Session C01: Poster Session

Quantum Confinement in the Topological Semimetal Bi­4Se3

Quantum confinement of the topological semimetal Bi₄Se₃ was observed as a giant enhancement of the optical bandgap in two characteristic length-reducing regimes: ultra-thin films and nanoplatelets. The films were prepared via DC magnetron sputtering and characterized using atomic force microscopy and ultraviolet-visible spectroscopy. Current work investigates correlations of reduced dimensionality to enhancements of the bandgap. By characterizing the dimensionality through microscopy, spectroscopy results can be mapped to changes in the band edge as determined through Tauc plots.   

Optimization of Coarse Approach for an Atomic Force Microscope (AFM) Design

Coarse approach for an AFM accomplishes the challenging task of moving a probe several millimeters with nanometer precision. We designed a coarse approach mechanism using the koala drive design with three sapphire disks clamped to the probe by molybdenum springs, each separately moved by a piezoelectric tube. Each disk should slip when moved, but stick when another leg is moved. Procedures for measuring when disks were sticking and slipping using interferometry were developed. The results have shown us that motion is improved using four points of contact instead of our original six points of contact. Interferometry also allows us to make informed adjustments to the spring clamping strength to improve motion.   

Impact of silane flow on donor traps in MOCVD Grown Gallium Oxide

Optimizing Gallium Oxide growth conditions will allow better quality material to be used in high power devices. Ideal thin-film Gallium Oxide material improves upon the tradeoff relation between the breakdown voltage and the specific on resistance which results in better performance than ordinary silicon transistors due to Gallium Oxide’s ultra-wide bandgap of 4.8 eV. To improve upon growth parameters, defects within Gallium Oxide must be minimized or prevented. The 100 meV donor defect is a recently new defect detected within the material that currently has no known basis as to what causes its presence. The purpose of this experiment was to see whether increasing the silane flow concentration within the Metal Organic Chemical Vapor Deposition (MOCVD) growth process influences the presence of the 100 meV donor. Both Hall measurements and Thermal Admittance Spectroscopy (TAS) data were used in conjunction to uncover the results of the hypothesis. While some of the samples did have the 100 meV donor appear in the Hall data, there seems to be no systematic trend with the silane flow and the defect according to Hall Effect data. Furthermore, there was not clear evidence of the 100 meV donor in the TAS data, which is still under investigation.  The data may suggest no direct relation between Silane flow and the defect regardless of concentration used. An alternative origin for the 100 meV defect could lay in extrinsic impurities or a lattice defect within antisites or intersitials of the substrate, however, further testing is recommended to verify that no relationship between the silane flow concentration and the 100 meV donor exists.   

Auger Electron Spectroscopy of Au(111) Films

In thin metal films, the predominant model used to explain adsorbate induced changes to resistivity is the Free Electron Point Scatters (FERPS) model. Previous experiments where dimethyl sulfide was thought to have adsorbed onto Au(111) thin films that were cleaned with argon ion sputtering and annealing failed to show the predicted change in resistivity.  Current research using Auger spectroscopy has revealed that the sample surface was likely contaminated with carbon such that no dimethyl sulfide had adsorbed.   

Conversion of Bi4Se3 to Bi2Se3 via post-annealing under Se flux

Bi₄Se₃ and Bi₂Se₃ are materials with topologically protected surface states that are currently the subject of extensive research as potential next-generation quantum technologies. In this study, DC magnetron sputtering was used to grow Bi₄Se₃ thin films. Conversion of Bi₄Se₃ to Bi₂Se₃ was explored via post-annealing under Se flux at moderate temperatures. The Se flux was generated by RF magnetron sputtering of a Se target onto Bi₄Se₃ films. Bi₄Se₃ was grown under two morphological distinct regimes, an atomically flat surface and faceted surface to contrast the Se incorporation through flat surfaces or grain boundaries. Energy dispersive x-ray spectroscopy was used to analyze film composition, while X-ray diffraction was used to verify crystal structure and orientation. Scanning electron microscopy additionally verified morphological changes post annealing. Determining Se percolation into the film under different temperatures and geometries allows an expansion of thin film sputtering capabilities by further control of film composition under various conditions. This work can lead to techniques in tailoring the fermi-level in chalcogenides.   

