Femtosecond excitation of radial breathing mode in 2-D arrayed GaN nanorods
Date Issued
2012
Date
2012
Author(s)
Chen, Hung-Pin
Abstract
GaN nanorod is a popular structure for the optoelectronic and microlectromechanical devices. Under the nano-scale, the rod has some interesting phenomenon about optical and mechanical properties. In this thesis, we successfully fabricated 2-D arrayed GaN nanorods with E-Beam lithography and dry-etching. The rod diameter can be scaled down to 35nm with this fabrication method. Some modifications of optical and mechanical properties were observed in our measurement, compared with the bulk GaN. In order to monitor the changes of optical and mechanical properties, the ultrafast optical pump-probe technique was adopted to generate and detect the nano-confined acoustic modes. The size-dependence experiment also pointed that non-radiative surface recombination or surface depletion region on the side wall might be a dominator factor for the relaxation time of carrier recombination.
Radial breathing oscillation of 2-D arrayed GaN nanorods was successfully excited and identified in rods with different diameters by using femtosecond transient reflectivity measurement. Through analyzing thus measured diameter dependent oscillation frequency, we discovered that modification of the mechanical property appeared in the 2-D arrayed piezoelectric GaN nanorods, fabricated on top of a bulk substrate, when the rod diameter was on the order of or less than 50 nm. Our measurement showed a much reduced elastic stiffness constant (C11) of 193±24 GPa in 35nm diameter nanorods, compared with the 365±2 GPa in bulk GaN. This size-reduction induced mechanical modification would be a critical factor to be considered for future sensing and energy applications. Our study also provides a new spectroscopic method to explore the size-reduction-induced softening effect through the measurement of the radial breathing oscillations.
Subjects
GaN nanorod
Femtosecond laser
elastic stiffness contant
Type
thesis
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