| AlN based deep ultraviolet(DUV)light-emitting diodes(LEDs)are important in epidemic prevention and control,sewage purification,medical health and other fields.However,the use of AlN based LEDs is limited because it is difficult to achieve high luminous efficiency,which is caused by Quantum-Confined Stark effect(QCSE).The use of nonpolar a-plane AlN(?-AlN)is a promising method to eliminate the QCSE of Al Ga N based devices,which will improve the efficiency of LEDs.However,it is hard to obtain high quality nonpolar ?-AlN due to the large and inhomogeneous mismatch between the heterogeneous substrate and the ?-AlN epitaxial layer.High temperature annealing(HTA)is an effective and simple method to improve the crystal quality of ?-AlN.Although the crystal quality of the ?-AlN has been greatly improved by HTA,the physical mechanism is still not clear on how the HTA method decreases the defect density and influences the surface morphology of ?-AlN.In this work,the related mechanism has been investigated by experiment and theory.Based on this point,the defect density of ?-AlN is further reduced by Metal-Organic Chemical Vapor Deposition(MOCVD).The main results are listed as follows:1.Control the defects in ?-AlN by HTA and revealing the mechanism of defect evolutionThe defect evolution in high temperature annealed ?-AlN has been studied.From the X-ray diffraction(XRD)results,the crystal quality of ?-AlN is significantly improved by HTA.The calculated basal stacking faults density(BSFD)of the ?-AlN annealed at 1700°C for 180 min is about 5500 cm-1.The minimum energy pathway during the elimination of the I1 stacking fault has been calculated by first-principles calculations.The calculations indicate that the HTA process can provide energy for the elimination of stacking faults and reduce the BSFD.It can be indicated from the calculations that the I3 stacking fault is the intermediate state to eliminate the I1stacking fault.2.Grow ?-AlN on the HTA ?-AlN template by metal organic chemical vapor depositionThe ?-AlN layers are grown on the annealed ?-AlN templates by MOCVD.Comparing with the traditional two-step growth method,the crystal quality of ?-AlN epitaxial layers have been significantly improved by adopting the annealed ?-AlN templates.By optimizing the growth parameters,the stacking fault density of ?-AlN epitaxial layer has been decreased to about 3900 cm-1.3.Reveal the mechanism on the surface evolution of ?-AlN during HTAThe surface evolution of the ?-AlN during HTA process has been investigated experimentally,and the mechanism on the surface evolution has been explored by first-principles calculations.The results indicate that the stripes along the c-axis direction will appear on the surface of the ?-AlN.It can be indicated from the calculations that the binding energy is-4.41 e V/-8.41 e V,-2.75 e V/-2.47 e V,and-2.67 e V/-0.83 e V for the Al/N atoms on c-AlN surface,m-AlN surface and ?-AlN surface.As a result,the Al/N atoms tend to decompose along a-/m-plane AlN and reabsorb along c-plane AlN,which result in the phenomenon that the area of ordered stripes along c-axis increases with higher annealing temperature or longer annealing time. |
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