School of Electrical, Computer and Energy Engineering
PhD Final Oral Defense
Growth and Characterization of Multisegment Chalcogenide Alloy Nanostructures
for Photonic Applications in a Wide Spectral Range
by
Sunay Turkdogan
05/05/2015
9:30am
GWC 208
Committee:
Dr. Cun-Zheng Ning (chair)
Dr. Hongbin Yu
Dr. Joseph C. Palais
Dr. A. John Mardinly
In this dissertation, I described my research on the growth and characterization of
various nanostructures, such as nanowires, nanobelts and nanosheets, of different
semiconductors in a Chemical Vapor Deposition (CVD) system.
In the first part of my research, I selected chalcogenides (such as CdS and CdSe) for a
comprehensive study in growing two-segment axial nanowires and radial nanobelts/sheets
using the ternary CdSxSe1-x alloy. Since the ultimate goal is for full-color emission
applications, I worked on growing CdS-rich and CdSe-rich segments in a monolithic fashion
as a first step towards achieving this objective. I report that the morphology of the structures
strongly depends on the substrate temperatures, and to a lesser extent on the source
temperatures, and it changes from nanowire to nanobelt, and finally to nanosheet, as the
growth temperature increases. In addition, the interplay between VLS and VS was found to
be critical to determining the morphology of the products. Finally, the substitution of S by Se
in CdS, a process called anion exchange, is also very effective.
I demonstrated simultaneous red (from CdSe-rich) and green (from CdS-rich) light
emission from a single monolithic heterostructure with a maximum wavelength separation of
160 nm. I also demonstrated the first simultaneous two-color lasing from a single nanosheet
heterostructure with a wavelength separation of 91 nm under sufficiently strong pumping
power.
In the second part, I considered several combinations of source materials with
different growth methods in order to extend the spectral coverage of previously demonstrated
structures towards shorter wavelengths to achieve full-color emissions. I achieved this with
the growth of multisegment heterostructure nanosheets (MSHNs), using ZnS and CdSe
chalcogenides, via our novel growth method. Our novel growth method relies on the atomic
replacement process at high growth temperatures and the new layer formation process at low
growth temperatures. By utilizing this method, I demonstrated the first growth of ZnCdSSe
MSHNs with an overall lattice mismatch of 6.6%, emitting red, green and blue light
simultaneously, in a single furnace run using a simple CVD system. The key to this growth
method is the dual ion exchange process which converts nanosheets rich in CdSe to
nanosheets rich in ZnS, demonstrated for the first time in this work.
I demonstrated that these structures can potentially exhibit any visible color emission
with appropriate changes to the growth positions, due to the temperature-dependent
composition deposition method utilized for the growth. I show that tri-chromatic white light
emission with different correlated color temperature values was achieved under different
growth conditions. We demonstrated multicolor (191 nm total wavelength separation) from a
single monolithic semiconductor nanostructure for the first time. Due to the difficulties
associated with growing semiconductor materials of differing composition on a given
substrate using traditional planar epitaxial technology, our nanostructures and growth method
are very promising for various device applications, including but not limited to: illumination,
multicolor displays, photodetectors, spectrometers and monolithic multicolor lasers.