Preparation Of Germanium Thin Films On Heterogeneous Substrates

Because of the increasingly prominent problem of traditional fossil energy, the development and utilization of renewable energy, especially using the solar cell to generate electricity, have become the inevitable choice in the worldwide development of economic currently. At present, the highest efficiency solar cell is Ⅲ-Ⅴ multi-junction solar cell, which generally chooses single crystal germanium as the substrate. However, considering that germanium is extremely scarce and expensive, germanium thin films were prepared on heterogeneous substrate instead of the single germanium in order to save germanium materials and reduce the cost. Therefore, the research content of this article is mainly focused on the deposition of high quality germanium films with compatible technology, low dislocation density on graphite substrates and monocrystalline silicon substrates.Firstly, the germanium films were fabricated on the graphite substrate by using magnetron sputtering technology and rapid thermal annealing (RTA).Then according to the experimental results of germanium films deposited on the graphite substrates, the germanium films were prepared on the single crystal silicon substrates with the transition layer of graphite. In the end, the germanium films deposited on graphite substrates and the silicon substrates were respectively characterized and analyzed by scanning electron microscope (SEM), X-ray diffraction (XRD) and step profiler. The main achievements are as follows:(1) The germanium films were prepared on the graphite substrates by using magnetron sputtering technology, and the influence of the substrate temperature on the microstructure of germanium film was studied. XRD results showed that the substrate temperature has a great influence on the crystallization of germanium and when the substrate temperature is below 500℃, germanium films are amorphous, therefore, it can be assumed that 500℃ is the critical temperature when the amorphous germanium film on silicon substrate turn into crystalline germanium.(2) The germanium films prepared on the graphite substrates at 450℃ were annealed by RTA and the influence of RTA on the microstructure of germanium films was studied. It was found that RTA plays an important role on the quality of the germanium film, which means that each diffraction peak of germaniums was heightened when the annealing temperature was increased and it has the greatest influence on the diffraction peak of Ge(111). In addition, it was found that the germanium films preferred Ge(111) orientation, and the best annealing temperature is 900 ℃.(3) The germanium films were prepared on the silicon substrate and the transition layer of graphite was introduced between them, which relieved the lattice mismatch and the thermal expansion coefficient mismatch between silicon and germanium. The scanning electric microscope(SEM) observation showed that the structure of germanium/graphite/ silicon are uniform and continuous; And the XRD results indicate that 450℃ is the critical substrate crystallization temperature when the amorphous germanium films on silicon substrate turn into crystalline germanium.(4) The germanium films prepared on the silicon substrates at 450℃ were annealed by RTA and the influence of RTA on the microstructure of germanium film was studied. The XRD tests showed that the crystallization degree of germanium film increase obviously when the annealing temperature increased to 750 ℃, and 30 seconds is the optimal annealing time when the substrate temperature is 750℃, meanwhile,900℃ is the optimal annealing temperature.