Study On The Carrier Transport Properties And Polarization Effect In Cd_1-xZn_xTe Detector

Ternary II-VI compound semiconductor Cd_1-xZn_xTe,shorten as Cd Zn Te or CZT,possesses high atomic number,high band-gap,high ressistance and high charge mobility and life-time product(??).These properties make it a most important new material for X-ray and?-ray detector fabrication.A deep understand on the charge transportation properties and the factors effecting the transportation behaviors,revealing the relationship between the charge transportation properties and the device performance are of great importance for the applications of the detectors in the real equipment with ultrahigh radiation dosages.The present dissertation studies the transportation behaviors of CZT crystal with different crystalline and the detector applications.The major achievements are as followings:(1)Photo counting X-ray detectors were fabricated from CZT crystals.The detectors were tested at ultra high X-ray dosages.Two typical crystals with high crystalline quality and poor crystalline quality respectively were comparatively studied.It was found that CZT detector made from high quality crystal produces high counting rate.For the detector with high quality,the counting rate peeks increasing with the increase of X-ray dosage.But for the crystal with poor quality,the counting rate increases to a peak when increasing the X-ray dosage.After that,the counting rate decreases rapidly when the further increase X-ray dosages.The working principles of the detector made of high quality were discussed.And a procedure for the selection of CZT crystals suitable for the fabrication of CZT detector was proposed.(2)Three CZT detectors with apparent different counting properties and photo-electrical responses were tested using thermal stimulated current spectra(TSC)method.Based on the results,the mobility life-time product was fitted.Simultaneously,the type and concentration of point defects studied.It was found that the stimulated current can be changed severely when the point defect concentration is high due to the trapping and acceptance of photos by the defects.CZT crystals with high point defect concentration produces low counting rate.The deep level defects are found to have the most important effect the spectra properties and the counting rate.The shallow energy levels have lesser effects on the spectra properties.But both the deep levels and shallow levels will deteriorate the counting properties.We obtained the quantities results of charge distribution,space charge accumulation and internal electrical field in CZT at ultra high X-ray dosages.(3)To study the effects of sub-band irradiation on the detection properties of CZT crystals,we selection two CZT wafers for the experiments.The wafers were first studied with TSC to characterize the type and concentration of defects.It was found the one with low deep level defects produced the counting rate about one order higher than the one with high deep level defects.In the I-V curves,the current was significantly increased when the sub-band radiation is applied.An inflection point was observed for all the I-V curves.When 1550nm radiation is applied,the inflection point keeps unchanged.It means the radiation did not change the internal electrical field.When the sub-band light of 1200nm and 1300nm in wavelength are applied,the inflection point moves to the lower bias voltage,which is favorable for reduce the distortion.The effect increases with the increase of light density.If the light of 1050nm and 980nm are used,the inflection point moves opposite,i.e.it move to the direction of higher bias voltage,which means the internal electrical field was distorted more severely.(4)We studied the internal electrical field in detail for the case of CZT detectors with high X-ray dosage.The charge was accumulated near the cathode due to the vacancy trapping,an opposite internal electrical field will form and the field will be distorted.(5)We designed and fabricated the X-ray imaging system based on photo counting CZT detectors,which can be used for 2-D imaging analysis.For different objects,a single energy range and 5 energy range imagines can be obtained.The images can reflect the size,morphology and the difference of materials.With this system,the counting rate of the detectors and their difference in different energy ranges are obtained.