A T J van Helvoort

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.

Wurtzite GaAs/AlGaAs core–shell nanowires grown by molecular beam epitaxy

We report the growth of GaAs/AlGaAs core-shell nanowires (NWs) on GaAs(111)B substrates by Au-assisted molecular beam epitaxy. Electron microscopy shows the formation of a wurtzite AlGaAs shell structure both in the radial and the axial directions outside a wurtzite GaAs core. With higher Al content, a lower axial and a higher radial growth rate of the AlGaAs shell were observed. Room temperature and low temperature (4.4 K) micro-photoluminescence measurements show a much higher radiative efficiency from the GaAs core after the NW is overgrown with a radial AlGaAs shell.

Controlling crystal phases in GaAs nanowires grown by Au-assisted molecular beam epitaxy.

Control of the crystal phases of GaAs nanowires (NWs) is essential to eliminate the formation of stacking faults which deteriorate the optical and electronic properties of the NWs. In addition, the ability to control the crystal phase of NWs provides an opportunity to engineer the band gap without changing the crystal material. We show that the crystal phase of GaAs NWs grown on GaAs(111)B substrates by molecular beam epitaxy using the Au-assisted vapor-liquid-solid growth mechanism can be tuned between wurtzite (WZ) and zinc blende (ZB) by changing the V/III flux ratio. As an example we demonstrate the realization of WZ GaAs NWs with a ZB GaAs insert that has been grown without changing the substrate temperature.

Imaging of out-of-plane interfacial strain in epitaxial PbTiO3∕SrTiO3 thin films

In this study, we rely on low-angle annular dark-field scanning transmission electron microscopy to probe the interface strain profile in epitaxial PbTiO3∕SrTiO3 thin-films. All samples displayed a compressively strained layer at the PbTiO3∕SrTiO3 interface, with the strain vector parallel to the polarization direction. The width of the strained layer was found to be ∼15–30A, dependent on the electrode environment. Our findings open a perspective to use interface strain engineering in combination with control of electrostatic boundary conditions as a tool to monitor the effective interface polarization. These findings have implications for future use of ferroelectrics in electronic and mechanical devices.

Compositional characterization of GaAs/GaAsSb nanowires by quantitative HAADF-STEM.

The Sb concentration in axial GaAs(1-x)Sb(x) inserts of otherwise pure GaAs nanowires has been investigated with quantitative high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The Sb concentration was quantified by comparing the experimental image intensities normalized to the incident beam intensity with intensities simulated with a frozen lattice multislice approach. Including static atomic displacements in the simulations was found to be crucial for correct compositional analysis of GaAs(1-x)Sb(x). HAADF intensities of individual nanowires were analysed both across the nanowires, exploiting their hexagonal cross-sectional shape, and along the evenly thick central part of the nanowires. From the cross-sectional intensity profiles, a decrease in the Sb concentration towards the nanowire outer surfaces was found. The longitudinal intensity profiles revealed a gradual build-up of Sb in the insert. The decrease of the Sb concentration towards the upper interface was either gradual or abrupt, depending on the growth routine chosen. The compositional analysis with quantitative HAADF-STEM was verified by energy dispersive X-ray spectroscopy.

Z-contrast imaging of the arrangement of Cu in precipitates in 6XXX-series aluminium alloys

High-resolution annular dark field imaging has been used to study small precipitates in the Al matrix of 6XXX-series aluminium alloys. Cu-containing columns in precipitates were imaged by atomic number contrast along a ⟨100⟩ matrix direction. At an under-aged condition, large, lath-shaped particles of the Cu-containing Q′-phase were observed at grain boundaries. At over-aged conditions, Q′ was the main matrix phase. In both cases the laths showed an Al {150} habit plane. A precursor phase was found, with different arrangement of Cu-rich columns from that of the Q′-phase and sharing Al {100} habit planes. Precipitates containing elements of both the Q′-phase and the precursor were found to be common. Annular dark field imaging can differentiate directly between phases. The method complements and aids interpretation of conventional high-resolution TEM images and nano beam diffraction observations. The results obtained contribute to an improved understanding of the crystallography of the precursor and details of the structural transformation taking place during the precipitation process.

Compositional analysis of GaAs/AlGaAs heterostructures using quantitative scanning transmission electron microscopy

We demonstrate a method for compositional mapping of AlxGa1–xAs heterostructures with high accuracy and unit cell spatial resolution using quantitative high angle annular dark field scanning transmission electron microscopy. The method is low dose relative to spectroscopic methods and insensitive to the effective source size and higher order lens aberrations. We apply the method to study the spatial variation in Al concentration in cross-sectioned GaAs/AlGaAs core-shell nanowires and quantify the concentration in the Al-rich radial band and the AlGaAs shell segments.

