Investigations On Growth And Properties Of CdTe-based Compound Crystals For Nuclear Radiation Detection

CdTe-based compounds are considered to be one of the most promising materials for room temperature nuclear radiation detectors.The radiation detectors prepared by the materials have the advantages of fast response and high sensitivity to X/gamma ray at room temperature.Since the CdTe-based compounds have strong iconicity,it has bringing about the decreases of thermal conductivity.The volatilization of Cd caused the melt to deviate from stoichiometric ratio,which makes the preparation of high-quality CdTe-based compounds difficult,and thus affects the application of radiation detectors.Therefore,CdTe:In and CdTeSe:In crystals were grown by the Modified Vertical Bridgeman?MVB?method.By optimizied the crystal growth process,the high-quality crystals were obtained.On this basis,three kinds of crystals?CdTeSe:In,CdTe:In and CdZnTe:In?were compared and analyzed the crystalline quality,optical and electrical properties.First of all,CdTe:In and CdTeSe:In crystals were grown by MVB method.The molar ratios of materials were n?Te?:n?Cd?=1.015:1 and n?Cd?:n?Te?:n?Se?=1:0.9:0.1,respectively.and the doping concentration of indium was 10-15 ppm.The CdTe:In and CdTeSe:In crystals were successfully grown with a diameter of 15 mm and a length of 65 mm.Among them,fractures occur in the initiation stage of CdTe:In crystal,resulted in a large number of holes in the latter stage.Te wrapped phenomenon was more serious in the1#CdTeSe:In crystal,and the inclusion size is smaller.The 2#CdTeSe:In crystal have relatively less Te inclusion and larger Te inclusion size.Besides,the size and density of Te inclusions in CdTe:In crystals were calculated.It found that the size of Te inclusions was mainly in the range of 0-20?m.After isothermal annealing,we found that the size of Te inclusions in the1#CdTeSe:In crystal was significantly smaller than 2#CdTeSe:In crystal without isothermal annealing.The size and distribution of Te inclusions in 2#CdTeSe:In crystal were larger and uneven,with an average surface density of 10⁴ cm^-2.By fitting the UV-vis-NIR transmission spectroscopy curve,we can know that the band gaps of CdTe:In,CdTeSe:In and CdZnTe:In crystals were 1.45 eV,1.40 eV and 1.51 eV,respectively.The reason caused the bonding mechanism and electronic structure of the three crystals.Infrared transmission spectra show that the transmittance curves of CdTeSe:In crystals are linear.The maximum average transmittance of CdTeSe:In crystals is 64%in the wave number range of 500-4000 cm^-1,followed by 57%of CdZnTe:In crystals and 47%of CdTe:In crystals.The main reason that there are large Te inclusions in CdTe:In crystals.One of the reason is the larger size Te inclusions distributed in CdTe:In crystals.Low-temperature photoluminescence spectra show that CdTe:In and CdZnTe:In crystals can effectively compensate for Cd vacancies the In doping.Therefore,the number of donor-acceptor composite centers in crystals was small.However,Cd vacancy concentration of CdTeSe:In crystal were the highest,and In was not effectively compensated.The results of I-V measurements show that the resistivity of CdZnTe:In crystal reaches10⁹?·cm,which was higher than CdTe:In and CdTeSe:In crystal.The fact that the band gap of CdZnTe:In crystal is larger than CdTe:In and CdTeSe:In crystal,the intrinsic transition at room temperature decreases,thus the concentration of free carriers decreases and the resistivity decreases;What's more,the existence of more Cd vacancies in CdTe:In crystal and CdTeSe:In crystal caused the decrease of resistivity.As a substitute atom for Cd,Zn in CdZnTe:In crystals can effectively reduce the concentration of Cd vacancies,thereby increased the resistivity of crystals.I-t measurements show that the decrease of current in CdZnTe:In crystal and the increase of current in CdTe:In and CdTeSe:In crystal are mainly the different doping amount of In.Hall measurements show that CdTe:In,CdZnTe:In and CdTeSe:In crystals are n-type semiconductors,which electrons are the main carriers.In the TOF measurements,as the applied bias voltage increases,the amplitude of the pulse voltage signal also increases.The bias voltage of CdTeSe:In crystal was lower than CdTe:In and CdZnTe:In,which mainly its relatively low resistivity and high leakage current.The carrier drift time t_drr decreases with the increase of applied bias voltage,and the fitted result shows that the?_e of CdTe:In is about 877 cm²·V^-1·s^-1,which was larger than CdZnTe:In(818 cm²·V^-1·s^-1)and CdTeSe:In(527 cm²·V^-1·s^-1),indicated that the electronic transport performance of CdTe:In crystal was better than CdZnTe:In and CdTe:In crystals.