Vidar Tonaas Fauske

Position-Controlled Uniform GaAs Nanowires on Silicon using Nanoimprint Lithography

We report on the epitaxial growth of large-area position-controlled self-catalyzed GaAs nanowires (NWs) directly on Si by molecular beam epitaxy (MBE). Nanohole patterns are defined in a SiO2 mask on 2 in. Si wafers using nanoimprint lithography (NIL) for the growth of positioned GaAs NWs. To optimize the yield of vertical NWs the MBE growth parameter space is tuned, including Ga predeposition time, Ga and As fluxes, growth temperature, and annealing treatment prior to NW growth. In addition, a non-negligible radial growth is observed with increasing growth time and is found to be independent of the As species (i.e., As2 or As4) and the growth temperatures studied. Cross-sectional transmission electron microscopy analysis of the GaAs NW/Si substrate heterointerface reveals an epitaxial growth where NW base fills the oxide hole opening and eventually extends over the oxide mask. These findings have important implications for NW-based device designs with axial and radial p-n junctions. Finally, NIL positioned GaAs/AlGaAs core-shell heterostructured NWs are grown on Si to study the optical properties of the NWs. Room-temperature photoluminescence spectroscopy of ensembles of as-grown core-shell NWs reveals uniform and high optical quality, as required for the subsequent device applications. The combination of NIL and MBE thereby demonstrates the successful heterogeneous integration of highly uniform GaAs NWs on Si, important for fabricating high throughput, large-area position-controlled NW arrays for various optoelectronic device applications.

Rapid estimation of catalyst nanoparticle morphology and atomic-coordination by high-resolution Z-contrast electron microscopy.

Heterogeneous nanoparticle catalyst development relies on an understanding of their structure-property relationships, ideally at atomic resolution and in three-dimensions. Current transmission electron microscopy techniques such as discrete tomography can provide this but require multiple images of each nanoparticle and are incompatible with samples that change under electron irradiation or with surveying large numbers of particles to gain significant statistics. Here, we make use of recent advances in quantitative dark-field scanning transmission electron microscopy to count the number atoms in each atomic column of a single image from a platinum nanoparticle. These atom-counts, along with the prior knowledge of the face-centered cubic geometry, are used to create atomistic models. An energy minimization is then used to relax the nanoparticles 3D structure. This rapid approach enables high-throughput statistical studies or the analysis of dynamic processes such as facet-restructuring or particle damage.

Vertically Oriented Growth of GaN Nanorods on Si Using Graphene as an Atomically Thin Buffer Layer.

The monolithic integration of wurtzite GaN on Si via metal-organic vapor phase epitaxy is strongly hampered by lattice and thermal mismatch as well as meltback etching. This study presents single-layer graphene as an atomically thin buffer layer for c-axis-oriented growth of vertically aligned GaN nanorods mediated by nanometer-sized AlGaN nucleation islands. Nanostructures of similar morphology are demonstrated on graphene-covered Si(111) as well as Si(100). High crystal and optical quality of the nanorods are evidenced through scanning transmission electron microscopy, micro-Raman, and cathodoluminescence measurements supported by finite-difference time-domain simulations. Current-voltage characteristics revealed high vertical conduction of the as-grown GaN nanorods through the Si substrates. These findings are substantial to advance the integration of GaN-based devices on any substrates of choice that sustains the GaN growth temperatures, thereby permitting novel designs of GaN-based heterojunction device concepts.

In Situ Heat-Induced Replacement of GaAs Nanowires by Au

Here we report on the heat-induced solid-state replacement of GaAs by Au in nanowires. Such replacement of semiconductor nanowires by metals is envisioned as a method to achieve well-defined junctions within nanowires. To better understand the mechanisms and dynamics that govern the replacement reaction, we performed in situ heating studies using high-resolution scanning transmission electron microscopy. The dynamic evolution of the phase boundary was investigated, as well as the crystal structure and orientation of the different phases at reaction temperatures. In general, the replacement proceeds one GaAs(111) bilayer at a time, and no fixed epitaxial relation could be found between the two phases. The relative orientation of the phases affects the replacement dynamics and can induce growth twins in the Au nanowire phase. In the case of a limited Au supply, the metal phase can also become liquid.

