Darrell L. Niemann

Effects of cathode structure on the field emission properties of individual multi-walled carbon nanotube emitters

We report the effect of cathode structure on the field emission properties of individual carbon nanotubes. Experimental field emission data are obtained for two well-defined cathode structures: a multi-walled carbon nanotube (MWNT) attached to an etched Ni metal wire and a MWNT attached to a flat Ni-coated Si microstructure. We observed different macroscopic turn-on fields of 1.6 and 2.5 V μm -1 , respectively, for the aforementioned experimental structures. This effect is investigated by detailed finite element analysis. We demonstrate that the geometry of the cathode structures significantly affects the microscopic tip field, leading to different turn-on voltages and field distributions for such individual MWNT emitters. Simulations show that changing the support geometry from a hemispherically capped shank to a cylindrical shank produces an increase in the macroscopic threshold field of 0.91 V μm -1 . This effect is further investigated by varying the support radius from 0.5 to 30 μm for a cylindrically shaped support structure. The results show that such a variation in the radius of the support structure produces an increase in the macroscopic turn on field from 0.72 to 5.89 V μm -1 . We also report quantitative evidence for the nonlinear relationship between the field enhancement factor as a function of support structure radius for nanostructures of three different aspect ratios.

Modeling intrinsic fluctuations in decananometer MOS devices due to gate line edge roughness (LER)

Intra-die random fluctuation outcomes inherent to fabrication processes such as gate LER give rise to corresponding fluctuations in device characteristics. These fluctuations become significant for devices with channel length less than 50 nm, a feature size rapidly approaching practical interest. At this scale, the fringe electric field and the charge confinement near the interface play dominant roles in determining MOS device properties and their fluctuations. In this work, we first characterize LER as a lognormal probability density function (pdf) in spatial frequency. Then we apply a 3D quantum mechanically corrected variational principle (VQM) to obtain closed-form expressions for standard deviation of threshold voltage and device capacitance due to LER. Our approach provides a simple physics based alternative to the presently available TCAD simulation for investigating these complex issues as functions of gate size, oxide thickness, and channel doping level.

Empirically Verified Thermodynamic Model of Gate Capacitance and Threshold Voltage of Nanoelectronic MOS Devices With Applications to

A thermodynamic variational model derived by minimizing the Helmholtz free energy of the MOS device is presented. The model incorporates an anisotropic permittivity tensor and accommodates a correction for quantum-mechanical charge confinement at the dielectric/substrate interface. The energy associated with the fringe field that is adjacent to the oxide is of critical importance in the behavior of small devices. This feature is explicitly included in our model. The model is verified using empirical and technology-computer-aided-design-generated capacitance-voltage data obtained on MOS devices with ZrO2, HfO2, and SiO2 gate insulators. The model includes considerations for an interfacial low-k interface layer between the silicon substrate and the high-k dielectric. This consideration enables the estimation of the equivalent oxide thickness. The significance of sidewall capacitance effects is apparent in our modeling of the threshold voltage (Vth) for MOS capacitors with effective channel length at 30 nm and below. In these devices, a variation in high-k permittivity produces large differences in Vth. This effect is also observed in the variance of Vth, due to dopant fluctuation under the gate.

\hbox{HfO}_{2}

In this article, we present a new methodology to characterise surface nanostructures of thin films. The methodology focuses on isolating nanostructures and extracting quantitative information, such as their shape and size, based on atomic force microscopy (AFM) data. We start by compensating the AFM data for some specific classes of measurement artefacts. After that, the methodology employs two distinct approaches. The first, which we call the overlay approach, proceeds by systematically processing the raster data that constitute the scanning probe image in both vertical and horizontal directions. The second approach, based on fuzzy logic, relies on a fuzzy inference engine to classify the surface points. Once classified, these points are clustered into surface structures. We have applied both approaches to characterise organic semiconductor thin films of pentacene on different substrates. In this article, we present results employing both approaches to pentacene films deposited on mica. Using only the overlay approach, we have compared the pentacene film characteristics grown on different substrates. These results are discussed and compared along with the challenges of the two approaches.

