Tuning the Optical and Electrical Properties of ALD-Grown ZnO Films by Germanium Doping

In this work, we report on the fabrication of ZnO thin films doped with Ge via the ALD method. With an optimized amount of Ge doping, there was an improvement in the conductivity of the films owing to an increase in the carrier concentration. The optical properties of the films doped with Ge show improved transmittance and reduced reflectance, making them more attractive for opto-electronic applications. The band gap of the films exhibits a blue shift with Ge doping due to the Burstein-Moss effect. The variations in the band gap and the work function of ZnO depend strongly on the carrier density of the films. From the surface studies carried out using XPS, we could confirm that Ge replaces some of the Zn in the wurtzite structure. In the films containing Ge, the concentration of oxygen vacancies is also high, which is somehow related to the poor electrical properties of the films at higher Ge concentrations.