The Study On Carrier Transport In Doped Si Nanocrystals/SiO₂ Multilayers

Silicon-based optoelectronic materials have always been the most important material in modern industry and information industry.With the continuous reduction of the size of the device,the research on nano-sized silicon material has aroused widespread concern,and since nano-silicon material presents lots of new physical properties,it has been applicated in both the silicon photonics devices and new generation of solar cells device.In order to further stabilize and improve the performance of nano-silicon devices,especially in optoelectronic integrated circuits,new generation of solar cells and floating gate memory applications,people need to dope the nano-silicon devices and explore its electrical properties and carrier transport properties.In this thesis,doped Si nanocrystals/SiO2 multilayers were prepared by plasma enhanced chemical vapor deposition(PECVD)and high temperature thermal annealing.The structural characterization was carried out by means of high resolution transmission electron microscopy,Raman,X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy.At the same time,we also investigated the effects of temperature,doping concentration,annealing temperature and grain size on the carrier transport properties of doped Si nanocrystals/SiO2 multilayers by Hall effect system.In addition,we also compare the similarities and differences of boron and phosphorus doping.The main contents and results of this thesis are as follows:1.We have successfully prepared the phosphorus doped Si nanocrystals/SiO2 multilayers of high-density,size-controlled and high conductivity up to 110S/cm.The layered structure and the formation of nano-silicon grains were confirmed by TEM.By XPS,we find that the phosphorus atoms doped into the nano-silicon are mostly in the form of Si-P bonds.The effective distribution of phosphorus atoms in nanocrystalline silicon grains was demonstrated by scanning EDX spectra of individual nano-silicon grains in the films.Finally,we demonstrate that part of the phosphorus atoms tend to passivate the surface dangling bonds by Raman spectroscopy and ESR,and part of them tend to dope into the nano-silicon grains in a substitutional way.2.The conductivity characteristics of phosphorus-doped Si nanocrystals/SiO2 multilayers were studied.We found that there are three different transport mechanisms in phosphorus doped Si nanocrystals/SiO2 multilayers between 40-660K by analyzing the conductivity-temperature curves.In the temperature of 40K to 80K,the acoustic phonon is frozen in low-temperature,so the carrier transport mechanism of phosphorus doped Si nanocrystals/SiO2 multilayers is Mott variable-range hopping transport mechanism.Between 80K and room temperature,the film behaves as multiple phonon hopping transport mechanism dominated by the weak interaction of carrier-phonon.At room temperature and the temperature above,the conductivity is described by the Arrhenius relationship which suggests the transport mechanism in phosphorus doped Si nanocrystals/SiO2 multilayers is thermally activated transport mechanism.3.We have further investigated the effect of phosphorus doping concentration on the photoelectric properties of Si nanocrystals/SiO2 multilayers.It was found that the Fermi level of the films gradually migrated with the phosphorus doping concentration.At the same time,we also found that the properties of Si nanocrystals/SiO2 multilayers meet the Mott criterion at high doping concentrations,and the film transits from semiconductive to metalloid states.We have confirmed the occurrence of this transition by theoretical model and measurement of mobility,carrier concentration and ESR integral intensity,respectively.4.We also explored the microstructure and electrical properties of boron doped Si nanocrystals/SiO2 multilayers.We found that the phosphorus atom can promote the crystallization of nano-silicon film while the boron atoms restrain the crystallization.In addition,the increase of annealing temperature also promoted the crystallization of nano-silicon film.Finally,we found the double activation energy caused by tunneling in small-sized Si nanocrystals/SiO2 multilayers.