ATMOSPHERIC IMPREGNATION OF POROUS SILICON AT ROOM-TEMPERATURE

Microporous and mesoporous Si layers contain a very large surface area that affects both their optical and electrical properties. Secondary ion mass spectroscopy (SIMS) analysis is used for the first time to simultaneously monitor all the major impurities on that surface. SIMS data on a microporous layer demonstrate that its chemical composition changes dramatically with time during ambient air exposure. Similar trends are observed for mesoporous layers. Extended storage in air at room temperature converts the hydride surface of freshly anodized layers to that of a contaminated native oxide. Characterization techniques need to take the metastability of the hydride surface into account since the structural, optical, and electrical properties of porous Si can consequently change with time upon exposure to ambient air. Low-temperature photoluminescence and spectroscopic ellipsometry data on freshly anodized and "aged" microporous and mesoporous layers are chosen to illustrate typical changes in optical properties and the timescales involved. Spreading resistance analysis is also shown for the first time to provide depth information on the resistivities of porous layers and their variation with extended exposure to air. Implications for other characterization techniques are briefly discussed.