The Study Of Growth And Properties Of GaN By MOCVD

GaN as a representative ? nitride material has been widely applied in photoelectronic devices such as photodectors,light emitting diodes,and high electron mobility transistors,due to the wide bandgap,high electron mobility,and high thermal conductivity.However,the heteroepitaxy in GaN growth leads to many problems,such as high-density threading dislocations,native defects and unintentionally doped impurities.These problems strongly affect the electrical and optical properties of GaN film,and even degrade the performance of the GaN based photoelectronic devices.Therefore,it is important to study the role of these threading dislocations and defects in the GaN material.In this thesis,GaN films on sapphire substrates have been prepared by metal organic chemical vapor deposition(MOCVD).Firstly,the effect of?/? in low temperature buffer layers on threading dislocations and defect involved luminescence has been studied.Secondly,the effect of thermal annealing on defect_involved luminescence has been studied.Finally,the effect of the GaN and Al2O3 substrate on Ni self-assembly nanoislands has been discussed.The contents and results in this work are as below.1 The effect of the pressure,?/? in low temperature buffer layer and thickness on threading dislocations in GaN:The optimization shows that the optimized parameter(?/?:1859,Pressure:500 Torr)is for the crystal quality and morphology of the GaN.The analysis by the mosaic model indicates that the densities of the screw and edge threading dislocations gradually increase with the increase of the ?/? in the low temperature buffer layers.In addition,the densities of the screw and edge threading dislocations gradually decrease with the increase of the thickness,and the density of the screw threading dislocations decreases faster than that of the edge threading dislocations.2 The effect of the ?/? in the low temperature buffer layer on the yellow and blue luminescence in GaN:The results indicate that the mechanism of the yellow and blue luminescence is the transition between shallow donors and deep acceptors.As the ?/?increases,the FWHM of(102)and(002)planes increases,but its ratio(FWHM(102)/FWHM(002))is kept at 1.44�0.03.The densities of the yellow luminescence and blue luminescence increases with the increase of the ?/?,but its ratio remains the same(0.70�0.02).The explaination is that the defects trapped in the screw and edge threading dislocations enhance the blue and yellow luminescence,respectively.3 The effect of the annealing temperature and ambient on the yellow and bule luminescence in GaN:The results show that the near-band edge emission dramatically increases,and the yellow and blue luminescence decreases after GaN annealing at 700?.It is mainly attributed to a reduction of the Ga and N vacancies by the annealing.As the temperature increases to 900?,the yellow and blue luminescence starts to enhance,and the near-band edge emission starts to decrease for the nitrogen annealed GaN.It is ascribed to an increase of the Ga and N vacancies in the GaN decomposition.For the oxygen annealed GaN,the PL intensity dramatically decreases at the temperature higher than 900?,including the yellow luminescence,the blue luminescence,and the near-band edge emission.It is attributed to the high-density surface states,which is induced by the incorporation of oxygen atoms in the GaN decomposition.4 The effect of the annealing temperature and ambient on the band bending on the GaN surface:The results show that the upward band bending on the surface is?0.48 eV for the as-grown GaN.For the oxygen annealed GaN,the extent of the band bending on the surface is close to stable below 850? compared to the as-grown GaN,and increases dramatically above 900?.The upward band bending even exhibits an increment of 0.34 eV at 1000?.This is attributed to the high-density surface states from the oxygen incorporation.For the nitrogen annealed GaN,the band bending on the surface is also close to stable from 700? to 850?.The upward band bending just shows a small increment above 900?,and exhibits an increment of 0.07 eV.The small increment is attributed to the increment of the Ga and N vacannies from the GaN decomposition.5 Effect of GaN and sapphire substrate on the magnetic and structure property of the Ni self-assembly nanoislands:The results show that a thin NixN film appears at the interface of the GaN and the Ni self-assembly nanoislands.The corresponding hysteresis loop shows a step due to a low energy barrier for the magnetic reversal process in the NixN film.For the Ni self-assembly nanoislands on sapphire substrates,the diameter of the Ni self-assembly nanoislands increases,and the density decreases as the annealing temperature increases.The Ni nanoislands on the sapphire show superparamagnetism.The blocking temperature is about 400 K due to the nanoislands with large size.