We report on a detailed investigation of molecular beam epitaxy and characterization of GaN epilayers on Si (0 0 1) substrate through controlled nanowire coalescence. By varying the substrate temperature, the axial/ lateral growth ratio of GaN nanowires on Si (0 0 1) can be modulated by nearly two orders of magnitude. As such, the transition from GaN nanowire arrays to coalesced GaN epilayers can be obtained through a relatively thin (similar to 150 nm) intermediate layer, which leads to the subsequent formation of crack-free GaN epilayers on Si (0 0 1) substrate. Detailed scanning transmission electron microscopy (STEM) studies suggest that the resulting GaN epilayers are nearly free of dislocations and stacking faults. Controlled p-type conduction is further achieved for Mg-doped GaN epilayers. Hole concentrations similar to 5.6 x 10(17) cm(-3) were measured at room temperature, with mobility values similar to 3 cm(2)/V.s. Moreover, we have demonstrated functional InGaN/GaN LEDs on Si (0 0 1) substrate, wherein the active region and p-contact layer consist of InGaN/GaN disks-in-nanowires and Mg-doped GaN epilayers, respectively. The devices exhibited a turn on voltage of 2.7 V and strong emission at 525 nm.