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Posted: Mar 28, 2014
Need your dielectric to stick to graphene? Just add water
(Nanowerk News) Ever since physicists Andre Geim and Konstantin Novoselov first used adhesive tape in 2004 to pull a single layer of carbon atoms off of a block of graphite, scientists, engineers and manufacturers have sought ways to utilize the material we now call graphene's unique properties. One such application highly desired by the semiconductor industry is graphene's potential role as a replacement for silicon as a substrate in microchips.
The International Technology Roadmap for Semiconductors, a collaborative effort involving the five leading chip manufacturing regions in the world - Europe, Japan, Korea, Taiwan and the United States - considers graphene to be among the top candidate materials for post-silicon electronics. In order to realize graphene-based devices, high-quality and high-k (high-electrostatic-shielding ability) dielectrics must be successfully layered atop the monolayer carbon molecule. One way to do this is atomic layer deposition (ALD); however, the chemical inertness of graphene makes it difficult to grow a uniform thin layer of a dielectric on it. Methods to overcome the problem, including placing a transition layer between the dielectric and the graphene, introduced undesired impurities or destroyed the substrate. What was needed was a different means of growing a high-k dielectric directly on the graphene surface.
To accomplish the task, a team led by Xinhong Cheng at the Shanghai Institute of Microsystem and Information Technology (part of the Chinese Academy of Sciences), turned to a compound with a very unusual name: Tetrakis(ethylmethylamino)hafnium, or TEMAH. At 80 degrees Celsius (176 degrees Fahrenheit), TEMAH is a gas from which hafnium oxide (HfO2), a proven high-k dielectric can be derived. Unfortunately, getting the HfO2 to stick to graphene wasn't easy.
"Using a traditional ALD techniques, we blew TEMAH into the deposition chamber with the hope that the HfO2 produced would be absorbed by the graphene substrate; but it would not stick," says Li Zheng, lead author on the JVSTA paper. "So, we pre-treated the substrate with water because we knew it would be absorbed onto the graphene and likely act as a nucleation [growth initiation] site. And that's what we found. TEMAH is attracted to the absorbed water, allowing a HfO2 layer to grow directly - and tightly - on the graphene surface."
Unlike past attempts to grow a dielectric directly on graphene, the water-based ALD technique deposited the desired uniform thin film of dielectric material (the HfO2) while leaving the substrate free of defects. Microscopic examinations revealed that the dielectrics grown on the graphene possessed both high quality and high permittivity (the ability to reduce any electric field present and therefore act as an efficient insulator).
Now that water-based ALD has been shown to work, Zheng says that the next step will be to test the electric properties of graphene affected by the high-k dielectrics. Once that is accomplished, he says, the researchers will then fabricate some graphene-based electronic devices and put them through their paces.