Deposition, Recrystallization, and Epitaxy of Silicon, Germanium, and GaAs on Fibers and Metal Wires for Optoelectronic
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Deposition, Recrystallization, and Epitaxy of Silicon, Germanium, and GaAs on Fibers and Metal Wires for Optoelectronic Device Applications Michael G. Mauk*, Bryan W. Feyock, Jeremy R. Balliet, and Todd R. Ruffins AstroPower, Inc. Solar Park, Newark, Delaware USA 19716-2000 *author for correspondence: mauk @astropower.com ABSTRACT Semiconductor p-n junctions formed in a cylindrical geometry as concentric cladding layers surrounding a wire or fiber ‘substrate’ could have significant advantages for optoelectronic devices such as LEDs and solar cells, especially with regard to optical coupling efficiency and high-throughput manufacturing. Fiber-based semiconductor device components may also prove useful in conformable electronics or electrotextiles, and for giant-area flexible circuits. We describe techniques and results for chemical vapor deposition and melt coating to form 2- to 50-micron thick cladding layers of silicon or germanium on various types of fibers and refractory metals. These Ge or Si cladding layers can be recrystallized to achieve large (several millimeters or greater) grains oriented along the axis of the fiber. Additional GaAs cladding layers are grown on the recrystallized Ge or Si by vapor-phase epitaxy or metallic solution growth. p-n junctions are formed by diffusion or epitaxy. Light-sensitive diodes have been fabricated in these structures. INTRODUCTION Fiber-shaped semiconductor devices are of general interest for an emerging technology area that has been broadly termed electronic textiles. Applications include wearable, drapable or conformal electronics, solar cells, and solid-state lighting and displays, as well as for electrodes in electrochemical systems. Most work in this area appears to focus on new materials or on unconventional applications of polymers and other materials primarily developed for nonelectronics applications. On a different tack, we are looking for ways to make conventional semiconductor materials, such as silicon, germanium, or GaAs, suitable for such applications. To this end, we are developing device structures in a cylindrical geometry comprised of several cladding layers of semiconductor material formed on conducting wires or fibers. This approach represents a considerable departure from the conventional semiconductor technology based on planar processing of semiconductor wafers, and our initial aim is to assess its feasibility, limitations, and options. We first introduce several new device concepts and discuss the potential advantages and practical difficulties of this approach. Some previous work by others on cylindrical-shaped solar cells, although using different materials technologies than described here, has been reported over the last two decades [1-3]. We next describe specific implementations based on coating carbon or silicon carbide fibers, or metal wires (such as tungsten), with cladding layers of silicon or germanium. For this, we use a chemical vapor transport process with iodine as the transport agent to deposit the silicon or germanium layer
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