Improve The Optical Quality Of KDP Crystal And Study The Growth Mechanism Of CMTD Crystal

Potassium dihydrogen phosphate (KH₂PO₄ or KDP) is one of the most ancient Non-linear Optical (NLO) crystals that people ever found and there have been more than 80 years for research on the KDP crystals. People show their considerable interest in investigating the dielectric properties of KDP-type crystals due to their promising applications in electrooptic and memory devices, optical communication, temperature control devices and ceramic industry. The key problems to be solved are centered on the two aspects: growing rapidly large aperture crystals and improving optical quality of KDP crystals. In the nineties of twenty century the crystal growth expert Zaiteva grown successfully 540×550mm² cross-section lager KDP crystals with the growth speed rate of 12-15mm/d using point seed technology and the first problem was solved. Point-seed technology has been used exoterically in America and Russia. But for the second problem it is difficult to work out an effective quality control operating discipline so far because of the complexity of crystal growth especially for the rapid growth KDP crystals whose optical qualities are clearly worse than that of crystals growing with traditional methods. The most important parameters of optical quality are optically homogeneity and the laser damage threshold.In my thesis, we studied the laser damage mechanisms of KDP crystals and gave some viewpoints of myself for how to improve optically homogeneity and the laser damage threshold from the crystal growth processes and post treatment methods, in which the primary coverage is as follows,1. The formation energies and the equilibrium concentration of vacancies, interstitial H, K, P, O, and antisite structure defects with P and K are investigated by ab initio total-energy calculations. The calculation results show that H, O, K point defects may be the dominant point defect at room temperature in KDP. The formation energy of O vacancy (5.25eV) is much higher than that of O interstitial (0.60eV).2. We have shown that interstitial oxygen has a much lower formation energy (0.60 eV) than an oxygen vacancy (5.25 eV), indicating that interstitial O is more easily formed in KDP than an O vacancy in the chosen reference state. First-principles total-energy calculations were also performed to study the electronic structures of the neutral and charge states of interstitial oxygen in KDP. An interstitial O atom in the neutral and -1 charge states induces defect states in the band gap. For the neutral O interstitial, the empty states are induced in the energy gap; the interstitial O bonds with a host O atom and attracts a neighboring host H atom generating an O-O-H complex. The addition of an electron breaks the O-O bond and an isolated O-H bond forms and defect states located around 1.4eV above Ef are induced in the energy gap. In sharp contrast, the addition of two electrons leads to the formation of an H₂O molecule in KDP simultaneously accompanying the breaking of two hydrogen-bonded chains, and there are no defects states induced in the energy gap. The results are contributed to explain the recently reported decomposition of KDP during laser-induced breakdown.3. We present the ab initio calculations results of K vacancy in KDP crystals. The electronic structures and formation energy as well as the relaxing configuration of K vacancy were detailed studied. The properties of Density of States and band structure on KDP with K vacancy were discussed. The formation energy of K vacancy was calculated to be about 6.5eV and much lower than K interstitial (13.07eV). The cell parameters increase due to K vacancy and enlarge the volume of cavum surrounded by neighbored eight O atoms to be 3.2%. That is in favor of the transference for K atoms and is benefit for the impurity atoms to fill the cavum in KDP. The increasing of mobility ratio of K induces the increasing of ionic conductivity. And therefore laser-induced damage threshed will be decreased.4. First-principles total-energy density-functional theory electronic structure calculations for the defect of Na substituting for K in KDP crystals are presented. The calculated formation energy for this defect is about 0.46eV. No defect states are induced in the energy gap, whereas two extra occupied states are induced in the valence band, which are located at-49eV and-21.5eV below the Fermi energy level, respectively.5. The effects of many factors on the temperature and fluid fields in the growth tank have been analyzed. The influential factor are: the diameter and height of the growth tank, sizes and rotate speed of the growing crystals, flow rate of the inlet and outlet, direction of the inflow current, the location of the outlet tube as well as the tape of the bottom of the growth tank.6. On the other hand, the temperature and fluid field of the annealing equipment of KDP crystals are also analyzed.CdHg(SCN)₄(H₆C₂OS)₂(CMTD) is a highly efficient organmetallic nonlinear optical (NLO)material for generating blue-violet light by laser frequency doubling. And it was grown from aqueous solution by a temperature lowering method. Its powder SHG intensity is higher than that of CMTC. In this thesis we investigated the microcosmic growth mechanisms of CMTD crystals, in which the primary coverage is as follows,7. A new dual elementary-steps growth mechanism was observed during our research on the CMTD crystals. Namely two types of elementary-step sources with different step height can be produced on the same crystal surface during the growth of some crystals. Furthermore the sum of these elementary-step heights is equal to the corresponding interplanar distance d_hkl. The heights of these elementary steps were determined by crystal structure as well as growth motif and they have no relation to crystal defects. The elementary steps referred in this paper are completely different to sub-step mentioned by Min in 1988. Only one step height h was included in normal growth rate formulas for the dominant crystal growth theories at present such as BCF spiral growth mechanisms and the classic two-dimensional nuclear growth modes. Without considering the effects of the dual elementary-steps h₁ and h₂ to the growth procedure, the classic formulas containing the parameter h (step height) should be modified when they are applied to the research on crystals growing with dual elementary-steps growth mechanism.8. Some fanciful cordis-shape screw protuberances were observed using atomic force microscopy (AFM) on the (001) faces of CMTD crystals. These cordis-shape protuberances are different to the Frank-Read circle arose from the anti-direction double screw dislocation. We suggested that the alternate growth of adjacent layers of these fanciful protuberances is associated with the structure domain formed in the step sources besides the 2₁ screw axis along the (001) direction. Many perpendicular nucleuses were observed on the terraces in both sides of the beeline extended from the cordis gaps. These nucleuses confirmed the beeline as the 90°structure domain. The experimental results showed that crystal structure and the orientation and intensity of the chemical bonds possess different crystals of unique rules of growth.9. During the research on the CMTD crystals using ex situ atomic force microscopy, we found a growth phenomenon with mono-terraced nuclear model. Namely the crystal was developed with only one terraced nuclear protuberance. This growth phenomenon cannot be explained by the classic two-dimensional nuclear growth modes. So we present a new terraced nuclear model for the complement of the classic crystal growth modes.10. The evolving of crystal habits of CMTD crystals during the growth was investigated using an inverted phase contrast micro-scope (PCM). At the same time the surface morphology on the (001) face was observed ex situ by AFM. The measured height of the elementary steps was about 1.411nm, which equals half of the cell parameter c. several kinds of spiral hillocks and defects were found, and the hollow cores of the hillocks created by dislocation sources were preliminarily analyzed. The formation mechanism of peculiar crotch-shaped and line-segment-shaped defects of CMTD crystals was discussed.11. Using in situ AFM we have succeeded in observing consecutive dynamic growth processes in CMTD crystals with a relatively high kinetic growth coefficient. Two different types of elementary-steps with step height h₁=0.69nm and h₂=0.72nm, respectively, were observed for the first time on the same surface of a crystal. Two differential types of straight edges formed on the top of the screw dislocation that were observed to be periodic. The growth velocity of the ACTIVE layers is about 8 times that of the LAZY layers and it is difficult to explain by means of existing crystal growth theory.