Zinc and tin oxides are both earth-abundant materials with demonstrated applicability as electrodes in several optoelectronic devices. The presence of grain boundaries in these polycrystalline films generally limits the electron mobility. By a combinatorial study of ZnO and SnO2, a transparent conducting amorphous zinc tin oxide (ZTO) electrode, free of grain boundaries, with a dense (void-free) microstructure has been developed. We show how tuning the stoichiometry (Zn4.5Sn30.2O65.3) and film's microstructure during sputtering deposition, allows achieving electron mobilities up to 25 cm(2)/Vs and free carrier concentrations of similar to 7 x 10(19) cm(-3). The effects of post-deposition thermal treatments are furthermore studied. The ZTO films keep their dense amorphous microstructure upon annealing up to 500 degrees C, as confirmed by cross-section TEM and XRD, while presenting a clear improvement in electron mobility up to 35 cm(2)/Vs when annealed in oxygen-rich atmospheres.