Study On Deep Level Transient Spectrum Test Method For Interface Defects Of Oxide MIS Structure

Strontium titanate（SrTiO₃,STO）is an insulator with typical perovskite structure.In recent years,various experiments show that the quasi two-dimensional conducting layer on the surface of SrTiO₃ has the properties of photoluminescence,magnetoresistance effect,persistent photoconductivity effect,metal insulation phase transition,spin orbit coupling.These are mainly related to the oxygen vacancy on the surface of SrTiO₃.The oxygen vacancy is an important defect in the conductive layer of STO,but the related research on the defect energy level of the oxygen vacancy is very rare,which needs further research.The deep level transient spectroscopy is one of the most effective methods to detect the deep level impurities and defects.By using the deep level transient spectroscopy,we can get the information of oxygen vacancy defects in samples.Because of the above background,this paper mainly carries out the following work:1.Based on the technology of lock-in amplification,a DLTS test system is built to realize the automatic DLTS test function.The transient capacitance test is made up of a lock-in amplifier and a current amplifier,and the instrument parameters are adjusted to reduce the noise.When the time constant T_C of the lock-in amplifier is determined to be100⁵00and the gain of current amplifier is determined to be 1k or 10k,the measurement accuracy of transient capacitance reaches a high level.Temperature control is carried out by using temperature controller,and the method of slow and continuous temperature rise is adopted to make the temperature regulation automatic.Using LabVIEW software,a set of software which can make DLTS test process automatic and include data post-processing is written to complete DLTS test automation.At the same time,the red photodiode is used to show the data processing process.The defect energy level of the red photodiode is 0.36 eV.2.The MIS devices with HfO₂/SrTiO₃ structure were prepared and their field effect properties were characterized.The substrate of MIS structure sample is a conductive layer formed by Ar⁺bombarding STO.The insulating layer and electrode are respectively hafnium oxide film and platinum/titanium metal film prepared by magnetron sputtering.The samples were both evaluated by Hall effect test and field effect test.The results of Hall effect test show that the conductive layer formed by Ar⁺bombarding STO is n-type semiconductor layer,and its conductivity and Hall mobility will increase sharply at low temperature.The carrier of semiconductor layer in the test sample can be modulated by applying negative gate voltage.And the results of field effect test show that the test sample is a threshold electricity n-type FET with voltage of-5V and switch ratio of 12:1.3.The test method of MIS structure samples was studied,and the trap characteristic parameters in HfO₂/SrTiO₃ structure were analyzed.The complexity of the sample with MIS structure is higher than that of the sample with PN or Schottky structure,so there will be more factors leading to errors in the DLTS measurement of MIS structure.In the DLTS measurement of this experiment,when t₂/t₁ is the fixed value,the peak height and temperature of the DLTS spectrum obtained from the measured sample have a strong dependence,and there are different rules at high and low temperature.Especially at low temperatures,the peak height of the DLTS peak has a very clear gap when the emission time constantdoes not change significantly,which is obviously not in line with the theoretical law.To solve this problem,a new DLTS test and data processing method is proposed in this paper.This method can modify the influence of the strong dependence of dielectric constant and temperature of the material（STO in this paper）in the MIS structure sample on the DLTS measurement signal,and make the corrected defect information more reasonable.The experimental results show that at low temperature,the defect energy levels of the samples before and after correction differ by 30 meV,and the intercepted area is greatly different,and the apparent trap concentration is 4 times different.At high temperature,the difference of defect energy level before and after correction is 20 meV,and the apparent trap concentration is only 1.5 times different,which is obviously smaller than the difference at low temperature.Through the improved DLTS analysis method,the defect energy level of sample 1 is 0.17 eV,the capture area is8.4×10^-16 cm²,and the trap concentration is 3.0×10¹² cm^-3;the defect energy level of sample 2 is 0.38 eV,the capture area is 1.5×10^-16 cm²,and the trap concentration is2.2×10¹² cm^-3.Based on the improved DLTS analysis method,the interface defects of HfO²/SrTiO³ structure were further studied.The results show that there are trap centers formed by interface defects in sample 2 and sample 3,with energy levels of 0.38 eV and0.52 eV,respectively.At the same time,it is found that the density of interface state in sample 3 is greater than 10¹² cm^-2·eV^-1.The higher density of interface state affects the modulation ability of gate voltage to electron,and finally reduces the performance of the device.