Structural and optical properties of GaN/AlGaN multiple quantum wells (MQWs) grown by plasma-assisted molecular-beam epitaxy on (1 (1) over bar 0 0) plane free-standing GaN substrates and (0 0 0 1) GaN quasi-substrates have been compared. Atomic force microscopy studies indicate that the films and MQW structures grown on both substrates replicate the surface morphology of the substrates. MQWs with AlGaN barriers grown in the presence of In flux have stronger photoluminescence (PL) intensity than those with AlGaN barriers without In. X-ray diffraction spectra of MQWs grown on the (0 0 0 1) GaN substrates show larger number of superlattices peaks than those grown on (1 (1) over bar 0 0) substrates suggesting that the former have smoother interfaces. The PL spectra of MQWs deposited on (0 0 0 1) GaN substrates, where the growth is in a polar direction, exhibit a red-shift as well as a decrease in peak intensity with increase in well widths. Similar MQW structures on the (1 (1) over bar 0 0) GaN, on which the growth is in a non-polar direction, do not exhibit this phenomenon, which we attribute to the absence of internal electric fields in these structures. PL intensity of MQWs with a well width of 75 A is 20 times stronger for those grown on the (1 (1) over bar 0 0) plane than on the (0 0 0 1) plane GaN substrate. (C) 2002 Elsevier Science B.V. All rights reserved.