Structural evolution and optical properties of hydrogenated germanium carbonitride films

Although the control of bond structure and optical properties in hydrogenated amorphous germanium carbonitride films (a-GeC1−xNx:H) is important for technological applications, the composition dependence of chemical bonds, especially hydrogen-containing bonds, is not yet well explored. The evolution in refractive index (n) and Urbach tail width (E0) remains unclear. Here, we show that nitrogen content (CN) exerts a significant effect on bonding structure and optical properties of a-GeC1−xNx:H films. As CN increases, the fraction of NH increases, whereas that of CH and GeH bonds reduces, and GeN bonds form at expense of GeC bonds. The replacement of carbon by nitrogen induces a substantial decrease in n from 3.0 to 2.3 because of decrease in electronic polarizability. With increasing CN, a significant increase in E0 from 198.8 to 327.9 meV takes place. This behavior arises from decrease in dielectric coefficient (ε), rather than the change in the degree of disorder previously believed. The change in E0 is proportional to the variation in 1/ε2, which agrees well with hydrogen-like atom model. This study discovers that a-GeC1−xNx:H films have the apparent tunability of n and E0 over a wide range, which is useful in controlling the optical transmission and absorption characteristics of these films.