Search

Graphene epitaxy on the Si face of a SiC wafer offers monolayer graphene with unique crystal orientation at the wafer-scale. However, due to carrier scattering near vicinal steps and excess bilayer stripes, the size of electrically uniform domains is limited to the width of the terraces extending up to a few microns. Nevertheless, the origin of carrier scattering at the SiC vicinal steps has not been clarified so far. A layer-resolved graphene transfer (LRGT) technique enables exfoliation of the epitaxial graphene formed on SiC wafers and transfer to flat Si wafers, which prepares crystallographically single-crystalline monolayer graphene. Because the LRGT flattens the deformed graphene at the terrace edges and permits an access to the graphene formed at the side wall of vicinal steps, components that affect the mobility of graphene formed near the vicinal steps of SiC could be individually investigated. Here, we reveal that the graphene formed at the side walls of step edges is pristine, and scattering near the steps is mainly attributed by the deformation of graphene at step edges of vicinalized SiC while partially from stripes of bilayer graphene. This study suggests that the two-step LRGT can prepare electrically single-domain graphene at the wafer-scale by removing the major possible sources of electrical degradation.

It is now widely recognized that continued performance gains in electronic computing will require new materials, both in the short and long term. In the short term, the silicon channel in transistors will be replaced by materials with higher mobility that are easier to “scale” (make thinner). In data storage, the goal is to have fast, non-volatile memory with a smaller cell size. In the long term, new architectures and new types of logic devices will be needed in order to further reduce power consumption. New materials cannot only boost performance, but can also add new functionalities, such as on-chip photonics, which can vastly improve interchip interconnects. The need for new materials is a big opportunity for materials research, but also a challenge. Replacement technologies must outperform conventional silicon technology, but also be compatible with the vast infrastructure of silicon manufacturing. Examples of some of the materials advances in the areas of computation, memory, and communication are given in this issue of MRS Bulletin.

We describe a new design for an aberration-corrected low energy electron microscope (LEEM) and photo electron emission microscope (PEEM), equipped with an in-line electron energy filter. The chromatic and spherical aberrations of the objective lens are corrected with an electrostatic electron mirror that provides independent control over the chromatic and spherical aberration coefficients C(c) and C(3), as well as the mirror focal length, to match and correct the aberrations of the objective lens. For LEEM (PEEM) the theoretical resolution is calculated to be approximately 1.5 nm (approximately 4 nm). Unlike previous designs, this instrument makes use of two magnetic prism arrays to guide the electron beam from the sample to the electron mirror, removing chromatic dispersion in front of the mirror by symmetry. The aberration correction optics was retrofitted to an uncorrected instrument with a base resolution of 4.1 nm in LEEM. Initial results in LEEM show an improvement in resolution to approximately 2 nm.

▪ Abstract The use of low-energy electron microscopy (LEEM) to study reversible surface phase transitions is reviewed. Representative experiments are described that highlight the key advantages of LEEM: the ability to image surfaces in situ, at elevated temperature, with good spatial and temporal resolution. With these capabilities, the evolution of individual surface features—domains, facets, islands, steps, etc.—can be measured. Real-time and real-space imaging make LEEM a powerful tool for characterizing the thermodynamics and kinetics that govern surface phase transformations.

Monolayer Graphene via Two Transfers Oriented monolayers of graphene containing some bilayer regions can be formed on silicon carbide crystal surfaces, but, to be cost effective, the graphene needs to be exfoliated and transferred to other substrates so that the silicon carbide crystal can be reused. Kim et al. (p. 833 , published online 31 October) used a nickel film grown to a thickness designed to impart a particular surface stress as a “handle” to exfoliate the graphene layer for transfer to a silica substrate. An additional gold layer was then used to remove the excess monolayer from the bilayer regions to create a monolayer suitable for electronics applications.

Organic light-emitting diodes are emerging as leading technologies for both high quality display and lighting. However, the transparent conductive electrode used in the current organic light-emitting diode technologies increases the overall cost and has limited bendability for future flexible applications. Here we use single-layer graphene as an alternative flexible transparent conductor, yielding white organic light-emitting diodes with brightness and efficiency sufficient for general lighting. The performance improvement is attributed to the device structure, which allows direct hole injection from the single-layer graphene anode into the light-emitting layers, reducing carrier trapping induced efficiency roll-off. By employing a light out-coupling structure, phosphorescent green organic light-emitting diodes exhibit external quantum efficiency60%, while phosphorescent white organic light-emitting diodes exhibit external quantum efficiency45% at 10,000 cd m(-2) with colour rendering index of 85. The power efficiency of white organic light-emitting diodes reaches 80 lm W(-1) at 3,000 cd m(-2), comparable to the most efficient lighting technologies.

In Part I we described a new design for an aberration-corrected Low Energy Electron Microscope (LEEM) and Photo Electron Emission Microscope (PEEM) equipped with an in-line electron energy filter. The chromatic and spherical aberrations of the objective lens are corrected with an electrostatic electron mirror that provides independent control of the chromatic and spherical aberration coefficients Cc and C3, as well as the mirror focal length. In this Part II we discuss details of microscope operation, how the microscope is set up in a systematic fashion, and we present typical results.

