We compare the results of strain state analysis (SSA) and photoluminescence (PL) of six InGaN/GaN quantum well samples with un-doped, well-doped, and barrier-doped structures. Based on the SSA images, a strain relaxation model is proposed for describing the nanostructure differences between the three sets of sample of different doping conditions. In the barrier-doped samples, the hetero-structure-induced strains are fully relaxed such that spinodal decomposition is effectively induced. Therefore, strongly clustering nanostructures are observed. In the well-doped samples, strains are partially relaxed and the spinodal decomposition process can be slightly induced. Hence, weaker composition fluctuations are observed. Then, in the un-doped samples, the un-relaxed strains result in higher miscibility between InN and GaN, leading to the relatively more uniform composition distributions. Between the low- and high-indium samples, higher indium content leads to a stronger clustering behavior. The strain relaxations in the well-doped and barrier-doped samples result in their unclear S-shaped behaviors of PL spectral peaks. The enhanced carrier localization and reduced quantum-confined Stark effect in the barrier-doped samples are responsible for their significant increases of radiative efficiency. (c) 2005 Elsevier B.V. All rights reserved.