This work discusses the use of gallium nitride (GaN)-based solid-state devices for high-power, high-frequency, and high-temperature technology. The article presents the results of an investigation into the Al fraction of AlGaN as a function of GaN cap growth time through µ-Raman and µ-Photoluminescence (µ-PL) spectroscopy under λ=325 and 266 nm laser source. The data exhibit that the detected Al fraction decreases as the GaN cap layer size increases, consistently with the surface quantum well effect in the layer stack. The study confirms that the GaN cap layer is acting as a potential well and enables the design of a non-destructive and quantitative assessment of the grown thickness of the GaN cap layer through UV laser spectroscopy. The interpretation of the data also rules out the possibility of thermal migration of Al in the adjacent GaN layers during MOCVD growth.