AlOx passivation of ion implanted boron emitters using batch AlOx process

In the present paper we highlight the results on passivation of the ion implanted boron emitters using a batch atomic layer deposition (ALD) - AlO
_x
process. We demonstrate that through optimization of the batch ALD process we can achieve extremely low levels of surface recombination uniformly across both sides of multiple wafers, processed during a single run. The double-sided nature of the batch AlO
_x
process is of particular value when passivating symmetric samples, which constitutes an essential step of development and optimization of ion implanted boron emitters. In this work we explore the effect of various ALD process parameters (deposition temperature, pulse and purge duration) on the coverage and passivation quality of the AlO
_x
films using the stacks of symmetric samples, i.e. ion implanted B emitters and high lifetime n-type wafers. We find that passivation quality shows a strong dependence on the film coverage/thickness and is primarily driven by the trimethyl aluminum (TMA) pulse pressure. Additionally, we have investigated the effect of separation distance between the wafers and showed that we can achieve uniform coverage with separation distance as small as 700um. This parameter (aspect ratio) is critical as it can lead to precursor starvation during the batch ALD process. However, to maximize the throughput of the lab scale equipment, the distance between wafers needs to be minimized. Using the optimized ion implant conditions and batch AlO
_x
process, we achieve an excellent value for implied Voc of ~688mV and emitter saturation current density J
_0e
as low as 23fA/cm
²
for a ~100Ω/sq implanted B emitter. Furthermore, we show that excellent inversion layer passivation can be achieved for lightly doped n-type wafers, with S
_eff
<;5 cm/s using this process, which could make batch AlO
_x
process an attractive option for fabricating high efficiency implanted n-type rear contact cells.