1. Strong scattering of light in silicon nanowires formed by metal-assisted chemical etching A. Efimova Moscow State M.V. Lomonosov University, Physics Department Nanomeeting2005, Minsk, 26—29 May, 2015 A. Tkachev A. Eliseev D. Presnov V. Timoshenko T. Nychyporuk Yu. Ryabchikov

2. Minsk, 26-29 May, 2015 Why MACE silicon nanowires?  Why MACE silicon nanowires? What for?  APPLICATION What’s different? What’s new? What’s novel? What’s peculiar? PROPERTIES LINEAR OPTICS EXTREMELY LOW or EXTREMELY LOW or HIGH REFLECTION  How comes?

3. Minsk, 26-29 May, 2015 Just a simple, reliable and cheap procedure 1. Ag nanoparticles deposition onto Si substrate in the aqueous solution of AgNO 3 2. Ag nanoparticles-assisted chemical etching of Si substrates in the aqueous HF and H 2 O 2 solution 3. Ag nanoparticles removal by rinsing in concentrated HNO 3 Silicon nanowires (SiNWs) First formed in the middle-1960s by vapour-liquid-solid (VLS) technique Metal-assisted chemical etching - MACE

4. Minsk, 26-29 May, 2015 V. Sivakov, F. Voigt, B. Hoffmann, et al. Wet - Chemically Etched Silicon Nanowire Architectures: Formation and Properties // Nanowires - Fundamental Research – 2011, p. 45-80. V. Sivakov, S. Christiansen. Novel discovery of silicon // J. Nanoelectron. Optoelectron. 2012. V.7. N6. P. 583-590. К.В. Буньков, Л.А. Головань, К.А. Гончар и др.. Зависимость эффективности комбинационного рассеяния света в ансамблях кремниевых нанонитей от длины волны возбуждения // Физика и техника полупроводников – 2013. Т.47. В.3, С. 329-333. Columnar Prismatic Bunched Pyramidal Tilted Zigzag Cotton-like Flaky Structure: Si(100) and Si(111) What are MACE SiNWs?

5. Minsk, 26-29 May, 2015... What’s peculiar? - Variety of individual SiNW shapes - Variety of SiNW lateral dimensions BUT - Dense arrays of SiNWs with comparable lateral dimension and interwire space - Free of catalyst contamination

6. Minsk, 26-29 May, 2015 Low reflection at normal incidence  Black silicon Pyramidal shape, thin layers PROPERTIES: Low reflection at normal incidence  Black silicon STRUCTURE: Pyramidal shape, thin layers, p.A286-A292 J.-Y. Jung, Zh. Guo, S.-W. Jee, et al. A strong antireflective solar cell prepared by tapering silicon nanowires // Optics Express – 2010. V.18. N103, p.A286-A292 Bunched Tapered c-Si Black silicon

7. Minsk, 26-29 May, 2015 High reflection Columnar shape, thick layers PROPERTIES: High reflection in Near-Infrared STRUCTURE: Columnar shape, thick layers A=1–T–R 15-20% increase in R Entirely diffuse c-Si substrate

8. Minsk, 26-29 May, 2015 STRUCTURE: What’s peculiar? Lateral dimensions Columnar wires 1 m1 m SEM Interwire space 1 m1 m SEM Inner channels 100 nm TEM, SEM Surface nanostructures 5–10 nm TEM 8 1 m1 m1 m1 m 1 m1 m1 m1 m What are SiNWs under study?

9. Minsk, 26-29 May, 2015 Radiative transport equation&Diffusion approximation: Applicability - a media  - Radiative transport ls generated a media  - Light diffusion is generated before absorption  - Light diffusion is generated in a restricted layer  - Scatterers are pointwise and independent  How comes? PROPERTIES: How comes?

10. Minsk, 26-29 May, 2015 Diffusion approximation: Diffuse reflectance. Statics Types of variables 1) Table:  substrate 2) Directly measured: L, R d 3) Obtained from the interference fringers n eff in the effective medium spectral range: n eff 4) Calculated: z 1,2 ( n eff,  substrate ), 5) a 5) Estimated or got from photoacoustics:  a Infinite media (solid line) and a layer of the thickness L in the air (dash line), L = 5l tr O. L. Muskens, J.G. Rivas, R.E. Algra, et al. Design of Light Scattering in Nanowire Materials for Photovoltaic Applications // Nano Letters – 2008, V.8, N.9, p. 2638-2642. How comes? PROPERTIES: How comes? Transport mean free path to be extracted

11. Minsk, 26-29 May, 2015 Transport mean free path of light. Static results Transport mean free path value meets the demands of diffusion approximation  0,  m 1.250 0.800 1000 cm 12  m L,  m 8 24 8 l tr  m

12. Minsk, 26-29 May, 2015 Diffusion approximation: Time-resolved reflection. Dynamics Cross-correlation function Cr 4+ :forsterit laser ultrashort pulses (60 fs), repetition rate 80MHz At long times

13. Minsk, 26-29 May, 2015 Transport mean free path of light: Static and dynamic results L = 24  m,  = 1,250  m Statics Dynamics

14. Minsk, 26-29 May, 2015 Conclusion Conclusion The reflection of thick silicon nanowire layers in the near-infrared range below and above the band gap is well understood by assuming the diffusion-like radiative transport in silicon nanowire array.

15. Minsk, 26-29 May, 2015 Stone Forest of Shilin Thank you for your attention