Numerical Simulation And Defect Analysis For The Growth Of Alkaline-earth Fluorides

CaF₂ crystals are the most important lens materials for laser lithography because of its high transmittance in VUV region.Furthermore,alkaline-earth fluorides can also serve as laser host materials which can be doped with various rare-earth ions.Neodymium doped Ca F₂ and Sr F₂ crystals are suitable for working as high-energy pulsed laser amplifiers for generating high-power ultrafast lasers.Extreme optical properties are required for laser lithography and high-power laser,so it is important to improve crystal quality to meet the requirement.Main works conducted on this thesis are as follows:The effect of point defects on crystal properties were studied.The main reason for the UV absorption was identified as Ce³⁺and Pb²⁺impurities,the former stem from the impureness of raw materials and the latter stem from the residual of oxygen scavenger.The segregation of solute in Ca F₂ and Sr F₂ crystal were investigated.It turns out that Nd³⁺and Gd³⁺ions were uniformly distributed in Ca F₂ crystal.However,the concentration of Nd³⁺and Gd³⁺ions decrease obviously in Sr F₂ crystal with the increase of solidification fraction.The effective segregation coefficient of Nd³⁺and Gd³⁺ions are 1.489 and 1.233 in 0.3%Nd,5%Gd:Sr F₂crystal.On the other hand,point defects can also have positive effect on crystal performance.By co-doping with two different types of regulating ions,the absorption spectra and emission spectra could be regulated controllably.The CW tuning range from 1043nm to 1077nm was obtained,which means that the tri-doped Ca F₂ crystals are suitable for the ultrafast mode-locked laser output.The causes of striations and scattering particles were investigated.It was found that two kinds of striations exist in the crystal,namely,parallel striations and vein-like striations.The former origin from the slip bands of dislocations and only exist in undoped Ca F₂ and Sr F₂ crystals,and the latter is related to the sub-grain boundaries.The slip bands are mainly induced by thermal stress,and the sub-grain boundaries in doped crystals can attribute to the cellular growth of the crystal induced by constitutional supercooling.It was found that there are many scattering centers in the crystal,and the scattering on the grain boundary is more serious.The reason for the formation of scattering centers remains obscure and need to be studied further.The temperature field,flow field and stress field in the process of Ca F₂ crystal growth were simulated using CGSim software.The effect of the internal radiation in crystal and melt to the temperature field,flow field and interface shape were investigated.Then different temperature fields were designed by changing the structural parameters of low temperature zone and gradient zone.It was found that changing the thickness of diaphragm is a rational method to obtain high temperature gradient near the interface while maintain low heater power and low thermal stress in the crystal.