Kim and Cho Nanoscale Research Letters 2012, 7:408 http://www.nanoscalereslett.com/content/7/1/408 experimental conditions such as doping levels, applied biases and current densities. The charge carrier transfer during etching reactions occurs differently depending on the doping levels. In addition to the morphological fea- tures, the nanostructural variation with the electrical conditions needs to be examined to understand the op- tical and electrical properties. The electrolytes consist of deionized water (DIW), HF and solvents such as ethanol and glycerin [17-20]. The solvent must be completely mixed in the deionized water. Methanol (CH4O) ethanol (C2H6O) and isopropyl alcohol (C3H8O, IPA) have been frequently used for solvent materials. Especially, IPA has the highest viscos- ity (2.4 cP) at room temperature and the lowest surface tension (20.8 dyn/cm) among them [21]. So, IPA is con- sidered a proper solvent material to help form a homogeneous-etched surface. This study investigated the variation of morphological features under a range of etching conditions, such as wafer resistivity and electrolytes. The nanostructural and optical characteristics were examined along with the morphological properties and related to the etching con- ditions such as doping levels and electrolyte solvents. Testing the hypothesis Experimental Porous silicon samples were prepared by electrochemical etching p-type (100) silicon wafers with a resistivity of 0.005, 1, 4 and 15 Ω cm. These etched samples are re- ferred to in this manuscript as samples 1, 2, 3 and 4, re- spectively. The electrolyte consisted of HF, IPA and DIW. To examine the wettability of the electrolyte to Si wafers, the IPA volume fractions of 0 %, 5 %, 33 %, 50 % and 66 % were applied to the etchant. Electrochemical Page 2 of 8 etching was carried out at 50 mA for 30 min. The elec- trolyte in a Teflon beaker was heated in a temperature- controlled hot plate; the electrolyte temperature was fixed to 40 °C. Platinum, as the contact electrodes, was pasted on the back side to improve the uniformity of the etching current. During the etching process, the electro- lytes were agitated to obtain a homogeneous etchant condition near the wafer surface. The etched samples were dried by blowing N2 gas (99.999 %) after electro- chemical anodic etching. The optical features were analyzed by photolumines- cence (PL; SPEX 1403 He-Cd Laser 325 nm, Horiba Ltd., Tokyo, Japan). The morphology of the etched sur- face and microstructural features of the PS wafer were examined by scanning electron microscopy (SEM; S- 4300, Hitachi, Ltd., Tokyo, Japan) and polarizing micros- copy (1161, Nikon Co., Tokyo, Japan). Contact angle analysis (PHX-300, SEO, UT, USA) was performed to examine the wettability of the electrolytes to Si wafers. The nanostructural features were analyzed by X-ray dif- fraction (XRD, POSCO Beamline 8C1 at Pohang Light Source, Kyungbuk, Korea), Raman spectroscopy (T.64000, Jobin Yvon, NJ, USA), and field emission transmission electron microscopy (JEOL, JEM-2100 F). Morphological features Figure 1 shows optical microscopy images of the etched wafers. The etching reaction occurred heterogeneously when the electrolyte did not contain a solvent (IPA). In Figure 1a, a non-etched region was observed on the sur- face of PS, indicating that some region was not etched or less etched compared to the other regions. In Figure 1a, region N appears to be in poor contact with the electrolytes, and as a result, etching did not occur in Figure 1 Polarizing microscopic images of etched wafers. Micrographs (a) and (b) were obtained from the samples etched in the etchant with IPA ratios of 0 % and 50 %, respectively. In micrograph (a), region E is a well-etched region, whereas etching reaction rarely occurred in region N. An enlarged image of the marked region is illustrated in the inset.