Metallisation-associated degradation of silicon coated with thin dielectric layers

Metallisation of semiconductor and dielectric-coated semiconductor surfaces is a key step in device production, such as in the fabrication of photovoltaic solar cells. Metal deposition can, however, be detrimental to the dielectric, interface and underlying substrate quality. This work studies damage created by thermal and electron-beam evaporation of metals on silicon passivated with thin dielectric layers. Metals are deposited on one side of silicon samples coated with aluminium oxide grown by atomic layer deposition with different thicknesses (∼1–25 nm). Samples are characterised from the other side using photoluminescence (PL) imaging with the radiative PL signal being highly sensitive to charge carrier density and thereby effective charge carrier lifetime. We find that thermal evaporation of aluminium can degrade the PL signal when the aluminium oxide films are 5 nm or thinner. In some cases, annealing at 400 °C in forming gas provides a partial repair of the damage. By performing superacid re-passivation experiments on previously metallised samples, we find that the damage is shallower than ∼6 µm from the surface. Electron-beam evaporation of metals, including aluminium, reduces the effective lifetime even in samples with ∼25 nm thick Al2O3 passivation layers. With electron-beam evaporation, annealing in forming gas at 400 °C only repairs the damage when the Al2O3 films are ⩾20 nm thick, with the damage in thinner films shown to be >3 µm deep into the silicon. This work shows that the choice of process used for metal contact deposition can affect the properties of the sample structure under investigation.