| For meeting the challenges of "Beyond Moore" Era,one of the possible solutions is to realize Si-based optoelectronic integrated circuits by combining the micro-electronic with photonic components on a single chip.So far,the most challenging work is to find an efficient and stable Si-based light source.Meantime,the study on further improvement of linear and nonlinear optical properties in novel Si-based materials is necessary as a potential approach to overcome this technical barrier and it has attracted much attention in the recent years.This thesis contains two main aspects.On the one hand,the amorphous Si/SiO2 multilayers with various amorphous silicon thicknesses were deposited in Plasma Enhanced Chemical Vapor Deposition(PECVD)system and the nano-crystalline Si/SiO2 multilayers(nc-Si/SiO2)were obtained after high temperature thermal annealing.The linear and nonlinear optical properties of nc-Si/SiO2 multilayers were systemieally studied.On the other hand,the oxygen doped hydrogenated amorphous Silicon-Nitride(a-SiN:0:H)films were prepared in PECVD system under the different conditions.The influences of the substrate temperature and the diluted gas on the microstructures and optical properties are systematically investigated while the recombination process of photo-excited carriers was discussed.Moreover,the role of annealing temperatures on the microstructures and optical properties of a-SiN:O:H was discussed.The main results and highlights of this thesis are listed as follows:1.The thickness controlled nc-Si/SiO2 multilayers were prepared by PECVD system and post-annealing technique.The cross-sectional transmission electron microscopy(X-TEM)and Raman spectroscopy were used to characterize the microstructure of multilayers and it is found that the periodic structures were kept well even after high temperature annealing.In addition,the average size and the crystallinity of the nc-Si dots were increased with increasing the annealing teruperature.Under the excitation of He-Cd laser with wavelength of 325nm,a broad photoluminescence emission band centered at 900nm was observed,which can be ascribed to the recombination of photo-excited carriers via luminescence centers at the interfacial region of nc-Si/SiO2 multilayers.Meanwhile,the nonlinear optical properties of the multilayers were investigated through Z-scan technique by using a Ti-sapphire laser with 50fs pulse duration as a pump laser.It is found that with increasing the annealing temperature,the nonlinear absorption turns from the two-photon absorption to saturation absorption while the nonlinear optical refraction is also changed simultaneously firom self-defocusing to self-focusing property.Moreover,with decreasing the thickness of amorphous Si layer,the nonlinear absorption turns from saturation absorption to the reverse saturation absorption while the nonlinear optical refraction is also changed from self-focusing to self－defocusing property.The two step absorption proeess via the localized states at the nc-Si/SiO2 interfacial was proposed to explain the observed switching behaviors.When the sample was pumped by femtosecond laser,the measured nonlinear absorption coefficient and nonlinear refraction index are in the order ofβ=10-7cm/W and n2=10-12cm2/W,respectively,which is one order of magnitude larger than that reported previously.2.The nonlinear optical response of the nc-Si/SiO2 multilayers excited under the various laser pulse durations from ns to fs was investigated through Z-scan technique.When the sample was excited by nanosecond laser(A=532nm,tp=8ns),it showed saturation absorption with nonlinear absorption coefficient β=-1.0×10-3cm/W and self-defocusing property with nonlinear refractive index n2=-7.9×10-8cm2/W;Under excitation of picosecond laser(λ=1.06um,tp=25ps),the nonlinear absorption turned reverse saturation absorption into saturation absorption when the laser intensity increased from I0=1.75×109W/cm2 to 5.72×109W/cm2.With further increasing the laser intensity,both the nonlinear absorption coefficient and nonlinear refraction index increased;When pumped by the femtosecond laser(λ=800nm,tP=50fs),the nonlinear absorption turmed reverse saturation absorption into saturation absorption when the laser intensity increased fromI0=3.54×1011W/cm2 to 3.54×1012W/cm2.Moreover,with further increasing the laser intensity,the nonlinear refraction index increased.It is found that the different nonlinear optical response under various pulse duration play the great role in the nonlinear optical properties of the nc-Si/SiO2 multilayers.Under the excitation of the nanosecond laser,the single photon resonance absorption and the free carries dispersive effect dominated the nonlinear optical properties.When pumped by the picosecond and femtosecond lasers,there is competition between the two step absorption and saturation process.The variation of free carrier density due to the competition makes the nc-Si/SiO2 multilayers have different nonlinear optical properties.3.We have prepared the a-SiN:O:H films in PECVD system under different substrate temperature(100℃ and 250℃)and diluted gas(H2 and Ar).The chemical bond configurations were characterized by x-ray photoelectron spectroscopy(XPS)and Fourier transform infrared spectroscopy(FTIR)technique,We compared the optical properties of the films under different deposition conditions.Under the low substrate temperature(100℃),it is found that the O,H contents were higher and the corresponding optical bandgap was larger than that which deposited at high substrate temperature.As consequence,the peak position of photoluminescence was blue-shifted.According to the temperature-dependent and time-resolved photoluminescence measurements,the carrier excitation and recombination model was proposed.When pumped by the 325nm laser,the electrons transit from the valence band to band-tail states,then relaxed between band-tail states and localized luminescence center,finally recombine from the localized luminescence center.We also discover two photoluminescence decay processes,one is the "fast" nanosecond PL decay component and the other is the "slow" microsecond radiative recombination component.By changing the dilution gas from H2 to Ar,the bandgap of prepared a-SiN:O:H films became smaller.The nonradiative recombination rate reduced due the increase of the structure disorder,which results in the longer lifetime of the radiative recombination.4.We study the influence of the thermal annealing temperature on the a-SiN:O:H films by using Ar dilution gas.With increasing the annealing temperature,the content of Si-H and N-H bonds decreased while the Si-N bonds increased.It was found that the films kept their amorphous nature even after annealing at 1000oC.The photoluminescence properties for samples with and without thermal annealing were studied.It was found that the luminescence intensity was gradually reduced with increasing the annealing temperature.A new photoluminescence band centered at 430nm appeared when the films annealed at 1000oC.Under the excitation of picosecond laser pulse durations(2=375nm,6＝70ps),the difference at the "fast"nanosecond PL decay is small for annealed and as-deposited samples.However,when excited by nanosecond laser pulse durations(λ＝410nmm,tp＝sns),the "slow"microsecond PL decay component reduced with increasing the annealing temperature.It was attributed to the increase in the nonradiative recombination rate due to the breakdown of Si-H and N-H bonds during the annealing process,which increases the non-radiative recombination sites.Furthermore,the lumninescence mechanism is different for 1000℃ annealed saomple from the as-deposited and 800℃ annealed one. |
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