Investigation On Luminescent Properties Of A-SiN_x Films And Charge Storage Effect Of A-SiN/nc-Si/a-SiN Structure.

Recently,Si-based amorphous thin films and nc-Si embedded in amorphous films have been extensively studied due to their potential application in many kinds of nanoelectronic and nanophotonic devices,such as memory devices,solar cells and Si-based light emitting devices.This thesis studied the properties of amorphous SiNx(a-SiNx)films and nc-Si embedded in a-SiNx films.The first part was about the luminescent properties of a-SiNx films.We fabricated a-SiNx films with different N/Si ratios,and investigated the relations between luminescent lifetimes and N/Si ratio through time-resolved photoluminescence spectra.Then,we measured the photoluminescence spectra collected in the various time scale.Further,the recombination mechanism was discussed.Based on these studies,electroluminescence devices were fabricated and the brightly tunable light was achieved at room temperature.The second part focused on the properties of nc-Si based floating memories.We first fabricated nc-Si embedded in amorphous SiN matrix by thermal annealing a-SiNx films.The optical and electronic properties were studied for nc-Si embedded in a-SiNx films.The carrier transport process was investigated and discussed.Moreover,we prepared the a-SiN/nc-Si/a-SiN sandwiched structures.The charging characteristics and the passivation effect of amorphous SiN on nc-Si were discussed.The main results of this thesis are listed as follows:(1)We prepared luminescent amorphous SiNx films with various N/Si ratio.The tunable photoluminescence behaviors were observed.By controlling the N/Si ratio from 0.58 to 0.81,the luminescent peaks were changed in the spectral range of 540nm to 690nm.The large Stokes shift between the absorption edge and luminescent peak indicated that the origin of luminescence signals in the present amorphous SiNx films was associated with the localized states in the gap.The photoluminescence spectra collected in the nanosecond and microsecond time range were identical for each samples,suggesting that the fast and slow PLs were originated from the same luminescence center,which was the localized O-Si-N related states as reported previously.So,we concluded the PL dynamics of a-SiNx films as(1)fast process:carrier relaxes rapidly from band tail states to localized O-Si-N related states in several nanoseconds,and then(2)slow process:carrier recombines and emits PLs in the localized O-Si-N related states in microseconds.(2)On the basis of investigation of photoluminescence,the electroluminescence devices were fabricated based on the a-SiNx films with various N/Si ratio.It was found that the turn-on voltage was as low as 4V.by reducing N/Si ratio of the films,the injection current density was enhanced significantly which was attributed to the increased film conduction as well as the reduced injection barrier between Si and a-SiNx film.As a consequence,the electroluminescence intensity was enhanced by 5 folds.Our present work suggested that the tunable a-SiNx luminescence characteristics could be achieved by controlling the film composition,which provided the great advantage to improve the performance of Si-based light emitting devices.(3)Based on the deposition of Si-rich SiN0.52 films,we found that nc-Si with size of about 4nm was formed after annealing at 1000?.The room temperature conductivity was increased by over three orders of magnitude compared with that of as-deposited one due to the formation of high conductive nc-Si dots,while the carrier transport process in the present case was dominated by the trap-assisted tunneling mechanism.A large memory window up to 7V and the charge density of 1012/cm2 were achieved by obtaining a-SiN/nc-Si/a-SiN floating gate structures.Frequency-independent C-V measurement indicated that amorphous SiN could well passivate the interface states of formed nc-Si,which can improve the charge storage effect obviously in nc-Si/SiN system.