Understanding the evolution of the pop-out effect in Si-based structures for photovoltaics

The most interesting phenomenon observed during nanoindentation of Si is the strain burst which occurs during unloading. This feature is referred to as a “pop-out” effect, and it is linked to the phase transformation that occurs underneath the indenter at high stresses. One of the peculiarities of this effect is the observed linear dependence between the depth at which the “pop-out” effect occurs and the maximum penetration depth. By performing a systematic study on the doped Si as well as on photovoltaic structures such as ITO/Si and SnO₂/Si, it is shown that this linearity holds for different unloading rates, which strongly affects the “pop-out” appearance and contact pressure. Furthermore, it is illustrated that that phase transformation in the doped Si is delayed when it is coated with a thin film due to the tension preservation in the imprint under the film coating. This observation suggests that the mechanical properties at the interface between thin films and Si substrates in coated systems (MEMs and photovoltaics) should be further investigated.