In a paper to be presented at the forthcoming Materials Research Society Fall Meeting, Fujitsu Laboratories claims a major breakthrough in lowering the graphene fabrication-temperature to 650 °C, thus allowing for graphene transistors to be formed directly on a variety of insulator substrates, including substrates that are sensitive to the higher temperatures. Graphene has extremely high electron mobility compared to silicon and could be used to produce fast, low-voltage transistors.
Fujitsu Laboratories developed novel technology that allows for graphene to be formed on insulating film substrate via CVD at the low fabrication-temperature of 650°C, enabling graphene-based transistors to be directly formed on the entire surface of a 75mm silicon wafer. However, Fujitsu claims the process is scalable to 300mm wafers.
An iron film catalyst is deposited on a silicon substrate with oxide film, and on top of that, a layer of graphene is formed to a controlled thickness via CVD with acetylene as the feedstock. As illustrated below, this technique can be used to form graphene in varied thicknesses anywhere from 1nm - or few-layer graphene - to 50nm, for multi-layer graphene.
Source and drain electrodes of titanium-gold film are formed at both ends of the graphene, thereby "fixing" the graphene. Next, just the iron catalyst is removed using acid, leaving the graphene suspended between the source and drain electrodes, with the graphene ‘bridged’ between the electrodes.
Using atomic-layer deposition (ALD), a layer of hafnium dioxide (HfO2) is grown on top of the graphene to stabilize the graphene. Finally, a gate electrode is formed on top of the graphene and through the HfO2, resulting in the formation of a graphene transistor.
In order for the performance of graphene transistors to be improved, it is essential that graphene can be formed so that it is suspended in air, so that both sides of the graphene can be cleaned. The ALD method employed by Fujitsu Laboratories allows for the formation of insulator film so that it covers the graphene suspended in air.
This new process enables the formation of graphene transistors across the entire surface of a large substrate. An example of the relationship (dependency) between drain current and gate voltage for a top-gate graphene transistor is indicated, clearly showing ambipolar characteristics that are particular to graphene.
As graphene at the thickness of a few nanometers is transparent and passes light well, it is a candidate for use as the channel and electrode material in thin-film transistors used in video displays. Fujitsu noted that the graphene-forming temperature was not far from the temperature of approximately 550°C at which graphene could be formed directly on conventional glass. Thus, it is feasible that in the near future it may be possible to form graphene transistors directly on large glass substrates.