Brian G. Willis

Nanometer spaced electrodes using selective area atomic layer deposition

Nanoelectrodes with spacing controlled between 1 and 10nm with subnanometer increment have been achieved using atomic layer deposition. Field emission and metal-vacuum-metal tunneling are used to characterize the electrode properties in situ during growth. The current-voltage data is modeled and gives electrode spacing of 1.0±0.2nm, a barrier height of 4.5eV, and electrode radius of 10nm. Temperature variation from 26to235°C changes the spacing by 0.05nm, as calculated from electrical data. This is close to 0.1nm expected from thermal expansion. Exposing to air reduces the barrier height to 2.15eV, which is explained by the growth of a thin metal oxide layer.

Nanoscale Devices for Rectification of High Frequency Radiation from the Infrared through the Visible: A New Approach

We present a new and viable method for optical rectification. This approach has been demonstrated both theoretically and experimentally and is the basis fot the development of devices to rectify radiation through the visible. This technique for rectification is based not on conventional material or temperature asymmetry as used in MIM (metal/insulator/metal) or Schottky diodes, but on a purely sharp geometric property of the antenna. This sharp “tip” or edge with a collector anode constitutes a tunnel junction. In these devices the rectenna (consisting of the antenna and the tunnel junction) acts as the absorber of the incident radiation and the rectifier. Using current nanofabrication techniques and the selective atomic layer deposition (ALD) process, junctions of 1 nm can be fabricated, which allow for rectification of frequencies up to the blue portion of the spectrum. To assess the viability of our approach, we review the development of nanoantenna structures and tunnel junctions capable of operating in the visible region. In addition, we review the detailed process of rectification and present methodologies for analysis of diode data. Finally, we present operational designs for an optical rectenna and its fabrication and discuss outstanding problems and future work.

Characterization of ALD copper thin films on palladium seed layers

A method for fabricating monolithic nanoscopic tunnel junctions (MNTJs) for tunneling spectroscopy measurements using atomic layer deposition (ALD) of Cu on Pd seed layers has recently been introduced [Gupta and Willis, Appl. Phys. Lett. 90, 253102 (2007)]. The ALD grown layers are characterized here using planar thin films as models for the nanoelectrode composition and structure. ALD Cu films grown on Pd seed layers using a varying number of deposition cycles were characterized using transmission electron microscopy, Auger electron spectroscopy (AES), and glancing incidence x-ray diffraction (GIXRD) to investigate the chemical composition and structure of the nanoelectrodes. Electron diffraction and GIXRD show that as Cu is deposited, the bulk composition progresses from being Pd rich to becoming predominately Cu. In contrast, AES data show that significant Pd consistently remains on the surface of the growing film. The divergence in surface and bulk behaviors is attributed to Pd surface segregation tha...

In-situ spectroscopic ellipsometry study of copper selective-area atomic layer deposition on palladium

Selective area copper atomic layer deposition on palladium seed layers has been investigated with in-situ real-time spectroscopic ellipsometry to probe the adsorption/desorption and reaction characteristics of individual deposition cycles. The reactants are copper bis(2,2,6,6-tetramethyl-3,5-heptanedionate) vapor and hydrogen gas. Self-limiting atomic layer deposition was observed in the temperature range of 135–230 °C in a low pressure reactor. Under optimal conditions, growth occurs selectively on palladium and not on silicon dioxide or silicon nitride layers. Based on in-situ ellipsometry data and supporting experiments, a new mechanism for growth is proposed. In the proposed mechanism, precursor adsorption is reversible, and dissociatively adsorbed hydrogen are the stable surface intermediates between growth cycles. The mechanism is enabled by continuous diffusion of palladium from the seed layer into the deposited copper film and strong H* binding to palladium sites. Less intermixing can be obtained at low growth temperatures and short cycle times by minimizing Cu/Pd inter-diffusion.

DNA gold nanoparticle nanocomposite films for chemiresistive vapor sensing.

Chemiresistive vapor sensors combining functionalized gold nanoparticles and single-stranded DNA oligomers are investigated to enhance specificity in chemical sensing. Sensors are made by depositing DNA-functionalized gold nanoparticles onto microfabricated electrodes using four distinct sequences. Sensor performance is evaluated for response to relative humidity and exposure to vapor analytes including ethanol, methanol, hexane, dimethyl methylphosphonate, and toluene under different relative humidity. It is found that sensors display a nonmonotonic resistance change toward increasing humidity due to the combined effects of hydration induced swelling and ionic conduction. Responses to vapor analytes show sequence-dependent patterns as well as a strong influence of humidity. Overall, the findings are encouraging for using DNA oligomers to enhance specificity in chemical sensing.

