Study On Minority Carrier Lifetime Enhancement Technology Of N Type SiC Epitaxial Material

In recent years,due to the superior performance of silicon carbide?SiC?bipolar devices in the ultra-high voltages?>5000 V?applications,SiC PiN diodes,bipolar junction transistor?BJTs?and insulated gate bipolar transistors?IGBTs?bipolar power devices have received more attention.As a typical representative of high voltage power switch device,SiC based insulated gate bipolar transistors?IGBTs?possess low on resistance,high operating frequency,safe operation area and simple gate driver circuit characteristics,which is much more suitable for ultra-high voltage domains such as locomotive traction and smart grid.Minority carrier lifetime is one of the important parameters to character the quality of semiconductor material,which plays a key role on the optimization design of SiC IGBT device.In order to realize effective conductivity modulation effect,it requires the drift layer to have enough minority carrier lifetimes.However,large amounts of carbon vacancy exists in the SiC epitaxial layer now,which presented as a certain concentration of deep level defects(Z_1/2,EH_6/7)in the energy band,seriously limits the material lifetime.At present,the deep level defects'characterization of the epitaxial layer and the improving method of minority carrier lifetime in the research is not enough in-depth,limiting the ultra-high voltage 4H-SiC devices performance.In this paper,the carbon vacancy defects and characterization methods and improvement measures of the minority carrier lifetime in N type 4H-SiC epitaxial materials are investigated by combining ESR?Electron Spin Resonance?method and?-PCD?Microwave Photoconductivity Decay?method.Firstly,the model of the minority carrier concentration distribution of SiC materials and the minority carrier lifetime decay is set up based on?-PCD method,analyzing the influence of surface recombination and bulk recombination on the minority carrier lifetime of SiC materials measured by?-PCD method.For the epilayer with 50?m thickness and 1×10¹⁵cm^-3 doping concentration,if the surface recombination rate is lower than 1000 cm/s,the measured effective minority carrier lifetime is almost equal to the set up bulk lifetime?1?s?;when it is higher than 1000 cm/s,the measured effective minority carrier lifetime is only 0.52?s,much less than the bulk lifetime.Simultaneously,it is also studied that the influence of different epitaxial layer thickness on the minority carrier lifetime and the influence of surface recombination rate on the minority carrier lifetime of the 200?m epitaxial layer.The results show that the epitaxial layer is thicker,the lifetime is higher;the epitaxial layer is thicker,the effect of surface recombination is smaller.Secondly,experiments of n type SiC materials with different thermal oxidation time and temperature are also carried out.The ESR test results show that:?1?Samples'g factors are between1.99635 and 2.00817,illustrating that the defects in the samples belong to the carbon vacancy.The variation between each sample is tiny,thus,it is speculated that thermal oxidation did not produce new types of defects.?2?The ESR intensity decreases with the prolongation of the oxidation time and the increase of the oxidation temperature.The?-PCD test results show that the sample which is oxidized for 10 h at 1050?obtains the maximum minority carrier lifetime,while its ESR line width and intensity is the minimum in all samples.It shows that thermal oxidation is helpful for reducing the surface recombination rate and the concentration of carbon vacancy defects,thereby improving the minority carrier lifetime.Besides,the longer the oxidation time and the higher the temperature will induce to the lower the defect concentration and more beneficial to the improvement of minority carrier lifetime.At the same time,it also shows that the ESR method can quantify the defect correlative to the minority carrier lifetime and its characterization results can match the minority carrier lifetime.In order to further improve the minority carrier lifetime,designing the scheme of C+ion implantation with different conditions,the samples are processed by C+ion implantation-high temperature annealing-thermal oxidation process.The designed experiment of C+ion implantation with different conditions is tested by ESR method,it shows that:?1?At room temperature,the defects concentration of the implanted SiC samples having a low target concentration of 2×10¹⁸ cm^-3,shallow implantation depth of 300 nm and long time's oxidation of 35 h,can be reduced to the minimum;?2?At high temperature,the defects concentration of the implanted SiC samples having a higher target concentration of 2×10²⁰cm^-3,deeper implantation depth of 540 nm and long time's oxidation of 35 h,can be reduced to the minimum.The nondestructive measurement of?-PCD is used to obtain the minority carrier lifetime of samples from the fitted decay curve.However,due to the influence of surface recombination rate,the minority carrier lifetime of original 4H-SiC epitaxial wafer is 800 ns,while the minority carrier lifetime of samples under different C+ion implantation and annealing is rather low,which is about 470 ns.The experimental results show that the C+ion implantation can reduce the deep level defects in 4H-SiC materials,but the increase of surface recombination rate caused by C+ion implantation can seriously affect the minority carrier lifetime measured by?-PCD method.The next step is required to reduce the effects of surface recombination through effective surface passivation technology.