Porous silicon strain during in situ ultrahigh vacuum thermal annealing

Abstract

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 °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.

Stress effects in meso‐porous silicon nanostructures

“…This result is in good agreement with numerous X-ray diffraction measurements [17] detecting relative increase of the lattice parameter z a a ∆ in the aged meso-PS samples up to 2×10 -3 compared to bulk Si. Such proximity of the macroscopic and sub-nanoscale strains can be explained by an elastic nature of the inter-atomic distance increase induced by the nanoscale porosification of bulk Si and air storage of the formed porous nanostructures.…”

“…Such proximity of the macroscopic and sub-nanoscale strains can be explained by an elastic nature of the inter-atomic distance increase induced by the nanoscale porosification of bulk Si and air storage of the formed porous nanostructures. The origin of the initial strain is shown to be related to the presence of hydrogen (fresh PS) and/or oxygen (aged PS) covering the Si nanocrystallites surfaces [12,[17][18][19]. Indeed, H-terminated Si atoms situated at the nanocrystallites surfaces relax toward the formed nano-pores because (i) of the decrease of the binding energy of the surface Si atoms and (ii) of the mutual attraction between the hydrogen covered surfaces due to van der Waals forces [18].…”

Size dependence of nanostructures: Impact of bond order deficiency

“…The lattice contraction was previously ascribed to the effect of hydrostatic pressure arising from surface stress [165] and the intrinsic compressibility of the material [154,166e168]. The central lattice expansion is expected to be the consequence of surface oxidation.…”

Simulations of nanocrystals under pressure: Combining electronic enthalpy and linear-scaling density-functional theory

“…The average Si-Si bond length is found to be contracted compared to the bulk; the contraction is reduced and tends towards the bulk value as the size of the nanocrystal is increased. Looking at individual Si-Si bonds, it is found that the outer shell is contracted, which has been interpreted as due to surface stress, 63,64 61 In classical linear elasticity, inhomogeneities, whether in vacuo or embedded in a material, have size-independent elastic fields. 65 This is the result of neglecting surface energies which can be justified when the ratio of surface to volume atoms is small.…”