Defects in GaN Nanowires

High resolution and cross-sectional transmission electron microscopy (HRTEM, XTEM) were used to characterize common defects in wurtzite GaN nanowires grown via the vapor-liquid-solid (VLS) mechanism. High resolution transmission electron microscopy showed that these nanowires contained numerous (001) stacking defects interspersed with cubic intergrowths. Using cross-sectional transmission electron microscopy, bicrystalline nanowires were discovered with two-fold rotational twin axes along their growth directions, and were concluded to grow along high index directions or vicinal to low index planes. A defect-mediated VLS growth model was used to account for the prevalence of these extended defects. Implications for nanowire growth kinetics and device behavior are discussed

Polarity in GaN and ZnO: Theory, measurement, growth, and devices

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Rev. 3, 041303 (2016) and may be found at https://doi.org/10.1063/1.4963919.The polar nature of the wurtzite crystalline structure of GaN and ZnO results in the existence of a spontaneous electric polarization within these materials and their associated alloys (Ga,Al,In)N and (Zn,Mg,Cd)O. The polarity has also important consequences on the stability of the different crystallographic surfaces, and this becomes especially important when considering epitaxial growth. Furthermore, the internal polarization fields may adversely affect the properties of optoelectronic devices but is also used as a potential advantage for advanced electronic devices. In this article, polarity-related issues in GaN and ZnO are reviewed, going from theoretical considerations to electronic and optoelectronic devices, through thin film, and nanostructure growth. The necessary theoretical background is first introduced and the stability of the cation and anion polarity surfaces is discussed. For assessing the polarity, one has to make use of specific characterization methods, which are described in detail. Subsequently, the nucleation and growth mechanisms of thin films and nanostructures, including nanowires, are presented, reviewing the specific growth conditions that allow controlling the polarity of such objects. Eventually, the demonstrated and/or expected effects of polarity on the properties and performances of optoelectronic and electronic devices are reported. The present review is intended to yield an in-depth view of some of the hot topics related to polarity in GaN and ZnO, a fast growing subject over the last decade

Determination of local crystal symmetry in complex, multielement, ferroelectric perovskites and alloys

Structural change arising from correlated lattice and charge interaction in complex, multi-element, crystals has a profound effect on their physical properties. Examples include charge ordering in complex oxides, polarization nanodomains in ferroelectrics, and vortex matter. A common scheme to these systems is that there exists a distinction between local crystal symmetry and the average, macroscopic symmetry imposed by fluctuations in the crystal lattice. Before we can control and exploit these fluctuation-induced emergent properties, the crucial first step is to fully characterize any correlation that may exist. This thesis explores the crystallographic aspect of local charge, polarization, and lattice interactions in complex, multi-element, crystals by developing scanning convergent beam electron diffraction (SCBED) based techniques. The applications of SCBED characterization demonstrated here include: ferroelectric BaTiO3 single crystal, relaxor-ferroelectric (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (x=0.08) single crystal, and multi-principal-element alloy Al0.1CrFeCoNi. First, we show the local crystal symmetry and polarization fluctuations in BaTiO3 single crystals as determined using SCBED. An improved algorithm for CBED symmetry quantification is used to map the ferroelectric domains and local symmetry across the ferroelectric phase transition temperatures. The symmetry in BaTiO3 was found inhomogeneous; regions of a few tens of nanometers retaining almost perfect symmetry are interspersed in regions of lower symmetry. The SCBED results suggest the coexistence of displacive and order-disorder phase transition, affected upon by the local structure. Next, we examine the local symmetry, polarization nanodomains, and the domain wall (DW) structures in relaxor-ferroelectrics. Nanometer-sized domains having the monoclinic Pm symmetry in PZN-8%PT single crystals are identified by performing SCBED along the [100], [001], and [111] zone axes. Intensity distribution in the (000) disks in the CBED patterns is used to determine lattice-rotation at the precision of ±0.012° by performing SCBED on a standard Si sample. A careful examination of the polarization DWs revealed the presence of lattice-rotation vortices of ~15nm in diameter in PZN-8%PT, which can be attributed to bound charge discontinuity and depolarization fields. The lattice distortion effect in high entropy alloys (HEAs) is explored as a model of multi element alloys. Lattice distortion is one of the four core effects of HEAs, which results from different atom sizes and influences solid solution hardening. However, so far quantification of lattice distortion effects by X-ray and neutron diffraction has provided contradictory results. Using SCBED, we visualize the sub-nanometer strain fluctuations and local symmetry breaking in single phase Al0.1CrFeCoNi. Our results reveal 10±3nm, disc-shaped, clusters having ~7.1% tensile displacements along directions distributed throughout the specimen; local strain, on the contrary, was found to be fluctuating within ±1.3% and slow-varying over ~50nm. The observed inhomogeneous lattice distortion using scanning electron diffraction thus provides a new perspective on structure and property relations in multi-principal-element systems

