Session Y28: Nanotechnology I

Nanoscale Assembly of Actuating Cilia-Mimetic

The cilium is among the smallest mechanical actuators found in nature. We have taken inspiration from this design to create magnetic nanochains, measuring approximately 1-5 $\mu $m long and 25 nm in diameter. Fabricated from the self-assembly of cobalt nanoparticles, these flexible filaments actuate in an oscillating magnetic field. The cobalt nanoparticles were functionalized with a polystyrene/benzaldehyde surface coating, thus allowing the particles to form imine bonds with one another in the presence of a diamine terminated polyethylene glycol. These imine bonds effectively cross-linked the particles and held the nanochains together in the absence of a magnetic field. Using design of experiments (DOE) to efficiently screen the effects of cobalt nanoparticle concentration, crosslinker concentration, and surface chemistry, we determined that the morphology of the final structures could be explained primarily by physical interactions (i.e. magnetic forces) rather than chemistry.   

Amplification by 1/f noise with stochastic resonance in silicon-based nanomechanical resonators.

We report signal amplification by 1/f noise with stochastic resonance in a nanomechanical two-state system of a nonlinear silicon resonator. The addition of 1/f noise to a sub-threshold modulation signal enhances the likelihood of an electrostatically driven resonator switching between its two states in the hysteretic region. Considering the prevalence of 1/f noise in integrated circuits, signal enhancement demonstrated here, using a fully on-chip electronic actuation/detection scheme, suggests potentially beneficial use of the otherwise detrimental noise.   

High Frequency Antennas for Wireless Transmissions of Audio and Video Signals Using Threads Spun From Long Multi-Wall Carbon Nanotubes.

We have used threads spun from long multiwall carbon nanotubes (MWCNT) to make antennas for audio and video broadcasts (transmission and reception) at GHz frequencies. The MWCNT used to make the threads have outer diameters from about 6 nm to 30 nm. These MWCNT's have been grown in lengths up to 18 mm. The diameter of the CNT threads used to fabricate the high frequency antennas was 25 microns. Initial measurements consist of 1) transmission and reception of a CW signals at f$_{ }$= 694 MHz and 1388MHz , 2) the transmission and detection of a CW signal plus sidebands at $\pm $ 100kHz, 3) the broadcast and reception of an AM modulated audio signal, 4) the broadcast and reception of composite video images, 5) the simultaneous broadcast and reception of audio signals from a single CNT antenna, and 6) the simultaneous transmission and/or reception at multiple frequencies from a single CNT thread antenna. The results of using the CNT thread antenna for these transmissions will be discussed.   

Noise color and asymmetry in stochastic resonance with silicon nanomechanical resonators

Stochastic resonance (SR) with white noise has been well established as a potential signal amplification mechanism in nanomechanical two-state systems. While white noise represents the archetypal stimulus for SR, typical operating environments for nanomechanical devices often contain different classes of noise, particularly colored noise with a $1/f$ spectrum. As a result, improved understanding of the effects of noise color is necessary in maximizing device performance. Here, we report measurements of SR in a silicon nanomechanical resonator using $1/f$ noise and exponentially correlated Ornstein-Uhlenbeck noise. Power spectral densities and residence time distributions provide insight into asymmetry of the bistable amplitude states, and evidence suggests that $1/f^{\alpha }$ spectra with increasing noise exponent $\alpha $ may lead to increasing asymmetry in the system, reducing the achievable signal-to-noise ratio. Furthermore, we explore the effects of correlation time $\tau $ on SR with the use of exponentially correlated noise. We find monotonic suppression of the spectral amplification as the correlation time increases.   

Room-Temperature Single-Electron Transistors fabricated using CMOS-compatible processes

A critical requirement for the fabrication of single-electron devices is that the device components (Coulomb island, source, drain, and gate electrodes) be arranged with nanometer scale precision. We present a new single-electron device structure which consists of vertically stacked source and drain electrodes separated by a thin dielectric film. Using this configuration, we were able to control the gap between the electrodes with nanometer scale precision over an entire wafer, thereby allowing the concurrent fabrication of many device units in parallel processing. Coulomb islands (10 nm Au nanoparticles) were positioned in the gap between the source and the drain electrodes. Individually addressable gate electrodes were then incorporated in these devices, also in complete parallel processing. These devices have yielded clear single-electron transport characteristics (Coulomb blockade/staircase and Coulomb oscillations) at room temperature as well as at low temperatures ($\sim $10 K). The experimental data is in excellent agreement with the orthodox theory of single-electron tunneling. This study suggests that the fabrication of chip-level integrated systems of single-electron devices may now be possible using current CMOS fabrication technology. (ONR (N00014-05-1-0030), NSF CAREER (ECS-0449958), and THECB ARP (003656-0014-2006)).   

