With a band gap value of 1.7 eV, Al 0.2 Ga 0.8 As is one of the ideal III-V alloys for the development of nanowire-based Tandem Solar Cells on silicon. Nevertheless, growing self-catalysed AlGaAs nanowires on silicon by solid-source molecular beam epitaxy is a very difficult task due to the oxidation of Al adatoms by the SiO 2 layer present on the surface. Here we propose a nanowire structure including a p.i.n radial junction inside an Al 0.2 Ga 0.8 As shell grown on a p-GaAs core. The crystalline structure of such self-catalysed nanowires grown on an epi-ready Si(111) substrate (with a thin native SiO 2 layer) was investigated by transmission electronic microscopy and photoluminescence. I(V ) measurements performed on single nanowires have shown a diode-like behaviour corresponding to the radial p.i.n junction inside the Al 0.2 Ga 0.8 As shell. Moreover, a current generation under the electron beam was evidenced over the entire radial junction along the nanowires by means of electron beam induced current (EBIC) microscopy. The same structure was reproduced on patterned substrates with a SiO 2 mask, producing an ordered hexagonal array. High and uniform yields from 83% to 87% of vertical nanowires were obtained on 0.9×0.9 cm 2 patterned areas. EBIC mapping performed on these nanowires confirmed the good electrical properties of the radial junction within the nanowires.

It is well known that the crystalline structure of the III-V nanowires (NWs) is mainly controlled by the wetting contact angle of the catalyst droplet which can be tuned by the III and V flux. In this work we present a method to control the wurtzite (WZ) or zinc-blende (ZB) structure in self-catalyzed GaAs NWs grown by molecular beam epitaxy, using in situ reflection high energy electron diffraction (RHEED) diagram analysis. Since the diffraction patterns of the ZB and WZ structures differ according to the azimuth [11 ̅ 0], it is possible to follow the evolution of the intensity of specific ZB and WZ diffraction spots during the NW growth as a function of the growth parameters such as the Ga flux. By analyzing the evolution of the WZ and ZB spot intensities during some NW growths with specific changes of Ga flux, it is then possible to control the crystal structure of the NWs. ZB GaAs NWs with a controlled WZ segment have thus been realized. Using a semi-empirical model for the NW growth and our in situ RHEED measurements, the critical wetting angle of the catalyst droplet for the structural transition is deduced.

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In this work we show that the incidence angle of group-III elements fluxes plays a significant role on the diffusion-controlled growth of III-V nanowires (NWs) by molecular beam epitaxy (MBE). We present a thorough experimental study on the self-assisted growth of GaAs NWs by using a MBE reactor equipped with two Ga cells located at different incidence angles with respect to the surface normal of the substrate, so as to ascertain the impact of such a parameter on the NW growth kinetics. The as-obtained results show a dramatic influence of the Ga flux incidence angle on the NW length and diameter, as well as on the shape and size of the Ga droplets acting as catalysts. In order to interpret the results we developed a semi-empirical analytic model inspired by those already developed for MBE-grown Au-catalyzed GaAs NWs. Numerical simulations performed with the model allow to reproduce thoroughly the experimental results (in terms of NW length and diameter and of droplet size and wetting angle), putting 1 arXiv:1907.03226v1 [cond-mat.mes-hall] 7 Jul 2019 in evidence that under formally the same experimental conditions the incidence angle of the Ga flux is a key parameter which can drastically affect the growth kinetics of the NWs grown by MBE. IntroductionGaAs nanowires (NWs) are one of the most promising materials for the integration of III-V semiconductors on Si, since they can be grown by molecular beam epitaxy (MBE) on Si substrates via self-assisted vapor-liquid-solid (VLS) mechanism 1-8 preventing the use of Au catalyst which would jeopardize the electronic and optoelectronic properties of these semiconductors, forming deep-level states in both of them. 9-14 When it comes to MBE, both Au-catalyzed and self-assisted growths of NWs are diffusion-controlled processes. Many theoretical and experimental studies were carried out to understand the growth mechanisms and to identify the parameters influencing the NW structure and the growth kinetics. 2,3,7,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] First works have shown that the catalyst droplet volume or shape 15,16,18,27,31 and, more recently, that the droplet wetting angle 20,22,24 do control the NW crystal structure through the location of the nucleation site. Moreover, the volume of the catalyst droplet controls the kinetics of the axial growth through the related capture surface, 25,26,28 and in particular, through the capture area for As in the case of self-assisted GaAs NWs. 18,22 Concerning the NW growth kinetics, the growth models developed so far take into account the Ga flux incidence angle 17,18,22,23,25,26,28,30 but do not demonstrate its influence. In particular, the model of Glas et al 17 for self-assisted GaAs NWs was based on the assumption that the Ga flux adopted is always high enough to supply the Ga droplet, therefore neglecting to consider the influence of the incidence angle of the Ga flux on the amount of

