Polytypism in semiconductors

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)

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.

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.

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.

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.

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).

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.

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).

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.

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.

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.

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.

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.

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.