K. Garre

Physical properties of lanthanum monosulfide thin films grown on (100) silicon substrates

Thin films of lanthanum monosulfide (LaS) have been deposited on Si (100) substrates by pulsed laser deposition. The films are golden yellow in appearance with a mirrorlike surface morphology and a sheet resistance around 0.1Ω∕◻, as measured using a four-probe measurement technique. The thin films are characterized by atomic force microscopy (AFM), x-ray diffraction (XRD) analysis, high resolution transmission electron microscopy (HRTEM), ellipsometry, and Raman spectroscopy. The root-mean-square variation of (1μm thick) film surface roughness measured over a 1μm2 area by AFM was found to be 1.74nm. XRD analysis of fairly thick films (micrometer size) reveals the growth of the cubic rocksalt structure with a lattice constant of 5.863(7)A, which is close to the bulk LaS value. HRTEM images reveal that the films are comprised of nanocrystals separated by regions of amorphous material. Two beam bright field TEM images show that there is a strain contrast in the Si substrate right under the interface with the...

Characterization and field emission properties of lanthanum monosulfide nanoprotrusion arrays obtained by pulsed laser deposition on self-assembled nanoporous alumina templates

Three distinct types of nanostructures—nanodomes, nanodots, and nanowires—have been simultaneously self-assembled by pulsed laser deposition of lanthanum monosulfide on anodic alumina films containing hexagonal arrays of pores about 50nm wide and 500nm deep. The nanostructures have been characterized by x-ray diffraction, atomic force microscopy (AFM), and field emission scanning electron microscopy (FE-SEM). Nanodomes preferentially grow on the boundary separating regions (grains) of the alumina template that have near perfect pore ordering, and their density is ∼109∕cm2. The diameter of a nanodome at the base is about 100nm and their aspect ratio (height/diameter at the base) is between 1 and 3. Additionally, nanodots nucleate on top of the alumina walls that separate adjacent pores. They have a diameter of ∼50nm, a density equal to the pore density (1010∕cm2), and an aspect ratio less than 1. Finally, cross sectional FE-SEM images of the templates indicate that LaS nanowires grow inside the pores with ...

Field emission properties of metallic nanostructures self-assembled on nanoporous alumina and silicon templates

Two types of nanoscale field emitter arrays have been self-assembled using either flexible alumina templates containing hexagonal close packed pores that are 50nm wide and 500nm long or nanoporous Si templates. The first type is an array of gold “nanopinetrees” obtained by e-beam evaporation of a few nanometers of gold on bare alumina templates. The second type is a nickel “nanoblade” array formed by e-beam evaporation of a few nanometers of nickel on nanoporous Si templates. The field emission characteristics of both mesoscopic structures were measured using a scanning field emission microscope, and results were analyzed in terms of the cathode material emission. This synthetic approach for producing nanoscale field emitters could lead to a versatile and inexpensive technology for synthesizing flexible arrays of nanoscale cold cathode emitters.

Field emission from lanthanum monosulfide thin films grown on the (100) magnesium oxide substrates

Lanthanum monosulfide (LaS) films were grown by pulsed laser deposition on the (100) magnesium oxide (MgO) substrates at an elevated substrate temperature and in a background gas of H2S. The thin films have been characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), and high resolution transmission electron microscopy. The film surface is composed of grainlike features with an average size of approximately 34nm. The root-mean-square variation of the film surface roughness measured over a 2×2μm2 area by AFM was found to be approximately 1.5nm. XRD data indicate that the average size of the nanocrystalline grains in the film is about 26nm, which is about twice the size of the grains found in LaS thin films deposited at room temperature on Si. The field emission (FE) properties of the films have been characterized by scanning anode field emission microscopy and are interpreted in terms of a recently developed patchwork FE model. The FE data indicate that there is roughly a seven times incre...

Correlated growth of organic material tris (8-hydroxyquinoline) aluminum (Alq3) and its relation to optical properties

We report slow correlated growth mode in energetic cluster vapor deposited organic light emissive material tris(8-hydroxyquinoline) aluminum from 5 to 100 nm. Phase modulated atomic force microscopy shows very slow grain growth with thickness, with very small phase differences within the film. Fractal dimension calculated from correlation function shows growth process above 10 nm consistent with diffusion-limited aggregation. For low thickness (5 nm), photoluminescence measurement shows the emission peak is shifted by ∼0.4 eV toward lower wavelength.

