The surface morphology, microstructural, and optical properties of indium gallium nitride (InGaN) films grown by plasma-assisted molecular beam epitaxy under low growth temperatures and slightly nitrogen-rich growth conditions are studied. The single-phase InGaN films exhibit improved defect density, an absence of stacking faults, efficient In incorporation, enhanced optical properties, but a grain-like morphology. With increasing In content, we observe an increase in the degree of relaxation and a complete misfit strain relaxation through the formation of a uniform array of misfit dislocations at the InGaN/GaN interface for InGaN films with indium contents higher than 55-60%.