False color AFM image of a semiconductor nanowire transistor with a polymer electrolyte gate. Courtesy of Damon Carrad/Adam Micolich.
False color AFM image of a semiconductor nanowire transistor with a polymer electrolyte gate. Courtesy of Damon Carrad/Adam Micolich.

Researchers at the University of New South Wales, Australian National University and Lund University have developed a method for nanoscale patterning of polymer electrolyte films using an electron beam. Polymer electrolytes consist of a salt dissolved in a solid polymer, for example, LiClO4 in poly(ethylene oxide), and are commonly used to enhance the efficiency of high-capacity Li-ion batteries and organic transistors. When used in a transistor, the voltage applied to the polymer electrolyte gate drives ion motion such that the charge on the gate is effectively transferred to within ~1 nm of the semiconductor channel. The result is an extremely high dielectric constant, as high as 103-104 compared to ~25 for HfO2. This gives significantly reduced operating voltages; an essential requirement for energy efficient transistors.

Fabrication is a double-edged sword for polymer electrolytes – while they are easily patterned by inkjet printing or photolithography, these methods are difficult to implement at the nanoscale. Nanoscale patterning of polymer electrolytes is an important step towards coupling them with key nanotechnological materials such as semiconductor nanowires or carbon nanotubes for making new nanoscale device architectures. In work published in the journal Nano Letters [Carrad et al. Nano Letters 14, 94 (2014)], the team report the ability to define lines as narrow as 650 nm in a thin film of 10:1 poly(ethylene oxide): LiClO4 using a standard electron-beam lithography system. Their method relies solely on the crosslinking of polyethylene oxide (PEO) by electron-beam exposure. It was demonstrated by making the first nanowire transistor featuring a nanoscale patterned polymer electrolyte gate (see image), and can be extended to making chips containing a number of separate nanowire transistors, each with multiple independently controllable electrolyte gates.

The combination of traditionally ‘soft’ materials, e.g., polymer electrolytes, with traditionally ‘hard’ materials, e.g., III-V semiconductors, in nanoscale devices is relatively new. Such hybrid devices bring some enticing advantages. The key benefit is that the device can still operate even if the metal gate electrode is several microns away from the transistor’s conducting channel. This in turn lessens the need for careful alignment of the metal features to tiny channel structures and provides an avenue to reduce gate leakage. Patterning of the PE, such as through the method developed by the team, further enhances these devices by minimizing or eliminating the overlap between the polymer electrolyte and other metal features. This mitigates problems common in devices with unpatterned polymer electrolytes, e.g., parasitic capacitance and contact corrosion.

This new ability to pattern polymer electrolytes at the nanoscale using basic electron-beam lithography opens the way to their more widespread use in nanoscale electronics, and broadens the horizons for new generations of hybrid devices that draw together the best features of both soft and hard electronic materials.

This story is reprinted from material from the University of New South Wales, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier.