Researchers from the Gwangju Institute of Science and Technology, the Korea Photonics Technology Center and the University of Cambridge have demonstrated how the surface morphology and diameter of nanowires can be used to tune the electronic properties of the wires.1

Semiconductor nanowires are promising for the realization of future electronic devices, such as light emitting diodes and transistors. Here, the research led by Takhee Lee and graduate student Woong-Ki Hong was focussed on ZnO nanowires.

Fig. 1: Transistor fabricated with a single ZnO nanowire.

Nanowires were grown on different types of substrates and depending on the substrate used, the resulting nanowires exhibited corrugated surfaces with a relatively small diameter, or smooth surfaces with larger diameter (Fig. 1). Electrical measurements of the transport properties of the wires showed that smooth and large wires were conducting even at zero gate voltage—this is called normally-on or depletion-mode transistor—while the rough and small diameter wires only conducted when a positive gate bias was applied—normally-off or enhancement-mode.

Their results were attributed to the effect of surface states and defects in the nanowires. It is known that electrons tend to be trapped at the surfaces of nanowires which results in electron depletion of the inner part of the wire structures. The researchers believe that in the smooth and large wires this effect—although present—was relatively small and the wires still contain free electrons derived from donor-type impurities. However, surface traps are much more numerous in rough wires, corresponding to a much larger depletion region. In addition, the smaller diameter implies a relatively stronger effect of the surface.

This proposition was confirmed by the optical emission experiments, where the small and rough nanowires emitted a relatively strong broad emission in the visible range, originating from charge trapped in surfaces states.

“The importance of this work is that the surface morphology and diameter of ZnO nanowires strongly influences their electronic properties, particularly the operational modes of nanowire transistors,” says Lee. “We are currently working on building actual electronic circuits with these two different modes of ZnO nanowires.”