A. Kaan Kalkan

Characteristics of amorphous and polycrystalline silicon films deposited at 120 °C by electron cyclotron resonance plasma-enhanced chemical vapor deposition

Amorphous and polycrystalline silicon (poly-Si) films, deposited by an electron cyclotron resonance plasma-enhanced chemical vapor deposition system at 120 °C, have been investigated. All films have been grown with either hydrogen or argon dilution. Using the films with the hydrogen dilution, the effect of rf (13.56 MHz) substrate bias has also been studied. Analysis with x-ray diffraction shows that films grown with Ar dilution and no rf bias do not show any crystallinity while the corresponding films deposited with H2 dilution and no rf bias contain a significant amount of the crystalline phase. With only a 3:1 H2 to silane ratio, highly crystallized films can be grown at 120 °C. In the presence of rf (13.56 MHz) substrate bias, there is a decrease of crystallinity in films. It has been found from cross-sectional transmission electron microscopy that films deposited without rf bias develop a very uniform columnar structure whereas films made with rf bias develop a closely packed, continuous but more amo...

Gold Nanowires for the Detection of Elemental and Ionic Mercury

We demonstrate for the first time the use of Au nanowires for the electrical detection of elemental and ionic mercury. By monitoring changes in resistance upon exposure to Hg vapor, concentrations as low as 6.25 μg/m 3 (2 X 10 -10 M or 5 ppb) were detected. Similarly, we used HgCl 2 in an aqueous solution as a source for Hg ions and were able to detect Hg 2+ concentrations as low as 10 -8 M. We also demonstrate the impact of the sensor surface to volume ratio, substantiate the model of a surface origin for the sensitivity, and underscore the advantages of the Au nanowire structure over the thin-film structure for Hg sensing.

Nanocrystalline Si thin films with arrayed void-column network deposited by high density plasma

High porosity nanocrystalline Si thin films have been deposited using a high density plasma approach at temperatures as low as 100 °C. These films exhibit the same unique properties, such as visible luminescence and gas sensitivity, that are seen in electrochemically etched Si (i.e., porous Si). The nanostructure consists of an array of rodlike columns normal to the substrate surface situated in a void matrix. We have demonstrated that this structure is fully controllable and have varied the porosity up to ∼90% (as derived from optical reflectance) by varying the deposition conditions. In particular, the impact of plasma power has been found to reduce porosity by increasing the nuclei density and therefore the areal density of columns. Humidity sensors have been demonstrated based on the enhanced conductivity of our films (up to 6 orders of magnitude) in response to increase in relative humidity. Depending on the porosity, the conductivity-relative humidity behavior of our films shows variations which can...

Laser-activated surface-enhanced Raman scattering substrates capable of single molecule detection

Surface-enhanced Raman scattering (SERS) substrates were obtained through silver reduction on and by nanostructured Si films. The absence of any chemical agents on the Ag nanoparticle surfaces allows analyte adsorption and SERS detection immediately with spotting. These SERS substrates have the further unique and useful attribute of being laser activated; i.e., laser impingement causes Ostwald ripening and formation of aggregates (e.g., dimers and trimers), which are essential for single molecule detection. Single molecule detection of fluorescein characterized with intermittent spectral fluctuations as well as a dramatic decrease in inhomogeneous Raman linewidth was demonstrated.

Charge-selective Raman scattering and fluorescence quenching by "nanometal on semiconductor" substrates.

Ag nanoparticles synthesized on n and p-type Si were shown to exhibit charge-selective surface-enhanced Raman scattering and fluorescence quenching. As revealed by electric force microscopy, the polarity and magnitude of the nanoparticle charge is controllable with the metal-semiconductor Fermi level difference and nanoparticle size. It is inferred that the Fermi level alignment is dominantly contributed by the charge-induced nanoparticle voltage. Nanoparticle charging also accounts for self-inhibition of coalescence during chemical reduction, allowing strong plasmon hybridization.

