Oxide electronics: A robust gate
High-quality transistors can be fabricated using an oxide interface and a polymer gate
Silicon and other covalent bonding semiconductors have been the primary materials used in electronic components for several decades, but the need to improve performance and develop novel functionalities has spurred research efforts into new materials. Particularly promising are the highly conductive ‘two-dimensional electron gases’ that form at the interfaces between insulating oxides — for example between LaAlO3/SrTiO3 and MgxZn1–xO/ZnO — in which electrons move freely in two dimensions.
Incorporating such interfaces into electronic devices such as transistors requires the ability to tune the carrier concentration by applying a voltage between a gate electrode and the interface itself. This is what Masashi Kawasaki and colleagues at the Advanced Institute for Materials Research (AIMR) have now successfully demonstrated by constructing a transistor using a polymer gate and a MgxZn1–xO/ZnO interface as the active layer1.
When a metallic gate is deposited on a semiconductor, the difference in work function — the energy required for an electron to leave the surface — between the two parts leads to the formation of a Schottky junction, which allows the carrier concentration in the semiconductor to be altered with minimal leakage current from the gate. As Kawasaki explains, however, “Schottky junctions involving ZnO and noble metals have not been adequately reproducible and the device characteristics are far from excellent. We suspect that the metals react with oxygen in the ZnO somehow, which induces defects.” The issue is even more delicate in the case of interfaces, as any small defect creates a path for the current to leak from the gate, which has a very damaging effect on the properties of the two-dimensional electron gas.
“Polymers are usually very stable against oxidation, and atoms in polymers are already saturated with chemical bonds,” says Kawasaki. The team fabricated a transistors using a high-mobility MgxZn1–xO/ZnO interface and a layer of a conducting polymer for the gate (Fig. 1). The polymer, PEDOT:PSS, is commercially available and widely used in organic displays.
The two-dimensional electron gas exhibits sharp resistance oscillations under a magnetic field due to the quantum Hall effect, which confirms the excellent quality of the device. In addition, the oscillations are clearly modulated by the application of a gate voltage, demonstrating that the carrier concentration can be tuned externally.
The results could be very significant for practical applications. “This interface could be one of the important ingredients for future transparent circuitry. The fabrication method is extremely simple and the materials are of low cost,” says Kawasaki. There is also no reason why the same route could not be followed for other conducting oxides: the team has already explored the possibility of using the same polymer as a gate for oxides such as SrTiO3 or TiO2.
Nakano, M., Tsukazaki, A., Ohtomo, A., Ueno, K., Akasaka, S., Yuji, H., Nakahara, K., Fukumura, T. & Kawasaki, M. Electric-field control of two-dimensional electrons in polymer-gated–oxide semiconductor heterostructures. Advanced Materials Published online: 24 Nov 2009 | doi: 10.1002/adma.200902162 | article
This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.