Temporal stability of a-Si:H and a-SiNx:H on crystalline silicon wafers

The temporal stability of single layer thin films of hydrogenated amorphous silicon (a-Si:H) and silicon nitride (a-SiNx:H) passivated crystalline silicon wafers have been investigated over 18 months. The thin films were deposited at low temperature with plasma-enhanced chemical vapor deposition (PECVD) onto different substrates, including float zone (FZ) p-type and Czochralski (CZ) n-type silicon. For a-SiNx:H, we investigate the temporal stability dependence on the flow rate of silane (SiH4) used during deposition. This was varied from 13 sccm to 50 sccm. The thickness of the a-Si:H and a-SiNx:H thin films was characterized by spectroscopic ellipsometry (SE), whereas the minority carrier effective lifetime (τeff) and the uniformity of the wafer were measured through carrier density imaging (CDI). We found that for both p-type FZ and n-type CZ silicon substrates, the a-Si:H passivation performance degrades after 150 h and reached a minimum value around 350 h, whereafter the lifetime recovered to a level of 1.1 ms (-16% compare to the initial state) and 1.6 ms (-4%), respectively. Similar trends were also seen on a-SiNx:H passivated samples, but the lowest value was reached after around 550 h. For both p- and n-type substrates passivated by a-SiNx:H passivation, as the flow rate of SiH4 increased, the temporal stability of passivated samples enhanced.