Semiconductor Nanocrystals Embedded in SiO_2 for Nanophotonic and Nanomemory Devices

摘要

Structures consisting of Si nanocrystals embedded in SiO_2 are candidates for optoelectronic and nanophotonic applications. A Si-based optoelectronic technology is being pursued to combine optical and electronic functionalities in the same monolithic circuits. The advantages of the Si/SiO_2 composites over other materials rely mainly on their robustness and surface stability in comparison with porous materials and on their full compatibility with mainstream CMOS technology. The opening of the band-gap of Si due to quantum confinement is nowadays an unquestionable fact. The photolumines-cence (PL) emission of Si nanocrystals in SiO_2 consists of an intense and wide spectral emission peaking in the near infrared or the visible spectrum. It is at large admitted that there exists a direct correlation between the structural (average nanocrystal size, density and volume fraction) and optical properties (band-gap and PL emission) of the composites. Thus, it is possible to tune the spectral emission of the material almost over the whole visible spectrum. On the other hand, MOS structures with Si nanocrystals have recently been investigated due to their promising applications in nano-memory devices. Charge storage effects are observed for the nanocrystals which are located close to the SiO_2/Si interface. The charge and discharge is performed by direct tunneling from the Si substrate and is controlled by the voltage applied to the gate. These devices have potential advantages in relation to the current EEPROM technologies, which are related to their simpler structure, lowering of write/erase voltage window, possibility to increase endurance and retention times, and the ability to achieve single electron devices by charge storage in a single nanocrystal. In this work we report the ion beam synthesis and characterization of Si nanocrystal/SiO_2 structures for nano-photonic and nanomemory device applications. Ion beam synthesis has already been reported as one of the most promising technologies for such applications. This is due to the high repetitivity and ease of control inherent to any ion implantation based technique, together with the high structural quality of the SiO_2 matrix and the good Si/SiO_2 interface passivation achieved after high temperature annealing. The detailed characteriza- tion performed on the synthesized structures has allowed us to propose a new model for the optical emission of the Si nanocrystals. Besides, this process has been successfully applied to the fabrication of thin film MOS based devices with memory effects. The obtained data demonstrate the potential of ion beam based technologies for nanomemory device applications.