The Preparation And Luminescence Properties Of Si Nanocrystals

Silicon rich silicon nitride (SRSN) and free-standing silicon nanocrystals (Si-nc)are very valuable for the applications of silicon-based light emission devices andbiological labeling, respectively. However, there still are some fundamental problemsincluding material preparation, sample characterization, luminescence mechanism andso on. The aims of this thesis are to develop an efficient method to identify the Si-ncsize distribution, prepare free-standing Si-ncs with size control in large quantities andunderstand the luminescence mechanism of SRSN.In order to determine the Si-nc size distribution with Raman scattering efficiently,the effect of Si-nc size distribution on Raman spectrum is studied in detail with thephonon confinement model. It is found that the Raman frequency shift is mainlydetermined by the most probable diameter of Si-ncs, while the width of Ramanspectrum is determined by both the most probable diameter and the size distribution. Aset of analytic relationships between Raman frequency shift/FWHM (full width at halfmaximum) and Si-nc size distribution is set up and applied to measure the sizedistribution.In order to prepare free-standing Si-ncs with controlled size in large quantities, theannealing effect and etching treatment of bulk silicon monoxide (SiO) are investigated.Firstly, the structure evolution of SiO and the growth of Si-ncs under various annealingconditions (temperature from800to1150oC and time from1to16h) are experimentallystudied. It is found that the critical conditions are the annealing temperature of900oCand annealing time of several hours. The relationship between Si-nc average sizes andannealing conditions are established. Then Si-ncs with an average size from less than3nm to about10nm are obtained by proper annealing treatments, while the sizedistribution width is about65%of the average value. Large quantities of free-standingSi-ncs are synthesized by etching annealed SiO with HF and HNO3. Both thecontinuous shift of luminescence spectrum due to quantum confinement effect andluminescence resulting from oxygen surface state are observed.SRSN films embedded with and without Si-ncs are prepared by PECVD andpost-annealing treatment. It is found that the Si-nc size is mainly determined by the annealing conditions and independent to the silicon content of samples. However, theluminescence intensities of samples embedded with Si-ncs are weak. The sampleswithout Si-ncs exhibit much stronger luminescence intensities, especially after about700oC short time (15s) annealing treatment. The luminescence centers are determined asradiative defects. It is found that, after annealing, the concentration of Si-Si bondsincreases. The defects of Si dangling bonds and Si-Si bonds originate the multi-peaksand redshift of the spectrum with increasing silicon content. Ultra-wide luminescencespectra with FWHM of about250nm are obtained with radiative defects.