Microscopy Images Photocurrent in Nanowire Detector

Daniel S. Burgess

Using a near-field scanning optical microscope, researchers at Northwestern University in Evanston, Ill., have produced two-dimensional maps of the photocurrent in a metal/semiconductor/metal photodetector constructed from a single nanowire. The approach, called near-field scanning photocurrent microscopy, has potential applications in the study of nanowire-based devices, such as transistors, sensors and LEDs.

The investigators produced N-type CdS nanowires, which they selected for the experiments for its visible bandgap and because its bulk photoresponse is well documented, by thermal chemical vapor deposition and gold-catalyzed vapor/liquid/solid growth. To fabricate the electrodes, they evaporated titanium and gold on a degeneratively doped silicon substrate that had been coated with a 400-nm-thick SiO₂ film and processed using electron-beam lithography. A frequency-doubled Ti:sapphire laser, the output of which passed through a 1-kHz optical chopper, served as the excitation source for the photodetector.

The completed device was 100,000 times more conductive under uniform illumination of 0.7 W/cm² of 400-nm laser light than it was in the dark. The measured room-temperature dark current was less than 2 pA, and the response time was less than 15 μs.

To map the local photoresponse, the researchers illuminated the detector through the fiber probe of a near-field scanning optical microscope, collecting photocurrent measurements at the fixed forward or reverse electrode biases while the excitation spot was scanned along the nanowire. The results suggest that the nanowire device differs fundamentally from a planar metal/semiconductor/metal photodetector and should be interpreted as behaving like a pair of back-to-back Schottky diodes.

The investigators propose that the performance of a nanowire-based photodetector is limited by the diffusion of photogenerated electrons. Accordingly, a device in which the length of the nanowire is comparable to the size of the photocarrier collection region should behave like a conventional bulk photodetector, and they are investigating the relationship between nanowire length and device performance.

Applied Physics Letters, July 25, 2005, 043111.