Photoluminescent porous silicon films were prepared and their microstructure investigations showed a double scale
porosity, the walls of the micropores being formed by a nanowires network. The temperature dependence of both the
electrical transport and photoluminescence processes in these films, as well as the spectral distribution of the
photoluminescence, were measured. The results prove a clear correlation between the two processes. A simple quantum
confiiement model was proposed for the calculation of the electronic energy in nanocrystalline silicon. The model
explains the observed experimental behavior of both the electrical transport and the photoluminescence and justifies their
correlation. Its quantitative predictions are in excellent agreement with the microstructure investigations. The model can
be applied to a wide class of materials.
Nanocrystalline silicon is studied with a view to obtaining a new photonic material. Non-equilibrium electronic processes in such materials play a significant role. We have studied trapping phenomena in nanocrystalline porous silicon and nanocrystalline silicon-based Multi-Quantum Wells structures by means of Optical Charging Spectroscopy method, which is a very good and sensitive method. We have also analyzed the modeling of the processes that occur during our measurements. This modeling allows us to separate the relative contribution of the different types of discharge currents that can appear: ohmic conduction currents of either equilibrium or non-equilibrium carriers, displacement currents, diffusion currents and tunneling currents through insulating layers (in Multi-Quantum Wells structures). It also allows us to increase the accuracy of the determination of the experimental trap parameters and to determine parameters that are not directly measurable. The model can be applied to other nanocrystalline semiconductors and can be easily generalized for other high resistivity materials.
The influence of the thermal annealing on the optical properties of the porous silicon films was revealed by photoluminescence (PL) and spectroellipsometric measurements. As result of 200 degree(s)C annealing small changes of the dielectric functions could be understood by desorption process of some molecules from Si skeleton surface. Strong changes of PL and dielectric function spectra after the thermal annealing at high temperatures (up to 800 degree(s)C) were explained by the change of the passivation from hydrogen to oxygen and then the beginning of the oxidation process. This oxidation process produces the disappearance of the PL slow component, an important enhancement of PL (2-3 orders of magnitude) and a shift of maximum position to higher energies, corresponding to the thinning of the nanocrystallites from the Si skeleton.
The photoluminescence (PL) decay measurements were performed on porous silicon films. It was observed that the two components of PL, one of them fast (ns) and the other slow (microsecond(s) or ms sometimes) have different contributions to PL signal, depending on the wavelength of the excitation light. The slow component of PL was in details investigated. Time decay cures for different excitation (337.1 nm, 470 nm, and 550 nm) and emission (550, 650, 700, 800 and 860 nm) wavelengths and also for different excitation intensities were taken. All decay curves were fitted with a stretched exponential. The slow component of PL was proposed to be attributed to the radiative recombination on surfaces.