Monte Carlo Simulation Study Of Electron Yields From Solids

Intensive experimental studies have been carried out since the 1950s on several important aspects related to electron emission,especially on energy distribution,secondary electron yield(SEY)and backscattering electron coefficient(BSC).The data measured by different researchers are largely scattered as some of those data were not measured under the ultrahigh vacuum condition so that the surface contamination may influence SEY and BSC significantly through the change of work function and electron affinity.Due to the reason that the available data from accurate experimental measurements for clean surfaces are quite limited,the mechanism and process related to secondary electron cascade generation and emission are still not fully understood yet.Moreover,there is absence of data for compound materials while many of them have wide practical applications.Considering the fact that the theoretical modelling of electron-solid interaction has been advanced significantly in recent years,it is therefore very necessary to retrieve this study for compound materials and/or for elemental solids in order to derive more reliable theoretical data.In this thesis,a systematic Monte Carlo simulation has been performed based on the up-to-date Monte Carlo model for the investigation of the BSCs,SEYs and total electron yields for elemental and compound semiconductor materials at different incident electron energies.Calculations for the EXDDF,EMDDF,and their combining effect in DDF at different primary energies for the excited and emitted secondary electrons in these materials also studied in this thesis.Some semiconductors including GaAs have now been widely used for photovoltaic materials,the electron yields are important parameters for the scanning electron microscopic characterization and,particularly,the recently developed scanning ultrafast electron microscopic characterization of charge carrier characteristics near the compound semiconductor surface.The currently used energy production methods will not be able to satisfy the energy needs of humanity in the long run.In the absence of a rapid increase in energy storage efficiency,it is becoming increasingly urgent to develop an environmentally friendly solution to the new energy sources.One of the best solutions in the future would be the implementation of fusion power plants.Beryllium is a major candidate to the first wall material in fusion reactors,and one of the key elements in the realization of the fusion power plant.Although beryllium is extremely important in nuclear research and high-tech applications,there are very limited and old data available for studies of its interactions with charged particles,especially electrons,in the incident energy range of 0.1-100 keV.Only a few investigations exist for the determination of the electron BSC for Be and therefore it is considered in this work.In 1st chapter,we introduced a short overview of the interaction between the electron beam and solid.The different types of electron signals emitted during the interaction process are discussed in detail.The energy spectrum and different peaks are introduced.The development and basic principle of scanning electron microscope(SEM)and other different types of microscopes are discussed briefly.The effects of different factors on the emission of electrons are also introduced.2nd chapter presented a detailed explanation of the Monte Carlo model.A very complex transport process of electron solid interaction will produce secondary electrons.It will be very difficult to obtain the related spectrums and yields by using analytical formalism.It is therefore we need an updated Monte Carlo simulation for getting accurate calculation results of concerned physical quantities.The transport process of electrons in a solid is a combination of electron elastic or/and electron inelastic scattering between the electron and atom/electron of the sample.Different theories are available for the description of these two scattering.We considered Mott's elastic cross-section for the elastic scattering.For the description of electron inelastic cross-sections,we used the dielectric function approach introduced by Penn,where the ELF obtained with the optical data is contributed from phonon excitation,the interband transition of the loosely bound valance electrons and inner-shell electron excitations.We used the above model to perform the calculation for the below work:In 3rd chapter,an up-to-date Monte Carlo simulation model with scattering potential is used for the calculation of the electron BSC of beryllium at an impact energy range between 100 eV and 100 keV.Simulation of backscattering spectra was carried out with and without Auger electron emission.This Monte Carlo simulation physical model enables us to derive accurate theoretical values of the BSC for beryllium.They are found significantly smaller than the previous published experimental data,particularly at energies below 10 keV.To verify our simulation results,we have also calculated electron BSCs for amorphous boron and carbon.A comparison with the available experimental data shows reasonable agreement.Further simulation for Be sample covered by carbon/water thin layer has shown that the surface contamination with several atomic/molecular layers can drastically alter the measurement values at low energies.The low BSC values of beryllium are partly attributed to the extremely strong forward elastic scattering.In the 4th chapter,we aim at studying secondary electron emission from several compound semiconductor materials by using the up-to-date Monte Carlo simulation model.The semiconductors are important industrial materials;on the other hand,due to the existence of an energy gap the electron inelastic scattering process in semiconductors is quite different from that of metals.Therefore,it is also our focus to know if there is any significant difference in electron mean escape depth between semiconductors and metals.In the surface analysis technique,surface sensitivity of the emitted signal is an important issue.Due to the secondary electron cascade process,the understanding of the average escapes depth becomes difficult.Traditionally,escape depth calculation is approximately expected by inelastic scattering mean free path or the maximum escape depth.The two processes of secondary electron excitation and emission are covered with the new definition.We have performed a systematic Monte Carlo simulation of primary and secondary electron trajectories to predicate the mean escape depth of secondary electron emission for compound semiconductors.We have calculated the EXDDF,EMDDF,and their combining effect in-DDF at different primary energies for the excited and emitted secondary electrons in some semiconductor materials.The calculation leads to the primary energy dependence of mean escape depth whose values are found in the range of 0.4-1.4 nm for these materials.In the 5th chapter,we calculate SEY and BSC of several compound semiconductor materials by using the up-to-date Monte Carlo simulation model.When a primary electron beam incident into a solid material,electron signals of different types are generated and emitted from the surface of the material as a consequence of elastic and inelastic scattering of electrons.The majority of the emitted electrons are secondary electrons and backscattered electrons,which are defined according to their energy being less or greater than 50 eV,respectively.The secondary electron emission phenomenon plays a very important role in many technical applications,such as photon multiplier,scanning electron microscope,spacecraft,atomic clock,plasma display panel,magnetrons,and cross-field multiplier.The materials used for different applications are usually evaluated by the SEY value.A systematic study has been performed based on a Monte Carlo simulation for investigation of the SEYs,BSCs and total electron yields for compound semiconductor materials at different incident electron energies ranging 0.1-10 keV.We also considered the effect of the surface depletion layer due to oxidation on the electron yields.As experimental data on BSCa for these compound materials are not available,therefore,we have compared calculation results for compounds with experimental data for elements having the nearest mean atomic numbers.The simulation predicted much larger BSC values than the empirical Staub's formula.In the 6th chapter,the calculation of electron BSCs for iron and tungsten is performed for incident energy ranging 100 eV-100 keV.Our present simulated data are fit well with the experimental data for both Fe and W at high energies;however,these data are higher than the experimental data at low incident energies.We also considered the carbon and water with few atomic/molecular layers as surface contaminations to see the effect of these layers on the values of BSCs.We observed that surface contamination strongly affect the resultant values of the backscattering coefficients.We also found that the BSCs depend on atomic numbers and found higher for higher atomic numbers(for tungsten in the present case.).For the more explanation and understanding of the BSCs,we also calculated the backscattered electron energy spectra,angular distributions,and depth distributions.7th chapter summarizes the works in the thesis.