Zinc Germanium Phosphide Infrared Crystal Growth And Its Nature

Continuously tunable mid-infrared sources is of vital important in such field of application as infrared guidance, infrared countermeasures, optical communications, infrared remote sensing, infrared imaging, infrared rangefinder, laser collimation, environmental protection, biology, medicine and so on. Frequency conversion with nonlinear optical crystals, such as ZnGeP₂, CdSe, AgGaS₂, AgGaSe₂, CdGeAs₂, is an important method to convert existing sources to mid-infrared wavelengths generate mid-infrared laser. However it is critical to grow and research nonlinear optical crystals to achieve mid-infrared laser.Zinc Germanium Phosphide (ZGP) is considered to be one of the most promising infrared nonlinear optical materials. It exhibits high nonlinear optical coefficient, wide potential transmission range of MIR region, adequate birefringence for phase matching, high thermal conductivity, little thermal lens effect, stable physicochemical property, high performance of mechanical character, and easy to be processed, etc. After few decades of research, large size high quality zinc germanium phosphide crystal can be obtain by optimizing growth and anneal technics abroad. Research of growing zinc germanium phosphide crystal still at preliminary stage at home. Because of the military applications of zinc germanium phosphide crystal, the growth technics about zinc germanium phosphide crystal is in secret state. Because Russia, U.S.A. can offer a small amount of zinc germanium phosphide single crystals at present, the price of ZnGeP₂ crystal is very high. In this thesis, the synthesis of ZnGeP₂ polycrystalline, the growth, structure, defects and properties of ZnGeP₂ crystal are studied. The outline of this dissertation is as follows: 1. Synthesis of Zinc Germanium Phospide polycrystallineThe ZnGeP₂ has two volatile components, Zn and P. In the reaction procedure, high pressures can be generated by P element. ZnGeP₂ polycrystalline material was successfully synthesized by means of a direct reaction with 5N pure Zn, Ge and P4. The product was characterized by XRD, XRF and EDS. The effects of the field of temperature, the proportion of the elements, reaction time, reaction temperature, the vacuity and the thickness of the quartz ampoules on the final product were discussed.2. Crystal growthZnGeP₂ single crystals were successfully grown by using the Bridgman method under the condition of spontaneous nucleation.Since crystal growth is a complex process, the factors that affect quality of crystals during the growth were analyzed, according to the theories of thermodynamics and dynamics, including the field of temperature, the optimal technics, the shape of the quartz ampoules, and spontaneous nucleation. We also presented the resolvents according to the characteristics of these factors.3. Crystal Components, Structure and DefectsThe concentrations of Zn, Ge and P elements in as-grown ZnGeP₂ single crystal were measured by the X-ray fluorescence and energy dispersive X-ray spectrometer analysis method. The measured results are different are caused by the means of prepare sample and analysis method characteristic. The measured results indicate that as-grown ZnGeP₂ single crystal deviates from stoichimetric composition to some degree.The structure of as-grown ZnGeP₂ single crystal was studied by X-ray powder diffraction and four-circle diffraction methods. The X-ray powder diffraction confirmed that as-grown ZnGeP₂ single crystal belongs to the tetragonal crystal system, I42d space group with unit cell dimensions a=b=5.4661 A, and c=10.7031 A. The result of the four-circle diffraction method is nearly the same as the above.Defects in the as-grown ZnGeP₂ single crystal were studied under a optical microscope to chemical etched cut-slice and measured by high-resolution X-ray diffraction. The results of the observations and discussion of growth defects and formation mechanisms in as-grown ZnGeP₂ single crystal were discussed in this thesis. The main defects in as-grown ZnGeP₂ single crystals are dislocations and sub-grain boundaries. These defects affected the perfection of crystals seriously.4. Crystal orientation and process The crystal orientation was determined by power and four-circle X-ray diffract ometers. It can be concluded from the orientation results that as-grown ZnGeP2 crystal grew along about c direction. The as-grown ZnGeP₂ crystal often cracks. The direction of eracture is vertical to (001).5. Basic PropertiesThe density, hardness and thermal properties of the as-grown ZnGeP₂ crystal were measured. The influences of these properties on crystal growth and applications were also discussed.The density of ZnGeP₂ crystal measured by using the buoyancy method at room temperature is 4.169 g.cm^-3.The Mohs hardness of ZnGeP₂ crystal was about 6.5.The weight of as-grown ZnGeP₂ crystal increases during heating process according to the result of differential thermal analysis and therrno-gravimetric analysis. The reason of weight increased was discussed. The thermal expansion of ZnGeP₂ single crystal was measured by using a thermal dilatometer. The average linear thermal expansion coefficient for the a direction and c direction were calculated in the temperature range from 298.15 to 764.15K, which values areα_a=2.4138×10^-6/K andα_c=35.1063×10^-6/K. The influences of these thermal properties on crystal growth and optical applications were also discussed.The transmission spectra of ZnGeP₂ were measured with Hitachi U-3500 and NEXUS670 FTIR spectrometer at room temperature and the results show that ZnGeP₂ single crystal has a wide spectral transparency range which is 1800nm～11947nm. The wide optical transparency region makes ZnGeP₂ crystal suitable for to achieve continuously tunable mid-infrared laser. Low transmission rate and high absorbing coefficient owned to poor crystal quality and process technics.