Andrew B. Yankovich

Stable p-type conduction from Sb-decorated head-to-head basal plane inversion domain boundaries in ZnO nanowires.

We report that Sb-decorated head-to-head (H-H) basal plane inversion domain boundaries (b-IDBs) lead to stable p-type conduction in Sb-doped ZnO nanowires (NWs) due to Sb and O codoping. Aberration-corrected Z-contrast scanning transmission electron microscopy shows that all of the Sb in the NWs is incorporated into H-H b-IDBs just under the (0001) NW growth surfaces and the (0001) bottom facets of interior voids. Density functional theory calculations show that the extra basal plane of O per H-H b-IDB makes them electron acceptors. NWs containing these defects exhibited stable p-type behavior in a single NW FET over 18 months. This new mechanism for p-type conduction in ZnO offers the potential of ZnO NW based p-n homojunction devices.

Electron scattering mechanisms in GZO films grown on a-sapphire substrates by plasma-enhanced molecular beam epitaxy

We report on the mechanisms governing electron transport using a comprehensive set of ZnO layers heavily doped with Ga (GZO) grown by plasma-enhanced molecular-beam epitaxy on a-plane sapphire substrates with varying oxygen-to-metal ratios and Ga fluxes. The analyses were conducted by temperature dependent Hall measurements which were supported by microstructural investigations as well. Highly degenerate GZO layers with n > 5 × 1020 cm−3 grown under metal-rich conditions (reactive oxygen-to-metal ratio <1) show relatively larger grains (∼20–25 nm by x-ray diffraction) with low-angle boundaries parallel to the polar c-direction. For highly conductive GZO layers, ionized-impurity scattering with almost no compensation is the dominant mechanism limiting the mobility in the temperature range from 15 to 330 K and the grain-boundary scattering governed by quantum-mechanical tunnelling is negligible. However, due to the polar nature of ZnO having high crystalline quality, polar optical phonon scattering cannot b...

Donor behavior of Sb in ZnO

Electrical behavior of Sb in ZnO:Sb layers doped in a wide concentration range was studied using temperature dependent Hall effect measurements. The layers were grown by plasma-enhanced molecular beam epitaxy, and the Sb concentration was changed by varying the Sb flux, resulting in electron concentrations in the range of 1016 to nearly 1020 cm−3. Upon annealing, the electron concentration increased slightly and more notable was that the electron mobility significantly improved, reaching a room-temperature value of 110 cm2/V s and a low-temperature value of 145 cm2/V s, close to the maximum of ∼155 cm2/V s set by ionized impurity scattering. Hall data and structural data suggest that Sb predominantly occupies Zn sublattice positions and acts as a shallow donor in the whole concentration range studied. In the layers with high Sb content (∼1 at. %), acceptor-type compensating defects (possibly Sb on oxygen sites and/or point-defect complexes involving SbO) are formed. The increase of electron concentration ...

Integrated Computational and Experimental Structure Refinement for Nanoparticles

Determining the three-dimensional (3D) atomic structure of nanoparticles is critical to identifying the structures controlling their properties. Here, we demonstrate an integrated genetic algorithm (GA) optimization tool that refines the 3D structure of a nanoparticle by matching forward modeling to experimental scanning transmission electron microscopy (STEM) data and simultaneously minimizing the particle energy. We use the tool to create a refined 3D structural model of an experimentally observed ∼6000 atom Au nanoparticle.

Impurity distribution and microstructure of Ga-doped ZnO films grown by molecular beam epitaxy

We report microstructural characterization of heavily Ga-doped ZnO (GZO) thin films on GaN and sapphire by aberration-corrected scanning transmission electron microscopy. Growth under oxygen-rich and metal-rich growth conditions leads to changes in the GZO polarity and different extended defects. For GZO layers on sapphire, the primary extended defects are voids, inversion domain boundaries, and low-angle grain boundaries. Ga doping of ZnO grown under metal-rich conditions causes a switch from pure oxygen polarity to mixed oxygen and zinc polarity in small domains. Electron energy loss spectroscopy and energy dispersive spectroscopy spectrum imaging show that Ga is homogeneous, but other residual impurities tend to accumulate at the GZO surface and at extended defects. GZO grown on GaN on c-plane sapphire has Zn polarity and no voids. There are misfit dislocations at the interfaces between GZO and an undoped ZnO buffer layer and at the buffer/GaN interface. Low-angle grain boundaries are the only threadin...

