Growth and Electronic Properties in Hot Wire Deposited Nanocrystalline Si Solar Cells
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0989-A21-06
Growth and Electronic Properties in Hot Wire Deposited Nanocrystalline Si Solar Cells Kamal Muthukrishnan1,2, Vikram Dalal3, and Max Noack4 1 Electrical and Computer Engr, Iowa State University, Coover Hall, Ames, IA, 50011 2 Microelectronics Res. Ctr., Iowa State University, Ames, IA, 50011 3 Electrical and Computer Engr., Iowa State University, Coover Hall, Ames, IA, 50011 4 Microelectronics Res. Ctr., Iowa State University, 1925 Scholl Rd, Ames, IA, 50011 ABSTRACT We report on the growth and properties of nanocrystalline Si:H grown using a remote hot wire deposition system. Unlike previous results, the temperature of the substrate is not significantly affected by the hot filament in our system. The crystallinity of the growing film and the type of grain structure was systematically varied by changing the filament temperature and the degree of hydrogen dilution. It was found that high hydrogen dilution gave rise to random nucleation and grain growth, whereas lower hydrogen dilution led to preferable growth of grains. Similarly, a high filament temperature gave rise to preferential growth compared to lower filament temperature. The electronic properties such as defect density and minority carrier diffusion length were studied as a function of the degree of crystallinity. It was found that the lowest defect density was obtained for a material which had an intermediate range of crystallnity, as determined from the Raman spectrum. Both highly amorphous and highly crystalline materials gave higher defect densities. The diffusion lengths were measured using a quantum efficiency technique, and were found to be the highest for the mid-range crystalline material. The results suggest that having an amorphous tissue surrounding the crystalline grain helps in passivating the grain boundaries. INTRODUCTION Nanocrystalline Si:H is an important electronic material with wide applications in solar cells [1-4] and thin film transistors [5-7]. The material is generally deposited using a plasma process, though it can also be deposited using a hot wire process [8-10]. High quality solar cells have been fabricated with both processes, producing solar cells with conversion efficiencies of ~ 10%. The hot wire process is interesting because, in principle, one can get higher growth rates, higher grain size and higher mobilities in hot wire films [11]. Most of the previous work on hot wire systems has been done using a proximity type hot wire, where the filament is only a few cm away from the substrate. For such cases, it is reported that the substrate temperature can be increased significantly because of the radiation for the proximate hot filament. Klein et al have reported that the substrate temperature increased by 185 °C when the hot filament was turned on in their system[8]. In this paper, we use a remote hot wire system so that we can better control the substrate temperature, with temperature rise being limited to ~10 °C . We report on the relationship between hydrogen dilution and filament temperature and grain crystall
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