The Study On Seeded Growth And Optical And Electrical Properties Of Aluminum Nitride Crystal

With the rapid development of new energy,communication,rail transit,aerospace and other fields,the semiconductor industry plays an increasingly important role.Aluminum nitride(Al N)has become a leader in the third generation of semiconductor materials because of its wide direct bandgap,high thermal conductivity,high thermal stability,high breakdown field strength and strong anti-radiation ability,which is suitable for manufacture of high frequency,high temperature resistance,high radiation resistance,high power electronics and optoelectronic devices.Al N has low lattice mismatch and thermal mismatch with III-nitrides such as Al Ga N and Ga N,so it is ideal as substrate material for epitaxial growth of III-nitrides.Meanwhile,extremely wide direct bandgap of 6.2e V makes Al N own great ultraviolet transmittance and broad application prospects in the field of ultraviolet optoelectronic devices especially UVC(210?280nm)devices.Therefore,the preparation of Al N crystals is particularly important.The physical vapor transport(PVT)method has been developed as the optimal method for growing Al N bulk single crystals,however,relevant technological problems affect the preparation of large-sized and high-quality Al N single crystals.The preparation of high-quality Al N seed crystals and the mastery of seeded growth techniques have become the keys to size enlargement and quality improvement of Al N single crystals.Based on the self-designed growth system,this paper explores the PVT growth mechanism and optimized growth conditions of Al N crystals.First,the temperature-gradient adjustment method is used to effectively control the nucleation to achieve significant reduction of nucleation amount and dominant growth of only one bulk Al N single crystal on the tungsten substrate.This method can provide centimeter-sized and high-quality Al N seed crystals through spontaneous growth.For spontaneous growth,polar facet(like c-plane)and non-polar facet(like m-plane and a-plane)Al N crystals tend to grow under relatively higher(1?2 mm/?)and lower(0.2?1 mm/?)axial temperature gradient,respectively.Second,combined with the temperature-field simulation and analysis by COMSOL Multiphysics software,the appropriate axial and radial temperature gradient can be controlled by adjusting power ratio of heaters and crucible position in seeded growth to reduce crystalline defects while ensuring diameter-enlargement of the Al N single crystal.Third,the Al N single crystal boule obtained by seeded growth is processed into wafers after cutting,grinding,and polishing.The wafer as seed can be fixed on the tungsten substrate by high-temperature bonding method.Finally,inch-sized Al N single crystals can be prepared by optimized seeded growth techniques and the maximum diameter of the wafers after process reaches 40mm.Furthermore,X-ray diffraction(XRD),scanning electron microscopy(SEM),electroluminescence(EL)spectroscopy,Raman spectroscopy,energy disperse spectroscopy(EDS)and other characterizations are used to study crystalline nature,growth characteristic,optical and electrical properties of the prepared bulk Al N single crystals.For the prepared inch-sized Al N wafers,the peak width at half height(FWHM)of X-ray rocking curve can be less than 100 arcsec and the FWHM of E₂(high)mode in the Raman spectrum can be less than 10cm^-1,showing that the methods of seed preparation and seeded growth process proposed in this paper provide great technical reserve for preparation of large-sized and high-quality AlN.