Zinc Phosphide Nanowires for Photon Detection

Michael J. Lander

Although not dirt, zinc phosphide is cheap and abundant. Carefully synthesized structures of the material also have good optical efficiency and resist the buildup of an oxide layer on their surface. Together, these properties give the substance great potential for optoelectronic applications.

Zhong Lin Wang and colleagues at Georgia Institute of Technology in Atlanta have produced and studied Zn₃P₂ nanostructures that meet these specifications. Although other researchers have analyzed zinc phosphide as a thin film, the team is one of a small number that have worked with it at the nanoscale.

Researchers graphed the on/off ratio for this PN photodiode as a function of time under green (523 nm), red (680 nm) and white light. The device, composed of crossed Zn₃P₂ and ZnO nanowires (NWs), is seen here in an electron micrograph (insert).

To begin their investigation, the scientists assembled a thermal-assisted pulsed laser deposition system to synthesize the compound of interest. They placed a mixture of ZnO and graphite in the middle of the device and a target of Zn₃P₂, ZnO and Zn at its end. While the temperature in the chamber was above 900 °C, they directed a Lambda Physik (now Coherent) excimer laser at the target. A crystalline silicon wafer placed beneath it caught the dark yellow Zn₃P₂ nanostructures as they formed.

Examination with a scanning electron microscope from Leo Electron Microscopy Ltd. (now Carl Zeiss) and a transmission electron microscope from Hitachi showed that the compound formed tree-shaped structures with sixfold symmetry, nanobelts of uniform thickness and nanowires of up to 100-nm diameter on the silica substrate.

For experimental purposes, the scientists focused on the nanowires. As with the other structures, they had a bandgap between 1.4 and 1.6 eV — a range ideal for solar energy conversion. They spanned a single wire between two gold electrodes via dielectrophoresis and created platinum-top electrodes with focused ion-beam microscopy to optimize it for photoconductivity studies. They measured current versus voltage (I-V) and the on/off ratio — the ratio of current under light to that in dark conditions — as a function of time under white, green (523 nm) and red (680 nm) light. The wire’s resistance was lowest and its ratio highest for higher-energy 523-nm light, but red and white lights also caused a significant response.

Wang and his team also laid a ZnO nanowire perpendicular to another Zn₃P₂ wire to create a PN photodiode. They plotted I-V curves and recorded on/off ratios for the system under reverse and forward bias in the dark and under the three types of light used in the single-wire experiments.

Most indicative of the heterojunction’s high sensitivity were the on/off ratios — 13 for green and about five for red and white lights. Both single- and crossed-wire systems also demonstrated a response time of less than 1 s when exposed to each light source. Quick response time and sensitivity, combined with low cost and resistance to oxidization, make Zn₃P₂ nanowires well-suited for compact light-sensing applications.

“I want to see if this can be a prototype for ultrasensitive photon detectors, imaging plates and other types of detectors,” Wang said in reference to the heterojunction. He added that substituting a nanowire of a different material for one made of ZnO can yield photodiodes capable of detecting light from the near-infrared to the UV.

“We are currently trying to improve the [synthesis] process to produce pure samples without such a high amount of other structures in them,” he said. The group also aims to improve the detection limit of the nanowire devices. Its primary goal, however, is to create an array of photodiodes. In modified form, such arrays could serve as nano-optoelectronic components in cameras, solar cells and scientific instruments.

NanoLetters, ASAP Edition, Dec. 19, 2006, doi: 10.1021/nl062228b.