The cleaved {11¯00} mirror facets of III-nitride ridge waveguide laser diodes grown on free-standing GaN substrates have been characterized using atomic force microscopy. The measurements indicate that the exposed facets are atomically smooth and therefore they have a much lower roughness than has been reported for cleaved surfaces of III-nitride heterostructures grown on other substrates. Individual heterostructure layers—including InGaN quantum wells only 3nm thick—could be identified from small variations in the height of the exposed surface. These variations are attributed primarily to the partial relaxation of strained layers at the free surface.