Optical and optoelectronic properties of Si nanocrystals embedded in dielectric matrix.
Date of Issue2009
School of Electrical and Electronic Engineering
It is the intent of this work to investigate the optical and optoelectronic properties of Si nanocrystals (nc-Si) embedded in dielectric matrix. Si nanocrystals embedded in dielectric films have been synthesized with the technique of Si ion implantation and plasma enhanced vapor deposition (PECVD). A comprehensive optical study has been performed with SE analysis based on the effective medium approximation (EMA) theories and proper optical dispersion model. The dielectric functions of nc-Si have been experimentally determined for the first time from the spectroscopic ellipsometric (SE) analysis. The bandgap and dielectric constant of nc-Si have been also obtained. Based on the optical constants of nc-Si obtained and EMA theories, the depth profiling of optical constants of SiO2 film containing nc-Si have been determined, which is very important to device modeling. Thermal annealing effect on optical properties of nc-Si embedded in SiO2 films has been studied also. The influence of nanocrystal size on the band gap and dielectric function has been investigated. Furthermore, a comprehensive study has been carried out on the dielectric function of dispersed nc-Si and densely-stacked nc-Si layer embedded in SiO2 fabricated with the technique of ion implantation. Strong room-temperature photoluminescence (PL) has been observed from SiO2 thin films embedded with nc-Si. Thermal annealing effect on the PL properties of nc-Si embedded in SiO2 has been investigated. It has been found that thermal annealing at 1100 oC exhibits PL related to the formation of nc-Si. Size-dependent PL emission has been realized through adjusting implantation recipes. The PL energy decreases with the increasing nanocrystal size in accordance with the quantum confinement concepts. However, the size-dependent PL could not originate from the direct band to band transition of nc-Si. Based on the knowledge of band structure of nc-Si obtained in this work, the size-dependent PL band could be attributed to the indirect band-to-band transition of the nc-Si assisted by the Si-O vibration at the interface of nc-Si/SiO2. Visible and near infrared (IR) electroluminescence (EL) has been observed from a metal–oxide–semiconductor-like (MOS-like) structure with nc-Si embedded in the gate oxide. The nanocrystal distribution is found to play an important role in the EL. The influence of the applied voltage, the implantation dose, and implantation energy on the EL properties has been investigated. The current transport of the device follows a power law, and it is determined by the concentration and distribution of the nc-Si in the oxide. A linear relationship between the EL intensity and the current transport is observed. The current transport evolves with both the concentration and distribution of the nc-Si, and so does the EL. For MOS capacitors with nc-Si embedded in the gate oxide, both the stress voltage and stress duration play important roles on the flatband voltage shift and the conductance peak, due to the charging and discharging effect of nc-Si. XPS analysis is used to study the charging mechanism of nc-Si embedded in SiO2. The results indicate that the charging effect mostly takes place inside the nc-Si rather than at the nc-Si/SiO2 interface.
DRNTU::Engineering::Materials::Microelectronics and semiconductor materials