Friday, January 2, 2015

The Gifts of the Magi


Many of us know of the tale of the Three Wise Men or Magi in the New Testament, trekking from distant lands as they followed a guiding star in the heavens to a manger in Bethlehem. The image of that event is forever etched in my mind by the nativity scene that my mother always sets up the week before Christmas and leaves on display in the living room until January 7, the day after the Magi were said to have completed their journey. It is an image that even now tugs at my heart.

The Magi brought with them precious items as gifts to celebrate the arrival of the newborn - gold, frankincense and myrrh. With 2014 now having closed and the new year opening up before us with all its trials, concerns and opportunities,can we hope that 2015 may bring us similar precious gifts in High Tech? 

In the editorial before Christmas, we discussed a few of the basic properties of graphene which suggest a miraculous potential for semiconductors and, by association, the entire High Tech sector. One must ask, though: is all the work being done with graphene on microelectronic applications the same kind of 'pie in the sky' dreams and aspirations that have been pursued by nuclear fusion researchers for the last seven decades (where commercial applications have always been 'thirty years in the future'), or will graphene prove to be the material that lifts High Tech out of the winter season it has just entered?

I think it's fair to conclude that graphene is likely in better shape than nuclear fusion - there are already working microelectronics products in the marketplace. Silicon Carbide is a widely used industrial material known for its hardness. By exposing a graphene sheet to base silicon, a band gap is created that allows the formation of bipolar, n-FET and p-FET transistors with suitable doping. Though this exposure to silicon does change the properties of pure graphene by creating a 3D crystal, it still exhibits excellent thermal and electrical conductivity. These first applications are for power electronics, where Silicon Carbide MOSFETs demonstrate the ability to deliver much higher voltages and currents in far smaller surface areas than silicon devices. There is also active research in RF applications, as SiC MOSFETS can theoretically operate at Terahertz frequencies.

There are other materials that have been used as well to affect graphene and form band gaps for transistors - for instance, hexagonal boron nitride (h-BN), similar in many interesting ways to graphene, has shown promise in multiple research initiatives. But what about forming actual gates of digital logic, or the equivalent of SRAM and Flash memory cells? Can graphene ever be expected to support VLSI designs with tens and hundreds of millions of gates, along with memories and interconnect?

There are investigations underway that seek to understand if graphene can be chemically modified in a pinpoint fashion so as to fashion circuit elements in a sheet - transistors, memories, interconnect, capacitors and so forth. The idea is to exploit the high reactivity of carbon atoms in the graphene matrix in a controlled manner to, in a fashion, chemically 'etch' appropriate circuit designs. The fact that this would be done on an atomic scale, combined with the intrinsic properties of graphene in both its original and modified states, suggests the possibility of gigantic leaps in achievable 3P (power, price and performance) levels of integrated circuits.

The ultimate realization of graphene's potential would be in its affinity for quantum computing. The basis of quantum computing is the 'spin' of an electron, which can create an angular momentum that is either 'up' or 'down' and is distinct from the angular momentum generated by its orbit around an atomic nucleus. Measuring this is a bit of a problem in silicon and involves an elaborate setup involving special ferromagnetic films, but carbon is extremely amenable to spin state observation. Because of this and its distinct behavior in magnetic fields, graphene is considered an ideal candidate for the implementation of spintronics and quantum computing. 

Let's look at a very simplified case of how this might apply to graphene. Recall that a sheet of graphene is made up of interlocking hexagons of carbon atoms in 2D space. Each of these hexagons has three electrons zipping around within it that keep the 6 carbon atoms bound together and give graphene its wonderful electrical, thermal and mechanical properties. If we look at just one of these electrons, we can say that it has more than one state based on its axial spin - up, down or a superposition of the two (which generates a value that can be characterized as a wave function.) (Side Note: Please don't ask me to explain that last part any further, as the quantum physics of it is currently beyond me and even now makes me want to reach for a bottle of tylenol.) 

If we simply ignore the superposition possibilities, we would have a minimum of 8 different 'states' that could be represented by the three electrons in each hexagon. One can immediately see the possibilities - these states could be made to represent the truth table for anything from an inverter to, in a very crude form, an 8b MCU. All of this, at the scale of 6 carbon atoms.

Such 'cells' could also be used for more mundane purposes - as a storage/memory element, for instance. This could also be achieved (theoretically) by the conjunction of graphene sheets with small carbon nanotubes, which have been studied for not only the same amazing physical properties that nanotubes by and large share with graphene, but as a means of confining particles such as electrons in three dimensions to form Quantum Dots. Such structures could store at least 2 bits of information, and possibly more (again, in theory.) 

Carbon nanotubes also facilitate the implementation of Quantum Wires, which confine 'excitons' in two dimensions while allowing free movement in a third. The conductivity of such channels appears to approach superconducting levels, even at room temperature. Since nanotubes and graphene are made from the same stuff, the transfer of that capability down to the graphene sheet (perhaps thru some of the same pinpoint chemical modifications described above) would then permit local and 3D routing to connect digital circuits/functions and memory cells with extremely high performance.

There are undoubtedly solid state physics researchers and quantum mechanics experts who would view the above description as such a moronic simplification of an otherwise sublime quantum phenomenon on which so much research is being performed that they would consider me to be intellectually lower than an amoeba. I simply provide the example as a very simplistic illustration of the colossal potential of graphene to High Tech.

We are such stuff
As dreams are made on, and our little life
Is rounded with a sleep. - Shakespeare, "The Tempest"

Research in these areas has taken on strategic significance at the international level. China currently leads the world with over 2000 patents on graphene for a vast range of applications, but is being chased energetically by the USA, Europe and Korea. At a more humble level of interested pursuit, Graphene is a guiding star for myriad solid state physicists, chemists, materials science researchers and other Wise Men the world over - a star which they follow in hopes of discovering something at the end of their journey that will change the world. 

There are a myriad selection of technologies also being scrutinized for their potential to break the current stagnation of process technology in silicon. Some of these "More than Moore" research initiatives include Germanium, Gallium Nitride, Silicene (a silicon version of graphene) and other extraordinarily exotic combinations of materials such as molybdenum and sulphur. I will try to cover these and more in future posts.

Next week, however, I'm off to CES in Las Vegas! I'll be spending at least two days there and will likely spend a majority of my time at the exhibits in the Sands (where a great deal of IoT - oriented companies will be present.) Next week's report will consist of the insights and information I will have gathered at the show.

Between now and then - Happy New Year, everyone! :-)


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