Effects of UV ns laser annealing on the electrical and microstructural properties of indium-based transparent conductive oxides

This paper investigates layer-selective laser annealing of thin transparent conductive oxides (TCO) for heat-sensitive substrates. UV nanosecond laser pulses are shown to be selectively absorbed in the layer, permanently altering microstructural and electrical properties of two different TCO thin films with minimal impact on the underlying solar cell structure. With increasing laser pulse fluence and repetitions the sheet resistance Rsh is continuously reduced for both TCOs. Hall- and X-ray diffraction (XRD) measurements reveal different annealing mechanisms, depending on the as-deposited properties of the films. An amorphous fluorine-doped indium oxide exhibits a dominating increase in carrier mobility due to induced crystallization. In contrast, for a polycrystalline tin-doped indium oxide, the reduced Rsh originates from an increased carrier concentration due to the creation of oxygen vacancies. In this case, laser annealing reduces Rsh to 90 ± 20 Ω/sq, thereby outperforming conventional thermal treatments (Rsh of 167 ± 8 Ω/sq). This advantage may be related to the short laser processing time, which could minimize depassivation of grain boundaries or defect formation in the TCOs. XRD and scanning electron microscopy analysis indicate structural and possibly stoichiometric gradients in the TCO thin films, suggesting surface-near modification in the range of several nanometers.