1. 1
Application of Photoluminescence
In Analyzing Optimal Growth Factors in Quantum Nanowires
I. Introduction
Despite the increasing predominance of solar energy in the search for alternative
energy sources, the uneconomical nature of solar energy has been hindering the energy
source from greater prevalence and popularity. Although the use of solar energy has
gradually been rising for several years, it is still widely criticized for its costliness and
inefficiency. Currently, many photovoltaic cells exploit planar semiconductors to conduct
energy for applicable use. Planar semiconductors utilize semiconductor materials, often
layered vertically on top of one another. This process requires intricate construction in
Molecular-Beam Epitaxy Labs as well as careful consideration of factors such as
substrate material and lattice structure of each element used to construct the
semiconductor. [5] In planar semiconductors, materials are arranged in a manner such
that the material of the top layer contains the greatest band gap, allowing it to absorb the
highest energy photons, leaving the layers beneath with smaller band gaps to absorb the
remaining photons. Materials play a vital key role in the efficiency of the semiconductor.
Materials such as amorphous silicon and organic materials offer an economical
alternative, but are greatly inefficient in obtaining energy. On the other hand, materials
such as Gallium Arsenide (GaAs) are often very costly but whose efficiency offer the
highest percentage of 28 percent in planar semiconductors. [21] The uneconomical and
impractical approach to producing planar semiconductors has been a major drawback in
the production of such use of semiconductors in devices such as solar cells; instead,

2. 2
recently researchers have been turning to explore the advantages of utilizing nanowire
semiconductors to resolve the economics of solar energy.
Quantum Nanowires
Nanowires specialize in their ability to harvest great amounts of energy in a
relatively miniscule length-to-width ratio. Nanowires are often approximately 100
nanometers in length; because of the size of these structures, such technology often
ranges into the field of quantum mechanics. Given its material, nanowires have the ability
to act as insulators, semiconductors, and conductors.
Unlike planar semiconductors, nanowire semiconductors present a more
pragmatic alternative because the only the nanowires, which range from 1 to 4
micrometers in height, require materials such as Gallium Arsenide to achieve efficiency.
Figure 1: A Scanning Electron Microscope (SEM) images Gallium Arsenide nanowires grown on a
Silicon substrate. The transmission electron microscope (TEM) illustrates a single nanowire; the final
Scaning Transmission Electron Microscope (STEM) image displays the atomic structure of an
individual nanowire.

3. 3
(Duan, Wang, Lieber, 2000) The substrate on which these nanowires are produced is
often made of more inexpensive materials, such as silicon dioxide (SiO2). (Dick, 2008)
Several benefits that nanowires provide include long absorption path lengths and short
distances for carrier charge transport; a strong ability to capture light; and alterations of
material properties and cell efficiencies through dimension and composition deviations of
the nanowires. (Yang, Yan, and Fardy, 2010)
The three-dimensional structure of nanowires allows more photons to be captured
than seen with planar two-dimensional structures. The size of these nanowires, in
addition to its potential efficiency in capturing light and converting the light to usable
energy, decreases the cost per watt of devices utilizing such structures.
Unfortunately, the current obstacle inhibiting the success of nanowires is the low
efficiency level of 6 percent primarily due to it being relatively new idea in comparison
to planar semiconductors. Nevertheless, nanowire semiconducors present a promising
future for solar energy as research advances.
Semiconductor Applications in Solar Cells
When a source of light strikes a photovoltaic cell, the semiconductor absorbs the
photons. The energy of the photons is able to knock loose electrons in the semiconductor;
thus allowing the electrons, know as carrier charges, to “jump” in the conduction band.
Semiconductors use a process known as “doping” to increase the efficiency of generating
current. The process of doping involves adding impurities to a material, most often
Silicon. [1] In GaAs semiconductors, N-type doping involves adding a minute quantity of
Arsenic, which contains five electrons in its valence shell, also known as valence band.

4. 4
Because Silicon only contains four outer electrons, the fifth electron from the Arsenic is
very loosely bound. As a result, relatively small amounts of energy, such as the energy
from photons, are able to “knock loose” these electrons into the conduction band,
creating a flow of current. The conduction band is the range of electron energies in which
electrons are delocalized and are able to conduct electricity.
In addition to N-type doping, P-type doping involves adding Gallium, which
contains only three outer electrons. When these three outer electrons bond to silicon, a
fourth electron from Silicon is unable to bind with another electron from Gallium, this
creating a “hole” where the electron is absent. The absence of an electron creates a
positive charge, which is also able to conduct a current in the semiconductor.
Photoluminescence
Figure 2: Semiconductors possess a relatively small band gap between the
valence band and the conduction band. The band gap represents the amount of
energy required to “knock loose” an electron from its valence shell. In insulators,
the size of the band gap is much greater; therefore, electrons are not easily
delocalized, resulting in the nonconductive nature of insulators. On the other
hand, the conduction band and valence band in metal conductors often overlap,
resulting in the conductive nature of conductors.

