ANALYSIS OF THE SURFACES STRUCTURE IN POROUS SI

We have studied the microscopic structure of the internal surfaces of porous Si by electron paramagnetic resonance (EPR) and XPS techniques. In particular we have determined the orientation and the passivation states of this surfaces in the as-prepared, aged and thermal annealed states. The EPR results for the as-prepared (p(+), p, p(-)) samples of 75% porosity give evidence exclusively for(lll) surfaces with misorie ntations relative to the substrate of less than similar to 1 degrees. In the as-prepared state the fraction of oxidized (111) surfaces is le ss than 1% of the total surface. Aging at ambient conditions, which in creases the PL efficiency in p(-) material, modifies the surface struc ture differently in p(+) and p(-) materials: for the p(+) layers a con tinuous oxidation is observed over a six-month period, whereas the p(- ) layers do not oxidize further. However, amorphous inclusions, the fr action of which increases to 0.6% during six months' storage, are form ed in p(-) material. Both the EPR and the XPS results show that 450 de grees C vacuum annealing strongly increases the oxidized surface fract ion: up to 6% and 50% for the p(-) and p(+) materials, respectively, b ut still showing (111) surfaces only. Contrary to the aged state, now both the p(+) and p(-) materials have disordered inclusions, the fract ion of which is up to 40% of the total silicon content. In agreement w ith the EPR results the XPS results show no evidence for surfaces othe r than (111).