Nanoscale hafnium oxide passivation for photovoltaic and electronic devices

This thesis presents a collection of work investigating the application of hafnium oxide (HfO2) thin films as both wet-chemical and surface-state passivation layers for photovoltaic (PV) and electronic devices. Surface-state passivation layers are an essential aspect of silicon (Si) based PV devices, preventing carrier recombination that would otherwise occur for direct metal-Si contacts; thus increasing conversion efficiency. Such layers – often dielectric thin films – can either be used as ‘passivation layers’ whereby the films are regionally etched to allow direct metal-Si contact, as in the case of Passivated Emitter and Rear Contact (PERC) cells, or ‘passivation interlayers’ where the dielectric is kept in place between the metal and Si in a passivating contact structure, with the Tunnel Oxide Passivated Contact (TOPCon) cell. HfO2 is shown to have excellent potential as a passivation layer, producing surface recombination velocities (SRVs) <1 cm/s with just 2.5 nm of material, which is competitive with current commercially used dielectric passivation layers. However, the conflicting annealing temperature dependence of passivation and conductance suggests that the use of HfO2 as a passivating interlayer is limited. A novel form of passivating contact structure is introduced, that utilises dielectric stacks with HfO2, which could provide a low-temperature alternative to the current silicon dioxide (SiOx)/poly-Si passivating contact structure used in TOPCon. This novel structure relies upon the unique etching characteristics of HfO2 when combined with an aluminium oxide (Al2O3) layer, which results in pinhole formation. HfO2 is found to be highly resistant to HF etching when crystallised, but etches rapidly whilst in an amorphous state. This varying etch resistance could potentially provide a route to wafer patterning, through a combination of laser annealing and a simple HF dip. This technique is more easily scaled-up for industrial application than existing photolithography processes. This strong etch resistance suggests great potential for HfO2 thin films to be utilised as protective barrier layers in complex device fabrication for both electronic and photovoltaic applications

Activation of Al2O3 surface passivation of silicon : separating bulk and surface effects

Understanding surface passivation arising from aluminium oxide (Al2O3) films is of significant relevance for silicon-based solar cells and devices that require negligible surface recombination. This study aims to understand the competing bulk and surface lifetime effects which occur during the activation of atomic layer deposited Al2O3. We demonstrate that maximum passivation is achieved on n- and p-type silicon with activation at ∼ 450 °C, irrespective of annealing ambient. Upon stripping the Al2O3 films and re-passivating the surface using a superacid-based technique, we find the bulk lifetime of float-zone and Czochralski silicon wafers degrade at annealing temperatures > 450 °C. By accounting for this bulk lifetime degradation, we demonstrate that the chemical passivation component associated with Al2O3 remains stable at activation temperatures of 450─500 °C, achieving an SRV of 300 °C, the interface becomes Si/SixAlyO2/Al2O3 due to diffusion of aluminium into the thin silicon oxide layer