Previous Research
Three weeks into my first undergraduate semester, I was wandering the laboratory
hallways and reading posters about optical research conducted on different nanostructures. A
PhD student caught me outside of his lab perusing one of his previous projects on the absorption
spectrum of single wall carbon nanotubes (CNT), and he was quick to begin a conversation and
explain the process to me. After realizing that we had a common interest in electrical properties
of such novel nanostructures, he invited me to tour the lab facilities. I quickly joined the
Optoelectronics research group, working alongside one other undergraduate student, several
graduate students pursuing master’s degrees, and several more PhD students. Thanks to my
particular interest in nanotubes, I was allowed to help with their newly-established CNT growth
project. The theoretical goal was to synthesize nanotubes, couple them with Cadmium Selenide
core/shell nanocrystals, and use conductive polymers to fabricate organic solar cell devices.
Understanding the atomic interactions between CNTs and quantum dot nanocrystals could
potentially increase solar efficiency of flexible plastic solar cells. Our synthesis method used
high temperature chemical vapor deposition under a weak vacuum in a quartz tube furnace with
a mass flow controller. We passed a consistent gas flow of carbon dioxide mixed with argon
over a series of crucibles containing a metallic nanoparticle catalyst for carbon chain formation.
Results were analyzed by measuring change in mass and Raman spectroscopy of the sample. My
task was to characterize the relationship of resulting Raman peak intensities vs. variable gas flow
input and total reaction time in the furnace. I was able to derive a pattern between reaction rate
output and the two input variables of mass flow and time.
During the spring of 2010, I began training for my next project by conducting a short
literature review of III-V semiconductors, specifically Gallium Arsenide (GaAs), and by learning
how to operate all the equipment available in the Optoelectronics lab. Several of the graduate
students in this research group have conducted experiments with semiconductor nanomaterials
which are grown using our on-campus molecular beam epitaxial growth chamber (MBE). I was
able to spend several weeks shadowing a graduate student who taught me the physical process of
the growth chamber and let me assist with mechanical preparation of the growth process.
Understanding the atomic layer-by-layer structure of these experimental semiconductors is the
necessary background for fabricating solar cell devices, which was my research task during the
summer of 2010. I was awarded a $1250 undergraduate research grant from the University of
Arkansas Honors College for performing the following project.
I was given a sample of repetitive InAs quantum dot layers deposited on a GaAs substrate,
which was designed by a graduate student to test the introduction of an intermediate energy band
into the semiconductor structure. My task was to characterize the photoluminescence (PL) and
capacitance-voltage (CV) of the raw sample, fabricate a device using wet etching techniques, and
then measure the photoresponse of the completed solar cell device. PL was measured using a
Bomem DA8 FT-IR spectrometer with a PL accessory. This allowed me to determine the band
gap of the sample by measuring the emitted photon wavelength. Using liquid nitrogen cooling,
the operating temperature of the sample was varied from 300K (room temperature) to 77K. With
this data I could observe the increasing band gap energy with respect to decreasing temperature
and find the optimal operating temperature which yields the highest intensity of emission. CV
measurements were performed with an Accent Electrochemical Capacitance-Voltage Pro. First,
an accurate CV curve was obtained for the sample by determining the least amount of deviation
between two testing models, Series and Parallel. Then the CV was measured at small intervals
throughout the thickness of the sample structure. This test was useful for determining good
ohmic contact capability and for checking for a correct correlation between measured carrier
concentrations and the designed sample profile carrier concentrations.
Fabricating the solar cell device from raw sample substrate was the next phase of my
project. It was the most time-consuming portion of my work, and the detailed steps are quickly
summarized in this report. . This sample utilized a two-step photolithography method, followed
by metallization and liftoff. The sample was cleaned and layered with photoresist using a Cee®
100 spin coater, and a grid pattern of 500um square mesas was transferred on the sample using a
Karl Suss MJB3 mask aligner. After etching the patterned surface, metallization was done using
an e-beam evaporator to deposit respective layers of AuGe, Ni, and Au for optimal ohmic
contact. Finally liftoff was performed by ultrasonic bath, and a K&S
4523 Wedge Bonder was used to connect the device to a standardized
testing platform. Photoresponse of the device was measured using a
Bruker IFS 125HR Fourier Transform-Infrared spectrometer (FTIR) at
300K, after dark current was initially measured using the Keithley 4200
Semiconductor Characterization System. By plotting photoresponse
Figure 1. Device mesa
intensity over wavelength and comparing with the solar emission spectrum,
with gold wire bonded
I could determine the effectiveness of the device as a solar cell. The last
step in this process was measuring the power output of the device, which was done using an
AM1.5 One Sun simulator. Plotting an I-V curve and measuring the fourth quadrant area under
the curve, where measured current is negative and voltage is positive, allowed me to find the
power output of the device and the theoretical operating efficiency.
Intellectual Merit and Broader Impacts: These projects have helped me prepare for further
research of nanomaterials and experimental solar cell devices. I am now energized for my
current research project, in which I will be examining the effect of colloidal metal nanoparticle
thin film deposition on existing III-V semiconductor nanostructures. The broader impacts of my
previous research are easily found in the potential technology to be gained from these materials;
advanced nanofabrication will make thinner, less expensive, and more efficient solar cells
available for technologically underdeveloped areas of the world. My work as an undergraduate
researcher displays intellectual merit through the diversity of scientific and analytical skills I
have acquired, and this knowledge base is highly applicable to many important fields of research.
I am excited to use these skills to continue working towards a future where renewable energy
sources and their scientific educational background are both readily available.