. Jiang et al. Adv. Mater. 15 (2003) 1195?1198. [4] S. Kumar et al. Nano Res. 2021.

Authors : Adrián Francisco-López, Bethan Charles, M. Isabel Alonso, Miquel Garriga, Mariano Campoy-Quiles, Mark T. Weller, Alejandro R. Goñi
Affiliations : A. Francisco-López; M.I. Alonso; M. Garriga; M. Campoy-Quiles; A.R. Goñi, Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus UAB, 08193 Bellaterra, Spain; B. Charles, Dept. of Materials Science & Metallurgy, University of Cambridge, CB3 0FS, UK; M.T. Weller, Dept. of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK, Dept. of Chemistry, Cardiff University, Wales CF10 3AT, UK; A.R. Goñi, ICREA, Passeig Lluís Companys 23, 08010 Barcelona, Spain

Resume : A salient feature of hybrid lead halide perovskites is the peculiar interplay between the organic and inorganic degrees of freedom, which has important consequences for the structural, vibrational, optical and transport properties of the material. Here we probed, on the one hand, the structural phase behavior of high quality single crystals of organic-cation solid-solutions of lead iodide perovskites (FAxMA1-xPbI3, where MA stands for methylammonium and FA for formamidinium) with x ranging from 0 to 1 in steps of 0.1 by combining Raman scattering and photoluminescence (PL) measurements as a function of temperature in the range from 10 to 365 K [1]. On the other hand, the pressure-induced changes on the crystal structure of the archetypal hybrid perovskite MAPbI3 were elucidated also by optical means for hydrostatic pressures up to ca. 10 GPa [2]. Both PL and Raman spectra show simultaneous changes in their profiles that indicate the occurrence of different phase transitions, which allowed us to construct the phase diagram of the solid solutions as a function of temperature, pressure and composition. At ambient conditions, only the perovskites with very high MA content x=1 and 0.9 crystallize in a tetragonal phase, whereas the rest exhibit a cubic crystal structure. At low temperatures and MA contents higher or equal to 0.6 a first order phase transition occurs from tetragonal to orthorhombic. At this phase transition, the Raman spectra exhibit a pronounced reduction in linewidth of the phonon modes of the inorganic cage, as a consequence of the locking of the organic MA cations in the cage voids, due to the reduced volume and symmetry of the unit cell [3]. In contrast, for high FA content the material exhibits an isostructural tetragonal-tetragonal transition rather than transforming into the orthorhombic phase. The degree of dynamical disorder of the MA cations has also strong impact on the high-pressure phenomenology of MAPbI3: At 2.7 GPa it undergoes a transformation into a phase characterized by a pronounced reduction of the Raman linewidths, which is interpreted as evidence of the locking of the MA cations inside the inorganic cage voids. Furthermore, no signs of amorphization is found at higher pressures. [1] A. Francisco-López, B. Charles, M. I. Alonso, M. Garriga, M. Campoy-Quiles, M. T. Weller, and A. R. Goñi, J. Phys. Chem. C 124, 3448-3458 (2020). [2] A. Francisco-López, B. Charles, O. J. Weber, M. I. Alonso, M. Garriga, M. Campoy-Quiles, M. T. Weller, and A. R. Goñi, J. Phys. Chem. C 122, 22073-22082 (2018). [3] A. M. A. Leguy, A. R. Goñi, J. M. Frost, J. Skelton, F. Brivio, X. Rodríguez-Martínez, O. J. Weber, A. Pallipurath, J. Sibik, A. Zeitler, M. I. Alonso, M. Campoy-Quiles, M. T. Weller, J. Nelson, A. Walsh, and P. R.F. Barnes, Phys. Chem. Chem. Phys. 18, 27051-27066 (2016).

Authors : Maurice, J.-L.*(1), Wang, W.(1), Ngo, É(1), Florea, I.(1), Bulkin, P.(1), Foldyna, M.(1), & Roca i Cabarrocas, P.(1).
Affiliations : (1) LPICM, École polytechnique, CNRS, Institut Polytechnique de Paris, 91128 Palaiseau cedex, France

Resume : The VLS (vapour-liquid-solid) and VSS (vapour-solid-solid) modes of semiconductor nanowire (NW) growth present the possibility of naturally delivering metastable polytypes directly, such as the hexagonal 2H polytype, without any additional treatment [1-3]. The mechanisms, for that natural occurrence of out-of-equilibrium structures, are fairly well understood for compound semiconductors [4, 5]. However, if such polytypes sometimes also appear in elemental semiconductor nanowires, like in very narrow SiNWs [3], they are much rarer, and the conditions for their growth remain unknown. Our goal with the present project was to understand the occurrence of the hexagonal 2H polytype in the case of SiNWs. Our approach, centred on the use of plasma-enhanced chemical vapour deposition (PECVD) and bimetallic catalysts, was twofold: on the one hand, we studied the growth parameters in a standard PECVD reactor [6]; on the other hand, we reproduced growth conditions as close as possible to PECVD in situ, in a modified Thermo Fisher environmental transmission electron microscope Titan ETEM “NanoMAX”, so as to visualise the growth mechanisms at the atomic scale [7]. As such polytypes had been obtained in NWs grown with Sn catalyst on a Cu substrate [3], we studied Cu-Sn alloy catalysts. To reproduce some of the PECVD conditions in NanoMAX, we implemented an electron cyclotron resonance plasma source (Aura-wave from SAIREM) on the H2 line of the ETEM. We managed to obtain 2H domains in narrow SiNWs, both ex-situ in the standard PECVD reactor and in the NanoMAX ETEM, where we could watch the nucleation of that phase atomic plane by atomic plane. The aim of the talk is to present the main results of the project, with an emphasis on the NanoMAX movies of the growth. This work was funded by the ANR, through the TEMPOS Equipex (ANR-10-EQPX-50), pole “NanoMAX” and the HexaNW project (ANR-17-CE09-0011). Thanks are due to the CIMEX – Centre interdisciplinaire de microscopie électronique de l’X, for the use of the electron microscopes at École polytechnique, and to F. Panciera (C2N) for giving Si cantilever substrates. 1. P. Rueda-Fonseca, E. Bellet-Amalric, R. Vigliaturo, M. den Hertog, Y. Genuist, R. André, E. Robin, A. Artioli, P. Stepanov, D. Ferrand, K. Kheng, S. Tatarenko and J. Cibert, Structure and morphology in diffusion-driven growth of nanowires: The case of ZnTe, Nano Lett. 14, 1877-1883 (2014). 2. J.-C. Harmand, G. Patriarche, F. Glas, F. Panciera, I. Florea, J.-L. Maurice, L. Travers and Y. Ollivier, Atomic step flow on a nanofacet, Phys. Rev. Lett. 121, 166101 (2018). 3. J. Tang, J. L. Maurice, F. Fossard, I. Florea, W. Chen, E. V. Johnson, M. Foldyna, L. Yu and P. Roca i Cabarrocas, Natural occurrence of the diamond hexagonal structure in silicon nanowires grown by a plasma-assisted vapour-liquid-solid method, Nanoscale 9, 8113-8118 (2017). 4. F. Glas, J.-C. Harmand and G. Patriarche, Why does wurtzite form in nanowires of III-V zinc blende semiconductors?, Phys. Rev. Lett. 99, 146101 (2007). 5. D. Jacobsson, F. Panciera, J. Tersoff, M. C. Reuter, S. Lehmann, S. Hofmann, K. A. Dick and F. M. Ross, Interface dynamics and crystal phase switching in GaAs nanowires, Nature 531, 317-322 (2016). 6. W. Wang, Plasma-enhanced CVD growth of cubic and hexagonal diamond silicon nanowires with liquid-solid mixed catalysts for photovoltaic applications, PhD thesis, Institut Polytechnique de Paris, 2021. 7. É. Ngo, In situ growth of silicon and germanium nanowires in the metastable hexagonal-diamond phase, PhD thesis, Institut Polytechnique de Paris, 2021.

