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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. [ABSTRACT FROM AUTHOR]

Highlights • Ga nanodroplets on SiO 2 /Si substrate are studied by XPS. • Ga oxides are formed due to the Ga/SiO 2 interaction. • Ga nanoparticles induce the formation of nanoholes in the SiO 2 layer. • GaAs nanowires are grown from these nanoholes and studied by XPS. Abstract In this paper the early stages of the self-catalyzed Vapor-Liquid-Solid (VLS) growth of GaAs nanowires on Si substrates by Molecular Beam Epitaxy (MBE) are studied. The interaction of Ga nanodroplets (NDs) with the silica overlayer is investigated by X-ray Photoemission Spectroscopy (XPS) and Atomic Force Microscopy (AFM). We show how Ga NDs drill the silica overlayer and make contact with bulk Si to allow GaAs nanowires (NWs) epitaxial growth by studying each of the three steps of the NW growth process sequentially: Ga NDs pre-deposition, post-deposition annealing to reach the NW growth temperature, and finally NW growth itself. The pre-deposition temperature allows to control the density and morphology of the NDs. A high enough annealing temperature enhances the reduction of SiOx by Ga oxidation and leads to the formation of holes which is seen by a new component in XPS spectra, assumed to be the consequence of the Ga Si interaction. Finally, these nano-holes act as nucleation sites for the epitaxial GaAs VLS growth. During the GaAs growth, three different chemical environments of Ga are identified in Ga2p core level: metallic Ga due to the droplets at the top of the NWs, Ga oxide in contact with the SiOx overlayer and Ga arsenide from NWs. [ABSTRACT FROM AUTHOR]

Hexagonal crystal phase silicon-germanium (hex-SiGe) features efficient direct bandgap emission between 1.5 and 3.4 µm. For expanding its application potential, the key challenge is to demonstrate material gain for enabling a hex-SiGe semiconductor laser. Here we report the transition from the spontaneous emission regime to the stimulated emission-dominated amplified spontaneous emission regime in the optically excited part of a hexagonal Si0.2Ge0.8 nanowire. We observe narrow resonance peaks arising above a spontaneous emission background, which show lasing signatures such as a threshold and a superlinear increase of the emission. A Hakki-Paoli analysis of the height of the cavity resonances provides the gain spectrum of hex-SiGe, showing evidence for a positive material gain. Measurements of the cavity line widths provide an independent assessment of the total cavity loss. While lasing has not been reached, the observation of optical amplification and amplified spontaneous emission provides a clear roadmap toward lasing in hexagonal SiGe. This opens a new pathway for the monolithic integration of a Si-compatible laser within electronic chips.Hexagonal silicon-germanium features efficient direct bandgap light emission. Here, the authors demonstrate the presence of stimulated emission and optical gain in hexagonal silicon germanium and provide a roadmap to reach lasing. [ABSTRACT FROM AUTHOR]

Hexagonal crystal phase silicon-germanium (hex-SiGe) features efficient direct bandgap emission between 1.5 and 3.4 µm. For expanding its application potential, the key challenge is to demonstrate material gain for enabling a hex-SiGe semiconductor laser. Here we report the transition from the spontaneous emission regime to the stimulated emission-dominated amplified spontaneous emission regime in the optically excited part of a hexagonal Si0.2Ge0.8 nanowire. We observe narrow resonance peaks arising above a spontaneous emission background, which show lasing signatures such as a threshold and a superlinear increase of the emission. A Hakki-Paoli analysis of the height of the cavity resonances provides the gain spectrum of hex-SiGe, showing evidence for a positive material gain. Measurements of the cavity line widths provide an independent assessment of the total cavity loss. While lasing has not been reached, the observation of optical amplification and amplified spontaneous emission provides a clear roadmap toward lasing in hexagonal SiGe. This opens a new pathway for the monolithic integration of a Si-compatible laser within electronic chips.Hexagonal silicon-germanium features efficient direct bandgap light emission. Here, the authors demonstrate the presence of stimulated emission and optical gain in hexagonal silicon germanium and provide a roadmap to reach lasing. [ABSTRACT FROM AUTHOR]

Direct bandgap group IV semiconductors, like strained Ge, GeSn, or hexagonal SiGe, are considered promising for photonic integration on silicon. For group IV semiconductor lasers, it is crucial to understand the carrier cooling efficiency toward the band edges. From a fundamental perspective, a study of carrier cooling within the Γ-valley of direct bandgap group IV semiconductors is particularly interesting since the Fröhlich interaction is expected to be very weak or even absent in these materials due to the nonpolar lattice. Intravalley carrier relaxation within the Γ-valley of a nonpolar semiconductor has not been experimentally accessible before since it has always been overshadowed by intervalley processes between energetically close indirect conduction band minima. Here, we study carrier cooling in direct bandgap hexagonal silicon-germanium (hex-SiGe) nanowires, allowing us to study carrier cooling in an isolated Γ-valley that is sufficiently separated from the indirect minima. We obtain a hot carrier cooling time of 180 ps in the Γ-valley of hex-SiGe. Although the cooling is much slower than in bulk polar group III/V materials due to the absence of Fröhlich interaction, it is comparable to the cooling time in an InGaAs MQW laser structure. We conclude that carrier cooling does not inherently limit hex-SiGe to serve as a laser gain material. This result is an important insight into the field of group IV semiconductor lasers. [ABSTRACT FROM AUTHOR]

Background: Cities are health promoters, although many of them lack of spaces for sport practice. In the meanwhile, for instance, several recreational and sport spaces, that can be open to the community, are offered by the network of churches. A research is developing a tool for making more efficient the assets of sport activities in parishes in a multi-generational, inclusive and health education perspective, in response to increasingly evident needs related to social and sports demand, education health, enhancement of the built heritage and social inclusion. The social and technical value based tool merges several issues related to Public Health, Urban Health, sport facilities design, urbanism and social impact. Methods: The first part of the tool refers to an urban analysis of the city to be investigated and it highlights the correlations among the parishes, sport infrastructures, population density and social inequalities, physical activity and green areas. All the analysis refer to the GIS systems in which all the data are collected and organized by districts. Results: The application was done in the city of Milan (Italy). The analysis with GIS system gives rise to several maps (a-population health status with a focus on physical activity, bsport infrastructures availability, c-parishes available, etc.) with different results and they have been merged together. Some areas show some critical aspects related to the lack of public sport spaces, green areas and social inequalities. It permits to select the case studies to be taken in consideration for the application of the Strategic Design Framework, and it will allow to give rise to a detailed examination of the parishes’ potentialities for improving the sport practice. Conclusions: The tool is working progress and it will be validated through the application on 5 case studies in the city of Milan. The research is funded by Politecnico di Milano - PoliSocial Award 2019, entitled ‘‘SPe`S’’ (Sport is Society). Key messages: Social governance, health promotion and urban inclusion, through the reactivation system of the sport infrastructures in the parishes of Milan. Local parishes as health promoters of sport practice: sport infrastructures for the local communities in Milan. [ABSTRACT FROM AUTHOR]