Study On Minority Carrier Lifetime Enhancement Mechanism Of P Type 4H-SiC

In recent decades,due to the thermal stability,high breakdown voltage and high thermal conductivity of silicon carbide?SiC?materials,SiC bipolar devices have been widely used in high voltage,high frequency and high power applications.As a typical representative of high voltage power switch device,minority carrier lifetime is one of the most important parameters to character the quality of semiconductor material and device,which plays a vital role in ameliorating and optimizing performance of SiC bipolar device.Conductivity modulation effect is key to reduce the conduction resistance of the high voltage device,in order to realize effective conductivity modulation effect,minority carrier lifetime of the drift layer is expected to be high enough.However,it is reported that large amounts of carbon vacancy,which presents as an equivalent concentration of deep level defects(Z_1/2and EH_6/7)in the energy band of N type 4H-SiC,and cause limits on carrier lifetime.But study of P type SiC is not enough,and the deep level type which influence carrier lifetime of P type 4H-SiC are not determined,which limits the performance improvement of 4H-SiC power devices.In this thesis,PL?Photoluminescence?,ESR?electron spin resonance?method and?-PCD?microwave photoconductive decay?method are used to study deep level defects and minority carrier lifetime of P type 4H-SiC epitaxial materials.Firstly,we have carried out the experiments of different thermal oxidation time and high temperature annealing for SiC materials.The results were found by the test of?-PCD method:?1?The average lifetime of the original sample is 223.2ns,lifetime of samples with high temperature thermal oxidation treatment under 1150?for 5 hours,15 hours,25 hours are 219.82ns,212.84ns and217.27ns,they are not significantly increased compared with the original samples;?2?the lifetime of sample with high temperature annealing is 276.38ns,and the lifetime of sample with high temperature annealing after 5 hours,15 hours,25 hours oxidation are 366.24ns,370.11ns and 443.29ns,show a obvious increase compared to sample with high temperature annealing only,indicate the effect of high temperature annealing after thermal oxidation on the lifetime of SiC samples.Moreover,the lifetime of the sample is increased with the increase of the oxidation time.In addition,ESR method is used to detect the defect of P type 4H-SiC,we get conclusions combined with the oxidation and high temperature annealing:?1?The carbon vacancy concentration in the sample after high temperature thermal oxidation was reduced significantly,indicating that the thermal oxidation could effectively reduce the carbon vacancy defects in the SiC epitaxial layer.?2?Carbon vacancies in the sample with high temperature annealing after oxidation have no significant change compared with the thermal oxidation of the sample,indicating that the high temperature annealing would not affect the carbon vacancy defects effectively.?3?Carbon vacancy is also the main defects which affect lifetime of P type 4H-SiC,these two defects were found been under the conduction band 0.905eV and 1.203eV by the PL test,and thermal oxidation annealing can effectively reduce the carbon vacancy and increase the lifetime.At the same time,open circuit voltage decay?OCVD?is applied to test lifetime of 10kV package SiC PiN diode,whose thickness and the donor concentration of the n-drift layer are 100?m and 3×10¹⁴ cm^-3,to confirm the validity,the value?720ns?is also applied in ISE TCAD software as parameter of the device simulation model,while the simulated forward characteristics are compared with the measured static performance.It is believed that this method is helpful for the analysis of the package device characteristics.The experimental results show that high temperature annealing after high temperature oxidation can effectively reduce deep level defects in the P type SiC epitaxial material,which bring considerable enhancement to minority carrier lifetime,and also show that the OCVD method can well compensate for the inadequacies of the?-PCD for testing lifetime of SiC devices.