Interfacial Oxides at Metal/TCO Junctions and Ultra-Low Contact Resistivity Determination by Micro Transfer Length Measurements Based on Selective Laser Ablation

Novel silicon solar cell concepts utilize transparent conductive oxides (TCOs) as charge transport and contact layers. In particular, ZnO:Al is treated as potential replacement of indium tin oxide (ITO) currently used in high-efficiency cells. Recently, a solar cell composed of ZnO/Si as active junction was proposed and its performance was simulated. Thereby, a grid of Al on top of the ZnO was used as metallization scheme. Electronic properties of involved interfaces play a crucial role on transport losses and depend on interfacial composition on the nanometer scale. In this work, we investigate Al-metallized ZnO:Al layers on glass substrates as model system for interfacial oxide formation and related electrical transport properties. Al/ZnO:Al layer stacks on glass were deposited by magnetron sputter deposition using different O2 partial pressures in Ar atmosphere. An increase in layer resistivity of more than one order of magnitude was found from four-point-probing (4pp), when the oxygen partial pressure had risen from 0% to 6% of total process pressure. Correspondingly, the optical band gap featured a redshift and the plasma frequency decreased. Stoichiometries and chemical bindings at the Al/ZnO:Al interfaces were studied by X-ray photoelectron spectroscopy (XPS) depth profiling as well as energy-dispersive X-ray spectroscopy (EDXS) in a scanning transmission electron microscope (STEM). A 2-3 nm amorphous Al2O3 or Al(OH)3 interlayer could be proven by both techniques in conjunction. Its thickness is not dependent on the oxygen flux during ZnO:Al deposition. Selective layer removal by ultrashort laser pulse ablation enabled to fabricate TLM patterns on the micron scale (μ-TLM), which allowed to resolve contact resistivities pc of the Al/ZnO:Al interface in the 0.001 mΩcm2 range. A jump of pc from ~0.01 to 0.001 mΩcm2 was observed, when the oxygen partial pressure rose from 0% to 2%. Further increase to 6% oxygen led to only a minor increase of pc to ~0.005 mΩcm2 in agreement with the observation that almost no interfacial chemistry changes happened. Sheet resistances of the buried ZnO:Al layer within the layer stack could be measured with the μ-TLM technique with high reproducibility and in good accordance with reference measurements from single layer samples. With this, a highly adaptable methodology was established, that is possible to resolve even the lowest contact resistivities in photovoltaics as encountered in metal/TCO junctions with high precision.