Research On Optical And Electrical Characteristics Of Non-Polar A-Plane AlGaN-Based Ⅲ-Nitrides

The non-polar a-plane AlGaN-based group-Ⅲ nitrides have been considered promising candidates for the fabrication of ultraviolet light-emitting diodes（UV-LEDs）due to the elimination of quantum-confined Stark effect（QCSE）along the growth direction.In this thesis,the epitaxial growth and characterization of the non-polar（?）a-plane AlGaN thin film grown on the semi-polar r-plane sapphire substrates with metal-organic chemical vapor deposition（MOCVD）technique were extensively investigated.The high-quality non-polar a-plane AlGaN epilayers were grown successfully with various newly developed techniques,such as application of indium surfactant,NH₃-flow-modulation technique,the insertion of the Al-composition-graded AlGaN intermediate layer with various thicknesses,and the Si Nx intermediate layer.The non-polar AlGaN epilayers were characterized with atomic force microscopy（AFM）,cathodoluminescence（CL）spectroscopy,high-resolution X-ray diffraction（HR-XRD）,photoluminescence（PL）spectroscopy,and Hall effect measurement.The major research achievements are listed below.1.High-quality non-polar a-plane AlGaN epilayers with dual Mg N interlayers were successfully grown on semi-polar r-plane sapphire substrates with the indium- surfactant-assisted MOCVD technique and characterized with AFM,CL spectroscopy,and X-ray diffraction（XRD）.It was found that both the surface morphology and crystalline quality of the non-polar AlGaN films were strongly dependent on the mass flow of indium surfactant in the MOCVD growth process. In fact,the great suppression of the deep energy level impurity-related transitions in the CL spectra indicates a significant enhancement in crystalline quality for the non-polar AlGaN films.Moreover,with the optimization of the indium surfactant mass flow,a root mean square value as small as 10.9 nm was achieved, demonstrating a remarkable improvement in the surface morphology of the non- polar a-plane AlGaN epilayer.2.An un-doped non-polar a-plane AlGaN epilayer was successfully grown on the semi-polar r-plane sapphire substrate with the MOCVD technique.In order to improve the surface morphology and crystalline quality of the non-polar AlGaN epilayer,an Al-composition-graded Al_xGa_1-xN（x=0.0–1.0）intermediate layer with varying Aluminium concentration and the layer thickness varied from 260 to 695 nm was inserted in between the high-temperature-grown Al N intermediate layer and the un-doped non-polar a-plane AlGaN epilayer.The structural and optical characteristics of the non-polar a-plane AlGaN epilayers were characterized using AFM,HR-XRD,PL spectroscopy,scanning electron microscope（SEM）,and Hall effect measurement.Significant improvements in the surface morphology and crystalline quality were achieved with the non-polar a- plane AlGaN epilayers grown by introducing an Al-composition-graded AlGaN intermediate layer.In fact,an AFM root mean square（RMS）value as low as 3.5 nm was obtained while the ratio of near-band-edge（NBE）and defect-related （yellow band）emission intensity determined from room temperature PL spectrum was found to have a high value of 1.22.Moreover,a relatively low background carrier concentration down to-4.4380×10¹⁷ cm^-3 was obtained from Hall effect measurement3.High crystalline quality non-polar a-plane AlGaN epilayer was successfully grown on r-plane sapphire substrate by MOCVD technique.In particular,the effect of Ammonia（NH₃）flow-modulation on the crystalline quality and surface morphology of the non-polar a-plane AlGaN epilayer was studied extensively by HR-XRD,AFM,and SEM.It was found that the optimization of the interruption time for the NH₃-flow was very powerful to decrease the dislocation density and to improve the non-polar a-plane AlGaN epilayers surface morphology and crystalline quality.In fact,an RMS value as small as 1.8 nm and a dislocation density lower than 2×10⁹ cm^-3 were achieved by carefully optimizing the NH₃- flow interruption time.Meanwhile,it was demonstrated that the X-ray rocking curves full width at half maximum（FWHM）value along with c-and m-directions could be reduced from 2410 to 1512 arc sec and from 2355 to 1645 arc sec, respectively.These results give us a direction to fabricate high efficiency non- polar AlGaN-based UV-LEDs in the near future.