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 commercialization," said Joseph Perry, professor of Georgia Tech's Center for Organic Photonics and Electronics. "Applications include everything from higher-capacities for storage capacitors to high-k gate dielectrics for organic FETs to plastic radio-frequency 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 field-effect transistors (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 45-nanometer 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, thereby 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 20-kVolts per centimeter," said Perry. "But polymers have 100-MVolt breakdown voltages [5000-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, thereby 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-to-120 nanometer range, thereby making the resulting composite uniform enough for commercialization.
"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-to-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.
