John Grazul

Artificial charge-modulationin atomic-scale perovskite titanate superlattices

The nature and length scales of charge screening in complex oxides are fundamental to a wide range of systems, spanning ceramic voltage-dependent resistors (varistors), oxide tunnel junctions and charge ordering in mixed-valence compounds. There are wide variations in the degree of charge disproportionation, length scale, and orientation in the mixed-valence compounds: these have been the subject of intense theoretical study, but little is known about the microscopic electronic structure. Here we have fabricated an idealized structure to examine these issues by growing atomically abrupt layers of LaTi3+O3 embedded in SrTi4+O3. Using an atomic-scale electron beam, we have observed the spatial distribution of the extra electron on the titanium sites. This distribution results in metallic conductivity, even though the superlattice structure is based on two insulators. Despite the chemical abruptness of the interfaces, we find that a minimum thickness of five LaTiO3 layers is required for the centre titanium site to recover bulk-like electronic properties. This represents a framework within which the short-length-scale electronic response can be probed and incorporated in thin-film oxide heterostructures.

Atomic-scale imaging of nanoengineered oxygen vacancy profiles in SrTiO3

At the heart of modern oxide chemistry lies the recognition that beneficial (as well as deleterious) materials properties can be obtained by deliberate deviations of oxygen atom occupancy from the ideal stoichiometry. Conversely, the capability to control and confine oxygen vacancies will be important to realize the full potential of perovskite ferroelectric materials, varistors and field-effect devices. In transition metal oxides, oxygen vacancies are generally electron donors, and in strontium titanate (SrTiO3) thin films, oxygen vacancies (unlike impurity dopants) are particularly important because they tend to retain high carrier mobilities, even at high carrier densities. Here we report the successful fabrication, using a pulsed laser deposition technique, of SrTiO3 superlattice films with oxygen doping profiles that exhibit subnanometre abruptness. We profile the vacancy concentrations on an atomic scale using annular-dark-field electron microscopy and core-level spectroscopy, and demonstrate absolute detection sensitivities of one to four oxygen vacancies. Our findings open a pathway to the microscopic study of individual vacancies and their clustering, not only in oxides, but in crystalline materials more generally.

Atomic-scale imaging of individual dopant atoms and clusters in highly n-type bulk Si.

As silicon-based transistors in integrated circuits grow smaller, the concentration of charge carriers generated by the introduction of impurity dopant atoms must steadily increase. Current technology, however, is rapidly approaching the limit at which introducing additional dopant atoms ceases to generate additional charge carriers because the dopants form electrically inactive clusters. Using annular dark-field scanning transmission electron microscopy, we report the direct, atomic-resolution observation of individual antimony (Sb) dopant atoms in crystalline Si, and identify the Sb clusters responsible for the saturation of charge carriers. The size, structure, and distribution of these clusters are determined with a Sb-atom detection efficiency of almost 100%. Although single heavy atoms on surfaces or supporting films have been visualized previously, our technique permits the imaging of individual dopants and clusters as they exist within actual devices.

HfO2 and Al2O3 gate dielectrics on GaAs grown by atomic layer deposition

High-performance metal-oxide-semiconductor field effect transistors (MOSFETs) on III–V semiconductors have long proven elusive. High-permittivity (high-κ) gate dielectrics may enable their fabrication. We have studied hafnium oxide and aluminum oxide grown on gallium arsenide by atomic layer deposition. As-deposited films are continuous and predominantly amorphous. A native oxide remains intact underneath HfO2 during growth, while thinning occurs during Al2O3 deposition. Hydrofluoric acid etching prior to growth minimizes the final interlayer thickness. Thermal treatments at ∼600°C decompose arsenic oxides and remove interfacial oxygen. These observations explain the improved electrical quality and increased gate stack capacitance after thermal treatments.

Imaging individual atoms inside crystals with ADF-STEM.

The quantitative imaging of individual impurity atoms in annular dark-field scanning transmission electron microscopy (ADF-STEM) requires a clear theoretical understanding of ADF-STEM lattice imaging, nearly ideal thin samples, and careful attention to image processing. We explore the theory using plane-wave multislice simulations that show the image intensity of substitutional impurities is depth-dependent due to probe channeling, but the intensity of interstitial impurities need not be. The images are only directly interpretable in thin samples. For this reason, we describe a wedge mechanical polishing technique to produce samples less than <50 A thick, with low surface roughness and no amorphous surface oxide. This allows us to image individual dopants as they exist within a bulk-like silicon environment. We also discuss the image analysis techniques used to extract maximum quantitative information from the images. Based on this information, we conclude that the primary nanocluster defect responsible for the electrical inactivity of Sb in Si at high concentration consists of only two atoms.

