Impurity Gettering by Atomic‐Layer‐Deposited Aluminium Oxide Films on Silicon at Contact Firing Temperatures

Abstract

Aluminium oxide (Al 2 O 3 ) thin films deposited on silicon surfaces, synthesised by plasma-assisted atomic layer deposition, are recently reported to possess impurity gettering effects for the silicon wafer bulk during annealing at 425 C, a typical temperature used for activating the surface passivation quality of the Al 2 O 3 films. This paper investigates the gettering effects of Al 2 O 3 films at higher temperatures of 700-900 C, which are commonly used for contact firing in silicon solar cell fabrication. Iron is used as a marker impurity in silicon to study the gettering effectiveness. Results show that Al 2 O 3 films also generate strong impurity gettering effects at 700-900 C, through a segregation gettering mechanism. The as-deposited Al 2 O 3 films are found to be more effective at gettering than the 425 C-activated Al 2 O 3 films, demonstrating gettering processes that are largely limited by the impurity diffusivity in silicon. For both as-deposited and activated Al 2 O 3 films, gettering during high temperature annealing occurs by impurity accumulation at the Al 2 O 3 /Si interfaces, similar to the gettering action at 425 C. However, some iron is found to redistribute into the bulk of the Al 2 O 3 films after long annealing at a high temperature.

Direct Observation of the Impurity Gettering Layers in Polysilicon-Based Passivating Contacts for Silicon Solar Cells

“…The samples were then coated with intrinsic amorphous silicon (a-Si) films from plasma-enhanced chemical vapor deposition (PECVD), at an on-sample temperature of ∼250 °C. The samples were held at ∼250 °C for a total of 1 h. Deposited dielectric films such as PECVD silicon nitride and atomic-layer-deposited (ALD) aluminum oxide have been reported to cause impurity gettering effects at elevated temperatures where the metals are sufficiently mobile. − It is presently unknown whether a-Si films possess similar gettering effects or not. Nevertheless, the moderate diffusivity of Fe i in silicon at 250 °C limits any possible surface gettering effects to less than 20% during the PECVD deposition process, which is rather small compared to the amount of Fe i being gettered during the polysilicon passivating contact formation (>99.9%) .…”

Impurity Gettering by Silicon Nitride Films: Kinetics, Mechanisms, and Simulation

“…It is well known that LPCVD SiN x is more stoichiometric and less defective than PECVD SiN x . Therefore, the density of gettering sites may have a correlation with the defect density associated with the SiN x films, similar to the gettering effects of atomic-layer-deposited aluminium oxide films …”

“…42 Therefore, the density of gettering sites may have a correlation with the defect density associated with the SiN x films, similar to the gettering effects of atomic-layerdeposited aluminium oxide films. 43…”

The Role of Charge and Recombination‐Enhanced Defect Reaction Effects in the Dissociation of FeB Pairs in p‐Type Silicon under Carrier Injection

“…It was found previously that the deposition or the activation of the AlO x films does not affect [Fe i ] significantly. [ 27 ] The measurements described earlier were then conducted again. The AlO x passivation allowed reliable lifetime data to be obtained from all the samples.…”