CHARACTERIZATION OF POROUS SILICON BY SOLID-STATE NUCLEAR-MAGNETIC-RESONANCE

Solid-state nuclear magnetic resonance (NMR) was used to characterize porous silicon (PS) surfaces. On freshly prepared samples, the range o f hydrogen content measured by H-1 NMR was equivalent to 0.5-3 monolay ers, while fluorine concentrations were below the F-19 NMR detection l imit. The H-1 nuclei were used to selectively cross-polarize (CP) Si-2 9 near the hydrogen passivation. This method was used to study the pas sivation of an as-prepared, thick (116 mu m), high surface area (893 m (2)/g), photoluminescent (700 nm) PS sample. CP followed by polarizati on inversion (CPPI) provided some spectral editing. Changes resulting from low-temperature annealing in air and an HF soak were followed by both NMR and infrared spectroscopy. The features of the Si-29 NMR spec tra are assigned as (O)(2)(Si)Si-H (-50 ppm), (O)(3)Si-H (-84 ppm), (S i)(3)Si-H (-91 ppm), (Si)(2)Si-H-2 (-102 ppm), and (O)(4)Si (-109 ppm) . These assignments are discussed in relationship to experimental meas urements and correlations of Si-29 NMR chemical shifts for other mater ials. The Si-29 NMR line widths for PS fall between those for crystall ine silicon and those for amorphous hydrogenated silicon (a-Si:H), sug gesting that disorder near the PS surface is intermediate between thes e extremes. However, comparision of the isotropic chemical shift value s shows that the bonding in the disordered regions of PS differs from that found in a-Si:H. In addition, the sharp Si-29 NMR resonance obser ved in the bulk single crystal starting material cannot be resolved in the spectra of PS. Thus, well-ordered silicon nanocrystallites in the PS are several bond lengths removed from hydrogen or comprise only a small fraction of the PS layer.