Kim and Cho Nanoscale Research Letters 2012, 7:408 http://www.nanoscalereslett.com/content/7/1/408 Page 7 of 8 Figure 8 PL spectra of the porous silicon. Spectra (1), (2), (3) and (4) were obtained from samples 1, 2, 3 and 4, respectively. the distance from the atomic defects/dopant sites. Under fixed electric and chemical etching conditions, a higher electric current density is to be applied at lattice defects, such as dopant sites, and an etching reaction is expected to start at these sites. The change in electric field strength with the distance from the defects produces a different electric flux, resulting in a particular crystallite size distribution. The intensity of photoluminescence from sample 1 was lower than those of the samples with ‘mountain’- or column-type surface morphologies. In particular, etching occurred simultaneously along the horizontal and verti- cal directions on the samples with mountain-type sur- face morphologies. Nanocrystallites contributing to the photoluminescence were located over the etched surface of sample 3 [30]. In other words, nanocrystallites were spread widely over the etched area and easily contribute to the photoluminescence of sample 2 or 3. In contrast, sample 1 had a wide size distribution, and most were present considerably deep from the surface of the porous layers. As a result, the PL intensity is relatively lower and spreads into the IR region. Similarly, in sample 4 with a column-like surface morphology, it is evident that vertical etching occurred only after the critical etching depth had been reached, and as a result, most of the nanocrystallites were located at the bottom of the etched column holes. The PS sam- ples with a column-type surface morphology in this ex- periment showed a 10-μm-etched depth with most nanocrystallites present at the lower inside of the pores. Owing to such a geometrical reason, the intensity of PL light generated at the lower part of the holes, i.e., nanocrystalline regions, might be reduced through absorption before exiting the top surface of the PS samples. Consequently, the PL intensity of sample 3 with a mountain-type structure was higher than that of the samples with sponge and column- type structures. Implication of the hypothesis The change in the morphological, nanostructural and optical characteristics of PS prepared by the electro- chemical anodic etching of p-type (100) Si wafers were investigated as a function of the etching conditions. The wettability of electrolytes was increased with the addition of IPA in the etchant. Therefore, the etchant appears to be in good contact with the wafer surface. As a result, homogeneously etched morphologies could be achieved over the entire surface of PS. When the Si wafer resistivity was varied, etched sur- faces exhibited different types of morphologies, includ- ing sponge, mountain and column-type morphologies. The sponge-type structured sample exhibited the largest surface area per unit volume. Silicon nanocrystallites with sizes ranging from 5.3 to 2.0 nm were confirmed in the porous layers. The PL peaks varied in the wavelength range of 615 to 722 nm. The changes in the maximum peak position as well as the FWHM of the spectra were related to the nanostructural features of PS. In particular, the size dis- tribution of Si nanocrystallites and the PL intensity were discussed in terms of the doping levels of the Si wafers. The doping concentration of the wafers significantly affects the size distribution of Si crystallites as well as the light-emitting behavior of the etched Si, which con- tains nanoscale Si crystallites. Competing interests The authors declare that they have no competing interests. Acknowledgment This work was supported by Inha University Grant. Authors’ contributions NC conceived of the study and participated in the design of the experimental procedures. HK carried out the etching experiment and drafted the manuscript. All authors read and approved the final manuscript. Authors’ information HK is a PhD candidate under Professor NC in the Department of Materials Engineering, Inha University, Republic of Korea. Received: 30 April 2012 Accepted: 20 July 2012 Published: 20 July 2012 References 1. Cullis AG, Canham LT: Visible light emission due to quantum size effects in highly porous crystalline silicon. Nature 1991, 353:335–338. 2. Cullis AG, Canham LT, Calcott PDJ: The structural and luminescence properties of porous silicon. J Appl Phys 1997, 82:909–965.