Structural Evolution And Optoelectronic Properties Of Metal Doped Amorphous Silicon Thin Films

Crystalline silicon?c-Si?has been a fundamental material for micro-electronics industry because of its excellent electronical properties,mature processing technology and low cost.However,limited by its indirect bandgap and fixed optical constants,the optical properties of c-Si are poor.Metallic doping,which can bring localized electron states such as deep level defects and bound excitons,is an effective way to achieve band modification of Si.Amorphous silicon?a-Si?holds the specific optoelectronic properties which c-Si does not have,such as controllable bandgap,variable refractive index and high absorption coefficient.Moreover,a-Si has higher solubility of metallic impurities than c-Si,making it a better Si-based host material for metallic doping.So,in this paper we take metal doped a-Si thin films as the direction of research.Efforts have been focused on the study of amorphous silicon ruthenium(a-Si_1-x Ru_x)thin films and amorphous silicon silver(a-Si_1-x Ag_x)thin films.Multiple deposition technologies and characterization methods have been used for deep researches on the variations of micro-structures and optoelectronic properties as well as the mechanism of these variations.Meanwhile,simulation designs have been applied for the exploration of potential applications of these films.The details are as follows:1. A-Si_1-x Ru_x thin films have been deposited by RF co-sputtering.The variation of a-Si network with Ru concentration and the distribution of Ru atoms in the as-deposited films have been thoroughly studied by multiple characterization methods.The structural changes of a-Si network and nanocrystals after annealing treatment have been carefully analyzed as well.Hydrogenated amorphous silicon ruthenium(a-Si_1-x Ru_x:H)thin films have been prepared by introducing H₂ during co-sputtering.The relationship of Si-H bonds and Ru concentration has been clarified.Results show that the short-range order and medium-range order of a-Si network decrease in different degrees with the increase of Ru concentration.Ru atoms can exist both in the amorphous network as substitutional atoms and in the precipitated ruthenium silicide nanocrystals.Phase change of ruthenium silicide nanocrystals is caused by high annealing temperature,and these ruthenium silicide nanocrystals make a-Si transfers into nc-Si through metal induced crystallization process.In a-Si_1-x Ru_x:H thin films,Ru atoms capture dangling bonds at the innerface of voids,resulting in the decrease of H content.The remaining H atoms mainly hold isolated Si-H bonds configurations in the bulk.2.The activation energy,resistivity and temperature coefficient of resistance?TCR?of a-Si_1-x Ru_x thin films have been studied combining with the obtained results of micro-structure.The optical models of thin films have been set up using spectroscopy ellipsometry?SE?to analyze refractive index,extinction coefficient and dispersion relation.The optical bandgap has been optimized as well.To achieve high infra-red absorption in limited thickness of thin film,two-dimensional disordered media has been created on the micro-structured surface of Si substrate.Results show that the activation energy and resistivity of a-Si_1-x Ru_x thin films decrease with the increase of Ru concentration.By controlling Ru concentration,the thin films can obtain both low resistivity and high TCR.Meanwhile,refractive index and extinction coefficient of a-Si_1-x Ru_x thin films increase with the increase of Ru concentration while optical bandgap decrease with Ru concentration.The absorption coefficient of a-Si_1-x Ru_x thin films is higher than that of Si Ge in the infra-red range.Two-dimensional disordered media containing random distributed nano-holes arrays can bring in-plane multiple scattering of optical modes to achieve broadband absorption enhancement.Base on this unique structure,a-Si_1-x Ru_x thin films could realize high absorption and avoid unnecessary carrier recombination.3.A-Si_1-x Ag_x thin films have been deposited by RF co-sputtering.The influence of Ag atoms on a-Si network and the morphology of embedded nanocrystals have been analyzed according to the features of Si-Ag system as eutectic alloy.The resistivity variation range of a-Si_1-x Ag_x thin films with different Ag concentration has been studied and compared with the on/off ratio of Ag/a-Si memristor.With SE methods,the dispersion relations of a-Si_1-x Ag_x thin films have been studied in details.The mechanism of the optical constants variations and potential application of a-Si_1-x Ag_x thin films have been clarified.Photoluminescence?PL?of a-Si_1-x Ag_x thin films in room temperature has been achieved with the deep level defects of Ag in Si.Results show that Ag atoms in the films cause disorder in a-Si network and form nanocrystals distributing uniformly in matrix.The resistivity of the films decreases with the increase of Ag concentration.Its variation range covers the typical on/off ratio of Ag/a-Si memristor.The variation of refractive index can be seen as a superposition including a broadband increase and an additional anomalous dispersion in visible range.The extinction coefficient increases with Ag concentration and an absorption peak is found around 700nm.Meanwhile,the transmittance decreases with Ag concentration in visible,near infra-red,far infra-red and THz range.Room temperature PL in near infra-red range is achieved.With the increase of Ag concentration,the PL intensity increases at first but then decreases while the location of PL peak appears red shift.4.With plasma enhanced chemical vapor deposition?PECVD?and sputtering technologies,Ag/a-Si/p-Si memristor structure has been built and its resistance switch property has been studied.Optical switch and optical readout synapse based on a-Si_1-x Ag_x thin film memristor have been designed,verified and optimized through simulation software.Results show that the resistance switch of Ag/a-Si memristor is influence by device area and preparation technologies and related to the voids and density of a-Si thin film.By optimizing structure features,the new optical switch shows low insertion loss and high extinction ratio.The optical readout synapse can read the synaptic weight through the variation of light intensity.By utilizing the dispersion of a-Si_1-x Ag_x thin film,a single node can achieve multiple channels in different wavelengths.