Emma Tevaarwerk

Electronic transport in nanometre-scale silicon-on-insulator membranes

The widely used ‘silicon-on-insulator’ (SOI) system consists of a layer of single-crystalline silicon supported on a silicon dioxide substrate. When this silicon layer (the template layer) is very thin, the assumption that an effectively infinite number of atoms contributes to its physical properties no longer applies, and new electronic, mechanical and thermodynamic phenomena arise, distinct from those of bulk silicon. The development of unusual electronic properties with decreasing layer thickness is particularly important for silicon microelectronic devices, in which (001)-oriented SOI is often used. Here we show—using scanning tunnelling microscopy, electronic transport measurements, and theory—that electronic conduction in thin SOI(001) is determined not by bulk dopants but by the interaction of surface or interface electronic energy levels with the ‘bulk’ band structure of the thin silicon template layer. This interaction enables high-mobility carrier conduction in nanometre-scale SOI; conduction in even the thinnest membranes or layers of Si(001) is therefore possible, independent of any considerations of bulk doping, provided that the proper surface or interface states are available to enable the thermal excitation of ‘bulk’ carriers in the silicon layer.

Toward conductive traces: Dip Pen Nanolithography® of silver nanoparticle-based inks

Low cost, direct writing of conductive traces is highly desired for applications in nanoelectronics, photonics, circuit repair, flexible electronics, and nanoparticle-based gas detection. The unique ability of Dip Pen Nanolithography (DPN®) to direct write a variety of materials onto suitable surfaces with nanoscale resolution and area-specific patterning is leveraged in this work. We present a direct-write approach toward creating traces with commercially available silver nanoparticle (AgNP)-based inks using DPN. In this work we demonstrate submicron AgNP feature creation together with a discussion on the ink transport mechanism.

Quantitative analysis of electric force microscopy: The role of sample geometry

Quantitative electric force microscopy (EFM) is usually restricted to flat samples, because vertical sample topography traditionally makes quantitative interpretation of EFM data difficult. Many important samples, including self-assembled nanostructures, possess interesting nanoscale electrical properties in addition to complex topography. Here we present techniques for analysis of EFM images of such samples, using voltage modulated EFM augmented by three-dimensional simulations. We demonstrate the effectiveness of these techniques in analyzing EFM images of self-assembled SiGe nanostructures on insulator, report measured dielectric properties, and discuss the limitations sample topography places on quantitative measurement.

Electrically isolated SiGe quantum dots

A variation of electric force microscopy (EFM) is used to measure the electrical isolation of SiGe quantum dots (QDs). The SiGe QDs are grown on mesas of ultrathin silicon on insulator. Near the mesa edges, the thin silicon layer has been incorporated into the QDs, resulting in electrically isolated QDs. Away from the edges, the silicon layer is not incorporated and has a two-dimensional resistivity of less than 800 TΩ per sq, resulting in relatively short RC times for charge flow on the mesa. The EFM technique we use here is a powerful probe of samples and devices with floating-gate geometries.

Commercially Available High-Throughput Dip Pen Nanolithography ®

Dip Pen Nanolithography® (DPN®) is an inherently additive SPM-based technique which operates under ambient conditions, making it suitable to deposit a wide range of biological and inorganic materials. Massively parallel two-dimensional nanopatterning with DPN is now commercially available via NanoInks 2D nano PrintArrayTM, making DPN a high-throughput, flexible and versatile method for precision nanoscale pattern formation. By fabricating 55,000 tip-cantilevers across a 1 cm2 chip, we leverage the inherent versatility of DPN and demonstrate large area surface coverage, routinely achieving throughputs of 3x107 μm2 per hour. Further, we have engineered the device to be easy to use, wire-free, and fully integrated with the NSCRIPTORs scanner, stage, and sophisticated lithography routines. In this talk we discuss the methods of operating this commercially available device, subsequent results showing sub-100 nm feature sizes and excellent uniformity (standard deviation < 16%), and our continuing development work. Simultaneous multiplexed deposition of a variety of molecules is a fundamental goal of massively parallel 2D nanopatterning, and we will discuss our progress on this front, including ink delivery methods, tip coating, and patterning techniques to generate combinatorial libraries of nanoscale patterns. Another fundamental challenge includes planar leveling of the 2D nano PrintArray, and herein we describe our successful implementation of device viewports and integrated software leveling routines that monitor cantilever deflection to achieve planarity and uniform surface contact. Finally, we will discuss the results of 2D nanopatterning applications such as: 1) rapidly and flexibly generating nanostructures; 2) chemically directed assembly and 3) directly writing biological materials.

Dip Pen Nanolithography: a maturing technology for high-throughput flexible nanopatterning

Precision nanoscale deposition is a fundamental requirement for much of current nanoscience research. Further, depositing a wide range of materials as nanoscale features onto diverse surfaces is a challenging requirement for nanoscale processing systems. As a high resolution scanning probe-based direct-write technology, Dip Pen Nanolithography® (DPN®) satisfies and exceeds these fundamental requirements. Herein we specifically describe the massive scalability of DPN with two dimensional probe arrays (the 2D nano PrintArray). In collaboration with researchers at Northwestern University, we have demonstrated massively parallel nanoscale deposition with this 2D array of 55,000 pens on a centimeter square probe chip. (To date, this is the highest cantilever density ever reported.) This enables direct-writing flexible patterns with a variety of molecules, simultaneously generating 55,000 duplicates at the resolution of single-pen DPN. To date, there is no other way to accomplish this kind of patterning at this unprecedented resolution. These advances in high-throughput, flexible nanopatterning point to several compelling applications. The 2D nano PrintArray can cover a square centimeter with nanoscale features and pattern 107 &mgr;m2 per hour. These features can be solid state nanostructures, metals, or using established templating techniques, these advances enable screening for biological interactions at the level of a few molecules, or even single molecules; this in turn can enable engineering the cell-substrate interface at sub-cellular resolution.

Scanning Tunneling Microscopy of Ultrathin Silicon-on-Insulator

We present near-atomic-resolution scanning tunneling microscopy (STM) images of the surface of a 10 nm thick Si template layer in silicon-on-insulator (SOI), demonstrating that ultra-thin SOI, which is typically described as fully depleted of charge carriers, can indeed be imaged. We attribute the ability to image to our cleaning process, which results in a Si (001) free of oxide and defects. Electronic conduction in this type of very thin Si film is enabled by the interaction of Si (001) surface bands caused by the Si (001) 2×1 reconstruction with the bulk Si bands.