The Study Of Growth And Defects Of Aluminum Nitride Single Crystal

As one kind of third-generation semiconductor material,aluminum nitride(AlN)has many excellent properties,such as ultra-wideband gap,high UV transparency,and a high coefficient of thermal conductivity.AlN also has a similar lattice mismatch and thermal expansion coefficient as AlGaN,which is considered an ideal substrate for the epitaxial growth of AlGaN layers.The physical vapor transport(PVT)method is a promising approach for obtaining AlN substrate for its advantages of fast growth rate and high crystal quality.However,there are still many challenges in the crystal growth mechanism and process,seed crystal growth strategy,and defect characteristics study of AIN single crystal by PVT method.In this thesis,the growth process of AlN crystal by the PVT method is systematically studied.Combined with the results of thermal field simulation,the crucible structure is optimized,and the spontaneous nucleation growth process of highquality AlN single crystals is studied to prepare self-standing AIN seed crystals.In addition,the growth mechanism and optical characteristics of non-polar AlN crystalline are also studied.The main work and results of the thesis are as follows.1.The phase transformation mechanism and carburization process of Ta crucible:TaC crucibles are obtained by high-temperature carburization treatment on commercially purchased Ta crucibles.The XRD results show that the surface of the crucible is completely transformed into the TaC phase after the carburizing treatment.The EDS results show a gradient distribution of carbon content along the crucible thickness.According to the difference in carbon content,TaC,Ta4C3,and Ta2C phases are successively formed in the carburized layer.The change of carburized layer thickness with carburizing time during carburizing was studied.It was found that the internal phase transformation mechanism of the Ta crucible was a typical reactiondiffusion process,with a reaction-diffusion coefficient k of 0.055 mm2·h-1.2.The effect of different crucible states on the thermal field in the growth zone:The effects of crucible structure,crucible spatial position,and heating power on the thermal field distribution are studied by using finite element simulation software.The results show that the internal temperature field can be adjusted by changing the distribution of the insulation material around the crucible.The addition of bottom insulation material can form a single direction of mass transport and obtain a temperature field distribution with an axial temperature gradient of 1.04℃/mm,while the addition of side insulation material can further increase the axial temperature gradient to 1.98℃/mm.In addition,the growth temperature increased with the increase of heating power,but the temperature difference between high and low temperatures in the growth zone remained the same.The corresponding growth process windows for different thermal field states are calculated to guide subsequent growth experiments.3.Growth mechanism and characterization of AlN crystals grown by spontaneous nucleation:The effects of growth parameters,such as growth temperature,pressure,and temperature gradient,on the growth of AlN crystals were investigated.The results show that the crystal size and growth rate increased with the increase in growth temperature and the decrease in gas pressure.Nearly 3-inch AlN polycrystalline ingot was obtained by several iterations under the optimal growth process with a growth temperature of 2300℃ and a growth pressure of 6.0 ×104 Pa.By optimizing the thermal field,the spontaneous nucleation of independent bulk AlN crystals on the raw materials was realized under the condition of large temperature gradient thermal field with non-uniform temperature distribution,and the optimal growth process was studied.The freestanding c-plane AlN substrate with a maximum size of 5 mm and the semipolar and non-polar AlN substrates with a maximum size of 2～3 mm were prepared at a suitable supersaturation.The full width at half maximum(FWHM)values of the XRD rocking curves of the(0002)and(10-12)crystal plane of the c-plane sample is 58.8 arcsec and 30.1 arcsec,respectively.The peak position of the E2(high)phonon mode peak in the Raman spectrum is 657.0 cm-1 and the FWHM value is 3.2 cm-1.The results show that the AlN crystals grown by spontaneous nucleation have lower stress and better crystal quality.Cathodoluminescence(CL)tests show the presence of a complex CL pattern consisting of a large number of bright lines extending along the m-direction in the dark brown region of the substrate,which is associated with structural defects such as dislocations modified by point defects and small angular grain boundaries.The optical transmission spectroscopy results show that there is a distinct absorption peak in the dark brown region of the substrate at about 300 nm with an absorption spectrum cut-off edge of 345 nm.The direct spatial correspondence between the local distribution of structural defects and Optical absorption properties suggests that the enrichment of impurities associated with structural defects may lead to the formation of zonal structures.The hardness and Young’s modulus of the c-plane AlN substrate are(14.45±0.16)and(321.17±4.81)GPa,respectively.In addition,the multiplication and slip of U-shaped dislocation along the<11-20>result in a sixfold symmetric distribution stress pattern.4.The growth mechanism and optical properties of non-polar AlN crystals:The m-oriented AlN microwire(UW)crystals grown in low nitrogen pressure environments have a rectangular shape consisting of c and a crystallographic planes and polyhedral tips.The non-polar AlN UW crystals were obtained in an Al-rich growth atmosphere during long-time cooling down.The TEM tests show that the AlN UW has high quality and does not have significant structural defects.The polyhedral tip morphology of AlN UW has various intermediate morphologies due to the surface energy differences of different crystallographic surfaces and the effect of anisotropic migration of Al adatoms on the non-polar surface.The study of the optical properties of AlN UW reveals a strong localized CL luminescence near the UW sidewalls.Based on the difference in impurity adsorption ability on different crystal planes,this particular localized CL luminescence pattern may be related to the local enrichment of C and O impurities within the crystal.The PL results show that high-quality AlN UW can be used as a Fabry-Perot(F-P)resonant cavity to achieve resonant enhancement of CL luminescence,and the anisotropic light-emitting property of the AlN crystal also contributes to the formation of the CL pattern in the AlN UW.