Study On Basal Plane Dislocations And Micropipe Defects In SiC Single Crystal

Silicon carbide(SiC),known as the third-generation semiconductor material,has become an excellent choice for fabricating high temperature,high-power and microwave radio-frequency devices due to unique properties such as wide band gap,high breakdown voltage,high thermal conductivity and excellent chemical inertness.Nowaday,micropipes and high density of dislocations in SiC substrates hinder the application of SiC.In recent years,the technology for preparing SiC substrates has become increasingly mature.Some breakthroughs have been made,especially in suppressing micropipes generation.Many institutions have successfully fabricated SiC substrates with low micropipe density or even zero micropipe.However,due to the harsh growth conditions of SiC,it is difficult to control the growth parameters such as temperature and pressure accurately in real time.There are still a small number of micropipes in the SiC single crystal grown in China.Up to now,the micropipe formation mechanisms have not been successfully clarified.From the etch pit morphologies,it is difficult to accurately distinguish micropipes from threading screw dislocations(TSDs).In addition,Raman spectroscopy is becoming an increasingly common analysis method for the microstructure of material,and offers fast and contact-free measurements with easy sample preparation.One can obtain the lattice vibrational spectra and future know the microstructure of material by means of Raman spectroscopy based on the analysis of inelastically scattered light.It has been applied to SiC materials for the characterization of polytypes,stacking faults,stress and doping.Few researchers have focused on micropipes by means of Raman spectroscopy in recent years.Therefore,it is necessary to apply Raman spectroscopy to characterize the micropipe etch pits.An important factor affecting the large-scale application of SiC devices is the high density of dislocations in SiC single crystals.Among these dislocations,basal plane dislocations(BPDs)has a particularly significant effect on device performance.According to the accurate definition of dislocation density,considering the premise that the dislocation line should be perpendicular to the surface of the sample,it is believed that only the number of BPDs observed in the longitudinal slices of SiC single crystal can give the dislocation density in the SiC crystal ingot.We employed the wet etching method to distinguish micropipe etch pits from those of TSDs in 4H-SiC and 6H-SiC wafers.We confirmed that the under-etched state was the best etching condition to distinguish micropipe etch pits from those of TSDs.The size of the micropipe etch pits was linearly related to the etching time,and the etching rate showed a polytype dependence.The etching rate was positively related to the hexagonality in 4H-SiC and 6H-SiC.The micropipe etch pits were classified in detail.Most importantly,the spectra of 4H-SiC and 6H-SiC crystals containing micropipes were examined using Raman scattering.In the Raman spectrum of the micropipe etching pits,an accompanying peak of approximately-784 cm-1 was a sensitive peak of micropipe etch pits.In this thesis,SiC(1100)plane longitudinal slices were prepared,and smooth SiC longitudinal slices with a surface roughness of 1.12 nm were obtained after doublesided grinding and mechanical polishing.The polytype of the(1100)plane of SiC was confirmed to be 4H-SiC by Raman Mapping.The defect-preferential etching method was used,and the etchant was molten KOH to expose the longitudinally sectioned BPD.The morphology of BPD in the(1100)plane,the etching depth and width as a function of etching time were observed with the help of LEXT confocal scanning microscope.It is found that there are three types of BPD with different morphologies in the early stage of etching,and they will eventually become inverted triangular pyramid-shaped etch pits after a period of etching experiments.The width of the three types of BPD etching pits increases with the increase of etching time,while the depth decreases with the increase of etching time.And a possible etching mechanism was investigated.The distribution of BPD in the entire longitudinal slice is calculated,and the dislocation density of BPD is finally obtained to be 8.59×105/cm2.