Electronic-structure modulation transistor: A new switch with few kT supply voltage

We present a novel electronic-structure modulation transistor (EMT) for post-CMOS logic applications. The device is based on the electronic structure modulation of the channel by an external gate voltage. Its functionality is theoretically analyzed using single-band tight-binding model and non-equilibrium Green's function formalism. We report that the EMT is expected to have very large ON/OFF current ratio with reasonable self gain using a few kT Vdd. We provide an experimental proof-of-concept device of the proposed mechanism in a double gated structure using a 20 nm long and 10 um wide channel consisting of Au nanocrystals (NCs) and nitride traps. Putting negative charge on the NCs is results in wavefunction extension over larger distance due to lifting of the energy levels, resulting in reduction of the effective barrier. In transfer characteristics, we find a nonlinear dependence of the drain current on gate voltage and charge stored in the channel, which we attribute to the wavefunction modulation of the Au NCs due to charging.   

The electronic structure of diodes probed under bias

Chemists have known for decades that when metal nano-particles are affixed to a catalytically inactive oxide surface, the catalytic turnover rate of the array is more than 10 times that of a metal surface alone. However, the mechanism behind the effect is not clear. To understand the catalytic activity of the interface between the metal nano-structures and the oxide substrate, we have investigated the electronic structure of Pt and Pd doped diodes on a TiO$_2$ substrate. The devices were put under bias during the measurements in an attempt to reproduce the potential differences found over the diode when used as a catalyst. This is challenging for electron based measuring techniques, but using photon-in, photon-out techniques we have successfully probed the electronic structure of Pt and Pd doped diodes under bias. The results from soft x-ray absorption and emission will be presented.   

Spatial Wavefunction Switched Field-Effect Transistors (SWS-FETs): A Novel Device with Multiple States and Functionality

An asymmetric coupled quantum well transport channel FET is shown to confine carriers in either the lower of the two wells, both wells, or upper well (adjacent to the gate insulator) depending on the gate voltage. That is, as the gate voltage is increased above threshold in n-channel FET, the electron wavefunction is spatially switched, which in turn change the operating characteristics. A Spatial Wavefunction Switched (SWS) FET, having two coupled wells in the channel, provides four states 00, 01, 10, 11 corresponding to wavefunction location. No wavefunction being the OFF (00 state), electrons in well W2 (01 state), in well W1 (10 state), and both Wells W1-W2 (11 state). Simulation has verified the spatial switching in SiGe as well as InGaAs coupled well FET structures. The wavefunctions/carrier locations get more pronounced and result in additional states when the transport channel is configured as a quantum dot (QD) channel. Preliminary simulation of quantum dot gate 3-state structures [1], configured as SWS-QD channel FET, will also be presented. [1]. F.C. Jain, E. Heller, S. Karmakar, and J. Chandy, Device and Circuit Modeling using Novel 3-State Quantum Dot Gate FETs, ISDRS Proc 2007. *Supported in part by ONR Contract N00014-06-1-0016 and NSF ECS 0622068 grant.   

Ultrathin germanium-on-insulator tunneling field effect transistors

As the CMOS downscaling is approaching its limits, there is greater need for alternative and unconventional devices to continue enhancing the performance of electronics. We report on the fabrication and electrical characterization of a CMOS-compatible germanium-on-insulator (GeOI) tunneling field effect transistor (TFET) device that can in principle switch more sharply than a standard FET. The source-drain current in the TFET is based on interband tunneling between an inversion channel and a counterdoped drain electrode. Taking advantage of the narrower bandgap of germanium, the devices are fabricated in ultra-thin GeOI and consist of a heavily $p$-type doped, epitaxially grown drain, an $n$-type ion-implanted source and a standard high-\textit{$\kappa $} dielectric gate stack with channel lengths down to 400 \textit{nm}. The devices exhibit a reasonable on-off current ratio of more than 10$^{2}$ and improved on current compared to silicon-on-insulator TFETs. Current-voltage measurements at room and low temperatures will be presented to characterize the behavior of the fabricated transistors.   

Full Recovery of PFET NBTI and NFET PBTI of high-k metal gate MOSFETs with high temperature bake

High-k metal gate MOSFET devices exhibit Bias Temperature Instability (BTI) degradation mechanisms. The p-channel Field Effect Transistor (PFET) has NBTI which results in threshold voltage (Vt) decrease and drive current (Ion) decrease when the gate is biased negatively with respect to the channel. The n-channel Field Effect Transistor (NFET) has PBTI which results in threshold voltage (Vt) increase and drive current (Ion) decrease when the gate is biased positively with respect to the channel. The amount of NBTI and PBTI is process dependent and depends on temperature, gate voltage, time and gate oxide thickness. PBTI has stronger dependence on voltage than NBTI and NBTI has stronger temperature dependence than PBTI. However, with a high temperature (370 $^{\circ}$C) bake, full recovery of both NBTI and PBTI is achieved and the devices behave like unstressed devices on repeated BTI stress.   

