Preparation And Properties Of Photoconductive X-Ray Detectors Based On Wide-Bandgap Semiconductors

X-rays are electromagnetic waves with very short wavelengths and strong penetrating ability,which are widely used in industrial non-destructive testing,medical imaging and radiotherapy,security systems and aerospace.The ever-increasing application demands raise higher performance requirements on X-ray detectors.The early X-ray detection technology was mainly traditional detection technology such as film and calorimeter.In recent year,two methods are used for X-ray inspection:indirect detection based on media such as scintillators and direct conversion detection based on ionization chambers,etc.Recently,as another new direct detection technology,semiconductor X-ray detector has attracted much attention in the industry due to its excellent response characteristics,good digital signal compatibility and radiation resistance.However,the sensitivity,signal-to-noise ratio（SNR）,response time,low detection limit and other performance parameters of commercial semiconductor X-ray detectors still need to be enhanced.It is of great scientific significance and technical value to study high-performance,low-cost semiconductor direct detectors for X-ray detection.Firstly,a remote controllable X-ray photoelectric response measurement system was built by using digital source meter,X-ray accelerator radiation source and virtual instrument platform.This system can accurately measure the performance parameters of X-ray detection devices.Secondly,through the systematic investigation of the existing research results at home and abroad,a series of prototype devices of photoconductive X-ray direct detector based on wide-bandgap semiconductors MgZnO and Bi₂O₃ are designed and developed by using semiconductor physics and semiconductor device theory.The effects of semiconductor physical properties,device structure and preparation technology on the X-ray detection performance of devices are comprehensively illustrated and discussed.The process and mechanism of streamer transport,separation,recombination and collection under 6 MV hard X-ray irradiation are also analyzed.A new scientific scheme to optimize the device structure and improve the detection performance is explored.The details of the research are as follows:（1）Combined with the existing hardware equipment and LabVIEW software platform,a remote controllable photoelectric response test system of X-ray detector is built.The hardware module consists of tumor radiotherapy equipment（X-ray source）,digital source meter and computer.The software module is a virtual instrument control system based on LabVIEW.The measurement system realizes the functions of volt ampere characteristics,transient response characteristics and constant voltage/current measurement of the detector under dark and irradiation conditions.（2）MgZnO thin film samples were prepared by RF magnetron sputtering technology.On this basis,the photoconductive detector prototype device with Metal-Semiconductor-Metal（M-S-M）structure was prepared.The physical properties of the absorption layer and the detection performance of devices are characterized,and the influence mechanism is analyzed.Firstly,X-ray detectors with Mg_xZn_1-xO absorber layers of different Mg components were fabricated.The results showed that the signal-to-noise ratio（SNR）of the devices increase continuously with the increase of Mg content in the film,and the sensitivity（R_s）increases first and then decreases.when x=0.188,the device obtains the highest R_sof 64.5nC/(Gy_air·cm²)under 6 MV hard X-ray irradiation with a dose rate of 100 mGy_air/s,and the SNR is as high as 260.The reason for this trend in R_sis that with the increase of Mg content,the bandgap of Mg_xZn_1-xO films becomes larger,which is beneficial to suppress the growth of dark current caused by thermal excitation.Due to the low atomic number of Mg and the change of carrier transport characteristics of Mg_xZn_1-xO thin films caused by Mg,the R_sof devices decreases when the Mg content is too high.Secondly,detectors with different Mg_0.188Zn_0.812O absorber layer thicknesses were developed,and the results showed that the R_sand SNR of the devices showed an increasing trend with the increase of the film thickness,possessing the highest R_sof 89.8 nC/(Gy_air·cm²)at the absorber layer thickness of 292 nm.The reason for this phenomenon is that as the thickness of the film increases,the grain size increases and the grain boundary scattering decreases,which leads to an increase in the mobility of the absorber layer film and an increase in the resistivity of the film.Finally,Mg_0.188Zn_0.812O detectors with different substrate temperatures were fabricated.The results showed that the R_sand SNR of devices showed an increasing trend as the substrate temperature increased,and the R_sof the devices reached 108.4 nC/(Gy_air·cm²)and the SNR was 303 at a substrate temperature of 120℃.The response and recovery times were 0.2/0.3 s,respectively,with the low detection limit was below 6.67 mGy_air/s.（3）High atomic number wide-bandgap semiconductorα-Bi₂O₃ was used to prepare M-S-M photoconductive X-ray detector to explore its application potential in the field of X-ray detection.The effects of substrate temperature and sputtering power on the performance of Bi₂O₃-based X-ray detectors were investigated.When the absorber layer sputtering power was 70 W,the device has the best detection performance with an R_sof 24.6nC/(Gy_air·cm²)under 6 MV hard X-ray irradiation with a dose rate of 100 mGy_air/s,a response and recovery time of about 0.2/0.4 s,a SNR of 61,and a low detection limit of less than 6.67mGy_air/s at an and the net photocurrent of the device varies linearly at different X-ray dose rates.The device with a substrate temperature of 100℃ shows the best performance with R_sof 25.0 nC/(Gy_air·cm²)and SNR of 56.The above results indicate that the Bi₂O₃-based X-ray detector has the potential to detect hard X-rays.In this study,wide-bandgap semiconductor-based photoconductive detectors for direct-conversion hard X-ray detection were fabricated and demonstrated.Through the structure design,device development and performance characterization of X-ray detector,the devices with good sensitivity,short response time and high SNR is obtained.The influence mechanism of physical properties and device structure of MgZnO,Bi₂O₃ and other wide-bandgap semiconductors on X-ray detection performance has been proved.It provides a scientific reference for the related research in the field of X-ray and high-energy radiation detection technology.