Coordination flexibility as key factor for variability of local structure in amorphous Zn–Sn–O and Cu–Sn–O thin films

Earth-abundant amorphous oxide semiconductors like Zn–Sn–O and Cu–Sn–O feature electricalconductivities that can be tuned over several orders of magnitude by adjusting the metal composition and oxygen content. This study investigates the element-specific local structure and amorphous nature of Zn–Sn–O and Cu–Sn–O thin films with varying Zn/Sn and Cu/Sn ratios, grownat different oxygen partial pressures by room temperature pulsed laser deposition. X-ray absorption spectroscopy was performed at the Zn, Cu and Sn K-edges, providing local coordination andstructural parameters for each metal species by in-depth analysis of near edge and extended finestructures. Except for very high copper contents, all thin films are fully amorphous with no noticeable order beyond the first coordination shell. Zinc atoms keep a tetrahedral configuration withan average bond length slightly larger than that of crystalline ZnO. Tin atoms prefer an octahedral coordination similar to crystalline SnO2 but with reduced average coordination number andslightly smaller average bond length. Copper atoms feature a CuO-like coordination with reducedaverage coordination number and bond length for high copper contents, while further reductionof these structural parameters for low copper contents indicates a broad distribution of local environments. The variability of the Zn–O, Sn–O and Cu–O local building blocks is thus determinedby the coordination flexibility of the respective metal, being small for zinc with only one stableoxide but large for tin and copper with two stable oxides at ambient conditions. The dominanteffect of the metal composition on the electrical properties of the material is given by the changingfraction of Zn–O, Cu–O and Sn–O building blocks. Undercoordinated tin atoms provide negativecharge carriers, leading to increasing n-type conductivity with increasing tin content. In contrast,CuO-like environments promote p-type conductivity for Cu–Sn–O thin films with high enoughcopper contents.