We investigate the spontaneous emission from ordered arrays of GaN nanowires (NWs) with well-defined diameters, representing the building blocks for light-emitting as well as light-detecting devices integrated on Si substrates. The luminous efficiency of these arrays is observed to decrease by more than an order of magnitude when the NW diameter is increased from 120 nm to 240 nm. A detailed analysis of both steady-state and transient photoluminescence data reveals that this quenching is not caused by a corresponding decrease of the internal quantum efficiency. Hence, we examine the coupling of light into and out of the NW arrays by appropriate numerical simulations. While the change in absorbance is minor in the investigated diameter range, the extraction efficiency for thin NWs is enhanced by an order of magnitude as compared to thick NWs. This phenomenon primarily originates in the efficient …