The accurate control of the crystal phase in III–V semiconductor nanowires (NWs) is an important milestone for device applications. Although cubic zinc-blende (ZB) GaAs is a well-established material in microelectronics, the controlled growth of hexagonal wurtzite (WZ) GaAs has thus far not been achieved successfully. Specifically, the prospect of growing defect-free and gold catalyst-free wurtzite GaAs would pave the way towards integration on silicon substrate and new device applications. In this article, we present a method to select and maintain the WZ crystal phase in self-assisted NWs by molecular beam epitaxy. By choosing a specific regime where the NW growth process is a self-regulated system, the main experimental parameter to select the ZB or WZ phase is the V/III flux ratio. Using an analytical growth model, we show that the V/III flux ratio can be finely tuned by changing the As flux, thus driving the system toward a stationary regime where the wetting angle of the Ga droplet can be maintained in the range of values allowing the formation of pure WZ phase. The analysis of the in situ reflection high energy electron diffraction evolution, combined with high-resolution scanning transmission electron microscopy (TEM), dark field TEM, and photoluminescence all confirm the control of an extended pure WZ segment, more than a micrometer long, obtained by molecular beam epitaxy growth of self- assisted GaAs NWs with a V/III flux ratio of 4.0. This successful controlled growth of WZ GaAs suggests potential benefits for electronics and opto-electronics applications.

Electron Beam Induced Current (EBIC) analyses of single NWs have validated the formation of a homogeneous radial p–n junction over the entire length of the NWs.

The hexagonal‐2H crystal phase of Ge has recently emerged as a promising direct bandgap semiconductor in the mid‐infrared range providing new prospects of additional opto‐electronic functionalities of group‐IV semiconductors (Ge and SiGe). The controlled synthesis of such hexagonal‐2H Ge phase is a challenge that can be overcome by using wurtzite GaAs nanowires as a template. However, depending on growth conditions, unusual basal stacking faults (BSFs) of I3‐type are formed in the metastable 2H structure. The growth of such core/shell heterostructures is observed in situ and in real time by means of environmental transmission electron microscopy using chemical vapor deposition. The observations provide the first direct evidence of a step‐flow growth of Ge‐2H epilayers and reveal the growth‐related formation of I3‐BSFs during unstable growth. Their formation conditions are dynamically investigated. Through these in situ observations, a scenario can be proposed for the nucleation of I3‐type BSFs that is likely valid for any metastable hexagonal 2H or wurtzite structures grown on m‐plane substrates. Conditions are identified to avoid their formation for perfect crystalline synthesis of SiGe‐2H.

Platelets are central elements of hemostasis and also play a pivotal role in the pathogenesis of thrombosis in coronavirus disease 2019. This study was planned to investigate the effects of different severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) recombinant spike protein variants on platelet morphology and activation. Citrated whole blood collected from ostensibly healthy subjects was challenged with saline (control sample) and with 2 and 20 ng/mL final concentration of SARS-CoV-2 recombinant spike protein of Ancestral, Alpha, Delta, and Omicron variants. Platelet count was found to be decreased with all SARS-CoV-2 recombinant spike protein variants and concentrations tested, achieving the lowest values with 20 ng/mL Delta recombinant spike protein. The mean platelet volume increased in all samples irrespective of SARS-CoV-2 recombinant spike protein variants and concentrations tested, but especially using Delta and Alpha recombinant spike proteins. The values of both platelet function analyzer-200 collagen-adenosine diphosphate and collagen-epinephrine increased in all samples irrespective of SARS-CoV-2 recombinant spike protein variants and concentrations tested, and thus reflecting platelet exhaustion, and displaying again higher increases with Delta and Alpha recombinant spike proteins. Most samples where SARS-CoV-2 recombinant spike proteins were added were flagged as containing platelet clumps. Morphological analysis revealed the presence of a considerable number of activated platelets, platelet clumps, platelet-monocyte, and platelet-neutrophils aggregates, especially in samples spiked with Alpha and Delta recombinant spike proteins at 20 ng/mL. These results provide support to the evidence that SARS-CoV-2 is capable of activating platelets through its spike protein, though such effect varies depending on different spike protein variants.