Localized Direct Metal-Silicon Contacts through Controlled Pinhole Formation in Si/Al ₂ O ₃ /HfO ₂ Stacks

Forming highly controlled and localized direct metal-silicon contacts through dielectric layers with controlled passivation is a critical challenge in advanced semiconductor devices, ranging from optoelectronics to low-resistance carrier extraction in high efficiency silicon solar cells. However, minimizing unwanted interfacial recombination requires the direct contact area to be tightly controlled. While laser ablation is effective for opening localized contacts, it can introduce damage and process complexity. We have used a novel alternative approach to create localized direct metal-silicon contacts based on pinholes that can be engineered to occur in Al2O3/HfO2 stacks grown by plasma-enhanced atomic layer deposition followed by annealing at 450 °C. These pinholes have an areal density of order 105 cm−2 and can be intentionally expanded by tetramethylammonium hydroxide (TMAH) etching. The average pinhole diameter—measured by atomic force microscopy— was found to increase linearly from ∼0.5 μm after 5 min to ∼2.5 μm after 20 min of etching. The effective carrier lifetime at an excess carrier density of 1 × 1015 cm−3 was ∼15 ms for unetched stacks and reduced monotonically with etching time as surface recombination increases with the growth of the pinholes, reducing to ∼100 μs after 20 min. Metal contacts were formed to the samples using radio frequency sputtering. We observed a trade-off between contact quality and surface passivation level, with a 5 min etch giving a lifetime of 2 ms, thus providing a suitable compromise between metal contact area and surface recombination. By optimizing the dielectric deposition parameters and TMAH etching conditions, it is possible to engineer different levels of surface passivation and further enhancements could result in low-resistance carrier extraction for high efficiency silicon solar cells.