Study On Defect Evaluation And VGF Growth Technique Of CdZn Te Materials

Mercury cadmium telluride(Hg_1-yCd_yTe,HgCdTe)is an excellent material for preparing high performance infrared detectors,which have important applications in the fields of weather forecast,resource exploration,astronomical observation,and so on.Cadmium zinc telluride(Cd_1-xZn_xTe,CdZnTe)is the optimum substrate material for high quality HgCdTe thin-film epitaxy as it can match with HgCdTe's lattice perfectly through adjusting Zn composition.Although HgCdTe epitaxy technique on alternative substrates has made much progress in recent years,it is mainly employed to fabricate SWIR(Short Wavlength Infrared)and MWIR(Medium Wavelength Infrared)detectors working in large thermal radiation.High performance LWIR(Long Wavelength Infrared),VLWIR(Very Long Wavelength Infrared),dual-band,avalanche photodiode(APD)and high operating temperature(HOT)HgCdTe detectors still need to use low defect density epitaxial materials grown on CdZnTe substrates.The epitaxy using CdZnTe substrates with large size and low defect density is still the mainstream technique for the fabrication of advanced HgCdTe infrared focal plane arrays(IRFPA).3^rd generation HgCdTe IRFPA technology has high requirements for the size and quality of CdZnTe materials.Howerver,it is quite difficult and costly to obtain such substrates.The difficulties come from the high density dislocations and second-phase defects produced by the crystal growth process,and the instability of the growth.Due to the limitation of measurement methods,there are few researches on the structural characteristics of CdZnTe defects,such as the structures of extended defects,the extending directions of the dislocations,and the features of second-phase defects in micrometer and sub-micrometer scales in CdZnTe materials,which makes it difficult to analyze the influence of CdZnTe crystal growth conditions on the defect properties comprehensively.In aspect of growth technology,vertical gradient freezing(VGF)method has become an advanced technique for the growth of CdZnTe single crystal.However,vertical Bridgman(VB)method is still the mainstream technique to manufacture the domestic CdZnTe substrates.There are few reports on VGF growth of CdZnTe crystals,and the technique is still immature.In this dissertation,the extended defects and second-phase defects in CdZnTe materials were studied by etch pit real-time observation technique(EPRTO)and infared transmission microscopy(IRTM)respectively.Some new measurement methods and characterization parameters were proposed for the evaluation of the defect properties of CdZnTe materials.VGF growth method of CdZnTe crystal using seeds with specific crystal orientation was explored.The heat transfer,melt flow,Zn transport and segregation during VGF growth process were computed with numerical simulation software.CdZnTe ingot with diameter of 90 mm and percentage of monocrystalline up to 90%was obtained by the seed-oriented VGF growth technique.The main research contents and innovative results obtained in this dissertation are summarized as follows:(1)The geometric kinematics describing the evolution process of etch pits on the surface of CdZnTe substrate was studied.The experiment results show that most of the tips of the etch pits on CdZnTe(111)A surface developed by Nakagawa etchant migrate along a straight line.There are three different types of pyramidal etch pits which correspond to three different kinds of orientated dislocation lines in the material.The study shows that the connecting line from the center to the tip of the etch pit can not accurately represent the extending direction of the dislocation line.The extending direction of the dislocation line should be determined by the displacement vector of the etch pit tip.The measurement method and calculation equations of the displacement vector of the etch pit tip in crystallographic coordinate system were built.By using the kinematics parameters of the dynamic etch pits,the crystal orientations of the dislocation lines corresponding to the three types of Nakagawa etch pits were obtained,which were<110>,<112>and orientations between<110>and<112>.The experiment results also show that the projections of the extending direction of the defect and the connecting line from the center to the tip of the etch pit on the substrate surface coincide so that the extending direction can be determined by the geometric structure of the pyramidal etch pit.(2)The orientation distributions of the extended defects in CdZnTe crystals were studied.By plotting the tip displacement vectors of Nakagawa pyramidal etch pits on the(111)A surface on the[111]pole figure,the orientation distributions of the extended defects in the samples were obtained.It is found that the extending directions of dislocations revealed by Nakagawa etchant are just located near the three connecting lines of[110],[101]and[011]on the[111]pole figure.It means that Nakagawa etchant can not reveal dislocations passing through CdZnTe(111)A substrate surface at larger angles,and the pyramidal etch pit density(PEPD)is not equal to the dislocation density of the material.It is also found that the extended defects in CdZnTe material can have inhomogeneous and asymmetric orientation distribution.In addition to using the orientation pole figure,some parameters can be also introduced to describe the property of the extended defects in CdZnTe materials.These parameters include the ratio of the dislocation densities on different{111}planes(RDD),the density of the dislocations passing through the surface(DDPS)and the bulk density of the dislocation lines(BDDL).Calculation results show that DDPS₍₁₁₁₎is twice as much as PEPD₍₁₁₁₎on CdZnTe(111)surface,and BDDL is about4.2 times as much as PEPD₍₁₁₁₎for CdZnTe materials with uniformly distributed defects.