AVS 52nd International Symposium
    Nanometer-Scale Science and Technology Monday Sessions
       Session NS2-MoM

Paper NS2-MoM8
Near-field Scanning Photocurrent Microscopy of a Nanowire Photodetector

Monday, October 31, 2005, 10:40 am, Room 210

Session: Nanowires
Presenter: Y. Gu, Northwestern University
Authors: Y. Gu, Northwestern University
E.-S. Kwak, Northwestern University
J.L. Lensch, Northwestern University
J.E. Allen, Northwestern University
T.W. Odom, Northwestern University
L.J. Lauhon, Northwestern University
Correspondent: Click to Email
One-dimensional nanomaterials such as semiconductor nanowires (NWs) are being considered for a variety of device technologies, including nanoscale photodetectors (PDs). The mechanisms of carrier photogeneration in nanoscale PDs have been addressed in a number of studies, but the charge transport and collection mechanisms have received comparatively little attention and are not well understood. In this regard, photoconductivity measurements with uniform illumination (spot size larger than the device) may be insufficient to establish the operational principles of NW devices because (1) the internal electric fields may be highly non-uniform, and (2) similarities between conventional and NW device characteristics may be fortuitous. To understand the global response and the ultimate potential of NW PDs, an understanding of the photoresponse on a smaller length-scale is desirable. We have developed a new technique, near-field scanning photocurrent microscopy (NSPM), to explore the local photoresponse of semiconductor NW devices. A near-field scanning optical microscope (NSOM) was used to image the photocurrent induced by local illumination (excitation spot size less than device size) along the length of a metal-semiconductor-metal (MSM) PD based on a single CdS NW. Under uniform monochromatic illumination, the MSM PDs exhibited photocurrents ~10@super 5@ larger than the dark current (< 2 pA). Under local illumination, the response of the devices was limited to regions near the M-S contact. Analysis of the spatial variation and bias dependence of the local photocurrent allowed the mechanisms of photocarrier transport and collection to be identified. The NSPM technique we describe can be readily extended to other NW-based devices with similar geometries, and provide insight into the operation principles of these devices. NSPM therefore has the potential to significantly advance the understanding and development of NW device technology.