Accelerated discovery of two crystal structure types in a complex inorganic phase field

The discovery of new materials is hampered by the lack of efficient approaches to the exploration of both the large number of possible elemental compositions for such materials, and of the candidate structures at each composition1. For example, the discovery of inorganic extended solid structures has relied on knowledge of crystal chemistry coupled with time-consuming materials synthesis with systematically varied elemental ratios2,3. Computational methods have been developed to guide synthesis by predicting structures at specific compositions4,5,6 and predicting compositions for known crystal structures7,8, with notable successes9,10. However, the challenge of finding qualitatively new, experimentally realizable compounds, with crystal structures where the unit cell and the atom positions within it differ from known structures, remains for compositionally complex systems. Many valuable properties arise from substitution into known crystal structures, but materials discovery using this approach alone risks both missing best-in-class performance and attempting design with incomplete knowledge8,11. Here we report the experimental discovery of two structure types by computational identification of the region of a complex inorganic phase field that contains them. This is achieved by computing probe structures that capture the chemical and structural diversity of the system and whose energies can be ranked against combinations of currently known materials. Subsequent experimental exploration of the lowest-energy regions of the computed phase diagram affords two materials with previously unreported crystal structures featuring unusual structural motifs. This approach will accelerate the systematic discovery of new materials in complex compositional spaces by efficiently guiding synthesis and enhancing the predictive power of the computational tools through expansion of the knowledge base underpinning them

Crystal Cartography: Mapping Nanostructure with Scanning Electron Diffraction

Nanostructure describes the network of defective and distorted atomic structure existing on the nanoscale within materials. This nanostructure bridges the gap between idealised crys- talline structure and real materials, playing a deterministic role in tailoring physico-chemical properties, as well as providing a basis for mechanistic understanding of complex processes such as mechanical deformation and phase transformation. Characterising nanostructure, to develop understanding of materials, requires experimental techniques capable of probing the structure with spatial resolution on the order of nanometres and across regions of interest up to micrometres. Recent developments in electron microscopy, enabling the acquisition of numerous diffraction patterns in a spatially resolved manner, combined with modern computational power, provides a route to meet this need as developed in this work. Scanning electron diffraction (SED) involves the acquisition of a two-dimensional elec- tron diffraction pattern at each probe position in a two-dimensional scan of a specimen. An information rich 4-dimensional (4D-SED) dataset is obtained that can be analysed extensively post-facto using a wide-range of computational methods. The acquisition of such 4D-SED data from the specimen at numerous orientations may also enable the reconstruction of nanostructure in three-dimensions via tomographic methods. In this work, methods for the acquisition and analysis of 4D-SED data are developed and applied to reveal nanostructure in two and three-dimensions. These methods are applied to various prototypical characterisation challenges in materials science, particularly: strain mapping in two and three dimensions, revealing inter-phase crystallographic relationships, mapping grains in two-dimensional materials, and probing nanostructure in polyethylene

Prospects for versatile phase manipulation in the TEM: beyond aberration correction

In this paper we explore the desirability of a transmission electron microscope in which the phase of the electron wave can be freely controlled. We discuss different existing methods to manipulate the phase of the electron wave and their limitations. We show how with the help of current techniques the electron wave can already be crafted into specific classes of waves each having their own peculiar properties. Assuming a versatile phase modulation device is feasible, we explore possible benefits and methods that could come into existence borrowing from light optics where so-called spatial light modulators provide programmable phase plates for quite some time now. We demonstrate that a fully controllable phase plate building on Harald Rose's legacy in aberration correction and electron optics in general would open an exciting field of research and applications.Comment: 9 pages, 4 figures, special Ultramicroscopy issue for PICO2015 conferenc

Local symmetry and polarization in relaxor-based ferroelectric crystals

Relaxor-ferroelectric single crystals, such as (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT), have the potential to transform technologies of medical imaging, actuation, and sensors, due to their extraordinary high piezoelectric effect. It has been suggested that polarization rotation driven by external electric fields is responsible for the large piezoelectric response. Polarization rotation is accompanied by a change in crystal symmetry and/or orientation. However, the nature of symmetry in PMN-xPT and other relaxor-ferroelectric crystals, despite extensive study by x-ray and neutron diffraction, is still controversial. Extensive studies have suggested the crystal symmetry varies on the nanoscopic scale and understanding the nature of local symmetry and its variations is thus critical to correlate the structure with polarization properties. In this thesis, the symmetry is measured based on the size of the crystal volume and the volume position in PMN-xPT single crystals. The study is enabled by the use of probes of different length scales to examine the symmetry of PMN-xPT single crystals. Local symmetry recorded by different probes is measured using quantitative convergent beam electron diffraction analysis (CBED) and transmission electron microscopy (TEM). Symmetry in CBED patterns is correlated with polarization direction with help of simulations. Furthermore, local symmetry fluctuation is observed using a new CBED method developed during this research. The technique enables the quantification of local symmetry development