Formation of silicon nanostructures in silicon nitride thin films for use in solar cells
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The increase in the world s demand for energy, and the fact that at one point we will run out of oil and gas which are two major contributers of the world supply of energy toady, are two reasons for why new and reliable energy sources are needed. The solar industry is one of the fastest growing industires, but the price of energy delivered by solar cells is still too high compared to other alternatives. More research is therefore needed in order to drive the price of solar energy down.In this report seven silicon nitride films with different stoichiometry are deposited on silicon substrate by the plasma enhanced chemical vapor de- position (PECVD) method. The deposition conditions are selected in order to enhance the formation of silicon nanoclusters. Silicon nanostructures have interesting properties due quantum effects observed at these dimensions. The most interesting of these properties is the ability to tune the silicon nanos- tructures to absrob ligth at different wavelengths. High energy light cannot be utilized in silicon solar cells. With the application of silicon nanostruc- tures, this light can be absorbed and down-converted to usable light which is then transmitted into the solar cell. This would increase the efficiency of the soalr cell, which results in cheaper energy. Two ensembles of as-deposited and annealed (annealed at 1050◦ C) samples were characterized with dif- ferent techniques in order to find the thickness, composition, light emitting sources and optical constants of the films. The techniques used were ellipsom- etry, photo-luminescence (PL) and transmission electron microscopy (TEM).The results obtained shows that all films are porous (indicated by the low index of refraction). One of the effects of annealing is an increase in the refractive index for all samples, which is an indication that the films have become more compact as a result of the annealing process. PL is obtained for samples with a high flow of ammonia, while samples of a low flow have little or no PL. The annealing process increase the PL observed for samples with a high ammonia flow, while a reduction is observed for the samples with a low flow. TEM images reveals that only one sample has any nanostructures present, so the observed PL is likely related to defect states.