Biomedical/analytical applications of deposited nanostructured Si films

We report on various biomedical applications of our deposited nanostructured column–void Si films including respiratory monitoring, quick mass analysis for proteomics, and cell attachment. These applications exploit certain unique attributes of our nanostructured Si material that are not present for bulk Si, such as molecular immobilization, enhanced coupling with electromagnetic radiation, high surface area, and pronounced hydrophobicity. A brief review of morphology and film growth is also given according to our latest understanding. Our capability of controlling columnar separation and porosity by varying film growth conditions allows the tailoring of the properties of our films as well as the optimization of the performance in an application. The fact that these films can be deposited on low processing temperature substrates such as plastics further enhances their versatility.

Thermoset-Cross-Linked Lignocellulose: A Moldable Plant Biomass

The present work demonstrates a high biomass content (i.e., up to 90% by weight) and moldable material by controlled covalent cross-linking of lignocellulosic particles by a thermoset through epoxide-hydroxyl reactions. As an example for lignocellulosic biomass, Eastern redcedar was employed. Using scanning fluorescence microscopy and vibrational spectroscopy, macroscopic to molecular scale interactions of the thermoset with the lignocellulose have been revealed. Impregnation of the polymer resin into the biomass cellular network by capillary action as well as applied pressure results in a self-organizing structure in the form of thermoset microrods in a matrix of lignocellulose. We also infer permeation of the thermoset into the cell walls from the reaction of epoxides with the hydroxyls of the lignin. Compression tests reveal, at 30% thermoset content, thermoset-cross-linked lignocellulose has superior mechanical properties over a commercial wood plastic composite while comparable stiffness and strength to bulk epoxy and wood, respectively. The failure mechanism is understood to be crack propagation along the particle-thermoset interface and/or interparticle thermoset network.

Band-tail photoluminescence in nanocrystalline Si

The characteristic subgap photoluminescence (PL) observed in nanocrystalline Si films was found to shift to higher energies with increasing optical gap and decreasing crystallite size. This behavior, along with the temperature dependence of the PL, is consistent with transitions between band-tail states in the energy gap of the crystallites. The PL bandwidth is too broad to be explained by the tail width as deduced from the temperature behavior. Hence, the broadening is ascribed to electron–phonon coupling, while the related Stokes shift was found to increase with decreasing crystallite size, possibly due to decreasing exciton size.

Nano- and microchannel fabrication using column/void network deposited silicon

Nano- and microchannels are fabricated using a novel deposited column/void network silicon film as a sacrificial material. This nanostructured silicon consists of nanometer-sized columns defined normal to the substrate in a void matrix, where the voids are continuously connected with each other, forming a network. The void network structure results in a high sacrificial layer etch rate due to the void network-enhanced transport of reactant and reaction products during the etching process, and high effective surface area. The use of our unique deposited column/void network material coupled with lift-off processing results in a manufacturable process for nano- and microchannel and nano- and microcavity fabrication. The approach provides extremely flat surfaces without a chemical–mechanical polishing process, and allows for multiple layers of channel or cavity structures with crossovers.

Stress and Dopant Activation in Solid Phase Crystalized Si Films

Defect creation mechanisms during solid phase crystallization (SPC) of Si thin films were investigated with PECVD amorphous precursor samples produced with various deposition temperatures and thicknesses. These precursor films were implanted with dopant and then crystallized to obtain both SPC and dopant activation. The doping efficiency was found to decrease with the tensile stress level as measured by Raman shift. The stress shows a decrease as the precursor deposition temperature and thickness are lowered. Furthermore, a lower level of stress is induced by rapid thermal annealing when the annealing temperature is high enough to soften the glass substrate on which the films were deposited. We show that by control of stress during the SPC step, intragrain defect density can be lowered and electronic quality of the resulting polycrystalline Si films can be improved. Based on these observations, we propose the following tentative model to explain the defect creation: during SPC, tensile stress evolution is considered to result from the volumetric contraction of Si film when it transforms from the amorphous to crystalline phase. This contraction is retarded by the substrate, which imposes a tensile stress on the film. A high level of stress leads to formation of structural defects inside the grains of the resulting polycrystalline material. These defects trap carriers or complex with the dopant reducing doping efficiency.