Surface effects in uniaxially stressed crystals: the internal-strain parameters of silicon and germanium revised

Abstract
All previous determinations of internal-strain parameters have been based on the measurement of the intensity of 'forbidden' X-ray reflections as a function of uniaxial stress, with the stress taken as the quotient of applied force and the cross sectional area of the crystal. The authors report the results of monitoring directly several inter-planar spacings in the regions of the surface where the X-ray diffraction actually occurs. Values 35% larger than those calculated from the mean stress were found consistently for both silicon and germanium. Together with support from a re-examination of the data on which previous values were based these investigations lead to revised values of the internal-strain parameters: for both materials A=-0.134+or-0.008. The corresponding bond-bending parameters are zeta =-4A=0.54+or-0.04. There is now good agreement with the most recent theoretical calculations.