Analysis of Structural Transitions in Magnetron Sputtered Bi4Se3 Films

Recently, bismuth chalcogenides have been of interest as topological insulators with uses in spintronic and other devices. Bi₄Se₃ presents a topologically insulating material which contains a native heterostructure of Bi₂Se₃ and Bi₂ layers, each with its own topological state. In spite of these valuable properties, the growth of Bi₄Se₃ via magnetron sputtering remains less studied. This study elaborates the properties of Bi₄Se₃ films grown in a magnetron sputtering system with an external magnetic field. Films were grown with varying deposition temperatures, external field orientations, and annealing procedures. Films were characterized using X-ray Diffraction and Electron Back Scatter Diffraction. The deposition temperature played a central role in determining film structure, and a transition from amorphous to polycrystalline to a primarily basally-oriented film occurred as temperature varied. Analysis of X-ray diffraction revealed clear variation in microstructure with external field orientation, and an increase in film ordering with annealing.   

Construction of a Radio Telescope Designed to Observe HI 21 cm Line

We constructed a radio telescope capable of observing the 1420 MHz emission line of neutral hydrogen (HI) following guides from the Digital Signal Processing in Radio Astronomy group at West Virginia University. We used basic building materials such as wood, fasteners, aluminum flashing, and aluminum foam board; a low noise amplifier; and software defined radio to collect the HI signal. We present observations of Galactic HI and discuss its motion as determined by Doppler shift calculations. Using our signal detected and velocity measurements, we calculated HI column densities in various directions. We compare our results to published values and discuss discrepancies.   

Logarithmic confocal distance measurement

This research presents a method and results based on confocal microscopy for noncontact axial distance measurement from a lens to a partially reflective surface using an array of

photodetectors. The ratio of the photodetector signals is independent of the surface reflectivity and light source variations over a range of distance. By using logarithmic photodetector signals the object distance is proportional to the difference between the signals. This technique potentially allows measurement of sound produced by single living cells, or the high velocity surface shape changes in the hohlraum in laser fusion experiments.   

Current-Voltage Characteristics of a Silicon Solar Cell

Solar cells are the source of fascination to students. It is profitable to introduce the fundamentals of photovoltaic devices to students at an early stage of their undergraduate program. Here, we make a simple experiment where students learn to plot the I-V characteristics of a solar cell and measure important photovoltaic parameters such as the fill factor and light conversion efficiency. To complete the experiment a commercial polycrystalline silicon solar cell with an active area of ~100 mm x 80 mm was illuminated from a non-standard light source with an intensity of ~50 mW/cm². To measure photovoltage and photocurrent, a circuit was completed using a voltmeter, an ammeter, and potentiometer. Short circuit current (I_SC) was first measured keeping the resistance in potentiometer minimum; current was then slowly decreased to a minimum by increasing the resistance, at which the measurement of open circuit voltage (V_OC) was taken. Incandescent light bulbs of power 150 W, 60 W and 14 W were used to illuminate the device. The I-V measurement of the device confirmed open circuit voltage of 20.805 volts, short circuit current of 22.3 mA and fill factor of 74%. Maximum photo-conversion efficiency of 8.5% was recorded when the device was illuminated with 150 W power source.  The I-V characteristics were also completed by varying the distance of the solar cell from the light source, by changing the angle of incidence, and by illuminating the solar cell with light of particular colors using translucent material color filters. Output powers of the device in each measurement condition were also calculated. Finally, dark J-V characteristics were also demonstrated to understand the diode behavior of the solar cells.    

Threshold Switching in CdTe Photovoltaics

The current manufacturing process of thin-film Photovoltaics involves laser scribing, an exhaustive and unreliable method, to establish interconnects. In this study, we investigate the threshold switching phenomenon of Cadmium Telluride (CdTe) photovoltaics. In our experimental studies, applying different voltage-bias configurations, a change in resistance was observed. This may potentially lead to a scribe-less technology by eliminating the use of laser or other similar conventional methods.