Justin Paulsen
Jul 23, 2012

Graphene Inching Forward

Graphene alone has a wide variety of useful properties, but semiconductivity is not one among them.Graphene, the much-touted miracle material over recent years, seems to demonstrate a promising consistency with regards to progressing towards applicability. The most recent development comes from a study in Nature Communications, which is a follow up on previous progress to leverage the heating of silicon carbide (SiC) crystals to ultimately produce quasi-free-standing bilayer graphene (QFBLG). But let’s backtrack a moment.

The roadmap to graphene-based semiconductors is littered with consistent steps forward, etching new challenges as old ones find themselves solved. The hurdles still standing strong primarily revolve around a fairly critical concept if we are talking semiconductors: semiconductivity. Graphene has an intrinsically excellent conductivity, it just doesn’t lend itself well to the one way street of semiconducting. The lowest energy state for graphene rests inconveniently on par with the highest energy of a bound electron. That means no bandgap, and no bandgap means no semiconductor switch.

Creating this switch has thus been the fantasy of many a physicist, and strategies focused on layering the one-atom thick materials have shown substantial promise. Still, this layering leaves much to be desired in achieving the speed and efficiency potential promised upon graphene’s discovery. So the cutting edge with graphene is how to minimize the drawbacks of layering to create the most conductive layered graphene while retaining the ability to control the flow. Now let’s get back to cooking up SiC in hydrogen.Creating semiconducting graphene structures is hard enough, but doing it without disrupting the standard conducting graphene is more tricky.

This newer development infuses hydrogen atoms at about 850 °C, resulting in the covalent bonds between the SiC and graphene breaking up. During the separation, the dangling bonds snap up the hydrogen atoms and create a buffer layer. This new layer results in a positively charged epitaxial graphene bilayer with the capacity to act as a gate graphene layer to the SiC.
Now past projects have demonstrated a similar capacity to create gate graphene. One of the trickier aspects is to accomplish this without converting all of the graphene to gate graphene, as much of it must remain contact graphene (see figure from Nature Communications to the right). This is where the specific temperature becomes relevant, as heating the materials at the lower temperature of 850 °C allows for only specific contact points to convert via hydrogen infusion. Finding balance between these two different graphene layers side-by-side is the objective, with plenty of potential experimenting ahead.

Additionally, achieving high-speed switching as a result of the design is one of the critical areas in need of development before graphene becomes a viable material in semiconducting. This specific SiC and graphene design demonstrated off/on ratios above 104 and virtually no damping at up to 1 MHz. Scaling down the size and optimizing drain resistances will continue to be researched and improved upon, and graphene will likely keep inching towards application.  When improvements will reach the level of implementation is a tricky question, but past progress gives reason enough for a certain degree of optimism.

Bottom Line

Improving these switching speeds, along with controlling various contact points for a variety of applications, will still require a sizable amount of trial, error, and frustration. As usual, this snapshot in graphene development shows a step forward along with plenty more footholds ahead. Still, as scaling approaches increasingly more costly limits in the current manufacturing paradigm of silicon, seeing these advancements is reassuring.