Strain in porous Si with and without capping layers

摘要

The strain of porous Si formed onp+hyphen;Si wafers without and with GeSi or CoSi2capping layers were analyzed by double crystal xhyphen;ray diffractometry, MeVthinsp;4He backscattering spectrometry, and Auger electron spectroscopy. The parallel strain is zero for all the samples. The perpendicular strain of uncapped porous Si is positive, and decreases from sim;0.3percnt; to sim;minus;0.6percnt; upon vacuum annealing at 600thinsp;deg;C for 30 min. That of GeSihyphen;capped porous Si is negative (sim;minus;0.1percnt;) and remains unchanged upon the same annealing. The perpendicular strain of CoSi2hyphen;capped porous Si also decreases due to the pinholes in the CoSi2layer. We propose that the decrease of strain upon thermal annealing in vacuum is a result of the desorption of physisorbed gas molecules from the native oxide of the pore walls in the porous Si layer. The strain of annealed uncappedhyphen;porous Si increases again when stored in ambient air at room temperature, with a time constant on the order of days, while no change was detected for the annealed GeSihyphen;capped porous Si. A little increase was observed for the annealed CoSi2hyphen;capped porous Si due to the existence of the pinholes. We attribute the increase of strain to adsorption of gas molecules (H2O in particular) from air.