Kim and Cho Nanoscale Research Letters 2012, 7:408 http://www.nanoscalereslett.com/content/7/1/408 N A N O I D E A Morphological and nanostructural features of porous silicon prepared by electrochemical etching Hyohan Kim and Namhee Cho* Open Access Abstract Porous layers were produced on a p-type (100) Si wafer by electrochemical anodic etching. The morphological, nanostructural and optical features of the porous Si were investigated as functions of the etching conditions. As the wafer resistivity was increased from 0.005 to 15 Ω cm, the etched region exhibited ‘sponge’, ‘mountain’ and ‘column’-type morphologies. Among them, the sponge-type structured sample showed the largest surface area per unit volume. Silicon nanocrystallites, 2.0 to 5.3 nm in size, were confirmed in the porous layers. The photoluminescence peaks varied in the wavelength range of 615 to 722 nm. These changes in the maximum peak position were related to the size distribution of the Si crystallites in the porous silicon. The doping levels of the wafers significantly affect the size distribution of the Si crystallites as well as the light-emitting behavior of the etched Si, which contains nanoscale Si crystallites. Keywords: Electrochemical etching, Nanocrystal, Photoluminescence, Porous silicon. Background Silicon has not been considered a proper material for op- tical device applications owing to its indirect band struc- ture. On the other hand, as luminescence behavior has been observed from chemically etched porous silicon (PS) at room temperature, many studies have examined the origin of the light emitting properties of such indirect band structured semiconductors [1,2]. In addition, PS is considered a strong candidate material for applications in the optoelectronic industries. Moreover, PS can also be used for a variety of sensing systems, such as detecting gasses, pressures and biomedical molecules, as well as anode materials of Li rechargeable batteries owing to its larger surface [3-7]. When Si crystallites embedded in PS become less than approximately 5 nm in size, the lumi- nescence of PS occurs due to the quantum confinement effect [1,2]. Some researchers attribute this to the pres- ence of surface/interface defects [8,9]. * Correspondence: nhcho@inha.ac.kr Department of Materials Science and Engineering, Inha University, 253 Yonghyundong, Incheon 402-751, Republic of Korea Presentation of the hypothesis Many fabrication methods have been assessed to pro- duce PS [10]. In particular, electrochemical anodic etch- ing has been the main method for producing PS because electrical variables can be added to the process condi- tions to obtain better control of the physical and chem- ical features of the PS. This method has benefits of being simple and economical to produce PS compared to vacuum system-based thin film fabrication facilities, such as e-beam lithography [10]. Some researchers reported the structural variations of PS electrochemically etched under a range of experi- mental conditions [11-16]. Zhang reported the variation of the morphologies of PS with the relationship between the Si wafers and electrolytes [13]. Depending on the electrical resistivity of Si wafers compared to that of the electrolyte, the formation of pores, such as macro- or micro-pores, can be initiated at a critical current density [17]. The critical current density is changed when the composition and physical characteristics of the electro- lytes are varied. The variation in the morphological features has been investigated with the formation of space charge regions (SCR) in PS [13,16]. The SCR width is varied with © 2012 Kim and Cho; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.