Non-rigid registration and non-local principle component analysis to improve electron microscopy spectrum images

Image registration and non-local Poisson principal component analysis (PCA) denoising improve the quality of characteristic x-ray (EDS) spectrum imaging of Ca-stabilized Nd2/3TiO3 acquired at atomic resolution in a scanning transmission electron microscope. Image registration based on the simultaneously acquired high angle annular dark field image significantly outperforms acquisition with a long pixel dwell time or drift correction using a reference image. Non-local Poisson PCA denoising reduces noise more strongly than conventional weighted PCA while preserving atomic structure more faithfully. The reliability of and optimal internal parameters for non-local Poisson PCA denoising of EDS spectrum images is assessed using tests on phantom data.

Poisson noise removal from high-resolution STEM images based on periodic block matching

Scanning transmission electron microscopy (STEM) provides sub-ångstrom, atomic resolution images of crystalline structures. However, in many applications, the ability to extract information such as atom positions, from such electron micrographs, is severely obstructed by low signal-to-noise ratios of the acquired images resulting from necessary limitations to the electron dose. We present a denoising strategy tailored to the special features of atomic-resolution electron micrographs of crystals limited by Poisson noise based on the block-matching and 3D-filtering (BM3D) algorithm by Dabov et al. We also present an economized block-matching strategy that exploits the periodic structure of the observed crystals. On simulated single-shot STEM images of inorganic materials, with incident electron doses below 4 C/cm 2, our new method achieves precisions of 7 to 15 pm and an increase in peak signal-to-noise ratio (PSNR) of 15 to 20 dB compared to noisy images and 2 to 4 dB compared to images denoised with the original BM3D.

High-precision scanning transmission electron microscopy at coarse pixel sampling for reduced electron dose

Determining the precise atomic structure of materials’ surfaces, defects, and interfaces is important to help provide the connection between structure and important materials’ properties. Modern scanning transmission electron microscopy (STEM) techniques now allow for atomic resolution STEM images to have down to sub-picometer precision in locating positions of atoms, but these high-precision techniques generally require large electron doses, making them less useful for beam-sensitive materials. Here, we show that 1- to 2-pm image precision is possible by non-rigidly registering and averaging a high-angle dark field image series of a 5- to 6-nm Au nanoparticle even though a very coarsely sampled image and decreased exposure time was used to minimize the electron dose. These imaging conditions minimize the damage to the nanoparticle and capture the whole nanoparticle in the same image. The high-precision STEM image reveals bond length contraction around the entire nanoparticle surface, and no bond length variation along a twin boundary that separates the nanoparticle into two grains. Surface atoms at the edges and corners exhibit larger bond length contraction than atoms near the center of surface facets.

Metasurfaces and Colloidal Suspensions Composed of 3D Chiral Si Nanoresonators

High-refractive-index silicon nanoresonators are promising low-loss alternatives to plasmonic particles in CMOS-compatible nanophotonics applications. However, complex 3D particle morphologies are challenging to realize in practice, thus limiting the range of achievable optical functionalities. Using 3D film structuring and a novel gradient mask transfer technique, the first intrinsically chiral dielectric metasurface is fabricated in the form of a monolayer of twisted silicon nanocrescents that can be easily detached and dissolved into colloidal suspension. The metasurfaces exhibit selective handedness and a circular dichroism as large as 160° µm-1 due to pronounced differences in induced current loops for left-handed and right-handed polarization. The detailed morphology of the detached particles is analyzed using high-resolution transmission electron microscopy. Furthermore, it is shown that the particles can be manipulated in solution using optical tweezers. The fabrication and detachment method can be extended to different nanoparticle geometries and paves the way for a wide range of novel nanophotonic experiments and applications of high-index dielectrics.

High Precision STEM Imaging by Non-Rigid Alignment and Averaging of a Series of Short Exposures

Precision in both high-resolution TEM and STEM imaging is fundamentally limited by signal to noise, but STEM encounters practical limits before the fundamental limit is reached. Because of the serial acquisition of the image, instabilities in the position of the probe or the sample introduce random and systematic errors in the positions of the atomic columns. As a result, 1 pm precision has been reported in TEM [1], but the best reported precision in STEM is 5 pm [2].