| Threading dislocations are known to have an adverse effect on carrier mobility and behave as recombination sites in nitride semiconductor devices. The conventional, two step epitaxy for the growth of gallium nitride (GaN) involves the deposition of a low temperature buffer layer, followed by a high temperature growth. The low temperature nucleation layer provides a high density of nuclei but has poor crystalline quality as a result of low adatom mobility at a low growth temperature. The high temperature layer of GaN that is deposited on this nucleation layer has improved crystal quality but the defects present in the buffer layer are incorporated in the GaN as threading dislocations. In this thesis, a study of high temperature growth of GaN on (0001) sapphire by metal organic chemical vapor deposition (MOCVD) is presented. The nanopillars that form upon a short direct high temperature growth are believed to have built in mechanisms that lead to the reduction of threading dislocations. Transmission electron microscopy (TEM) revealed that few dislocations, stacking faults and inversion domains were present in the pillars. The behavior of defects is proposed to be closely related to the growth mode and morphology of the pillars. Inversion domains result from exposure of sapphire to ammonia at a high temperature. Trends in density, height, lateral dimension and spacing between the pillars have been correlated with temperature, V/III ratio, pressure and time as variables. Nitridation of the sapphire substrate significantly affects the morphology of GaN growth. Using the data on density of pillars, a value of about 4 eV was estimated for the activation energy for diffusion of a Ga adatom on the nitrided sapphire surface.;This dissertation represents a part of the work done to obtain this degree. |
