Porous silicon strain during in situ ultrahigh vacuum thermal annealing

In situ synchrotron radiation measurements of porous silicon (PS) strain have been performed during ultrahigh vacuum (UHV) thermal annealing. For a p(+) sample, the initial lattice expansion shifts toward a contraction above 270 degrees C in relation with hydrogen desorption. For a p(-) sample, the strain variation is similar to that of a p(+) one, but with effects five times larger: after hydrogen desorption, the contraction strain is large (>1.5%) and inhomogeneous. In both cases, most of these strains are elastic as an HF etch re-establishes the initial expansion with a narrow diffraction peak. For p(+) samples, the lattice constant exhibited a slow variation during subsequent exposure to air due to a slow oxidation of the annealed porous samples. The origin of these strain variations is discussed in relation with the presence of hydrogen or oxide coverage. The observation of similar variations in other PS properties is also discussed. Finally, the absence of a strain effect during the introduction of water vapor in UHV is discussed as possibly due to a contamination of the PS sample by residual water during the long time passed under UHV at high temperature. (C) 1999 American Institute of Physics. [S0021-8979(99)05510-3].