Japan develops new graphene transistors for use in channel layers
November 07 01:08:52, 2023
Northeastern University of Japan announced that its researchers have developed a channel layer using a new carbon material, a graphene transistor, with a turn-on/turn-off ratio of over 104. Graphene generally has no band gap, so even if it is made into a transistor, it cannot be turned off, and the on/off ratio is very small. Northeastern University has solved this problem through this technology development, and it has also achieved high integration of transistors beyond current silicon semiconductor technology. A paper detailing the technology has been published in the academic journal NatureNanotechnology, which was released on September 9, 2012. The technology was developed by the Honorary Professor of the Graduate School of Engineering, Tohoku University, Japan, and the assistant professor Kato Junzen. The reason why a high on/off ratio can be obtained with graphene having no band gap is because the graphene of the channel layer is successfully synthesized into a thin strip shape called "nanoribbon". The formation of a thin strip of graphene produces a band gap, so it is possible to use it as a transistor for a logic circuit. The prototype graphene nanoribbon transistor has a channel length of about 500 nm, an on/off ratio of 104 or more, and a carrier mobility of about 40 cm 2 /Vs. The on/off ratio is dramatically improved compared to the original graphene transistor. "In the future, it is expected to be refined to a channel width of several nm and a channel length of several tens of nm" (Kato). The transistor manufacturing process for forming graphene before Ni is melted to achieve the above performance index is a "rapid heating diffusion plasma chemical vapor deposition (CVD) method". The specific steps are as follows: First, a pattern of source/drain electrodes and a channel layer is formed on a SOI (Si-on-Insulator) substrate with a nickel (Ni) film. The channel layer portion is formed into a nanobelt shape called "Ni nanorod". Then, the temperature was raised to 900 to 1000 ° C in 1 minute while graphene was formed on the Ni nanorods based on methane (CH 4 ) gas using a plasma CVD method. According to reports, by controlling the shape of the Ni nano-bars, graphene nanoribbons of any shape can be formed. According to Kato of Northeastern University, the key point of this technology is the two processes of “rapid heating†and “diffusion plasma†in the manufacturing process. Ni has long been known as a catalyst for forming graphene. Moreover, researchers have previously tried to make Ni into a nano-belt and synthesize graphene on it. However, this attempt did not yield satisfactory results. The reason is that a high temperature process of approximately 1000 ° C causes the Ni nanorods to melt or sublimate and destroy them. Therefore, this time, by rapidly heating and cooling, and using plasma to make CH4 have high catalytic activity, the synthesis rate of graphene is increased, and thus graphene is synthesized before the Ni nanorods are destroyed. This brings opportunities for breakthroughs. Graphene quantum dots or contribute to the formation of band gaps In the graphene nanoribbon theory, the shape of the edge of the graphene is left and right with the presence and size of the band gap. Previously, there was also a view that it was necessary to make an edge shape in units of one atom, otherwise it would be difficult to use graphene nanoribbons for transistors. However, according to Kato, in the technology developed this time, "Because it is very difficult to make an edge shape with one atom or one atom, a method of randomly mixing a plurality of edge shapes is used." In this technique, the band gap is primarily determined by the width of the nanoribbon. Kato said that the reason why the shape of the band is not controlled by the shape of the edge may be that the band gap obtained is strictly a "transport band gap" rather than the theoretical band gap of the semiconductor. This is the band gap that occurs when a circular "graphene quantum dot" is combined with a nanoribbon on a synthetic graphene nanoribbon. It can be said that the result of not obtaining the edge shape and also obtaining the band gap is a great advancement in the practical use of the graphene nanoribbon transistor.
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