Phononic Amorphous Silicon: Theory, Material, and Devices
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0910-A05-06
Phononic Amorphous Silicon: Theory, Material, and Devices Samrat Chawda1, Jose Mawyin1, Harv Mahan2, Charles Fortmann3, and Gary Halada1 1 Materials Science and Engineering, SUNY at Stony Brook, Stony Brook, NY, 11794 2 National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO, 80401 3 Applied Mathematics and Statistics, SUNY at Stony Brook, Stony Brook, NY, 11794 ABSTRACT The prospect of using phonons in amorphous silicon to convey information from one location to another is investigated. It is known that the phonon lifetime in amorphous silicon is anomalously long and the phonon diffusivity is relatively large as compared to crystal silicon and other materials. A commercial Raman spectrometer measuring from the film side operating at 785 nm was used in conjuncture with a 470 nm bias light illuminating the glass side of amorphous silicon films deposited onto glass substrates. All measurements were conducted at liquid nitrogen temperature. Analysis indicates a phonon diffusion length of a least 0.5 µm. These results directly lead to tantalizing prospects for phonon engineered amorphous silicon technology. INTRODUCTION Typically the phononic properties of a given material are immutable and the phonons have such a limited diffusion length (sub-micron’s) as to be unsuitable for engineered purpose. Phonon diffusion lengths can measure centimeters in length (e.g., crystal SiO2) at temperature less than 10K but drop to sub micron length at room temperature. Amorphous silicon, owing to the incoherent scattering structures and owing to localization of at least some phonon bands, has anomalously large phonon lifetimes. For example, Scholten, et al [1] reported that the phonon associated with the anti-Stokes 525 cm-1 peak had a lifetime of at least a nanosecond. Orbach explained the long lifetime in terms of localization [2]. Elsewhere, Scholten et al. reported that heat transport studies on amorphous silicon films were consistent with a large (1 cm2/s) phonon diffusivity [3]. These lifetimes and diffusivities may be indicative of a large phonon diffusion length and/or a fast phonon hop rate from one domain to the next. These reports suggest that phonon transport is better in the incoherent amorphous silicon comprised of an aggregate of similar but relatively independent oscillators than the crystal silicon case comprised of an organized set of identical oscillators. Therefore, the question becomes whether a distribution of dissimilar oscillators each having a particular phonon cross section is less deleterious to phonon propagation than a organized system having a well defined energy? The transport of the 490 cm-1 transverse acoustic optical phonon is investigated in hot wire CVD deposited hydrogenated amorphous silicon films on glass. The prospects for a technology involving phonon transport and device design itself are considered. EXPERIMENT A Thermo Nicolet Almega® micro-Raman spectrometer with a 785 nm probe beam was used to analysis amorphous silicon deposited onto glass substrate by Hot Wire
CVD
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