| Owing to the superior properties such as wide bandgap,high breakdown electronic field,high saturated electron mobility,high thermal conductivity,and chemical stabilities,4H silicon carbide（4H-Si C）holds great promise for the development of high-power and high-frequency electronic devices.High-quality 4H-Si C is the basis for the rapid development of new energy vehicles,railway transportations and 5G communications.However,there still exist high-density dislocations in 4H-Si C single crystals,which significantly limits the performance and reliability of devices based on 4H-Si C.Therefore,it is critical to understand the basic properties of dislocations in 4H-Si C,and reduce the dislocation density in 4H-Si C single crystals to improve the performance and reliability of devices based on 4H-Si C.In this thesis,the intrinsic mechanism of different etching behaviors of silicon（Si）face and carbon（C）face is explored.On this basis,the effect of dislocations on the electrical properties of 4H-Si C is revealed,and the importance of reducing dislocation density in the single crystals is explained.The evolution of dislocations is established,which paves the way to reduce the dislocation density in 4H-Si C.Finally,the effect of different annealing conditions on the height of macro steps and width of the growth terraces of the surface of the seed crystal formation surface is explored,which is beneficial for the design of step-flow growth and thereby reducing dislocation density of 4H-Si C single crystals.The main contents are as follows:（1）Combined with molten-alkali etching,Arrhenius equation,x-ray photoelectron spectroscopy（XPS）analysis,and first-principles calculations,the anisotropic etching mechanism of molten alkali etching 4H-Si C is studied.The results indicate that the activation energies of molten KOH etching rate for the Si face and C face of 4H-Si C are 35.75 kcal/mol and 25.09 kcal/mol,respectively.The molten alkali etching 4H-Si C is realized by the cycling of oxidation and removal of oxides.The faster etching rate of the C face is caused by the fact that the oxides on the C face is unstable,and is easier to be removed by molten alkali,rather than the C face being easier to be oxidized.Therefore,dislocations is revealed at the Si face of molten-alkali etched 4H-Si C via preferential etching along the dislocation lines of dislocations.（2）The local leakage current of each type of dislocations in 4H-Si C is detected by the tunneling atomic force microscopy（TUNA）equipped in an atomic force microscopy（AFM）.The results show that the threading dislocation increases the reverse leakage current of 4H-Si C.The leakage current decreases in the order to threading mixed dislocations（TMD）,threading screw dislocations（TSD）,threading edge dislocations（TED）,and basal plane dislocations（BPDs）.Since almost all threading dislocations in the substrate are inherited into the epitaxial layer during the homoepitaxy of 4H-Si C,we investigate the evolution of dislocations during the physical vapor transport（PVT）growth of 4H-Si C single crystals.It turns out that in the middle and later stages of single crystal growth,the TSD density decreases significantly due to the interaction between TSD with opposite signs and dislocation transformation.The BPD density gradually decreases as the BPD extends out of the crystal along the step flow direction during growth.Due to the macroscopic step causing BPD to transform into TED,the TED density increases sharply in the later stages of single crystal growth.（3）In order to reduce the dislocation density,we investigate the effect of high-temperature thermal annealing on the height and width of the surface steps and terraces of the 4H-Si C seed crystals.The results showed that the step height and width of the seed crystal increased with the increase of annealing temperature.After annealing in an H2atmosphere,the C face of the seed crystal formed an uneven and discontinuous macroscopic step morphology,and excessively high annealing temperature would seriously damage macroscopic steps,while the Si face of the seed crystal was less affected by temperature.By improving the annealing process,the C face of the seed crystal formed a uniform,continuous,and narrower macroscopic step in a vacuum atmosphere after being heated to 1200℃under a pressure of 400 mbar.The average height of the steps was 19.8 nm,and the average width of the step surface was 244 nm.These results provide guidance for the design of the step-flow growth mode to reduce the dislocation density in 4H-SiC single crystals. |
