X-ray diffraction investigation of the low temperature thermal expansion of porous silicon

Using high resolution x-ray diffraction, the porous silicon lattice paramet er was measured in vacuum, as a function of temperature in the range 90-300 K, showing that the thermal expansion of porous silicon is larger than tha t of bulk silicon. We then estimate the differential thermal expansion coef ficient delta alpha of p(+)-type porous silicon samples of various porosity (from 60% to 80%). Between 90 and 250 K, delta alpha is constant as a func tion of the temperature but increases linearly with the sample porosity. Th e porous silicon thermal expansion is related to the temperature variation of the silicon nanocrystallite surface stress. (C) 2000 American Institute of Physics. [S0021-8979(00)00705-2].