GaN/Al0.1Ga0.9N quantum wells (QWs) are grown by molecular beam epitaxy on (0001) sapphire and (0001) GaN single-crystal substrates. Their optical properties are investigated by temperature-dependent photoluminescence (PL). Ar room temperature, the integrated PL intensity of the homoepitaxial QW is 20 times higher than that of the heteroepitaxially grown QW. In the latter case, the PL intensity rapidly decreases even in the low-temperature range (10-100 K), This is ascribed to the non-radiative recombination of excitons on threading dislocations. In contrast, the PL intensity quenching of the homoepitaxial QW is purely governed by carrier thermal escape. These results demonstrate that GaN bulk substrates offer new opportunities for UV optoelectronics.