Bonding Structures And Properties Of Germanium Carbide Films With And Without Nitrogen Incorporation

Germanium carbide(Ge_1-xC_x) films have excellent optical and mechanical properties, which make Ge_1-xC_x films promising candidates for application as antireflective and protective coatings on far-infrared windows. Ge_1-xC_x films have drawn researchers’ attention. So far, significant progress has been made in the study of germanium carbide films. However, there are still some shortcomings:（1） In previous study, carbon content remarkably influnced the hardness of grmanium carbide films. However, the results of hardness were disagreenment among these investigations. The correlation between carbon content and hardness has not been well understood yet.（2） Ge_1-xC_x films have low hardness and narrow optical band gap, which make the films difficult to use at high speed. Therefore, it is necessary to improve the hardness and optical band gap of Ge_1-xC_x films.According to the problems above, we prepared Ge_1-xC_x and Ge_1-x-yC_yN_x:H films via magnetron sputtering in the mixed gases of CH₄, Ar and（or） N₂. The bonding structures, optical properties and mechanical properties have been studied by XPS, FTIR, Raman, AFM, SEM, UV-VIS-NIR, profiler and nanoindentation. We come to conclusions as follows:（1） Carbon content（x） remarkably influnces the bonding strcture and hardness of Ge_1-xC_x films. For the films with low x（x<0.17）, excess Ge–Ge bonds make the Ge_1-xC_x network more similar to that for germanium. However, for the films with high x（x>0.40）, a large fraction of sp² C–C bonds causes the formation of the structure more similar to that of graphite. In contrast, for the Ge_1-xC_x films with mediate x（0.17<x<0.40）, sufficient sp3 C–Ge bonds promote formation of a strong covalently bonded network, which contributes to the high film hardness. This investigation suggests that the density of sp3 C–Ge bonds is an important parameter that determines the hardness of Ge_1-xC_x films.（2） The introduction of nitrogen can significantly improve the optical and mechanical properties of germanium carbide films. As the introduction of nitrogen, the contents of nitrogen and carbon experience a linear increase and decrease, while the CGe remains almost unchanged. That is to say, carbon in the films is gradually replaced by nitrogen. The refractive index of the films decreases from 3.0 to 2.3 with increasing nitrogen content as a result of decreasing in electronic polarizability. The optical band gap increases from 1.11 e V to 1.56 e V, due to the improved average binding energy of the electron. The hardness increases from 6.25 GPa to 11.73 GPa, which contribute to the substitution of strong Ge-N bond and the forming of covalent network. The Urbach tail width increases from 198.77 me V to 327.94 me V, due to the decrease of dielectric coefficient, rather than the influnce of degree of disorder. However, with high nitrogen content（9.7%）, the far infrared transmittance decreases, due to the replace of Ge-C bond by Ce-N bond, causing the increasing of basic vibration frequency. In summary, there is an optimal range of nitrogen content.