Research On Preparation And Performance Of Zinc Oxide-based Photoconductive X-ray Detectors

X-ray is an electromagnetic wave with an extremely short wavelength and strong penetrating power,which has a wide range of applications in medical diagnosis,radiotherapy,space detection,security,environmental monitoring,industrial detection,and basic scientific research.The increasing requirements of application introduce higher performance demands of X-ray detectors,such as sensitivity,response/recovery speed,dose detection limit,energy/spatial resolution,and stability.At the beginning of this century,the new X-ray detection technology began to attract attention of researchers;especially in the past 5 years,the related research has made rapid development.New materials and new structures have been constantly reported,and the performance continue improving.However,the detection performances of X-ray detectors such as sensitivity,signal-to-noise ratio(SNR)and response time still need to be improved,and device stability is still not satisfactory.X-ray detectors based on wide-bandgap semiconductors are still far away from large-scale industrialization.In this thesis,based on the investigation of existing research and scientific analysis of semiconductor device theory,a series of photoconductive X-ray detectors based on wide-bandgap semiconductor ZnO were designed and fabricated.The energy band structures and carrier transport characteristics of the absorber layers were controllably adjusted by the doping engineering and energy band engineering.An X-ray photoelectric remote measurement system was independently built using Trilogy linear accelerator,digital source meter,Lab VIEW software platform,etc.,and comprehensive performance measurements were carried out on the developed prototype devices to obtain the sensitivity,SNR and response time of detectors.In addition,the process and mechanism of carrier transport,separation,recombination and collection under X-ray irradiation were systematically analyzed.The new scientific method of optimizing device structure was explored and detection performance was improved.The specific research contents are as follows:First of all,a series of ZnO thin films were prepared using sol-gel technology.ZnO-based metal-semiconductor-metal(MSM)photoconductive X-ray detectors were fabricated,and the influence of sol concentration on the detection performance was systematically analyzed.The results show that as the sol concentration increases,the dark current and photocurrent of the device also increase,and the response of each device to X-rays is not obvious.When the sol concentration is too high,the device does not respond to X-rays.This phenomenon is attributed to the excessive defects of thin films deposited by sol-gel technology,resulting in a large leakage current of the device,and seriously affects the detection performance.Then,in order to obtain high quality ZnO films,MSM photoconductive X-ray detector based on ZnO was fabricated by radio frequency magnetron sputtering,and the influence of the thickness of the absorption layer on the detection performance was systematically studied.The results show that when the thickness of the absorption layer increases,the response time of the device shows a trend of shortening,while the sensitivity and SNR start to increase and then decrease.When the sputtering time is 90 min,the device has the best X-ray detection performance:the sensitivity is 25.2n C·Gy^-1_air·cm^-2,the SNR is 111,and the response/recovery time are 0.3/0.5 s.Finally,in order to further increase the bandgap of the film and reduce the intrinsic carrier concentration,X-ray detectors based on Mg_xZn_1-xO MSM structures with various Mg contents were fabricated by magnetron sputtering,and the influence of the Mg contents in Mg_xZn_1-xO on the detection performance was systematically studied.The results show that the performance of the device after ZnO doped with Mg has been significantly improved,and the device based on Mg_0.19Zn_0.81O film has the best performance with a sensitivity of 90.9 n C·Gy^-1_air·cm^-2,a SNR of278,tresponse/recovery time of 0.2/0.3 s.The performance of X-ray direct-conversion photoconductive detectors based on(Mg)ZnO fabricated in this thesis is better than that of a commercialized a-Se-based X-ray detector.These studies provide practical experience and scientific reference for the development and optimization of X-ray detectors based on wide-bandgap semiconductors,and provide new ideas for the commercialization of wide-bandgap-semiconductor X-ray detectors with high performance.