The effects of Sb concentration variation on the optical properties of GaAsSb/GaAs heterostructured nanowires

This thesis presents a quantitative high-angle annular dark eld scanning transmission electron microscopy (HAADF STEM) study on heterostructured GaAs-based nanowires (NWs). Quantitative HAADF STEM was employed to investigate Sb concentration variations in axial GaAsSb inserts within GaAs NWs and in GaAsSb NWs, as well as Al concentration variations in the AlGaAs shell in GaAs/AlGaAs core-shell NWs. The NWs were grown with the vapor-liquid-solid (VLS) growth mechanism using Ga- and Au-assisted molecular beam epitaxy (MBE).Compositional characterisation by quantitative HAADF STEM was realized by comparing experimental image intensities normalized to the incident beam intensity with simulated intensities. The HAADF STEM image simulations were performed using the frozen-phonon multislice approach. For correct compositional analysis of GaAs1-ySby, static atomic displacements (SAD) had to be included in the simulations, however this was not the case with AlxGa1-xAs.GaAsSb inserts within GaAs NWs and GaAsSb NWs were studied with non-corrected STEM at a relatively low magnication using NWs in plane. In this case the known thickness prole of the hexagonal NWs could be exploited in the quantitative HAADF STEM analysis. In the GaAsSb inserts, concentration gradients axially along as well as radially across the insert were identied. The Sb concentration in the insert decreased axially towards the upper and lower interfaces with GaAs, and radially towards the outer surfaces. The axial concentration gradients were attributed to the reservoir eect related to the VLS growth process. The eects of the axial concentration gradients on the NWs optical properties were investigated.The radial concentration gradients in the GaAsSb inserts result from a combined eect of radial GaAs overgrowth and out-diusion of Sb during the post-insert axial GaAs growth, causing an increased surface depletion of Sb with increasing post-insert GaAs growth time. In GaAsSb NWs, increased surface depletion of Sb towards the bottom of the NW was observed, and this was attributed to out-diusion of Sb during the NW growth.GaAs/AlGaAs core-shell NWs were studied with aberration corrected STEM, using cross-sectional specimens prepared with ultramicrotomy. A method for mapping Al concentration in AlxGa1-x As at unit cell spatial resolution using the atomic resolution HAADF STEM images was developed. The method is independent of the eective source size and higher order lens aberrations. With the method, Al concentration variations in the AlGaAs shell could be quantied at unit cell spatial resolution.

Correlated micro-photoluminescence and electron microscopy studies of the same individual heterostructured semiconductor nanowires.

To correlate optical properties to structural characteristics, we developed a robust strategy for characterizing the same individual heterostructured semiconductor nanowires (NWs) by alternating low temperature micro-photoluminescence (μ-PL), low voltage scanning (transmission) electron microscopy and conventional transmission electron microscopy. The NWs used in this work were wurtzite GaAs core with zinc blende GaAsSb axial insert and AlGaAs radial shell grown by molecular beam epitaxy. The series of experiments demonstrated that high energy (200 kV) electrons are detrimental for the optical properties, whereas medium energy (5-30 kV) electrons do not affect the PL response. Thus, such medium energy electrons can be used to select NWs for correlated optical-structural studies prior to μ-PL or in NW device processing. The correlation between the three main μ-PL bands and crystal phases of different compositions, present in this heterostructure, is demonstrated for selected NWs. The positions where a NW fractures during specimen preparation can considerably affect the PL spectra of the NW. The effects of crystal-phase variations and lattice defects on the optical properties are discussed. The established strategy can be applied to other nanosized electro-optical materials, and other characterization tools can be incorporated into this routine.

Electron energy loss spectroscopy investigation of Pb and Ti hybridization with O at the PbTiO3/SrTiO3 interface

Studies of oxide interfaces, and how they affect physical properties, have recently gained large attention. For ferroelectric materials, the evolution of the order parameter close to an interface is important to understand regarding the stability of the ferroelectric phase, and how to optimize devices taking advantage of the polarization at the interface. Here we employ electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy to compare the electronic and structural properties in both bulk and interface regions of epitaxial PbTiO3 thin films grown on SrTiO3 substrates. At the interface, changes in EELS spectra of the Ti-L3,2 and O-K edges, as compared to the bulk of the thin film, reveal a reduction in the hybridization of Ti 3d and Pb 6sp states with O 2p, and thus tetragonal distortion of the TiO6 octahedron. Real-space multiple-scattering calculations of the O-K edge support the experimental results. Moreover, the analysis of the Ti-valence reveals that the change is gradua...