Single-Molecule Observation of Protein Adsorption onto an Inorganic Surface

Understanding the interactions between silicon-based materials and proteins from the bloodstream is of key importance in a myriad of realms, such as the design of nanofluidic devices and functional biomaterials, biosensors, and biomedical molecular diagnosis. By using nanopores fabricated in 20 nm-thin silicon nitride membranes and highly sensitive electrical recordings, we show single-molecule observation of nonspecific protein adsorption onto an inorganic surface. A transmembrane potential was applied across a single nanopore-containing membrane immersed into an electrolyte-filled chamber. Through the current fluctuations measured across the nanopore, we detected long-lived captures of bovine serum albumin (BSA), a major multifunctional protein present in the circulatory system. Based upon single-molecule electrical signatures observed in this work, we judge that the bindings of BSA to the nitride surface occurred in two distinct orientations. With some adaptation and further experimentation, this approach, applied on a parallel array of synthetic nanopores, holds potential for use in methodical quantitative studies of protein adsorption onto inorganic surfaces.

Epitaxial growth and electronic structure of LaTiOx films

LaTiOx films have been grown on (001) perovskite oxide substrates by pulsed-laser deposition. Both single-phase perovskite LaTiO3 and layered La2Ti2O7 films could be stabilized by varying the oxygen partial pressure and substrate temperature during growth. We have obtained a crystallographic and electronic phase diagram for LaTiOx films, demonstrating the ability to vary the titanium valence from 3+ to 4+ in thermodynamically unfavorable growth conditions by utilizing interface energies.

Block Copolymer Self-Assembly–Directed Single-Crystal Homo- and Heteroepitaxial Nanostructures

Polymer Templating for Metals Polymer templating has been used to fabricate a wide range of ordered materials, both due to the ability to pattern the polymers easily over a large area and their facile removal. However, the process is somewhat limited to the incorporation of materials that will flow easily into the templated areas. Arora et al. (p. 214) show that current techniques can be extended to the patterning of metals, through guided epitaxial growth. An excimer laser was used to control the flow of material into patterned templates formed from block copolymers. Patterns created on surfaces by phase-separating polymers direct the growth of crystalline inorganic nanostructures. Epitaxy is a widely used method to grow high-quality crystals. One of the key challenges in the field of inorganic solids is the development of epitaxial single-crystal nanostructures. We describe their formation from block copolymer self-assembly–directed nanoporous templates on single-crystal Si backfilled with Si or NiSi through a laser-induced transient melt process. Depending on thickness, template removal leaves either an array of nanopillars or porous nanostructures behind. For stoichiometric NiSi deposition, the template pores provide confinement, enabling heteroepitaxial growth. Irradiation through a mask provides access to hierarchically structured materials. These results on etchable and non-etchable materials suggest a general strategy for growing epitaxial single-crystal nanostructured thin films for fundamental studies and a wide variety of applications, including energy conversion and storage.

A Closer “Look” at Modern Gate Oxides

Using high resolution TEM (HRTEM), we identified some process induced ‘weak spots’ in SiO 2 layers: First, we observed thinning in the periphery of the transistor, i. e. near the boundary to the shallow trench isolation. At the boundary to the shallow trench, the Si substrate gradually changes its orientation from to , which results in an unexpected oxidation behavior in this region. Secondly, we observed the intrusion of poly-Si grains from the gate into the gate oxide, resulting in local thinning of the dielectric. Using image simulations, we show that conventional high resolution TEM can reveal the interface roughness only to a very limited extend.

Structural and Initial Optical Characterization of Basalia Spicules in the Glass Sponge Euplectella

We report on the structural properties of siliceous spicules, found in the hexactinellid sponge Euplectella . Selective chemical etching of Euplectella spicules in bleach reveals the presence of a striated arrangement of silica spheres (∼50-200 nm in diameter) placed radially around a central, high-organic content cylindrical region. Complementary etching experiments with HF indicate a graded variation of silica content in these shells. The presence of such a multi-layered and variable-composition structure affords these spicules not only with enhanced structural integrity but could also be reflected in their optical properties. Qualitative, index-matched, refractive index measurements are shown to support this expectation.