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Long Minority Carrier Diffusion Lengths in Bridged Silicon Nanowires Dong Yu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl503870u • Publication Date (Web): 26 Dec 2014 Downloaded from http://pubs.acs.org on December 28, 2014
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Long Minority Carrier Diffusion Lengths in Bridged Silicon Nanowires
Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:
Nano Letters nl-2014-03870u.R2 Communication 16-Dec-2014 Triplett, Mark; UC Davis, Physics Yang, Yiming; UC Davis, Physics Leonard, Francois; Sandia National Laboratories, Talin, A. Alec; SNL, Materials Physics Islam, M. Saif; UC Davis, Electrical and Computer Engineering Yu, Dong; UC Davis, Physics
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Nano Letters
Long Minority Carrier Diffusion Lengths in Bridged Silicon Nanowires M. Triplett,†, ‡, ǁ Y. Yang,† F. Léonard,ǁ A. Alec Talin,ǁ M. Saif Islam,‡ and D. Yu*,† †
Department of Physics, University of California, Davis, CA 95616, USA
‡
Department of Electrical and Computer Engineering and Center for Nano and Micro
Manufacturing, University of California, Davis, CA 95616, USA ‖
Sandia National Laboratories, Livermore, CA 94551, USA
Abstract Nanowires have large surface areas which create new challenges for their optoelectronic applications. Lithographic processes involved in device fabrication and substrate interfaces can lead to surface defects and substantially reduce charge carrier lifetimes and diffusion lengths. Here, we show that using a bridging method to suspend pristine nanowires allows for circumventing detrimental fabrication steps and interfacial effects associated with planar device architectures. We report electron diffusion lengths up to 2.7 µm in bridged silicon nanowire devices, much longer than previously reported values for silicon nanowires with a diameter of 100 nm. Strikingly, electron diffusion lengths are reduced to only 45 nm in planar devices incorporating nanowires grown under the same conditions. The highly scalable silicon nano-bridge devices with the demonstrated long diffusion lengths may find exciting applications in photovoltaics, sensing and photodetectors.
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Keywords: Nanowire, minority carrier diffusion length, surface effect, scanning photocurrent microscopy.
NWs are promising building blocks for optoelectronic applications such as solar cells and photodetectors.1-4 Planar devices with single nanowires (NWs) lying on substrates and contacted by top metal electrodes are often used to characterize the optoelectronic properties of the NWs.5-7 However, because of the large surface to volume ratios of the NWs, the device properties can be affected by fabrication procedures and/or substrate effects.8,
9
As one of the most promising
techniques to assemble NWs into realistic device architectures, the bridge method combines aspects from both top-down and bottom-up paradigms and enables mass-production of nanodevices in a CMOS compatible and inexpensive fashion.10-13 In addition, the bridge architecture provides an excellent platform for investigating charge transport and recombination in NWs, as this technique produces pristine NWs free of harsh microfabrication processes and isolated from substrates.14-16 Minority carrier diffusion lengths (LD) are an important figure of merit for optoelectronics. To date, the highest LD value achieved in silicon (Si) NWs is an electron diffusion length of 10 µm in 2 µm-diameter Si NWs grown by the vapor-liquid-solid (VLS) method.9 On the other hand, thinner Si NWs usually have much shorter diffusion lengths, presumably due to surface recombination. For example, hole diffusion lengths from 25 to 80 nm for Si NWs 30 to 100 nm in diameter, respectively, have been reported.17 Using a cold wall growth method, Mohite et. al. were able to improve LD up to 1.0 µm in Si NWs with a diameter of