(3)The characteristics of Everson etch pits on CdZnTe(112)B surface were researched.By analyzing the relationships between Everson etch pits on the(112)B surface and(111)B surface,the correspondence between Everson etch pits on CdZnTe(112)B substrate surface and the extended defects in CdZnTe materials is revealed.The etch pits on the(112)B surface with the shapes of nested triangle,trapezoid and rod correspond to the flat-bottom etch pits on the(111)B surface etched by Everson etchant,which originate from the bulk defects in the material or the residual etch pits after the dislocation defects terminate.Three typical kinds of pyramidal etch pits observed on the(112)B surface come from the dislocations with the extending directions in[011],[112]and[101].There are also some pyramidal etch pits originated from the dislocation defects with the orientations between[011]and[101].Experimental results show that Everson etchant used for CdZnTe(112)B substrate can only reveal the dislocations with the orientations located near the connecting line of[011]and[101]on the[112]pole figure.The relationship between the etch pit density on CdZnTe(112)B surface and the defect density was discussed.Calculation results show that PEPD₍₁₁₂₎on CdZnTe(112)B surface is 35%of that on the(111)B surface,DDPS₍₁₁₂₎is 6.2 times of PEPD₍₁₁₂₎,and BDDL is about 12.6 times of PEPD₍₁₁₂₎for CdZnTe materials with uniformly distributed defects.The difference between PEPD₍₁₁₂₎and PEPD₍₁₁₁₎for the same CdZnTe material can be explained by the inhomogeneous and asymmetric orientation distributions of the extended defects.(4)The measurement methods of second-phase defects in CdZnTe crystal were researched.A 3D inspection technique is developed based on the infrared transmission microscopy,which can precisely obtain the data of the positions and sizes{(x,y,z,d)}of the micrometer second-phase defects in CdZnTe substrates.The contrast transfer function(CTF)of the second-phase defects on the infrared transmission micrographs were analyzed.It is found that the maximum defocus distance where the second-phase defect is observable in the micrographs is proportional to the defect size.When microscope's N.A.and threshold of CTF were set to 0.45 and 0.02 respectively,the maximum defocus distances of the observable second-phase defects with different sizes were measured experimentally.The result shows that the ratio of the maximum defocus distance to the defect size is 23.4.Then the volume density and area density of second-phase defects at a focus position can be calculated according to their grayscales on the observed transmission micrograph at that position.Based on such technique,a detailed procedure to measure the distribution of the second-phase defects was established.In addition,some etch pits which disappeared very rapidly were observed on CdZnTe substrate surface during dynamic etching processes.It is confirmed that these shallow etch pits relate to sub-micrometer second-phase defects.According to the kinematic model of the etch pits,the sizes of these sub-micrometer defects are in the range of 0.15?m to 1?m,and the volume density of the defects is about 10⁸ cm^-3.This result is consistent with the previous result measured by the laser scattering topograph(LST)technique.It provides an economical and convenient way to test sub-micrometer second-phase defects in CdZnTe materials.(5)The growth technique of CdZnTe crystal by seed-oriented VGF method was studied.The nominal translation velocities of the temperature fields under the control modes of constant gradient and varied gradient were analyezed theoretically.By using crystal growth simulation software,the thermal field,melt flow field,solid-liquid interface,growth rate and Zn concentration in VGF growth process of CdZnTe crystal were computed.Based on the simulation results,the VGF furnace was designed,manufactured and applied for the practical crystal growth.The simulation results are basically consistent with the measured results.Statistical result of the orientations of dominant grains in CdZnTe ingots shows that<1h1>is the normal direction of the solid-liquid interface where CdZnTe crystal can maintain stable growth.By using VGF growth technique with<1h1>oriented seed,CdZnTe single crystal of 90 mm in diameter and 100 mm in length was obtained.(6)The distributions of EPD,infrared transmittance(IRT),second-phase defect density(SPDD)and Zn compostion in the as-grown crystal were measured.The results show that the average of the EPDs of the crystal grown by VGF method is less than 5.0×10⁴ cm^-2.The distributions of IRTs and SPDDs on the cross section of the ingot have notable nonuniformities and can be divided into two different regions with a distinct boundary.In the outer region of the boundary,the IRTs are in normal levels corresponding to no absorption.The SPDDs are small.In the middle part of the outer region,no second-phase defect is observed by the transmission microscope.But in the inter region of the boundary,the IRTs decrease obviously.The second-phase defects with density of 1000 cm^-2 and size of 5?m can be observed.The formation mechanism of the second-phase defects during the growth process was discussed.Zn composition measurement results show that there is a high composition region in the shoulder of the ingot.The isosurface of Zn compostion is relatively flat in the middle of the ingot and warps gradually in the tail of the ingot.The difference in the absolute value of Zn composition is less than 0.004 in the central region(60 mm×60 mm)of a wafer cut perpendicular to the ingot axis.According to the simulation result,the enrichment effect of Zn in the shoulder and center of the ingot due to the melt flow vortices near these regions was discussed.The above research results will help us better understand the etch pits on CdZnTe substrate surface and the characteristics of dislocation defects in CdZnTe materials.The improvement of the measurement technique using non-scanning transmission micrograph provides a practical and fast quantitative method to inspect micrometer second-phase defects.The research result also provides a new detection method for sub-micrometer second-phase defects.The new measurement methods and characterization parameters obtained in this dissertation will be useful to evaluate the defect properties of CdZnTe materials more comprehensively,and will be of practical value to the quality inspection and screening of CdZnTe substrate.They will be also helpful to study the influence of crystal growth process on the defect properties of the materials.The research results of seed-oriented VGF growth technique and successful acquisition of CdZnTe single crystal provide a good foundation for the application of VGF technique.