The Study On The Electric And Optical Properties Of Phosphorus Or Boron Doped Si Nanocrystalline/SiO₂ Multilayers

Si nanocrystalline(Si NCs)based microelectronic devices,optoelectronic devices,and photovoltaic devices demonstrate favorable developing prospect.In order to modulate the electric and optical performance of Si NCs based devices,precisely controlling the properties of Si NCs has significant research implication.Among all of the methods,doping can notably change the optical and electric properties of Si NCs.This thesis mainly studied the optical and electric properties of phosphorus-doped Si NCs/SiO2 multilayer structures as well as the electric properties of boron-doped Si NCs/SiO2 multilayer structures.Firstly,the doped amorphous Si(a-Si)/SiO2 multilayer structures were precipitated by plasma-enhanced chemical vapor deposition.Subsequently,after the process of dehydrogenation and high temperature annealing,the doped Si NCs/SiO2 multilayer samples were obtained.The structures of the multilayers were characterized by Raman and transmission electron microscopy.The chemical constituents of the multilayers were measured by X-ray photoelectron spectroscopy.Also electron spin resonance was utilized to study the bonding configuration of the multilayers.On the aspect of electric properties,Hall system was utilized to measure the carrier concentration,Hall mobility,and conductivity of the multilayers at variable temperature.For the doped Si NCs,the size-dependent doping effect and the carrier transport behavior were systematically studied.On the aspect of optical properties,photoluminescence spectrum was used to investigate two light emission signals,which separately located in the visible spectrum and the infrared spectrum.The main research contents and results of the thesis are listed below:(1)Phosphorus-doped Si NCs/SiO2 multilayers were obtained with controllable Si NCs sizes and well crystallinity,in which significant size-dependent doping effect was observed:As change the Si NCs sizes,all of the conductivity,carrier concentration,and Hall mobility are obviously varied.When the sizes of Si NCs increase from 2 nm to 20 nm,the room temperature conductivity increases from 6×10-5 S/cm to 1034 S/cm,and the corresponding carrier concentration also enhances with 7 orders of magnitude.For the Si NCs with small sizes such as 2 nm and 4 nm,the conductivities of the sample increase with temperature,which exhibit semiconductor characteristics.For the Si NCs with large sizes such as 8 nm and 20 nm,the conductivities decrease slightly with increasing temperature,which behaves as the metal-like electric properties.(2)For the phosphorus-doped Si quantum dots with small size(2 nm),the photoluminescence signal at 820 nm,related to the recombination centers at the interface of Si NCs,is stronger than that of phosphorus-doped Si NCs with size of 4 nm.In addition,near-infrared(NIR)luminescence at 1250 nm can only be observed in the phosphorous doped Si NC s with with size of 2 nm.Furthermore,as the sizes of Si NCs increase,the carrier concentrations enhance remarkably,and the localized surface plasmon resonance absorption signal can be detected.With the sizes of Si NCs enlarging,the absorption becomes stronger and the corresponding peak shifts towards the higher wavenumber.For the larger size(8 nm)phosphorus-doped Si NCs samples,significant absorption peaks can be observed in the infrared(500-2000 cm-1)region.(3)Boron-doped Si NCs based multilayers with uniform sizes but different boron doping concentrations have been successfully obtained.The introduction of boron impurities can reduce the grain boundary barriers between quantum dots,which effectively improve the Hall mobility of the materials.For Si NCs with a nominal boron doping ratio of 1%,the room-temperature Hall mobility can reach as high as 8.7 cm2/Vs.In addition,due to most of the boron atoms in Si NCs cannot be effectively doped in Si NCs,boron dopants have higher opportunities to occupy at the grain boundary.(4)For the boron-doped Si NCs,variable temperature Hall effect measurements exhibit that the carrier transport mechanisms are dominated by Mott-variable range hopping(Mott-VRH)in the low temperature region(T<120 K).Whereas in the high temperature region(T>300 K),the carrier transport behaviors of the samples are mainly dominated by thermally activation.In the high temperature region,the mobility of carriers decreases with the enhancement of temperature.Furthermore,as increases the boron doping concentration,the activation energies of Si NCs are gradually reduced.In the transition region(120-300 K)between high temperature region and the low temperature region,the conductivities of boron-doped Si NCs are influenced by both of Mott-VRH and thermal activation conduction.(5)All of the structures of p-n junctions based on phosphorus or boron doped Si NCs/SiO2 multilayer and phosphorus-doped Si NCs monolayer have been successfully prepared.Compared with the doped a-Si NCs/SiO2 multilayers p-n junction,doped Si NCs enable the p-n junction to have higher conductivities,which also improves the forward current significantly.As the forward voltage is 5 V,for the phosphorus doped Si NCs/SiO2 multilayer based p-n junction with a nominal doping ratio of 10%,the total current of reaches 380 mA.In addition,the multilayers-based p-n junctions exhibit better rectification characteristics than a monolayer-based p-n junction.