We investigated the mask-pattern and trimethylalminium (TMAl)-flow-rate dependence of the Al composition in AlGaN selective growth for novel InGaN multiple quantum well lasers having selectively grown ridge structures (RiS-type lasers). The Al composition decreased with increases of the local coverage ratio. This result was explained quantitatively by a simple model in which the Al is deposited as polycrystals on the mask without migration occurring and the Ga is concentrated into the window. Under a high TMAl flow-rate condition, this effect of lowering the Al composition was weakened by the high Ga consumption on the mask, and an desirable Al composition for GaN-based lasers was realized. The model can be used to control the composition of p-AlGaN layers for RiS-type lasers.