5. 5
The process of photoluminescence is utilized to characterize semiconductors.
Photoluminescence utilizes Einstein’s Photoelectric Effect, in which photons and
maximum kinetic energy are released as the energy of a beam of light surmounts the band
gap of the semiconductor. In the process of photo-excitation, electrons “jump” into their
excited states; as the electrons assume back into their ground state, excess energy is
emitted in the form of photons. The amount of energy in emitted light from the sample,
known as photoluminescence, can also be used to measure the band gap of new
compound semiconductors for characterization.
The purpose of this study is to observe the photoluminescence of diverse GaAs
quantum wire samples grown in Molecule Bean Exitaxy (MBE) labs under various
conditions, including temperature, etc. The photoluminescence of each sample measures
the luminosity or amount of photons that are emitted from the nanowire sample as an
incident light source, such as a Helium-Neon laser or a white light source, strikes the
sample. In addition to measuring the efficiency of the sample, photoluminescence is also
imperative in characterization of semiconductors and recognition of contamination often
found during its epitaxial growth stages. This measurement is directly related to the
efficiency of the nanowire sample; as the amount of photons yielded from the sample
increases, the greater the effectiveness of a particular sample.
Figure 3: Einstein’s Photoelectric
Effect demonstrates the release of
photons as electrons fall back to their
ground state after a process of photo-
excitation occurs.

6. 6
II. Methodology
Growth and Fabrication of Nanowire Semiconductors
There are two methods to fabricate nanowires: “top-down” and “botton-up”. The
top-down technique involves carving down a bulk of the desired material to the desired
size. Although this process had been more widely used in the past several decades,
problems arise when technology begins to demand smaller and smaller structures.
The “bottom-up” technique, often associated with epitaxial growth, is the more
prevalent method used for nanowire growth today and also utilized for the growth of
GaAs samples in this experiment.[25] Epitaxial growth involves the oriented growth of
crystalline structures, usually grown on a crystal substrate. This technique allows for a
controlled chemical composition in addition to the ability to easily fabricate smaller
structures unlike the “top-down” technique.
In the experiment, Molecular Beam Epitaxy (MBE) was used to grow the
semiconductor nanowire samples. MBE requires a high vacuum environment, in which a
low deposition rate of elemental beams of material occurs. [25] First, Gallium and
Arsenide are heated to a sublimation temperature to become gaseous atoms. During this
stage, the two materials remain in separate gaseous chambers; the term “beam” signifies
that the two materials do not interact with one another until both materials reach the
wafer. At this point, the two materials will condense on the Si wafer to form a single
crystal using quantum wells to direct the growth of GaAs. [22]

7. 7
In addition to the Gallium and Arsenide, an additional elemental particle is
essential in the growth of the nanowires. [5] In this particular experiment, gold
nanoparticles were applied to promote the growth of the nanowire in one dimension. Au
particles are currently the most commonly used materials, largely due to the extensive
research concentrated on this material.[24]
A problem that arises with the use of Au particles is the rise of contamination in
the Silicon wafer. [5] As Au particles hit the surface of the wafer, often the particles will
submerge itself into the bandgap of the Si wafer, thus negatively affecting the electrical
Figure 4: Molecular Beams of Gallium
and Arsenide coat the Si wafer in this
“bottom-up” technique
Figure 5: SEM photograph of GaAs
nanowires grown in MBE lab used in this
experiment.

8. 8
conductivity of the semiconductor. As a result, small quantities of Au particles must be
applied at a time in order to minimize the diffusion of Au into the Silicon surfaces.
Procedure
In determining the photoluminescence of the given GaAs nanowires samples, an
optical set-up was required to direct the incident and photoluminescent light in the
following path.
The original source of light is emitted from a helium-neon laser or a white light
source through a series of mirrors and lenses to the optical chopper, which modulates the
intensity of the incident light.[19]
Next, the light was directed to the cryostat containing the nanowire semiconductor
samples, which absorb the incident light. The cryostat is responsible for lowering the
Figure 5: Conceptual diagram of optical photoluminescence set-up; beams
of light originating from either the He-Ne laser or white light box follow
the pattern of optics to the cryostat, where the semiconductor nanowires
are contained.

9. 9
temperature of the nanowire samples to increase emitted photons. The lower temperature
will increase the intensity of emitted light and reduce random scattering of electrons and
holes.
Then, the photons released from the nanowire samples were then directed to a
monochrometer and photomultiplier tube. The monochrometer is responsible for
narrowing the range of light to a select wavelength.[16] Following the monochrometer,
the photomultiplier multiplies the current of the wavelength selected by the
monochrometer by Einstein’s Photoelectric Effect and Secondary Emission.
Next, the current was delivered to the lock-in amplifier, which modified the signal
to reduce obstructive noise from the surrounding environment. The lock-in amplifier
multiplies the input reference signal (ωr), the signal from the optical chopper, by the input
signal (ωs), the current from the photomultiplier tube, to generate two Alternating Current
waves (ωr+ωs and ωr-ωs). These two waves then pass through a low-pass filter, which
allows low frequency waves to bypass while removing out any frequencies higher than
the set cutoff frequency. The low-pass filter generally eliminates the two Alternating
Current s unless the ωr and ωs are equivalent, which results in a Direct Current that can
generate a voltage and is proportional to the signal amplitude.
Figure 6: conceptual diagram of
interactions between reference signal
and input signal; in the case where both
signals are equal, a direct current is
produced

