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Laser picosecond spectroscopy was used for studying the electronic structure of silicon surfaces and films. The method of bandgap determination by estimating the exponential decay time was proposed. The oxidation of silicon surface was important for quantum beats phenomena and connected with defect production in grain boundary of silicon nanocrystallites. Also, the career density decay process in irradiated polycrystalline silicon films was studied by using the pump-probe laser scheme.Polycrystalline silicon ͑poly-Si͒ films, porous silicon, and silicon surface with dominant orientations ͑111͒ and ͑100͒ were studied by laser time-resolved laser picosecond spectroscopy. This method illustrates the correlation function of deflected radiation as a function of the delay time between the pump and probe pulses. This method of measurements is similar to the laser fluorescence spectroscopy of molecules. 1,2 In addition, the signal time evolution reflects the energy positions of defects or impurities levels. Our previous research work 3 was devoted to the photoluminescence ͑in the photon energy range 1.6-2.4 eV͒, nonlinear optical properties ͑3.2-3.4 eV͒, and absorption properties of polycrystalline silicon films. However, the low energy spectra ͑for photon energies in range 1.1-1.6 eV͒ were not measured due to the limit of spectral technique. But most silicon surfaces show the great optical response around bandgap energy value ͑1.17 eV for crystalline Si͒. Also, the recent researches of polycrystalline silicon films deposited by plasma-enhanced chemical vapor deposition ͑PECVD͒ show the great role of the structural properties and oxidation by deposition process 4,5 and their correlation with optical properties.The present work illustrates the study of composite silicon materials such as poly-Si, amorphous silicon ͑a-Si͒ films, and porous silicon structures for their bandgap estimation and possible investigation of impurity or defect level. Experimental Techniques and ResultsThe block diagram of laser pump-probe technique was shown on Fig. 2 of Ref. 6. A mode-locked YAG:Nd 3ϩ laser radiation with wavelength 1064 nm was used as a optical pump of media, but the second-harmonic radiation ( ϭ 532 nm) was used for probing the sample surface. The pulse duration was 120 ps. The pulse repetition rate was 100 MHz and the frequency of Q-switched modulation of second-harmonic radiation was 6.2 MHz.According to our previous experiments in Ref. 3, we supposed that the time evolution corresponds to the surface electronic band structure. Also, we proposed the formula for estimation of recombination rate from the Fermi golden rule 4 Ϫ1 ϭ 16 2 3 n e 2 h 2 m 2 c 3 E͉͗i͉p͉ j͉͘ 2 ͓1͔where the E is transition energy, and ͗i͉ and ͉j͘ are the initial and final states of electron transition. The time evolution reflects the superposition of an ensemble of different kinds of transitions such as band-to-band transition, transition through the surface or defect states. The width of the energy level depends on the chemical bond structure of silicon su...

Polycrystalline silicon (poly-Si) films were deposited by plasma-enhanced chemical vapor deposition with a mixture of silane (SiH4) and silicon tetrafluoride (SiF4) diluted with hydrogen, and their structural, optical, and bonding properties were investigated. Poly-Si films with a crystalline volume fraction of 80% at a substrate temperature no more than 3 00°C can be deposited, and lowering the substrate temperature resulted in decreased crystalline grain size. Also, a decrease in the size from microcrystalline to nanocrystalline grains can be achieved by increasing the hydrogen dilution ratio to the SiH4/SiF4 mixture. On the other hand, the increase in the hydrogen causes an increase in the crystalline volume fraction which was estimated from Raman scattering spectra. The structural properties for the surface of crystallites in the poly-Si films were investigated by means of X-ray diffraction and Fourier transform infrared spectroscopy. Furthermore, the photoluminescent response measured for the poly-Si films was investigated in connection with the structural properties.Polycrystalline silicon (poly-Si) films have become an important material for thin film transistors (TVrs), image sensors, and integrated circuits. Recently, it has been reported that poly-Si films can be grown at temperatures less than 400°C by plasma-enhanced chemical vapor deposition (PECVD) using a gas mixture of SiF4/SiH4/H2.13 It is known that grain size for poly-Si films decreases as the deposition Present address: The Institute of Physical and Chemical Research (RIKEN), Saitama 351-0198, Japan.temperature (Td) decreases,4 but that the crystalline volume fraction also decreases. However, with increasing hydrogen dilution ratio, it has been found that the grain size decreases and the crystalline fraction increases.2 Using these two ways, the average size of grains could be decreased to a level of magnitude up to 5 to 10 nm, keeping a high crystalline fraction. Ultralow temperature conditions in the growth of PECVD of poly-Si films using SiH4/H2 or SiR4! SiF4/H2 mixtures allowed us to produce micro-or nanocrystalline Si ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 137.99.31.134 Downloaded on 2015-06-03 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 137.99.31.134 Downloaded on 2015-06-03 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 137.99.31.134 Downloaded on 2015-06-03 to IP