Fabrication And Characterization Of Heterojunctions Based On FeSi₂Thin Films

Among the transition metal silicides, the environment-friendly semiconducting FeSi₂thin film has a narrow energy band gap of0.80eV-0.89eV. It shows light emission andphotoresponse near1.5m. The absorption coefficient of FeSi₂is in the order of105cm-1.Therefore, FeSi₂thin film has prominent optoelectronic properties.In this dissertation, the a-Si/-FeSi₂/c-Si, a-FeSi₂/c-Si, a-Si/a-FeSi₂and ZnO:Al/a-FeSi₂heterojunctions were prepared by magnetron sputtering. The structural, surface morphology,optical, and electrical properties of FeSi₂thin films and the electrical and photovoltaicproperties of FeSi₂heterojunctions were measured. The results show that:Both the a-Si/-FeSi₂/c-Si double heterojunction and the-FeSi₂/c-Si heterojunctionshow the rectifying property of diode. The rectifying ratios of a-Si/-FeSi₂/c-Si doubleheterojunction and-FeSi₂/c-Si heterojunction are66.3and46.3, respectively. The a-Si/-FeSi₂/c-Si double heterojunction shows the open-circuit voltage of0.26V, short-circuitcurrent density of2.90mA/cm2, fill factor of0.356and energy conversion efficiency of0.268%. The measured conversion efficiency of a-Si/-FeSi₂/c-Si heterojunction increases by59.7%compared with that of the-FeSi₂/c-Si heterojunction. The effective enhancement inphotovoltaic performance of a-Si/-FeSi₂/c-Si double heterojunction is ascribed to theextended built-in electric field distribution and the increased built-in potential by the a-Si thinfilm. Therefore, the collection efficiency of photo-generated carriers is improved.The-FeSi₂thin films in the a-Si/-FeSi₂/c-Si double heterojunctions are doped by Al, B,and P impurities. The impurities do not affect the formations of-FeSi₂crystalline structure.The Si vacancies in the-FeSi₂thin films can be occupied by the doped atoms. Therefore, thedoping can lead to the reductions of Si vacancy density and recombination of photo-generatedcarriers. The infrared response properties of-FeSi₂thin film can be improved by impuritydoping. By the optimization of the experimental parameters of doping, the energy conversionefficiencies of double heterojunctions based on Al-doped, B-doped, and P-doped-FeSi₂thinfilms can reach3.39,1.98, and3.04times to that of the un-doped-FeSi₂doubleheterojunction, respectively.The FeSi₂thin films, which were prepared on crystalline silicon, glass, and stainless steel substrates by co-sputtering without the substrate heating during sputtering and postannealing, are amorphous structures. Based on the a-FeSi₂thin films, the prepareda-FeSi₂/c-Si, a-Si/a-FeSi₂, and ZnO:Al/a-FeSi₂heterojunctions exhibits rectifying propertiesof the diode. The a-FeSi₂/c-Si heterojunction shows the rectifying ratio of37.6, the shuntresistance of458.6cm2and the diode ideal factor of2.07, which has better rectifyingproperties than other a-FeSi₂heterojunctions.In conclusion, the a-Si/-FeSi₂/c-Si double heterojunction can enhance the built-inelectric field and built-in electric potential. By doping the-FeSi₂absorption layer with III-Velements, the density of defect state of-FeSi₂can be reduced. It leads to the improvement ofthe photovoltaic properties of-FeSi₂double heterojunctions. The a-FeSi₂heterojunctionscan achieve the low temperature fabrication of FeSi₂heterojunctions.