Ultrafast Dynamics Of Nanodiamonds And Silicon Semiconductor Junctions Based On Ultrafast Electron Microscopy

Ultrafast electron microscopy(UEM)provides ultrafast temporal resolution by combining an ultrafast laser system with an electron microscope with nanoscale spatial resolution.It allows one to visualize the ultrafast dynamic process of materials excited by ultrafast laser pulses.Ultrafast electron microscopy could be used to study charge transport,recombination,and electron-phonon coupling processes of materials.It also is of great significance to understand the internal mechanism of energy transfer and transformation on the surface and also inside the materials.In this study,the lattice dynamic process of nanocrystalline diamonds excited by femtosecond laser pulses is observed using ultrafast transmission electron microscopy(UTEM)to study the electron-phonon coupling mechanism and the lattice thermal expansion process in nanodiamonds.It reflects the energy transfer and transformation mechanism in nanodiamonds;The carrier transport process on the surface of semiconductor junctions excited by a femtosecond laser pulse is visualized using the ultrafast scanning electron microscopy(USEM)to probe the surface carrier dynamics of the silicon semiconductor junction.The carrier transport characteristics of different types of silicon semiconductor junctions excited by a femtosecond laser with different fluence are explored and modeled using the ballistic carrier dynamics model.The main research contents include:(1)Ultrafast transmission electron microscope equipment:The transmission electron microscope is converted into an ultrafast transmission electron microscope with ultrafast time resolution,which is divided into three important steps:conversion of the electron gun chamber and sample chamber of the transmission electron microscope,ultrafast laser system and photoelectric connection(the connection of laser system and transmission electron microscope).The structure of the transmission electron microscope is modified to introduce pump laser and probe laser;The ultrafast laser system is divided into two parts:the pump laser and probe laser systems.The pump laser is used to excite the sample to initiate ultrafast dynamics,and the probe laser is used to excite the photocathode to produce ultrafast electron pulses.The ultrafast dynamics of the sample are obtained by controlling the time delay between the pump laser and the probe laser;The photoelectric connection links the ultrafast laser system with the modified transmission electron microscope to ensure that the pump laser accurately excites the sample and the probe laser accurately excites the photocathode.(2)Ultrafast lattice dynamics of nanodiamonds:The ultrafast lattice dynamics of nanodiamonds in femtosecond to nanosecond time scale excited by a femtosecond laser pulse is studied by using the electron diffraction function of the ultrafast transmission electron microscope.The results show that the relaxation time of nanodiamonds is about 8μs and the phonon-driven lattice expansion time constant is about 4～5 ps under the excitation of femtosecond laser pluse with 80 m J/cm~2 fluence.There are significant differences in the expansion rates of(111)and(220)crystal planes excited by the femtosecond laser under the same laser fluence,indicating that there is anisotropic expansion of lattice in nanodiamonds.The theory of lattice dynamics shows the relationship between lattice expansion and lattice anharmonic vibration.The results indicate the difference between the lattice expansion and the elasticity of nanodiamonds.The physical mechanism of anisotropic expansion of nanodiamonds is expounded,which is of great significance for understanding the energy transfer and transformation of nanodiamonds.(3)Ultrafast carrier dynamics of silicon semiconductor junctions:The ultrafast carrier dynamics of p-n type silicon semiconductor junctions excited by high fluence(40 m J/cm~2)femtosecond laser was observed by ultrafast scanning electron microscopy.It is found that the plasma waves formed due to carrier density oscillations are obviously different from the previous simple expansion at low fluence(1.28 m J/cm~2).In order to analyze the carrier oscillation phenomenon excited at high fluence,a carrier dynamics model including the Coulomb interactions between carriers(electrons and holes)and the actual potential field of the depletion zone is proposed.The calculation results show that the carrier density increases significantly under the excitation of a high fluence femtosecond laser,and the Coulomb interaction between carriers plays an important role.The oscillation frequency and propagation velocity of plasma waves increase with the carrier density,which is directly proportional to the femtosecond laser fluence(K represents laser fluence).The carrier dynamics model illustrates the mechanism of carrier oscillations and plasma wave propagation on the surface of p-n type silicon semiconductor junctions excited by a high fluence femtosecond laser.The frequencies of the plasma wave oscillations are found to be in the THz range,indicating potential applications as a new THz source.(4)Regulation of ultrafast carrier transport in silicon semiconductor junctions:Two different methods are used to realize carrier transport regulation.Firstly,a forward or reverse DC voltage bias is applied on the two sides of p-n type silicon semiconductor junctions.The calculated results of the carrier dynamics model show that when a reverse bias is applied,the carrier oscillation amplitude is significantly enhanced compared with p-n junction;Secondly,two different types of semiconductor junctions are investigated,including the p-n-p type silicon semiconductor junction and the p-n-p junction sandwiched by an insulating layer on each side(quantum well junctions).The model calculation results show that the p-n-p junction well confines the movement of electrons inside the n-type region.Compared with the p-n junction,the oscillation frequency and propagation velocity of the plasma waves on the p-n-p junction increase.The p-n-p junction sandwiched by insulating layers limits the movement of electrons and holes inside the n-type and p-type regions,respectively.Compared with the p-n junction,the oscillation frequency of the plasma waves increase,and there is an obvious echo phenomenon in the p-type region,indicating stronger radiation performance.The carrier transport regulation realized by different approaches shows that the propagation of the plasma waves can be effectively controlled,which indicates this terahertz radiation method induced by femtosecond laser on silicon semiconductor junctions is more flexible and effective.