Silicon and Graphene: Two Great Materials That Stay Great Together

The use of graphene as a transparent conducting film has been hotly pursued of late, in large part because it offers a potentially cheaper alternative to indium tin oxide (ITO) where a bottleneck of supply seems to be looming.

It has not been clear whether photovoltaic manufacturers have taken any interest in graphene as an alternative for transparent conducting films. This lack of interest may in part be the result of there being little research into whether graphene maintains its attractive characteristic of high carrier mobility when used in conjunction with silicon.

Now researchers at the Helmholtz Zentrum Berlin (HZB) Institute in Germany have shown that graphene does not lose its impressive conductivity characteristics even when mated with silicon.

“We examined how graphene’s conductive properties change if it is incorporated into a stack of layers similar to a silicon based thin film solar cell and were surprised to find that these properties actually change very little,” said Marc Gluba of the HZB Institute for Silicon Photovoltaics in a press release.

The research, which was published in the journal Applied Physics Letters (“Embedded graphene for large-area silicon-based devices”), used the method of growing the graphene by chemical vapor deposition on a copper sheet and then transferring it to a glass substrate. This was then covered with a thin film of silicon.

The researchers experimented with two different forms of silicon commonly used in thin-film technologies: amorphous silicon and polycrystalline silicon. In both cases, despite completely different morphology of the silicon, the graphene was still detectable.

“That’s something we didn’t expect to find, but our results demonstrate that graphene remains graphene even if it is coated with silicon,” said Norbert Nickel, another researcher on the project, in a press release.

In their measurements, the researchers determined that the carrier mobility of the graphene layer was roughly 30 times greater than that of conventional zinc oxide-based contact layers.

Although the researchers concede that connecting the graphene-based contact layer to external contacts is difficult, it has garnered the interest of their thin-film technology colleagues. “Our thin film technology colleagues are already pricking up their ears and wanting to incorporate it,” Nickel adds.


The Market for Nanomaterial Solutions for ITO Replacement Gets Crowded

With the introduction of Apple’s iPhone and then all the other smart phones, and then the introduction of Apple’s iPad followed by all the other tablets, touch screen displays have experienced enormous growth over the last six years. However, from the beginning of that growth, concern was developing about what could be done about the relatively scarce resource of indium-tin oxide (ITO) that these devices need to operate.

ITO is used as a transparent conductor to control display pixels. What was a clear challenge and concern for display manufacturers actually served as a new ray of hope for nanomaterial producers. Companies like Cambrios Technologies, which had been launched back in 2002 with the aim of getting man-made viruses to pattern inorganic materials for a host of electronic applications, finally saw an application that was driven by “market pull” rather than “technology push”.

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New Form of Carbon from Boston College and Nagoya University

Nearly three decades ago, our understanding that there were three basic forms of carbon—diamond, graphite, and amorphous carbon—was stood on its head with the introduction of fullerenes, then carbon nanotubes, and more recently graphene.

Now researchers at Boston College and Nagoya University have synthesized another new form of carbon unofficially dubbed “grossly warped nanographenes.” The research, which was published in the journal Nature Chemistry (“A grossly warped nanographene and the consequences of multiple odd-membered-ring defects”), has led to creating a material that is essentially defects in the two-dimensional hexagonal honeycomb-like arrangements of trigonal carbon atoms found in graphene. These defects consist of non-hexagonal rings that force distortions out of the two-dimensional plane.

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Method to grow epitaxial nitride semiconductors on graphene

The US Naval Research Laboratory (NRL) in Washington DC has developed a method to grow epitaxial nitride semiconductors on graphene [Neeraj Nepal et al, Appl. Phys. Express, vol6, p061003, 2013]. The researchers hope that this could lead to high-speed current-switching applications using devices such as hot-electron transistors (HETs).

