Systhesis And Characterization On Mechanical Properties Of Gallium Nitride And Alumimum Nitride Crystals And Nanostructures

Gallium nitride(GaN)is a typical wide-band-gap semiconductor.Due to its high thermal conductivity,high saturation electron mobility and band gap of 3.39 e V at room temperature,GaN has a significant application in high power high temperature integrated circuits.Among group III nitrides,aluminum nitride(AlN)has a relatively wider band gap(6.2 e V)and a transition of energy band which can reach deep ultraviolet wavelength.Therefore,it has important applications in violet and ultraviolet devices.However,the research level of GaN and AlN crystals at home is still relatively backward.Moreover,the high brittleness of the AlN crystals not only restricts its application in devices,but also limits its potential performance.For the above questions,GaN single crystals and AlN polycrystalline boules were obtained by means of flux method and physical vapor transportation(PVT)method respectively.GaN nanowires were synthesized by chemical vapor deposition(CVD)method.AlN nanohelices and AlN hexagonal rings were synthesized by PVT method.Crystal growth technology of AlN,properties of GaN and AlN crystals were studied emphatically.The mechanical properties of AlN crystals and nanostructures were further studied by in-situ mechanical tests.This paper provides a theoretical basis for further improving the performance of GaN-based and AlN-based devices.The X ray diffraction(XRD)pattern showed that the crystal structure of GaN single crystal was hexagonal wurtzite.Raman spectra conducted at different temperature revealed lifetime of A1(LO)and E2(high)mode declined with the increasing temperature.Additionally,it was found that phonons of A1(LO)and E2(high)mode of the high-quality GaN single crystal are mainly symmetrically decayed into two phonons with equivalent energy(Klemens mode).By crystal growth experiment and VR-AlN software simulation,the optimal growth process of AlN crystals was obtained: the optimum relative displacement from heater to the coil was 0 cm;the relative displacement from crucible to heater is 30 cm in height;2000~2200 ? is most suitable temperature for the crystals growth,1800~2000? is suitable temperature range for micro(nano)structures growth.An AlN polycrystalline ingot with a diameter of 25 mm and a height of 12 mm was prepared by PVT method.The wafer of 8× 8×1 mm3 was obtained by dicing.Nano indentation tests were conducted on the polycrystalline wafer.Its young's modulus was 385~410 GPa.XRD proved GaN nanowires were hexagonal wurtzite structure.Analysis of HRTEM showed that GaN nanowires grew along c axis;XRD revealed nanohelices were hexagonal wurtzite structure.Morphological characterizations showed that its basic unit is oblique hexagonal prism.Six prisms constructed a circle of nanohelices.The corresponding 2H phase atomic stacking model was established.The dominating growth driving force of AlN nanohelices was polar plane driving.In-situ mechanical tests were conducted on individual AlN nanohelix;deformation mechanism was also analyzed.Firstly,tensile tests were performed in focused ion beam/scanning electron microscope(FIB/SEM)system,which revealed Young's modulus of nanohelices was 330 GPa and the maximum linear elastic deformation of nanohelices was 4.7 %.The nanohelices fractured into two parts,fractured plane is(0001)plane.Finite element analysis(FEA)showed load mainly distributed at the inner circle of the nanohelices.Subsequently,bending tests were also performed in FIB/SEM,the nanohelix fractured into three parts after going through linear-plastic deformation stage and elastic-plastic deformation stage.Maximum of bent strain is 54.5%.Facet slip is the main reason for the plastic deformation of nanohelices.Finally,in-situ nanoindentation tests were performed in scanning electron microscope/scanning probe microscope(SEM/SPM)hybrid system,Young's modulus of nanohelices was 332~335 GPa.Engineering elastic coefficients of AlN single crystal were used to the checking calculation of Young's modulus of nanohelices.The result of it turned out to be 292.17 GPa.The error between the checking calculation and the experimental result mainly came from the size effect and the special structure of the nanohelices.Mechanical performance tests have also been carried out on individual AlN nanowire,including tensile tests,nanoindentation tests,and three-point bending tests,the experimental Young's modulus is 373.71 GPa,372 GPa and 369.7 GPa respectively.Comparison of Young's modulus value between the literature and this research has been made,which showed that the order of size of young's modulus in different grain size was: polycrystalline wafer/ceramic> single crystals ?nanowires> nanohelices.This further shows that the special structure of the nanohelices enhanced the ability to resist non-destructive deformation of AlN.By using PVT method,AlN hexagonal rings were synthesized.XRD patterns showed that their phase were hexagonal wurtzite structure.By microscopic morphology characterization,growth mechanism of AlN hexagonal rings was studied:At the initial stage,AlN grew into straight hexagonal prisms in step flow mode.The step plane was(0002)plane,and growth was towards <10?10> direction.Due to the presence of spontaneous polarization,the straight hexagonal prism moved along the <11?20> direction and subsequently grew towards <10?10> direction.After six cycles of growth-movement-growth,AlN finally formed a hexagonal ring structure.The atomic model of AlN hexagonal rings was established,which was perfectly consistent with the SEM images.The dominating growth driving forces of AlN nanohelices was polar plane driving and catalyst driving.Raman spectra of AlN hexagonal rings were carried out and were compared with that of AlN polycrystalline wafers and nanohelices.Raman spectra reflected the preferred orientation of AlN samples: semi-polar plane {10?10} for AlN polycrystalline wafer,polar plane {0001} for hexagonal rings,and semi-polar plane {10?11} for AlN nanohelices.The tensile stress of various AlN samples was calculated by the stress coefficient of E2(high).Stress of pure AlN polycrystalline wafer and nanohelices specimen was the lowest,compared with that of hexagonal ring samples.The stress of hexagonal ring samples increased the increasing the nominal doping content.The FWHM of the E2(high)increases with the increase of the doping content.