The Effect Of Regulation And Modification Of Interface Layers On Improving The Performance Of Si Quantum Dots/Si Nanowires LED

Stable and efficient silicon based light source is of great importance to the issues of solid-state illumination,panel display,biological detection,photoelectric interconnection and so on.Because of the quantum size effect,the band gap of silicon quantum dots can be adjusted in the range of visible and infrared wavelength and corresponding room-temperature light emitting has been observed.Silicon quantum dots fabricated through confining the crystallization in multilayers are compatible with micro-nano processing technology,which have special advantages in device fabrication.However,compared to devices based on colloidal silicon quantum dots with good surface modification,at present reports about electroluminescent devices based on silicon quantum dots fabricated through confining the crystallization in multilayers focus more on the simple "back-side electrode/flat substrate/light emitting layer/front-side electrode" structures,which have a lower electro-optical conversion efficiency.Therefore,it is desirable to develop this kind of devices with more complex and advanced structures.In our previous work,it is found that compared to flat ones,the current injection efficiency and the light extraction efficiency are improved for devices with nano structures based on silicon quantum dots/SiO2 multilayers.Therefore,this paper has established the down-top structures of electroluminescent devices based on silicon quantum dots/SiO2 multilayers on nano structures(silicon nanowires),with band engineering and interface modification.The device photoelectric performances have been further improved.The main research contents and results of the paper are listed below:1.Silicon nanowires are fabricated with the metal-assist chemical etching method.Silicon quantum dots/SiO2 multilayers are deposited on silicon nanowires through the plasma enhanced chemical vapor deposition and subsequent high-temperature annealing.Silicon quantum dots/silicon nanowires hybrid electroluminescent devices are obtained with the back-and front-side electrodes.In order to suppress the influence of nonradiative recombination centers related to interface defect states between silicon nanowires and silicon quantum dots/SiO2 multilayers,a 7 nm ultra thin Al2O3 layer is inserted through the atomic layer deposition.The device current injection efficiency is obviously improved with Al2O3.When the injection currents are the same,the forward applied voltage becomes lower while the electroluminescence becomes intenser for devices with Al2O3.The highest electroluminescence intensity is 8.5-fold stronger than that of the original devices.Furthermore,metal/silicon nanowires/Al2O3/metal MOS devices are fabricated to quantificationally calculate the defect state density related to silicon dangling bonds with the conductive method and the chemical passivation effect of Al2O3 to silicon nanowires is correspondingly proved. Meanwhile,the field-effect passivation effect of Al2O3 to silicon nanowires is also proved by the capacitance-voltage curves of the MOS devices.These results give the reason of the improvement of device performances with Al2O3.2.Based on the research of Al2O3-passivated electroluminescent devices,a?30 nm Ag thin film is coated on the silicon nanowire/Al2O3 substrates through magnetron sputtering.Ag nanoparticles,with the diameter of dozens of nanometers,are formed in silicon quantum dots/SiO2 multilayers during high- temperature annealing because of Ostwald ripening and coalescence.When Ag nanoparticles are introduced,both photoluminescence and electroluminescence performances are obviously improved.When the injection currents are the same,the applied voltage is greatly reduced with Ag nanoparticles for electroluminescence.We have proved that the localized surface plasmon resonance effect introduced by Ag nanoparticles can distinctly enhance the absorption cross section of nanowire devices,which improves the photoluminescence performances.Meanwhile,more conductive paths are established in silicon quantum dots/SiO2 because of the existence of Ag nanoparticles and the current injection efficiency is correspondingly improved.Moreover,we also have proved that Ag nanoparticles can promote the wave-guide effect of nanowires and improve the light extraction efficiency,with the finite-different time-domain method.Therefore,both electrical and optical properties of devices are improved by Ag nanoparticles.3.Based on the research of Al2O3-passivated electroluminescent devices,we also try to optimize the top-side electrode.After preparing silicon quantum dots/SiO2 multilayers,a?10 nm Au layer is coated on the device through magnetron sputtering.Then,the top-side ITO electrode is prepared.By doing this,the current injection efficiency is improved.When the Au buffer layer is inserted,the threshold voltage of Fowler-Nordheim tunneling is obviously reduced,leading to the improvement of device performances.Furthermore,considering the large difference between the Fermi level of Au and the conduction band bottom of SiO2,a TiO2 electron transport layer is inserted between Au and silicon quantum dots/SiO2.The photoelectric properties of TiO2 are also measured and the weak n-type conductive property of TiO2 is analyzed through XPS.Meanwhile,through the electroluminescence performance and the reverse current-voltage curves,TiO2 is also proved to be able to act as the hole barrier layer,which can confine holes in silicon quantum dots and enhance the radiative recombination possibility.The highest electroluminescence intensity is 2.4-fold stronger than that of the original devices under the same applied voltage.4.Initial researches on the photoelectric properties of boron doped silicon-rich silicon carbide thin films are also carried out.Silicon quantum dots with the diameter from several to more than a dozen nanometers are formed and embedded in amorphous silicon carbide after high-temperature annealing.It is found that the crystalline degree is dominated by silicon/carbon ratios and boron doping plays a subordinate role in crystalline.Correspondingly,the optical band gap,Hall mobility,carrier concentration and conductivity are also affected by silicon/carbon ratios and boron doping concentrations.The grain boundary energy level plays an important role in the carrier transport and the optical band gap.The conductivity can be reached to 91 S cm-1 and the corresponding optical band gap is 2.7 eV.Based on the photoelectric properties of boron doped silicon-rich silicon carbide,we try to exploit it in silicon quantum dots/silicon nanowires light emitting devices.When boron doped silicon-rich silicon carbide is inserted between silicon nanowires and silicon quantum dots/SiO2 multilayers,device photoluminescence and electroluminescence performances become obviously better.We initially think that boron doped silicon-rich silicon carbide may has the "field-effect passivation" effect to the nanowire devices.However,the mechanism still needs to be further discussed.