Characteristics of ZnON films and heterojunction diodes with varying O:N ratios

Abstract

Zinc Oxynitride (ZnOxNy) thin films display high mobilities and a considerable tunability of both the free electron concentration and optical band gap. The properties achievable for this material system makes ZnOxNy an intriguing n-type absorber candidate in silicon-based tandem solar cells. We have studied how the O:N ratio affect structural, optical and electrical properties of magnetron sputtered ZnOxNy thin films, as well the electrical behavior of ZnOxNy-Si pn heterojunction diodes. X-ray diffraction of the ZnOxNy films indicate either a Zn3N2-like structure or ZnO-like grains in combination with structural disorder. As the O:N ratio is increased, the optical band gap of ZnOxNy films increase from 1.1 to 1.9 eV. Hall effect measurements of the n-type ZnOxNy films show free electron concentrations varying with the O:N ratio, from 9.8 × 1017 to 1.5 × 1016 cm−3. Hall mobility up to 88 cm2∕Vs is achieved. We observe the formation of pn heterojunction diodes between ZnOxNy-films and p-Si. The electrical characteristics of these diodes are shown to depend on the ZnOxNy anion composition. Current rectification of 3.7 orders of magnitude is achieved between -1 and 1 V at room temperature. However, the built in voltage extracted from capacitance–voltage measurements are higher than theory suggest, implying an influence of defects on the electrical characteristics.