12:30 Q&A live session / Break    
Session : tba
Authors : Federico Panciera 1, Zhaslan Baraissov 2, Gilles Patriarche 1, Laetitia Vincent 1, Charles Renard 1, Vladimir G. Dubrovskii 3, Frank Glas 1, Laurent Travers 1, Utkur Mirsaidov 2, Jean-Christophe Harmand 1
Affiliations : 1. Université Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies, 91120, Palaiseau, France. 2. Centre for BioImaging Sciences, Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, 117557, Singapore 3. ITMO University, Kronverkskiy pr. 49, 197101 St. Petersburg, Russia

Resume : Synthetizing III-V semiconductor nanowires (NWs) using the vapor-liquid-solid (VLS) method can result in the growth of crystal structures different from their bulk phase. In GaAs NWs, for example, the stable phase zinc-blende (ZB) coexists with the metastable wurtzite (WZ) structure. Remarkably, the valence and conduction bands are misaligned in the two phases, so that small sections of one phase within the other effectively confine charge carriers along the wire axis. Therefore, the possibility of controllably switching between phases opens up opportunities to create novel heterostructures commonly identified as crystal phase quantum dots (CPQDs). In contrast to compositional heterojunctions, crystal phase heterostructures have the intrinsic property of minimizing residual strain and alloy intermixing at interfaces. Here, we present experimental observations of the growth of self-catalyzed GaAs nanowires using a modified environmental transmission electron microscope (ETEM) equipped with molecular-beam-epitaxy (MBE) and chemical-vapor-deposition (CVD) sources. NWs are grown directly inside the microscope, and their growth is monitored in situ and in real-time with high spatial and temporal resolution. First, we will show that by changing the contact angle of the catalyst droplet, we can control the switching between crystal phases. We will then present the main differences between the phase control during growth by MBE and CVD. In the light of these results, we will discuss the existing models, and we will propose a new model to describe the phase selection in III-V nanowires. Finally, we will present new strategies to synthesize CPQDs with unprecedented accuracy.

Authors : Fadaly, E.M.T.*(1), Marzegalli, A. (2), Ren, Y. (1), Sun, L. (3), Dijkstra, A. (1), de Matteis, D. (4), Scalise, E. (2), Sarikov, A. (2), De Luca, M. (4), Rurali, R. (5), Zardo, I. (4), Haverkort, J.E.M. (1), Botti, S. (3), Miglio, L., Bakkers, E.P.A.M. (1), Verheijen, M.A.(1)
Affiliations : (1) Eindhoven University of Technology, the Netherlands (2) Università di Milano-Bicocca, Italy (3) Friedrich-Schiller-Universität Jena, Germany (4) Universität Basel, Switzerland (5) Institut de Ciència de Materials de Barcelona, Spain * lead presenter

Resume : Recently synthesized hexagonal group IV materials are a promising platform to realize efficient light emission that can be closely integrated with electronics. A high crystal quality is essential to assess the intrinsic electronic and optical properties of these materials unaffected by structural defects. Here, we report on a new type of partial planar stacking fault and its termination in hexagonal (Hex) group IV semiconductors. To comprehensively characterize the nature of this defect, the crystal structure of the defect has been extensively studied in both materials systems (Hex- Si and Ge) with Transmission Electron Microscopy (TEM) supported by atomistic modelling to reconstruct and visualize the planar stacking fault and its terminating dislocations in the crystal. We also present ab initio density functional theory (DFT) calculations to supplement the TEM findings and study the influence of this type of defect on the electronic and optical properties of this material system. In addition, photoluminescence (PL) measurements of Hex-Ge crystals have been performed to verify the ab initio DFT calculations and experimentally examine the influence of the defects on the optical properties. From this study, we conclude that this defect is a partial I3 basal stacking fault (BSF) with no mid-gap states associated with it. This clearly leads to no change in the electronic properties of Hex-Ge rendering the I3 defect -the dominant defect in our Hex-group IV nanocrystals- not being detrimental to their electronic and optical properties. Finally, shedding light on the structural and optical properties of this novel defect is of great interest to the community of hexagonal materials such as-III-Ns, in which this defect is also present.

