Growth Kinetics And Thermal Stress Modeling And Simulation During AlN Crystal Growth By PVT Method

As a typical representative of the third-generation semiconductor materials,aluminum nitride?AlN?has a wide bandgap?6.2 e V?,high thermal conductivity?340W/?m?K??,high breakdown field?11.7 MV/cm?,excellent ultraviolet?UV?transparency and high chemical and thermal stability.Due to such excellent characteristics,AlN is an ideal candidate for high-temperature,high-frequency,high-power electronic,and deep-UV optoelectronic devices with high Al content,such as power devices,DUV-LEDs,UV lasers and sensors.Physical vapor transport method?PVT?has been shown to be a very promising technique for the growth of large-size and high-quality bulk AlN single crystals.Similar as more mature Si C growth,the research of AlN growth was started in the middle of 20 century.Nevertheless,the progress of the AlN growth is very slow due to its extraordinary growth difficulty,growth complexity and high cost.Since any extensive experiments in the hostile crystal growth environment are time consuming and extremely expensive,numerical modeling and simulation has become an essential and indispensable tool for the prediction and optimization of the crystal growth processes.In this paper,the properitary in-house convection and mass tranfer modules for the AlN PVT growth was firstly developed using finite element method?FEM?by C++language based on growth kinetics.Based on the developed modules,the influences of the crucible position,crucible shape and sublimation surface on the growth kinetics,such as the mass transfer,gas flow,growth supersuation and growth rate were studied.The simulation results for our in-house PVT growth reactor with two resistive heaters show that a high crucible position is beneficial to improve the temperature uniformity and Al gas flux along the seed surface,thus to ensure the crystal growth rate uniformity,while a low crucible position is helpful to avoid polycrystalline parasitic growth.The average growth rates increase considerably with increasing crucible sidewall expansion angles,while the growth rates near the seed center almost keep unchanged since it is dominated by the specified growth conditions.The optimal expansion angle is found to be between 15°and 18°in order to obtain a slight convex interface.In addition,the rate limiting step of AlN crystal growth by PVT method is still controversially discussed in the AlN crystal growth community.Our numerous analysises based on a series of simulations conclude that the Al local pressure gradient,not the N₂ gas,in the growth system could be the most likely rate-limiting step during AlN PVT growth process,while the Al local pressure is mainly controlled by the temperature distribution.The generation of various defects during the AlN crystal growth is strongly dependent on the stress level in the crystal.Therefore,an anisotropic stress module for the AlN single crystal by the PVT growth was developed based on the thermal elastic-plastic theory for the first time,including the FEM modules of the von Mises stress?VMS?,resolved shear stress?RSS?,total resolved shear stress?TRSS?.Employing the developed modules,3D anisotropic stresses in AlN single crystal during the PVT growth process under different geometrical designs and growth conditions were investigated,and the generation and multiplication rules of dislocations and cracks are qualitatively predicted.The simulation results show that the VMS stress inside the crystal exceeds 1.11 GPa during the AlN growth,and there is a high risk of micro-cracking and dislocation generation for AlN crystals grown along the[100]orientation.An inversion of the shear stress from tensile to compressive along the-a₃ slip direction is observed when the growth transitions from the[001]orientation to the[100]orientation.For the homoepitaxial growth,the magnitude of the TRSS stress increases tremendously after the crystal thickness reaches 6 mm,and the maximum TRSS stress is always closed to the growth surface during the AlN growth.For the heteroepitaxial growth,a high VMS stress layer with strong fluctuations near the Si C/AlN interface is observed,which implies that the generation of cracking could easily occur at this region during the cooling process.After the heteroepitaxial thickness reaches 4.3 mm,the location of the maximum TRSS stress abruptly changes from the seed/crystal interface to the side surface of the epitaxial crystal in a very short growth time,implying that the dislocation generation region with high risk changes from the seed/crystal interface to the side surface.