Fabrication, Characterizations And Optical Properties Of β-FeSi₂Nanostructures

Beta-phase iron silicide (β-FeSi2) is a performance fine and environmentally friendly semiconductor material, a strong contender for silicon-based light emitting diodes (LED) and suitable for optical fiber communication due to the band gap of0.81-0.87eV of β-FeSi2semiconductor.β-FeSi2also possesses many unique physical and chemical properties such as good physico-chemical stability under high temperature/pressure, strong light absorption and able to grow epitaxially on Si substrates. In addition,β-FeSi2has other excellent features such as the rich abundance of its constituents in natural resources and no toxicity. These advantages render β-FeSi2a promising candidate for the use in near infrared detectors, light emitters, solar cell material, etc. In this dissertation, we devote our attention to the fabrication of β-FeSi2nanocrystals and films and pay more attention to how to enhance the PL efficiency. The obtained main results are described as follows:1. By chemically etching microscale β-FeSi2powders in a mixture of hydrofluoric acids and ethanol (1:10ratio), the needle-shaped β-FeSi2nanowhiskers with diameters of2-5nm and lengths of20-100nm are obtained. The TEM observations show that the formed nanowhiskers mainly remain the <400> growth direction. With the characterization of SEM and XPS, we infer the formation mechanism of β-FeSi2nanowhiskers:The β-FeSi2has an anisotropic etching process in hydrofluoric acids. The etching rates in different facets are not the same. We found in hydrofluoric acid that silicon in the β-FeSi2can be dissolved more easily than iron. The facets with more Fe atoms (higher Fe ion density) have lower etching rates, whereas those with more Si atoms have higher etching rates. Some facets perpendicular to the <400> direction are all Fe atoms and have the highest Fe ion densities. Hence, the etching rate of the <400> direction is the lowest and the nanowhisker mainly grows along the <400> direction. The result is of great interest in optoelectronic devices and fluorescent biological probes. The absorbance spectra show that the energy gap of the β-FeSi2nanowhiskers increases from0.81(bulk) to0.94eV, demonstrating distinct quantum size effect.2. High quality β-FeSi2films on Si (111) were fabricated by pulse laser deposition (PLD) and annealing. The target of β-FeSi2was prepared in high pressure of6GPa and high temperature of900℃, so the target is very compact and firm. The films we fabricated by bombarding the compact target by pulse laser have very smooth surface and there are no a large number of irregular microscale particles. The sizes of β-FeSi2nanocrystals are about40-70run. The photoluminescence (PL) characteristics of β-FeSi2films were obtained at different measurement and annealing temperature. The films have strong PL at low temperature, the PL vanishes until the temperature is up to200K.3.In order to improve the PL efficiency of β-FeSi2. We fabricated hybrid β-FeSi2/Si films and pure β-FeSi2films by PLD and investigated the PL characteristics at20K. We found that the intensity of hybrid β-FeSi2/Si films at1.54μm PL is enhanced distinctly than pure β-FeSi2films, but the enhancement vanishes when the excitation wavelength is larger than the widened band gap of Si nanocrystals. Time-resolved PL decay measurements reveal that the lifetime of the photo-excited carriers in the hybrid β-FeSi2/Si film is longer than that in the pure β-FeSi2film, providing evidence that the PL enhancement results from the resonant charge transfer from nanocrystalline Si to β-FeSi2.