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Composite nanomaterials to double capacitor charge storage

Posted: 03 May 2007     Print Version  Bookmark and Share

Keywords:Georgia composite nanomaterials  high-k gate dielectrics  high-k dielectric gate 

Composite nanomaterials, which encapsulate inorganic dielectrics in an organic polymer matrix, promise to double the charge storage capabilities of capacitors, as well as supercharge plastic circuits with high-k dielectric gate oxides, according to researchers at the Georgia Institute of Technology.

"We think our new composite nanomaterials are nearly ready for commercialisation," said Joseph Perry, professor of Georgia Tech's Centre for Organic Photonics and Electronics. "Applications include everything from higher-capacities for storage capacitors to high-k gate dielectrics for organic FETs to plastic RF components like filters."

The ability of a material to store charge—its permittivity—is measured by what is called the dielectric constant "k"—the ratio of a material's permittivity divided by the permittivity of a vacuum. For capacitors, the higher the k, the higher the capacitance. For FETs, the higher the k, the thinner the gate-oxide can be made—a requirement for advanced semiconductor nodes.

High-k dielectrics are being pursued by semiconductor fabricators moving beyond the 45nm node, but reluctantly because they are still considered unproven for high-volume production in some cases. Traditional gate oxides like silicon dioxide have a ''k factor'' of 3.9, but dielectric constants as high as 25 have been measured for materials such as hafnium oxide, zirconium oxide and barium titanate.

The key to Georgia Tech's technique of making high-k dielectrics more reliable is reducing them to nanoparticles, rather than growing them in crystalline lattices. The nanoparticles can then be embedded into the matrix of a polymer, thus retaining their high-k but strengthening the composite material to make it more robust.

"If you have a thin film of crystalline barium titanate, its dielectric constant will be very, very high, but its electrical breakdown voltage is too low for typical semiconductors—about 20kV/cm," said Perry. "But polymers have 100MV breakdown voltages [about 5,000 times greater]."

Other research groups have embedded barium titanate in a polymer and measured the resulting high-k of the composite dielectric, but the nanoparticles tended to form micron-sized clumps that caused the thin film to crack, thus reducing its ability to resist electrical breakdown.

To solve that problem, researchers tailored an organic phosphonic acid ligand, which the Georgia Tech researchers say keeps clumps in the 30-120nm range, making the resulting composite uniform enough for commercialisation.

"At first we tried to duplicate the coatings for barium titanate nanoparticles used by other labs, but they kept cracking and flaking," said Perry. "Now we have found a molecule that solves this coating problem by reducing the size of aggregates three-to-four times. One end of the molecule anchors onto the nanoparticle, and the other end can be chemically tailored to provide application-specific functionality, such as bonding to a specific polymer matrix."

Today, polycarbonate—the polymer with which Perry's group has found the most success—is already used to separate the metal-foil plates of capacitors. By embedding the barium titanate nanoparticles into the polycarbonate's matrix, the Georgia Tech researchers hope to enable super-capacitors that store much more energy, and which can rapidly discharge high currents.

"Polycarbonate by itself has a permittivity of 3-4, but by adding barium titanate nanoparticles to the polymer, its permittivity can be raised to 20," said Perry. "For capacitors, you want a high dielectric constant, but you also want a high breakdown voltage, so that you can store a large amount of charge on them [since capacitance is proportional to the square of the voltage]. Now you can have both."

On the downside, introducing barium titanate nanoparticles slightly lowers the breakdown voltage of polycarbonate alone. But on the upside, by increasing the dielectric constant of the compound, the capacitors can store more energy per unit area—about twice as much in the current material. "The take-home message for EEs, is that by adding our coated barium titanate nanoparticles to polycarbonate, we can essentially double the amount of energy you can store on a capacitor," said Perry.

Next, Perry's group plans to fabricate plastic transistors using their high-k dielectric composite nanomaterial as gate oxide, hopefully increasing the performance of the organic transistors which have always been dismally slow compared to inorganic silicon transistors.

"Now we are looking to use these polycarbonate composite nanomaterials in the fabrication of organic semiconductor circuitry. In particular, we want to use our nanocomposite as the gate dielectric for organic FETs," said Perry.

Perry's group is also fabricating larger samples, which today are only produced on 2-by-3-inch wafers, so that the films can be shared with other researchers working on organic transistors and RF applications. "We see a potential big win for those types of devices, because the gate dielectrics that people are using today [for organic transistors] are very low-k compared to our nanocomposites," said Perry.

Perry's group also plans to experiment with adding more nanoparticles per unit volume and with using different polymer matrices, to see if the permittivity of the resulting composite nanomaterials can be raised yet higher. "We are now experimenting with using more nanoparticles in our composites, and with using other types of polymers, to further increase the k-values of the matrix," said Perry. "Plus we are going to test our current materials at higher frequencies—we want to know how well they perform at radio- and micro-wave frequencies, which would open a whole new range of applications."

Today, organic semiconductors typically operate at low frequencies, compared to inorganic silicon circuitry. Currently Perry's group has only tested its nanocomposites up to 1MHz, but the group's goal will be to achieve GHz-like performance.

Perry's research was funded by the Office of Naval Research and the National Science Foundation. Georgia Tech has a patent pending on the nanoparticle encapsulation process.

- R. Colin Johnson
EE Times

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