Figure 4.

Time-dependent voltage of the piezoelectric transducer. Structural parameters of the porous silicon layers are (a) l_por = 50 μm and ε = 65%; (b) l_por = 30 μm and ε = 65%; (c) l_por = 50 μm and porosity ε = 60%; and (d) l_por = 30 μm and ε = 60%