Single Crystal Growth Of Large Size CdSe For Manufacturing Of Mid-infrared Wave Plate

The high precision polarization observation in the 8-12.5μm band of mid-infrared is the trend of future development,and the wave plate plays an irreplaceable role as the core element of polarization observation.Therefore,it is of great significance to manufacture the mid-infrared wave plate with high delay accuracy and it is also a scientific problem that needs to be solved urgently.Cadmium Selenide（Cd Se）single crystal is a II-VI compound semiconductor crystal material with good optical properties.It has wide infrared transmittance band（0.750-25μm）,high transmittance（about 70%）and suitable birefringence index,which is an ideal material for preparing mid-infrared wave plates.In this paper,a price of high quality Cd Se single crystal with diameter of 37 mm and length of 60 mm was grown by the modified vertical unseeded sublimation method.The XRD test results showed that the experimental growth Cd Se single crystal was of high purity,its crystal structure was the six-party wurtzite,through Raman further verify it was for wurtzite structure;EDS test results showed that the components of Cd Se crystals grown by experiment had little deviation,and the elements of Se and Cd were close to the stoichiometric ratio.The（001）plane was accurate orientation by X-ray diffractometer,and the Cd Se wafer with the size of 20 mm×20 mm×5 mm was cut along the optical axis.The polishing treatment was very important to improve the performance of the Cd Se wave plate.The p H value of the polishing solution was adjusted by weak acid hydrochloric acid.After the polishing solution was polished under the acidic condition of p H=5,the thickness of the wave plate was 3.0786 mm,and better surface parameters were obtained,the average surface roughness was Ra=0.73 nm.After polishing,the Cd Se wafer was annealed at 650°C for 48h in Se atmosphere.The infrared transmittance of the annealed Cd Se wafer in the range of 6-12.5μm was increased from 50%to 69%,and the absorption coefficient was about 0.04cm-1 after annealing.It was shown that annealing can improve the integrity of crystal lattice perfection and reduce the impurity content and free carrier concentration in the crystal.In order to further improve the infrared transmittance of Cd Se wave plate,YF₃ and Zn S were selected as antireflection coating materials,and Essential Macleod software was used to assist the design to optimize the optimal film system thickness.The thermal evaporation vacuum coating machine and particle beam assisted deposition were used for coating.The infrared transmittance test results showed that the transmittance of the coated Cd Se wave plate reaches 90%in the band of 6-12.5μm,and the highest transmittance at 10.5μm,with a peak of 99%.According to the design requirements of the solar spectral line polarization observation system,the delay accuracy of the wave plate at 12.4μm was required to be±1°.The phase retardation of Cd Se wave plate was measured by spectral scanning method.The measured results showed that the phase retardation was 90.71°at 12.4μm,and the phase retardation accuracy was 0.71°,which was indicated that the Cd Se wave plate met the design requirements of solar spectral line polarization observation.