The Effect of Surface Roughness on Photoluminescence of Porous Silicon
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THE EFFECT OF SURFACE ROUGHNESS ON PHOTOLUMINESCENCE OF POROUS SILICON Shunsuke OGAWA*, Nobutomo UEHARA**, Masato OHMUKAI*, Yasuo TSUTSUMI* *Department of Electrical Engineering, Akashi College of Technology, Hyogo 674-8501, Japan **Graduate school of Engineering, Hokkaido University, Kita-13 Nishi-8 Kita-ku, Sapporo 060-8628, Japan ABSTRACT We studied the effect of surface roughness of Si wafers on porous silicon by means of photoluminescence (PL), Fourier transformed infrared (FTIR) absorption and Raman spectroscopy. We prepared several kinds of Si wafers with a different surface roughness, and then the anodization was performed at a same condition. PL spectra show a blue shift with the increase of surface roughness. The particle size of porous silicon nanostructure becomes the smaller with increasing surface roughness at the same time. On the other hand, FTIR absorption spectra show no difference regardless of surface roughness. The PL emission dependent on the surface roughness originates from a quantum size effect. We infer that the surface roughness causes the concentration of the current during anodization in the area where the radius of the curvature at the surface is small. INTRODUCTION Since the discovery of visible photoluminescence (PL) from porous silicon layers at room temperature, a variety of studies have been reported in order to understand its physical properties. Electroluminescense has been also observed, and light-emitting diode was realized with porous silicon [1][2]. Main features of the advance can be found in the literature [3]. However, light-emitting mechanism from porous silicon has not been explicated and has been a matter of controversy. Various explanations have been suggested: quantum confinement effect [4], Si-H [5], and siloxene [6]. We reported previously that PL spectra from porous silicon are composed of three individual peaks at 850 (peak A), 730 (peak B), and 660 (peak C) nm. We considered consistently that peaks B and C stem from Si-H and Si-H2 species, respectively. The origin of peak A was not clarified yet [7]. In this article, we report the effect of surface roughness of Si wafers on porous silicon in the viewpoint of photoluminescence, Fourier transformed infrared (FTIR) absorption and Raman spectroscopy.
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EXPERIMENTAL DETAILS All silicon wafers used in the experiment were (100) oriented boron doped p-type silicon wafers whose conductivity was in the range of 1-10 Âcm. We polished the wafers with water diluted Al2O3 abrasive to obtain various surface roughnesses. The surface roughness was measured with a surface profiler (Surfcom 590A by Tokyo Seimitsu Co. Ltd.) and denoted by the arithmetical mean deviation of the surface profiles. The range of the surface roughness was between 0.2 and 1.0 m. We then performed the anodization of the surface roughened Si wafers at the same conditions: a solution of HF (50 wt.%) : ethanol = 1 : 1, the current density of 20 mA/cm2 and the anodization time of 30 min. RESULTS AND DISSCUSSION Surface roughness of Si wafers bef
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