Silicon surface passivation properties of aluminum oxide grown by atomic layer deposition for low temperature solar cells processes

Abstract : This thesis focuses on the passivation properties provided by thin Al2O3 films grown by atomic layer deposition (ALD) from TMA and H2O for silicon solar cells having process temperatures lower than 400 °C. The first part of this doctoral work aims at identifying the formation mechanisms of negative electrostatic charges in aluminium oxide. Thus, the effects of post-deposition illumination (namely photon flux and photon energy), as well as substrate temperature were investigated. It was found that at least 70 % of what are generally named “fixed charges” are in fact trapped charges resulting from the injection of carriers from the silicon substrate into the aluminium oxide. From this result, we studied the influence of Al2O3 deposition parameters and post-deposition treatments on charge trapping and resulting passivation performances within an Al2O3/a-SiNX:H stack on p-type c-Si. The dependence of passivation performance (and stability) on Al2O3 thickness has been highlighted. Best compromise has been found to be around 60 ALD cycles (~6 nm), providing a lifetime up to 4500 µs. The second part of this PhD deals with the degradation mechanisms of passivation. Blistering at the c-Si/Al2O3 interface is the first studied degradation mechanism. Thanks to coloured picosecond acoustic microscopy, the Al2O3/c-Si adhesion has been confirmed to be reduced by Al2O3 thickening but also by the reduction of its deposition temperature, i.e. an increase of hydrogen content. A thermal drift during ALD (TD-ALD) has been used to solve this blistering issue. Gradual increase of the substrate temperature during the growth favours the release of hydrogen from the wafer/Al2O3 interface. For 60 ALD cycles, TD-ALD increased the lifetime up 5500 µs. Finally, the weakening of the electrostatic passivation arising from the positive charges in a-SiNX:H capping layer has been underlined by finite element simulations. The a-SiNX:H properties have been experimentally tuned thanks to a design of experiment approach. New a-SiNX:H capping containing 50 % less positive fixed charges resulted in a lifetime of 8800 µs for 60 TD-ALD cycles, i.e. an outstanding surface recombination velocity of 0.8 cm.s-1.
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Fabien Lebreton. Silicon surface passivation properties of aluminum oxide grown by atomic layer deposition for low temperature solar cells processes. Material chemistry. Université Paris-Saclay, 2017. English. ⟨NNT : 2017SACLX109⟩. ⟨tel-01698990⟩

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