Porous Silicon Photoluminescence Versus HF Etching: No Correlation with Surface Hydrogen Species
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POROUS SILICON PHOTOLUMINESCENCE VERSUS HF ETCHING: NO CORRELATION WITH SURFACE HYDROGEN SPECIES M.B. ROBINSON, A.C. DILLON, AND S.M. GEORGE Dept. of Chemistry and Biochemistry, Univ. of Colorado, Boulder, CO 80309 Abstract The photoluminescence and infrared absorbance of electrochemically anodized porous silicon samples were examined as a function of hydrofluoric acid (HF) etching time. Transmission FTIR spectroscopy measurements revealed that the infrared absorbance from silicon hydrogen surface species was largest for the initial porous silicon samples and immediately decreased with HF etching. In contrast, the photoluminescence did not appear until after HF etching times of 20-80 minutes, depending on initial sample porosity. Subsequently, the photoluminescence intensity increased, reached a maximum, and then progressively decreased versus HF etching time. These HF etching results demonstrate that there is no direct correlation between the photoluminescence and the silicon hydrogen surface species. Introduction Porous silicon is an intriguing material consisting of silicon nanostructures that can be prepared by electrochemically anodizing single' crystal silicon in solutions of hydrofluoric acid (HF) (1,2). However, interest in porous silicon has been limited since its discovery in 1956. Since 1990, though, when Canham revealed that under certain conditions porous silicon can become photoluminescent (3), efforts to understand the material and its potential applications has exploded. This photoluminescence (PL) was initially attributed to quantum confinement in the silicon nanostructures (3), but various research groups have proposed mechanisms based on quantum confinement (3-6), the presence of an amorphous silicon (7) or surface species such as hydrogen or polysilanes (8-11) and siloxene (12,13). Porous silicon has extremely high surface areas of - 200 m 2 /cm 3 (14,15) and its surface is terminated with only hydrogen immediately after preparation (14). H2 desorption from the SiH and SiH2 surface species on porous silicon has been studied by transmission Fourier Transform Infrared (FTIR) Spectroscopy (14). Following hydrogen removal, the porous silicon surface is very reactive to a variety of semiconductor processing gases (16-18). The hydrogen-passivated porous silicon surface is also susceptible to chemical contamination and instability that has led to confusion regarding the surface species that may influence or dictate photoluminescence. For example, the siloxene mechanism (12,13) should never have been seriously considered because the freshly prepared porous silicon surface contains no oxygen. Thermal annealing of photoluminescent porous silicon leads to a degradation of the PL at temperatures that are close to the temperatures for H2 desorption from the SiII2 surface species. However, contrary to earlier thermal annealing results (8-10), the photoluminescence is not directly dependent on the SiH2 surface species (19). Recent thermal annealing studies have demonstrated that the PL decreases at temperature
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