Authors : Dursap, T.*(1), Dudko, I.(1)(2), Vettori, M.(1), Botella, C.(1), Regreny, P.(1), Blanchard, N.(3), Gendry, M.(1), Chauvin, N.(4), Walia, S.(2), Bugnet, M.(5), Danescu, A.(1), Penuelas, J.(1)
Affiliations : (1) Univ Lyon, CNRS, INSA Lyon, ECL, UCBL, CPE Lyon, INL, UMR 5270, 69130 Ecully, France ; (2) Functional Materials and Microsystems Research Group and the Micro Nano Research Facility, RMIT University, Melbourne 3000, Australia ; (3) Univ Lyon, CNRS, UCBL, ILM, 69622 Villeurbanne, France ; (4) Univ Lyon, INSA Lyon, ECL, CNRS, UCBL, CPE Lyon, INL, UMR 5270, 69621 Villeurbanne, France ; (5) Univ Lyon, CNRS, INSA Lyon, UCBL, MATEIS, UMR 5510, 69621 Villeurbanne, France

Resume : III-V semiconductor nanowires (NWs) obtained by the vapor-liquid-solid (VLS) mechanism exhibit a zinc-blende (ZB) or a wurtzite (WZ) structure [1] depending on the growth conditions, and more particularly on the amount of III and V element fluxes [2]. Controlling precisely the growth of the crystal phases in self-assisted GaAs NWs by molecular beam epitaxy (MBE) would be an important achievement for device applications [3]. Nevertheless, the optimized growth of WZ segments in NW geometry is still in its infancy, and major achievements have been reported only very recently [4-7]. In this work, we used an analytical growth model to show the existence of a stationary regime allowing the formation of a pure WZ phase, by a precise tuning of the V/III ratio. A slight change of the As flux indeed induces a modification of the catalyst droplet wetting angle in a desired range of values. The combination of an in situ reflection high energy electron diffraction (RHEED) evolution with high-resolution scanning transmission electron microscopy (STEM) and dark field TEM, as well as photoluminescence analysis confirmed the successful control of a micron-long pure WZ segment, obtained by MBE growth of self-assisted GaAs NWs [8]. 1 F. Glas, et al, Physical Review Letters, 99 (2007), 146101 2 F. Panciera, et al, Nano Lett., 20 (2020), 1669-1675 3 E. M. T. Fadaly, et al, Nature, 580 (2020), 205-209 4 T. Dursap, et al, Nanoscale Adv., 2 (2020), 2127-2134 5 M. M. Jansen, et al, ACS Appl. Nano Mater., 3, 11 (2020), 11037-11047 6 T. Dursap, et al, Nanotechnology, 32 (2021), 155602 7 V. G. Dubrovskii, et al, Nano Lett., 21 (2021), 3139-3145 8 This work was done as part of the ANR BEEP project (ANR-18-CE05-0017) and received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 801512.

Authors : C. Renard*, D. Bouchier, F. Panciera, L. Vincent
Affiliations : Centre de Nanosciences et Nanotechnologies, UMR 9001, CNRS-Université Paris Saclay, 91128, Palaiseau, France

Resume : NANOMAX TEM facility has been developed with the aim of implementing original experiments to study in situ the crystal growth mechanisms of nanoobjects. The growth can be observed in real time at the atomic scale using an environmental electron microscope equipped with an image aberration corrector. The microscope can be implemented either with molecular beam epitaxy (MBE) sources or with a gas injector for chemical vapor deposition (CVD). For this last case, numerous precursors as hydride and metalorganic have been installed to enable the growth of various semiconductor heterostructures. Here we present the growth of GaAs/Ge core/shell structures obtained with CVD. Our goal is to synthesize the hexagonal crystal phase Ge-2H which offer a direct band-gap optical transition. GaAs nanowires are used as template to epitaxially transfer the wurtzite (W) structure to the Ge shell. The GaAs-W and Ge-2H structures take advantage to present almost the same lattice constants. VLS (vapor-liquid-solid) Au catalyzed GaAs nanowires are grown directly in the microscope at 400-500°C using TMGa and TBAs precursors. By changing the III/V flux ratio, we manage to monitor the wurtzite/zinc-blende polytytpism transition in the GaAs nanowire, and we are able to grow portion of pure wurtzite phase (i.e without stacking faults formation) of several tens of nanometers length. Then, simultaneously to the shutdown of Ga and As precursors, Ge2H6 is injected to start the lateral Ge overgrowth. We evidence the different epitaxial growth modes of Ge-2H on GaAs-W. Step-flow growth is favourable for defect free Ge-2H. Depending on growth conditions the formation of intrinsic stacking faults of I3-type is observed randomly during growth. We discuss the correlation of these faults with growth modes related to surface diffusion and propose a model of formation. In addition, the VLS and VSS growths of Au catalysed Ge-2H branches are observed on the sidewalls of the GaAs-w nanowires with the <1-100> direction. The structure of those branches is presented and discussed.

Authors : Vladimir G. Dubrovskii, Federico Panciera, Zhaslan Baraissov, Gilles Patriarche, , Frank Glas, Laurent Travers, Utkur Mirsaidov, and Jean-Christophe Harmand
Affiliations : Faculty of Physics, St. Petersburg State University, Universitetskaya Embankment 13B, 199034, St. Petersburg, Russia Université Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies (C2N), 91120 Palaiseau, France Centre for BioImaging Sciences, Department of Biological Sciences and Centre for Advanced 2D Materials and Department of Physics, National University of Singapore

Resume : Recent experimental and theoretical studies have revealed the droplet contact angle as the key parameter controlling the crystal phase of vapor-liquid-solid GaAs and other III-V nanowires, which can be either cubic zincblende or hexagonal wurtzite. In this talk, I will present an energetic model [V. G. Dubrovskii, Cryst. Growth Des. 2017, 17, 2544] which explains the importance of the contact angle and allows one to plot out the crystal phase diagrams versus the contact angle for different III-V materials. Further, I will review in situ data on the growth monitoring of Ga-catalyzed GaAs nanowires obtained in NanoMax TEM [F. Panciera et al., Nano Lett. 2020, 20, 1669]. It will be shown that GaAs nanowires form in the wurtzite phase within an intermediate range of contact angles (from 100 to 125 degrees), with vertical sidewalls and planar growth interface. Zincblende GaAs nanowires form at smaller or larger contact angles, with either tapered sidewalls (for smaller contact angles) or with truncated growth interface under the droplet (for larger contact angles). I will also discuss how the picture changes for GaP nanowires, where the wurtzite range is narrower compared to GaAs. The contact angle can easily be changed by tuning the V/III flux ratio in molecular beam epitaxy, which provides a simple way for controlling the crystal phase or forming crystal phase quantum dots in self-catalyzed or Au-catalyzed III-V nanowires.

