Crystal properties of low-temperature grown AIN (LT-AlN) combined with low temperature GaN (LT-GaN) grown by metal organic vapor phase epitaxy (MOVPE) were investigated to obtain a high quality GaN/AlN/GaN structure with a few-nm-thick AIN layer. LT-AIN suppresses unintentional Ga incorporation and can be pseudomorphically grown on GaN with a relatively smooth surface morphology. The lattice of LT-AlN coherent to GaN, however, was found to relax after reactor conditions were changed to grow the subsequent GaN layer at higher temperature. The top GaN layer grown on the relaxed LT-AIN, thus, exhibited a rough surface morphology and a threading dislocation density (TDD) higher than 109 cm(-2) estimated from an X-ray diffraction measurement. An LT-GaN capping layer was found to be highly effective for avoiding such lattice relaxation of LT-AlN. The combination of LT-AIN and LT-GaN enables us to obtain a GaN/AIN/GaN junction with high Al content, a low TDD, and abrupt interfaces. As a result, introducing an LT-GaN layer improved the photoelectrochemical (PEC) property of a polarization engineered un-doped GaN/A1N/n-type GaN (u-GaN/ AIN/n-GaN) photocathode for water splitting.