Study On Growth And Properties Of AlN Crystal By PVT

Aluminum nitride(AIN)is a very important ultra-wide band gap semiconductor material.It has excellent physical and chemical properties,which has broad application prospects in the field of manufacturing high-frequency high-power devices and deep ultraviolet optoelectronic devices.As a new semiconductor material,the domestic research of AIN is still in the early stage.Growth of high-quality AIN single crystal,optimization of its properties,and exploration of its devices and applications still need systematic research.Physical vapor transport method(PVT)has the advantages of fast growth rate,high crystal quality,and the ability to grow bulk AIN crystals,and it is considered to be an ideal method for commercializing AIN crystals in the future.In order to obtain AlN crystals with large-size and high-quality,a suitable temperature field in the growth system is a key factor.In this paper,numerical simulation was used to study the temperature field distribution under different assemble conditions,the change rules of temperature field were summarized,with which guided the optimization of the growth process by using spontaneous nucleation and 6H-SiC seeds to grow AIN crystals by PVT method.The main research works and results of the thesis are as follows1.The temperature fields under different assemble conditions were simulated,the effects of assemble factors on temperature field distribution were summarized.CrysMas software was used to simulate the influence of heating power,the relative position of the crucible and the induction coil,and the size of the temperature measurement hole on the temperature field distribution.The simulation results showed that the heating power was proportional to the growth temperature.With the growth temperature increased,the axial temperature gradient and radial temperature gradient both were increased.When the crucible continued to move upward from the relative position of 0,the high temperature zone moved towards the lower part of the crucible,the axial temperature gradient gradually increased,and the radial temperature gradient gradually decreased.With the diameter of the temperature measuring hole increased,the axial temperature gradient,radial temperature gradient of the growth chamber and the heat of the crucible radiated to the ambient through the temperature measuring hole also continued to increase.2.The effect of growth temperature on the morphology of AIN crystals was studied,high-quality and high aspect ratio AIN nanowires(AIN NWs)were obtained by controlling the growth temperature.The morphology changes of AIN crystals by PVT method with spontaneous nucleation at different growth temperatures were studied.The results showed that the mixture of nanowires and bands was obtained at 1800?1900?,with the increase of growth temperature,the morphology of AIN crystals gradually changes from nanowires to bulk.High-quality,uniform AIN NWs with high aspect ratio(?2000)were obtained under 1800? for 3h.The optical,electrical and field emission properties of AIN nanowires have been studied.The AIN nanowires had an intensive deep ultraviolet absorption peak at 5.94 eV and exhibited a relatively high electrical conductivity(1.29×10-3 ?-1cm-1),low turn-on field(6.2 V ?m-1)and threshold field(8.5 V ?m-1).These results indicated that the AIN nanowires not only play an important role in deep ultraviolet photoelectric devices but also have tremendous potential as a candidate for field-emission nanodevices.3.Based on the numerical simulation results of the effect of the relative position between the crucible and the induction coil on the temperature field distribution,a 2-inch AIN boule was obtained via optimizing the growth process.According to the thermodynamic and kinetic analysis of crystal growth and the numerical simulation results of the effect of the relative position between the crucible and the induction coil on the temperature field distribution,the effect of different crucible positions on the size of AIN crystals obtained by spontaneous nucleation growth was studied.The results showed that when the relative position of the crucible was 16%,a larger AIN single crystal grain could be obtained,due to the axial temperature was moderate,and the radial temperature gradient was large.The c-plane AlN crystal was obtained with a size up to 3×3 mm by spontaneous nucleation.According to the HRXRD results,the FWHM values of the(002)and(102)planes were 66.4 arcsec and 94.4 arcsec,respectively,which illustrated the high crystalline quality of as-obtained AIN crystal.Raman spectroscopy analysis showed that the stress of the AlN crystal gradually decreased as the crystal thickness increased.A 2-inch AIN boule with a thickness of 25 mm was obtained by AIN seed and a 5×6 mm,600 ?m thick AIN crystal wafer was obtained by slicing and polishing.The FWHM value of the(102)plane was 66.4 arcsec,which indicated that the crystal quality was high.By characterizing the absorption spectrum and transmittance spectrum of the AIN wafer,the absorption peak at 294 nm,the transmittance was about 80%in the visible light range,and was also above 60%in the ultraviolet band of 300 to 400 nm.4.Studied the growth mechanism of AIN crystal on the Si polar surface of 6H-SiC seed and optimized the growth process,a 2-inch AIN crystal with the thickness about 500 ?m was obtained successfully.A 2-inch AIN crystal with the thickness about 500 ?m was successfully obtained on the SiC seed at 1810? for 20 h.The growth mechanism of AIN crystal grown on the Si-polar of SiC seed was summarized.The c-plane AIN crystal was grown on c-plane 6H-SiC seed,and EBSD results detected in the AIN crystal and SiC seed revealed that the cross-sections both were(10-10)plane,therefore,the crystallographic orientation relationship could be expressed as[0001]AIN//[0001]6H-sic and[10-10]AIN//[10-10]6H-SiC.A photodetector with an excellent light response to ultraviolet light was prepared based on the AIN crystal grown from the SiC seed.The proposed growth mechanism and the confirmation of crystallographic orientation relationship of AIN crystals on the Si-polar of SiC seed have important value to guide the growth of AIN crystals.