High-Gain and Low-Power ZnO Nanowire Photodetectors

Anne L. Fischer

Zinc oxide (ZnO) is frequently used as an alternative to GaN in optoelectronics because of its low cost, ease of manufacturing and wide bandgap. The proliferation of ZnO nanowire devices such as optically pumped lasers, chemical and biological sensors, and field-effect transistors has prompted additional research into the material. A group from the University of California, San Diego, recently looked at the two main factors that contribute to the high photosensitivity of ZnO nanostructures.

This image shows p-type ZnO nanowires. Courtesy of the University of California, San Diego.

The high photosensivity arises from the large surface-to-volume ratio and the presence of deep-level surface trap states in nanowires, which prolong the lifetime of the photocarrier. The second factor is the reduced dimensions of the active area in the nanowire devices, which shortens the carrier transit time. The researchers investigated the photoconductive gain that results from the combined long lifetime and short transit time of charge carriers.

They studied the photoconductivity of ZnO nanowires grown by chemical vapor deposition by taking time-resolved measurements in different ambient conditions, such as in air or under vacuum. To determine the charge carrier lifetime, they studied the photocurrent relaxation via time-resolved measurements using UV illumination at low excitation intensity.

They quantified the photoconduction mechanism that leads to the substantial photoconductive gain measure (G = 2 × 10⁸). They found that the relaxation dynamics of photogenerated carriers consist of a fast decay component in the nanosecond time range, which is a result of the fast carrier thermalization and hole trapping by surface states, followed by a photocurrent, which was found to decay within several seconds.

In terms of time spans, they found that the photosensitivity and lifetime are greater when oxygen is deficient. The nanowire mechanism was effective even at the shortest time scale investigated, which was less than a nanosecond.

The researchers developed a model that shows the gain and predicts its dependence on excitation intensity and frequency. Using this model, they can demonstrate the uniqueness of nanowires as photodetectors for applications such as sensing, imaging, optical communications and memory storage, which use low-dimensional semiconductors with high-density surface trap states

Nano Letters, published online March 15, 2007.