Bismuth-oxide nanoparticles: study in a beam and as deposited.

Bi2O3 is a promising material for solid-oxide fuel cells (SOFC) due to the high ionic conductivity of some phases. The largest value is reached for its δ-phase, but it is normally stable at temperatures too high for SOFC operation, while nanostructured oxide is believed to have more suitable stabilization temperature. However, to manufacture such a material with a controlled chemical composition is a challenging task. In this work, we investigated the fabrication of nanostructured Bi2O3 films formed by deposition of free Bi-oxide nanoparticles created in situ. The particle-production method was based on reactive sputtering and vapour aggregation. Depending on the fabrication conditions, the nanoparticles contained either a combination of Bi-metal and Bi-oxide, or only Bi-oxide. Prior to deposition, the free particles were probed in the beam - by synchrotron-based photoelectron spectroscopy (PES), which allowed assessing their composition "on the-fly". The nanoparticle films obtained after deposition were studied by PES, scanning electron microscopy, transmission electron microscopy, and electron diffraction. The films' chemical composition, grain dimensions, and crystal structure were probed. Our analysis suggests that our method produced Bi-oxide films in more than one polymorph of Bi2O3.

Tuneable 2D surface Bismuth incorporation on InAs nanosheets.

The chemical bonding at the interface between compound semiconductors and metals is central in determining electronic and optical properties. In this study, new opportunities for controlling this are presented for nanostructures. We investigate Bi adsorption on 2D wurtzite InAs (112̄0) nanosheets and find that temperature-controlled Bi incorporation in either anionic- or cationic-like bonding is possible in the easily accesible range between room temperature and 400 °C. This separation could not be achieved for ordinary zinc blende InAs(110) surfaces. As the crystal structures of the two surfaces have identical nearest neighbour configurations, this indicates that overall geometric differences can significantly alter the adsorption and incorporation. Ab initio theoretical modelling confirms observed adsorption results, but indicate that both the formation energies as well as kinetic barriers contributes to the observed temperature dependent behaviour. Further, we find that the Bi adsorption rate can differ by at least 2.5 times between the two InAs surfaces while being negligible for standard Si substrates under similar deposition conditions. This, in combination with the observed interface control, provides an excellent opportunity for tuneable Bi integration on 2D InAs nanostructures on standard Si substrates.

Optical-Beam-Induced Current in InAs/InP Nanowires for Hot-Carrier Photovoltaics.

Using the excess energy of charge carriers excited above the band edge (hot carriers) could pave the way for optoelectronic devices, such as photovoltaics exceeding the Shockley-Queisser limit or ultrafast photodetectors. Semiconducting nanowires show promise as a platform for hot-carrier extraction. Proof of principle photovoltaic devices have already been realized based on InAs nanowires, using epitaxially defined InP segments as energy filters that selectively transmit hot electrons. However, it is not yet fully understood how charge-carrier separation, relaxation, and recombination depend on device design and on the location of optical excitation. Here, we introduce the use of an optical-beam-induced current (OBIC) characterization method, employing a laser beam focused close to the diffraction limit and a high precision piezo stage, to study the optoelectric performance of the nanowire device as a function of the position of excitation. The photocurrent response agrees well with modeling based on hot-electron extraction across the InP segment via diffusion. We demonstrate that the device is capable of producing power and estimate the spatial region within which significant hot-electron extraction can take place to be on the order of 300 nm away from the barrier. When comparing to other experiments on similar nanowires, we find good qualitative agreement, confirming the interpretation of the device function, while the extracted diffusion length of hot electrons varies. Careful control of the excitation and device parameters will be important to reach the potentially high device performance theoretically available in these systems.

Improved Electrostatics through Digital Etch Schemes in Vertical GaSb Nanowire p-MOSFETs on Si.

Sb-based semiconductors are critical p-channel materials for III-V complementary metal oxide semiconductor (CMOS) technology, while the performance of Sb-based metal-oxide-semiconductor field-effect transistors (MOSFETs) is typically inhibited by the low quality of the channel to gate dielectric interface, which leads to poor gate modulation. In this study, we achieve improved electrostatics of vertical GaSb nanowire p-channel MOSFETs by employing robust digital etch (DE) schemes, prior to high-κ deposition. Two different processes, based on buffer-oxide etcher (BOE) 30:1 and HCl:IPA 1:10, are compared. We demonstrate that water-based BOE 30:1, which is a common etchant in Si-based CMOS process, gives an equally controllable etching for GaSb nanowires compared to alcohol-based HCl:IPA, thereby realizing III-V on Si with the same etchant selection. Both DE chemicals show good interface quality of GaSb with a substantial reduction in Sb oxides for both etchants while the HCl:IPA resulted in a stronger reduction in the Ga oxides, as determined by X-ray photoelectron spectroscopy and in agreement with the electrical characterization. By implementing these DE schemes into vertical GaSb nanowire MOSFETs, a subthreshold swing of 107 mV/dec is obtained in the HCl:IPA pretreated sample, which is state of the art compared to reported Sb-based MOSFETs, suggesting a potential of Sb-based p-type devices for all-III-V CMOS technologies.