In Situ Process Promises Mass Production of Nanowire-Based Photodetectors

Daniel S. Burgess

A fundamental hurdle in the race to develop viable optoelectronic devices based on nanoscale structures is finding an easy way to place those structures as desired and to connect them with much larger components. Now John T.L. Thong and Jaslyn B.K. Law of National University of Singapore have constructed a photodetector based on ZnO nanowires using an in situ growth process that they suggest overcomes this hurdle and may enable the mass production of nanowire detector arrays for applications including optical communications, target acquisition, and radiation calibration and monitoring.

Law, a postgraduate student in the department of electrical and computer engineering and an A*Star research scholar, explained that most nanowire-based photodetectors have been single-nanowire devices, constructed using a number of time-consuming steps. “The nanowires are typically synthesized on a substrate, harvested, redeposited and individually contacted for electrical characterization.”

Although this approach has enabled researchers to construct operational proof-of-concept devices, she said, they are not suitable for mass production. Moreover, they have tended to suffer from relatively low collection efficiencies because of the difficulties involved in stimulating an individual nanoscale detector element.

Thong and Law believe that they have a solution to both of these problems. In their approach, they combine the directed synthesis and assembly of the nanowires into one in situ fabrication step, using a self-catalytic growth process. The result is the generation of multiple interconnected nanowires that extend across the gap between the prepatterned microscale electrodes.

Figure 1. An optical micrograph shows the patterned zinc electrodes extending from the gold electrodes and bond pads. The inset offers an enlarged view of the gap between the electrodes. Courtesy of Jaslyn B.K. Law.

To create their demonstration device, they deposited gold on a Si₃N₄-coated silicon substrate to form microscale bond pads and electrodes. Next, they deposited zinc on the electrodes, narrowing the gap to 3 μm (Figure 1). Finally, they heated this assembly for three hours at 700 °C in the presence of oxygen and argon gas at 1 atmos.

The scientists suggest that the zinc melts and forms droplets at the apexes of the hexagonal grains of the metal as its temperature passes approximately 419 °C, which then react with the oxygen to produce ZnO. The low wettability of the ZnO with the zinc leads to the oxide migrating outward from the zinc across the gap, creating a web of 20- to 40-nm-diameter ZnO nanowires (Figure 2).

Figure 2. The ZnO nanowires form between the electrodes as the zinc is heated beyond its melting point in the presence of oxygen (a). A bright-field transmission electron micrograph reveals a single-crystal ZnO nanowire (b).

They investigated the time-dependent photoresponse of the device at room temperature and under a bias of 5 V using a 370-nm LED driven by a 600-Hz square wave from a Thurlby Thandar Instruments Ltd. function generator to offer pulsed operation. A Tektronix oscilloscope monitored the response of the detector at a sampling interval of 0.4 μs. At ambient conditions, the detector displayed a response time of less than 0.4 ms. Other photodetectors based on ZnO nanowires, they note, have displayed photoresponse times ranging from less than 1 s to thousands of seconds.

Law said that the team is investigating the effects on device responsivity of using networks of interconnected nanowires. The scientists also hope to explore the potential applications of the detectors in trace gas sensing, noting the influence of the presence of water vapor on the response curve of the photodetector in the experiments.

Applied Physics Letters, March 27, 2006, 133114.