Electron Microscopy (Big and Small) Data Analysis With the Open Source Software Package HyperSpy

Francisco de la Peña, Tomas Ostasevicius, Vidar Tonaas Fauske, Pierre Burdet, Petras Jokubauskas, Magnus Nord, Mike Sarahan, Eric Prestat, Duncan N. Johnstone, Joshua Taillon, Jan Caron, Tom Furnival, Katherine E. MacArthur, Alberto Eljarrat, Stefano Mazzucco, Vadim Migunov, Thomas Aarholt, Michael Walls, Florian Winkler, Gaël Donval, Ben Martineau, Andreas Garmannslund, Luiz-Fernando Zagonel and Ilya Iyengar

In situ electronic probing of semiconducting nanowires in an electron microscope

For the development of electronic nanoscale structures, feedback on its electronic properties is crucial, but challenging. Here, we present a comparison of various in situ methods for electronically probing single, p‐doped GaAs nanowires inside a scanning electron microscope. The methods used include (i) directly probing individual as‐grown nanowires with a sharp nano‐manipulator, (ii) contacting dispersed nanowires with two metal contacts and (iii) contacting dispersed nanowires with four metal contacts. For the last two cases, we compare the results obtained using conventional ex situ litho‐graphy contacting techniques and by in situ, direct‐write electron beam induced deposition of a metal (Pt). The comparison shows that 2‐probe measurements gives consistent results also with contacts made by electron beam induced deposition, but that for 4‐probe, stray deposition can be a problem for shorter nanowires. This comparative study demonstrates that the preferred in situ method depends on the required throughput and reliability.

In-situ electrical and structural characterization of individual GaAs nanowires

A method for probing the electrical and structural characteristics of individual as-grown III-V nanowires was studied. In-situ electrical characterization was performed in a focused ion beam / scanning electron microscopy system by using a fine nano-manipulator and ion beam assisted deposition. Transmission electron microscopy specimens of probed nanowires are prepared afterwards. This method would potentially allow the correlation of electrical and structural characteristics (e.g. crystal faults such as twinning) of the nanowire-substrate system. The challenge is in contacting the nanowires so that the electrical characteristics of the nanowire-substrate system can be extracted correctly.

Radial composition variations in the shells of GaAs/AlGaAs core-shell nanowires

GaAs nanowires (NWs) are seen as promising building blocks for future optoelectronic devices. To ensure reproducible properties, a high NW uniformity is required. Here, a substantial number of both position-controlled and randomly grown self-catalyzed GaAs/AlGaAs core-shell NWs are compared. Single NWs are characterized by correlated microphotoluminescence (µ-PL) spectroscopy and transmission electron microscopy (TEM). TEM is done in the 〈110〉- and 〈112〉-projections, and on the 〈111〉-cross-section of the NWs. The position-control grown NWs showed a higher degree of uniformity in morphology. All NWs on both samples had a predominantly stacking fault free zinc blende structure, with a main optical response around the GaAs free exciton energy. However, NW-to-NW structural variations in the tip region and radial compositional variations in the shell are present in both samples. These structural features could be the origin of variations in the optical response just below and above the free exciton energy. This correlated study demonstrates that the observed distinct, sharp PL peaks in the 1.6 - 1.8 eV energy range present in several NWs, are possibly related to radial compositional variations in the AlGaAs shell rather than the structural defects in the tip region.

Visualising the Three-dimensional Morphology and Surface Structure of Metallic Nanoparticles at Atomic Resolution by Automated HAADF STEM Atom Counting

Because of their large proportion of surface atoms and favourable chemical activity, metallic nanoparticles are used to catalyse a wide range of technologically important reactions [1]. However, many industrial catalysts utilise expensive and/or rare metals, leading to the desire to account for their content and efficiency at the atomic scale. High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) proves a powerful tool here, as this readily interpretable incoherent imaging mode shows direct mass-thickness contrast. Using aberration correctors such dark-field data can be recorded at atomic resolution and subsequently analysed on an atomic column by column basis.