| Energy crisis and environmental pollution have become urgent problems to be solved in the 21st century.All countries in the world are actively promoting the transition of energy resources,developing and utilizing clean energy.Due to its pollution-free and sustainable characteristics,solar energy has been highly valued and vigorously utilized by all governments.In the foreseeable future,solar energy will be bound to occupy a major position in the energy resources.Solar cells have been paid more and more attention because their high efficiency of conversion and relatively low cost.At present,the solar cells with the highest efficiency of conversion are GaAs based multi-junction solar cells.The GaAs based multi-junction solar cells are generally fabricated on germanium single crystal substrates,which are also used as the bottom sub-cell.However,germanium is a typically rare metal and is extremely expensive.In order to save germanium sources and reduce costs for fabricating multi-junction solar cells,it deserves to be developed that growing high-quality germanium films on silicon substrates instead of germanium single crystal wafers.Therefore,the primary mission of this thesis is to prepare germanium films with highly preferred orientation and smooth surface on mono-crystalline silicon substrates.Germanium films aere prepared using techniques of magnetron sputtering,rapid photo-thermal annealing,and conventional thermal annealing on graphite buffer layers and silicon substrates with deviation angles respectively.XRD,Raman spectroscopy,Step Profiler,and other instruments are utilized for characterizing the germanium films.The main achievements of this thesis are as follows:(1)Germanium films are prepared by use of magnetron sputtering technique on graphite buffer layers in order to reduce the mismatch between germanium and silicon.The effects of substrate temperature on the crystallization of germanium films are studied systematically,and the critical crystallization temperature of germanium film is 430℃.(2)The amorphous germanium films deposited on graphite buffer layers are crystallized with rapid photo-thermal annealing technique,and the mechanism of crystallization has been explored.It is found that not only the rapid photo-thermal annealing technique can crystallize amorphous germanium films,but it may de-crystallize the crystallized germanium films.(3)The amorphous germanium films are directly deposited on(001)orientation mono-crystalline silicon substrates with deviation angles,and the amorphous germanium films are crystallized with rapid photo-thermal annealing afterwards.The results of characterization indicate that the crystallization of amorphous germanium films deposited of silicon substrates with deviation angles in one direction is better than those deposited on the silicon substrates without deviation angles,and the crystallization of amorphous germanium films deposited of silicon substrates with deviation angles in two directions is better than those deposited on the silicon substrates with deviation angles in one direction.(4)The amorphous germanium films deposited on graphite buffer layers are crystallized with conventional thermal annealing technique,and the effects of conventional thermal annealing on the crystallization of amorphous germanium films have been explored.The measurements of Raman and XRD indicate that the grain of germanium grows up with the temperature becoming higher.It is found that the preferred orientation will be changed from(100)to(111)with the extension of annealing time under temperature of 800 ℃,and the crystallized germanium films with highly preferred(111)orientation are obtained after 10h of conventional thermal annealing. |
