NEWS ANALYSIS
Graphene: Opening The Bandgap
A host of lesser-known compound semiconductors are being used to exploit graphene’s electronic properties. Reports Rebecca Pool.
SOME TEN YEARS AGO, UK-based University of Manchester researchers, Kostya Novoselov and Andre Geim, published their seminal Science paper on graphene. Using scotch tape, the physicists peeled layers of graphene from a graphite block, dissolved the tape to leave ultra-thin flakes of graphene that they used to fabricate rudimentary silicon oxide-based transistors.
The rest is history. Just one atom thick, graphene forms a high quality crystal lattice with no vacancies and no dislocations, yielding remarkable properties. Studies by Geim and Novoselov – since awarded the Nobel Prize for Physics – revealed the material’s conducting electrons arrange into quasi-particles that behave more like neutrinos, moving close to the speed of light. With their first transistors displaying ballistic transport, linear current-voltage characteristics and huge sustainable currents, the researchers could only conclude: “graphene may be the best possible metal for metallic transistor applications”.
However, a clear obstacle to the use of graphene as an alternative to silicon electronics is its lack of bandgap; with electrons flowing at any energy, low power dissipation in the off state is very difficult. Given this, researchers worldwide have been exploring ways to create an artificial bandgap – applying an electric field, modifying graphene’s structure or doping with impurities - but device characteristics have fallen short of the wonder material’s intrinsic properties. On a different track, Novoselov and a handful of other researchers have been fabricating heterostructures based on single layers of graphene and novel compound semiconductors. These structures comprise layers of alternating 2D crystals - graphene and the insulating material - stacked up to form a device. As Novoselov says: “Research on heterostructures is gaining momentum, and the possibilities for controlling properties of heterostructures might become very useful for future applications.”
Novoselov and his team at the University of Manchester have largely focused on using 2D crystals of hexagonal boron nitride to fabricate graphene heterostructures. Just over two years ago they unveiled a bipolar field-effect transistor based on layers of graphene and atomically thin boron nitride with on-off ratios an order of magnitude greater than planar graphene FETs. At
28
www.compoundsemiconductor.net June 2014
the same time, other researchers have fabricated devices with mobilities almost an order of magnitude better than devices on silicon dioxide.
As Novoselov’s colleague, Colin Woods, explains, hBN has a very similar lattice to graphene, making it an attractive candidate for building graphene heterostructures. “It’s also a high bandgap semiconductor and has a high crystal purity so we’ve been getting very good results with these heterostructures,” he adds. Excitingly, recent research published in Nature Physics, shows that growing graphene on hBN can alter its crystal structure, opening a gap between its electron energy bands.
Novoselov, Woods and colleagues found that when the two lattices are oriented in a certain way, the distance between graphene’s carbon atoms is stretched so its lattice matches that of hBN, and the bandgap is opened. If this alignment is off, the carbon atoms revert to original positions and the bandgap disappears.
“It was extremely exciting to see the properties of graphene change so dramatically by simply twisting the two crystals only a fraction of a degree,” says Woods. “We’re not yet sure whether the bandgap is a result of [lattice] strain, but obviously a bandgap in graphene is very desirable.”
“It is very small but you never know where this might go in the future,” he adds. Crucially, as the researcher points out: “This discovery provides another way to control the properties of graphene heterostructures. Not all the questions are answered and this is certainly a phenomenon worth investigating.”
But the research doesn’t stop with hBN. Novoselov’s group is looking at other 2D crystals – from molybdenum disulphide to niobium diselenide – all of which hold considerable promise for graphene.
“Each material has its own set of properties and its difficult to single out a single crystal and say ‘yes its properties are going to be good’ as they change as you alter the number of layers in the heterostructure,” says Woods. “But this is a very exciting time to be looking at graphene; people honestly don’t know what’s going to happen until they do it.”
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108