Synthesis and Characterization of Si, Ge, and SixGe1-x Nanowires by Fiber Drawing
Floyd, Adam R.
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This research provides a method of using a mixed powder in tube approach for producing and characterizing large quantities of highly oriented, high aspect ratio semiconductor nanowires in an inherently safe and contained manner. This work modifies the previously used mixed powder method to produce significantly smaller features below 100nm in diameter. For the first time SiGe alloys are produced in optical fiber from a mixture of the two powders across the entire compositional range. A discussion of the properties of silicon and germanium and their alloys is given with emphasis on the differences between properties at the bulk scale and at the nanoscale. The limitations of silicon and germanium for photonic applications, due to their indirect band gap nature, is removed when these materials are reduced to the nanoscale. A brief discussion of ways that these properties can be modified is given with size, composition, and strain all being viable factors of control. The optical and electrical properties of these nanowire arrays is evaluated as a function of the size, number of wires, and composition. A clear dependence between size and quantity of wires was observed with respect to composition. The nanowires were found to have complex interactions with light showing high absorption as well as unique transmission characteristics. Arrays of these fibers were able to create a measurable photocurrent and provide potential uses for detection of light and other photonic applications. An understanding of the etching necessary to both expose these nanowires for analysis as well as completely remove them from the glass matrix was developed. Etch rates in these areas was observed to be higher than reported etch values. Etching with dilute solutions was found to allow removal of the wires cleanly and allow recovery of them for other applications.
General Audience Abstract
This research provides a method of using a mixed powder in tube approach for producing and characterizing large quantities of highly oriented, high aspect ratio semiconductor nanowires in an inherently safe and contained manner. These wires are over 1000 times smaller than thickness of a human hair are made using traditional fiber drawing methods or pulling at high temperatures. These fibers differ from traditional optical fibers in that they are produced from a tube filled with powder instead of a solid glass rod. This is similar to the same method used to produce wires in other materials such as copper. The use of the glass to contain the semiconductor material allows us to increase the temperature it is pulled at above the melting point. The liquid material is then drawn into the very small sizes using pores in the glass powder it is mixed with. This allows these wires to be produced in much longer lengths, larger quantities, and easier than previous methods. These nanowires are produced from silicon and germanium, which are two of the most important materials currently used in electronics. These semiconductors are used in most electronics, solar cells, and LEDs that are used in everyday life. Silicon and germanium while very important materials have limitations in photonic applications, interactions with light. The properties of the materials for these applications can be improved by reducing them in size to the nanoscale. The wires produced in this research were evaluated to determine if they possessed the more ideal properties. The wires were found to have detectable photocurrent, electricity generated from light. This is the primary property that is needed in solar cells. The wires produced in this method are an important early step to improving solar cells efficiency and reliability. These v wires have benefits over other forms of silicon because they are produced with protective glass coating in a single step.
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