Nancy Ruzycki

Epitaxial growth and properties of ferromagnetic co-doped TiO2 anatase

We have used oxygen-plasma-assisted molecular-beam epitaxy (OPA-MBE) to grow CoxTi1−xO2 anatase on SrTiO3(001) for x=∼0.01–0.10, and have measured the structural, compositional, and magnetic properties of the resulting films. Whether epitaxial or polycrystalline, these CoxTi1−xO2 films are ferromagnetic semiconductors at and above room temperature. However, the magnetic and structural properties depend critically on the Co distribution, which varies widely with growth conditions. Co is substitutional in the anatase lattice and in the +2 formal oxidation state in ferromagnetic CoxTi1−xO2. The magnetic properties of OPA-MBE grown material are significantly better than those of analogous pulsed laser deposition-grown material.

Calcium adsorption on MgO(100): energetics, structure, and role of defects.

The adsorption of Ca on the MgO(100) surface at 300 K has been studied using microcalorimetry, in combination with LEED, AES, ISS, work function, sticking probability measurements, and density functional theory (DFT) calculations. The MgO(100) thin films (approximately 4 nm thick) were grown epitaxially on a 1 microm thick Mo(100) single-crystal. The sticking probability of Ca on MgO(100) at 300 K is unity. On the basis of AES and ISS measurements, it was determined that Ca grows mainly as 3D particles on the MgO(100) surface with a density of approximately 1 x 10(12) islands/cm2. Ca adsorbs initially at defect sites with a very high heat of adsorption (approximately 410 kJ/mol). DFT calculations attribute this high initial heat to Ca binding to kink sites (376 kJ/mol), step sites (205 kJ/mol), and lower concentrations of stronger binding sites. The heat of adsorption decreases rapidly with coverage, reaching a minimum of 162 kJ/mol at approximately 0.3 ML, where Ca is mainly adding to small 3D Ca clusters. Afterward, it increases to the value of bulk Ca heat of sublimation (178 kJ/mol) at approximately 1.2 ML, attributed to the increase in stability with increasing Ca particle size. A 1.0 eV decrease of the work function with Ca coverage from 0 to 0.3 ML indicates that Ca adsorbed at defects is cationic, in agreement with calculations showing that Ca donates electron density to the MgO. Light ion sputtering of the MgO(100) surface generates point defects, but these do not change the heat of adsorption versus coverage, implying that they do not nucleate Ca particles. Oxygen vacancies are a likely candidate; DFT calculations show that F and F+ center vacancies bind Ca more weakly than terrace sites. More extensive sputtering creates extended defects (such as steps and kinks) that adsorb Ca with heats of adsorption up to approximately 400 kJ/mol, similar to that at the intrinsic defect sites.

Current-controlled channel switching and magnetoresistance in an Fe3C island film supported on a Si substrate

A film of magnetic Fe3C islands separated by nanochannels of graphite was prepared with pulsed laser deposition on a Si substrate with a native SiO2 surface. When the temperature is increased above 250 K the resistance suddenly drops because electron conduction switches from the film to the Si inversion layer underneath. The film shows a negative magnetoresistance. The inversion layer exhibits a large positive magnetoresistance. The transition to the low resistance channel can be reversed by applying a large measuring current, making possible current-controlled switching between two types of electron magnetotransport at room temperature.

Diffusion of TiN into aluminum films measured by soft x-ray spectroscopy and Rutherford backscattering spectroscopy

Understanding the atomic bonding properties at the interface between thin films is crucial to a number of key modern technical devices, including semiconductor integrated circuits, magnetic recording media, batteries, and even solar cells. Semiconducting materials such as titanium nitride (TiNx) are widely used in the manufacturing of modern electronic devices, requiring a wealth of information about its electronic structure. We present data from soft x-ray emission, soft x-ray absorption, and Rutherford backscattering spectroscopy experiments involving a sample consisting of a 40 nm TiN layer on top of an aluminum film 600 nm thick. Soft x-ray emission spectroscopy and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy are tools that provide a nondestructive, atomic site-specific probe of the interface, where the electronic structure of the material can be mapped out element by element. Rutherford backscattering spectroscopy (RBS) measurements supply data on the elemental composition and depth profiling of the sample. From these measurements, we show that the Ti and the N diffuse into the Al film to form an equivalent material depth of about 4.5 nm, and the NEXAF structure reveals that the nitrogen has probably formed AlN, and the Ti has also diffused to form a titanium–aluminum compound.Understanding the atomic bonding properties at the interface between thin films is crucial to a number of key modern technical devices, including semiconductor integrated circuits, magnetic recording media, batteries, and even solar cells. Semiconducting materials such as titanium nitride (TiNx) are widely used in the manufacturing of modern electronic devices, requiring a wealth of information about its electronic structure. We present data from soft x-ray emission, soft x-ray absorption, and Rutherford backscattering spectroscopy experiments involving a sample consisting of a 40 nm TiN layer on top of an aluminum film 600 nm thick. Soft x-ray emission spectroscopy and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy are tools that provide a nondestructive, atomic site-specific probe of the interface, where the electronic structure of the material can be mapped out element by element. Rutherford backscattering spectroscopy (RBS) measurements supply data on the elemental composition and dep...

Model study of soft x-ray spectroscopy techniques for observing magnetic circular dichroism in buried SmCo magnetic films

X-ray emission and absorption spectroscopy (XES and XAS, respectively) are important and powerful techniques for determining the electronic properties of materials. Both are used to study magnetic circular dichroism (MCD) which is especially useful for analyzing the magnetic properties of materials. We present XAS and XES measurements and a MCD model study of two thin film layered samples containing SmCo layers in order to report on the applicability of soft x-ray spectroscopic techniques to determine the composition, layer thickness, and electronic structure of such materials. Using a transmission by fluorescence attenuation (TFA) technique we determined the composition and thickness of the SmCo layer to be consistent with the intended composition and thickness. We also confirmed the thickness of the other layers by comparing the XES from the thin film with that of a bulk sample. We showed by a model study that TFA could be used to obtain MCD, and thus the anisotropy of the sample, for film thicknesses b...

Diffusion of TiN into aluminum films measured by soft x-ray spectroscopy

Understanding the atomic bonding properties at the interface between thin films is crucial to a number of key modern technical devices, including integrated circuits, magnetic disk read/write heads, batteries, and solar cells. Semi-conducting materials such as titanium nitride (TiNx) are widely used in the manufacturing of modern electronics, requiring a wealth of information about its electronic structure. We present data from soft x-ray emission and absorption experiments involving a sample consisting of a 40 nm TiN layer on top of an aluminum film 550 nm thick. Soft x-ray emission spectroscopy (XES) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy are tools that provide a non-destructive, atomic site-specific probe of the interface, where the electronic structure of the material can be mapped out element by element. From these measurements, we show that the Ti and the N diffuse into the Al film to form an equivalent material depth of about 4.5 nm, and the NEXAF structure reveals that...