Moh'd Rezeq

Theoretical and experimental investigations of nano-Schottky contacts

Formation of metal-semiconductor (M-S) contacts at sub-20 nanometer range is a key requirement for down-scaling of semiconductor devices. However, electrical measurements of M-S contacts at this scale have exhibited dramatic change in the current-voltage (I-V) characteristics compared to that of conventional (or planar) Schottky contacts. This change is actually attributed to the limited metal contact region where the transferred charge from the semiconductor into the metal is confined to a small surface area, which in turn results in an enhanced electric field at the nano-M-S interface. We here present detailed theoretical models to analyze the nano-M-S junctions at 10 nm contact range and then implement this analysis on the experimental data we conducted under these conditions. Both theoretical and experimental results demonstrate a significant effect of the contact size on the electronic structure of the M-S junctions and thus on the I-V characteristics. This effect is rather prominent when the size of...

Characterization of nano Schottky junctions for a new structure of nano-electronic devices

There is an increasing interest in reducing the size of semiconductor devices to sub 20 nm scale for technical requirements, like low power consumption and high switching speed. Electronic devices based on nano Schottky junctions have the potential to address these issues. This is because nano metal-semiconductor contacts are expected to have narrower barriers compared to conventional Schottky diodes. Nano Schottky junctions have been investigated experimentally using gold (Au) coated AFM tips in contact with different silicon (Si) substrates. For nano-tips with an apex radius around 7 nm, the current-voltage (I-V) curves on low n-dope Si substrates have showed a reversed rectification diode behavior compared to the high n-dope Si samples. We have used a new theoretical model to study the electric field enhancement at the nano metal-semiconductor interface, and thus the enhancement of the tunneling current. We have found out that the tunneling current at the reverse bias is dominant on low dope substrates and very small on high dope substrates. This accounts for the reversed I-V rectification behavior on low dope Si Schottky contacts. The calculated I-V curves showed good agreement with the experimental results for both types of Si samples.

The significant effect of the size of a nano-metal particle on the interface with a semiconductor substrate

Reduced metal-semiconductor contacts to sub 10 nm range have exhibited interesting characteristics that drastically deviate from those related to planar Schottky contacts. We present a theoretical model with analytical analyses that describe the crucial effect of the size of the metal contact, with a semiconductor substrate, on the energy band structure at the interface. We present a direct method for calculating the reduced depletion width, the enhanced built-in potential and enhanced electric field at the interface. The calculations showed that these parameters are direct functions of the radius of the nano metal particle when the substrate is moderately doped and this particles radius effect diminishes when the sample is highly doped.

Numerical and Finite Element Simulations of Nanotips for FIM/FEM

Due to their crucial applications in nanotechnology, several methods have been developed for fabricating nanotips. Such nanotips can be fabricated and characterized in the field ion microscope (FIM), and can be tapered down to a single atom apex. As only a top view of the tip apex can be captured and analyzed in the FIM the overall nanotip shape is still undefined. The FIM images or field emission microscope (FEM) images of single-atom tips (SATs) made by different methods have been found to span a wide range of applied voltages for the respective mode. Here we present theoretical and numerical methods to analyze the distribution of the electric field in the vicinity of the nanotip apex that holds the topmost single atom. We use two different geometries for the nanotip apex, spherical and ellipsoidal shapes, to analyze its electric field. We demonstrate that the electric field at the center of the nano-protrusion is still significantly dominated by the nanotip base and enhanced further at the center of the apex by the nanotip protrusion. The analyses explicitly show that nanotips with broad bases produce even less field than some regular tips, at the same applied voltage. This pronounced effect of the tip base accounts for the relatively high voltages needed for imaging some nanotips in FIM or FEM. In addition, the overall nanotip shape can be estimated based on the radius-voltage relationship. This approach helps in the selection of nanotips for particular applications in ion and electron microscopy, nano lithography, nano characterization and other aspects of nanotechnology.

