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

Using high resolution x-ray diffraction, the porous silicon lattice parameter 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 that of bulk silicon. We then estimate the differential thermal expansion coefficient δα of p+-type porous silicon samples of various porosity (from 60% to 80%). Between 90 and 250 K, δα is constant as a function of the temperature but increases linearly with the sample porosity. The porous silicon thermal expansion is related to the temperature variation of the silicon nanocrystallite surface stress.