Delivered by Ingenta to: McMaster University IP: 191.101.55.112 On: Fri, 24 Jun 2016 15:13:18 Copyright: American Scientific Publishers RESEARCH ARTICLE Copyright © 2012 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 12, 3954–3958, 2012 Formation Mechanism and Characterization of Black Silicon Surface by a Single-Step Wet-Chemical Process Li-gong Li 1 3 , Shu-man Liu 1 ∗ , Xiao-ling Ye 1 , Marius Hossu 2 , Ke Jiang 2 , Wei Chen 2 ∗ , and Zhan-guo Wang 1 1 Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China 2 Department of Physics, The University of Texas at Arlington, P.O. Box 19059, Arlington, TX 76019, USA 3 Department of Physics, Tsinghua University, Beijing 100084, China We report a simple single-step etching method for formation of black surface on silicon wafer by using HAuCl 4 -HF-H 2 O 2 etching solution, in which catalytic Au particles were deposited in situ. The black surface suppresses the reflectivity in a wide spectral region. The formation mechanism involved has been discussed. Keywords: Nanostructure, Silicon, Anodization, Electroless Deposition. 1. INTRODUCTION One of the special nanoscaled features of black silicon (Si) is that its refractive index can be made to vary gradually, which results in significantly suppressed sur- face reflection. 1 This low reflectivity is very attractive for Si-based solar cells, due to its expected increase in the efficiency of the absorption of sunlight. Therefore it is essential for terrestrial solar applications if black Si could be fabricated by cost-saving methods. In fact, nanoscaled black Si structures have been demon- strated by a simple metal-assisted wet-chemical etching technique, which might be promising for solar energy har- vesting in low-cost Si solar cells compared to the reac- tive ions etching, electrochemical etching or laser-assisted etching methods. Previous researchers have introduced nanosized noble metals (e.g., Au, Ag, Pt, etc.) as cata- lysts into etching solution containing HF, H 2 O 2 to produce black Si. 2–4 The noble metals such as Au were deposited on Si wafer before etching by evaporating thin Au lay- ers or by coating Au nanoparticles (NPs) from colloidal solution. 5–7 It has been accepted that the nanosized Au acts as a micro cathode, on which an oxidant like H 2 O 2 is reduced catalytically to produce holes, and Si beneath the Au NP acts as a micro anode to which holes injec- tion and reaction of HF with Si occur. 4 5 8 After etch- ing, un-reacted parts of the Si surface give nanocones, ∗ Authors to whom correspondence should be addressed. nanowires, or nanoholes depending on the experimental parameters. 4 On the other hand, the diffusion of Au into silicon was observed in several studies, 3 9 10 which will have bad effects on the performance of solar cells. In this work, we study the formation mechanism of black Si formed by a single-step low-cost etching method to enable full control over the synthesis. The in situ depo- sition mechanism of Au nanoparticles and the reactions involved in the etching process are discussed. The depen- dence of black Si morphology, crystalline properties and Raman resonances on etching duration is described. A sig- nificant reduction of reflection in a broad range has been observed. 2. EXPERIMENTAL DETAILS Polished single crystalline (100) Si wafers were used as substrates. In a typical synthesis, Si substrates were sequentially degreased in acetone, isopropyl alcohol, and boiled in a Piranha solution for 30 minutes. After clean- ing, Si substrates were soaked in a 5% HF solution for 5 minutes to remove the natural oxide. Then Si sub- strates with fresh surfaces were immersed in a HAuCl 4 solution, to which a same volume of etching solution of HF:H 2 O 2 :H 2 O was added. The final concentration of HF and H 2 O 2 were 1.4 mM and 3.0 M, respectively. Si surfaces were etched at different rate depending on the HAuCl 4 concentration between 0.2–2.0 mM. The optimized [AuCl 4 ] - content was 0.8 mM, by which the 3954 J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 5 1533-4880/2012/12/3954/005 doi:10.1166/jnn.2012.5853