15:40 Q&A live session    
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Session : tba
Authors : Claes Thelander
Affiliations : Solid State Physics, Lund University P.O. Box 118 S-221 00 Lund Sweden

Resume : In this presentation, I describe how control of polytypism during nanowire growth provides a path to create strongly confined structures with tunable symmetries, where new types of artificial molecules appear. During epitaxial growth of InAs nanowires, we introduce segments of wurtzite (WZ) structure within a zinc-blende (ZB) background to form local energy barriers for electrons. Structures with pairs of closely spaced WZ segments result in ZB quantum dots (QDs) with strong quantum confinement, where the WZ act as tunnel barriers in electrical characterization. Compared to chemical heterostructures, the band offset is relatively shallow and the barriers therefore need to be thick (~20 nm). The QDs can be approximated to thin discs, having the same diameter as the nanowire, but with a thickness of only a few nms. We have noticed that they are prone to split into two or more QDs, which is a process that can be controlled using nearby gate-electrodes. Owing to the strong confinement, excited state energies can be tracked over several meVs. Electrons accumulate at the surface of InAs materials, and we correspondingly find strong evidence for quantum ring formation in the disc-shaped QDs. In the case of a double-QD (DQD), this can lead to a unique symmetry, where the DQD is connected in two points to form a ring. Such an artificial molecule behave very differently from its typical counterpart which connects only in one point, and where hybridization leads to a stable bond ground state. Depending on the parities of the hybridizing orbitals, we find that having two coupling points can completely change the bond nature. In particular, when combining an odd and even orbital, the hybridization energy vanishes, which we attribute to the overlap integrals having different signs at the two connection points. The result is four nearly degenerate states (spin orbit), with a giant orbital contribution to the g-factor. However, we find that this contribution can be quenched by simply localizing the electron into one QD using a small electric field. Depending on the sign of the spin-orbit interaction, this also allows to electrostatically manipulate the ground state spin. In an even-electron regime, a standard DQD would correspondingly have a singlet ground state. In the DQD rings however, we find that when each QD has an unpaired spin, and the total electron number is 4n, the ground state is spin-triplet. By making one bond weaker, we show how the conventional singlet ground state can be recovered. Finally, I will discuss ongoing projects where we take advantage of such engineered spin states. By strongly coupling QD-rings to superconductors, we explore how a supercurrent is affected when changing the even-electron spin ground state. We also aim to create a spin filter embedded in a Cooper-pair splitter geometry, such that spin correlations of entangled electrons can be probed.

Authors : Philipp Staudinger, Noelia Vico Triviño, Kirsten E. Moselund, and Heinz Schmid
Affiliations : IBM Research Europe, Zurich, Säumerstrasse 4, 8803 Rüschlikon, Switzerland

Resume : Selective area epitaxy has been widely used in various implementations for creating unique device structures, and more recently as versatile platform for heterointegration of wide bandgap (GaN) and high mobility III-V semiconductor on Si. SAG on Si can lead to reduced defect densities which originate from the mismatched material properties towards Si. Inspired by our previous work on heterointegration, we explore the potential of selective area growth using MOCVD as a path to obtain III-V semiconductors in their thermodynamic less stable (wurtzite) crystal configuration. While most literature focus on vertical nanowire structures, our current emphasis is placed on understanding the growth towards planar structures of metastable materials, motivated by the potential impact of wurtzite III-V and lonsdaleite SiGe for electro-optical device applications. Acknowledgement: We gratefully acknowledge funding received from EU H2020 program SiLAS (Grant Agreement No. 735008).

Authors : N. Izitounene1, N. D. Le1, B. Davier1, P. Dollfus1, L. Paulatto2, J. Saint-Martin1
Affiliations : 1. Université Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies, 91120, Palaiseau, France ; 2. Sorbonne Université/CNRS/MNHN/IRD UMR 7590 , Institut de minéralogie, de physique des matériaux et de cosmochimie

Resume : Interfaces can be used to design low thermal conductance nanostructures to achieve optimized thermoelectric performance. Polytype nanowires [1], [2] that can be fabricated using both silicon and germanium are particularly attractive for thermal engineering as they can exhibit a high polytype interface density. In this works, we theoretically study the thermal transport of phonons across polytype heterojunction such as Si 3C/Si 2H and Ge 3C/Ge 2H by using particle Monte Carlo simulation to solve the Boltzmann transport equation for phonons. It includes a full-band phonon dispersion and phonon-phonon scattering rates calculated by using the density-functional theory (DFT). In this work, phonon transmission across interfaces that are perpendicular to the heat flux has been implemented in our home made Full Band Monte Carlo simulator for phonons [3] by using a Full-band version of the Diffusive Mismatch Model (DMM). First, for homogenous 3C and 2H Si and Ge bars, the Knudsen number, commonly used to characterize the different transport regimes (diffusive, ballistic and intermediate) is investigated. This parameter appears strongly correlated to the spectral contributions of the thermal flux. Then, single and double polytype Si and Ge heterostructures are studied from the micrometer scale down to the nanometer scale. The variation of the interface thermal conductance as a function of the geometric dimension as well as the effects of the spectral distribution of the flux are investigated. This local indicator of the phonon transport regime quantifies the importance and the characteristic length of out-of-equilibrium transport regime around the interfaces. References: [1] F. J. Lopez, U. Givan, J. G. Connell, et L. J. Lauhon, « Silicon Nanowire Polytypes: Identification by Raman Spectroscopy, Generation Mechanism, and Misfit Strain in Homostructures », ACS Nano, vol. 5, no 11, p. 8958‑8966, nov. 2011, doi: 10.1021/nn2031337. [2] L. Vincent et al., « Novel Heterostructured Ge Nanowires Based on Polytype Transformation », Nano Letters, vol. 14, no 8, p. 4828‑4836, août 2014, doi: 10.1021/nl502049a. [3] B. Davier et al., « Heat transfer in rough nanofilms and nanowires using full band ab initio Monte Carlo simulation », J. Phys.: Condens. Matter, vol. 30, no 49, p. 495902, déc. 2018, doi: 10.1088/1361-648X/aaea4f. Acknowledgment: This work was supported by the French National Research Agency (ANR) as part of the “Investissements d’Avenir” program (Labex NanoSaclay, reference: ANR-10-LABX-0035).

