A method of fabricating an epitaxial stack for Group IIIA-N transistors includes depositing at least one Group IIIA-N buffer layer on a substrate in a deposition chamber of a deposition system. At least one Group IIIA-N cap layer is then deposited on the first Group IIIA-N buffer layer. During a cool down from the deposition temperature for the cap layer deposition the gas mixture supplied to the deposition chamber includes NH3 and at least one other gas, wherein the gas mixture provide an ambient in the deposition chamber that is non-etching with respect to the cap layer so that at a surface of the cap layer there is (i) a root mean square (rms) roughness of ) 2 nm deep less than (