Piezoelectric properties of porous silicon

The theoretical analysis and direct experiments with samples based on $n$- and $p$-type porous silicon demonstrate that porous silicon becomes a piezoelectric because of the reduction of the symmetry of the crystal lattice. It is shown that both $n$ and $p$ types of porous silicon have piezoelectric properties. The piezoelectric properties of $n$-type porous silicon are $2.5$ times weaker than the piezoelectric properties of $p$-type porous silicon at the same porosity because pores in $p$-type silicon become wider and smoother with an increase in the distance from the surface, whereas pores in $n$-type porous silicon become narrower and more sinuous.