Andrea Cattoni

One of the greatest challenges in the monolithic integration of micro-magnets in Si-based microelectromechanical systems (MEMS) is the release of suspended structures through exposure to hydrofluoric acid. Ni80Fe20 (Py) has been mainly used in prototypes of these hybrid devices, essentially because of its high resistance to exposure to hydrofluoric acid (HF). However, a detailed investigation of the physical mechanisms leading to its resistance to corrosion, as well as a characterization of the impact of HF exposure on the magnetic properties of Py, is still missing. Here, we present a combined surface science and magnetic investigation of Py films exposed to HF vapors in the typical conditions of MEMS release. X-ray photoemission spectroscopy and scanning Auger microscopy measurements show the tendency of Py to self-passivation by the creation of a surface fluoride layer (a few nm thick), accompanied by the formation of blisters with diameter on the order of 10–20 μm. A decrease in the average static magnetization is observed by vibrating sample magnetometry upon HF exposure, compatible with the creation of an interfacial fluoride capping behaving as a magnetic dead layer. A slight increase in the Gilbert damping (from 0.0079 to 0.0095) is found by vector network analyzer-ferromagnetic resonance, which could be compatible with blistering. Overall, this analysis confirms that Py displays excellent intrinsic resistance to HF so that it can be effectively used for integration of thick magnetic elements in MEMS without protective layers. For thin films to be used in spintronic/magnonic devices, instead, an additional capping layer should be used to fully preserve dynamic magnetic properties.

The WZ and ZB phases of single GaAs nanowires are probed by cathodoluminescence (CL) mapping, CL polarimetry and time-resolved CL.

Hot carrier solar cells are a concept of photovoltaic devices, which offers the opportunity to harvest solar energy beyond the Shockley-Queisser limit. Unlike conventional photovoltaic devices, hot carrier solar cells convert excess kinetic energy into useful electrical power rather than losing it through thermalisation mechanisms. To extract the carriers while they are still “hot”, efficient energy-selective contacts must be developed. In previous studies, the presence of the hot carrier population in a p-i-n solar cell based on a single InGaAsP quantum well on InP substrate at room temperature has been demonstrated by means of complementary optical and electrical measurements, leading to an operating condition for this device beyond the limit for classical device operation. This result allows to design a new generation of devices to increase the hot carrier conversion contribution. In this work, we study InGaAs/AlInAs type II heterojunction as a selective contact for a future hot carrier solar cell device epitaxially grown on (001) oriented InP substrate. Two p-i-n solar cells have been grown by molecular beam epitaxy on InP. The absorber is a 50 nm-thick InGaAs layer surrounded by AlInAs barriers, all lattice-matched to InP. Two architectures are compared, the first with two symmetrical AlInAs barriers and the second with a single InGaAs quantum well in the center of the n-side barrier to allow electron tunneling across the barrier. Electrical characteristics under laser illumination with two different wavelengths have been measured to investigate the effect of the selective contact compared to the barrier. This preliminary study of InGaAs/AlInAs-based selective contacts show that such III–V combination is adapted for a future hot carrier solar cell in the InP technology.

Cyclic thermo-ellipsometry enables exploration of the effects of temperature variation rates on vapor physisorption in MOFs, a key insight for many adsorption-driven applications.

Spin waves represent the collective excitations of the magnetization field within a magnetic material, providing dispersion curves that can be manipulated by material design and external stimuli. Bulk and surface spin waves can be excited in a thin film with positive or negative group velocities and, by incorporating a symmetry-breaking mechanism, magnetochiral features arise. Here we study the band diagram of a chiral magnonic crystal consisting of a ferromagnetic film incorporating a periodic Dzyaloshinskii-Moriya coupling via interfacial contact with an array of heavy-metal nanowires. We provide experimental evidence for a strong asymmetry of the spin wave amplitude induced by the modulated interfacial Dzyaloshinskii-Moriya interaction, which generates a nonreciprocal propagation. Moreover, we observe the formation of flat spin-wave bands at low frequencies in the band diagram. Calculations reveal that depending on the perpendicular anisotropy, the spin-wave localization associated with the flat modes occurs in the zones with or without Dzyaloshinskii-Moriya interaction.

With their unique structural, optical and electrical properties, III–V nanowires (NWs) are an extremely attractive option for the direct growth of III–Vs on Si for tandem solar cell applications. Here, we introduce a core–shell GaAs/GaInP NW solar cell grown by molecular beam epitaxy on a patterned Si substrate, and we present an in-depth investigation of its optoelectronic properties and limitations. We report a power conversion efficiency of almost 3.7%, and a state-of-the-art open-circuit voltage (V_OC) for a NW array solar cell on Si of 0.65 V. We also present the first quantification of the quasi-Fermi level splitting in NW array solar cells using hyperspectral photoluminescence measurements. A value of 0.84 eV is obtained at 1 sun (1.01 eV at 81 suns), which is significantly higher than qV_OC. It indicates NWs with a better intrinsic optoelectronic quality than what could be expected from TEM images or deduced from electrical measurements. Optical and electronic simulations provide insights into the main absorption and electrical losses, and guidelines to design and fabricate higher-efficiency devices. It suggests that improvements at the n-type contact (GaInP/ITO) are key to unlocking the potential of next generation NW solar cells.

Cathodoluminescence mapping is used as a contactless method to probe the electron concentration gradient of Te-doped GaAs nanowires. The room temperature and low temperature (10 K) cathodoluminescence analysis method previously developed for GaAs:Si is first validated on five GaAs:Te thin film samples, before extending it to the two GaAs:Te NW samples. We evidence an electron concentration gradient ranging from below 1 × 10 ¹⁸ cm ⁻³ to 3.3 ×10 ¹⁸ cm ⁻³ along the axis of a GaAs:Te nanowire grown at 640 °C, and a homogeneous electron concentration of around 6–8 × 10 ¹⁷ cm ⁻³ along the axis of a GaAs:Te nanowire grown at 620 °C. The differences in the electron concentration levels and gradients between the two nanowires is attributed to different Te incorporation efficiencies by vapor–solid and vapor–liquid–solid processes.

Noble metal coordination xerogel exhibit solubility switch when exposed to X-rays. The films can be used as recyclable negative tone resists for deep X-ray lithography that can be converted into metallic nanoarchitectured films.