Manufacture And Optoelectric Response Properties Of The Photodetectors With CdTe And Cu₂ZnSnS₄ Thin Films

Photodetector is an optoelectronic device that converts ultraviolet,visible and infrared light signals into electrical signals.It has been widely used in communication engineering,remote sensing,process control,astronomy,national defense and other fields.The development of photodetectors with broad-spectrum response,high-sensitivity,specific wavelength selection,fast response and strong stability is the main goal in this field.Therefore,researchers continue to explore to find new material systems,construct new device structures,and develop devices with new principles to fabricate the new photodetectors with the above characteristics.Cadmium telluride(CdTe)and copper-zinc-tin-sulfur(Cu₂Zn Sn S₄,CZTS)are chalcogenide semiconductor optoelectronic materials with a band gap of 1.5 e V and high light absorption coefficient.They are typical semiconductor materials with a direct electronic transition band and have been used in the field of photovoltaic thin film cells.In recent years,CdTe and CZTS have been extensively studied because of their unique advantages in photodetectors.In addition,it has been reported that the device structure,such as channel size,detection principle,etc.,affects and even determines the response performance of the photodetector.Therefore,in this paper,we have developed photoconductive photodetectors based on CdTe films with different channel length and photovoltaic photodetectors based on CZTS films,and then studied the effect of channel length and built-in electric field on the response performance of a photodetector.The main research contents and results are summarized as follows:1.A set of photoelectric test system with variable temperature has been built during the postgraduate study.The system based on the Lake Shore TTPX probe station can control the temperature of the sample station in the range of 78 K-475 K through the precise temperature controlling.In addition,the photoelectric test system is mounted with laser,power meter,source meter,oscilloscope and other components,which can be selected and matched to measure most electrical and photoelectric performance tests such as current-voltage(I-V),capacitance-voltage(C-V),and dielectric spectrum(J-V)and so on.2.The microstructure and interface structure of the CdTe/STO film epitaxy system have been studied.The CdTe film has four uniformly distributed domain,through analyzing the XRD pole figure which are consistent with the results tested by the reflected high-energy electron diffraction.The interface between the CdTe film and the STO substrate is clear and there is no obvious element interdiffusion,which was analyzed by the high-resolution transmission electron microscope(HRTEM)and X-ray energy spectrum EDS.Furthermore,CdTe/STO thin-film photoconductive photodetectors with different channel lengths were fabricated.The response parameters of the detectors are strongly dependent on the channel length.The responsivity,detectivity and gain increase exponentially with the decrease of channel length while the response time shows a linear decrease trend with the decrease of channel length.In addition,the response time of the detector is only 1.7 ms,reaching the fastest response speed among the CdTe thin-film photodetectors.3.Photovoltaic photodetectors based on CZTS/CdS heterojunction with self-powerd effect has been fabricated,and the photoelectric response performance related to defects was studied.The on-off ratio and response time of the detector reached 6×10⁴and 18?s,respectively,indicating that the device has ultra-sensitive and fast response characteristics.In addition,abnormal phenomenon appeared in the photoelectric response of the device at the incident light power of 0.5?W,which is due to the fact that a small amount of photogenerated carries at low incident light power are easily trapped by defects or defect dipoles,which leads to a low responsivity.when the incident power is higher than 0.5?W,the defects or defect dipoles level is completely filled,and the detectivity and responsivity of the detector reached the maximum,which are 2.69×10¹⁰Jones and 220 m A/W,respectively.Furthermore,the detector was found to exhibit abnormal photoelectric response behavior through the I-V test at 180 K under the same optical power irradiation,which is likely due to the freezing effect of defects or defective dipoles at low temperatures.The capacitance of the CZTS/CdS heterojunction has an abnormal point through J-V testing and analysis at 180 K,which may be related to the conversion of the dipole moment of the defects during freezing and thawing.When the temperature is higher than 180 K,the responsivity and detectivity increase linearly with the temperature,which indicating the inherent physical mechanism between the abnormality of the photoelectric response characteristic and the freezing of the defects or the defect dipoles.