Band Structure Engineering of PtSi

PtSi is being considered as a contact material in field effect transistors. It has an additional advantage of having a low Shottky barrier to p-type Si. The relatively low conductivity of PtSi compared for example to pure Pt can be traced to the low density of states at the Fermi level. In this theoretical study we discuss a method to increase the conductivity of PtSi by manipulating the density of states through alloying. The scheme is based on substituting Pt atoms by Ti atoms to shift the Fermi level to a higher density of states region. We find identify a compound with the carrier concentration 2.7 times larger than that of bulk PtSi. We estimate the formation energies of the compounds and determine the solubility limit of Ti in PtSi at room temperature. We analyze the effect of doping with Ti on the work function for the (121) surface (the lowest energy surface orientation of PtSi). Moreover, we study possible schemes to lower the formation energies of the alloys by further doping with boron, carbon, gallium and aluminum. We identify a stable alloy in the case of aluminum doping. All calculations are done within the framework of density functional theory.   

Ideal SiC Schottky Barrier Diodes Fabricated Using Refractory Metal Borides

We present results of n-type 4H-SiC Schottky barrier diodes fabricated using several refractory metal boride Schottky contacts deposited on SiC held at various temperatures. From the electrical properties determined by current-voltage measurements, diodes with contacts deposited on SiC substrates held at 600 $^{o}$C had average ideality factors in the range 1.04 -- 1.09 and Schottky barrier heights of 1.02 eV -- 1.14 eV; and these values remained unchanged after the diodes were annealed at 600 $^{o}$C for 20 min. Diodes with contacts deposited on substrates held at 20 $^{o}$C had much higher ideality factors which decreased only slightly after the annealing. The Rutherford backscattering spectroscopy spectra of these contacts revealed a systematic decrease in oxygen with increase in the deposition temperature. The improved electrical properties and thermal stability are attributed to the removal of oxygen from the boride/SiC interface during high temperature deposition.   

Achieving a low interfacial density of states in atomic layer deposited Al$_{2}$O$_{3 }$on In$_{0.53}$Ga$_{0.47}$As

Atomic-layer-deposited (ALD) Al$_{2}$O$_{3}$ dielectrics on In$_{0.53}$Ga$_{0.47}$As with short air exposure between oxide and semiconductor deposition has been demonstrated nearly ideal capacitance-voltage ($C-V)$ characteristics with negligible frequency dispersion at flat-band and accumulation. A relationship of surface potential versus gate voltage derived by the excellent quasi-static $C-V$ curve shows high efficiency of 63{\%} for Fermi-level movement near the mid-gap. A low mean interfacial density of states ($\overline D it) \quad \sim $ 2.5x10$^{11}$ cm$^{-2}$eV$^{-1}$ was determined using the charge pumping method, which was also employed to probe the depth profile of bulk trap density ($N_{bt})$ and the energy dependence of $D_{it}$ measured at 50kHz: a low $N_{bt} \quad \sim $ 7x10$^{19}$ cm$^{-3}$ and $D_{it}$ of 2-4x10$^{11}$ cm$^{-2}$eV$^{-1}$ in the lower half of the band-gap and a higher $D_{it}$ of $\sim $10$^{12}$ cm$^{-2}$eV$^{-1}$ in the upper half of the band-gap. The employment of charge pumping method has given a more accurate determination of $D_{it}$, which is usually overestimated using other commonly methods such as Terman, conductance, and high-low frequencies, due to the influence of weak inversion at room temperature.   

Palladium and Palladium-Carbon Nanotube Composite Nanomechanical Resonator

For its bio-compatibility, conductivity and optical reflectivity, metallic thin films are an attractive choice to realize multifunctional micromechanical resonators. However, moderate elastic properties of metallic thin films are ill suited for high frequency applications. Meanwhile, Carbon nanotubes have shown great potential with superior electrical and mechanical properties. Combined Metal-CNT nanolaminates have increased strengths and are less susceptible to onset of mechanical nonlinearity compared to equivalent metal beams without CNT. With palladium's good affinity to CNT to further study the role of the metal-CNT interface, we realized doubly clamped beam and torsional resonators from Palladium and Palladium/CNT composite. Resonance frequencies were detected using optical modulation technique with different wavelength at room temperature under moderate vacuum. Comparing the dynamic flexural response of Pd and Pd/CNT doubly clamped beam and torsional resonators, we will also discuss the difference between Pd-CNT and Al-CNT resonators as well as actuating the resonators electrostatically and optically