Real-time spectroscopic ellipsometric investigation of adsorption and desorption in atomic layer deposition: A case study for the strontium bis(tri-isopropylcyclopentadienyl)/water process

The atomic layer deposition (ALD) of SrO thin films from Sr(C5iPr3H2)2 (g) and H2O (g) was studied using real-time spectroscopic ellipsometry (SE) investigations of adsorption and desorption during each half cycle. Adsorption of Sr(C5iPr3H2)2 was self-terminating at deposition temperatures of 150–350 °C and the saturated growth per cycle (GPC) highly depended on the deposition temperature, ranging from 0.05 to 0.33 nm/cycle at the lower and upper limits, respectively. Submonolayer sensitivity of SE was demonstrated by examining changes in the ellipsometric parameters and apparent thickness before and after precursor pulses. A comparison between experimental GPC and available theoretical models demonstrates that the deposition temperature has a marked effect on the reaction mechanism and indicates more than one operation regime for the ALD process of Sr(C5iPr3H2)2 and H2O.

Atomic layer deposition synthesis and evaluation of core–shell Pt-WC electrocatalysts

Pt-WC core shell particles were produced using atomic layer deposition (ALD) to deposit Pt layers onto WC particle substrates. A range of Pt depositions were used to determine the growth mechanism for the Pt-WC powder system. TEM imaging and Cu stripping voltammetry found that Pt ALD growth on WC powder substrates was similar to that on WC thin films. However, excess free carbon was found to affect Pt ALD by blocking adsorption sites on WC. The Pt-WC samples were evaluated for the oxygen reduction reaction using a rotating disk electrode to obtain quantitative activity information. The mass and specific activities for the 30 and 50 ALD cycle samples were found to be comparable to a 10 wt. % Pt/C catalyst. However, higher overpotentials and lower limiting currents were observed with ALD Pt-WC compared to Pt/C catalysts, indicating that the oxygen reduction mechanism is not as efficient on Pt-WC as on bulk Pt. Additionally, these Pt-WC catalysts were used to demonstrate hydrogen evolution reaction activity and were found to perform as well as bulk Pt catalyst but with a fraction of the Pt loading, in agreement with the previous work on Pt-WC thin film catalysts.

Oxygen pressure dependence of copper ion transport in SiO2 dielectrics

Electrical bias-temperature stress measurements were performed on copper oxide/SiO2/silicon metal-oxide-semiconductor capacitors to measure the oxygen partial pressure dependence of the copper ion transport rates through the SiO2 layer. Both copper(I) oxides (Cu2O) and copper(II) oxides (CuO) were investigated, and copper oxide film stoichiometry was characterized by glancing incidence x-ray diffraction and x-ray photoelectron spectroscopy. At relatively high oxygen pressures, copper ion transport is observed for both Cu2O∕SiO2∕Si and CuO∕SiO2∕Si, and the activation energies are comparable. Under reduced oxygen pressures and vacuum, the copper ion flux is reduced for both copper oxides. In vacuum, the activation energy for copper transport from CuO layers is reduced by approximately 0.7eV compared to the apparent activation barrier at high oxygen pressures, and the copper transport signals for Cu2O are below the detection limit of the experiment. A study of copper transport rates for both CuO and Cu2O ele...

Tunable plasmonic response of metallic nanoantennna heterodimer arrays modified by atomic-layer deposition

Abstract. We present a systematic study of tunable, plasmon extinction characteristics of arrays of nanoscale antennas that have potential use as sensors, energy-harvesting devices, catalytic converters, in near-field optical microscopy, and in surface-enhanced spectroscopy. Each device is composed of a palladium triangular-prism antenna and a flat counter-electrode. Arrays of devices are fabricated on silica using electron-beam lithography, followed by atomic-layer deposition of copper. Optical extinction is measured by employing a broadband light source in a confocal, transmission arrangement. We characterize the plasmon resonance behavior by examining the dependence on device length, the gap spacing between the electrodes, material properties, and the device array density, all of which contribute in varying degrees to the measured response. We employ finite-difference time-domain simulations to demonstrate good qualitative agreement between experimental trends and theory and use scanning electron microscopy to correlate plasmonic extinction characteristics with changes in morphology.

The Role of Geometry in Nanoscale Rectennas for Rectification and Energy Conversion

We have previously presented a method for optical rectification that has been demonstrated both theoretically and experimentally and can be used for the development of a practical rectification and energy conversion device for the electromagnetic spectrum including the visible portion. This technique for optical frequency rectification is based, not on conventional material or temperature asymmetry as used in MIM or Schottky diodes, but on a purely geometric property of the antenna tip or other sharp edges that may be incorporated on patch antennas. This “tip” or edge in conjunction with a collector anode providing connection to the external circuit constitutes a tunnel junction. Because such devices act as both the absorber of the incident radiation and the rectifier, they are referred to as “rectennas.” Using current nanofabrication techniques and the selective Atomic Layer Deposition (ALD) process, junctions of 1 nm can be fabricated, which allow for rectification of frequencies up to the blue portion of the spectrum (see Section 2).