Fine Structure of Visible Light Emitting Porous Silicon
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FINE STRUCTURE OF VISIBLE LIGHT EMITTING POROUS SILICON
KIYOKAZU NAKAGAWA, AKIO NISHIDA AND TOSHIKAZU SHIMADA Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185
ABSTRACT Porous Si formed by anodization is studied by means of high-resolution scanning electron microscopy, optical microscopy, micro-photoluminescence spectroscopy, highresolution transmisson electron microscopy, and micro-Raman spectroscopy. Surface morphologies of porous Si films are varied by changing the anodization conditions. Highresolution scanning electron microscopy measurements show that in all these films, porous Si fine structures of visible photoluminescent regions, which are confirmed with optical microscopy and micro-photoluminescence spectroscopy, are composed of particlelike structure of various sizes from several nm to several tens of nm. Transmission electron microscopy shows that these particles are composed of single-crystal Si spheres covered with an oxide layer. The peak of the Raman spectrum from the particlelike structure is shifted to lower energy and is wider than that for bulk Si, possibly due to lattice expansion around large surface fractions. These results suggest that the effective photoluminescence of porous Si is due to threedimensional carrier confinement in quantum boxes.
INTRODUCTION There have been various reports concerning the effective photoluminescence of porous Si at room temperature. Some of them have stated that porous Si has a wirelike structure and that the photoluminescence is due to two-dimensional carrier confinement[1-6]. Others have stated that chemical reactants, such as siloxane, are produced during porous Si formation and the effective photoluminescence is due to these reactants[7]. To understand the mechanism of this phenomenon, it is first necessary to determine the real microstructure of porous Si that emits visible light.
Since porous Si is fragile and chemically reactive, we have therefore performed scanning electron microscope (SEM) observation on cleaved surfaces of photoluminescent porous Si regions, which are confirmed with optical microscopy and micro-photoluminescence spectroscopy. In addition, micro-Raman scattering measurements of photoluminescent porous Si region are also conducted simultaneously with microphotoluminescence spectroscopy. For the TEM sample preparation, the anodization mode was transformed into an etching mode by increasing the current density and only a thin porous Si film was removed from the Si substrate without causing structural damage.
Mat. Res. Soc. Symp. Proc. Vol. 283. 01993 Materials Research Society
186
EXPERIMENTAL The Si wafers used here were p-type (about 30 W cm) single crystals with (100) mirror surfaces. Aluminum was deposited on the back of the wafers to form electrodes, and the wafers were anodized in a mixture of HF and 1H20. A platinum plate was used as the cathode, and current densities were between 5 and 100 mA/cm 2 . The macrostructures of the porous Si on cleaved surfaces were examined with an optical microscope. To determine
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