Evolution of threading dislocation density and stress in GaN films grown on (111) Si substrates by metalorganic chemical vapor deposition

We have studied the evolution of threading dislocations (TDs), stress, and cracking of GaN films grown on (111) Si substrates using a variety of buffer layers including thin AlN, compositionally graded Al _x Ga _1-x N (0 ≤ x ≤ 1), and AlN/Al _y Ga _1-y N/Al _x Ga _1-x N (0 ≤ x ≤ 1, y = 0 and 0.25) multilayer buffers. We find a reduction in TD density in GaN films grown on graded Al _x Ga _1-x N buffer layers, in comparison with those grown directly on a thin AlN buffer layer. Threading dislocation bending and annihilation occurs in the region in the graded Al _x Ga _1-x N grown under a compressive stress, which leads to a decrease of TD density in the overgrown GaN films. In addition, growing a thin AlN/Al _y Ga _1-y N bilayer prior to growing the compositionally graded Al _x Ga _1-x N buffer layer significantly reduces the initial TD density in the Al _x Ga _1-x N buffer layer, which subsequently further reduces the TD density in the overgrown GaN film. In-situ stress measurements reveal a delayed compressive-to-tensile stress transition for GaN films grown on graded Al _x Ga _1-x N buffer layers or multilayer buffers, in comparison to the film grown on a thin AlN buffer layer, which subsequently reduces the crack densities in the films.