Depth profiling of graphene with high-resolution ion beam analysis is a practical method for analysis of monolayer thicknesses of graphene. Not only is the energy resolution sufficient to resolve graphene from underlying SiC, but by use of isotope labeling it is possible to tag graphene generated from reacted ethylene. Furthermore, we are able to analyze graphene supported by oxidized Si(100) substrates, allowing the study of graphene films grown by chemical vapor deposition on metal and transfered to silicon. This introduces a powerful method to explore the fundamentals of graphene formation.

The authors report the results of a high-resolution Electron-Energy Loss Spectroscopy (EELS) investigation of Be(0001) surface. Two dispersive surface vibrational modes are observed in the {bar {Gamma}} to {bar M} direction: a sagittal-plane mode (the Rayleigh wave) and a shear horizontal mode. They have compared their experimental results to a 300-layer slab calculation and to a semi-infinite Green`s function calculation of Sameth and Mele. Compared with the experimental results, both calculations predict a higher energy for the Rayleigh wave and lower energy for the shear horizontal mode. These results are consistent with stronger in-plane bonding and weaker interplanar bonding at the surface, in accord with other theoretical predictions.

When epitaxial graphene layers are formed on SiC(0001), the first carbon layer (known as the ``buffer layer''), while relatively easy to synthesize, does not have the desirable electrical properties of graphene. The conductivity is poor due to a disruption of the graphene $\ensuremath{\pi}$ bands by covalent bonding to the SiC substrate. Here we show that it is possible to restore the graphene $\ensuremath{\pi}$ bands by inserting a thin oxide layer between the buffer layer and SiC substrate using a low temperature, complementary metal-oxide semiconductor-compatible process that does not damage the graphene layer.

The effects of channel length scaling on carbon nanotube field-effect transistor (CNTFET) performance was investigated by varying device lengths on the same nanotube. Results show that scaling improves performance with substantial increases in on-current, resistances closer to the quantum limit than have ever been reported, and the shortest (∼30 nm) well-behaving CNTFETs to date.

Carbon nanotubes (CNTs) have been widely studied as a channel material of scaled transistors for high-speed and low-power logic applications. In order to have sufficient drive current, it is widely assumed that CNT-based logic devices will have multiple CNTs in each channel. Understanding the effects of the number of CNTs on device performance can aid in the design of CNT field-effect transistors (CNTFETs). We have fabricated multi-CNT-channel CNTFETs with an 80-nm channel length using precise self-assembly methods. We describe compact statistical models and Monte Carlo simulations to analyze failure probability and the variability of the on-state current and threshold voltage. The results show that multichannel CNTFETs are more resilient to process variation and random environmental fluctuations than single-CNT devices.

Semiconductor nanowires formed using the vapor-liquid-solid mechanism are routinely grown in many laboratories, but a comprehensive understanding of the key factors affecting wire growth is still lacking. In this paper we show that, under conditions of low disilane pressure and higher temperature, long, untapered Si wires cannot be grown, using Au catalyst, without the presence of oxygen. Exposure to oxygen, even at low levels, reduces the diffusion of Au away from the catalyst droplets. This allows the droplet volumes to remain constant for longer times and therefore permits the growth of untapered wires. This effect is observed for both gas-phase and surface-bound oxygen, so the source of oxygen is unimportant. The control of oxygen exposure during growth provides a new tool for the fabrication of long, uniform-diameter structures, as required for many applications of nanowires.

We describe a process for the growth of a single, electronically decoupled graphene layer on SiC(0001). The method involves annealing in disilane to (1) prepare flat, clean substrates, (2) grow a single graphene layer, and (3) electronically decouple the graphene from the substrate. This approach uses a single process gas, at μTorr pressures, with modest substrate temperatures, thus affecting a drastic simplification over other processes described in the literature.

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

We describe a simple method to detect the formation of graphene during Si sublimation from SiC surfaces at elevated temperature. The method exploits differences in the thermionic emission current density between graphene and SiC. When graphene forms, the thermionic current from the sample increases by a factor of about 20. The increase in thermionic emission can be detected in situ using a biased plate located near the sample. The ability to detect when graphene forms during processing is useful in optimizing graphene synthesis processes.

Addition of an electron energy filter to low energy electron microscopy (LEEM) and photoelectron emission microscopy (PEEM) instruments greatly improves their analytical capabilities. However, such filters tend to be quite complex, both electron optically and mechanically. Here we describe a simple energy filter for the existing IBM LEEM/PEEM instrument, which is realized by adding a single scanning aperture slit to the objective transfer optics, without any further modifications to the microscope. This energy filter displays a very high energy resolution ΔE/E = 2 × 10(-5), and a non-isochromaticity of ∼0.5 eV/10 µm. The setup is capable of recording selected area electron energy spectra and angular distributions at 0.15 eV energy resolution, as well as energy filtered images with a 1.5 eV energy pass band at an estimated spatial resolution of ∼10 nm. We demonstrate the use of this energy filter in imaging and spectroscopy of surfaces using a laboratory-based He I (21.2 eV) light source, as well as imaging of Ag nanowires on Si(001) using the 4 eV energy loss Ag plasmon.

We have used real-time low-energy electron microscopy to observe the growth and shape evolutions of self-assembled stress domains on Si(111) surfaces. We find that elastic strain leads to dramatic transformations in the shapes of large domains that are not predicted by existing theoretical models. By comparing the experimental observations on the formation of the stress domains with dynamic growth simulations, we have developed a quantitative understanding of how a self-assembling system falls out of equilibrium. Our work shows the nonequilibrium shapes that a domain adopts during growth depending very strongly on the azimuthal dependence of its boundary energy.