Optical properties of Si nanowires: Dependence on substrate crystallographic orientation and light polarization
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ancisco Martín, Dietmar Leinen, Efrain Ochoa, and José R. Ramos-Barrado Lab. de Materiales y Superficies (Unidad Asociada al CSIC), Dptos. de Física Aplicada & Ingeniería Química, Universidad de Málaga, E29071 Málaga, Spain (Received 8 October 2014; accepted 10 February 2015)
Optical properties of Si nanowire (SiNW) arrays prepared on p-doped Si(111) and Si(100) substrates were studied. SiNWs were synthesized by self-assembly electroless metal deposition nanoelectrochemistry in an ionic silver HF solution through selective etching. Total reflectance (Rt) and total diffuse reflectance (Rdt) of SiNWs change drastically in comparison to polished Si. To understand these changes, diffuse reflectance (Rd) with polarized incident light was studied. For samples prepared on Si(111), the wave length integrated Rd (wIRd) shows maxima at certain angles of incidence h, regardless of the incident light polarization. For samples prepared on Si(100), wIRd increases with h and depends on incident light polarization. Also, Rd spectra show structures due to interference effects. Therefore, SiNWs prepared on Si(100) can be considered as thin films whose refractive index depends on light polarization. Moreover, Rdt of SiNWs prepared on Si(111) can be modeled as an ensemble of diffuse reflectors.
I. INTRODUCTION
Silicon nanowire (SiNW) arrays have important antireflective properties.1 One reason for this is their gradual variation of effective refractive index from air to substrate.2,3 Another one is light trapping by multiple scattering events,2,4 which increases the probability for light absorption.4 For these reasons their main application is for light enhanced absorption in Si photovoltaic solar cells.5,6 Another advantage for this application is the possibility of decoupling light absorption and charge carrier collection into orthogonal directions. This condition is satisfied with a junction in the radial direction, and the nanowire (NW) axis parallel to the incident light direction,7,8 for example, normal incidence of light when NWs are perpendicular to the substrate plane. In this way, the solar cell requirements on the minority carrier diffusion length of the absorber material are reduced significantly in comparison to planar geometry.7–10 Silicon nanowires (SiNWs) were first obtained in 1964 by Wagner and Ellis by a vapor–liquid–solid mechanism.11 Since this very first synthesis, many methods have been developed for the growth of SiNWs, such as pulsed laser deposition,12,13 gas-phase molecular beam
Contributing Editor: Winston Schoenfeld a) Address all correspondence to this author. e-mail: abadan@fing.edu.uy DOI: 10.1557/jmr.2015.47 J. Mater. Res., Vol. 30, No. 6, Mar 28, 2015
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epitaxy,14 electron beam lithography,15 and chemical vapor deposition.16,17 Among the many methods developed, self-assembly electroless metal deposition (SAEMD) nanoelectrochemistry in an ionic silver HF solution through selective etching is a simple way to prepare SiNW arrays.18–21 This method has several a
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