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Thin AlN spacer layer is widely adopted in Al(In)GaN/AlN/GaN heterostructures to improve the mobility of two-dimensional electron gas (2DEG) by increasing the 2DEG confinement and reducing the remote alloy scattering. However, the nominal AlN spacer is found to be an AlGaN spacer mainly due to the unintentional incorporation of Ga from the decomposition of the underlying GaN channel during the growth of AlN spacer and the temperature ramping process from the GaN channel to the Al(In)GaN barrier. The growth conditions of AlN spacer layer were varied systematically to study their influences on the unintentional Ga incorporation and the 2DEG mobility of AlInGaN/AlN/GaN heterostructures. Interestingly, the nominal AlN spacer (or AlGaN spacer in fact) can still effectively reduce the remote alloy scattering and deliver a 2DEG mobility substantially higher than that of AlInGaN/GaN heterostructures with no spacer or an AlGaN spacer uniformly containing a Ga content similar to the average Ga composition of the nominal AlN spacer. A simplified microstructure of the nominal AlN spacer with an inhomogeneous Ga incorporation was proposed to discuss about the influence of the spacer on the 2DEG mobility. [ABSTRACT FROM AUTHOR]