Authors : Dudko, I*(1,2), Dursap, T.(1), Regreny, P.(1), Botella, C.(1), Lamirand, A. (1), Danescu, A.(1), Bugnet, M. (4) Chauvin, N.(3), Walia, S.(2) Penuelas, J.(1)
Affiliations : (1) Univ Lyon, CNRS, INSA Lyon, ECL, UCBL, CPE Lyon, INL, UMR 5270, 69130 Ecully, France (2) Functional Materials and Microsystems Research Group and the Micro Nano Research Facility, RMIT University, Melbourne 3000, Australia (3) Univ Lyon, INSA Lyon, ECL, CNRS, UCBL, CPE Lyon, INL, UMR 5270, 69621 Villeurbanne, France (4) Univ Lyon, CNRS, INSA Lyon, UCBL, MATEIS, UMR 5510, 69621 Villeurbanne, France * lead presenter

Resume : Obtaining light emission from group IV materials is a topic of high interest because of their abundance and their compatibility with the silicon photonics [1-2]. However they have indirect-bandgap due to their natural cubic form that prevents them to realize efficient light emission. It has been shown that Ge and Si materials in hexagonal phase have different characteristics in comparison to their cubic phase [3-4]. Recently hexagonal SiGe shells were grown on wurtzite GaAs NW template using MOCVD and gold catalyst [5]. In this contribution we demonstrate the possibility to use gold-free GaAs nanowires on silicon substrate for subsequent growth of hexagonal germanium. GaAs nanowires were grown in a molecular-beam epitaxy (MBE) reactor via self-assisted growth following the VLS method using Ga as a catalyst. An extended pure wurtzite segment was maintained by tuning the V/III ratio to appropriate value [6]. Following the growth of the GaAs core, Ge shell has been grown with different thicknesses. The growth has been continuously monitored by in situ reflection high energy electron diffraction (RHEED). High-resolution scanning transmission electron microscopy (STEM) and dark field transmission electron microscope (TEM) confirmed the long extended hexagonal phase of Ge. The Ge surface chemistry was studied by X-ray photoelectron spectroscopy (XPS). 1 Z. Wang, et al, Appl. Phys. Lett, 118 (2021), 172101. 2 C. Rödl, et al, Phys. Rev. Materials, 3 (2019), 034602. 3 X. Cartoixà, et al, Nano Letters, 17 (8) (2017), 4753-4758. 4 D. De Matteis, et al, ACS Nano, 14, 6, (2020), 6845–6856. 5 E. M. T. Fadaly, et al, Nature, 580 (2020), 205-209. 6 T. Dursap, et al, Nanotechnology, 32 (2021), 155602. 7 This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 801512

10:30 Q&A live session / Break    
Session : tba
Authors : Riccardo Rurali
Affiliations : Institut de Ciència de Materials de Barcelona (ICMAB−CSIC), Campus de Bellaterra, 08193 Bellaterra, Spain

Resume : Crystal structure and interface engineering are acquiring an increasing importance in nanoscience because of their enormous potential to conceive new properties and functionalities. In the case of nanowires (NWs), the emergence of new stable polytypes of common semiconductors promises to have an important impact in materials design. Driven by this promising evidence, we use first-principles methods based on density functional theory to carry out an extensive scrutiny of several common dopants revealing similarities, but also unexpected differences with their behavior in cubic and hexagonal polytypes of III-V NWs. We show that an important class of dopants, i.e. acceptors, are much more easily incorporated in hexagonal lattices and have a smaller associated transition energy. Roughly speaking, this means that it is easier to p-type dope a wurtzite crystal and the charge carrier concentration at a given temperature and doping dose is larger in the wurtzite as well [1]. As for donors, we show that neutral chalcogen impurities have no clear preference for a specific crystal phase, while charged chalcogen impurities favor substitution in the zincblende structure with a transition energy that is smaller when compared to the wurtzite case (thus, charge carriers are more easily thermally excited to the conduction band in the zincblende phase) [1]. Similarities with the case of cubic vs hexagonal Si NWs are discussed [2]. We also present discuss Si homojunctions in NWs, with emphasis on the role of the diameter on the band alignment at the interface [3], and the prediction of optical emission in hexagonal SiGe allow NWs [4], recently confirmed experimentally [5]. [1] G. Giorgi, M. Amato, S. Ossicini, X. Cartoixà, E. Canadell, and R. Rurali, J. Phys. Chem. C 124, 27203 (2020) [2] M. Amato, S. Ossicini, E. Canadell, and R. Rurali, Nano Lett. 19, 866-876 (2019) [3] M. Amato, T. Kaewmaraya, A. Zobelli, M. Palummo, and R. Rurali, Nano Lett. 16, 5694 (2016) [4] X. Cartoixà, M. Palummo, H. I. T. Hauge, E. P. A. M. Bakkers, and R. Rurali, Nano Lett. 17, 4753 (2017) [5] E. M. T. Fadaly et al., Nature 580, 205 (2020)

Authors : Vettori, M.*(1), Fadaly, E.M.T.(1), Peeters, W.H.J.(1), Danescu, A.(2), Verheijen, M.A.(1), & Bakkers, E.P.A.M.(1).
Affiliations : (1) Eindhoven University of Technology, The Netherlands (2) Lyon Institute of Nanotechnology – Ecole Centrale de Lyon, France

Resume : GaAs nanowires (NWs) show crystal structures precluded in the bulk, with potential benefits to the development of applied nanotechnologies. For example, wurtzite (WZ) GaAs NWs are indispensable templates for photonic devices based on light-emitting hexagonal SiGe. For this purpose, the nanowires need to be long (>6 µm) and defect-free. Despite all progress made in recent years in the growth of NWs, for WZ GaAs these demands cannot be fulfilled and mechanistic insights into the limitations are missing. Supported by Transmission Electron Microscopy (TEM) analysis of the experimental results, we demonstrate how using a proper analytical model allows to understand the growth process of GaAs NWs, permitting to explain their morphology and crystal nature. The model is particularly performing when applied to the Molecular Beam Epitaxy (MBE) growth. In this case, beside explaining the experimental data, it also opens to the possibility of modulating the growth by properly tuning the precursor fluxes, ultimately introducing a way to obtain highly pure single-polytype NWs. We extend this approach also to the Metal Organic Vapor Phase Epitaxy (MOVPE) growth: the analysis of the experimental results highlights the existence of a critical length for the formation of defect-free WZ above which defects start to appear, thus explaining why controlling polytypism in MOVPE is still so difficult. In conclusion, here we show how the model can help to gain better understanding of the growth process, and how it could allow to modulate the growth for possibly overcoming the current limitations in the control of the crystal structure.

