Optical Characterization of Electrochemically Self-Assembled Compound Semiconductor Nanowires

• Main Author: Ramanathan, Sivakumar
• Format: Others
• Published: VCU Scholars Compass 2006
• Subjects: quantum dots; Zinc oxide; fluorescence; quantum confined Stark effect; Electrical and Computer Engineering; Engineering
• Online Access: http://scholarscompass.vcu.edu/etd/1436; http://scholarscompass.vcu.edu/cgi/viewcontent.cgi?article=2435&context=etd

Semiconductor nanowires have attracted considerable attention as possible source for lasers and optical storage media. We report the fabrication and optical characterization of ZnO and CdS nanowires. The former are produced by electrochemical deposition of Zn inside nanoporous alumina films containing regimented arrays of 10nm, 25nm and 50 nm diameter pores, followed by room temperature chemical oxidization. Fluorescence spectroscopy shows different characteristics associated with different sample diameter. The 50 nm ZnO nanowires show an exciton recombination peak and an additional peak related to the deep trap levels. 25 nm ZnO nanowires show a only the exciton recombination peak, which is red shifted, possibly due to quantum confined Stark effect associated with built in charges in the alumina. This feature can be exploited to produce light emitting devices whose frequency can be modulated with an external electric field. Such devices could be novel ultra-violet frequency modulators for optical communication and solar blind materials. In addition, we have investigated fluorescence spectra of 10-, 25- and 50-nm diameter CdS nanowires (relative dielectric constant = 5.4) self assembled in a porous alumina matrix (relative dielectric constant = 8-10). The spectra reveal peaks associated with free electron-hole recombination. The 10-nm wire spectra show an additional lower energy peak due to exciton recombination. In spite of dielectric de-confinement caused by the insulator having a higher dielectric constant than the semiconductor, the exciton binding energy increases almost 8-fold from its bulk value in the 10 nm wires. This increase is most likely due to quantum confinement accruing from the fact that the exciton Bohr radius (~5 nm) is comparable to or larger than the wire radius, especially if side depletion is taken into account. Such an increase in the binding energy could be exploited to make efficient room temperature luminescent devices in the visible range.