Investigation On Urbach Tail Width And Surface Morphology Of Ge_1-xC_x Films

Germanium carbon (Ge_1-xC_x) is a kind of ideal antireflection and protectioncoatings, which can be used in the infrared guidance, infrared imaging and other fields.The investigations on Ge_1-xC_xfilms have attracted more and more attention due to thepromotion of military demand. Although many researches on the structure, chemicalbonding, optical and mechanical properties of Ge_1-xC_xthin films have been performed,there are two shortcomings in these investigations. The first is that the Urbach tailwidth of amorphous semiconductor films significantly affects their photoelectricproperties and thus merits further research, but the research on Urbach tail width forGe_1-xC_xthin film is still not reported. The second is that there is no systematic studyon the surface morphology of Ge_1-xC_xthin films, and the influence of depositionconditions on the surface morphology of the film is not clear.In this paper, Ge_1-xC_xthin films have been prepared via magnetron sputtering inthe discharge of CH4/Ar, and their composition, bonding structure, optical andmechanical properties have been explored by X-ray photoelectron spectroscopy （XPS）,Raman spectrum, Fourier transform infrared spectroscopy （FTIR）, Atomic forcemicroscope （AFM）, Surface Profiler and Ultraviolet-visible-near infraredspectrophotometer （UV-VIS-NIR）. The systematic researches on Urbach tail widthand surface morphology for Ge_1-xC_xthin films have been carried out. The conclusionsare summarized as follows:（1） The change in Urbach tail width （E0） is not only related to the structuredisorder, but also to the dielectric coefficient （ε）, which has been proved through twosets of experiments: the first set of experiments on Ge_1-xC_xthin films is annealling invacuum at different temperatures, by which the important influence of structuredisorder on E0is observed. In this set of experiments, With the increase in thetemperature from400to600oC during annealing, the carbon content and ε of Ge_1-xC_xfilms keeps unchanged, whereas the structure disorder increases, which leads to theincrease in E0. The second set of experiments is to change the carbon content in Ge1-xCxfilms, through which the important influence of dielectric coefficient on E0isfound. In this experiment, with the increase in the carbon content, structure disorderdoes not increase significantly, but the dielectric coefficient decreases significantly,which results in a significant increase in E0. The quantitative relationship between E0 and ε can be well described by means of hydrogen-like atom model. It is shown thatE0is proportional to reciprocal of dielectric coefficient square （1/ε2）, which is in goodagreement with our experimental results. Furthermore, we find that the linearrelationship between1/ε2and E0is also suitable to explain the evolution in E0withcomposition for other amorphous alloy films such as a-Si1-xNx:H and a-Si1-xCx:H.（2） It is found that the the surface morphology of Ge_1-xC_xfilms can significantlybe affected by deposition conditions and vacuum annealling. As the substratetemperature increases gradually from60°C to500°C, the root mean square roughness（Rms） decreases gradually and reaches a minimum at300°C, and then it increaseswith further increasing temperature. This phenomenon can be attributed to that theappropriate temperature promotes the diffusion and reaction of deposited particles onthe substrate, and thus smoothes the film surface. However, the too high temperaturecauses in the rapid increase in the size of clusters, which increases the Rms roughnessof films. With the sputtering pressure gradually increase from0.5to2.0Pa, the Rmsroughness of the films increases continuously. The energy of sputtered particlesbecomes so low in a high sputtering pressure that the diffusion of deposited particlesis suppressed and particles agglomeration is formed, which results in the increase inRms roughness. The Rms roughness of films is found to decrease gradually wheneverRF power is increased gradually from80to250W, or annealing temperature is risedto600oC. This is because in above cases the surface diffusion of deposited particles isenhanced in different ways, so the film surface becomes smooth. These studies canprovide experimental guidance for obtaining smooth and compact Ge_1-xC_xthin films.