10. 10
Finally, the signals and data recorded by the various instruments are delivered to a
Labview program for data analysis.
III. Results and Discussion
The results of this experiment are still pending due to technical
miscommunications between laboratory instruments. Although data is currently
inconceivable, this setback does not hamper the significance of the project.
By analyzing the photoluminescence of each sample, we will be able to determine
which growth conditions will help yield the greatest amount of photons from the
semiconductor. In order to enhance the quality and efficiency of nanowire semiconductor
technology, it is essential to cultivate the process of producing an efficient
semiconductor. By combining the efforts of maximizing nanowire efficiency during its
growth stages with the work of various other labs to maximize efficiency in other areas of
the process, the effectiveness of nanowire semiconductors will begin to grow vastly.
The following photoluminescence samples illustrate the potential results of this
project. By comparing the various graphs in regards to growth conditions, rather than
Helium-Neon laser excitation power as displayed in the examples below, it becomes
possible to distinguish which growth condition yields the greatest count. The count on the
y-axis reflects the number of photons emitted from the semiconductor after the light
source strikes the cryostat. As figures 7 and 8 below display, the count can differ
immensely from one particular test to another. By garnering the greatest number of

11. 11
counts possible, it is directly reflecting the semiconductor’s ability to produce an efficient
amount of electricity.
In addition to obtaining the count of each GaAs semiconductor sample,
photoluminescence can also display any contaminations in the semiconductor during
growth procedures. Each visually significant peak apart from the middle peak represents
contaminations in the semiconductor. As previously mentioned, the Au particles that had
dissolved into the Silicon wafer during Molecular Beam Epitaxy display its negative
effects on the quality of the semiconductor in photoluminescence diagrams. [22]
IV. Conclusions
Despite promising future that nanowire semiconductors hold for photovoltaic
cells, the low efficiency of nanowires is nonetheless a major topic of research. In this
project, the photoluminescence measurements of each semiconductor sample, each
labeled with and grown under different conditions in the MBE lab, will suggest which
Figures 7 and 8: results of photoluminescence in a GaAs semiconductor
sample testing the effects of power of the light source. The graphs display
different counts of emitted photons as well as reveal contamination of the
Silicon substrate during epitaxial growth.

12. 12
growth conditions will benefit the efficiency and exploitation of photovoltaic’s using
nanowires. Apart from analyzing ideal growth conditions to optimize efficiency of
nanowire semiconductors, a relevant amount of research has also been conducted to
search for alternatives to optimizing semiconductors.
Many researchers continue to demonstrate an interest in increasing the efficiency
from solar power to electricity. Through the method of direct water electrolysis using p-n
junction doping, researchers have designed an effective technique to increase the
production of hydrogen.[14,20] Such materials have also proved to be effective in the
process of passivation, the coating of the junctions to preserve the condition of the cell. In
addision, using Indium Gallium Arsenside (InGaAs) for passivation not only provides
protection from environmental stresses on the cell, but has also increased the
effectiveness of power conversion.[21] By various arrays of design implementations of
Group III-V semiconductors, the collaborations of such research can ultimately lead to a
solution to the relative inefficiency in the power conversion of semiconductors in tandem
cells.
The success of GaAs nanowires are far from optimal, as demonstrated by the
ongoing research pertaining to such materials. In order to obtain greater success with
nanowires in solar cells, it is important to characterize the nanowire through
photoluminescence, which utilizes the photoelectric effect in displaying the bright light
spectrum emitted by the semiconductor when it is struck by light, such as a white light
box or a laser. After it is characterized, this information would facilitate the
understanding of how to construct a more efficient solar cell using the information
gathered from the photoluminescence. Research pertaining to the use of GaAs nanowires

13. 13
to power solar cells will further augment our abilities to fabricate a photovoltaic of
greater efficiency.
By observing the optical properties that improve the functionality such nanowires,
further research would be able to enhance the efficiency of devices that utilize
semiconductor nanowires. Apart from analyzing optimal growth factors, it is also
essential to comprehend other factors that would contribute to the progression of
nanowire semiconductors. Photovoltaics, which are often associated with
semiconductors, are also a major topic of study as researchers are searching for
techniques to enhance such devices.[9,10] The results of this project, as well as similar
research on the topic of cultivating the efficiency of nanowire semiconductors, will be
able to enhance the overall effectiveness of solar cells. By implementing the use of
semiconductor nanowires in photovoltaics in the future when the efficiency of nanowires
surmount that of planar semiconductors, the cost-friendly approach to solar energy could
potentially kindle a newfound interest in solar energy.

14. 14
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