Fabrication and Characterization of an Intermediate Band Material based on Ion Implantation and Pulsed Laser Melting of Ag in Fz-Si
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Intermediate band solar cells (IBSCs) are an innovative field of material engineering and their prospect of high efficiency may contribute to meet the demand for new energy alternatives. IBCSs are based on absorption of low energy photons to exploit a larger portion of the solar spectrum and enhance the photocurrent. This thesis introduces a novel experimental study of Ag as a potential candidate for IBSCs based on Si. This work also aims to reveal challenges related to processing and characterization of the IB material. In this work, two Fz-Si samples were implanted with Ag by a double implantation with a peak concentration of 1 ∙ 10^20 atom/cm3 and 5 ∙ 10^20 atom/cm3 in order to surpass the Mott limit. A double implantation was performed to obtain an implanted layer up to roughly 500 nm. The impacts from the implantation formed a non-crystalline structure assumed to be amorphous, detected by Electron Backscatter Diffraction (EBSD). The crystallinity of the material was recovered by a constructed Pulsed Laser Melting (PLM) set-up, using an excimer Krypton Fluoride (KrF) laser. The main restrictions for the set-up are inhomogeneous regions in the intensity profile of the beam and low laser energy. This lead to a small beam area, which restricted the characterization techniques due to sample size. The PLM experiment was conducted for two various fluences for Sample 1 and three fluences for Sample 5 employing 1, 10, 20, 50 and 100 number of pulses. The samples subjected to 0.675 ± 0.002 J/cm2 all displayed what resemble a polycrystalline surface structure, due to incomplete recovery. Sample 5 was additionally subjected to a fluence of 0.846 ± 0.002 J/cm2, which did not provide any recovery either. The samples subjected to 1.112 ± 0.003 J/cm2, showed areas of crystalline recovery with a <001> orientation according to the underlying substrate. However, the surface experienced extensive cracking due to multiple numbers of pulses. Hyperspectral images demonstrated an improvement in the band-to-band signal for the PLM samples, especially the recovered pulses supporting the results from EBSD. The photoluminescence (PL) spectra indicated features in regions where Ag luminescence should occur, which is highly encouraging. However, the amorphous features broaden the response significantly and no final conclusions can be made. The hyperspectral images on the other hand revealed an increase in Ag- related luminescence in the laser spot regions. Minority carrier lifetimes obtained from Carrier Density Imaging (CDI) revealed a distinguishable region for as-implanted and PLM samples for the un-passivated surface. Distinguishing these regions in a lifetime map is promising for Si samples hyperdoped with transition elements. In addition, transmission spectra revealed an increased absorption for sub-band gap photons, which is linked to the amorphous structure and hyperdoping of Ag. It can be concluded that states in the band gap has been produced. Based on these results, Ag implanted in Si above the Mott limit and recovered by PLM is a promising candidate for IBSC applications.