Doug D. Perovic

Field-emission SEM imaging of compositional and doping layer semiconductor superlattices

Field-emission scanning electron microscopy (FE-SEM) has been used to study several semiconductor multilayer heterostructures. Compositional superlattices based on Ge x Si 1−x /Si and Al x Ga 1−x As/GaAs have been studied in both cross-sectional and oblique plan-views after indentation. Secondary and backscattered electron images reveal strong atomic number contrast which is primarily structural in origin. Secondly, for the first time, heterostructures containing n and p doping have been directly imaged at low voltages (0.5-1 kV) including: (i) Si- and Be-doped GaAs layers and (ii) B- and As-doped Si layers. Secondary electron images reveal strong contrast at doping concentrations as low as 10 17 cm −3 . The results have been interpreted in terms of energy band-bending effects between n- and p-doped layers

Imaging elastic strains in high-angle annular dark field scanning transmission electron microscopy

Abstract High-angle annular dark field (HAADF) imaging in a dedicated scanning transmission electron microscope (STEM) has been applied to the study of imperfect crystals. Firstly, a study of B-doped layers in Si has revealed significantly stronger contrast and of opposite sign relative to simple atomic number contrast ( Z -contrast) predictions. It is shown that misfitting substitutional B atoms act as point defect sites in a Si matrix which enhance scattering to high angles via a static Debye-Waller effect. Multi-beam Bloch-wave theory has been used to quantitatively predict experimental contrast levels. Secondly, HAADF-STEM imaging of inclined dislocation segments revealed a number of novel contrast effects which depend on the specific position of the dislocation in the foil. Unlike conventional diffraction contrast from dislocations, HAADF dislocation contrast is neither similar nor complementary at the entrant and exit surfaces of the specimen. A qualitative Bloch-wave scattering description has been developed consistently to describe the dislocation contrast features.

Photomethanation of Gaseous CO2 over Ru/Silicon Nanowire Catalysts with Visible and Near‐Infrared Photons

Gaseous CO2 is transformed photochemically and thermochemically in the presence of H2 to CH4 at millimole per hour per gram of catalyst conversion rates, using visible and near‐infrared photons. The catalyst used to drive this reaction comprises black silicon nanowire supported ruthenium. These results represent a step towards engineering broadband solar fuels tandem photothermal reactors that enable a three‐step process involving i) CO2 capture, ii) gaseous water splitting into H2, and iii) reduction of gaseous CO2 by H2.

MEMS microgrippers with thin gripping tips

Gripping small objects requires tool tips of comparable dimensions. Current methods for miniaturizing an MEMS tool entirely down to sub-micrometer in dimensions, however, come with significant tradeoffs in device performance. This paper presents a microfabrication approach to selectively miniaturize gripping tips only to sub-micrometers in thickness. The process involves using the thin buried SiO2 layer of a standard silicon-on-insulator wafer to form gripping tips, and using the thick device silicon layer to construct high-aspect-ratio structures for structural, sensing, and actuation functions. The microgrippers with thin gripping tips (i.e. finger-nail-like) were experimentally characterized and applied to gripping 100 nm gold spheres inside a scanning electron microscope.

Low-temperature synthesis of nanoscale silica multilayers – atomic layer deposition in a test tube

Herein we demonstrate a simplified, ‘poor-mans’ form of the Atomic Layer Deposition (ALD) technique to grow uniform silica multilayers onto hydrophilic surfaces at low temperatures, including room temperature (RT). Tetramethoxysilane vapor is used alternately with ammonia vapor as a catalyst, with very common benchtop lab equipment in an ambient environment. This deposition method could be applied in a wide range of fields for growing nanoscale layers of silica from an inexpensive vapor source, without the sophisticated vacuum systems or high temperatures that are generally required for ALD. Conditions for uniform deposition are demonstrated for 20-nm-thick silica shells grown around polymer spheres at RT, and in the interstitial space of a colloidal crystal film. This approach is shown to provide a controlled means of sintering the silica spheres and thereby is an easy way to modify the photonic and mechanical properties of the resulting material. We believe this method has an advantage compared to other more sophisticated methods of ALD and provides a simple technique for broad applications in MEMs, nanoporous structures, sintering of components, cell encapsulation, and organic/inorganic layered composites.

Compositional control in molecular beam epitaxy growth of GaNyAs1−y on GaAs (001) using an Ar/N2 RF plasma

Abstract Single and multiple quantum wells of GaN y As 1− y were grown on GaAs substrates using solid-source molecular beam epitaxy (MBE) with an RF plasma cell. Dynamic gas switching was used to control the gas composition and active nitrogen flux produced using ultra-pure Ar/N 2 gas mixtures. Real-time optical spectroscopy of the plasma during growth was used to monitor the active nitrogen flux and to optimize the growth method. High-resolution X-ray diffraction, secondary ion mass spectrometry, transmission electron microscopy and low temperature photoluminescence spectroscopy were used to study the composition, structural and optical quality of the structures. We demonstrate the exceptional sensitivity of these techniques to the nitrogen profiles in the films and the high quality of the layers produced using the Ar dilution technique.

Electronic contribution to secondary electron compositional contrast in the scanning electron microscope

Abstract Scanning electron microscopy of cleavage surfaces through a variable thickness SiGe 0.25 Si 0.75 heterostructure is shown to reveal the high sensitivity of the secondary electron signal to small changes in band structure. Ge 0.25 Si 0.75 layers that are coherently strained appear brighter in secondary electron micrographs than equal thickness layers of the unstrained Ge 0.25 Si 0.75 alloy. This effect has been studied quantitatively and is explained in terms of the 0.1 eV strain-induced raising of the Ge 0.25 Si 0.75 valence band edge resulting in an increased secondary electron escape probability.

Absolute quantum yields in NaYF4:Er,Yb upconverters – synthesis temperature and power dependence

We investigate the temperature and power dependence of the emission quantum yield of NaYF4:Er,Yb upconverters. We show that the quantum yields may vary by almost three orders of magnitude. We demonstrate that formation of the hexagonal phase of this material is not in itself sufficient to guarantee high upconversion efficiency.

Deactivation and diffusion of boron in ion-implanted silicon studied by secondary electron imaging

Secondary electron (SE) imaging in a scanning electron microscope is used to map electrically active dopant distributions of B-doped superlattices in Si. By comparing SE contrast profiles with secondary ion mass spectroscopy data, it is shown that B is electrically deactivated when the damage caused during Si implantation falls onto a doped region. Following a 450 °C anneal, the effect of the implantation damage is severely reduced in the SE profiles and the B is partially reactivated. An 815 °C anneal results in transient enhanced diffusion of some of the B with the remainder trapped in an inactive immobile peak.

Plastic deformation under microindentations in GaAs/AlAs superlattices

Abstract Presented is a high-resolution scanning electron microscopy study of the mechanical behaviour of GaAs/AlAs superlattices that have been deformed through microindentation. Transverse cleavage through the indentation sites reveals the pattern of the deformed multilayers, showing in particular debonding between the GaAs and AlAs layers, regions of compressive strains of up to 30% and substantial slip on the {111} planes.