Strain in porous Si with and without capping layers

The strain of porous Si formed on p+-Si wafers without and with GeSi or CoSi2 capping layers were analyzed by double crystal x-ray diffractometry, MeV 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 ∼0.3% to ∼−0.6% upon vacuum annealing at 600 °C for 30 min. That of GeSi-capped porous Si is negative (∼−0.1%) and remains unchanged upon the same annealing. The perpendicular strain of CoSi2-capped porous Si also decreases due to the pinholes in the CoSi2 layer. 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 uncapped-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 GeSi-capped porous Si. A little increase was observed for the annealed CoSi2-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.