Composition and Structure of Spark Eroded Porous Silicon
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COMPOSITION AND STRUCTURE OF SPARK ERODED POROUS SILICON
R.E. Hummel, S.-S. Chang, M. Ludwig, and A. Morrone Department of Materials Science and Engineering, University of Florida, Gainesville, FL
ABSTRACT Porous silicon which has been prepared by a "dry" technique, that is, by spark erosion, yields similar photoluminescence spectra as anodically etched porous silicon which has been prepared in aqueous solutions. Fourier transform infrared spectra reveal that the dominant features in spark eroded silicon are the Si-O-Si stretching mode and the Si-O-Si bending mode. No infrared vibrational modes characteristic for siloxene have been found in spark eroded Si. Results from X-ray and electron diffraction studies suggest that spark eroded Si involves minute polycrystalline silicon particles which are imbedded in an amorphous matrix. INTRODUCTION In a recent publication [1], we demonstrated for the first time that "dry" spark erosion of n- or p-type single crystalline silicon in a nitrogen atmosphere or in air yields similar photoluminescence (PL) spectra as obtained by "wet" techniques. The latter methods utilize anodic etching in ethanol and HF or unbiased etching in solutions of various oxidizing reagents in aqueous HF. The wavelengths of the PL peaks of spark eroded Si were found to be essentially identical compared to those generally obtained by anodic etching. Spark eroded silicon which is illuminated by "black light," i.e., UV radiation, emits visible red light. We have further shown that silicon dioxide (quartz glass slides), untreated silicon (commercial Si wafers), and amorphous silicon (Si-ion implanted Si-wafers) do not yield PL peak intensities which are even remotely comparable in strength to those obtained by spark eroded Si. We therefore concluded that SiO 2 , amorphous Si, or pure Si are not the major causes for PL in spark eroded Si. It has been discussed in the literature whether or not siloxene (Si6 0 3H6) may be responsible for the room temperature photoluminescence in anodically etched silicon [2]. Indeed, it has been shown that siloxene, which has been produced by chemical synthesis (that is, by reacting CaSi 2 powder with HCI) displays quite comparable PL spectra as anodically etched Si [3]. Further, infrared (IR) absorption spectra of anodically oxidized (porous) silicon and "Kautsky siloxene" have been found to yield essentially identical characteristic IR peaks [3]. The present paper presents some new experimental results on spark eroded silicon. In particular IR spectra, X-ray diffraction and electron diffraction data will be given in order to clarify whether or not silotene is the predominant cause for PL in spark eroded silicon. EXPERIMENTAL PROCEDURE Sparks produced by a high voltage high frequency Tesla transformer were generated between two identical p- or n-type silicon wafers (about 5x10 mm2 in size) which were positioned a few mm apart. One of the wafers suffered erosion about 20-30 Am in depth whereas the counter wafer showed an accumulation of silicon in the form of an extended hil
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