Authors : Aymen Ben Amor, Doriane Djomani, Stefan Dilhaire, Laetitia Vincent and Stéphane Grauby.
Affiliations : Univ. Bordeaux, LOMA, CNRS UMR 5798, F-33400 Talence, France. C2N – CNRS, Univ. Paris-Sud, 91405 Orsay, France.

Resume : Nowadays an extraordinary control over the growth of nanowires (NWs) has been achieved, enabling also the integration of different types of heterostructures, which can lead to the engineering of the functional properties of the NWs. One of the many applications of NWs includes energy conversion. In this work, thermal characterization of Ge and Si allotrope heterostructured NWs is done using 3-SThM. These NWs are composed of successive hexagonal 2H and cubic diamond 3C crystal phase along the <111> axis and are embedded in a silica matrix. The thermal characterization of these NWs revealed a strong diameter dependent decrease in the thermal conductivity, which can be predominantly ascribed to boundary scattering. In addition, we have also studied the effect of the temperature of phase transformation, which influences the size and the number of 2H domains, on the NW thermal conductivity. We have deduced the NW thermal conductivity for different annealing temperatures and we have evidenced that this latter, can constitute an efficient parameter to tune the thermal conductivity. During the final presentation, I will present these two studies but also the influence of doping on the thermal conductivity. These results are the first experimental evidence of thermal conductivity reduction in such allotrope 2H/3C heterostructured NWs.

Authors : Sebastian Lehmann
Affiliations : Solid State Physics and NanoLund, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden

Resume : Au-seeded III-V nanowire growth offers the advantage of tuning the crystal phase of conventional III-V materials. Both zinc blende (Zb) and wurtzite (Wz) can now be achieved relatively easy by addressing fundamental growth parameters such as temperature, nominal V/III-ratio of incoming precursors, particle diameter, or external doping. Single crystal phase III-V nanowires are grown following these approaches but engineering the nanowires to deliberately switch the crystal structure of alternating segments on demand requires a high level of control and it is still a challenge to design advanced heterostructures such as crystal phase quantum dots [1–4]. Although not the only available approach, we have chosen to vary the group V flow of the incoming precursor in order to engineer the crystal structure of the resulting nanowire with a high V/III-ratio usually leading to Zb while a low V/III-ratio is required to reach Wz growth conditions. Together with the increased knowledge on the growth process [5–7] it is possible to design crystal phase heterostructures on demand using this strategy but still a couple of practical challenges remain. To name a few, competing radial growth, varying collecting area of material for growth, and technical details of the low pressure metal organic vapour phase epitaxy (MOVPE) system have to be carefully considered and counter balanced in order to reach the desired crystal phase heterostructure. An active research field is the investigation of such, designed crystal phase heterostructures in single III-V nanowires among which e.g. optical and electrical studies provide very interesting insights. Apart from investigating the fundamental properties of Wz and Zb III-V nanowires it is also possible to exploit differences such as in the surface energies during the formation of core-shell, compositional heterostructures. Since equivalent Wz facets usually adopt lower surface energies than their Zb counterparts [8], radial overgrowth can be tuned to occur with different ratios between the Wz and Zb facets. In view of a more fundamental investigation one can look at the deposition of elemental Sb on both Zb {110} and Wz {11-20} facets side by side [9]. Here, a preferred alloying into the Zb facet compared to the Wz facet was observed which supports the concept that similar surfaces adopt lower surface energies in the Wz structure compared to the Zb counterparts. Another example of an almost complete suppression of overgrowth is e.g. the GaSb shell growth on crystal structure tuned InAs core nanowires [10] or the case of InAs growth on GaAs nanowires [11]. For the case of InAs overgrowth of GaAs core nanowires, we have found that it is possible to switch between selective and non-selective InAs growth modes on Wz and Zb GaAs segments, respectively. In addition we have tuned the facet termination of the core GaAs nanowire from predominantly Zb-type {110} facets to predominantly Zb-type {112}B facets. As a result of changing the core GaAs nanowire properties, the InAs overgrowth can be controlled to being selective/non-selective not only to the crystal phase but also to the facet termination. This allows for the design of a large variety of InAs architectures on the engineered GaAs core nanowires. While the knowledge and experimental advancement in controlling the formation of crystal phase heterostructures has made tremendous steps forward, a need for adequate sample/nanowire characterization also exists. Specifically non-destructive methods to characterize single nanowire devices, which are set up in architectures not suitable for transmission electron microscopy (TEM), the standard method for investigating the structure-property relation, are required. Here we present an alternative to TEM which is carried out in a scanning electron microscope (SEM). The SEM offers an option named electron channelling contrast imaging (ECCI) [12,13]. This method is an excellent tool to non-destructively characterize the crystal structure of e.g. nanowire devices bound to a non-electron transparent substrate/chip, however, a couple of requirements have to be fulfilled to successfully apply this method. The main working principle is a contrast formation in backscattered electron detection based on the difference of channelling along different crystallographic directions in the Wz and Zb crystal segments. Therefore, imaging has to be carried out in specific crystallographic orientations, which is practically achieved by tilting the nanowires with respect to the incident electron beam. However, in order to know where to tilt, one has to take electron backscatter diffraction (EBSD) patterns, which is rather challenging due to multiple reasons. The other option is to take the known growth direction and facets formed and, thus, use the morphology of the nanowire to determine the crystallographic direction needed for ECCI like in this study. We have investigated a variety of nanowires to verify and understand the ECCI method in more detail in addition to determining characteristic features such as the resolution limit. References: [1] S. Assali, L. Gagliano, D. S. Oliveira, M. A. Verheijen, S. R. Plissard, L. F. Feiner, and E. P. A. M. Bakkers, Exploring Crystal Phase Switching in GaP Nanowires, Nano Lett. 15, 8062 (2015). [2] N. Akopian, G. Patriarche, L. Liu, J.-C. Harmand, and V. Zwiller, Crystal Phase Quantum Dots, Nano Lett. 10, 1198 (2010). [3] N. Vainorius, S. Lehmann, D. Jacobsson, L. Samuelson, K. A. Dick, and M.-E. Pistol, Confinement in Thickness-Controlled GaAs Polytype Nanodots, Nano Lett. 15, 2652 (2015). [4] B. Loitsch, D. Rudolph, S. Morkötter, M. Döblinger, G. Grimaldi, L. Hanschke, S. Matich, E. Parzinger, U. Wurstbauer, G. Abstreiter, J. J. Finley, and G. Koblmüller, Tunable Quantum Confinement in Ultrathin, Optically Active Semiconductor Nanowires Via Reverse-Reaction Growth, Adv. Mater. 27, 2195 (2015). [5] V. G. Dubrovskii, Development of Growth Theory for Vapor–Liquid–Solid Nanowires: Contact Angle, Truncated Facets, and Crystal Phase, Crystal Growth & Design 17, 2544 (2017). [6] P. Krogstrup, H. I. Jørgensen, E. Johnson, M. H. Madsen, C. B. Sørensen, A. F. i Morral, M. Aagesen, J. Nygård, and F. Glas, Advances in the Theory of III–V Nanowire Growth Dynamics, J. Phys. D: Appl. Phys. 46, 313001 (2013). [7] E. K. Mårtensson, S. Lehmann, K. A. Dick, and J. Johansson, Simulation of GaAs Nanowire Growth and Crystal Structure, Nano Lett. (2019). [8] V. Pankoke, S. Sakong, and P. Kratzer, Role of Sidewall Diffusion in GaAs Nanowire Growth: A First-Principles Study, Phys. Rev. B 86, 085425 (2012). [9] M. Hjort, P. Kratzer, S. Lehmann, S. J. Patel, K. A. Dick, C. J. Palmstrøm, R. Timm, and A. Mikkelsen, Crystal Structure Induced Preferential Surface Alloying of Sb on Wurtzite/Zinc Blende GaAs Nanowires, Nano Lett. 17, 3634 (2017). [10] L. Namazi, M. Nilsson, S. Lehmann, C. Thelander, and K. A. Dick, Selective GaSb Radial Growth on Crystal Phase Engineered InAs Nanowires, Nanoscale 7, 10472 (2015). [11] T. Rieger, T. Schäpers, D. Grützmacher, and M. I. Lepsa, Crystal Phase Selective Growth in GaAs/InAs Core–Shell Nanowires, Crystal Growth & Design 14, 1167 (2014). [12] S. Zaefferer, New Developments of Computer-Aided Crystallographic Analysis in Transmission Electron Microscopy, J Appl Cryst, J Appl Crystallogr 33, 10 (2000). [13] S. Zaefferer and N.-N. Elhami, Theory and Application of Electron Channelling Contrast Imaging under Controlled Diffraction Conditions, Acta Materialia 75, 20 (2014).