Field emission characteristics of a lanthanum monosulfide cold cathode array fabricated using microelectromechanical systems technology

Using microelectromechanical systems technology, an array of cold cathodes was fabricated by pulsed laser deposition of chemically and thermally stable lanthanum monosulfide (LaS) thin film anode and cathode contacts. The latter were defined via etching and processing of two different pieces of (100) Si wafers separated via a highly resistive sputter deposited aluminum nitride (AlN) layer, whose thickness was used to control the anode to cathode spacing. The top and bottom Si wafers were aligned and glued together using high temperature, vacuum compatible epoxy. Field emission characteristics were recorded in a vacuum chamber with a base pressure less than 10−7Torr. An average electric field threshold for Fowler-Nordheim field emission in the range of 100V∕μm was measured. The largest emission current measured was about 5×10−7A, above which thermal runaway occurred, leading to a failure of the cathode. The failure mechanism is analyzed in terms of a patchwork field emission model from the LaS thin film re...

Field emission from self-assembled arrays of lanthanum monosulfide nanoprotrusions

The field emission properties of LaS nanoprotrusions called nanodomes, formed by pulsed laser deposition on porous anodic alumina films, have been analyzed with scanning anode field emission microscopy. The voltage necessary to produce a given field emission current is ∼3.5 times less for nanodomes than for thin films. Assuming the same work function for LaS thin films and nanoprotrusions, that is, ∼1 eV, a field enhancement factor of ∼5.8 is extracted for the nanodome emitters from Fowler-Nordheim plots of the field emission data. This correlates well with the aspect ratio of the tallest nanodomes observed in atomic force micrograph measurements.

Organic nano-spintronics

We study spin transport in organic nanowires and show that the spin relaxation time in organics can be exceptionally long. This makes organics ideal for many applications in spintronics since the longevity of spin polarization is an important criterion for virtually all spintronic devices. We also discuss other important characteristics of spin transport in organics. Finally, we present results pertaining to the transverse spin relaxation time (T2 time) in organics since it is critical for quantum computing applications.

Self-assembled growth on flexible alumina and nanoporous silicon templates

Several nanoscale arrays of metallic, semiconductor, and organic carbon compounds (carbon nanopearls) have been fabricated on nanoporous flexible alumina and silicon templates based on a new self-assembly growth mode. They were obtained using pulsed laser deposition, thermal evaporation, e-beam evaporation, or RF magnetron sputtering. The different moieties that were observed include nanodomes and nanodots (gold, nickel, cobalt, and aluminum nitride), nanonecklaces (carbon nanopearl), and nanopinetrees (gold) self assembled on flexible alumina templates. A nanoneedle array was also self assembled by e-beam evaporation of nickel on silicon substrates that were rendered nanoporous by the use of a porous alumina mask. The physical processes underpinning the new self assembly growth mode have been studied based on extensive characterization of the templates prior to and after deposition of the various metallic, semiconductor, and organic compounds. These include atomic force microscopy (AFM), X-ray diffraction (XRD) analysis, Raman spectroscopy and field emission-scanning electron microscopy (FE-SEM). Some of the arrays have been tested as potential candidates for new cold cathode arrays for vacuum electronic applications using the scanning electron field emission microscopy (SAFEM) technique.

Field emission properties of multi-level self assembled mesoscopic structures on nanoporous templates

We report a new multi-modal self assembly technique in which a rich structural diversity of distinct moieties emerge when thin films of a wide variety of materials are deposited on nanoporous substrates using different techniques. We also report multi-level self assembly where one set of self assembled nanostructures seeds the self assembly of another set. The multi-modal self assembly has been observed using flexible alumina templates containing hexagonal arrays of cylindrical pores that are 50 nm wide and 500 nm in length and also on nanoporous Si templates. The latter were formed by anodization of thin film alumina templates deposited on platinum coated silicon substrates. The deposition parameters leading to the observation of each of the moieties will be discussed in detail and the field emission (FE) properties, measured by scanning anode field emission microscopy (SAFEM), will be reported.