| As a member of the wide bandgap semiconductor family with the widest bandgap,aluminum nitrate(AlN)crystal possesses a remarkable 6.1 eV bandgap and unparalleled physicochemical properties,making it a crucial material in the production of high temperature,high pressure,high frequency,high power,and ultraviolet optoelectronic devices.Although the growth of AlN crystal has been the subject of intense research worldwide for almost half a century,significant commercialization has been hindered by substantial technical barriers and exorbitant growth costs.Physical vapor transport(PVT)is widely regarded as the most effective method for the bulk crystal growth of AlN,due to its high growth rate,low dislocation density,and superior crystal quality.In this work,the temperature field in the PVT growth system was simulated based on the finite element analysis method.Based on this,AlN crystal was grown using SiC substrate as a heterogeneous substrate.AlN seeds are grown through spontaneous nucleation,and AlN crystal size was expanded through iterative growth.The physical,structural,quality,and optical properties of the resulting AlN crystal were thoroughly characterized.Additionally,the study prepared AlN substrate with flattened surfaces through crystal processing.The specific research content of this paper is as follows:1.A model was created to analyze the temperature field distribution during the growth of AlN crystal via the PVT method.The model was constructed based on the finite element abstraction of the system,and different parameters were examined to understand their effects on the temperature field distribution.Through mathematical approximation and finite element analysis,the complex multi-physics coupling system was accurately modeled to obtain the global temperature distribution of the AlN crystal growing system.The study then focused on the temperature field distribution in the crucible region,and examined the impact of different crucible parameters on the temperature field distribution.The study found that the location of the crucible has a significant impact on the temperature field distribution in the crucible.Increasing the thickness of the crucible cover led to a significant increase in the temperature distribution in the growing region of the crucible,while reducing the temperature gradient in the same region.However,this had little effect on the temperature distribution in the source region of the crucible.Furthermore,increasing the thickness of the crucible wall resulted in an overall increase in the temperature distribution in the crucible,as well as an increase in the temperature gradient in the growing region.Similarly,increasing the thickness of the crucible bottom led to an overall increase in the temperature distribution,and altered the temperature gradient in the crystal growth region,although the effect was not pronounced.Overall,the findings suggest that the temperature distribution pattern remains unchanged with increasing crucible thickness.2.AlN crystal was grown using SiC substrate as a heterogeneous substrate,and its structural integrity,quality,and stress levels were extensively discussed.The presence of a high concentration of C impurities in the AlN crystal was confirmed by SEM and EDS mapping analysis.XRD results revealed a shift in the(0002)diffraction peak of the AlN crystal towards a smaller angle,with the Raman patterns appearing distinct from the theoretical predictions of the AlN crystal,providing evidence of impurity-induced structural changes.The polarity of the AlN crystal was determined to be Al polarity using the wet etching method,which is unsuitable for further growth.EBSD was used to characterize the crystal quality of the AlN,demonstrating that it was structurally homogeneous and intact.HRXRD analyses showed that the AlN crystal has low-angle boundaries,with high dislocation densities of screw dislocations(6.98×108 cm2)and edge dislocations(5.18×109 cm-2).Based on EBSD texture analysis,the deviation between the AlN crystal and ideal crystal structure was determined.Theoretical and practical analyses were conducted to evaluate the stress variation of the AlN crystal.It was discovered that the substrate has a significant impact on the early growth of the AlN crystal,leading to high stress levels inside the crystal.However,as growth progresses,the substrate’s influence on the AlN crystal is diminished,resulting in a continuous decrease in the stress level within the crystal3.Independent AlN seeds were prepared through spontaneous nucleation,followed by iterative growth to expand the diameter of AlN crystals.The physical,structural,quality,and optical properties of the resulting crystals were thoroughly characterized.Remarkably,the growth rate of AlN was found to be predominantly controlled by temperature fields rather than crystallographic differences,as independently grown AlN seeds of varying orientations exhibited comparable growth rates.A comparative analysis of characteristic c-plane and mplane seeds was conducted,revealing the bonding mode and specific crystal structure of AlN seeds.Surface chemical states,oxide layer thickness,stress levels,and structural quality of AlN seeds were also examined.Notably,iterative growth led to the enlargement of AlN crystals,with the 3D island growth pattern on the crystal’s surface resulting from the growth advancement of 2D nucleation.The physical properties of the AlN crystals,including their density,hardness,magnetic properties,and thermal stability,were also fully characterized.The crystal demonstrated a density of 3.23487 g·cm-3 and a hardness of 8.3,along with diamagnetic behavior at room temperature and remarkable stability at higher temperatures.Analysis of the crystal structure revealed Al-N bonds as the primary chemical bond,with a significant presence of O-Al and O-N bonds.The orbital contributions from the top of the valence band and the bottom of the conduction band were also investigated.Notably,the crystal exhibited excellent structural consistency,high quality,and low dislocation density,albeit with some observed vacancy defects.Furthermore,the optical properties of the crystal were analyzed,and it was discovered that the PVT method affected the light transmission of AlN in the ultraviolet region.As the temperature increased,the peak width of the optical phonon mode in the crystal broadened,and the peak intensity decreased.4.The AlN substrate with flattened surface was prepared by cutting,grinding and polishing the growing AlN ingot.The contact type and wear type during the grinding process were analyzed,and the mechanism of the chemico-mechanical polishing(CMP)process was analyzed based on chemical thermodynamics and kinetics calculations of the degree and rate of reaction during the oxidation of AlN crystal.AlN ingot was wire cut to obtain AlN chips with excellent thickness consistency,and it was found that there were a lot of brittle damage on the surface of the cut chips and the surface quality was poor.In order to quickly reduce the thickness of the damage layer,AlN chips were mechanically ground.The type of contact and wear among grinding disc,abrasive material and processed crystal during the grinding process were analyzed by machining theory.The results show that the plastic scratches were evenly distributed in the same direction on the ground surface during the grinding process and grooves with brittle and plastic processing characteristics were observed.In order to further reduce the thickness of the brittle damage layer,the ground wafer was CMP.Based on the chemical thermodynamic and kinetic calculation,it was found that AlN crystal is more likely to generate a soft surface layer during the polishing process,so the oxidant with weak oxidation ability can be selected or the concentration of oxidant can be appropriately reduced.The process of CMP was analyzed in detail,and the micro-zone diagram of AlN crystal CMP was drawn.The polished crystal surface had excellent surface quality,with Ra and Rq values of 2.51 and 3.20 nm,respectively. |
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