Present HETs use heavily doped semiconductor or metal base regions. Heavy doping hampers ballistic transport due to impurity and carrier-carrier scattering effects. Metal base regions suffer from electron reflection effects at the base-collector interface. The NRL team believes that using graphene as the base region, in conjunction with nitride semiconductors, could lead to devices with cut-off frequencies greater than 1THz (1000GHz).

Up to now, growth of nitride semiconductors on graphene has resulted in non-uniform GaN crystallites and not a continuous film.

The NRL method includes a functionalization step that produces “for the first time” nitride semiconductor layers of a quality similar to that obtained by traditional growth methods on conventional sapphire substrates. In fact, the crystal quality is achieved with thinner layers of less than 1μm compared with layers on other substrates.

The researchers comment: “These results support a successful demonstration of electronic-quality, heteroepitxial wurtzitic GaN on graphene that is currently unavailable and can improve the performance of present state-of-the-art devices such as HETs.”

The initial epitaxial graphene (EG) layer was prepared on 4° 4H-polytype silicon carbide (1.6mm x 1.6mm squares) using silicon sublimation. Silicon nitride was then applied using plasma-enhanced chemical vapor deposition (PECVD). The silicon nitride was patterned into discs of various diameters between 50μm and 500μm.

Figure 1

Figure 1: (a) Schematic of GaN/AlN/graphene/SiC layered structure, (b) atomic force microscope (AFM) image as-synthesized epitaxial graphene, (c) AFM image 1.2nm ALE AlN/graphene, (d) AFM image GaN/graphene, (e) scanning electron microscope (SEM) image pristine graphene, (f ) SEM image 1.2nm ALE AlN/graphene, (g) SEM image GaN on AlE AlN/graphene. (h) Al atoms replace F atoms, creating an AlN nucleation site on graphene resulting in proposed crystalline alignments.

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Nanowires Grow Better on Graphene – from IEEE Spectrum

By Dexter Johnson
In an attempt to grow nanowires on a graphene substrate, researchers at the University of Illinois may have stumbled upon a new paradigm for epitaxy (the growth of crystals on a susbstrate).

Some believe that developing new manufacturing methods for nanoscale devices—like epitaxy—may be more crucial to meeting the demands of next generation chips than creating new materials, especially when feature sizes start falling below three nanometers. So, the Illinois researchers’ development of a new method of epitaxy may ultimately be more significant than creating a new material.

The research, which was published in the journal Nano Letters (“InxGa1–xAs Nanowire Growth on Graphene: van der Waals Epitaxy Induced Phase Segregation”), produced nanowires made from III-V compound semiconductors. Generally, III-V semiconductors like gallium arsenide don’t integrate well with silicon, but  recently  it was discovered that when these materials were brought down to the nanoscale that they were compatible.

Researchers have previously combined two of these semiconductors in gaseous form so that they deposit themselves on a graphene substrate (a process known as metalorganic chemical vapor deposition, or MOCVD) and self assemble into ordered crystalline form. However, the Illinois research marks the first time three of the semiconductors have been mixed together in this way.

The researchers discovered that something remarkable occurred when this third semiconductor was added to the mix. The materials began spontaneously to segregate into an indium arsenide (InAs) core with an InGaAs shell around the outside of the nanowire.

“This is unexpected,” says professor Xiuling Li, who led the research, in a press release. “A lot of devices require a core-shell architecture. Normally you grow the core in one growth condition and change conditions to grow the shell on the outside. This is spontaneous, done in one step. The other good thing is that since it’s a spontaneous segregation, it produces a perfect interface.”

This precise delineation between the core and the outside of the nanowire has to do with relationship between the atomic structure of the semiconductors and that of the graphene. The crystal structure of InAs has the same distance between its atoms as the carbon atoms in a sheet of graphene. As a result, the InAs fits in that space perfectly, leaving the gallium compound to form on the outside of that core.

The next step for the researchers will be to see if they can exploit their new manufacturing technique to create solar cells and other optoelectronic devices.

Image: Joshua D. Wood/University of Illinois at Urbana-Champaign