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Ali standing next to the chemical vapor deposition (CVD) system used to grow the Cadmium Sulfide nanopillars. |
Scholar's Journal
At the beginning of the Spring semester I began working in the Laboratory for Nano Materials and Electronics to conduct research on fabrication of a solar cell based on nanopillars (or vertical nanowires). After spending some time becoming familiar with the laboratory equipment and understanding the fabrication process, I started to plan a project of my own that would allow me to contribute to the lab's solar cell project while expanding the current fabrication capabilities. Originally, my project was an attempt to use a new material system, Gallium Arsenide, instead of the Cadmium Telluride and Cadmium Sulfide which had been successfully completed. After a number of attempts of fabricating the desired nanopillars, we realized that the Gallium Arsenide (GaAs) powder we were using was highly contaminated. Because of the nature of research in semiconductor devices, we would not be able to use the contaminated material at all. During this time, we also realized that switching to a GaAs may not be the appropriate direction to study use of nanopillars in solar cells. Because of the high surface recombination velocity of GaAs, and because of the high surface area of our device, a GaAs nanopillar based solar cell would suffer from poor electrical performance. For this reason, we decided to stay with Cadmium Telluride and Cadmium Sulfide because of their low surface recombination velocities. So instead of switching the material, we decided to change the geometry of the nanopillars. I wanted to try various other shapes of nanopillars, such as square or triangular cross-section nanopillars instead of the circular cross-section that was originally used. This change in the nano-scale geometry would surely have an effect on the optical and electrical properties of the solar cell, such as absorption and electron mobility. Additionally, the growth of square semiconducting nanopillars arrays has not been demonstrated, and has many potential applications for us to study in the future. Thus it was clear that we needed to develop a process for growing nanopillars of various shapes so that their properties could be studied with respect to their shapes.
With the new direction in mind, I began altering the previous method used for making circular cross-section nanopillars. In order to fabricate the desired nanopillars, we were growing them in cylindrical pores. I expected that if I could form square cross-section pores then I should be able to grow the square nanopillars inside without much alteration to the rest of the process. So by making various adjustments to the pore formation process, I was able to grow pores with square, rectangular, and diamond cross-sections. With these new pores, I began growing the Cadmium Sulfide nanopillars with the desired geometries. The first few runs were very successful; however I have noticed some variation from sample to sample, most likely caused by the initial imprinting procedure. A number of parameters must still be carefully optimized in order to insure robust control of this process. Once I am comfortable with the fabrication of these different geometries of the nanopillars, I will begin optical characterization of the pillars in order to see how they are different then there circular counterparts. Once we have collected the desired data, we will continue through to the rest of the fabrication procedures so that we have a fully functioning solar cell using different nanopillars.
Most importantly, the knowledge and experience I have gained as a Haas Scholar has given me the ability to confidently conduct research in a supervised yet independent environment. With this exposure to the graduate level research environment, I have become certain of my desire to continue my education in this field. The opportunity to become a laboratory scientist and work on highly technical and potentially groundbreaking research has been invaluable for me, and I hope to advance upon the progress I have made in this endeavor.
