Near-lattice-matched GaN/Al1-xInxN single quantum wells, grown using both free-standing GaN and conventional GaN-on-sapphire substrates, are studied by photoluminescence (PL) and PL excitation spectroscopies. PL spectra distinguish luminescence originating in the wells, barriers, and underlying GaN buffer layers. The spectra also reveal significant differences between structures grown simultaneously on the different substrates. The quantum well transition energy decreases as the well width increases due to the intense in-built electric fields, estimated to be 3.0 +/- 0.5 MeV/cm, that persist in strain free GaN/Al1-xInxN. Screening of these fields is studied using the excitation power dependence of the PL.