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This paper reports experimental data for individual multi-walled CNT (MWNT) emitters. For comparison, a thermodynamics-based variational method was used to obtain a closed-form expression for the radial electric field of a spherical cathode. Theoretical data generated from Technology Computer-Aided Design (TCAD) device simulation software is also reported. A fundamental understanding of the individual CNT emitter has important ramifications for electron microscope applications. Developing theoretical models for nanotube field emission properties using a thermodynamics-based variational approach (Gunther et al., 2004) as well as TCAD (Smith et al., 2005) would provide design rules for optimizing carbon nanotube film emitters for various applications

\hbox{ZrO}_{2}

In this work we investigate the effect of various structural geometry on the field enhancement factor of an individual carbon nanotube (CNT) field emitter. We employ a computational model, that has been experimentally verified, to investigate the influence of system component geometry. Our results show that, in certain cases, the influences of support length and anode-cathode separation distance are of comparable magnitude. Additionally, the effect of CNT radius dominates the system electrostatics as compared to other structural geometries, such as anodecathode distance, the cathode structure supporting the CNT emitter and the length of the emitter.

Gate Insulators

In this paper, we present an empirical study of dynamic behavior of an electron source system which incorporates an individual CNT. We propose a representative circuit model that is simple yet particularly valuable for emission current control for each CNT emitter in an array to facilitate high throughput maskless lithography.

Systematic quantitative characterisation of surface nanostructures by scanning probe microscopy of thin-films

The effects of shape and the surface area of cathode support structure on the field emission properties of carbon nanotube cathodes are investigated and quantified in this paper. This study demonstrates that the overall structural geometry, specifically the area of the cathode support structure, plays a significant role and should be considered in the design rules for optimization of electron source for advanced electron microscopy.

An Experimental Study and Modeling of the Field Emission Properties of an Isolated Individual Multi-Walled Carbon Nanotube

Recently thin film organic semiconductor (OS) materials such as poly-phenylene-vinylene (PPV), pentacene, etc., have attracted the attention of researchers for use in low cost alternatives to existing silicon applications including RFDDs as well as promising new frontiers such as flexible electronic displays. Typically, these films exhibit strong anisotropic electronic polarization effects and possess conduction properties similar to those in p-type amorphous silicon. The capacitance-voltage characteristic can be considered as one of the effective tools for investigating electronic polarization effects on the performance of devices using such films

Computational modeling of field enhancement in an individual carbon nanotube field emitter system

Random fluctuations in fabrication process outcomes such as Si-SiO2 interface surface roughness (SR) and gate line edge roughness (LER) give rise to corresponding fluctuations in scaled down MOS device characteristics. These fluctuations are intra-die and inherent even to ideal processes. As such, they represent fiudamental limitations to the die-level uniformity of the properties of otherwise identical devices. Modeling based on statistical characterization of fluctuations in the characteristics of small 3D devices is becoming increasingly important to understand the implications for product designers and process engineers. [1-5] Presently, TCAD numerical simulation is the only tool available for investigating the complex interaction of these issues. In this work, we employ a novel device modeling approach based on thennodynamics and on variational mathematical methods. [6] We obtain closed-form expressions for threshold voltage (Vth), and device capacitance (C) at Onset of Strong Inversion (OSI) for MOS devices in the deep sub-0.1 micron regime. In our model SR is assumed to affect only the gate area whereas LER affects only the gate perimeter of the device. Figure 1 shows the SEM of the fabricated line used in our analysis of SR and LER. [7-8] We take the roughness of this line to be representative and characterize it by Fourier transforming the digitized data. By choosing a pdf to represent the entire spectrm we can then identify the average frequency and the variance. Figure 2 shows the FFT of the digitized data (circles) from Fig. 1, together with the lognormal pdf (solid line) used to fit the data. We have looked in some detail at three possible candidate pdfs: exponential, gaussian, and lognormal. For a combination of reasons we prefer the lognormal. [9] Our variational model predicts that the random deviation of threshold voltage due to LER should increase as the square of the roughness amplitude. Figure 3 shows this variation for a MOSCAP of gate length 35 nm and a width of 50 nm. Our results are compared here with results from Kim et al. [10] In both cases the variation appears to be quadratic in roughness amplitude. Figure 4 shows our prediction of the random deviation of total capacitance of the device at OSI for both LER and SR against standard deviation of roughness wavenumber. Figure 5 shows the random deviation of Vt for LER and SR against average roughness wavenumber according to our model. These statistical characteristics are extremely difficult to capture using TCAD. In contrast, the novelty and strong advantage of our modeling approach is that it allows us to treat the situation with less difficulty by explicitly incorporating these statistical quantities. Oxide thickness is one of the key parameters that affect the variance in Vh. Figure 6 shows the effect of variation in the oxide thickness on the random deviation of Vh according to our model. The model predicts that the deviation is greater for LER than for SR. Interestingly, the deviation in Vh due to roughness reduces drastically for oxide thicknesses less than 4 nm.