| With the rapid development of energy,information,national defense,rail transit,electric vehicles and other fields,people put forward higher requirements for the performance of power devices.High voltage resistance,low loss,high power electronic devices have become the trend of power electronics industry.β-Ga2O3 is a potential material for power electronics and optoelectronics applications,with a band gap width of about 4.8 eV and a critical breakdown electric field of 8 MV cm-1.The low-cost growth of large-size,high-quality βGa2O3 bulk single crystal has greatly promoted its development in solar blind UV detectors,gas sensors and other fields.Substrate quality has an important influence on the quality of epitaxial films and device performance.Usually,before obtaining the substrate,the ingot needs to be cut and processed into a wafer with certain shape and size,and reduce surface damage by series of mechanical machining.Each step will introduce crack damage in the hard and brittle β-Ga2O3 wafer.As a new generation of semiconductor materials,research of β-Ga2O3 is still immature.At present,the reports on β-Ga2O3 processing mainly focus on process optimization,and the understanding of the processing process is not deep.Mathematical modeling is an ideal tool to study the machining mechanism.β-Ga2O3 has unique cleavage characteristics,and the growth rate of epitaxial films with the lowest surface energy on the main cleavage plane(100)is low.The introduction of the miscut angle changes the diffusion and nucleation mode of the epitaxial film,improving the growth rate and surface quality.The(100)plane substrate epitaxial layer with a 6° miscut angle is grown in a step flow manner.Substrate processing is an important basis for epitaxy and device fabrication.The surface quality and structure of the substrate affect the quality of the epitaxial layer.In order to grow high-quality films,the substrate surface is required to be atomically flat.The cleavage of gallium oxide crystal seriously hinders the ultra-precision machining of materials.It is very important to alleviate and eliminate the cleavage damage ofβ-Ga2O3(100)surface substrate during processing.The main research content and results are as follows:1.Establishment of numerical slicing damage layer model for β-Ga2O3A morphology model of the wire saw was established based on the randomness of the wire saw parameters,and a mathematical model was developed to predict the subsurface damage(SSD)depth based on fracture mechanics.In addition,the factors influencing the depth of SSD and abrasive grain indentation depth of cut were analyzed.As the abrasive grain position angle increased,the average indentation depth of cut caused by the bottom abrasive grain of the wire saw became deeper,and the material removal mode tended to be brittle.Abrasive grains with smaller position angle is the main cause of crystal crack damage.By adjusting the velocity ratio and the wire saw parameters,the indentation depth of abrasive grains can be reduced and crack damage can be alleviated.2.Study on the surface quality of β-Ga2O3(100)plane wafer with different miscut angle processingIn the process of(100)plane substrate processing,stress concentration is formed in the contact area between the abrasive particles and the substrate surface layer.When the maximum stress value exceeds the critical value,the crystal material in the stress area will be cleavage,which greatly increases the processing difficulty.In order to study the influence of the miscutangle on the(100)plane substrate processing and optimize the process conditions for forming ultra smooth surface,the(100)plane wafers with miscut angle of 0°,1° and 6°were obtained by diamond wire cutting,and the surface morphology of the wafers after cutting,lapping and polishing was analyzed.The inhibition of(100)plane chip cleavage damage with different miscut angles was analyzed.With the increase of the miscut angle,the removal mode of the substrate material showed a transition from brittle removal to brittle plastic mixed removal to plastic removal,and the surface roughness and material removal rate of the wafer after lapping decrease.When the miscut-angle is 6°,the surface roughness of the substrate after lapping was the lowest.Polishing pressure,polishing pad type and miscut angle also have great influences on the induction of cleavage damage in the polishing process of βGa2O3(100)plane wafers.Lower polishing pressure,the softer polishing pad and the miscutangle can help to suppress the(100)facet chip cleavage damage.The introduction of 6°miscut angle greatly improved the polishing efficiency,and the surface roughness Ra of the polished substrate was 0.189 nm.3.Comparison of etched morphology of mechanically peeled and(100)plane with miscut angle of 6°β-Ga2O3 crystals have low symmetry,which can lead to complex defects within the crystal.The existence of defects in crystals can affect the properties of devices.The(100)substrate with miscut angle of 6° substrate can improve the subsequent epitaxy rate.However,defects of the(100)plane with miscut angle of 6° substrate have been rarely reported.In this paper,wet chemical etching technology was used to study the defects of mechanically peeled and(100)plane with miscut angle of 6°.Macroscopic and microscopic analysis of the etch pits were performed using SEM,AFM,and FIB-TEM characterization methods.The morphology of etch pits in mechanically peeled and(100)plane with miscut angle of 6° were compared under low etching intensity.Scratches were found on the(100)plane with miscut angle of 6° surface after chemical etching,indicating the existence of subsurface damage.TEM was used to characterize the triangular etch pit on the mechanically peeled(100)plane,and it was found that there was dislocation in the lower part of the etch pit.The shape,distribution,density of the etch pit and surface morphology were compared between mechanically peeled and(100)plane with miscut angle of 6° at high etching intensity.This study provides defect basis for subsequent epitaxial growth and device application. |
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