Modeling of Nanotips Fabricated by Local Electron Bombardment

Nanotips have attracted increasing interest in several aspects of nanotechnology, particularly in nanotip-based microscopy and nano-characterization. Therefore, fabricating nanotips with well-defined shapes and in a highly controlled process is vital for these applications. Here, we present some characterization analysis of nanotips fabricated by the recently developed local electron bombardment method. We highlight the importance of using spherical crystal models to reproduce the nanotip atomic structure as a way to estimate the nanotip size, rather than using the conventional ring counting method. The nanotip shape is modeled by building the crystal structure of the nanotip apex to estimate its size. Then, the overall nanotip shape is estimated by sequentially destructing the entire nanotip and utilizing finite element simulation tools to build the nanotip model. The simulation model is made in a way to generate the same threshold electric field, in the field ion microscope, before and after the nanotip destruction.

Modeling of nanotips fabricated by local electron bombardment

Nanotips have attracted increasing interest in several aspects of nanotechnology, particularly in nanotip based microscopy and nano-characterization. Therefore, fabricating nanotips with well-defined shapes and in a highly controllable way is vital. Here we present some characterization analysis of nanotips prepared by the recently developed local electron bombardment method. The nanotip shape is modeled by building the crystal structure of the nanotip apex to estimate its size. Then the overall nanotip shape is estimated by sequentially destructing the entire nanotip and utilizing finite element simulation tools to build a nanotip model. The simulation model is made in a way to generate the same threshold electric field, in the field ion microscope (FIM), before and after the nanotip destruction.

Analysis of the interface barriers between nano metal particles and semiconductors substrates

Reducing the size of metal-semiconductor (M-S) contacts to sub 20 nm results in a deviation of the interface barrier characteristics from the those predicted by the conventional theory of (M-S) contacts. This is attributed to the enhancement of the electric field at the interface due to the charge confinement in the nano metal particle. We introduce analytical analysis and finite element simulations for calculating the interface parameters. The significant electric field enhancement and the reduction of the barrier thickness account for the reversed rectification behavior compared to the conventional I-V data of M-S junctions.

Observations of Individual Cu-Phthalocyanine Molecules Deposited on Nano-Tips in the Field Emission Microscope

Using a field emission microscope (FEM), the first image ever of isolated individual molecules was reported in the 1950s. At that time, the Cu-Phthalocyanine (Cu-Pc) molecule was imaged in different configurations, namely two- and four-leaf patterns. These various apparent shapes were linked to the location of the molecule on particular atomic planes of the relatively quite large FEM tip apex used at that time. We report here on how the fabrication of an extremely sharp FEM tip with an apex of the size of the molecule to be imaged provides a unique opportunity to study the behavior of one molecule at a time on the tip apex. Preliminary data are presented where two adsorption states have been observed according to the electronic cloud FEM images of the molecule. Since the atomic structure of the tip can be determined first from a field ion microscope image, the interaction of the molecule with tip apex surface atoms, and thus the molecule adsorption conformation can be readily determined.

Reproducibility and field emission characteristics of nanotips fabricated by local electron bombardment

The rapid advance in microscopy in the last few decades enabled researchers to fabricate, manipulate and characterize materials at nano-scale. The fabrication of ultra-sharp tips with an apex of few nanometers (referred to as nanotips) and single atom tips (SATs) considerably optimizes the image resolution of scanning probe microscopes and electron microscopes to reach the level of atomic resolution. Several techniques were introduced in order to optimize the fabrication process of nanotips. Here, we experimentally verify the reproducibility of nanotip fabrication using the recent local electron bombardment method. Also, we investigate the field emission characteristics of such nanotips using the field emission microscope.

Characterization and modeling of nanotips fabricated in the field ion microscope

Abstract Nanotips are considered significant elements in some of nanotechnology instruments. They are used in scanning probe microscopes and electron microscopes to characterize materials at the nano and atomic scales. Therefore, the size and profile of the nanotip determines the performance of these microscopes. The advancement of nanotip fabrication techniques has enabled the fabrication of ultra-sharp tips and even single-atom tips. However, the characterization of nanotips with an apex of a few nanometers is still premature, while the conventional characterization methods of the tip size, such as the ring counting method, have shown some limitation at this nano scale. In this paper, we review the various nanotip fabrication methods with a focus on the most recent one, which is called the local electron bombardment method. We demonstrate an approach for estimating the nanotip radius with good approximation using ball crystal models. We also model the overall nanotip profile using finite element simulation tools based on the hyperboloidal geometry. The modeling and radius estimation approach is applied on tips fabricated by the local electron bombardment method, which will be explained in detail as well.