Jul 2, 2003
New look for nanowire devices
German physicists have made a new type of transistor by growing semiconductor nanowires in a polymer stack. Jie Chen and Rolf Könenkamp at the Hahn-Meitner Institute in Berlin say that their process avoids many of the basic problems encountered in making such 'hybrid' structures, and that their transistors are much more robust than previous, similar devices (J Chen and R Könenkamp 2003 Appl. Phys. Lett. 82 4782).
Semiconductor nanowires are one-dimensional structures with novel electrical and optical properties that can be used as building blocks in nanoscale devices such as field-effect transistors, sensors and light-emitting diodes. Hybrid devices combine the flexibility of organic materials such as polymers with the useful electronic and optical properties of semiconductor materials. At present, however, these structures lack mechanical strength and this limits their use in practical applications.
Chen and Könenkamp prepared a polymer-metal-polymer stack that consisted of a metal layer sandwiched between two polymer films about 8 microns thick. They used fast-ion irradiation to drill holes in this stack, and then grew semiconductor nanowires in the holes. Each nanowire had a diameter of about 100 nanometres and as many as 100 million nanowires could be packed into a square centimetre. When source and drain contacts are added to the top and bottom of the stack it can act as a transistor, with the metal layer in the middle playing the role of the gate electrode.
The design ensures that the source, drain and gate electrodes are all firmly embedded in the polymer substrate, which means that the semiconductor device itself is not affected by external stresses. Moreover, high packing densities can be achieved without the traditional time-consuming lithographic techniques normally used to make such devices.
Although the transistor currently suffers from a relatively high leakage current, Chen and Könenkamp hope to reduce this in future experiments. The pair also plans to decrease the width of the gate electrode to below 100 nanometres by using thinner intermediate metallic layers. This should lead to faster switching and might even allow single-electron effects to be observed in the device.
About the author
Belle Dumé is science writer at PhysicsWeb.