Study On Several Critical Effects On The Quality Of Oxide Crystals Grown By Czochralski Method

Czochralski method,one of the most efficient and convenient method of crystal growth,has been developed over a hundred years.High purity silicon with hundreds of kilograms,large sized YAG and GGG crystals are also grown by Cz method.With the rapid development of Cz method,people are focused on the optimization and promotion of this technology.The most important one is Automatic Diameter Controlling technology,laser range acquisition method,up weight method and down weight method,and so on,which raise the automatic level of Cz method,especially for the automation of manufacturing in industry.The convenience and efficiency of Cz method give the illusion that Cz method is so easy to operate that make people overleap the details.This often leads to the low crystal quality,even failure in crystal growth.The topic of my doctoral dissertation is based mainly on the crystal grown projects.My research is focus on the strange but crucial important factor,which is often been neglected.We explore the essence of the problem in depth,and give the corresponding solutions.Improving the quality of single crystal to meet the national defense and industrial applications is our unswerving pursuit of the goal.With the demand-oriented and single crystal-based idea,we constantly strive for excellence,stay owning what better than others,and do our best to get high quality crystals.In this doctoral dissertation,I picked out several typical and representative cases in our research work of oxide crystal growth by Cz method.The dissertation focuses on the efforts of trace moisture,oxygen and melting viscosity on crystal growth.The main contents of my doctoral dissertation are as follows:A.The effect of trace moisture in crystal growth environment on the crystal growthThe unit cell of C12A7 crystal consists of two molecules that constitute a positively charged lattice framework containing 12 cages with a free space.The two oxygen ions rancdomly occupy 2 out of the 12 cages and the rest of the cages are empty.compensating the positive charges of the framework.Each oxygern ion in the cage is coordinated with the six Ca2+ ions constituting the cage framework,and the distance between the Ca2+ and the O2-is 50%longer than that in CaO crystals.These oxygen ions are in a very loosely bound state and are called "free" oxygen ions:they can be replaced by monovalent anions such as OH-,F-,and Cl-.Free oxygen ions can be successfully replaced by active anion species unstable in the atmosphere,i.e.,O-,O2-,H-,and electrons.Because of these exceptional characteristics.C12A7 crystals are expected to be widely used in many fields,such as cold electron emission,active anion beam emission,optical writing of conductive wires utilizing photoconversion from insulator to conductor,and as various catalytic agent for chemical reactions.To realize some of these applications,specifically for electron and ion beam emission,high-quality single crystals are indispensable.However,in the early stage of growing Ca12Al14O33 crystals,the crystals were found to be severely cracked,while the cracking was confined to the surface of crystals.Initially,such phenomenon was believed been caused by the unsuitable temperature gradient,improper ratio of raw materials,phase transformation and so on.However,after adjusting the temperature field structure,control parameters,the purity of raw materials,sintering methods and growth atmosphere,the obtained crystals were all epidermal cracking.When the samples cut from the cole of crystal were annealed in different annealing atmosphere,the cracking phenomenon was also observed.This unique phenomenon was not found been reported in the literature,however,it is obvious that Ca12Al14O33 crystal is very sensitive to the atmosphere,in particular,the crystal with cage structure can react with many atmosphere.Combining with the root cause of the cracking of the crystals,it can be suggested that crystals react with the water of the atmosphere during cooling process,O2-in the cage was replaced by OH-,resulting in difference of the lattice constants because of the different concentration of OH-,which make the cracking occur during cooling.In the Czochralski crystal growth process,especially the growth of oxide crystals,people did not attach great importance to water vapor in the air and adsorption of trace water in the heat insulation materials and furnace wall,ignoring the existence and influence on the grown crystals.In our current experimental conditions,the Ca12Al14O33 crystals without cracking were not obtained,but the great influence of trace water molecules on the crystal growth was observed,which was never been paid attention before in our studies.It provides a very important reference and guidance for our future study of crystal growth.B.The influence of atmosphere on oxide crystal growth1336.6 nm laser has important application in the task of "27Al+ ion optical clock".Nd:LGGG crystal can generate the specific wavelength laser at 1.3?m by different Lu ion concentration.In addition,the Nd:LGGG crystal also has the potential application as high power laser.In considering the importance of Nd:LGGG single crystal in high power laser and "27Al+ ion optical clock" task,longer Nd:LGGG crystals were grown.Although our research group has rich experience in the growth of GGG crystal,it is still very difficult to obtain the long and high quality Nd:LGGG single crystal due to the increase of the materials and the growth period for the long LGGG(-9 days totally),the high quality crystal rod cannot be grown according to the previous temperature field structure and the growth condition.One of the most difficult problems that should be overcome is the volatilization and decomposition of gallium oxide in the raw materials for the crystal growth.In order to prevent the volatilization and decomposition of gallium oxide,a variety of atmospheres for the crystal growth,such as high purity Ar+2%O2,high purity Ar+50%CO2,100%CO2,1%O2+99%CO2 and so on,have been selected.The experimental results show that the high purity Ar+2%O2 and high purity Ar+50%CO2 can inhibit the volatilization and decomposition of gallium oxide effectively.But the latter atmosphere is easy to cause oxidation of iridium crucible,so we tend to using 50%CO2 as the growth atmosphere.But the latter is easy to cause oxidation of iridium crucible,so we tend to use the former as the growth atmosphere.Finally,we have successfully grown long Nd:LGGG crystal rods with high quality.A good continuous laser output at 1.06 nm was obtained.Using this crystal,Chinese Academy of Sciences obtained long-pulse,high-energy,narrow-linewidth Nd:LGGG laser at 1336.63 nm.In order to prevent the oxidation of iridium crucible as well as maintain the relative stability of the system,high melting point oxides grown by Czochralski method usually using an inert gas as the growth atmosphere.In addition to keeping the growth environment stable,the interaction between the melts cannot be ignored either.From the example of Nd:LGGG,we can see that adjusting the ratio of the growth atmosphere can play a multiplier role on the quality of the crystal.We can use the effect of the growth atmosphere in a more reasonable way.C.The effect of melt viscosity on the quality of grown crystalsCalcium aluminate silicate Ca2Al2SiO7(CAS)single crystal,also known as the mineral gehlenite,can be grown from a melt using the Cz method.CAS crystals have been studied for laser and persistent phosphor applications.In the literature,only a few papers reported on the growth of CAS crystals,and the crystals contain bubbles,wrapping and easy to crack.In view of the potential application of CAS,we try to grow CAS single crystal to solve the problems.In the early stages of the experiment,we also encountered the difficulties.The as-grown crystal has bubbles,inclusions,impurity,cracks and scattered particles obviously.By lower growth rate,the crystal quality has been significantly improved.The key parameter that affects the crystal growth is thelarge viscosity of the melt.In the crystal growth by Cz method,the melting temperature of the crystal is relatively fixed,and the melt viscosity cannot be reduced by heating.According to our experience,the melt viscosity of Ca2Al,SiO7 may be reduced if part of Al2O3 is replaced by Ga2O3.Therefore,Ca2Al(2-x)GaxSiO7(CAGS)crystal was grown.The results indicated that the piezoelectric properties CAGS is similar to that of CAS,but can be grown with higher speed and quality.Generally,to speed up the melt flowing,increasing rotate speed and increasing the temperature gradient are the common approach.We demonstrated that the melt viscosity of CAS can be reduced by doping.And single crystal with the high quality can be obtained in a more convenient way,which laid a solid foundation for high-temperature piezoelectric applications in the future.