MOCVD GROWTH OF GALLIUM NITRIDE WITH INDIUM SURFACTANT

Open Access
Author:
Won, Dong Jin
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 13, 2010
Committee Members:
  • Joan Marie Redwing, Dissertation Advisor
  • Joan Marie Redwing, Committee Chair
  • Christopher Muhlstein, Committee Member
  • Elizabeth C Dickey, Committee Member
  • Francesco Costanzo, Committee Member
Keywords:
  • GaN
  • AlN
  • nitrides
  • MOCVD
  • Indium surfactant
  • epitaxy
  • heteroepitaxy
  • XRD
  • PL
  • AFM
  • Raman
Abstract:
Indium can act as a surfactant on the growing GaN surface without being incorporated due to its high vapor pressure at growth temperatures of 900 – 1000 oC. Thus, indium surfactant can modify the growth mode of GaN films by changing surface energy and surface kinetics of adsorbed Ga and N adatoms. In this thesis research, the effect of indium surfactant on Ga-polar and N-polar GaN films grown at 950 oC by MOCVD on various substrates such as Si-face SiC, bulk GaN, Si(111), and C-face SiC was studied to investigate the stress relaxation mechanism, structural, and optical properties of GaN films which were modified by the indium surfactant. The effect of indium surfactant on GaN films grown on SiC was studied first. In the 1.8 m thick Ga-polar GaN films grown on lattice-mismatched Si-face SiC substrates utilizing indium surfactant at 950 oC, inverted hexagonal pyramid surface defects, so-called V-defects which consist of six planes, formed at threading dislocations on the GaN surface, which gave rise to the relaxation of compressive misfit stress in an elastic way. Simultaneously, enhanced surface mobility of Ga and N adatoms with indium surfactant lead to improved 2D growth, which may be contradictory to the formation of surface defects like V-defects. In order to find the driving force for V-defect formation in the presence of indium, a nucleation and growth model was developed, taking into consideration the strain, surface, and dislocation energies modified by indium surfactant. This model found that the V-defect formation can be energetically preferred since indium reduces the surface energy of the plane, which gives rise to the V-defect formation and growth that can overcome the energy barrier at the critical radius of the V-defect. These Ga-polar GaN films were found to be unintentionally doped with Si. Thus, an investigation into the effect of intentional Si doping at a constant TMIn flow rate on GaN films was also performed. Si turned out to be another important factor in the generation of V-defects because Si may be captured at the threading dislocation cores by forming Si – N bonds, acting as a mask to locally prevent GaN growth. This behavior appeared to assist the initiation of the V-defect which enables V-defects to easily grow beyond the critical radius. Thus, introduction of indium surfactant and Si doping was found to be the most favorable conditions for V-defect formation in Ga-polar GaN films grown on Si-face SiC substrates. The nucleation and growth model predicted that V-defects may not form in homoepitaxy because the energy barrier for V-defect formation approaches infinity due to zero misfit stress. When indium surfactant and Si dopant were introduced simultaneously during the homoepitaxial growth, V-defects did not form in 1.8 m thick Ga-polar GaN films grown at 950 oC on bulk GaN that had very low threading dislocation density, as predicted by the nucleation and growth model. Ga-polar GaN films grown on Si(111) substrates using indium surfactant showed that additional tensile stress was induced by indium with respect to the reference GaN. Since cracking is known to be a stress relaxation mechanism for tension, the In-induced additional tensile stress is thus detrimental to the GaN films which experience the tensile thermal stress associated with the difference in coefficient of thermal expansion between GaN and the substrate during cooling after growth. The generation of tensile stress by indium seemed correlated with a reduction of V-defects since a high density of V-defects formed under the initial compressive stress at the GaN nucleation stage and then V-defect density decreased as the film grew. Even though the initial misfit stress of the GaN film grown on Si(111) was lower than that of GaN grown on SiC, a high density of V-defects were created under the initial compressive stress. Therefore, the high density of threading dislocations was believed to strongly drive the V-defect formation under In-rich conditions. Consequently, without using high quality bulk GaN substrates, V-defects could not be avoided in Ga-polar GaN films grown on foreign substrates such as Si-face SiC and Si(111) in the presence of indium surfactant and Si dopants during growth. Thus, N-polar GaN films were investigated using vicinal C-face SiC substrates because a theoretical study utilizing first-principles calculations predicted that V-defects are not energetically favored on the N-face GaN. When indium surfactant and Si doping were used during N-polar GaN growth, V-defects did not form, as predicted by theory. This observation suggests that V-defect free N-polar InGaN alloys also can be achieved, which may enable stable green laser diodes with long lifetime to be fabricated using the high indium composition N-polar InGaN films. Fundamental studies of N-polar GaN growth with varying growth parameters such as growth temperature, two-step process, and AlN buffer layer thickness, were conducted to optimize the surface morphology and structural properties of N-polar GaN epilayers for future N-polar InGaN growth.