12:30 Q&A live session / Break    
Session : tba
Authors : Y. Le Godec, S. Pandolfi, C. Renero-Lecuna, W. Crichton and Alexandre Courac
Affiliations : Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Sorbonne Université, UMR CNRS 7590, Muséum National d’Histoire Naturelle, IRD UMR 206, 4 Place Jussieu, 75005, Paris, France

Resume : Silicon is the second abundant element, after oxygen, in the Earth crust. It is essential for today’s electronics because of its ability to show various electronic behaviors that allow covering the numerous fields of cutting-edge applications. Moreover, silicon is not a pollutant and, therefore, is an ideal candidate to replace the actual materials in photovoltaics, like compounds based on the arsenic and heavy metals. It has not replaced them so far because Silicon is an indirect gap semiconductor and cannot absorb directly the solar photons without thermal agitations of crystal lattice (phonons). In this talk, we will show that the use of very high pressures and temperatures combined with the in situ probe by X-ray diffraction with synchrotron radiation is the methodological key to stabilize new crystal configuration structures and to control their nanostructure. I will give many examples from our recent studies, in particular the synthesis of Si-III [1] and nanostructured hexagonal 4H Polytype [2]. References : [1] Inorganic Chemistry, vol. 55, no 17, p. 8943-8950 (2016). [2] Nano Letters, 2018, 18 (9), pp.5989 - 5995.

Authors : Alexandre Courac (Kurakevych)
Affiliations : IMPMC - Sorbonne university

Resume : Dimond-polytype crystal structures are common for carbon, boron nitride BN and silicon. All together they cover a large domain of bandgaps (from 30 meV for Si-III with BC8 structure up to ~5 eV for diamond). Polytypism, often together with intrinsic nanostructuring, allows enhancement of typical semiconductive functional properties (bandgap, photoluminescence, electrical conductivity). We will consider synthesis and properties of numerous examples of recent high-pressure materials with crystal structures that are polytypes of archetypical structures of diamond (C, BN, Si [1-3]), graphite (C, BN [1]) and BC8 (Si [4,5]). [1] O.O. Kurakevych, V.L. Solozhenko. High-pressure design of advanced BN-based materials. Molecules 21[10], 1399 (2016) [2] S. Pandolfi, C. Renero-Lecuna, Yann Le Godec, B. Baptiste, N. Menguy, M. Lazzeri, C. Gervais, K. Spektor, W. A. Crichton, O. O. Kurakevych. Nature of Hexagonal Silicon Forming via High-Pressure Synthesis: Nanostructured Hexagonal 4-H Polytype. Nano Letters 18[9] 5989-5995 (2018). [3] A. Mukhanov, A. Courac, V.L. Solozhenko. Lattice Parameter and Properties of Cubic Boron Nitride. J. Superhard Mater. 42[6] 377-387 (2020). [4] O.O. Kurakevych, Y. Le Godec, W.A. Crichton, J. Guignard, T.A. Strobel, H. Zhang, H. Liu, C. Coelho Diogo, A. Polian, N. Menguy, S.J. Juhl, C. Gervais. Synthesis of Bulk BC8 Silicon Allotrope by Direct Transformation and Reduced-Pressure Chemical Pathways. Inorg. Chem. 55[17] 8943–8950 (2016). [5] H. Zhang, H. Liu, K. Wei, O.O. Kurakevych, Y. Le Godec, Z. Liu, G.S. Nolas, T.A. Strobel. BC8 Silicon (Si-III) is a narrow-gap semiconductor. Phys. Rev. Lett. 118[14] 146601 (2017).

