Porous silicon strain during in situ ultrahigh vacuum thermal annealing

In situ synchrotron radiation measurements of porous silicon (PS) strain ha ve 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 ti mes 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 p eak. For p(+) samples, the lattice constant exhibited a slow variation duri ng subsequent exposure to air due to a slow oxidation of the annealed porou s samples. The origin of these strain variations is discussed in relation w ith the presence of hydrogen or oxide coverage. The observation of similar variations in other PS properties is also discussed. Finally, the absence o f a strain effect during the introduction of water vapor in UHV is discusse d as possibly due to a contamination of the PS sample by residual water dur ing the long time passed under UHV at high temperature. (C) 1999 American I nstitute of Physics. [S0021-8979(99)05510-3].