| Silicon carbide(SiC)is one of the most important third-generation semiconductors,which exhibits excellent performance in fabricating high temperature,high frequency and high power devices that are widely used in military,civil,aerospace and other fields.Growing high-quality SiC single crystal is vital for the foundation of SiC industry.At present,the main method for growing SiC single crystal is the physical vapor transport(PVT)method that involves the vapor of Si,C and Si_xC_y.In recent years,growth of SiC crystal by high-temperature solution growth method has attracted much attention since it can grow high quality p-type SiC single crystal that are difficult to be obtained by previous PVT method.Nowadays,a lot of researches have been carried out and great progresses have been achieved.However,no good technical research has been reported in China.In this thesis,the characteristics of the solution growth method are systematically studied and the properties of SiC crystal grown by the solution growth method are studied.Meanwhile,the method for characterizing invisible scratches on SiC substrate wafers is also studied.First,we investigate some key issues in the growth of SiC crystal by high-temperature solution growth method.We have studied the auxiliary solution system used in liquid phase growth,and determined the auxiliary solution composition and the suitable proportion of each component.By designing the thermal insulation conditions,reasonable distributions of the growth temperature field is achieved in the radial and axial directions,respectively.During the growth process,important parameters such as the bonding of seed crystal,seed crystal rotation and lifting of the seed crystal are studied and optimized.At the same time,we also study the relevant parameters such as gas pressure and weight loss during growth.We have established a basic theoretical model of the growth process,which helps us further understand the physical process of crystal growth by the solution growth method.We successfully obtain a SiC single crystal with a diameter of 2 inches and a thickness of10 mm.Secondly,we characterize the properties of SiC crystals grown by the solution growth method.The Raman spectrum indicates that the addition of Al in the auxiliary solution can selectively stabilize the polytype to be 4H-SiC.Electrical and secondary ion mass spectrometry(SIMS)data show that high concentration doping of Al atoms is doped into the crystal and the free carrier type is hole,which demonstrate that we successfully obtain p-type SiC crystals by the solution growth method.The doping concentration of Cr and Ce in the crystal is very low,which are under their limit of detections of SIMS.We also find that the grown crystals exhibit the characteristics of dilute magnetic semiconductors.The Cuire temperature of Al-doped SiC is 30-35 K and doesn’t change with Al-concentration.For the first time,we measure the magnetic anisotropy of the Al-doped SiC single crystal and determine that its easy magnetization axis is in(0 0 0 1)plane.Combining with first-principles calculations,we identify that Si vacancies plusing Al doping plays a vital role in the formation of ferromagnetism.Finally,we have studied a method to characterize invisible scratches on the surface of SiC wafers using lasers.After irradiated by picosecond pulse laser with a wavelength of 355 nm at a speed of 10 mm/s,many invisible scratches can be clearly seen under optical microscope.Through the multiple tools,like scanning electron microscopy,transmission electron microscopy and atomic force microscopy,we find that laser irradiation can form a thin layer of amorphous SiC and amorphous Si O on the surface of the wafer with a thickness of 20 nm.Because the scratches are enriched with more defects,they are more unstable.Under laser irradiation,the depth of invisible scratches will be enlarged from 1 nm to 70 nm,making them visible by optical microscopy.We also determine the damage threshold energy density of laser irradiation for different polymorphism.The threshold energy densities for 4H-SiC and6H-SiC are 1.70 J/cm~2(85 GW/cm~2)and 1.28 J/cm~2(64 GW/cm~2)respectively.Experimental datas show that the threshold energy density is the optimal energy density for laser characterizing invisible scratches.Compared with the traditional method,this method is easy,fast and has good application prospects. |