Authors : E. Scalise, A. Sarikov, L. Barbisan, A. Marzegalli, D. B. Migas, F. Montalenti and L. Miglio
Affiliations : L-NESS and Department of Materials Science, Università degli Studi di Milano-Bicocca, via Cozzi 55, 20125, Milano, Italy V. Lashkarev Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 45 Nauki avenue, 03028, Kyiv, Ukraine L-NESS and Department of Materials Science, Università degli Studi di Milano-Bicocca, via Cozzi 55, 20125, Milano, Italy L-NESS and Department of Physics, Politecnico di Milano, via Anzani 42, 22100, Como, Italy Department of micro- and nanoelectronics, Belarusian State University of Informatics and Radioelectronics, P. Browka 6, 220013, Minsk, Belarus and National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe shosse 31, 115409 Moscow, Russia L-NESS and Department of Materials Science, Università degli Studi di Milano-Bicocca, via Cozzi 55, 20125, Milano, Italy L-NESS and Department of Materials Science, Università degli Studi di Milano-Bicocca, via Cozzi 55, 20125, Milano, Italy

Resume : By exploiting a synergic approach based on first-principles calculations and a state-of-the-art interatomic potential, we evidence a surface-related driving force in the formation of Si and Ge nanowires with the hexagonal diamond crystal structure. Indeed, the surface energy of the typical facets exposed in Si and Ge nanowires is lower for the hexagonal-diamond polytype than for the cubic one. Then, we focus on the stability of Si and Ge core-shell nanowires with hexagonal cores (made of GaP for Si nanowires, of GaAs for Ge nanowires). In this case, the stability of the hexagonal shell over the cubic one is also favoured by the energy cost associated with the interface that should form between the two phases. Interestingly, our calculations indicate a critical radius of the hexagonal shell much lower than the one reported in recent experiments, indicating the presence of a large kinetic barrier allowing for the enlargement of the wire in a metastable phase. Finally, we present a systematic study of the hexagonal polytype of the SiGe alloy, evidencing the effectiveness of the virtual crystal approximation to predict the variation of its electronic properties with the alloy composition.

Authors : Xavier Cartoixà
Affiliations : Departament d'Enginyeria Electrònica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain

Resume : The recently opened possibility to control the phase of a synthesized material during fabrication, including the combination of different phases [1-3], has enabled a new degree of freedom to tune material properties. In group IV semiconductors, the phases that can be regularly reached at ambient pressure are diamond (cubic) and lonsdaleite (hexagonal). Analogously, for III-Vs and II-VIs the zincblende and wurtzite phases can be combined into crystal phase nanostructures (barriers, superlattices, etc.). In transition metal dichalcogenides (TMDs), the phases that have been more commonly fabricated and combined are the semiconducting 2H together with the (semi)metallic 1T or its distortion, the 1T? [4,5]. In this talk I will focus on the consequences for lattice dynamics (i.e. Raman spectra and phonon transport) of material polytypism and the combination of different phases into nanostructures. It is well known that the lower symmetry of the hexagonal vs the cubic phases gives rise to extra peaks in Raman spectra. But the different structure also translates into different phonon-phonon interaction, which affects the peak shape and temperature dependence. Also, the advent of software capable of predicting the thermal conductivity from first-principles gives us unprecedented access into the relative importance of the factors that determine the value of the lattice thermal conductivity; namely, the phase space and the strength of the ph-ph interaction matrix element [6]. I will also discuss the phonon transport properties in ZB-WZ interfaces, relating them to the thermal boundary resistance. Finally, I will discuss specifics of phonon transport in 2D materials and 2H-1T homostructures. [1] Koguchi, M.; Kakibayashi, H.; Yazawa, M.; Hiruma, K.; Katsuyama, T.; Jpn. J. Appl. Phys., Part 1 1992, 31, 2061. [2] P. Caroff, K. A. Dick, J. Johansson, M. E. Messing, K. Deppert and L. Samuelson, Nat. Nanotechnol., 2009, 4, 50?55. [3] K. A. Dick, C. Thelander, L. Samuelson and P. Caroff, Nano Lett., 2010, 10, 3494?3499. [4] Eda G, Fujita T, Yamaguchi H, Voiry D, Chen M and Chhowalla M 2012 ACS Nano 6 7311?17. [5] Voiry D, Mohite A and Chhowalla M 2015 Chem. Soc. Rev. 44 2702?12. [6] Raya-Moreno, M.; Rurali, R.; Cartoixà , X. Phys. Rev. Mater. 2019, 3, 084607. [7] Carrete, J.; López-Suárez, M.; Raya-Moreno, M.; Bochkarev, A. S.; Royo, M.; Madsen, G. K. H.; Cartoixà, X.; Mingo, N.; Rurali, R.; Nanoscale 2019, 11, 16007?16016.

15:30 Q&A live session / Closing Remarks    

No abstract for this day

No abstract for this day

Symposium organizers
Claudia FASOLATOUniversità degli Studi di Perugia

Dipartimento di fisica e geologia - FIS/01 – Fisica Sperimentale - P.zza Università - 06123 Perugia, Italy
Ilaria ZARDOUniversity of Basel

Department of Physics, Klingelbergstrasse 82, 4056 Basel, Switzerland
Laetitia VINCENTCentre de Nanosciences et de Nanotechnologies (C2N) / CNRS

10 Boulevard Thomas Gobert - 91120 Palaiseau, France
Michele AMATOLaboratoire de Physique des solides (LPS), Université Paris Saclay

1 rue Nicolas Appert, Bâtiment 510, 91405 Orsay Cedex, France